Low Speed Wind Tunnel Testing Barlow Pdf To Jpg

Counterstrike download torrent software keygen crack serial patch. Wisbar advance desktop full version. Price shopper is a shopping list and price manager. Wisbar advance desktop 2. Dawn of war soulstorm 1. Arabian love poems nizar qabbani pdf to jpg. Low speed wind tunnel testing barlow pdf converter. S2p is a stylus.

A model with helium-filled bubbles showing of the A wind tunnel is a tool used in research to study the effects of past solid objects. A wind tunnel consists of a tubular passage with the object under test mounted in the middle. Air is made to move past the object by a powerful system or other means. The test object, often called a wind tunnel model, is instrumented with suitable sensors to measure aerodynamic forces, pressure distribution, or other aerodynamic-related characteristics.

The earliest wind tunnels were invented towards the end of the 19th century, in the early days of aeronautic research, when many attempted to develop successful heavier-than-air flying machines. The wind tunnel was envisioned as a means of reversing the usual paradigm: instead of the air standing still and an object moving at speed through it, the same effect would be obtained if the object stood still and the air moved at speed past it. In that way a stationary observer could study the flying object in action, and could measure the aerodynamic forces being imposed on it. The development of wind tunnels accompanied the development of the airplane. Large wind tunnels were built during World War II. Wind tunnel testing was considered of strategic importance during the Cold War development of supersonic aircraft and missiles. Later on, wind tunnel study came into its own: the effects of wind on man made structures or objects needed to be studied when buildings became tall enough to present large surfaces to the wind, and the resulting forces had to be resisted by the building's internal structure.

Determining such forces was required before could specify the required strength of such buildings and continue to be used for large or unusual buildings. Still later, wind-tunnel testing was applied to, not so much to determine aerodynamic forces per se but more to determine ways to reduce the power required to move the vehicle on roadways at a given speed.

In these studies, the interaction between the road and the vehicle plays a significant role, and this interaction must be taken into consideration when interpreting the test results. In an actual situation the roadway is moving relative to the vehicle but the air is stationary relative to the roadway, but in the wind tunnel the air is moving relative to the roadway, while the roadway is stationary relative to the test vehicle.

Some automotive-test wind tunnels have incorporated moving belts under the test vehicle in an effort to approximate the actual condition, and very similar devices are used in wind tunnel testing of aircraft take-off and landing configurations. Wind tunnel testing of sporting equipment has also been prevalent over the years, including golf clubs, golf balls, Olympic bobsleds, Olympic Cyclists, and race car helmets. Helmet aerodynamics is particularly important in open cockpit race cars (Indycar, Formula One). Excessive lift forces on the helmet can cause considerable neck strain on the driver, and flow separation on the back side of the helmet can cause turbulent buffeting and thus blurred vision for the driver at high speeds. The advances in (CFD) modelling on high speed digital computers has reduced the demand for wind tunnel testing.

However, CFD results are still not completely reliable and wind tunnels are used to verify CFD predictions. Contents • • • • • • • • • • • • • • • • • • • • • • • • • • Measurement of aerodynamic forces [ ] Air velocity and pressures are measured in several ways in wind tunnels. Sync2 2 11 Crackle. Air velocity through the test section is determined.

Measurement of the, the, and (for only) the temperature rise in the airflow. The direction of airflow around a model can be determined by tufts of yarn attached to the aerodynamic surfaces. The direction of airflow approaching a surface can be visualized by mounting threads in the airflow ahead of and aft of the test model.

Smoke or bubbles of liquid can be introduced into the airflow upstream of the test model, and their path around the model can be photographed (see ). Aerodynamic forces on the test model are usually measured with, connected to the test model with beams, strings, or cables. The pressure distributions across the test model have historically been measured by drilling many small holes along the airflow path, and using multi-tube to measure the pressure at each hole. Pressure distributions can more conveniently be measured by the use of, in which higher local pressure is indicated by lowered fluorescence of the paint at that point.

Pressure distributions can also be conveniently measured by the use of pressure-sensitive, a recent development in which multiple ultra-miniaturized pressure sensor modules are integrated into a flexible strip. The strip is attached to the aerodynamic surface with tape, and it sends signals depicting the pressure distribution along its surface. Pressure distributions on a test model can also be determined by performing a wake survey, in which either a single is used to obtain multiple readings downstream of the test model, or a multiple-tube manometer is mounted downstream and all its readings are taken. The aerodynamic properties of an object can not all remain the same for a scaled model. However, by observing certain similarity rules, a very satisfactory correspondence between the aerodynamic properties of a scaled model and a full-size object can be achieved. The choice of similarity parameters depends on the purpose of the test, but the most important conditions to satisfy are usually: • Geometric similarity: all dimensions of the object must be proportionally scaled; •: the ratio of the airspeed to the speed of sound should be identical for the scaled model and the actual object (having identical in a wind tunnel and around the actual object is -not- equal to having identical airspeeds) •: the ratio of inertial forces to viscous forces should be kept.

This parameter is difficult to satisfy with a scaled model and has led to development of pressurized and cryogenic wind tunnels in which the viscosity of the working fluid can be greatly changed to compensate for the reduced scale of the model. In certain particular test cases, other similarity parameters must be satisfied, such as e.g.. History [ ] Origins [ ] English military engineer and mathematician (1707–1751) invented a apparatus to determine drag and did some of the first experiments in aviation theory.

(1773–1857) also used a whirling arm to measure the drag and lift of various airfoils. His whirling arm was 5 feet (1.5 m) long and attained top speeds between 10 and 20 feet per second (3 to 6 m/s). Used a rotating arm to do measure accurately wing airfoils with varying, establishing their polar diagram, but was lacking the notions of and. Eiffel's wind tunnels in the Auteuil laboratory However, the whirling arm does not produce a reliable flow of air impacting the test shape at a normal incidence. Centrifugal forces and the fact that the object is moving in its own wake mean that detailed examination of the airflow is difficult.

(1824–1908), a Council Member of the, addressed these issues by inventing, designing and operating the first enclosed wind tunnel in 1871. Once this breakthrough had been achieved, detailed technical data was rapidly extracted by the use of this tool.

Wenham and his colleague John Browning are credited with many fundamental discoveries, including the measurement of l/d ratios, and the revelation of the beneficial effects of a high. Built an open-section wind tunnel with a centrifugal blower in 1897, and determined the drag coefficients of flat plates, cylinders and spheres.

Danish inventor applied wind tunnels in his process of developing and refining the technology of in the early 1890s. Used a wind tunnel when designing his from 1897 and onwards. In a classic set of experiments, the Englishman (1842–1912) of the demonstrated that the airflow pattern over a scale model would be the same for the full-scale vehicle if a certain flow parameter were the same in both cases. This factor, now known as the, is a basic parameter in the description of all fluid-flow situations, including the shapes of flow patterns, the ease of heat transfer, and the onset of turbulence.

This comprises the central scientific justification for the use of models in wind tunnels to simulate real-life phenomena. However, there are limitations on conditions in which is based upon the Reynolds number alone. The ' use of a simple wind tunnel in 1901 to study the effects of airflow over various shapes while developing their was in some ways revolutionary.

It can be seen from the above, however, that they were simply using the accepted technology of the day, though this was not yet a common technology in America. In, (1832-1923) built his first open-return wind tunnel in 1909, powered by a 50 kW electric motor, at Champs-de-Mars, near the foot of the tower that bears his name. Between 1909 and 1912 Eiffel ran about 4000 tests in his wind tunnel, and his systematic experimentation set new standards for aeronautical research. In 1912 Eiffel's laboratory was moved to Auteuil, a suburb of Paris, where his wind tunnel with a 2-metre test section is still operational today. Eiffel significantly improved the efficiency of the open-return wind tunnel by enclosing the test section in a chamber, designing a flared inlet with a honeycomb flow straightener and adding a diffuser between the test section and the fan located at the downstream end of the diffuser; this was an arrangement followed by a number of wind tunnels later built; in fact the open-return low speed wind tunnel is often called the Eiffel-type wind tunnel. Widespread usage [ ].

Wind tunnel test on a human subject, showing the effects of high wind speeds on the human face Later research into airflows near or above the speed of sound used a related approach. Metal pressure chambers were used to store high-pressure air which was then accelerated through a designed to provide supersonic flow. The observation or instrumentation chamber ('test section') was then placed at the proper location in the throat or nozzle for the desired airspeed.

In the United States, concern over the lagging of American research facilities compared to those built by the Germans lead to the of 1949, which authorized expenditure to construct new wind tunnels at universities and at military sites. Some German war-time wind tunnels were dismantled for shipment to the United States as part of the plan to exploit German technology developments.

For limited applications, (CFD) can supplement or possibly replace the use of wind tunnels. For example, the experimental was designed without any use of wind tunnels.

However, on one test, flight threads were attached to the surface of the wings, performing a wind tunnel type of test during an actual flight in order to refine the computational model. Where external flow is present, CFD is not practical due to limitations in present-day computing resources. For example, an area that is still much too complex for the use of CFD is determining the effects of flow on and around structures, bridges, terrain, etc. Preparing a model in the Kirsten Wind Tunnel, a subsonic wind tunnel at the The most effective way to simulative external turbulent flow is through the use of a boundary layer wind tunnel. There are many applications for boundary layer wind tunnel modeling.

For example, understanding the impact of wind on high-rise buildings, factories, bridges, etc. Can help building designers construct a structure that stands up to wind effects in the most efficient manner possible. Another significant application for boundary layer wind tunnel modeling is for understanding exhaust gas dispersion patterns for hospitals, laboratories, and other emitting sources. Other examples of boundary layer wind tunnel applications are assessments of pedestrian comfort and snow drifting. Wind tunnel modeling is accepted as a method for aiding in design.

For instance, the use of boundary layer wind tunnel modeling can be used as a credit for (LEED) certification through the U.S. Green Building Council. Fan blades of 's 16 foot wind tunnel in 1990, before it was in 2004 Wind tunnel tests in a boundary layer wind tunnel allow for the natural drag of the Earth's surface to be simulated. For accuracy, it is important to simulate the mean wind speed profile and turbulence effects within the atmospheric boundary layer. Most codes and standards recognize that wind tunnel testing can produce reliable information for designers, especially when their projects are in complex terrain or on exposed sites. In the United States, many wind tunnels have been decommissioned in the last 20 years, including some historic facilities. Pressure is brought to bear on remaining wind tunnels due to declining or erratic usage, high electricity costs, and in some cases the high value of the real estate upon which the facility sits.

On the other hand, CFD validation still requires wind-tunnel data, and this is likely to be the case for the foreseeable future. Studies have been done and others are under way to assess future military and commercial wind tunnel needs, but the outcome remains uncertain. More recently an increasing use of jet-powered, instrumented unmanned vehicles ['research drones'] have replaced some of the traditional uses of wind tunnels. How it works [ ].

Six-element external balance below the Kirsten Wind Tunnel Air is blown or sucked through a duct equipped with a viewing port and instrumentation where or geometrical shapes are mounted for study. Typically the air is moved through the tunnel using a series of fans. For very large wind tunnels several meters in diameter, a single large fan is not practical, and so instead an array of multiple fans are used in parallel to provide sufficient airflow. Due to the sheer volume and speed of air movement required, the fans may be powered by stationary engines rather than electric motors. The airflow created by the fans that is entering the tunnel is itself highly turbulent due to the fan blade motion (when the fan is blowing air into the test section – when it is sucking air out of the test section downstream, the fan-blade turbulence is not a factor), and so is not directly useful for accurate measurements.

The air moving through the tunnel needs to be relatively turbulence-free and. To correct this problem, closely spaced vertical and horizontal air vanes are used to smooth out the turbulent airflow before reaching the subject of the testing. Due to the effects of, the cross-section of a wind tunnel is typically circular rather than square, because there will be greater flow constriction in the corners of a square tunnel that can make the flow turbulent. A circular tunnel provides a smoother flow. The inside facing of the tunnel is typically as smooth as possible, to reduce surface drag and turbulence that could impact the accuracy of the testing. Even smooth walls induce some drag into the airflow, and so the object being tested is usually kept near the center of the tunnel, with an empty buffer zone between the object and the tunnel walls. There are correction factors to relate wind tunnel test results to open-air results.

The lighting is usually embedded into the circular walls of the tunnel and shines in through windows. If the light were mounted on the inside surface of the tunnel in a conventional manner, the light bulb would generate turbulence as the air blows around it. Similarly, observation is usually done through transparent portholes into the tunnel. Rather than simply being flat discs, these lighting and observation windows may be curved to match the cross-section of the tunnel and further reduce turbulence around the window. Various techniques are used to study the actual airflow around the geometry and compare it with theoretical results, which must also take into account the and for the regime of operation. Pressure measurements [ ] Pressure across the surfaces of the model can be measured if the model includes pressure taps.

This can be useful for pressure-dominated phenomena, but this only accounts for normal forces on the body. Force and moment measurements [ ].

A typical versus curve With the model mounted on a, one can measure lift, drag, lateral forces, yaw, roll, and pitching moments over a range of. This allows one to produce common curves such as versus angle of attack (shown). Note that the force balance itself creates drag and potential turbulence that will affect the model and introduce errors into the measurements. The supporting structures are therefore typically smoothly shaped to minimize turbulence. Flow visualization [ ] Because air is transparent it is difficult to directly observe the air movement itself. Instead, multiple methods of both quantitative and qualitative flow visualization methods have been developed for testing in a wind tunnel. Qualitative methods [ ].

Fog (water particle) wind tunnel visualization of a NACA 4412 airfoil at a low-speed flow (Re=20.000) • Smoke • Tufts are applied to a model and remain attached during testing. Tufts can be used to gauge air flow patterns and flow separation.

• Evaporating suspensions are simply a mixture of some sort or fine powder, talc, or clay mixed into a liquid with a low latent heat of evaporation. When the wind is turned on the liquid quickly evaporates, leaving behind the clay in a pattern characteristic of the air flow. • Oil: When oil is applied to the model surface it can clearly show the transition from laminar to turbulent flow as well as flow separation. • Fog (usually from water particles) is created with an. The fog is transported inside the wind tunnel (preferably of the closed circuit and closed test section type).

An electrically heated grid is inserted before the test section, which evaporates the water particles at its vicinity, thus forming fog sheets. The fog sheets function as streamlines over the test model when illuminated by a light sheet. • Sublimation: If the air movement in the tunnel is sufficiently non-turbulent, a particle stream released into the airflow will not break up as the air moves along, but stay together as a sharp thin line. Multiple particle streams released from a grid of many nozzles can provide a dynamic three-dimensional shape of the airflow around a body. As with the force balance, these injection pipes and nozzles need to be shaped in a manner that minimizes the introduction of turbulent airflow into the airstream. High-speed turbulence and vortices can be difficult to see directly, but and film cameras or high-speed digital cameras can help to capture events that are a blur to the naked eye. High-speed cameras are also required when the subject of the test is itself moving at high speed, such as an airplane propeller.

The camera can capture images of how the blade cuts through the particulate streams and how vortices are generated along the trailing edges of the moving blade. Classification [ ] There are many different kinds of wind tunnels. They are typically classified by the range of speeds that are achieved in the test section, as follows: • • • • • Wind tunnels are also classified by the orientation of air flow in the test section with respect to gravity.

Typically they are oriented horizontally, as happens during. A different class of wind tunnels are oriented vertically so that gravity can be balanced by drag instead of lift, and these have become a popular form of recreation for simulating: • Wind tunnels are also classified based on their main use. For those used with land vehicles such as cars and trucks the type of floor aerodynamics is also important. These vary from stationary floors through to full moving floors, with smaller moving floors and some attempt at boundary level control also being important.

Aeronautical wind tunnels [ ] The main subcategories in the aeronautical wind tunnels are: High Reynolds number tunnels [ ] is one of the governing similarity parameters for the simulation of flow in a wind tunnel. For less than 0.3, it is the primary parameter that governs the flow characteristics. There are three main ways to simulate high Reynolds number, since it is not practical to obtain full scale Reynolds number by use of a full scale vehicle. • Pressurised tunnels: Here test gases are pressurised to increase the Reynolds number. • Heavy gas tunnels: Heavier gases like and are used as test gases.

The transonic dynamics tunnel at Langley is an example of such a tunnel. • Cryogenic tunnels: Here test gas is cooled down to increase the Reynolds number. The uses this technique. • High-altitude tunnels: These are designed to test the effects of shock waves against various aircraft shapes in near vacuum. In 1952 the University of California constructed the first two high-altitude wind tunnels: one for testing objects at 50 to 70 miles above the earth and the second for tests at 80 to 200 miles above the earth.

V/STOL tunnels [ ] tunnels require large cross section area, but only small velocities. Since power varies with the cube of velocity, the power required for the operation is also less. An example of a V/STOL tunnel is the Langley 14' x 22' tunnel.

Spin tunnels [ ] Aircraft have a tendency to go to spin when they. These tunnels are used to study that phenomenon. Automotive tunnels [ ] Automotive wind tunnels fall into two categories: • External flow tunnels are used to study the external flow through the chassis • Climatic tunnels are used to evaluate the performance of door systems, braking systems, etc. Under various climatic conditions. Most of the leading automobile manufacturers have their own climatic wind tunnels built the first full-scale wind tunnel for motor vehicles. For external flow tunnels various systems are used to compensate for the effect of the boundary layer on the road surface, including systems of moving belts under each wheel and the body of the car (5 or 7 belt systems) or one large belt under the entire car, or other methods of boundary layer control such as scoops or perforations to suck it away. Aeroacoustic tunnels [ ] These tunnels are used in the studies of noise generated by flow and its suppression.

Vertical wind tunnel T-105 at, Moscow, built in 1941 for aircraft testing Aquadynamic flume [ ] The aerodynamic principles of the wind tunnel work equally on watercraft, except the water is more viscous and so sets greater forces on the object being tested. A looping is typically used for underwater aquadynamic testing. The interaction between two different types of fluids means that pure wind tunnel testing is only partly relevant. However, a similar sort of research is done in a towing tank.

Low-speed oversize liquid testing [ ] Air is not always the best test medium for studying small-scale aerodynamic principles, due to the speed of the air flow and airfoil movement. A study of fruit fly wings designed to understand how the wings produce lift was performed using a large tank of mineral oil and wings 100 times larger than actual size, in order to slow down the wing beats and make the generated by the insect wings easier to see and understand. Fan testing [ ] Wind tunnel tests are also performed to precisely measure the air movement of fans at a specific pressure. By determining the environmental circumstances during measurement, and by revising the air-tightness afterwards, the standardization of the data is ensured.

There are two possible ways of measurement: a complete fan, or an on a hydraulic installation. Two measuring tubes enable measurements of lower air currents (.

Main design criteria The general layout of the proposed wind tunnel is shown in. The airflow circulates in the direction indicated in the test chamber (counter clockwise in the figure). Upstream of the test chamber we find the other two main components of the wind tunnel: the contraction zone and the settling chamber.

The other crucial component is of course the power plant. The remainder of the components just serve the purpose of closing the circuit while minimising the pressure loss. Nevertheless, diffuser 1 and corner 1 also have an important influence on the flow quality and they are responsible for more than 50% of the total pressure loss. The design criteria are strongly linked with the specifications and requirements and those must be in accordance with the wind tunnel applications. The building and operation costs of a wind tunnel are highly related to the specifications and these are just a consequence of the expected applications. In the case of the so called Industrial Aerodynamics or educational applications, the requirements related to flow quality may be relaxed, but for research and aeronautical applications the flow quality becomes very important, resulting in more expensive construction and higher operational costs.

General layout of a closed circuit low speed wind tunnel. Figure labels indicate the part name, according to standards. The main specifications for a wind tunnel are the dimensions of the test section and the desired maximum operating speed. Together with this the flow quality, in terms of turbulence level and flow uniformity, must be specified in accordance with the applications. At this point it should also be defined whether all the components of the wind tunnel are going to be placed on the floor in a horizontal arrangement or in a vertical one, with only half of the circuit on the floor and the other half on top of it. Flow quality, which is one of the main characteristics, is a result of the whole final design, and can only be verified during calibration tests.

However, according to previous empirical knowledge, some rules can be followed to select adequate values of the variables that affect the associated quality parameters. The recommended values will be discussed in the sections corresponding to the Contraction, Settling Chamber, Diffusor 1 and Corner 1, which are the wind tunnel parts that have the greatest impact on the flow quality. Once these specifications are given, it is very important to obtain on one side the overall wind tunnel dimensions to check their compatibility with the available room, and on the other side a preliminary estimation of the overall cost.

The cost is mainly associated to the external shape of the wind tunnel and the power plant requirements. For the benefit of new wind tunnel designers, a tool has been devised and implemented in an Excel spreadsheet (visit web page Using this tool the designer will immediately get information about each part of the wind tunnel, the overall dimensions, the global and individual pressure loss coefficients, and the required power. This will be done according to the recommended input parameters and specification based on the intended use of the wind tunnel. Wind tunnel components definition In the following sections the design of each part will be thoroughly discussed and analysed in detail to get the best design addressing the general and particular requirements.

Before dealing with each component, some general comments are given for the most important parts. In the case of the contraction zone, its design is crucial for achieving the required flow quality in the test section. In this sense, its contraction ratio, length and contour definition determine the level of uniformity in the velocity profile, as well as the necessary turbulence attenuation. It is crucial to avoid flow separation close to the walls of the contraction zone. At the stage of design, the most adequate method to verify that design meets those criteria is computational fluid dynamics (CFD). Other important parts of the wind tunnel design worth mentioning here are the corners which incorporate turning vanes.

Their aim is to reduce pressure loss and, in the case of the corner 1, possibly improve flow quality in the test section. The parameters to be considered in their design are the spacing between vanes (whether the space ought to be constant or not) and the possibility of expanding the flow (increasing the cross-section). To complete the design process, the measurement equipment needs to be defined together with the complimentary calibration tests. Special attention needs to be devoted to the specification and selection of the balance for forces measurement, a device that is used to measure aerodynamic forces and moments on the model subjected to airflow in the test section.

Since the drag force on test subjects can be very small and significant noise may be coming from the vibration of the tunnel components, such as the model stand, the true drag value may become obscured. The choice of an appropriate force balance is therefore crucial in obtaining reliable and accurate measurements. The selection depends mainly on the nature of the tests.

Wind tunnel balances can be categorized into internal and external ones. The former offers mobility since it is usually only temporarily mounted to the test section and may be used in different test sections.

However, the latter has more potential in terms of data accuracy and reliability since it is tailored to a specific wind tunnel and its test section. Due to this reason, external force balances should be studied in greater depth. Test chamber The test chamber size must be defined according to the wind tunnel main specifications, which also include the operating speed and desired flow quality. Test chamber size and operating speed determine the maximum size of the models and the maximum achievable Reynolds number. The cross-section shape depends on the applications.

In the case of civil or industrial applications, in most of the cases, a square cross-section is recommended. In this case, the test specimens are usually bluff bodies and their equivalent frontal area should not be higher than 10% of the test chamber cross-sectional area in order to avoid the need of making non-linear blockage corrections. Accurate methods for blockage corrections are presented in ). Nevertheless, a rectangular shape is also recommended for aeronautical applications.

In the case of three-dimensional tests, a typical width to height ratio is 4:3; however, for two-dimensional tests a 2:5 ratio is advised in order for the boundary layer thickness in the test section to be much smaller than the model span. Taking into account that it is sometimes necessary to place additional equipment, e.g. Measuring instruments, supports, etc., inside the test chamber, it is convenient to maintain the operation pressure inside it equal to the local environment pressure. To fulfil this condition, it is recommended to have a small opening, approximately 1,0% of the total length of the test chamber, at the entrance of the diffuser 1. From the point of view of the pressure loss calculation, the test chamber will be considered as a constant section duct with standard finishing surfaces.

Nevertheless, in some cases, the test chamber may have slightly divergent walls, in order to compensate for the boundary layer growth. This modification may avoid the need for tail flotation correction for aircraft model tests, although it would be strictly valid only for the design Reynolds number. Layout of a constant section wind tunnel test chamber. Shows a design of a typical constant section test chamber.

With the typical dimensions and velocities inside a wind tunnel, the flow in the test section, including the boundary layer, will be turbulent, because it is continuous along the whole wind tunnel. According to Idel´Cik (1969), the pressure loss coefficient, related to the dynamic pressure in the test section, which is considered as the reference dynamic pressure for all the calculations, is given by the expression: ζ = λ L / D H, where L is the length of the test chamber, D H the hydraulic diameter and λ a coefficient given by the expression: λ = 1 / ( 1,8 log ⁡ R e - 1,64 ) 2, where Re is the Reynolds number based on the hydraulic diameter. Contraction The contraction or “nozzle” is the most critical part in the design of a wind tunnel; it has the highest impact on the test chamber flow quality. Its aim is to accelerate the flow from the settling chamber to the test chamber, further reducing flow turbulence and non-uniformities in the test chamber. The flow acceleration and non-uniformity attenuations mainly depend on the so-called contraction ratio, N, between the entrance and exit section areas. Shows a typical wind tunnel contraction.

General layout of a three-dimensional wind tunnel contraction. Although, due to the flow quality improvement, the contraction ratio, N, should be as large as possible, this parameter strongly influences the overall wind tunnel dimensions.

Therefore, depending on the expected applications, a compromise for this parameter should be reached. Quoting ), “The effect of a contraction on unsteady velocity variations and turbulence is more complicated: the reduction of x-component (axial) fluctuations is greater than that of transverse fluctuations. A simple analysis due to Prandtl predicts that the ratio of root-mean-square (rms) axial velocity fluctuation to mean velocity will be reduced by a factor 1/ N 2, as for mean-velocity variations, while the ratio of lateral rms fluctuations to mean velocity is reduced only by a factor of N: that is, the lateral fluctuations (in m/s, say) increase through the contraction, because of the stretching and spin-up of elementary longitudinal vortex lines.

Batchelor, The Theory of Homogeneous Turbulence, Cambridge (1953), gives a more refined analysis, but Prandtl's results are good enough for tunnel design. The implication is that tunnel free-stream turbulence is far from isotropic. The axial-component fluctuation is easiest to measure, e.g.

With a hot-wire anemometer, and is the 'free-stream turbulence' value usually quoted. However, it is smaller than the others, even if it does contain a contribution from low-frequency unsteadiness of the tunnel flow as well as true turbulence.” In the case of wind tunnels for civil or industrial applications, a contractions ratio between 4,0 and 6,0 may be sufficient. With a good design of the shape, the flow turbulence and non-uniformities levels can reach the order of 2,0%, which is acceptable for many applications. Nevertheless, with one screen placed in the settling chamber those levels can be reduced up to 0,5%, which is a very reasonable value even for some aeronautical purposes.

For more demanding aeronautical, when the flow quality must be better than 0,1% in non-uniformities of the average speed and longitudinal turbulence level, and better than 0,3% in vertical and lateral turbulence level, a contraction ratio between 8,0 and 9,0 is more desirable. This ratio also allows installing 2 or 3 screens in the settling chamber to ensure the target flow quality without high pressure losses through them. The shape of the contraction is the second characteristic to be defined. Taking into account that the contraction is rather smooth, one may think that a one-dimensional approach to the flow analysis would be adequate to determine the pressure gradient along it.

Although this is right for the average values, the pressure distribution on the contraction walls has some regions with adverse pressure gradient, which may produce local boundary layer separation. When it happens, the turbulence level increases drastically, resulting in poor flow quality in the test chamber. According to ), “The old-style contraction shape with a small radius of curvature at the wide end and a large radius at the narrow end to provide a gentle entry to the test section is not the optimum.

There is a danger of boundary-layer separation at the wide end, or perturbation of the flow through the last screen. Good practice is to make the ratio of the radius of curvature to the flow width about the same at each end. However, a too large radius of curvature at the upstream end leads to slow acceleration and therefore increased rate of growth of boundary-layer thickness, so the boundary layer - if laminar as it should be in a small tunnel - may suffer from Taylor-Goertler 'centrifugal' instability when the radius of curvature decreases”.

According to our experience, when both of the contraction semi-angles, α/2 and β/2 (see, take the values in the order of 12º, the contraction has a reasonable length and a good fluid dynamic behaviour. With regard to the contour shape, following the recommendations of ), two segments of third degree polynomial curves are recommended. Fitting polynomials for contraction shape. As indicated in, the conditions required to define the polynomial starting at the wide end are: the coordinates ( x W,y W), the horizontal tangential condition in that point, the point where the contour line crosses the connection strait line, usually in the 50% of such line, and the tangency with the line coming from the narrow end. For the line starting at the narrow end the initial point is ( x N,y N), with the same horizontal tangential condition in this point, and the connection to the wide end line. Consequently, the polynomials are: y = a W + b W x + c W x 2 + d W x 3, y = a N + b N x + c N x 2 + d N x 3.

Imposing the condition that the connection point is in the 50%, the coordinates of that point are [ x M, y M]=[( x W+ x N)/2,( y W+ y N)/2)]. Introducing the conditions in both polynomial equations, the two families of coefficients can be found. According to ), the pressure loss coefficient related to the dynamic pressure in the narrow section, is given by the expression: ζ = λ 16 s i n α 2 1 - 1 N 2 + λ 16 s i n β 2 1 - 1 N 2, where λ is defined as: λ = 1 / ( 1,8 l o g R e - 1,64 ) 2. The Reynolds number is based on the hydraulic diameter of the narrow section. Settling chamber Once the flow exits the fourth corner (see, the uniformization process starts in the settling chamber. In the case of low-quality flow requirements, it is a simple constant section duct, which connects the exit of the corner 4 with the entrance of the contraction. Nevertheless, when a high quality flow is required, some devices can be installed to increase the flow uniformity and to reduce the turbulence level at the entrance of the contraction (see.

The most commonly used devices are screens and honeycombs. Both devices achieve this goal by producing a relatively high total pressure loss; however, keeping in mind that the local dynamic pressure equals to 1/ N 2 of the reference dynamic pressure, such pressure loss will only be a small part of the overall one, assuming that N is large enough.

General layout of a settling chamber with a honeycomb layer. Honeycomb is very efficient at reducing the lateral turbulence, as the flow pass through long and narrow pipes.

Nevertheless, it introduces axial turbulence of the size equal to its diameter, which restrains the thickness of the honeycomb. The length must be at least 6 times bigger than the diameter. The pressure loss coefficient, with respect to the local dynamic pressure, is about 0,50 for a 3 mm diameter and 30 mm length honeycomb at typical settling chamber velocities and corresponding Reynolds numbers. Although screens do not significantly influence the lateral turbulence, they are very efficient at reducing the longitudinal turbulence.

In this case, the problem is that in the contraction chamber the lateral turbulence is less attenuated than the longitudinal one. As mentioned above, one screen can reduce very drastically the longitudinal turbulence level; however, using a series of 2 or 3 screens can attenuate turbulence level in two directions up to the value of 0,15%. The pressure loss coefficient, with respect to the local dynamic pressure, of an 80%-porous screen made of 0,5 mm diameter wires is about 0,40. If a better flow quality is desired, a combination of honeycomb and screens is the most recommended solution.

This configuration requires the honeycomb to be located upstream of 1 or 2 screens. In this case, the pressure loss coefficient, with respect to the local dynamic pressure, is going to be about 1,5. If the contraction ratio is 9, the impact on the total pressure loss coefficient would be about 0,02, which may represents a 10% of the total pressure loss coefficient.

This implies a reduction of 5% in the maximum operating speed, for a given installed power. The values of the pressure loss coefficients given in this section are only approximated and serve as a guideline for quick design decisions. More careful calculations are recommended for the final performance analysis following ) methods. Rectangular section diffuser.

Diffuser 1 pays an important role in the test chamber flow quality. In case of flow detachment, the pressure pulsation is transmitted upstream into the test chamber, resulting in pressure and velocity non-uniformities. In addition, diffuser 1 acts as a buffer in the transmission of the pressure disturbances generated in the corner 1.

It has been proved that in order to avoid flow detachment, the maximum semi-opening angle in the diffuser has to be smaller than 3,5°. On the other hand, it is important to reduce as much as possible the dynamic pressure at the entrance of the corner 1, in order to minimise the possible pressure loss. Consequently, it is strongly recommended not to exceed the semi-opening angle limit and to design the diffuser to be as long as possible. Diffuser 2 is a transitional duct, where the dynamic pressure is still rather large. Subsequently, the design criterion imposing a maximum value of the semi-opening angle must also be applied. The length of this diffuser cannot be chosen freely, because later it becomes restrained by the geometry of corners 3 and 4 and diffuser 5.

Diffuser 3 guides the flow to the power plant which is strongly affected by flow separation. In order to avoid it, the criterion imposing a maximum value of the semi-opening angle is maintained here as well. The cross-sectional shape may change along this diffuser because it must connect the exit of corner 2, whose shape usually resembles that of the test chamber, with the entrance of the power plant, whose shape will be discussed later.

The same can be said about diffuser 4 because pressure oscillations travel upstream and therefore may affect the power plant. Analogically to the previous case, it provides a connection between the exit of the power plant section and the corner 3, which has a cross-section shape resembling the one of the test chamber.

Diffuser 5 connects the corners 3 and 4. It is going to be very short, due to a low value of the dynamic pressure, which will allow reducing the overall wind tunnel size. This will happen mainly when the contraction ratio is high and the diffusion angle may be higher than 3,5°.

It can also be used to start the adaptation between the cross-section shapes of the tests section and the power plant. An accurate calculation of the pressure loss coefficient can be done with ) method. A simplified procedure, derived from the method mentioned above, is presented here to facilitate a quick estimation of such coefficient. The pressure loss coefficient, with respect to the dynamic pressure in the narrow side of the diffuser, is given by: ζ = 4,0 tan ⁡ α / 2 tan ⁡ α 2 4 ( 1 - F 0 F 1 ) 2 + ζ f. Α being the average opening angle, F 0 the area of the narrow section, F 1 the area of the wide section and where ζ f is defined as: ζ f = 0,02 8 sin ⁡ α / 2 1 - F 0 F 1 2. Corners Closed circuit wind tunnels require having four corners, which are responsible for more than 50% of the total pressure loss.

The most critical contribution comes from the corner 1 because it introduces about 34% of the total pressure loss. To reduce the pressure loss and to improve the flow quality at the exit, corner vanes must be added.

Shows a typical wind tunnel corner, including the geometrical parameters and the positioning of corner vanes. The width and the height at the entrance, W ent and H ent respectively, are given by the previous diffuser dimensions. The height at the exit, H exit, should be the same as at the entrance, but the width at the exit, W exit, can be increased, giving the corner an expansion ratio, W exit/ W ent.

This parameter can have positive effects on the pressure loss coefficient of values up to approximately 1,1. However, it must be designed considering specific geometrical considerations, which will be discussed, in greater details in the general arrangement. The corner radius is another design parameter and it is normally proportional to the width at the corner entrance. Easyusetools For Keygen Tomtom Navigator here.

The radius will be identical for the corner vanes. Although increasing the corner radius reduces the pressure loss due to the pressure distribution on corner vanes, it increases both the losses due to friction and the overall wind tunnel dimensions.

According to previous experience, it is recommended to use 0,25 W ent as the value of the radius for corners 1 and 2, and 0,20 W ent for the other two corners. Scheme of a wind tunnel corner, including vanes, flaps and nomenclature. The corner vanes spacing is another important design parameter.

When the number of vanes increases, the loss due to pressure decreases, but the friction increases. Equal spacing is easier to define and sufficient for all corners apart from corner 1. In this case, in order to minimise pressure loss, the spacing should be gradually increased from the inner vanes to the outer ones. The vanes can be defined as simple curved plates, but they can also be designed as cascade airfoils, which would lead to further pressure loss reduction. In the case of low speed wind tunnels the curved plates give reasonably good results. However, corner 1 may require to further stabilise the flow and reduce the pressure loss.

Flap extensions with a length equal to the vane chord, as shown in, is a strongly recommended solution to this problem. Other parameters, such as the arc length of the vanes or their orientation, are beyond the scope of this chapter. For more thorough approach the reader should refer to Idel´Cik (1969), Chapter 6. As mentioned above, the pressure loss reduction in the corners is very important. Therefore, an optimum design of these elements, at least in the case of corner 1 and 2, has a significant impact on the wind tunnel performance. In order to allow a preliminary estimation of the pressure loss in the corners we will follow the method presented in Diagram 6.33 from Idel´Cik (1969) mentioned above.

In this approach, we take an average number of vanes, n= 1,4* S/ t 1, S being the diagonal dimension of the corner, where t 1 is the chord of the vane. The pressure loss coefficient is given by the expression: ζ = ζ M + 0,02 + 0,031 * r W e n t. Ζ M depends on r/ W ent, and its values are 0,20 and 0,17 for r/ W ent equal to 0,20 and 0,25, respectively. As a result, the corresponding values of ζ are 0,226 and 0,198 respectively, always with respect to the dynamic pressure at the entrance.

This proves the validity of the recommendations given before with regard to the value of the curvature radius and the length of diffusor 1. Power plant The main aim of the power plant is to maintain the flow running inside the wind tunnel at a constant speed, compensating for all the losses and dissipation. The parameters that specify it are the pressure increment, Δp, the volumetric flow, Q, and the power, P.

Once the test chamber cross-section surface, S TC, and the desired operating speed, V, are fixed, and the total pressure loss coefficient, ζ, has been calculated, all those parameters can be calculated using: Δ p = 1 2 ρ V 2 ζ Q = V S T C P = Δ p Q η, where ρ is the operating air density and η the fan efficiency, accounting for both aerodynamic and electric motor efficiencies. In order to reduce the cost of this part by roughly one order of magnitude, we propose to use a multi-ventilator matrix, as presented in, instead of a more standard single ventilator power plant configuration. The arrangement of this matrix will be discussed later.

Layout of a multi-fan power plant. According to our experience, for a closed circuit wind tunnel eventually including settling chamber screens or/and a honeycomb, the total pressure loss coefficient is in the range of 0,16 to 0,24. Consequently, in the case of 1,0 m 2 test section area and 80 m/s maximum operating speed, assuming an average value of ζ to be in the range mentioned above, and for a typical value of η equal to 0,65, the data specifying the power plant are: Δp= 785 Pa, Q= 80 m 3/s, P= 100 kW. In this case we could use a 2,0m diameter fan specially designed for this purpose or 4 commercial fans of 1,0 m diameter, producing the same pressure increment, but with a volumetric flow of 20 m 3/s each. The latter option would reduce the total cost because the fans are a standard product. • Test chamber dimensions: width, W TC, height, H TC, and length, L TC. These parameters allow to compute the cross-sectional area, S TC= W TC H TC, and the hydraulic diameter, D TC=2 W TC H TC/( W TC+ H TC).

• Contraction ratio, N≈5 for low quality flow, and N≈9 for high quality flow (considering the drawbacks of choosing a higher contraction ratio, explained before). • Maximum operating speed, V TC. According to the impact on the wind tunnel dimensions and flow quality, shows a classification of the design variables divided into two categories: main and secondary design parameters. Main and secondary wind tunnel design parameters Now, following the guidelines given above, such as the convergence angle and the contour line shape of the contraction zone, the test and contraction chamber can be fully defined. In the case when both opening angles, α and β, are the same, the contraction length, L C, is given by the expression: L C = N - 1 W T C 2 tan ⁡ ( α C / 2 ). Continuing in the upstream direction, the next part to be designed is the settling chamber.

The only variable to be fixed is the length, because the section is identical to the wide section of the contraction. In the case when high quality flow is required, the minimum recommended non-dimensional length based on the hydraulic diameter, l SC, is 0,60. This results from the necessity to provide extra space for the honeycomb and screens. In all other cases, the non-dimensional length may be 0,50. Therefore, the length of the settling, L SC, chamber is given by: L S C = N W T C l S C.

To obtain all the data for the geometric definition of the corner 4 satisfying all the recommendations given above we only need to fix the non-dimensional radius, r C4. Its length, which is the same as its width, is: L C 4 = W C 4 = N W T C 1 + r C 4. Going downstream of the test chamber, we arrive at the diffuser 1.

Assuming that both semi-opening angles are 3,5°, its non-dimensional length, l D1, is the only design parameter. Although it has a direct effect on the wind tunnel overall length, we must be aware that this diffuser together with corner 1 are responsible for more than 50% of the total pressure losses. According to the experience, l D1>3 and l D1>4 is recommended for low and high contraction ratio wind tunnels respectively. The length of the diffuser 1, L D 1, and the width in the wide end, W WD 1, is defined by: L D 1 = W T C l D 1 W W D 1 = 1 + 2 l D 1 tan ⁡ ( α D 1 / 2 ) W T C. With regard to the corner 1, once its section at the entrance is fixed (it is constrained by the exit of diffuser 1), we must define the non-dimensional radius, r C1, and the expansion ratio, e C1. As a result, the width at the exit, W EC1, the overall length, L C1, and width, W C1, can be calculated using: W E C 1 = W W D 1 e C 1 L C 1 = W W D 1 e C 1 + r C 1 W C 1 = W W D 1 1 + r C 1. Therefore, we can already formulate the overall wind tunnel length, L WT, as a function of the test chamber dimensions, the contraction ratio, and other secondary design parameters: L W T = L T C + W T C [ N - 1 2 tan ⁡ ( α C / 2 ) + N l S C + N 1 + r C 4 + l D 1 + 1 + 2 l D 1 tan ⁡ ( α D 1 / 2 ) e C 1 + r C 1 ].

This quick calculation allows the designer to check whether the available length is sufficient to fit the wind tunnel. Taking into account all the recommended values for the secondary design parameters, a guess value for the wind tunnel overall length, with a contraction ratio N=9 (high quality flow), is given by the formula: L W T = L T C + 16 W T C. In the case when N=5 (low quality flow), the formula becomes: L W T = L T C + 11,5 W T C. The designer must be aware that any modification introduced to the secondary design parameters modifies only slightly the factor that multiplies W TC in the formulas above.

Consequently, if the available space is insufficient, the only solution would be to modify the test chamber dimensions and/or the contraction ratio. As we have already defined the wind tunnel length using the criterion of adequate flow quality, we can now devote our attention to designing the rest of the circuit, the so-called return circuit.

The goal is not to increase its length, intending also to minimize the overall width and keeping the pressure loss as low as possible. Keeping this in mind, the next step in the design is to make a first guess about the power plant dimensions. Following our design recommendations, a typical value for the total pressure loss coefficient of a low contraction ratio wind tunnel, excluding screens and honeycombs in the settling chamber, is 0,20, with respect to the dynamic pressure in the test chamber.

This value is approximately 0,16 for a large contraction ratio wind tunnels. If screens and honeycombs were necessary, those figures could increase by about 20%. As the power plant is placed more or less in the middle of the return duct, the area of the section will be similar to the mid-section of the contraction. Therefore, taking into account the volumetric flow, the total pressure loss, and the available fans, the decision about the type of fan and the number of them can be taken. Using this approach, the power plant would be defined, at least in the preliminary stage. We will return now to the example we started before for the power plant section.

To improve the understanding of the subject, we are going to present a case study. If the test chamber section was square and N=5, the mid-section of the contraction would be 1,67 x 1,67 m 2. This would allow us to place 4 standard fans of 0,800 m diameter each.

The maximum reduction in the width size would be obtained by suppressing the diffuser 5, obtaining the wind tunnel platform shown in. We have not defined the diffusion semi-angle in diffuser 3, but we checked afterwards that it was smaller than 3,5°. Is just a wire scheme of the wind tunnel, made with an Excel spreadsheet, and for this reason the corners have not been rounded and are represented just as boxes. In the case of a 4:3 ratio rectangular test chamber cross-section, the mid-section of the contraction would be 1,869x1,401 m 2 and for this reason we could suggest the use of 6 standard fans of 0,630 m diameter, organized in a 3x2 matrix, occupying a section of 1,890x1,260 m 2.

Shows the wire scheme of this new design. We can check that the diffuser 3 semi-angle is below 3,5° as well. Non-dimensional scheme of a wind tunnel with square section test chamber and low contraction ratio, N≈5. It is clear that the new design is slightly longer and wider, but it is because of the influence of the test chamber´s width, as shown above. Notice that in both cases corner 3 has the same shape as corner 4. Similarly, the entrance section of diffuser 4 is the same as of the power plant section, and using a diffuser semi-angle of 3,5°, this item is also well defined.

At this stage we have completely defined the wind tunnel centre line, so that we can calculate the length, L CL, and width, W CL, using: L C L = L C 1 - W E C 1 / 2 + L D 1 + L T C + L C + L S T + L C 4 - W E D 5 / 2 W C L = W C 4 - W E C 4 / 2 + L D 5 + W C 3 - W E D 4 / 2. The distance between the exit of the corner 1 and the centre of the corner 2, DC1_CC2, can be calculated through the expression (see: D C 1 _ C C 2 = W C L - W E D 1 r C 1 + 1 2. Scheme with the definition of the variable involving the design of diffuser 2 and 3, and corner 2. With this value, by substituting it into the previous expressions, we have all the parameters to design diffusers 2 and 3, and corner 2. Finally, it is necessary to check that the opening angles of diffuser 3 are below the limit. In case when the vertical opening angle, α, exceeds the limit, the best option is to increase the diffuser 1 length, if this is possible, because it improves flow quality and reduces pressure loss.

If the wind tunnel length is in the limit, another option is to add the diffuser 5 to the original scheme. However, it will increase the overall width. When the limit of the horizontal opening angle, β, is exceeded, then the best option is to adjust the values of the expansion ratio in corners 1 and 2, because it will not change the overall dimensions. The following case study is a wind tunnel with high contraction ratio, N≈9, and square section test chamber. In this case, the approximate area of the power plant section will be 2,000 x 2,000 m 2. In this case we have two compatible options to select the power plant.

We can just select a matrix of 4 fans, 1,000 m diameter each. However, if the operating speed is rather high, in order to be able achieve the required pressure increment and the mass flow, we may need to use 1,250 m diameter fans. Shows both options. Note that the overall planform is only slightly modified and the only difference is the position where the power plant is placed. The design of the diffusers 2 and 3, and the corner 2 will be done following the same method as for the previous cases. Wind tunnel construction One of the most important points mentioned in this chapter refers to the wind tunnel cost, intending to offer low cost design solutions. Up to now we have mentioned such modifications to the power plant, proposing a multi-fan solution instead of the traditional special purpose single fan.

The second and most important point is the wind tunnel’s construction. The most common wind tunnels, including those with square or rectangular test sections, have rounded return circuits, like in the case of the NLR-LSWT. However, the return circuit of DNW wind tunnel is constructed with octagonal sections. Although the second solution is cheaper, in both cases different parts of the circuit needed to be built in factories far away from the wind tunnel location, resulting in very complicated transportation operation.

Non-dimensional scheme of a wind tunnel with rectangular section test chamber and large contraction ratio, N≈9. To reduce the costs, all the walls can be constructed with flat panels, which can be made on site from wood, metal or even concrete, like in the case of ITER’s wind tunnel. Shows two wind tunnels built with wood panels and aluminium standard profile structure. Both wind tunnels shown in are open circuit. The one on the left is located in the Technological Centre of the UPM in Getafe (Madrid) and its test chamber section is 1,20 x 1,00 m 2. Its main application is mainly research. The right one is located in the Airplane Laboratory of the Aeronautic School of the UPM.

Its test chamber section is 0,80 x 1,20 m 2, and it is normally used for teaching purposes, although some research projects and students competitions were done there as well. Despite the fact that these tunnels are open circuit, the construction solutions can be also applied to closed circuit ones. Research and education purpose wind tunnels built with wood panel and standard metallic profiles, with multi-fan power plant. According to our experience, the manpower cost to build a wind tunnel like those defined in to could be 3 man-months for the design and 16 man-months for the construction. With these data, the cost of the complete circuit, excluding power plant, would be about 70.000,00 €. In our opinion, the cost figure is very good, considering the fact that the complete building time possibly may not exceed even 9 months. We have more reliable data with regard to the ITER wind tunnel, built in 2000-01.

The whole cost of the wind tunnel, including power plant, work shop and control room, was 150.000,00 €. This wind tunnel was almost completely built with concrete. Shows different stages of the construction, starting from laying the foundations to the almost final view. The small photos show the contraction, with the template used for wall finishing, and the power plant.