LOW SPEED WIND TUNNEL TESTING PDF
Barlow, Jewel B. Low speed wind tunnel testing I by Jewel B. Barlow, William H. Rae, Alan Pope. - 3rd ed. p. cm. Rev. Ed. of: Low-speed wind tunnel testing. 1-John Wiley & Sons ().pdf - Ebook download as PDF File .pdf), Text File . txt) or read Low speed wind tunnel testing I by Jewel B. Barlow, William H. Download as PDF, TXT or read online from Scribd all aspects of low-speed wind tunnel design, analysis, testing, and instrumentation in one.
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Ed. of: Low-speed wind tunnel testing / WiUiam H. Rae, Jr., Alan Pope. 2nd ed. ~ 1 9 8 4. "A Wiley-Interscience publication." Includes bibliographical references. new PDF Low-Speed Wind Tunnel Testing Full Online, new PDF Low -Speed Wind Tunnel Testing Full Page, new PDF LOW SPEED WIND TUNNEL TESTING by. M. T. Boyle. Assistant Professor of Mechanical Engineering. University of Maine. Orono, Maine and.
If a model experiment has the same Reynolds and Mach numbers as the full-scale application, then the model and the full-scale flows will be dynamically similar.
The nondimensional functions for fluid velocity components, pressure coefficient, density, viscosity, and temperature will then be the same for the model and the fullscale flows. In turn the force and moment coefficients will be the same for the model and full-scale flows.
The moments developed by'the model can. In practice it is seldom possible to match both Reynolds number and Mach number to full scale in a model experiment. In fact, it is frequently the case that neither Reynolds number nor Mach number can be matched. Choices must then be made on the basis of which parameter is known to be most important for the type of flow situation under consideration. The matching of Mach number usually applies only to flight vehicles in the high-speed flight region as Mach number effects predominate and the matching of Reynolds number effects is not as critical.
In the low-speed flight region Reynolds number effects predominate and matching of Mach number is not as critical. However, for any experiment a careful evaluation of the effect of Reynolds and Mach numbers should be made to ensure that the results can be applied to the full-scale problem.
Many wind tunnel experiments are seriously sensitive to Reynolds number effects, and no experiment should be attempted without knowledge of material like that found in Chapter 8 and a discussion with the experienced operators of the tunnel to be used. Despite the fact that it is difficult, if not impossible, to match both Reynolds and Mach numbers in most wind tunnel experiments, the wind tunnel still is one of the most useful tools an aerodynamics engineer has available to him or her.
Skillful use of the wind tunnel can make strong contributions to the aerodynamics engineer's goal of quickly and efficiently optimizing his or her design.
The more complex the flow phenomena involved, the more important will be the role of the wind tunnel. An interesting and useful fact that follows from the scaling relations is that the force on a body of a particular shape for which the flow characteristics are a function only of Reynolds number is the same regardless of the combination of size and speed that is used to produce the particular Reynolds number if the fluid, its temperature, and the free-stream pressure are unchanged.
This can be seen by writing the expression for a particular force component. Choosing drag, we have. This indicates that the drag on a particular shape with length of 10 ft at 20 rnph is the same as the drag on the same shape with a length of 1 ft at mph if the fluid temperature and pressure are unchanged. Or the force on a b-scale truck model at mph is the same as the force on the full-scale vehicle at 25 mph.
This would certainly start with a sound basis in the current state of aerodynamic theory. Appropriate results, if such exist, from previous experiments and from previous computational studies are typically of great value.
The aerodynamics engineer must then choose approaches to develop the specific information required to meet the objectives of the immediate program. Three broad categories are commonly recognized: The analytical approach plays a vital role in the background studies and in gaining an appreciation for possibilities, but it never suffices for a vehicle development program.
All development programs from the time of the Wright Brothers to the s were based on a combination of analytical and experimental approaches. During the s the evolution of the digital computer reached a point where solutions to approximate forms of the fluid dynamic equations could be obtained for vehiclelike geometries.
The development of methods and computing machinery have advanced rapidly and have led to many predictions that "computers will replace wind tunnels. It has turned out, however, that the continuing dizzy pace of development of computers notwithstanding, the complexity of real flows has only partially been tamed by the computational approach. Practical computations for complete vehicles for the foreseeable future will require "turbulence models" that up to now at least must be tailored for specific types of flow.
Hammond' presented a review of progress in application of computers to engineering development in both structural mechanics and fluid dynamics.
In the case of fluid dynamics he gave three aspects of development as pacing items for increasing the effectiveness of applications of the computer: The f i s t two continue to advance at a rapid rate. In the case of turbulence models, Hamrnond asserted that while many have been developed it is not clear that there has been progress in terms of achieving generality or significantly improved performance in the period from to Ockendon and OckendonZ assert that "modeling turbulence is the major unsolved problem of fluid dynamics.
According to Speziale, the direct numerical simulations of complex turbulent flows that are of technological importance could require the generation of databases with upward of loZonumbers. This is unlikely to be possible in the near future, and even should it become feasible, it is not clear how this would result in a technologically useful result. The next line of attack that has been expected to minimize requirements for modeling turbulence is large eddy simulation LES.
Speziale discusses the failure of LES to live up to its earlier promise and is proposing. Even small wind tunnels today will commonly have a dedicated computer to manage data gathering and presentation and possibly provide control of the experiment. In addition. This enables the aerodynamics engineer both to check the predicted results and. The most important parts of the matrix or conditions to be included in an experiment will be those parts that are most at variance with the analytical or computational predictions.
So what may appear to applied scientists to be mathematical formalities. Computational methods are now an important tool to be applied in aerodynamic development programs. Advances in computing power have contributed greatly to the capabilities and cost effectiveness of wind tunnels and other experimental facilities. The availability of increased computing power has contributed in other ways to the effectivenessof wind tunnel programs.
Whether or not the equations actually do display these pathologies remains an open problem: It's never been proved one way or the other.
This is true of all large wind tunnels. The process of model design and construc-. According to Doering and Gibbon!
This is an issue of far greater importance to analytical and computational efforts than to experimental work of the nature undertaken to support vehicle design. It is possible that the equations produce solutions which exhibit finite-time singularities. It has never been shown that the Navier-Stokes equations.
If this occurs. Emerging communication technology such as the World Wide Web when linked to the highly computerized wind tunnel of today and tomorrow offers a possibility of the wind tunnel as a virtual laboratory for people for whom physical presence is not convenient or cost effective.
This can shorten the time required to prepare for an experiment provided the wind tunnel facility is intimately involved with the model design so that tunnel mounting features are included in the initial model realization. Figure 2. There are new measurement methods that have been enabled by the availability of powerful dedicated computers and the potency of old methods has been amplified greatly. The air flowing through an open circuit tunnel follows an essentially straight path from the entrance through a contraction to the test section.
In general. The air flowing in a closed return wind tunnel. This is a role that is likely to become more important as project teams are increasingly diversified and information must be delivered with the absolute minimum time delay to a cross section of the development team who may be geographically dispersed. The tunnel may have a test section with no solid boundaries open jet or Eiffel type or solid boundaries closed jet or National Physical Laboratory NPL type.
As with any engineering design. The two basic testsection configurations are open test section and closed test section.
Virtually every wind tunnel with a test section larger than 2 s ft2 is one of a kind. Many of these will be discussed later in this book. An example of a closed circuit tunnel is shown in Figure 2. The two basic types are open circuit and closed circuit. The great majority of the closed circuit tunnels have a single return. This can be important for a tunnel used for developmental experiments with high utilization two or three shifts. If located in a room.
Open Return Wind Tunnels The following are advantages and disadvantages of an open return tunnel: Advantages 1. This is usually a factor only if used for developmental experiments where the tunnel has a high utilization rate. The initial cost is higher due to return ducts and comer vanes. If one intends to run internal combustion engines or do extensive flow visualization via smoke.
Closed Return W i d Tunnels The following are advantages and disadvantages of a closed return tunnel: For larger tunnels test sections of 70 ft2 and more noise may cause environmental problems. Disadvantages 1. Because of the low initial cost. Less energy is required for a given test-section size and velocity.
Open circuit designs are also frequently used by science fair participants who build their own wind tunnels. There is less environmental noise when operating. For a given size and speed the tunnel will require more energy to run. Construction cost is typically much less. Through the use of comer turning vanes and screens. If used extensively for smoke flow visualization experiments or running of internal combustion engines.
The cost of building and operating a tunnel of this size is staggering to contemplate. In fact. Open or Closed Test Section? An open test section in conjunction with an open circuit tunnel will require an enclosure around the test section to prevent air being drawn into the tunnel from the test section rather than the inlet.
Many open test-section. The most common geometry is a closed test section. A number of tunnels are available in which full-scale automobiles are routinely used as test articles. It is also noted that in larger size tunnels a rectangular test section is preferable because it is easier to change a model when working off a flat surface. If one uses the rule of thumb that the model span should be less than 0.
If tunnel has high utilization. The cost of building a model. For closed return tunnels of large size with an external balance. Slotted wall test sections are becoming more common as are test sections that can be converted among two or more configurations. This is an anomaly for aircraft experiments other than takeoff and landing.
For a given power input the use of Freon 12 can increase the Mach number by a factor of 2. Consideration of this goal is given in the following. A second approach is to change the working fluid. In practice most development experiments are done in tunnels with widths from 10 to 20 ft.
The VDT. Some basic issues related to cost of construction and cost of operation of pressurized tunnels must be considered. Despite these problems. Recalling the earlier discussions of flow similarity. The shell cost for a given size will be greater but the proper comparison is the shell cost for equal Reynolds number. Operation of pressurized facilities involves additional time to change the pressure condition and to access the model.
Necessary compressor equipment will add to the cost for pressurized tunnels. For high-speed tunnels capable of sonic speeds or more.
We provide synopses of a number of specialized classes of facilities. There are an increasing number of wind tunnels in use exclusively for other than aeronautical applications. If one increases the pressure by a factor of One of the oldest methods is to build a tunnel that can be pressurized.
These tunnels used an annular return duct. High-Reynolds-Number lhnnels It is often not practicable to obtain full-scale Reynolds numbers by use of a fullscale vehicle in an experimental facility. A drawing of the circuit is shown in Figure 2. This design avoids the high installed power required to drive the larger. Since power varies with the cube of velocity.
Low productivity of the NTF due to the long times required to cycle and stabilize the temperature has prevented it from being useful as a development facility. There will be. The NTF tunnel combines the ability to operate at cryogenic temperatures with the ability to change pressure up to 9 atm. This sort of facility is very expensive both to build and to operate. This was the solution in both the Boeing Helicopter Co. The first. By this technique it is possible to operate over a range of dynamic pressures and Reynolds numbers at a constant temperature to the tunnel's stagnation pressure limit.
The second test section has a cross section of ft2 and speeds from 58 to mph. Recent recognition of detrimental effectson the environment from the use of Freon has led to plans to use a different heavy gas.
The Lockheed low-speed wind tunnel has tandem test sections with two contractions. The range of unit Reynolds number and Mach number is impressive. The European Transonic Facility at Cologne. Flight velocities in the transition region are low.
The working fluid is nitrogen. The United Technologies Research Center has a large wind tunnel with interchangeable test sections. This could be the settling area ahead of the contraction cone or.
The speed available will also be determined by the original tunnel dimensions. These test sections may suffer from poorer flow quality than a tunnel built for the purpose.
The McDonnell-Douglas low-speed tunnel actually has three legs. With this arrangement. Lockheed Martin Aeronautical Systems tunnel in Marietta. The length of the tunnel is increased by this solution. This is attractive. In the design and construction of the tunnel.
These few examples show that there are many ingenious and practical solutions to adapting an existing facility for new experiments.
A dynamically similar model is inserted into the tunnel by an operator in a spinning attitude. The dynamic behavior of the model could be studied in these tunnels. The tunnels could be tilted to set the angle of the air stream to match the glide path of the model.
An example is shown in Figure 2. NASA Langley has performed a considerable number of free-flight experiments in the 30 X 6 0 4 tunnel" with powered models. Free-Flight Tunnels In the s several "free-flight" tunnels were built.
At present. A number of spin. Spin lhnnels or Vertical Wind k n e l s The tendency of some aircraft to enter a spin after a stall and the subsequent need to determine actions to achieve recovery from the spin have been perennial problems of the aircraft designer.
Some spin tunnels use an annular return with turning vanes while others are open circuit with the air drawn in at the bottom and emitted at the top. These tunnels were of the open return type and were arranged so that dimensionally and dynamically scaled models could be flown under the influence of gravity. The recovery from a spin is studied in a spin tunnel. In this way spin modes can be predicted from the measurements without the restrictive requirement of dynamic scaling of the model.
The second test section was curved to simulate turning flight. Germany tunnels have been built in several countries. Propeller lhnnels Propeller tunnels are similar to conventional tunnels with the exception that they usually have an open test section and a round cross section see Chapter 3. This tunnel was moved to the Virginia Polytechnic Institute and State University in where it continues to serve as both a general.
Rotary balances have also been installed in horizontal wind tunnels to carry out similar experimental programs. This tunnel had two interchangeable test sections about 6 ft in size. Similar results are obtained by using oscillating model techniques or free-flight experiments in conventional tunnels. Bihrle Applied Research Tunnel. Since the engine must be operated in the tunnel. Low-Turbulence Tunnels These tunnels usually have a wide-angle diffuser just ahead of the settling chamber in order to increase the size of the settling chamber without a corresponding increase in the overall circuit dimensions.
Two-Dimensional Tnnnels Two-dimensional tunnels are used primarily for evaluation of airfoil sections. Some low-turbulence tunnels of the closed return type have used " curved corners rather than the usual two 90' turns.
The altitude requirement necessitates pumps to provide the low density. These tunnels have. Besides propeller experiments. Propulsion 'hnnels Experimental evaluation of aircraft engines. A novel aspect of this facility is that the fan must be run at idle speed during model changes to prevent it from freezing.
The shell of this tunnel is heavily insulated to help keep the tunnel cold. Among the largest and most powerful experimental facilities in existence is the propulsion test facility at the U. The large settling chamber has honeycombs and a larger than usual number of screens to damp out turbulence.
They have been built both as open circuit and closed return types. The formation of ice on aircraft continues to be a serious safety problem for aircraft and helicopters that operate at low to medium altitudes. The tunnels are usually of the low-turbulence type and may be pressurized to increase the Reynolds numbers. Smoke is used for flow visualization in many general-purpose tunnels.
Wiley_ Low-Speed Wind Tunnel Testing, 3rd Edition - Jewel B. Barlow, William H
For research tunnels. It is also advantageous to use moderate scale such as 0. All of the major automobile manufacturers worldwide either own or have regular access to wind tunnels for such experiments of both model. Smoke has been injected both just before the model and at the tunnel inlet. For smoke sources current practice seems to favor vaporized light oils. External Flows There are two distinct classes of wind tunnels involved in aerodynamic experiments on automobiles.
Smoke tunnels used for research rather than demonstration purposes tend to have very large contraction ratios up to The General Motors Research Laboratory operates both model. In North America there are several wind tunnels used extensively for automobile aerodynamic experiments. The one that is the main focus of this book is concerned first and foremost with the external aerodynamic flow and'with internal flow to the extent it has a significant interaction with the external flow characteristics.
Both two. The Lockheed low-speed wind tunnel in Marietta. Usually these tunnels are of the nonreturn type. Unlike the case of aircraft.
In Europe. The primary manufacturers in Japan. The Chrysler Corp. The Glenn L. Martin wind tunnel GLMWT at the University of Maryland has worked extensively on automobile aerodynamics as well as heavy truck aerodynamics beginning in They are typically very heavily scheduled for their environmental purposes.
Steam Lances Lamps Section Vehicle Entrance An automotive "environmental" wind tunnel. The auto industry frequently refers to wind tunnels. Movable Corner Turning Vane These facilities have capability to heat and cool the airstream. Courtyard Cooling Tower 7. The size that - h- 6. Control Room An example of such a wind tunnel is shown in Figure 2. Ford Motor Co. Every manufacturer has several climatic wind tunnels. This and other issues will be addressed in more detail later in the book.
A major point concerning wind tunnel experiments on automobiles is the question of ground simulation.
J. B. Barlow, W. H. Rae, Jr, A. Pope-Low Speed Wind Tunnel Testing. 1-John Wiley & Sons ().pdf
While basically a singlereturn wind tunnel with a closed test section upstream of an open one. One result is that understanding these types of flow in wind tunnels continues to engage research personnel. The use of a wide-angle diffuser to permit a contraction ratio of A considerable amount of work has been carried out to address thequestionof when this is necessaryand when this conditioncan be relaxed. Generally speaking. Most automobiles are considered to be aerodynamically bluff bodies.
Wind tunnels for automotive experiments are increasingly required to have low-flow noise levels so that wind noise associated with flow around the vehicle can be measured with sufficient accuracy to allow assessment of proposed design variations. Special features include: The principal difference is the clearance between the bottom of the car and the ground.
Realizing the advantages of holding the model and the measuring instrumentation still and letting the fluid move. This leads tointeractions between the flow about the model and the wind tunnel walls or free jet boundary that are somewhat more complex than is the case for bodies with fully attached flow.
A sketch of the tunnel is shown in Figure 2. And the flow quality is generally less than is thought proper for external aerodynamic studies.
To provide a strict simulation. This permitted the length needed for the fan noise suppressors. Extremely heavy concrete constructions plus the use of noise suppression materials on walls. Many general-purpose wind tunnels have been modified to include noise absorption materials and other features to.
An example of a general-purpose facility that has received extensive acoustic treatment is the National Full Scale Facility. An anechoic chamber surrounding the open test section to yield by far the lowest noise levels achieved in a wind tunnel.
The wedges used for reflection cancellation are evident in the background. Section isolation is practiced throughout. Increased understanding of aeroacoustic principles. Water tunnels support direct investigation of cavitation phenomena that cannot be done in a wind tunnel. Courtesy Naval Surface Warfare Center. Water tunnels tend to be physically smaller than wind tunnels for achieving the same Reynolds numbers. It has been possible to achieve low dispersion of die streaks. There is a in.
There are few "large" water tunnels. This tunnel is used for underwater vehicle development. Small water tunnels have been widely used for flow visualization studies.
Many of these tunnels have been closed and some have been destroyed as work that had its origin in these tunnels moved more specialized facilities spawned by increasing understanding of particular problems.
Air Force has one. The most common of these is the 7 X ft class wind tunnel that have those approximate dimensions of their test sections. These facilities continue to be very good for a wide range of vehicle-related experiments and continue to be the best available for a range of special-purpose experiments that have not spawned their own specially designed aerodynamic facilities.
A selection of subjects of aerodynamic experiments not previously mentioned is added here to further emphasize the broad range of applications that arise for lowspeed wind tunnels.
For many purposes they are simply the most cost effective at carrying out exploratory investigations when no fully satisfactory capability is available. Navy has two. Its layout is shown in Figure 2. The U. People Attention has been paid to people-drag of the type encountered by bike racers and skiers.
Martin wind tunnel at the University of Maryland. The fascinating part of these experiments. Birds and Insects Over the years a number of wind tunnel experiments have been made of natural fliers. No such things have been found. It would be preferable to evaluate windmills in the wind gradient that they will eventually see.
In some cases. For skiers. Experiments have shown a change of drag with the number of wheel spokes and other details. Position changes are shown on frontal and side TV projectors. Experiments are usually run at low tunnel speeds. Experiments will probably encompass runs under various power loadings and at different Reynolds numbers. In some cases clothing has been evaluated to seek increases in drag when it is to be worn by an athlete seeking to increase load during training.
Differences that remain are of small magnitude and are within the uncertainty of experiments to date. Live insects have been somewhat more cooperative than live birds and have flown more extensively in tunnels for close observation.
A programmed computation in real time can also be presented so the skier or biker can be shown how much each change helps in terms of race time or distance results. Airborne troops have been trained in vertical wind tunnels so that they are familiar with the condition they will experience when jumping from aircraft.
Substantial improvements in clothing have resulted from such programs. The high landing angles of some buds have been duplicated with highly latticed wings. Initially experimenters were seeking mysterious and incredibly efficient devices that nature's creatures were supposed to have.
Bird fat turns out to have a similar energy content to jet fuel. The tunnel engineer should encourage the windmill promoter to have siting experiments PTior made in an environmental wind tunnel to get the best results in the field. Wind Power Devices Currently. Wind tunnel entries have proven beneficial for showing bike riders their lowest drag posture. At the moment. Measurement of side force is not normally made but should be. For estimating the maximum power coefficient one should use.
The lower maximum power coefficient for the Savonius type is offset in practice by its lower manufacturing cost. Courtesy Verdian-Calspan Operations. A long-term record cannot be established for the mountain location. Natural winds have many times the total power needed for the entire electrical needs of humans.
Solar Collectors Interest in solar energy has spawned a need for wind loads and moments on the various solar collectors. An example of an installation to evaluate the effect of wind screens is shown in Figure 2. Yawing moments and roll moments are measured but are usually of lesser import. Loads are reduced substantially by being shielded by a nearly solid fence around the array or other collectors. It turns out that a maximum of Since winds come from all directions.
The experimental program usually consists of force. Besides needing loads for strength and preservation of the proper focal distance. The tunnel speed range is chosen based on historical wind records at the proposed site.
Aeroacoustic Phased Array Testing in Low Speed Wind Tunnels
As local winds may be quite high. Large installations seem to suffer from making a disturbing buzzing sound. A major difference is that one would not expect to find them in arrays. Pressure data are needed for limiting local deflections of the reflector itself. Runs at several air speeds usually establish that there is little variation with Reynolds number. Windmills should never be put on a rooftop. Array spacing and distance above ground are additional variables.
The collector pitch angle is varied from. Windmills have. Radar Antennas and Satellite Television Receivers The same type of experiments described above for solar collectors may be made for radar antennas or other dish-type receivers. In particular. Radar antennas are common on ships that may expect to encounter high-wind conditions and at those very times may be dependent on the radar for safety. Ship experiments have been somewhat more extensive. It is quite common to evaluate the actual hardware in the wind tunnel to obtain the actual structural integrity and the capability of the drive motors to operate the antenna in the specified wind conditions.
If there is any question at all about structural integrity. Sail material should be varied during an experiment to see what effect. These usually embrace a floor or ground plane model cut off at the waterline. For wind tunnel experiments on exposed antennas the model should be mounted in the tunnel on top of the same structure it will see in the field-trailer. Material roughness and porosity will probably be out of scale. Measurements of drag. Sails and Above-Water Parts of Ships The performance of sails have been evaluated in wind tunnels in limited numbers over the years.
Experiments on speed boats are primarily to find a body shape that has minimum nose-up characteristics. Here the model is set at a range of pitch angles about the stem. Extreme care to duplicate model detail. A relatively recent problem that arises with tankers carrying liquid natural gas is ascertaining that the vents needed as the gas boils off do not constitute a fire hazard.
The current wide. Four types of experiments have been tried: Martin Wind Tunnel. Bridges The effect of natural winds is important to the proper design of long or even intermediate bridge spans.
The models should be kept small enough so that at a yaw angle of 90" bow and stem remain no less than half a ship length from the tunnel walls.
Courtesy Glenn L. Both sail and ship model experiments should have their data corrected for wake and solid blockage. There has been at least one case in which aerodynamic excitation at quite low wind speed led to fatigue cracking of high aspect ratio I beams in a bridge superstructure. Full models in turbulent air are the best.
Drag component includes support drag. 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[ edit ] 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[ edit ] Compilation of images taken during an alpha run starting at 0 degrees alpha ranging to 26 degrees alpha. Images taken at the Kirsten Wind Tunnel using fluorescent mini-tufts. Notice how separation starts at the outboard wing and progresses inward.
Notice also how there is delayed separation aft of the nacelle. Fluorescent mini-tufts attached to a wing in the Kirsten Wind Tunnel showing air flow direction and separation. China clay on a wing in the Kirsten Wind Tunnel showing reverse and span-wise flow Oil flow visible on a straight wing in the Kirsten Wind Tunnel.
Trip dots can be seen near the leading edge. Tufts can be used to gauge air flow patterns and flow separation. Tufts are sometimes made of fluorescent material and are illuminated under black light to aid in visualization.
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. Tempera Paint: Similar to oil, tempera paint can be applied to the surface of the model by initially applying the paint in spaced out dots.
After running the wind tunnel, the flow direction and separation can be identified. An additional strategy in the use of tempera paint is to use blacklights to create a luminous flow pattern with the tempera paint. Fog usually from water particles is created with an ultrasonic piezoelectric nebulizer. 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. Sublimation alternate definition : A flow visualization technique is to coat the model in a sublimatable material where once the wind is turned on in regions where the airflow is laminar, the material will remain attached to the model, while conversely in turbulent areas the material will evaporate off of the model.
This technique is primarily employed to verify that trip dots placed at the leading edge in order to force a transition are successfully achieving the intended goal. High-speed turbulence and vortices can be difficult to see directly, but strobe lights 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 stop-motion images of how the blade cuts through the particulate streams and how vortices are generated along the trailing edges of the moving blade.
Quantitative methods[ edit ] Pressure Sensitive Paint PSP : PSP is a technique whereby a model is spray coated with a paint that reacts to variations in pressure by changing color. In conjunction with this technique, cameras are usually positioned at strategic viewing angles through the walls, ceiling, and floor of the wind tunnel to photograph the model while the wind is on.
The photographic results can be digitized to create a full distribution of the external pressures acting on the model, and subsequently mapped onto a computational geometric mesh for direct comparison with CFD results.
PSP measurements can be effective at capturing pressure variations across the model however often require supplemental pressure taps on the surface of the model to verify the absolute magnitude of the pressure coefficients.
APPENDIX D: AERONAUTICAL SPEED REGIMES AND TEST PARAMETERS
An important property of well behaved PSP paints is they also should be insensitive to temperature effects since the temperature inside the wind tunnel could vary considerably after continuously running. Common difficulties encountered when using PSP include the inability to accurately measure the leading and trailing edge effects in areas where there is high curvature due to limitations in the cameras ability to gain an advantageous viewing angle.
Additionally application of PSP on the leading edge is sometimes avoided because it introduces a finite thickness that could cause early flow separation thus corrupting results. Since the pressure variations at the leading edge is typically of primary interest, the lack of accurate results in that region is very problematic. Once a model is painted with pressure sensitive paint, certain paints have been known to adhere and continue to perform for a matter of months after initially applied.
Finally PSP paints have been known to have certain frequency characteristics where some require a few moments to stabilize before achieving accurate results while others converge rapidly.
In the latter instance paints that have ability to reflect rapid changes in pressure can be used for Dynamic PSP applications where the intent is to measure unsteady flow characteristics. Particle Image Velocimetry PIV : PIV is a technique in which a laser sheet is emitted through a slit in the wall of the tunnel where an imaging device is able to track the local velocity direction of particles in the plane of the laser sheet.
Sometimes this technique involves seeding the airflow with observable material. This technique allows for the quantitative measurement of the velocity and direction of the flow across the areas captured in the plane of the laser.
Model Deformation Measurement MDM : MDM works by placing markers at known geometric locations on the wind tunnel model and taking photographs of the change in the marker's location as the wind in the tunnel is applied.
SlideShare Explore Search You. Submit Search. Successfully reported this slideshow. We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime. Upcoming SlideShare. Like this presentation? Why not share! An annual anal Embed Size px. Start on.According to Speziale, the direct numerical simulations of complex turbulent flows that are of technological importance could require the generation of databases with upward of loZonumbers.
This is a very important concept that leads to significant advantages in experimental work and in theoretical and computational work as well. Sagar Patel. Body forces will also arise in cases of noninertial reference frames. Test-Section Size This is commonly the starting point in the design of a wind tunnel. The first concrete for building was poured on 22 June on a site that eventually would become Calspan, where the largest independently-owned wind tunnel in the United States still operates.
Pope, K. In practice most development experiments are done in tunnels with widths from 10 to 20 ft.
Windmills and Wind Characteristics. A Product.
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