Flow separation should be avoided nents such as a turning section, vibration isolator, and jet inside the tunnel circuit to prevent flow unsteadiness and as- collector for open-jet test sections are also discussed. Fundamentals of Wind-Tunnel Design 5 Honeycomb removes swirl from the incoming flow and minimizes the lateral variations in both mean and fluctuat- ing velocity Mehta and Bradshaw, Honeycomb comes in differ- ent shapes such as circular, square, and hexagonal cross sec- tions.
Among these, hexagonal is usually the cross-sectional shape of choice, as it has the lowest pressure drop coeffi- cient Barlow, Rae and Pope, Honeycomb cells have been shown to have the best performance with a length-to- diameter ratio of between 7 and 10 Mehta and Bradshaw, Mehta and Bradshaw also state that the cell size should be smaller than the smallest lateral wavelength of the velocity variation. The honeycomb section should have sufficient structural rigidity to withstand applied forces dur- ing operation without significant deformation.
Special con- sideration may be required if the incoming Mach number to the honeycomb section is high enough that flow choking is possible. Figure 2. Flow chart for low-speed, low-noise, wind-tunnel design.
Tensioned screens are placed in the settling duct for the reduction of turbulence levels of the incoming flow. Screens break up the large-scale turbulent eddies into a number 3. Schubauer, In most tunnels, the flow-conditioning section contains a hon- Spangenberg and Klebanoff state that the Reynolds eycomb, screens, and a settling duct. An example honeycomb number based on the screen wire diameter should be less section is shown in Figure 3. The honeycomb aligns the flow than 60 to prevent additional turbulence generation due to with the axis of the tunnel and breaks up larger-scale flow vortex shedding.
The screens cascade large-scale turbulent fluc- The spacing between the screens should be of the order of tuations into smaller scales. These decay in the settling duct, the length scale of the large energy-containing eddies Mehta which must be sufficiently long to allow for sufficient de- and Bradshaw, Placing multiple screens in the settling cay while minimizing boundary-layer growth see Turbulent duct with varying porosity, with the coarsest screen being Boundary Layers.
A settling chamber is necessary after the screens so the smaller-scale fluctuations generated by the wires can decay before accelerating through the contraction. The contraction accel- erates and aligns the flow into the test section. The size and shape of the contraction dictate the final turbulence inten- sity levels in the test section Derbunivich et al.
The contraction stretches vortex filaments, which reduces axial but intensifies lateral turbulent fluctuations Tennekes and Lumley, The length of the contraction should be suf- ficiently small to minimize boundary-layer growth and cost but long enough to prevent large adverse pressure gradients Figure 3.
Schematic of a hexagonal honeycomb section. A common rule of thumb in test section sizing is to have rectangular dimensions with a ratio of about 1. Schematic of the contraction shape with matched poly- ducing the load of the drive system. The flow field within nomials. The orientation, size blockage , and wake lead to flow separation. While CFD may be used in modern development of the airfoil models are some of the factors design schemes, Morel suggested a simple analytical that affect the diffuser entrance flow.
The area of the diffuser method of matched polynomials. A schematic of the con- should increase gradually along its axis, so as to prevent flow traction shape polynomial is shown in Figure 4. The entrance separation. As with contraction sections, diffuser geometry height of the contraction is Hi , and the exit height is He. The can be optimized.
At the exit of the contraction, any remaining be less than 3. Contractions with mini- design of diffusers, as shown in Figure 5. The area ratio AR mal flow nonuniformities can be designed by iteratively se- between the exit and entrance of the diffuser is plotted versus lecting the entrance height, contraction ratio, match point, the ratio of diffuser length to the entrance height of the dif- and length of the contraction.
Techniques using 3-D poten- fuser. Three regions are shown on the plot. The test section design should allow for ease of ac- 2. Adapted from Runstadler, acoustic reverberation. Fundamentals of Wind-Tunnel Design 7 diffuser is conducted by selecting a length for the diffuser, One of the main components of background noise comes that is, within facility size constraints.
When designing a low- height is dictated by the test section size , the corresponding disturbance facility, fan noise must be attenuated. The blade value of AR is selected from the no-stall regions. A spectrogram con- tions. If facility constraints limit the length of the diffuser or tour plots of sensor signal power vs.
Such contamination can be reduced via acoustic treatment of the tunnel circuit and other acoustic paths in open-circuit case between the fan and test 3. The drive system generates a volume flow rate and com- pensates for the remaining pressure losses.
The driver can be a fan, blower, or a regulated compressed gas source. For an open-jet test section, a collector is required at the Fans are rated by the volume flow rate and the static pres- downstream end of the test section to capture the jet. The sure drop they can overcome. The the tunnel circuit, aiding in fan selection. Load curves are estimated for various fan bulent shear layer impingement on the collector.
The col- rotational speeds. The pressure loss calculation Barlow, Rae lector is acoustically treated in an acoustic wind tunnel. Its and Pope, leads to the wind-tunnel performance curve, effect on flow and acoustic measurements requires careful which is an estimate of the static pressure loss for various assessment.
Fans provide optimal significant pressure drop, flow unsteadiness, and noise gen- performance when the tunnel operating points fall near the eration. Turning vanes are installed in corner sections to mit- maximum efficiency of the fan, as shown in Figure 6. Collar and Salter developed some early designs for turning vanes. Gelder et al.
The chord, thickness-to-chord ratio, shape, and the number of turning vanes determine the efficiency, as defined by flow losses, of the turning section. Modern CFD can now be used to design advanced turning vanes that function well over the entire speed range of the tunnel. In addition, the turning vanes offer the possibility for cooling or heating the flow via heat transfer between the tunnel flow and fluid passing through the vane interior.
Mechanical vibrations generated by the drive system can propagate into the test section through ductwork and the ground. These vibrations must be minimized if aero-optic, transition, or acoustic measurements will be conducted. Vibration isolation devices consist of passive and active mounts, as well as a flexible bellows section Beranek and Figure 6. Fan load curve. The most obvious test is to check that the facility operates safely at desired flow speeds, but be- yond this a series of careful experiments are recommended.
Some of the most common characterization experiments are in the text below. Single-, dual-, and triple-wire CTA configurations. Reproduced with permission from Dantec Dynamics Clearly, uniform velocity in the test section is desired. The in- viscid core of the test section flow should have as little devia- and Mehta, , meaning the axial and lateral fluctuations tion from a plug profile as possible.
There are a large number can be drastically different. As such, a measurement scheme of experimental techniques available to measure the mean should be used to resolve these two different velocity com- velocity profile Tavoularis, , including traversing a ponents.
In the simplest configuration, a measurement can be pitot-static or pitot probe through the test section see Pres- conducted with a single wire normal to the incoming flow, sure and Velocity Measurements. This is a proven, inexpen- and then the wire can be rotated such that the test section ax- sive, but time-consuming technique.
A rake may be used as ial fluctuations would be rejected in a repeated experiment. Single or multiple hot wires configured for sired, a dual-wire probe configuration is usually used. For constant-temperature anemometry CTA , particle image ve- complete velocity steady and unsteady vector decomposi- locimetry PIV , and laser Doppler velocimetry LDV can tion, a triple wire assembly is required.
These configurations also be used. The flow uniformity is usually characterized as are shown in Figure 7. An array of hot wires can be used either as a min—max or an rms deviation from the mean veloc- if spatial—temporal data or wave number spectra are desired.
Turbulence intensity TI is typically computed as in equa- tion 1 expressed as a percent of the local mean velocity. When computing TI, it is common to high-pass filter the data 4.
While many full-scale applications may have sig- Mehta, Velocity spectra can be evaluated. An alterna- nificant incoming turbulence levels, wind-tunnel tests often tive method uses LDV, although it requires flow seeding that seek to isolate the effects of incoming freestream turbulence.
For example, in boundary-layer transition studies, signi- LDV can be used in single-, dual- as shown in Figure 8 , ficant levels of incoming turbulence can alter the behavior of or triple-beam configurations to resolve one, two, or three the boundary-layer transition location.
In aeroacoustic stud- components of velocity. A single hot wire can be traversed through the core of the test sec- tion, and the bridge signal measured and analyzed at each 4. Note that a single hot wire will resolve a velocity magnitude normal to the wire line, with ambigu- If the facility is used for acoustic measurements, background ous angle of incidence, while rejecting velocity fluctuations noise should be assessed.
Ideally, an aeroacoustic flow facility along the axis of the wire. It should also be noted that in most should have background noise levels at least 10 dB below wind tunnels, test section turbulence is anisotropic Bradshaw the acoustic source of interest in a test Duell et al.
Completed facilities must be carefully characterized to determine their characteristics. Two-beam LDV configuration for two-component veloc- ity measurements. Reproduced with permission from R. The He duct exit height m selection is typically based on where the majority of acous- Hi duct inlet height m tic measurements will be made. AIAA Paper Nashville, Tennessee, July Amiet, R. Brooks, T.
Bruun, H. Technical Report Pope, A. Derbunivich, G. Rasmi, S. Gaza, 2, — Duell, E. Hanover, New Hampshire. Breck- Salter, C. Reno, Nevada, January Saric, W. Fluid Wind tunnel turning vanes of modern design. Sylvain Mbutot. A short summary of this paper. Download Download PDF. Translate PDF. Recent developments in Formula SAE Society of Automotive Engineers have included the design and implementation of aerodynamic devices such as inverted wings and undertrays to improve performance.
In this work the literature of undertray technology is presented and a design of an undertray for the Global Formula Racing car is developed.
Computational Fluid Dynamics simulations are used to iterate the design and discover the effect on the downforce developed of various vehicle parameters such as speed, ride height and roll.
Predicted performance is then tested using on-track data and statistical analysis is preformed on lap times from a back-to-back comparison to identify the gain of the undertray. My signature below authorizes release of my thesis to any reader upon request.
Robert Paasch for his support and advice throughout this work as well as the Global Formula Racing team for their support of the project. I would also like to thank my committee members for their time and expertise. Current Undertray Technology Since the competitions inception in , the cars have been evolving and changing and there has been no single design that stands out as "the best. One development that seems to be more common of late is the use of downforce producing aerodynamic elements [].
Downforce is the vertical force that is produced from aerodynamic loads instead of mass. A tires coefficient of friction will decrease with added vertical force. This means that a lightweight car will be able to make more efficient use of its tires than a heavier car and will be able to accelerate faster in any direction. Aerodynamic elements, however, produce vertical load on the tires with very little added mass, giving the tires more grip and allowing the car higher acceleration [7].
These elements come in many forms, but the major contributors to downforce are inverted wings and underbody diffusers. Design of aerodynamic elements for race cars is complex due to the body interactions between the elements and the car, wheels, etc, and has in the past been mostly an experimental science [7, 8]. This simulation can greatly reduce the cost and time needed to test aerodynamic elements.
In this work the design of a Formula SAE undertray is developed using CFD and verified with on-track testing to determine actual vehicle performance increase. Like a venturi there is a nozzle that increases the velocity of the air underneath the vehicle, a throat where the maximum velocity is reached and a diffuser where the air is slowed back down to free stream velocity.
Bernoulli's Equation shows us that as the local velocity increases relative to the free stream velocity the local pressure is decreased. Using this lower pressure under the vehicle and the higher pressure on top, downforce can be created.
Like a venturi, the efficiency of an undertray is only as good as the efficiency of the diffuser section [12]. Due to its high visibility relative to the rest of the undertray, there are some common misconceptions in the race car industry to how a diffuser works [10]. Both of these concepts are false since the role of the diffuser is to slow the air under the vehicle back down to free stream to reduce the drag and increase the overall undertray efficiency, and as it is an open system with gaps around the edges it is unable to expand the air to cause a density change.
With these things in mind, it is the diffuser angle and entrance location that drives the undertray performance. The location of the entrance of the diffuser greatly affects where the low pressure occurs on the vehicle undertray. Data presented in Katz et al. To move the center of pressure of the undertray or the balance, the low pressure concentration can be moved by changing the location of the diffuser entrance more forward or rearward.
For a race car, balance is critical to vehicle performance due to its effects on understeer and oversteer characteristics []. The angle of the diffuser relative to the ground affects the magnitude of downforce that is created [7, ].
In general it is desired to have the highest angle without flow separation to generate maximum downforce. Once separation occurs the downforce is reduced and drag is greatly increased [10]. However in experiments and 3-dimensional simulation there is another effect that is occurring that changes this.
Starting at the diffuser entrance there is a vortex that forms that travels down the length of the diffuser. This vortex flow also adds energy to the flow and will delay separation allowing larger diffuser angles [7, ]. Vortices can also be used on other parts of the undertray. Large vortex generators can be placed at the entrance of the undertray so that the vortices travel along the length of the vehicle, reducing the pressure and increasing downforce [, 14].
These vortices can also be used along the sides of the undertray creating a "false seal" that also increases downforce [9]. All of these ideas can be used together to create an effective undertray that will produce large amounts of downforce with a relatively small increase to drag. The problem that occurs however is that there are complex interactions between all parts of the undertray as well as the car body, making design an uncertain area. Also, since racing is a competitive sport, most of the specific information about undertray design is not published.
The solution to the simulation can be used to observe pressure, velocity, downforce, drag and any other fluid properties of interest. Little work has been published using CFD to help design a Formula SAE car [2, , 15], and most of the work has been in 2-dimension simulation of airfoils. The complexity of the CAD geometry depends on what is of interest from the data and how much computing power is available.
For external aerodynamics there is a "wind tunnel" box that is placed around the model. The entrance to the wind tunnel is placed a few car lengths ahead of the geometry and is considered a velocity inlet [ 19].
The exit to the wind tunnel is then placed many car lengths behind the geometry and is considered a pressure outlet []. Since the simulation will be of an open wheeled car the tires should be rotating and the ground set to a moving ground or frictionless. From the literature review it was found that the simulation of the tires was important to the accuracy of the rest of the model [5, ]. For the CFD model it is critical that the tires are rotating and that the mesh will capture the behavior of the flow around them.
Since the airflow around a race car is very turbulent, a model needs to be selected for simulation of the turbulent flow. Mesh numbers vary widely depending on the simulation being done and the computational power available. For external aerodynamics it was found that a mesh on the order of a million cells is enough to predict aerodynamic forces [21]. These simulations can be very beneficial to the designer as they can give visual aids and data of the interactions that are occurring as well as flow trends that were not thought about before.
These numbers were chosen based from the literature of what the CFD simulations would predict with the resources available. Turn-around time also needed to be relatively quick, on the order of a few days, so that the design could progress and be finalized in the typical three month design cycle of Oregon State University Formula SAE.
Imported CAD geometry was used for all surfaces. This extra room was to insure that the boundary conditions could be met with the geometry of the vehicle included. To reduce the total cell count, and therefore computing time, a symmetry plane was used down the center of the vehicle.
Figure 1 Simulation geometry with labels. As seen in the literature, rotating tires have a significant effect on the flow field around the vehicle.
Mesh size was chosen such that all detail of the vehicle was accurately captured and overall cell count was on the order of a million. A prism layer mesh of five layers was included to capture near-wall effects, this detail can be seen in Figure 2.
The mesh was allowed to grow to large sizes away from the vehicle geometry to reduce cell count where the detailed solution was not important. Figure 2 Prism layer mesh detail.
It has been shown to produce the accuracy of results desired with minimal computing power required and good stability. Once it is verified the designers have more confidence in using the simulation as a tool. For automotive external aerodynamics there are two major areas of testing: wind tunnel and on track [, , ]. Wind tunnel testing is broken down into two areas of full-scale testing and model testing. Full- scale testing uses a full-scale model of the car, or the car itself, to test lift and drag of the vehicle [2, 5, 27, 28].
Scale models are usually cheaper to create parts for than a full-scale car and are easier to handle and store. The disadvantages are that you have to correct for the scale of the vehicle and that often the design used on the scale model will be conservative for the full scale car, meaning that it can be improved more [9].
Both wind tunnel methods have the advantage of doing flow visualization such as smoke, oil streaking and yarn tufts [5, ] and also that consecutive runs can be completed quickly. Since the underbody of the car is of most interest, some sort of boundary layer control needs to be used. Katz et al. The first option would be to just test lap times on a closed course [3].
This would give the representative gain or loss of the aerodynamic changes to the vehicle. The down-side is the factor of the human driver that can skew the data, as well as the time on the track is usually expensive in terms of labor, track use and also wear on the car. Flow visualization for on track testing is also limited and can generally only be oil streaking or yarn tufts [10]. In order to properly capture the data the car must be equipped with data acquisition and the proper sensors to detect lift, drag and balance [9].
Using the sensors installed on the car straight line testing was conducted with the undertray. These runs were conducted to test aerodynamic effects at different speeds of the undertray, and also closely matched simulation testing for verification of the model.
For back-to-back comparisons with and without the undertray an asymmetric oval course was set up. The asymmetric oval consists of a foot large diameter corner and a 30 foot small diameter corner with their centers placed feet apart.
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