The BMW AVZ Wind Tunnel Center

<div class="section abstract"><div class="htmlview paragraph">Open jet wind tunnels are normally tuned to measure “correct” results without any modifications to the raw data. This is an important difference to closed wall wind tunnels, which usually require wind tunnel corrections. The tuning of open jet facilities is typically done experimentally using pilot tunnels and adding final adjustments in the commissioning phase of the full scale tunnel. This approach lacked theoretical background in the past. There is still a common belief outside the small group of people designing and using open jet wind tunnels, that - similar to closed wind tunnels, which generally measure too high aerodynamic forces and moments without correction - open jet wind tunnels measure coefficient too low compared to the real world. The paper will try to show that there is a solid physical foundation underlying the experimental approach and that the expectation to receive self-correcting behavior can be supported by theoretical models.</div><div class="htmlview paragraph">During the past years an improved understanding of test section interference in open jet wind tunnels has been developed. In addition there is a growing number of new automotive open jet wind tunnels, making use of several specific advantages of this design for development work in the car industry. Therefore, in this paper an attempt is made to develop design criteria for open jet wind tunnels to systematically approach a self-correcting behavior of the test section, based on classical theory for the prediction of test section interference.</div><div class="htmlview paragraph">In order to achieve a balance between different blockage effects which is independent (or only weakly dependent) of the characteristics of the test object, it is important to set up the balance in an appropriate way. Test section interference effects should be balanced against each other in a way that parameters increasing the interference act on the two counterparts in a similar magnitude. For example, effects related to solid blockage should be balanced against each other and gradient effects should be treated in a second group. The approach of this paper results in design charts for the distance between the nozzle and the model and design strategies to cope with the influence of the collector. Finally, an ideal open jet wind tunnel with reliably self-correcting behavior is outlined.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The Ford Motor Company Rolling Road Wind Tunnel (RRWT) is a state-of-the-art aerodynamic wind tunnel test facility in Allen Park, Michigan. The RRWT has operated since January 2022 and is designed for passenger and motorsport vehicle development. The test facility includes an office area, three secure customer vehicle preparation bays, a garage area, a vehicle frontal area measurement system, and a full-scale ¾ open jet wind tunnel. The wind tunnel features an interchangeable single belt and 5-belt Moving Ground Plane (MGP) system with an integrated 6-component balance, a two-position nozzle, boundary layer removal systems, and two independent flow traverse systems. Each flow traverse has a large horizontal box beam and vertical Z-strut that can position the flow traverse accurately within the test volume. The individual traverse systems can also operate in tandem and have the capability to perform stereo PIV measurements and scale model vehicle testing, including vehicle passing or drafting simulations. The MGP can be quickly changed between single belt and 5-belt operations, requiring only a single eight-hour shift to transition between the two configurations. The efficient wind tunnel airline is capable of airspeeds up to 250 km/h in the large 26.4 m<sup>2</sup> nozzle configuration and 322 km/h in the small 18.6 m<sup>2</sup> nozzle configuration using a 12-blade, 8 m diameter fan with a 5.4 MW motor. The wind tunnel’s flow quality and aerodynamic performance compare well with similar facilities in the automotive industry. Ford Motor Company is currently correlating aerodynamic measurement data from the RRWT with other similar facilities, using 60 vehicles across multiple product lines.</div></div>

<div class="htmlview paragraph">In order to suppress the well-documented low frequency pressure fluctuations in open jet wind tunnels, termed ‘wind tunnel buffeting’, an Active Resonance Control (ARC) System was implemented in the Audi aero-acoustic wind tunnel several years ago. This ARC-Sys-tem reduces the periodic pressure fluctuations by up to 23 dB and completely eliminates the periodic velocity fluctuations using a simple feedback control scheme.</div> <div class="htmlview paragraph">To set up the ARC system in practice, the system's parameters are optimised once for each critical flow velocity, when the vortex shedding frequency coincides with an acoustic resonance mode of the wind tunnel.</div> <div class="htmlview paragraph">Due to the fact that both frequency and amplitude of the excited resonances not only depend on flow velocity but also on other parameters such as collector position and test-car geometry, the system has to be adjusted with regard to each of these cases. Furthermore, experimental data suggests that multiple resonances are excited simultaneously under certain conditions.</div> <div class="htmlview paragraph">For these reasons a new, self-adapting control scheme was developed which combines the utilisation of the same physical mechanism of damping with greater control flexibility.</div> <div class="htmlview paragraph">The first part of this paper deals with experimentally measured pressure fluctuations and the influence different variables have on this phenomenon. The configuration of the self-adapting Enhanced Active Resonance Control (EARC) together with experimental results is presented in the second part of the paper.</div>

<div class="section abstract"><div class="htmlview paragraph">Establishing data adjustments that will give an interference free result for bluff bodies in automotive wind tunnels has been pursued for at least the last 45 years. Recently, the Two-Measurement correction method that yields a wake distortion adjustment for open jet wind tunnels has shown promise of being able to adjust for many of the effects of non-ideal static pressure gradients on bluff automotive bodies. Utilization of this adjustment has shown that a consistent drag results when the vehicle is subjected to the various gradients generated in open jet wind tunnels. What has been lacking is whether this consistent result is independent of the other tunnel interference effects. The studies presented here are intended to fill that gap on the performance of the two-measurement technique. The subject CFD studies are designed to eliminate all wind tunnel interference effects except for the variation of the (linear) static pressure gradient. Zero gradients and linear gradients are generated by tapering the walls of solid wall test sections with a blockage ratio of 0.5%. Under these conditions, the variation in drag coefficient is observed with and without application of simple buoyancy adjustments, Glauert Factor based adjustments, and the two-measurement method adjustments. Conclusions are reached relative to the ability with each of these approaches to achieve interference free results on a range of vehicle body styles both simplified and with a moderate level of production vehicle detail.</div></div>

<div class="htmlview paragraph">The paper presents new data to document the effective test area in a model ¾ open jet wind tunnel with a long test section using drag measurements. The drag on MIRA Pickup models (up to 20% blockage) was tested and found to be virtually independent of blockage when tested in a non-resonant test section with L/D ratio of about 8. An analysis of these test results is presented to predict the required test section L/D as a function of blockage. In addition, the effects of plenum dimensions are presented. Based on these results, the test sections of present open jet test sections have a limited range for testing models without corrections for the effects of blockage, model location, and test section length. A longer test section enables direct measurement of drag even with high blockage models. Testing with higher blockage can mean lower test facility costs.</div> <div class="htmlview paragraph">The test section design technology described in this report is the subject of a pending patent application.</div>

Reduction of 3/4 open jet low-frequency fluctuations in the S2A wind tunnel

<div class="section abstract"> <div class="htmlview paragraph">This Recommended Practice provides a procedure for measuring and documenting the aerodynamic performance in a full-scale wind tunnel of passenger vehicles, i.e., mass-produced cars and light-duty trucks intended primarily for individual consumers. Testing or numerical modeling of pre-production and/or reduced-scale models is outside the scope of this document. Aerodynamic <i>development</i> procedures, i.e., methods to improve or optimize aerodynamic performance, are also excluded.</div> <div class="htmlview paragraph">It is well-known that aerodynamic performance results depend significantly on vehicle content and loading, as well as the wind tunnel itself (type, scale, and simulation qualities of the wind tunnel). Publication of non-standard test results causes unnecessary additional development work and incorrect perception of a vehicle’s anticipated aerodynamic performance by government, academia, and the general public.</div> <div class="htmlview paragraph">The intent of this document is to promote uniformity and traceability of published aerodynamic performance data acquired in a wind tunnel, thereby enhancing the public reputation and credibility of the aerodynamics discipline in the total vehicle development process.</div> <div class="htmlview paragraph">Measurement of the aerodynamic performance according to this Recommended Practice requires a full-scale wind tunnel test on a production-ready, full-scale vehicle according to specifications for vehicle loading, content, and wind tunnel conditions.</div> <div class="htmlview paragraph">The measurement and resulting documentation for a particular vehicle shall be reported only by the company that intends to produce the vehicle. Whenever there is an opportunity for interpretation of the document, a good faith effort shall be made to obtain the vehicle’s typical in-service performance and avoid finding the best possible performance under the best possible circumstances.</div> <div class="htmlview paragraph">Use of and compliance with this Recommended Practice is voluntary; however, it is mandatory for those advertising or otherwise publishing aerodynamic performance as “measured according to SAE J2881.” Conversely, any advertised or otherwise published result carrying this or a substantially similar designation must have the necessary documentation accepted by and on file with the SAE in accordance with the procedures defined in this Recommended Practice.</div> <div class="htmlview paragraph"><span class="xref">Appendices B</span> and <span class="xref">C</span> contain the necessary submittal documents for compliance with this Recommended Practice. When a submittal is made, the chairman of the Road Vehicle Aerodynamics Forum Committee (RVAC) or his/her designee shall review the submittal for proper completion according to the requirements herein, but without consideration of the technical merit of the claimed performance. After the chairman’s review, SAE will maintain an electronic database of all submittals.</div> <div class="htmlview paragraph">The database shall be available only to members and liasion members of the RVAC. Any RVAC member who retrieves information from the database shall do so with the understanding that the information is to be considered confidential within his or her company.</div></div>

<div class="htmlview paragraph">Thick airfoils are commonly used for wind turbine and propeller blade inboard sections, and for struts on aircraft. These airfoils are sometimes truncated for manufacturing convenience, or to save weight. The purpose of this investigation was to study the effects of systematic truncation on airfoil performance. In order to investigate these effects two basic airfoil shapes representative of wind turbine designs were tested: the NACA 23024 and the NACA 64<sub>3</sub>-621. The trailing edges of the models were truncated in steps up to the maximum thickness point. Aerodynamic forces and pitching moments were measured for angles of attack from -10° to 90° for each truncated model. Wind tunnel tests were conducted in the Wichita State University 2.13m × 3.05m (7′ × 10′) Walter Beech Memorial Wind Tunnel, using constant chord reflection plane models. Tests were conducted at a nominal dynamic pressure of 718 Pa (15 psf), which corresponds to a Mach number of 0.10 and a Reynolds number of 0.65 × 10<sup>6</sup> based on the full airfoil chord.</div> <div class="htmlview paragraph">Test results show relatively small changes in the aerodynamic force and moment coefficients for chord reductions up to 10%. For larger chord reductions, stalling angle and C<sub>LMAX</sub> values increase dramatically. C<sub>D</sub> shows large increases, as expected. C<sub>M</sub> changes are related to changes in camber associated with trailing edge truncation. For high angles of attack, the aerodynamic center moves aft for all configurations, an expected consequence of separation. Changes in C<sub>N</sub>, normal force coefficient, are similar to changes in lift. C<sub>S</sub>, leading-edge suction force coefficient shows surprisingly small change for moderate truncation. These results have important implications for rotor and strut design.</div>

<div class="section abstract"><div class="htmlview paragraph">China Automotive Engineering Research Institute Co., Ltd (CAERI) has completed a new vehicle aero-acoustic wind tunnel (AAWT), which is located in Chongqing, China, and has been in operation for 5 years. To help addressing the Chinese vehicle market’s need to improve fuel economy, reduce exhaust emissions, and decrease product development period, the wind tunnel was designed and implemented to achieve a high degree of automation for vehicle testing next to a high aerodynamic and acoustic test accuracy for product development. The CAERI wind tunnel was in operation in June 2019, achieving a top speed of 250 km/h. A 5-belt rolling road system with a long center belt for proper wake simulation is installed inside, a test section with very low static pressure gradient and background noise. Wind tunnel calibration and customized measurement activities can be performed with an overhead traversing system. In the present paper, the main facilities of the AAWT are described next to necessary auxiliary equipment. The aerodynamic and acoustic performances of the CAERI AAWT are summarized. Brief comparisons of wind tunnel flow field parameters to the WLTP certification demand are included. In addition, drag measurement results of one sedan and one SUV are compared with measurements in the FKFS wind tunnel. After employing appropriate wind tunnel correction methods for both wind tunnels, the measurements are in good agreement, demonstrating the accuracy and stability of the CAERI AAWT test results.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The aerodynamics of a rotating tire can contribute up to a third of the overall aerodynamic force on the vehicle. The flow around a rotating tire is very complex and is often affected by smallest tire features. Accurate prediction of vehicle aerodynamics therefore requires modeling of tire rotation including all geometry details. Increased simulation accuracy is motivated by the needs emanating from stricter new regulations. For example, the upcoming Worldwide harmonized Light vehicles Test Procedures (WLTP) will place more emphasis on vehicle performance at higher speeds. The reason for this is to bring the certified vehicle characteristics closer to the real-world performance. In addition, WLTP will require reporting of CO<sub>2</sub> emissions for all vehicle derivatives, including all possible wheel and tire variants. Since the number of possible derivatives can run into the hundreds for most models, their evaluation in wind tunnels might not be practically possible. Therefore, simulations are very appealing alternative especially since their use is allowed by WLTP.</div><div class="htmlview paragraph">As a first step in order to meet these escalating demands, the current study uses a Lattice Boltzmann method (LBM) based computational fluid dynamics (CFD) solver using an immersed boundary method (IBM) based approach to simulate and validate a standalone rotating treaded tire. Preliminary wake plane prediction results are in good agreement with experimental wake plane measurements. Effect of tire loading on wake results is also discussed.</div></div>

<div class="htmlview paragraph">Wind tunnel interference effects are still considered to be negligible - or at least undesired - in automotive aerodynamics. Consequently, up to now there is no standard correction method which is used in everyday wind tunnel testing although a lot of research has been done in recent years.</div> <div class="htmlview paragraph">In most full-vehicle CFD simulations, wind tunnel interference effects are not simulated. The flow about the car is computed under idealized conditions. The wind tunnel is designed to simulate these conditions but fails to do so to some degree due to its limited size.</div> <div class="htmlview paragraph">Therefore a comparison of blockage-free CFD results and wind tunnel measurements is deficient. Hence CFD simulations including wind tunnel interference effects should be favored in the future for validation purposes. Furthermore, CFD offers new possibilities to investigate individual contributions to wind tunnel interference effects and therefore could help to increase the understanding of the flow in the wind tunnel. This may result in the development of advanced correction methods. To be able to use these possibilities, it has to be ensured that all effects of the whole wind tunnel test section with a car present will be captured in a simulation.</div> <div class="htmlview paragraph">In this paper simulations of the IVK model scale wind tunnel including wind tunnel interference effects are presented. As a first parameter variation, the influence of different longitudinal pressure distributions in the virtual test section are investigated and compared with test data. This investigation is presented here. In a second step, different vehicle models will be tested. All simulations were carried out with EXA PowerFLOW™.</div>

This paper is intended to provide a summary of the necessary adjustments required for road-representative open-jet wind tunnel measurements on automobiles. The open-jet wind tunnel provides accurate measurements, but they are made in a finite-sized jet that differs from the unconfined open-road conditions. Furthermore, measurements on a given automobile made in different open-jet wind tunnels disagree with each other, and with measurements in closed-wall wind tunnels that were corrected for the influences of their solid boundaries. There appears to be reticence at some company levels to making ‘corrections’ to open-jet measurements. Perhaps non-specialist managers think that the need for a ‘correction’ means an erroneous measurement. It does not! Any high-quality wind tunnel measurement is accurate, but it needs to be ‘calibrated’ to on-road conditions through an appropriate set of procedures. Closed-wall wind tunnels measure higher drag coefficients, in comparison with those in an unconstrained on-road flow. Open-jet wind tunnels frequently measure a lower value. The closed-wall adjustments lower the drag coefficient to the unconstrained value. Open-jet adjustments should also adjust the drag coefficient to the same unconstrained value. This paper explores the range of effects from the finite jet and elucidates the effectiveness of a two-measurement correction procedure. It is shown that not every data point must be measured twice, only a small selected subset. Since approximately 20% of tunnel occupancy is in the fan-on condition, then the additional cost of correct accurate on-road-equivalent data is low.

<div class="section abstract"><div class="htmlview paragraph">The improvement of vehicle soiling behavior has increasing interest over the past few years not only to satisfy customer requirements and ensure a good visibility of the surrounding traffic but also for autonomous vehicles, for which soiling investigation and improvement are even more important due to the demands of the cleanliness and induced functionality of the corresponding sensors. The main task is the improvement of the soiling behavior, i.e., reduction or even prevention of soiling of specific surfaces, for example, windows, mirrors, and sensors. This is mostly done in late stages of vehicle development and performed by experiments, e.g., wind tunnel tests, which are supplemented by simulation at an early development stage. Among other sources, the foreign soiling on the side mirror and the side window depend on the droplet detaching from the side mirror housing. That is why a good understanding of the droplet formation process and the resulting droplet diameters behind the side mirror is necessary, including all the possible influencing factors. A first fundamental study was already investigated by the authors at a vehicle side mirror using a modified wind tunnel soiling test rig [<span class="xref">1</span>].</div><div class="htmlview paragraph">In the current study, a generic plate with a defined edge for droplet detachment is placed inside the open jet of a calibration wind tunnel. The aim of the investigation is to test different parameters of the detachment process and to analyze the resulting droplet sizes behind the generic edge. Parameters such as the radius of the detachment edge, the liquid, and the volumetric flow rate are varied and their influence on the atomization as well as soiling processes is analyzed using shadowgraphy measuring technique and high-speed video recording.</div><div class="htmlview paragraph">Our results show that the droplet detachment behind a generic plate can be divided into two separate topics: the detachment position and the detachment process. The detachment position is dependent on the air velocity and the edge radius. With an increased radius and an increased velocity, the water can stay longer wall-bound and detach further downstream. The detachment process at the edge shows two different ligament formation processes independent of the air velocity: ligament formation and ligament-bag formation. A secondary breakup process may occur for droplets in the air, where four different processes are observed, covering vibration breakup, bag breakup, bag-and-stamen breakup, and sheet thinning. In all tested variations, most of the droplets show a diameter of about 65 μm.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The use of devices to reduce aerodynamic drag on large trailers and save fuel in long-haul, over-the-road freight operations has spurred innovation and prompted some trucking fleets to use them in combinations to achieve even greater gains in fuel-efficiency. This paper examines aerodynamic performance and potential drag reduction benefits of using trailer aerodynamic components in combinations based upon wind tunnel test data.</div><div class="htmlview paragraph">Representations of SmartWay-verified trailer aerodynamic components were tested on a one-eighth scale model of a class 8 sleeper tractor and a fifty three foot, van trailer model. The open-jet wind tunnel employed a rolling floor to reduce floor boundary layer interference. The drag impacts of aerodynamic packages are evaluated for both van and refrigerated trailers. Additionally, the interactions between individual aerodynamic devices is investigated. Results indicate that combinations of devices can provide trailer aerodynamic drag reductions of up to twenty one percent in wind-averaged conditions. Van and refrigerated trailer configurations show relatively similar aerodynamic performance with refrigerated trailers seeing greater drag reductions in certain cross-wind conditions. The combinations investigated in this study indicate that trailer aerodynamic components can act synergistically, especially in certain cross-wind conditions, providing aerodynamic benefits greater than the sum of their individual impacts. Future study needs are explored.</div></div>

<div class="htmlview paragraph">A continuously growing demand comes from the automotive industry for optimization of materials and sound insulating product packaging inside the car, so as to propose the best acoustic performance at reduced costs.</div> <div class="htmlview paragraph">A new acoustical holography system provides part of the solution to meet such a demand. The capability of measuring the acoustic field inside a vehicle with high spatial resolution makes it an advanced tool for performing extensive studies of the acoustic transparency of car openings and interior components in various environmental conditions (acoustic chamber, on road, in wind tunnel). The method allows for:</div> <div class="htmlview paragraph"> <ul class="list disc"> <li class="list-item"><div class="htmlview paragraph">Detailed localization of noise sources or leaks,</div></li> <li class="list-item"><div class="htmlview paragraph">Knowledge of the acoustic energy distribution on elementary surfaces (such as doors, windscreen, roof, sealing components, …),</div></li> <li class="list-item"><div class="htmlview paragraph">Reconstruction of the energy radiated by elementary surfaces in order to predict the acoustic pressure at driver's and passengers' ears,</div></li> <li class="list-item"><div class="htmlview paragraph">Introduction of local modifications on subsystems and components in order to predict their effect on the acoustic field.</div></li> </ul> </div> <div class="htmlview paragraph">In conjunction with some numerical prediction methods, this technique will allow to apply integrated acoustic engineering design methods for a complete understanding and control of the large variety of components and systems involved in the car performance.</div>