Advances in Projection Moiré Interferometry Development for Large Wind Tunnel Applications

<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>

<div class="section abstract"><div class="htmlview paragraph">Efforts in aerodynamic optimization of road vehicles have been steadily increasing in recent years, mainly focusing on the reduction of aerodynamic drag. Of a car's total drag, wheels and wheel houses account for approx. 25 percent. Consequently, the flow around automotive wheels has lately been investigated intensively.</div><div class="htmlview paragraph">Previously, the authors studied a treaded, deformable, isolated full-scale tire rotating in contact with the ground in the wind tunnel and using the Lattice-Boltzmann solver Exa PowerFLOW. It was shown that applying a common numerical setup, with velocity boundary condition prescribed on the tread, significant errors were introduced in the simulation. The contact patch separation was exaggerated and the flow field from wind tunnel measurements could not be reproduced.</div><div class="htmlview paragraph">This investigation carries on the work by examining sensitivities and new approaches in the setup. A new validation case, which has the wheel rotating with small ground clearance, is tested both numerically and experimentally. Thus, the effect of tread rotation can be investigated separately from the effect of a deformed contact patch. In the simulation this is achieved by including the whole wheel in a sliding mesh region. For the wind tunnel a new motor-driven support sting was built to enable wheel rotation without ground contact. In order to validate the test case, flow field measurements are captured in the wind tunnel using a twelve-hole probe. Moreover, the treaded tire is compared to a smooth tire of the same outer contour in order to evaluate the tread's influence on the flow field.</div></div>

<div class="htmlview paragraph">Wind tunnel measurements on a rectangular vehicle-like shape and on two detailed, scale-model trucks have been employed to define the front and rear edge geometries that minimize aerodynamic drag. Optimum configurations are identified with sufficient detail for commercial vehicle design purposes. Comparisons of the model-scale measurements with limited measurements on a full-scale straight truck in a large wind tunnel support the interpretation of these test results.</div>

<div class="section abstract"><div class="htmlview paragraph">Icing wind tunnel testing was performed as part of the Republic of Korea certification of the Light Civil Helicopter (LCH) for inadvertent flight in icing conditions. The test was aimed at the compliance demonstration of the engine and air intake with dry-media Inlet Barrier Filter (IBF) and was performed with an Arriel 2C2 engine in turbojet configuration. Testing took place at the sea level ambient pressure Large Climatic Wind Tunnel (CWT) at Rail Tec Arsenal (RTA) in Vienna, Austria, by an integrated test team comprising engineers from the Royal Netherlands Aerospace Centre (NLR), Korea Aerospace Industries (KAI), and Safran Helicopter Engines. The test matrix covered the AC29-2C Appendix C 10,000 ft icing envelope, as well as simulated ground icing conditions, considering both a clean and artificially contaminated IBF. Beyond the aforementioned certification conditions, exploratory testing was performed in conditions with Supercooled Large Droplets (SLD) and rain. The test set-up consisted of the front section of the full-scale production cowling with internal modifications to accommodate the test engine and interface with the RTA exhaust gas handling system. Prior to tunnel entry, initial check-out and integration tests were performed with the same set-up at Safran, as well as in the Small CWT at RTA. Data gathering included the delta-pressure across the IBF, static pressures at the Aerodynamic Interface Plane (AIP), engine parameters, high-speed camera imagery, and 3-D scans of the critical ice shapes. The latter was used to support dry air flight testing with simulated ice shapes and blockage to demonstrate altitude effects and measure inlet distortion.</div></div>

<div class="htmlview paragraph">A continuously growing demand comes from the automotive industry for optimization of materials and sound insulating products implementation inside the car, so as to propose the best acoustic performance at reduced costs.</div> <div class="htmlview paragraph">The acoustical holography system dBVISION developed by 01dB Acoustics & Vibration provides part of the solution to such a demand. Its demonstrated capability of measuring the acoustic field inside a vehicle makes it an advanced tool for performing extensive studies of the acoustic transparency of car openings as well as wind tunnel measurements. The developed technique allows now for:</div> <div class="htmlview paragraph"> <ol class="list nostyle"> <li class="list-item"><span class="li-label">1-</span><div class="htmlview paragraph">Detailed localization of noise sources or acoustic weakness points inside the vehicle,</div></li> <li class="list-item"><span class="li-label">2-</span><div class="htmlview paragraph">Knowledge of the acoustic energy distribution on elementary surfaces (such as doors, windscreen, roof, sealing system, etc.),</div></li> <li class="list-item"><span class="li-label">3-</span><div class="htmlview paragraph">Reconstruction of the energy radiated by elementary surfaces in order to predict the acoustic pressure at the driver's and passengers' ears,</div></li> <li class="list-item"><span class="li-label">4-</span><div class="htmlview paragraph">Estimation of the acoustic incidence of local modifications on components of the tested car.</div></li> </ol> </div> <div class="htmlview paragraph">Technical background, together with the main features of this acoustical imaging system, will be exposed and the presentation will illustrate with practical examples how this fits to the project engineer process considering new acoustically driven design methods for a complete understanding and control of all components and systems involved in the car performance.</div>

<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>

In image-based measurements, quantitative image data must be mapped to three-dimensional object space. Analytical photogrammetric methods, which may be used to accomplish this task, are discussed from the viewpoint of experimental fluid dynamicists. The Direct Linear Transformation (DLT) for camera calibration, used in pressure sensitive paint, is summarized. An optimization method for camera calibration is developed that can be used to determine the camera calibration parameters, including those describing lens distortion, from a single image. Combined with the DLT method, this method allows a rapid and comprehensive in-situ camera calibration and therefore is particularly useful for quantitative flow visualization and other measurements such as model attitude and deformation in production wind tunnels. The paper also includes a brief description of typical photogrammetric applications to temperature- and pressure-sensitive paint measurements and model deformation measurements in wind tunnels.

<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>

Results are presented from a large eddy simulation (LES) and wind tunnel study of the turbulence regime in a horizontally evolving sheared atmospheric convective boundary layer (CBL) capped by a temperature inversion. The wind tunnel part of the study has been conducted in the thermally stratified tunnel of the University of Karlsruhe. For the numerical part a modified LES procedure that was originally designed for simulation of the horizontally homogeneous atmospheric CBL has been employed. The study focuses on the transition between the neutrally buoyant boundary layer in the initial portion of the wind tunnel flow and a quasi-homogeneous convectively mixed layer developing downwind. The character of the transition between the two boundary layers and the associated changes in the turbulence structure are found to be strongly dependent on the magnitude and distribution of disturbances in the flow at the entrance of the wind tunnel test section. For all simulated inflow conditions, the transition is preceded by accumulation of potential energy in the premixed CBL. The eventual energy release in the transition zone leads to turbulence enhancement that has a form of turbulence outbreak for particular flow configurations. The numerically simulated CBL case with temperature fluctuations introduced in the lower portion of the incoming flow appears to be the closest to the basic CBL flow case studied in the wind tunnel. Second-order turbulence statistics derived from the LES are shown to be in good agreement with the wind tunnel measurements. Main features of transition, including the turbulence enhancement within the transition zone, are successfully reproduced by the LES.

Volatilization of ammonia from field-applied animal slurry is a significant problem. Accurate emission measurements are needed for inventories and research, but are not provided by all measurement methods. Wind tunnels may give emission values substantially above or below micrometeorological results, which have been shown to be accurate. This limitation reduces the utility of wind tunnel results, which make up a large fraction of available measurements. The present work focused on understanding wind tunnel measurement error by comparing micrometeorological and wind tunnel measurements with the aid of a semi-empirical model. Ammonia loss from digestate after field application was measured in high time resolution in three field trials using wind tunnels and the backward Lagrangian stochastic (bLS) dispersion technique simultaneously. Differences in measured emission were interpreted using the ALFAM2 model, and measurements were used to evaluate the model. Results showed that wind tunnel and bLS methods provided different cumulative emission estimates, although there were similarities in measured emission dynamics. The ALFAM2 model was generally able to reproduce emission dynamics for both measurement methods, but only when differences in mass transfer between the two methods were incorporated in an experimental parameter set. This important result suggests that: 1) the simple structure of the ALFAM2 model captures the essential physical and chemical processes controlling emission and 2) the two measurement methods differ only (or mainly) through mass transfer above the slurry/soil surface and rain. Therefore, with careful selection of wind tunnel air flow it should be possible to approximately match emission that occurs under open-air conditions. But without temporal variation in air flow, actual emission dynamics cannot be captured. This work provides a template for integrating and comparing measurements from different methods, and suggests it is possible to use wind tunnel measurements for model evaluation and even parameter estimation.

<div class="section abstract"><div class="htmlview paragraph">Aerodynamics is of vital importance in the development of a high-performance super-sports motorcycle. It directly links with the product’s performance in terms of top speed, handling dynamics and user experience. The objective of this work is to achieve the best-in-class aerodynamic performance of a motorcycle using a comprehensive method, involving wind tunnel testing and CFD (computational fluid dynamics) modelling. Focus of the study is to understand the impact of aerodynamic forces on the ride comfort and handling (safety), in case of high-speed operation. In this work, a comprehensive CFD model is developed to assess the aerodynamic performance of a motorcycle. The model is validated with wind tunnel measurements – both for integral parameters, namely, coefficient-of-drag (????) and coefficient-of-pitching moment (????) and for a discrete parameter, which is coefficient-of-pressure (????), measured at 30 different locations on the motorcycle fairing. The coefficient-of-pressure is measured using pressure taps. A good agreement between the CFD model and wind tunnel measurement is observed. The results for integral quantities are within 5% error band, and for discrete parameters (????), the correlation is of the order of 98% between the measurement and model. We have analyzed about 150 design variations to achieve the best in class aerodynamic performance. Further, the CFD model is used to understand the transient forces experienced by rider head, because of vortex shedding from the wind shield (visor), head and its interaction. The study is conducted at two different heights of the rider head, with respect to wind shield, and at three different vehicle speeds. The results of this study, in terms of flow visualization and dynamic forces on the rider head and its frequency analysis are discussed. A vortex shedding from the helmet is observed at all speeds. Results are presented for two different position of the helmet with respect to visor.</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>

<div class="htmlview paragraph">The boundary interference of high blockage models inside open jet test section is studied in three phases. First, a wind tunnel test was performed using a high blockage automotive model inside the 1/15<sup>th</sup> scale Langley Full Scale Wind tunnel. Second, a CFD simulation was done using CFL3D code (developed by NASA Langley Research Center). Finally, a panel method was used to assess the boundary interference and to study the effect of the collector. The objective of the study is to highlight the challenges in assessing the boundary interference for high blockage models. A secondary object is to present a model to integrate all the available information from the wind tunnel test and the CFD simulation to solve the problem using a panel method.</div>

<div class="section abstract"><div class="htmlview paragraph">Conventional assessments of the aerodynamic performance of ground vehicles have, to date, been considered in the context of a vehicle that encounters a uniform wind field in the absence of surrounding traffic. Recent vehicle-platooning studies have revealed measurable fuel savings when following other vehicles at inter-vehicle distances experienced in every-day traffic. These energy savings have been attributed in large part to the air-wakes of the leading vehicles. This set of three papers documents a study to examine the near-to-far regions of ground-vehicle wakes (one to ten vehicle lengths), in the context of their potential influence on other vehicles.</div><div class="htmlview paragraph">Part one of this three-part paper documents principally the influence of vehicle shape on the development of its wake. A series of high-fidelity numerical simulations, based on a Lattice-Boltzmann approach, and a series of scaled-model wind-tunnel measurements are presented to examine the effects of four types of vehicles: a sedan, an SUV, a pickup truck, and a heavy-duty vehicle. The influence of using a stationary-ground-plane setup in the wind tunnel is examined using numerical simulations, to provide context for the wind-tunnel results.</div><div class="htmlview paragraph">The results of these investigations show that ground motion, or the lack thereof, has a greater influence on the wakes of the slant-back and step-back shapes than for a square-back shape due to an interaction of the wake vortex structures with a horseshoe vortex generated by the interaction of the vehicle pressure field with the oncoming boundary layer. The results also demonstrate two distinct types of wake regimes at low yaw angles for these different classes of vehicles shapes. Slant-back and step-back configurations like the car and pickup-truck models demonstrate the classic C-pillar vortex structure with central downwash, while the square-back shapes like the SUV model demonstrate a central upwash from a vortex pair of opposite sign. The mechanisms leading to these two opposing vortex pair orientations is examined. Drag reduction technologies applied to a heavy-duty-vehicle shape are shown to modify the wake structure such that slant-back and square-back wake characteristics can be generated.</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>