Drag Measurements with High Blockage

<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>

Flow studies of an open jet wind tunnel and comparison with closed and slotted walls

<div class="section abstract"><div class="htmlview paragraph">The new BMW Aerodynamisches Versuchszentrum (AVZ) wind tunnel center includes a full-scale wind tunnel, "The BMW Windkanal" and an aerodynamic laboratory "The BMW AEROLAB." The AVZ facility incorporates numerous new technology features that provide design engineers with new tools for aerodynamic optimization of vehicles. The AVZ features a single-belt rolling road in the AEROLAB and a five-belt rolling road in the Windkanal for underbody aerodynamic simulation. Each of these rolling road types has distinct advantages, and BMW will leverage the advantages of each system. The AEROLAB features two overhead traverses that can be configured to study vehicle drafting, and both static and dynamic passing maneuvers. To accurately simulate "on-road" aerodynamic forces, a novel collector/flow stabilizer was developed that produces a very flat axial static pressure distribution. The flat static pressure distribution represents a significant improvement relative to other open jet wind tunnels. In addition, improved performance for reducing open jet plenum pressure fluctuations was achieved by incorporating and tuning Helmholtz resonators to selected low frequencies. The AVZ also includes a 32,000 m₂ support building with space for workshops, vehicle preparation, storage and offices. The close coupling of the support building to the wind tunnels improves the internal workflow within the vehicle design development process.</div></div>

A detailed flow survey of the test section of the Langley Full-Scale Tunnel is described. Flow angularity, dynamic pressure, and turbulence intensity were measured in the test section volume over a matrix with three foot spacing. Instrumentation consisted of a rake of four five hole probes and two hot-wire anemometers. Details of probe calibration, installation, and use are presented in addition to survey results. Facility Description Old Dominion University (ODU), working under a Memorandum of Agreement with NASA Langley Research Center, has operated the Langley Full-Scale Tunnel (LFST) since 1997. This facility is the second largest in the United States in terms of test section size and is the largest university run wind tunnel in the world. During its seventy years of service, a wide variety of aerospace test programs have been undertaken. Sub-scale model testing has included high angle of attack testing of current U.S. front-line fighters, high-lift systems on supersonic and subsonic transports, parachutes, and even an airship, submarine, and building complex. Recently testing has included fullscale automobiles, trucks, and UAVs. The open jet test section of the LFST (formerly the NASA 30 by 60) is semi-elliptical in cross section with a width of 60 ft and a height of 30 ft as shown in figure I. The elevated ground board is 42.5 ft wide by 52.3 ft long and features a turntable with a diameter of 28.5 ft. * Graduate Research Assistant Student Member AIAA t Associate Professor of Aerospace Engineering, Senior Member AIAA Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. Power is supplied by two 4000 HP electric motors driving two 35.5 ft diameter fourbladed fans. The dual return tunnel circuit design has a nozzle contraction ratio of approximately 5:1, and no flow conditioning devices such as honeycombs or screens anywhere in the circuit. The test section ground board and balance shield represent the only major modifications to the circuit since the commissioning of the facility. Introduction The large test section and open jet design are ideal for testing large models and UAVs with negligible boundary effects. Special vents provide fresh air exchange, useful when testing with power on. There are two model aircraft support systems available depending on size and weight. Both systems allow a 100° angle of attack sweep and a ±90° yaw sweep. A large scale x-y-z traverse (survey carriage) can be used to position a second model or survey probes in the test section. Interest in a modern flow survey originally stemmed from a planned NASA Blended Wing Body (BWB) test using a large scale model. There have been several flow surveys in the history of the tunnel and much folklore surrounding the flow quality on the tunnel centerline.'' The merging streams of the dual return design were thought to have created a measureable shear layer on the centerline with an elevated turbulence intensity. This report describes the flow quality in the Langley Full-Scale Tunnel (LFST) in terms of flow angularity, dynamic pressure and turbulence intensity. Aerodynamic flow field surveys were made over the entire test section volume. The instrumentation consisted of four five hole hemispherical-head probes for dynamic pressure and flow angularity measurements, two constant-temperature hot-wire anemometers for 1 American Institute of Aeronautics and Astronautics (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

<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 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">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>

In linear cascade wind tunnel tests, a high level of pitchwise periodicity is desirable to reproduce the azimuthal periodicity in the stage of an axial compressor or turbine. Transonic tests in a cascade wind tunnel with open jet boundaries have been shown to suffer from spurious waves, reflected at the jet boundary, that compromise the flow periodicity in pitch. This problem can be tackled by placing at this boundary a slotted tailboard with a specific wall void ratio s and pitch angle α. The optimal value of the s-α pair depends on the test section geometry and on the tunnel running conditions. An inviscid two-dimensional numerical method has been developed to predict transonic linear cascade flows, with and without a tailboard, and quantify the nonperiodicity in the discharge. This method includes a new computational boundary condition to model the effects of the tailboard slots on the cascade interior flow. This method has been applied to a six-blade turbine nozzle cascade, transonically tested at the University of Leicester. The numerical results identified a specific slotted tailboard geometry, able to minimize the spurious reflected waves and regain some pitchwise flow periodicity. The wind tunnel open jet test section was redesigned accordingly. Pressure measurements at the cascade outlet and synchronous spark schlieren visualization of the test section, with and without the optimized slotted tailboard, have confirmed the gain in pitchwise periodicity predicted by the numerical model.

<div class="htmlview paragraph">This paper investigates the aerodynamic simulation accuracy of several types of wind tunnel test sections. Computational simulations were performed with a closed wheel race car in an 11.0 m<sup>2</sup> adaptive wall, a 16.8 m<sup>2</sup> open jet, and a 29.7 m<sup>2</sup> slotted wall test section, corresponding to model blockage ratios of 20.9%, 13.7%, and 7.7%, respectively. These are compared to a simulation performed in a nearly interference-free condition having a blockage ratio of 0.05%, which for practical purposes of comparison, is considered a free air condition. The results demonstrate that the adaptive wall most closely simulates the free air condition without the need for interference corrections. In addition to this advantage, the significantly smaller size of the adaptive wall test section offers lower capital and operating costs.</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>

At present, there is a great interest in the development of new airfoils for wind turbines and high-lift wings of unmanned aerial vehicles (UAV). The requirements for such airfoils differ from conventional aircraft airfoils, because of structural reasons and extreme operating conditions. So, wind turbine airfoils operate frequently under fully separated flow when stall is used for power regulation at high wind speeds. At the same time design of airfoils for wings UAV poses the problem of availability of high-lift at low Reynolds number. Modern airfoils are to a large extent developed from numerical methods. However, the complex flow conditions such as separation at high angles of attack, laminar separation bubbles and the transition from laminar to turbulent flow are difficult to predict accurately. Hence, testing of airfoils at a two-dimensional condition is an important phase in airfoil design. The development and validation of a 2D testing facility for investigation of single and multi-element airfoils in the wind tunnel Т-102 with open test section are considered in this article. T-102 is a continuous-operation, closed-layout wind tunnel with two reverse channels. The test section has an elliptical cross-section of 4 ×2,33 m and a length of 4 m. Two big flat panels of the L × H=3 ×3,9 m size installed upright on balance frame aligned with the free stream are used for simulating two-dimensional flow in the tunnel test section. The airfoil section in the layout of a rectangular wing is mounted horizontally between flat panels with minimum gaps to ensure 2D flow conditions. The aerodynamic forces and pitch moment acting on the model were measured by wind tunnel balance. To determine boundary corrections for a new test section of wind tunnel, the experimental investigation of three geometrically similar models has been executed. The use of boundary corrections has provided good correlation of the test data of airfoil NACA 6712 with the results obtained from the wind tunnel except for lift and drag coefficient values at high angles of attack.

<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>

Aerodynamic development for road vehicles is usually carried out in a uniform steady-state flow environment, either in the wind tunnel or in Computational Fluid Dynamics (CFD) simulations. However, out on the road, the vehicle experiences unsteady flow with fluctuating angles of incidence β, caused by natural wind, roadside obstacles, or traffic. In order to simulate such flow fields, the Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart (FKFS) swing® system installed in the quarter scale model wind tunnel can create a variety of time-resolved signals with variable β. The static pressure gradient in the empty test section, as well as cD values of the Society of Automotive Engineers (SAE) body and the DrivAer model, have been measured under these transient conditions. The cD measurements have been corrected using the Two-Measurement Correction method in order to decouple the influence of the unsteady flow from that of the static pressure gradient. The investigation has determined that the static pressure gradient in the empty test section varies greatly with different excitation signals. Thus, it is imperative to apply a cD correction for unsteady wind tunnel measurements. The corrected cD values show that a higher signal amplitude, as in, signals with large β, lead to higher drag forces. The influence of the signal frequency on drag values varies depending on the vehicle geometry and needs to be investigated further in the future.

The characterization of a new wind tunnel at the University of Florida’s Research and Engineering Education Facility is presented. The tunnel is specifically designed to operate in the low Reynolds number regime in which Micro Air Vehicles operate. The wind tunnel is driven by a 60 inch, 50 hp axial blower controlled by a variable frequency drive. The tunnel entrance consists of a flow conditioning section and a 8:1 area contraction ratio that results in a 42” square entrance to the open jet test section. The enclosure surrounding the test section has a volume of nearly 2000 ft with an axial length of 10 ft. The free stream velocities in the tunnel range from nominally 0 to 22 m/s by altering the frequency on the variable frequency drive. Flow uniformity studies for free stream velocities of 2 and 15 m/s were conducted demonstrating a potential core throughout the test section of at least 60% of the 1.14 m contraction exit. Experiments using hot wire anemometry were also performed and the turbulence intensities were found to be less 0.22% for free stream velocities greater than 1m/s.