Seal vibrissa-based three-fourths open-jet wind tunnel nozzle optimization

<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>

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

The present study aims to investigate the effect of the contraction profile on the flow quality such as the turbulent intensity, flow uniformity and pressure compared to baseline case (existing one). Primarily, losses of every wind tunnel part should be calculated. Hence, the total pressure losses of the whole wind tunnel circuit can be determined and all these losses are added up. This technique is required to evaluate the required power to operate the wind tunnel. As result, this method is well-suited with both of opened and closed-circuit wind tunnels. Moreover, two different profiles of the contraction are compared to the baseline wind tunnel (existing one) using the Computational Fluid Dynamics (CFD) as a basis for the comparison and analysis of the airflow properties through the whole wind tunnel design. The necessary parameters are the flow conditions within the test section that included the velocity, turbulent intensity and flow uniformity as well as the pressure distribution. The results showed that the turbulent intensity is decreased by 29.7 % with more uniform flow inside the optimized wind tunnel, when the “eye design contraction wall” is used. The contraction profile (wall design shape) plays an important role to effect on the flow quality inside the test section. Therefore, a special attention should be given when designing the wind tunnel with a specific contraction profile.

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.

Mechanism of compressor airfoil boundary layer flow control using nanosecond plasma actuation

<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">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>

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.

There is a need for hypersonic wind-tunnel testing at about mach 10 and above using natural air and simulating temperatures and pressures which are prototypic of flight at 50 km altitude or below. With traditional wind-tunnel techniques, gas cooling during expansion results in exit temperatures which are too low. Miles, et al., have proposed overcoming this difficulty by heating the air with a laser beam as it expands in the wind-tunnel nozzle. This report discusses an alternative option of using a high-power electron beam to heat the air as it expands. In the e-beam heating concept, the electron beam is injected into the wind-tunnel nozzle near the exit and then is guided upstream toward the nozzle throat by a strong axial magnetic field. The beam deposits most of its power in the dense air near the throat where the expansion rate is greatest. A conceptual design is presented for a large-scale system which achieves Mach 14 for 0.1 seconds with an exit diameter of 2.8 meters. It requires 450 MW of electron beam power (5 MeV at 90 A). The guiding field is 500 G for most of the transport length and increases to 100 kG near the throat to converge the beam to a 1.0-cm diameter. The beam generator is a DC accelerator using a Marx bank (of capacitors) and a diode stack with a hot cathode. 14 refs. 38 figs., 9 tabs.

Improvement of airflow quality in an open jet windtunnel

The effects of unsteady wind on the performances of a newly developed cross-axis wind turbine: A wind tunnel study

<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">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">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>

<div class="htmlview paragraph">A well-known problem with open-jet wind tunnels are low-frequency pulsations, which affect the flow quality and thus the quality of the measured data. This so-called wind tunnel buffeting is caused by large-scale vortices in the shear layer, which excite acoustic resonant modes of the wind tunnel circuit.</div> <div class="htmlview paragraph">This paper presents a novel approach to control low-frequency pulsations by means of active flow control at the nozzle exit. The final setup consists of oscillated flaps which are actuated by electrodynamic shakers. Single-frequency sinusoidal signals and band-limited random signals were used for excitation. The oscillated flaps prevent the synchronization of vortex shedding with acoustic resonance by generating forced perturbations in the shear layer at a frequency that is different from the acoustic resonance.</div> <div class="htmlview paragraph">The flaps - which were named FKFS-flaps by the authors - were installed at the IVK 1:4 model-scale wind tunnel. Experimental results from an ongoing experimental study are presented showing the reduction of low-frequency pulsations and the influence of the flaps on pressure distribution and force measurements.</div> <div class="htmlview paragraph">The small space requirements of the FKFS-flaps allows retrofit of existing wind tunnels without major modifications to the tunnel. The installation cost is considerably lower than that of other active systems. Very simple open-loop control is used, which is easier to implement and more stable than a closed-loop system.</div>