Wind Tunnel Wall Research Articles

Further Analysis of the Blockage Phenomenon during the Testing of Bluff Automotive Bodies in Closed Wall Wind Tunnels

<div class="section abstract"><div class="htmlview paragraph">The difficulties of testing a bluff automotive body of sufficient scale to match the on-road vehicle Reynolds number in a closed wall wind tunnel has led to many approaches being taken to adjust the resulting data for the inherent interference effects. But it has been impractical if not impossible to experimentally analyze the effects that are occurring on and around the vehicle when these blockage interferences are taking place. The present study is an extension of earlier work by the author and similarly to that study uses the CFD (computational fluid dynamics) analysis of several bodies of differing configurations to examine the interference phenomena in solid wall wind tunnels and the effects that they have on the pressures, forces and force increments experienced by the vehicle model. This is accomplished by executing a series of CFD configurations with varying sized cross sections from 0.2% to 13% blockage enabling an approximation of free air conditions as a reference. The earlier work is extended by using additional body configurations which include reference bodies from the Technical University of Munich, the University of Stuttgart, and Ford Motor Company to complement the simple shapes used in that study along with the use of a SAE Notchback model simple shape. Examination is made of the physical phenomena occurring around the vehicle as the proximity to the walls and ceiling is changed holding the test section aspect ratio and length constant. Static Pressure distortions, skin friction changes, velocity distortions and the distribution of forces on the vehicle body are examined. It is intended that this dataset be used by the SAE Road Vehicle Aerodynamics Forum Committee (RVAC) and the Subsonic Aerodynamic Testing Association combined activity, Commonized Automotive Aerodynamic Test Standards, or CAATS, to evaluate and/or develop wind tunnel blockage techniques for automotive bluff bodies.</div></div>

Direct Numerical Simulation of Flow over a Triangular Airfoil Under Martian Conditions

Martian conditions present various challenges when designing rotorcraft. Specifically, the thin atmosphere and low sound speed require Martian rotor blades to operate in a low-Reynolds-number (1000–10,000) compressible regime, for which conventional airfoils are not designed. Here, we use PyFR to undertake high-order direct numerical simulations (DNS) of flow over a triangular airfoil at a Mach number of 0.15 and Reynolds number of 3000. Initially, spanwise periodic DNS are undertaken. Extending the domain-span-to-chord ratio from 0.3 to 0.6 leads to better agreement with wind-tunnel data at higher angles of attack, when the flow is separated. This is because smaller domain spans artificially suppress three-dimensional breakdown of coherent structures above the suction surface of the airfoil. Subsequently, full-span DNS in a virtual wind tunnel are undertaken, including all wind-tunnel walls. These capture blockage and wall boundary-layer effects, leading to better agreement with wind-tunnel data for all angles of attack compared to spanwise periodic DNS. The results are important in terms of understanding discrepancies between previous spanwise periodic DNS and wind-tunnel data. They also demonstrate the utility of high-order DNS as a tool for accurately resolving flow over triangular airfoils under Martian conditions.

Lift measurement of airfoil AH93-157 from wall pressure distribution

Measurement of the lift coefficient of the AH93-157 airfoil was performed by measuring the static pressure distribution on the wind tunnel walls along the test-section. A correlation was established between the lift coefficient value, determined by integrating the static pressure distribution on the wind tunnel walls, and the lift coefficient value, determined by integrating the static pressure distribution on the airfoil surface. This method is useful for easy and quick determination of the lift coefficient on a simple airfoil model without complicated static pressure tubing from the surface. The measurement was made within a closed test-section. The correlation relationship also eliminates the problem of the finite span and the effect of the side walls. Comparison of pressure distribution on airfoil with CFD was performed as well.

Simulating the acoustic response of cavities to improve microphone array measurements in closed test section wind tunnels.

Cavities placed along wind tunnel walls can attenuate the turbulent boundary layer (TBL) fluctuations as they propagate into the cavity. Placing microphones within the cavities can thus improve the signal-to-noise ratio of acoustic data. However, standing waves form within these cavities distorting the acoustic measurements. This work uses a finite element (FE) solver to evaluate how cavity geometry (depth, diameter, and wall angle) and wall material (hard-walled and melamine foam) affect the amplitude and eigenfrequency of standing waves when excited by an incident acoustic plane wave. Good agreement between predicted and measured acoustic transfer functions is shown. Compared to cylindrical cavities, countersunk and conical cavities improve the overall response, i.e., reducing the quality factor quantifying the resonance and damping characteristics. Stainless steel coverings also reduce the quality factor. A finding is that the shape of the external foam holder rather than the cavity shape drives the standing wave characteristics for the melamine foam cavities. The optimization problem of minimizing the acoustic response while also attenuating the TBL is thus decoupled by using the melamine foam. Consequently, these considerations can be addressed independently by optimizing the outer cavity shape for acoustics and the melamine foam insert for TBL attenuation.

Airfoil lift calculation using wind tunnel wall pressures

An experimental method to calculate lift using static pressure ports on the wind tunnel walls and its associated limits has been explored in this article. While the wall-pressure measurement (WPM) technique for lift calculation has been implemented by other researchers, there is a lack of literature on the sensitivity of the WPM method to airfoil chord length, model thickness, surface roughness, and freestream conditions. Chord sensitivity studies showed that the airfoil chord to test section length ratio plays an important role in the accuracy of the measurements. Models need to be appropriately sized for optimum performance of the WPM method. Additionally, choosing the correct scaling ratio also ensures independence of lift measurements from freestream Reynolds number conditions. Finally, a combination of symmetric and cambered airfoils with thicknesses varying from 6 % − 21 % were tested and successfully validated against reference data for a freestream chord Reynolds number range of 100,000 to 550,000. The WPM method was found to be sensitive to varying surface flow conditions and airfoil thickness and has been shown to be a viable replacement to traditional lift measurement techniques using load balances or airfoils with surface pressure ports.

Aerodynamic interaction between propellers of a distributed-propulsion system in forward flight

This article describes an experimental investigation of the aerodynamic interaction that occurs between distributed propellers in forward flight. To this end, three propellers were installed in close proximity in a wind tunnel, and the changes in their performance, flow-field characteristics, and noise production were quantified using internal force sensors, total-pressure probes, particle-image velocimetry (PIV), and microphones recessed in the wind-tunnel wall. At the thrust setting corresponding to maximum efficiency, the efficiency of the middle propeller is found to drop by 1.5% due to the interaction with the adjacent propellers, for a tip clearance equal to 4% of the propeller radius. For a given blade-pitch angle, this performance penalty increases with angle of attack, decreasing thrust setting, or a more upstream propeller position, while being insensitive to the rotation direction and relative blade phase angle. Furthermore, the velocities induced by the adjacent propeller slipstreams lead to local loading variations on the propeller disk of 5% – 10% of the average disk loading. Exploratory noise measurements show that the interaction leads to different tonal noise waveforms of the system when compared to the superposition of isolated propellers. Moreover, the results confirm that an active control of the relative blade phase angles between propellers can effectively modify the directivity pattern of the system.

Aerodynamic Free-Flight Conditions in Wind Tunnel Modelling through Reduced-Order Wall Inserts

Parallel sidewalls are the standard bounding walls in wind tunnels when making a wind tunnel model for free-flight condition. The consequence of confinement in wind tunnel tests, known as wall-interference, is one of the main sources of uncertainty in experimental aerodynamics, limiting the realizability of free-flight conditions. Although this has been an issue when designing transonic wind tunnels and/or in cases with large blockage ratios, even subsonic wind tunnels at low-blockage-ratios might require wall corrections if a good representation of free-flight conditions is intended. In order to avoid the cumbersome streamlining methods especially for subsonic wind tunnels, a sensitivity analysis is conducted in order to investigate the effect of inclined sidewalls as a reduced-order wall insert in the airfoil plane. This problem is investigated via Reynolds-averaged Navier–Stokes (RANS) simulations, and a NACA4412 wing at the angles of attack between 0 and 11 degrees at a moderate Reynolds number (400 k) is considered. The simulations are validated with well-resolved large-eddy simulation (LES) results and experimental wind tunnel data. Firstly, the wall-interference contribution in aerodynamic forces, as well as the local pressure coefficients, are assessed. Furthermore, the isolated effect of confinement is analyzed independent of the boundary-layer growth. Secondly, wall-alignment is modified as a calibration parameter in order to reduce wall-interference based on the aforementioned assessment. In the outlined method, we propose the use of linear inserts to account for the effect of wind tunnel walls, which are experimentally simple to realize. The use of these inserts in subsonic wind tunnels with moderate blockage ratio leads to very good agreement between free-flight and wind tunnel data, while this approach benefits from simple manufacturing and experimental realization.

Cylinder-lamina system fluid–structure interaction problem solved with an original OpenFOAM code

In numerical simulations, the partitioned approach allows treating a multi-physics phenomenon as two different computational fields, which are solved separately with their respective mesh discretisation and algorithms. This is of particular interest to fluid–structure interaction (FSI) problems for their complex resolution. In this respect, through the partitioned approach, ModsFsiFoam (MODalSuperpositionFsiFoam) solver has been developed. It allows the application of two different methods for fluid and solid solutions. In particular, it is based on a theoretical approach, the modal superposition principle, for the structural solution; this method provides a certain result for the linear structural field. The FVM (Finite VolumeMethod), by far the most widely used method for fluid-dynamics and gas-dynamics problems, has been used instead to solve the Navier–Stokes equations for an incompressible laminar flow of Newtonian fluid. The interfacial conditions have been used to pass information between the domains through a coupling algorithm. Implemented in OpenFOAM, a development environment consisting of libraries and applications for Linux distributions, written in the C++ source code, ModsFsiFoam solver has been already applied with satisfactory results to a typical fluid–structure interaction case found in literature, i.e. a system composed by an inverted flag treated as a flexible lamina, clamped to a wind tunnel wall, in which airflow is introduced in the axial direction. In this work, ModsFsiFoam solver outputs are compared with a system, the results of which are known for both fluid-dynamics and structural fields. The configuration consists of a laminar incompressible channel flow around a solid structure with an elastic part.

Full-Span Topology of Trailing-Edge Separation at Different Angles of Attack

The velocity field over a two-dimensional wing at Reynolds number of 672,000 was investigated to characterize the three-dimensional topology and evolution of the separate flow with variation of the angle of attack. Planar particle image velocimetry over the full span of the wing demonstrated that with increasing angle of attack, isolated pockets of backflow, which appeared near the trailing edge, merged and formed an asymmetric stall cell at angle of attack of 9.7°. The asymmetry was mainly associated with the dissimilar boundary layers developed on the wind tunnel walls at the spanwise ends of the wing. Secondary structures were also observed between the stall cell and the spanwise end. The stall cell topology was characterized using large-scale three-dimensional particle tracking velocimetry measurements using helium-filled soap bubbles. The results showed that the separation bubble had a small wall-normal height with two wall-normal counter-rotating vortices extended up to the edge of the separation bubble. In addition, the investigations demonstrated that vortex generators can induce a symmetric stall cell by removing the secondary structures and isolating the stall cell from the flows at the spanwise ends.

Effects of inflow boundary layer on the wake of a radially non-uniform porous disk

This study presents the results of an experimental investigation focusing on the effects of the inflow boundary layer on the wake characteristics of a 0.12 m diameter porous disk with radially non-uniform porosity in terms of mean flow, turbulence, and wake scaling. Two-dimensional two-component particle image velocimetry measurements within the wake are performed up to 7.5 diameters downstream as the disk is lowered deeper into a boundary layer that is representative of a neutral atmospheric boundary layer over a flat terrain. Results show that otherwise symmetrical wake velocity profiles that exist outside the boundary layer get skewed and sheared around the disk centerline in the boundary layer due to the inflow wind shear. The turbulent kinetic energy, its production, and Reynolds shear stress levels in the wake get asymmetrical around the centerline of the disk such that the production of turbulent kinetic energy is observed to be higher above centerline. Due to the inflow shear, the wake centerline gets shifted downwards (i.e., toward the wind tunnel wall), which is in contrast to the observations on real wind turbine wakes in the literature where the wake actually lifts up. The asymmetrical and skewed velocity profiles both in the streamwise and cross-stream directions can be collapsed onto a single function by using proper wake scaling parameters based on the ratio of local strain to average strain within the velocity profile calculated separately for either side of the wake.

Control of the self-sustained shear layer oscillations over rectangular cavities using high-frequency vortex generators

The excitation of acoustic resonance by flow over a rectangular cavity can generate acute noise, cause damage to equipment, and interrupt operation. In this work, a passive control technique to suppress the excitation of acoustic resonance by the flow over rectangular cavities is experimentally investigated. A span-wise rod that generates high-frequency vortices is mounted upstream of the cavity leading edge to prevent the flapping of the shear layer. The effect of the rod parameters on the mechanism of acoustic resonance suppression is identified by means of acoustic pressure and particle image velocimetry (PIV) measurements. It is found that the effectiveness of this control technique is significantly dependent on the streamwise location of the rod with respect to the cavity leading edge, the gap between the rod and the wind tunnel wall, and the cavity aspect ratio. In addition, PIV measurements revealed that, in effective rod configurations, the vortices generated in the gap between the control rod and the wall alter the development of the shear layer. Moreover, analysis of the Reynolds stresses showed that fluctuations in the wake of the rod prevent the shear layer from impinging on the cavity downstream edge. Consequently, this interaction interrupts the initiation of the feedback mechanism responsible for the onset of acoustic resonance excitation. Finally, a universal criterion is developed to predict an optimum region to implement the control rod upstream of the cavity leading edge to effectively suppress the acoustic resonance excitation.

Lift Interference in Wind Tunnels with Perforated and Solid Walls

In order to obtain accurate results in wind tunnel testing it is necessary to determine the effect of interaction between the flow around the model and the test section walls. In this paper, the classical theory for wind tunnel wall corrections assessment is used to evaluate the wall induced change in the circulation caused by the presence of the test article in the wind tunnel. This primary correction, also known as lift interference is based on the test section geometry and it is applied to the test article angle of attack. The computations performed in this paper employ the assumption of the potential linearized flow between the test section walls and the model. As well, the principle of superposition is a key element in this analysis.

基于线性涡模型的风洞孔壁干扰特性分析

An estimation and correction method of perforated wall-induced interference characteristics for 2D airfoils was proposed. Based on the Prandtl-Glauert velocity potential equation and linear vortices arranged on the surfaces of the airfoil and tunnel walls, the effects of the perforated wind tunnel wall on pressure distributions on the airfoil surface were solved iteratively to analyze influences of air permeability parameters of the perforated wall. Previous reference data and wind tunnel test results were used for comparison to validate the accuracy of the proposed calculation method. The comparison shows that, the effects of the perforated tunnel walls on the flow around the airfoil mainly act on the upper surface between the suction peak and the maximum thickness, and lead to lower pressure coefficients and decreased integrated lift coefficients. With the increase of the air permeability parameter of the perforated wall, the wall interference characteristics develop from the solid wall to the open boundary sharply. With good efficiency and reliability, the proposed method applies to quick estimation of the pressure on the airfoil surface in the subcritical range, and evaluation of the wall interference in the 2D airfoil tests.

Large eddy simulation of aircraft at affordable cost: a milestone in computational fluid dynamics

Abstract While there have been numerous applications of large eddy simulations (LES) to complex flows, their application to practical engineering configurations, such as full aircraft models, have been limited to date. Recently, however, advances in rapid, high quality mesh generation, low-dissipation numerical schemes and physics-based subgrid-scale and wall models have led to, for the first time, accurate simulations of a realistic aircraft in landing configuration (the Japanese Aerospace Exploration Agency Standard Model) in less than a day of turnaround time with modest resource requirements. In this paper, a systematic study of the predictive capability of LES across a range of angles of attack (including maximum lift and post-stall regimes), the robustness of the predictions to grid resolution and the incorporation of wind tunnel effects is carried out. Integrated engineering quantities of interest, such as lift, drag and pitching moment will be compared with experimental data, while sectional pressure forces will be used to corroborate the accuracy of the integrated quantities. Good agreement with experimental $C_L$ data is obtained across the lift curve with the coefficient of lift at maximum lift, $C_{L,max}$ , consistently being predicted to within five lift counts of the experimental value. The grid point requirements to achieve this level of accuracy are reduced compared with recent estimates (even for wall modelled LES), with the solutions showing systematic improvement upon grid refinement, with the exception of the solution at the lowest angles of attack, which will be discussed later in the text. Simulations that include the wind tunnel walls and aircraft body mounting system are able to replicate important features of the flow field noted in the experiment that are absent from free air calculations of the same geometry, namely, the onset of inboard flow separation in the post-stall regime. Turnaround times of the order of a day are made possible in part by algorithmic advances made to leverage graphical processing units. The results presented herein suggest that this combined approach (meshing, numerical algorithms, modelling, efficient computer implementation) is on the threshold of readiness for industrial use in aeronautical design.

Unsteady aerodynamics of a wing in a novel small-amplitude transverse gust generator

A novel small-amplitude high-frequency gust generator has been developed that works by oscillating a small fence on the wind tunnel wall. The gust generator produces approximately constant local angle of attack in the chordwise direction. Due to the challenges of measuring small and slightly non-uniform gust angles the gust generator was calibrated using direct lift measurements on a symmetric wing set at zero geometric angle of attack. Unsteady lift force measurements and the Theodorsen's theory were used for the dynamic calibration of the gust angle. At nonzero geometric angles of attack, if the wing’s effective angle of attack remains below the stall angle, unsteady lift closely follows the static lift curve with very small hysteresis. Beyond the stall angle, dynamic stall and larger lift hysteresis are observed. Interestingly, in this regime, if flow is separated and a separation bubble is maintained on the wing throughout the cycle then increasing frequency reduces lift hysteresis. The slope of the lift curve, averaged over the cycle, may be greater than that of attached flow. The gust response is more sensitive to maximum effective angle of attack than the reduced frequency or the reduced pitch rate. The normalized lift change is much larger for separated flows than for attached flows.Graphic abstract

Nonlinear Wind-Tunnel Wall-Interference Corrections Using Data Assimilation

This paper presents a novel approach for correcting wind-tunnel wall interference in the nonlinear flow regime, that is, in the presence of phenomena such as flow separation and shocks. The methodology uses a gradient-based optimization to minimize the difference between experimental measurements and a Favre-averaged Navier–Stokes (FANS) simulation. The aim is to exploit the high-fidelity experimental data to correct turbulence-modeling errors in the FANS simulations, as well as to use the accurate angle of attack and Mach number from the FANS simulations to correct the in-tunnel flow conditions. The optimization is carried out directly in free air, thus avoiding the requirement to mesh the wind-tunnel walls and/or to model the ventilated-wall boundary condition. A byproduct of this method is the availability of flow information everywhere around the test object, which augments and complements the experimental data. The methodology is tested on two-dimensional and three-dimensional flow cases, demonstrating a significant improvement in the agreement between experimental and numerical data.

Application of the HPCMP CREATETM-AV Kestrel to an Integrated Propeller Prediction

This article presents the results of a computational investigation of an integrated propeller test case using the HPCMP CREATETM-AV Kestrel simulation tools. There is a renewed interest in propeller-driven aircraft for unmanned aerial vehicles, electric aircraft, and flying taxies. Computational resources can significantly accelerate the generation of aerodynamic models for these vehicles and reduce the development cost if the prediction tools can accurately predict the aircraft/propeller aerodynamic interactions. Unfortunately, limited propeller experimental data are available to validate computational methods. An American Institute of Aeronautics and Astronautics (AIAA) workshop was therefore established to address this problem. The objective of this workshop was to generate an open access-powered wind tunnel test database for computational validation of propeller effects on the wing aerodynamics, specifically for wing-tip-mounted propellers. The propeller selected for the workshop has four blades and a diameter of 16.2 in. The wing has a root and tip chord of 11.6 and 8.6 in, respectively. Two different simulation approaches were used: one using a single grid including wind tunnel walls and the second using a subset grid overset to an adaptive Cartesian grid that fills the space between the near-body grid and wind tunnel walls. The predictions of both approaches have been compared with available experimental data from the Lockheed Martin low-speed wind tunnel to investigate the grid resolution required for accurate prediction of flowfield data. The results show a good agreement for all tested conditions. The measured and predicted data show that wing aerodynamic performance is improved by the spinning tip-mounted propeller.

Numerical Study of Wind-Tunnel Wall Effects on Lift and Drag Characteristics of NACA 0012 Airfoil

Possible interference effects of the wind tunnel walls play an important role especially for measurements in closed-wall test sections. In this study, a numerical analysis of two-dimensional subsonic flow over a NACA 0012 airfoil at different computational domain heights, angles of attack from 0o to 10o, and operating Reynolds number of 6×106 is presented. The work highlights the role of computational fluid dynamics (CFD) in the investigation of wind tunnel wall effect on lift curve slope correction factor (Ka). The flow solution is obtained using Ansys Fluent software by solving the steady-state continuity and momentum governing equations combined with turbulence model k-v shear stress transport (SST-K?). The numerical results are validated by comparing with the available experimental measurements. Calculations show that the lift curve slope correction results are very close to the published data.

Mean Parameters of an Incompressible Turbulent Boundary Layer on the Wind Tunnel Wall at Very High Reynolds Numbers

The mean parameters of a two-dimensional incompressible turbulent boundary layer with zero pressure gradient were measured on the smooth wall of the TsAGI T-128 transonic wind tunnel in the range Reθ = 5.3 × 104–2.9 × 105 velocity profiles, skin friction coefficient, and shape factor. Novel data were obtained in the range Reθ ≈ 2.35 × 105–2.9 × 105. At the maximum Reynolds number, the record value of the von Karman number Reτ ∼1 × 105 was reached and the logarithmic law was maintained up to y+ ≈ 1.3 × 104. The equilibrium state of the boundary layer was estimated using Clauser’s equilibrium parameter, which was G = 6.4–6.8 for high Reynolds numbers. The results of the investigation confirm the conclusions of other studies on the universality of dimensionless velocity profile in the boundary layer outer region. The results were compared with semi-empirical dependencies, direct numerical simulation (DNS) results and the results of other studies performed both on wind tunnel walls and on a flat plate. At high Reynolds numbers the agreement between the shape factor was within ΔH = ± 0.012 (± 1%), with a skin friction coefficient—Δcf= ± 0.00006 (± 3.7%).

Wind Turbine Tip Vortices under the influence of Wind Tunnel Blockage Effects

The current paper describes the characteristics of the tip vortex in the near wake of a three-bladed upwind horizontal axis wind turbine with a rotor diameter of 3 m. Phase-locked stereo particle image velocimetry measurements were carried out under the influence of the wind tunnel walls that create a high blockage ratio. The location of the vortex, convection velocity, core radius, and strength were investigated and compared with similar investigations, including different blockages cases. Additionally, the same performance of the wind turbine model was simulated in the open source wind turbine tool QBlade, using the lifting line free vortex wake module in the absence of the walls.The results showed that the location of the tip vortices was more inboard the tip and more downstream the tunnel compared to the simulations and similar experiments. The convection velocity remained similar in the axial direction and changed in the lateral direction, contributing to the delay of the movement of the tip vortex outboard the tip. The strength, based on the circulation, was found with a difference of 4% between simulation and experiment.