Wind Tunnel Wall Effects Research Articles

Evaluation of the porosity parameter for a perforated wall wind tunnel using measured wall pressure distributions

The existing analytical methods for determining and correcting the effect of wind tunnel walls on experimental data are based on the hypothesis of potential flow. A useful simplification of the boundary conditions used to describe the perforated walls was to consider the wall as homogeneous, the solid and free portions not being treated separately, but as an equivalent permeable surface. The approximation of the wall behaviour during the experiment was possible by defining a porosity parameter. The purpose of this paper is to estimate the porosity parameter for the perforated walls of a trisonic wind tunnel by evaluating the pressure distributions measured on the walls of the test section.

Wind Tunnel Installation Effects on Japan Aerospace Exploration Agency’s Standard Model

This paper investigates the interference effects of wind tunnel floor and walls on the Japan Aerospace Exploration Agency’s High-Lift Configuration Standard Model (JSM) as an extension to a paper published in the Journal of Aircraft, Vol. 56, No. 3, May–June 2019, pp. 1080–1098. An unstructured hybrid grid generator, MEGG3D, was applied to JSM wing–body and wing–body–nacelle–pylon configurations to generate grids for free-air flow conditions. These grids were then reused in an investigation of the interference effects of Japan Aerospace Exploration Agency’s low-speed wind tunnel walls to better compare the computational results, both with each other and with experiment. The geometry of the wind tunnel used in our computational simulations is described in detail so that they can be reproduced. A Reynolds-averaged Navier–Stokes flow solver for unstructured hybrid grids, the Tohoku University Aerodynamic Simulation (TAS) code, was used for the simulations. The computational results indicated that the standoff and test section floor boundary layer significantly affected the aerodynamics of the JSM at high angles of attack. Better agreement with experiment was observed for the wing–body configuration in the aerodynamic characteristics near maximum lift if the effects of the standoff and floor boundary layer were considered.

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.

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.

Reynolds number and wind tunnel wall effects on the flow field around a generic UHBR engine high-lift configuration

RANS simulations of a generic ultra-high bypass ratio engine high-lift configuration were conducted in three different environments. The purpose of this study is to assess small scale tests in an atmospheric closed test section wind tunnel regarding transferability to large scale tests in an open-jet wind tunnel. Special emphasis was placed on the flow field in the separation prone region downstream from the extended slat cut-out. Validation with wind tunnel test data shows an adequate agreement with CFD results. The cross-comparison of the three sets of simulations allowed to identify the effects of the Reynolds number and the wind tunnel walls on the flow field separately. The simulations reveal significant blockage effects and corner flow separation induced by the test section walls. By comparison, the Reynolds number effects are negligible. A decrease of the incidence angle for the small scale model allows to successfully reproduce the flow field of the large scale model despite severe wind tunnel wall effects.

Computational investigation of wind tunnel wall effects on buffeting flow and lock-in for an airfoil at high angle of attack

Wind tunnel walls may significantly affect the aerodynamic characteristics of an airfoil if the distance between walls is not large enough and the experimental data may be influenced by the confinement effect. The purpose of this paper is to investigate the wall effects on a NACA 0012 airfoil at high angle of attack (the flow over the airfoil is assumed to be fully separated). The effects on both a static airfoil and also an oscillating airfoil (pitching motion) are studied. Reynolds number varies from 105 to 3×105. For a static airfoil, there is a dominant frequency of the flow oscillations called the Strouhal or buffet frequency. Strouhal number of an airfoil in confined flow is affected by blockage ratio and angle of attack. A new wall correction formula for the Strouhal number has been determined. For an oscillating airfoil, lock-in is determined by a combination of excitation frequency and amplitude of the oscillating airfoil. The reason of difference between computational and experimental lock-in regions are explained. Lock-in region decreases with increase of blockage ratio. Reynolds number has slight impact on lock-in region, both in unconfined flow and confined flow.

Updating of aerodynamic reduced order models generated using computational fluid dynamics

Reduced order models of computational fluid dynamics codes have been developed to decrease computational costs; however, each reduced order model has a limited range of validity based on the data used in its construction. Further, like the computational fluid dynamics from which it is derived, such models exhibit differences from experimental data due to uncertainty in boundary conditions and numerical accuracy. Model updating provides the opportunity to use small amounts of additional data to modify the behaviour of a reduced order model, which means that the range of validity of the reduced order model can be extended. Whilst here computational fluid dynamics data have been used for updating, the approach offers the possibility that experimental data can be used in future. In this work, the baseline reduced order models are constructed using the Eigensystem realisation algorithm and the steps used to update these models are given in detail. The methods developed are then applied to remove the effects of wind tunnel walls and to include viscous effects.

Numerical Simulations of Turbulent Flow over Airfoils Near and During Static Stall

Reynolds-averaged Navier–Stokes and hybrid large-eddy/Reynolds-averaged Navier–Stokes simulations of turbulent flow past an Aérospatiale A-Airfoil near stall at , , and a NACA 0012 airfoil under static stall conditions (, , ) are described in this paper. In the flow past the A-Airfoil, comparisons with surface skin-friction coefficient and pressure coefficient distribution are generally in good agreement with experimental measurements. Comparisons with experimental velocity profile data and Reynolds-stress data are also generally favorable. Leading-edge laminar separation and turbulent reattachment is predicted when the Menter–Langtry correlation-based transition model is used in combination with either Reynolds-averaged Navier–Stokes or large-eddy/Reynolds-averaged Navier–Stokes strategies, but the level of trailing-edge separation is underpredicted, relative to experimental data and to results obtained without the inclusion of transition model. For the case of flow past a NACA 0012 airfoil under static stall conditions, results from the large-eddy/Reynolds-averaged Navier–Stokes simulations exhibit a sensitivity to mesh refinement, with finer spanwise mesh resolution leading to light stall, characterized primarily by trailing-edge separation, and coarser spanwise mesh resolution leading to deep stall, characterized by the presence of a stabilized leading-edge vortex. Calculations that include wind-tunnel wall effects show better general agreement with experimental measurements but also display the aforementioned sensitivity to spanwise mesh refinement.

CFD Study of Solid Wind Tunnel Wall Effects on Wing Characteristics

Most of airplane structures include the components such as Fuselage, Wings, Empennage, Landing gear and Power plant etc. Wings are the important part of aircraft and these are airfoils attached to each side of the fuselage and are the main lifting surfaces that support the airplane in flight. SSD-2 airfoil, a modified version of NACA23015 airfoil is used to study the aerodynamic characteristics such as lift, drag, moment and wall effects with different wind tunnel blockage ratios. Generally, any new airfoil characteristics are derived from the extensive wind tunnel tests, however experiments are costly. Hence, to get the idea of aerodynamic characteristics and wall effects over airfoil with different wind tunnel blockage ratios, CFD tools are used such as ICEM CFD and ANSYS FLUENT. This paper presents the results of simulation of flow past SSD-2 airfoil. The study is carried out on a 2D airfoil section for free stream condition. Further the computations are made on 3D rectangular wing of solid wind tunnel simulation by varying wind tunnel height to get the different wind tunnel blockage ratios, keeping the other parameters constant. In solid wind tunnel simulation the lift and moment are less compared to free stream simulation results due to blockage effects. Finally, it is observed that increase in the wind tunnel blockage ratio, lift and moment decreases, drag increases with less difference in pressure; whereas decrease in wind tunnel blockage ratio, the lift and moment increases, drag decreases with more difference in pressure.

Numerical Investigation and Wind Tunnel Validation on Near-Wake Vortical Structures of Wind Turbine Blades

AbstractComputational fluid dynamics (CFD) has been used by numerous researchers for the simulation of flows around wind turbines. Since the 2000s, the experiments of NREL phase VI blades for blind comparison have been a de-facto standard for numerical software on the prediction of full scale horizontal axis wind turbines (HAWT) performance. However, the characteristics of vortex structures in the wake, whether for modeling the wake or for understanding the aerodynamic mechanisms inside, are still not thoroughly investigated. In the present study, the flow around N-REL phase VI blades was numerically simulated, and the results of the wake field were compared with the experimental ones of a one-to-eight scaled model in a low-speed wind tunnel. A good agreement between simulation and experimental results was achieved for the evaluation of overall performances. The simulation captured the complete formation procedure of tip vortex structure from the blade. Quantitative analysis showed the streamwise translation movement of vortex cores. Both the initial formation and the damping of vorticity in near wake field were predicted. These numerical results showed good agreements with the measurements. Moreover, wind tunnel wall effects were also investigated on these vortex structures, and it revealed further radial expansion of the helical vortical structures in comparison with the free-stream case.

Numerical Investigation of Wind Tunnel Wall Effects on a Supersonic Finned Missile

Numerical Investigation of Wind Tunnel Wall Effects on a Supersonic Finned Missile

Mitigation of wind tunnel wall interactions in subsonic cavity flows

The flow over an open aircraft bay is often represented in a wind tunnel with a cavity. In flight, this flow is unconfined, though in experiments, the cavity is surrounded by wind tunnel walls. If untreated, wind tunnel wall effects can lead to significant distortions of cavity acoustics in subsonic flows. To understand and mitigate these cavity–tunnel interactions, a parametric approach was taken for flow over an L/D = 7 cavity at Mach numbers 0.6–0.8. With solid tunnel walls, a dominant cavity tone was observed, likely due to an interaction with a tunnel duct mode. An acoustic liner opposite the cavity decreased the amplitude of the dominant mode and its harmonics, a result observed by previous researchers. Acoustic dampeners were also placed in the tunnel sidewalls, which further decreased the dominant mode amplitudes and peak amplitudes associated with nonlinear interactions between cavity modes. This indicates that cavity resonance can be altered by tunnel sidewalls and that spanwise coupling should be addressed when conducting subsonic cavity experiments. Though mechanisms for dominant modes and nonlinear interactions likely exist in unconfined cavity flows, these effects can be amplified by the wind tunnel walls.

Aerodynamic Modeling of Swept-Bladed Vertical Axis Wind Turbines

A computationally light time- and space-accurate two-dimensional free vortex model capable of simulating straight- and swept-bladed vertical axis wind turbines operating in a fluctuating wind has been developed. Fast solution times are achieved via vortex merging, limiting the number of calculations necessary at each time step, while solution integrity is maintained through the use of second-order time stepping, vortex-blob rediscretization, and the use of a “vortex curtain” preventing vortex mergers within a predetermined distance from the rotor. Finite span and spoke drag corrections are applied to account for three-dimensional effects in addition to a formulation for extending the two-dimensional model to arbitrary three-dimensional geometries. The two-dimensional free vortex model is validated against unswept and swept-bladed vertical axis wind turbine experimental data, with allowances for the effects of wind-tunnel walls made within the free vortex model in the case of the swept-bladed data set.

Unsteady transonic aerodynamics during wing flutter

Abstract Unsteady pressure distributions of a two-dimensional super-critical wing while it was fluttering were measured in the transonic flow regime. The results were compared with those by the Navier-Stokes code which includes wind-tunnel wall effects. Although there were discrepancies between the experimental results and the analytical model for the pressure phase delay distribution, no disagreements were observed for the pitching first harmonics provided that there was no large flow separation. In the tests, the flutter was forced to be suppressed soon after its onset before it reached a limit cycle oscillation (LCO) where the amplitude of the pitching angle was supposed to be over 2 degrees.

Considerations for Numerical Modeling of Inverted Wings in Ground Effect

T HE series of experiments conducted by Zerihan [1], Zerihan and Zhang [2,3], and Zhang and Zerihan [4–6] on an 80% scale model of the front wing of the 1998 Tyrell Formula One car (here referred to as the T-026 profile, based on a modified NASA GA(W) LS(1)-0413) have justifiably become the go-to source of validation for researchers investigating single and double element wings in ground effect (for example, [7–13]). This is due to the comprehensive nature of the tests, which included chordwise and spanwise pressure measurements for a large range of angles of attack and ground clearances (in terms of height-to-chord ratio, h=c), flow visuali­ zation, information about turbulent transition, offsurface measure­ ments of wakes, vortices, and the general flowfield using both particle image velocimetry and laser doppler anemometry and force balance data for drag and downforce (negative lift). Subsequent numerical investigations modeling this geometry have tended to focus on two-dimensional geometry representing the semispan point of the full wing (z=b = 0), and several studies have made comparisons between turbulence models and meshes in an attempt to determine the best approach for such flows [7–12]. While the original experiments for the wing detailed three-dimensional effects in considerable detail with regard to vortex behavior, spanwise pressure distributions, etc. [1], the wing had a fixed aspect ratio of 4.92 (corresponding to the real-world Formula 1 wing) and thus the true two-dimensionality of the wing at the semispan was not fully investigated. For this reason, some methodological issues associated with the interpretation of the data in two-dimensional comparisons have not been properly resolved, and to date it is not clear if a truly objective comparison of numerical schemes has been determined. While the influence of aspect ratio and wind-tunnel wall effects are well established for other oft-cited airfoils [14], lifting

Development and Validation of a CFD Technique for the Aerodynamic Analysis of HAWT

This paper demonstrates the potential of a compressible Navier–Stokes CFD method for the analysis of horizontal axis wind turbines. The method was first validated against experimental data of the NREL/NASA-Ames Phase VI (Hand, et al., 2001, “Unsteady Aerodynamics Experiment Phase, VI: Wind Tunnel Test Configurations and Available Data Campaigns,” NREL, Technical Report No. TP-500-29955) wind-tunnel campaign at 7 m/s, 10 m/s, and 20 m/s freestreams for a nonyawed isolated rotor. Comparisons are shown for the surface pressure distributions at several stations along the blades as well as for the integrated thrust and torque values. In addition, a comparison between measurements and CFD results is shown for the local flow angle at several stations ahead of the wind turbine blades. For attached and moderately stalled flow conditions the thrust and torque predictions are fair, though improvements in the stalled flow regime are necessary to avoid overprediction of torque. Subsequently, the wind-tunnel wall effects on the blade aerodynamics, as well as the blade/tower interaction, were investigated. The selected case corresponded to 7 m/s up-wind wind turbine at 0 deg of yaw angle and a rotational speed of 72 rpm. The obtained results suggest that the present method can cope well with the flows encountered around wind turbines providing useful results for their aerodynamic performance and revealing flow details near and off the blades and tower.

Wind-tunnel interference effects on a 70° delta wing

AbstractThis paper considers the effects of both wind-tunnel walls and a downstream support structure, on the aerodynamics of a 70° delta wing. A RANS model of the flow was used with the wind-tunnel walls and supports being modelled with inviscid wall boundary conditions. A consistent discretisation of the domain was employed such that grid dependence effects were consistent in all solutions, thus any differences occurring were due to varying boundary conditions (wall and support locations). Comparing solutions from wind-tunnel simulations and simulations with farfield conditions, it has been shown that the presence of tunnel walls moves the vortex breakdown location upstream. It has also been seen that vortex strength, helix angle, and mean incidence also increase, leading to a more upstream breakdown location in wind-tunnels. The secondary separation line was also observed to move outboards. It was observed that for high Reynolds numbers, with a support downstream of the wing, vortex breakdown can be delayed due to blockage effects providing the vortices do not impinge on the support This was observed to be the case for smaller supports also.

Stability of a Soft Plate in Channel Flow. Aerodynamic Aspects of Palatal Flutter.

The present paper gives a computational method for the flutter analysis of a soft plate placed in two-dimensional subsonic channel flow, remarking the palatal flutter in snoring. The computations were carried out first for the prelimanary case of a hard plate oscillating in windtunnel and then for the case of a soft plate oscillating with standing- or travelling-wave mode, simulating the palatal oscillation. The results obtained for a hard plate are in good agreement with those for the windtunnel wall effects, showing the validity of the present method. The results for a soft plate show that the palatal flutter can be caused by the oscillation of travelling-wave mode, and that the flutter is slightly promoted by the channel walls but slightly suppressed by the hard plate attached fore the soft plate. The effect of mechanical damping is also discussed.

Numerical simulation of wind tunnel wall effects on wind turbine flows

A low-order panel method and a prescribed wake vortex model have been combined into a coupled model capable of assessing the basic effect of wind tunnel walls on wind turbine flow and performance. The wind tunnel walls are discretised into a series of panels, and sources distributed on these panels simulate the constraint effect of the wind tunnel walls. The wind turbine and its wake are represented by the prescribed wake model. The source strengths are related to the induced velocities at the panel control points due to the turbine vortex system by satisfying the boundary condition of zero normal velocity on the solid tunnel wall. The vortex-induced and source-induced velocities at the blade are summed to determine an interim turbine wake using the prescription functions. The effects of the disturbance velocities due to the source panels are then superposed upon the prescribed wake to obtain the final wake geometry under the influence of wind tunnel wall interference. The model developed in this study is compared with wind turbine wake measurements made in a low-speed wind tunnel. Generally, the model is found to compare well with experiment, although some discrepancies are noted. Finally, possible future improvements to the combined method are recommended. Copyright © 2001 John Wiley & Sons, Ltd.

Prediction of tunnel wall upwash for delta wings including vortex breakdown effects

AbstractAn incompressible method is presented to predict the upwash corrections associated with vortical flow as a result of wind-tunnel side wall effects. An image system is used to simulate the tunnel side walls which are assumed to be solid. An integral expression is formulated, representing the average upwash induced over the wing by the image system. Wall effects may be determined for flows with and without vortex breakdown. Comparisons of the results with upwash predictions from a Navier-Stokes study show close accord. The upwash expression also displayed the ability to successfully predict corrections for flows involving vortex breakdown.