Abstract
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.
Highlights
The configuration of wind-tunnel sidewalls affects the pressure-gradient distributions over wind-tunnel models [1,2,3,4,5,6]
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
A streamline is selected to separate the flow around the wing and the theoretical infinite region encountered in free-flight conditions, thereby the real infinite flow field is split into two parts with a streamtube (ignoring the boundary-layer (BL) growth on the test-section walls) [9]
Summary
The configuration of wind-tunnel sidewalls affects the pressure-gradient distributions over wind-tunnel models [1,2,3,4,5,6]. The occurrence of the compression shock in the wind tunnel, which was later denoted as ‘transonic blockage’, was resolved based on the principle of wall streamlining [8] In this method, a streamline is selected to separate the flow around the wing and the theoretical infinite region encountered in free-flight conditions, thereby the real infinite flow field is split into two parts with a streamtube (ignoring the boundary-layer (BL) growth on the test-section walls) [9]. It is known that both the free streamlines and the BL growth, have nonlinear trajectories, which imposes much complexity to the design of the ideal sidewalls Such an accuracy does not seem to be required in the case of moderate wall-interference as it is the case in subsonic wind tunnels and for low blockage ratios, as the streamlining method is not normally applied for these cases. Seeking for a reasonable solution for reducing wall interference in subsonic wind tunnels, this research studies the possibility of a reduced-order approximation of streamlined inserts
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