Jet-perforated boundaries as an effective method to reduce wall interference for airfoil tests in a transonic wind tunnel
Aeronautical and Space-Rocket Engineering
Central Aerohydrodynamic Institute named after N.E. Zhukovsky, TsAGI, 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia
Elimination of the influence of the wind tunnel test section walls on the flow over the model is one of the important problems in experimental aerodynamics. The flow near the model placed in the test section of the wind tunnel is different from the flow existing over the model in the unbounded flow. The shape of the streamlines is distorted at the location of the model due to the presence of the test section walls. The problem of interference between the model and the walls becomes most urgent due to the phenomenon of the test section blockage in transonic wind tunnels with solid walls. The using of permeable (perforated or slotted) walls of the test section is the most common method to reduce wall interference. However, permeable walls allow only to reduce their influence on the flow over the model, but not to completely exclude it. In addition, perforation is a source of low-frequency noise, large-scale eddies are generated due to slot boundaries.
Jet boundaries have been shown to be effective compared to existing methods to solve the wall interference problem in transonic wind tunnel. However, this approach has not become widespread due to the technical complexity of the jet installations implementation.
The approach based on the using of a controlled boundary layer is quite effective and technically easy to implement that is shown both experimentally and numerically. However, in some cases, the tested models are oversized, and the thickness of the boundary layer turns out to be insufficient to eliminate the solid wall interference.
A new approach to solve the wall interference problem is presented in the paper — combined jet-perforated boundaries. The proposed method combines the advantages of perforated boundaries and the controlled boundary layer. In addition, it is technically easy to implement, economically profitable and does not exclude the possibility of using it in existing wind tunnels.
Experimental study was carried out with a drained symmetric NACA-0012 airfoil with a chord 150 mm in TsAGI T-112 wind tunnel.
The experiment was carried out in solid walls with spoilers, in perforated boundaries with an open-area ratio of 0%, 2%, 10% and 23% and in jet-perforated boundaries with similar permeability coefficients and the spoiler height of 30 mm. The Mach number was 0.6; 0.65; 0.7 and 0.74. The angle of attack varied from −4° to 6°. As a result, the pressure distribution was obtained. The main aerodynamic characteristics of the model were calculated based on the obtained data on the pressure distribution.
This article presents the results of the airfoil model characteristics under the unbounded flow that was conducted in ANSYS CFX software by numerically solving the Reynolds averaged Navier — Stokes (RANS) equations. The SST turbulence model was used for the approximation. Numerical calculations of the flow over the NACA 0012 airfoil were carried out under conditions corresponding to the experimental one (Mach number: 0.6; 0.65; 0.7; 0.74; angle of attack: 0°, 1°, 2°, 3°, 4°).
The analysis of the results made it possible to draw a number of conclusions about the possibility to reduce the wall interference in transonic wind tunnel by using jet-perforated boundaries. It is shown that with relatively moderate level of disturbances introduced into the flow by the model (at Mach numbers up to 0.74 and angles of attack from −4° to +4°), the optimal combination of the perforated wall with the open-area ratio of no more than 2% with the controlled boundary layer generated wedge-shaped spoilers with a height of 30 mm (10% of the test section half-height of the T-112 wind tunnel). The selected combination of parameters made it possible to practically eliminate wall interference when the models’ chord does not exceed 25% of the test section height. The perforation ratio or boundary layer thickness should also increase with the increase in the model size or lift force.
Keywords:wind tunnel (WT), wall interference, boundary layer, computational fluid dynamics (CFD)
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