An investigation on a supercritical aerofoil with a wavy leading edge in a transonic flow

Abstract

A compressible large-eddy simulation is performed to study the effects of a wavy leading edge (WLE) applied on a supercritical aerofoil in a transonic flow at Re∞ = 5.0 × 105 and M∞ = 0.7. The wavelength and peak-to-trough amplitude of the WLE used in this study are 5% and 2.5%, respectively, of the mean aerofoil chord. The primary aim of this study is to understand the aerodynamic characteristics of the modified aerofoil over a range of incidence angles. For this reason, a slow heaving motion is imposed where the geometric angle of attack is gradually increased from αg = 2° to 7° without a significant dynamic (added mass) effect, i.e., a quasi-linear range. The new transonic flow study shows significantly different findings (with some similar features) to the previous low-speed flow studies. It is observed in the quasi-linear range that the modified aerofoil achieves a performance improvement at low and moderate angles because of a drag reduction in the leading edge region and downstream of the laminar–turbulent (L–T) transition point. The leading edge (LE) analysis shows that the maximum pressure coefficient remains equal to that of the baseline case only at the trough and peak sections. The relative decrease in pressure at the LE results in the drag reduction. The transonic flow at the LE is analyzed in further detail to show a reversed flow region at the trough and its influence on the boundary layer development over the aerofoil. In addition, the spanwise variation of the boundary layer characteristics over the modified aerofoil is evaluated and analyzed. One of the most notable findings in this paper is that the flow at the trough becomes supersonic even at low angles of attack, and this results in an enhanced LE flow acceleration spread across the span, which seems facilitated by using a short WLE wavelength. This flow behavior is qualitatively explained by using an analogy between a channeling effect and a convergent–divergent nozzle in a transonic flow. Another notable observation is that there is an upstream movement of the laminar–turbulent transition point seemingly related to the flow distortion around the WLE. Interestingly, the flow distortion introduces a three dimensionality into the laminar boundary layer, but it keeps the flow laminar, so the benefits of the laminar supercritical aerofoil are not lost. These LE phenomena have a major impact on the shock structure at high incidence angles where the more energetic laminar boundary layer changes the shock structure over the modified aerofoil. This can be crucial to control the shock buffet phenomenon.