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.

Mitigation of transonic shock buffet on a supercritical airfoil through wavy leading edges

“…The beneficial effect for the drag is observed downstream of the modified aerofoil L-T transition point. These results are consistent with previous studies 26 . The region of L-T transition on the modified aerofoil has a negative contribution on drag coefficient but it decreases when the wavy amplitude is increased.…”

“…The grid is validated with three different grids on the straight aerofoil, they are reported in Table I. G1 is the grid used for the following analysis and it is consistent with a previous study 26 . Since no turbulence model is included, a sufficiently high level of near-wall grid resolution is maintained in order to properly resolve the boundary layers throughout the surface.…”

“…The spanwise aerodynamic performance of a wavy aerofoil in transonic flows has been analysed in previous works 26 . The objective of this section is to extend the previous analysis to a higher incidence angle and to the influence of wavy amplitudes.…”

“…§II introduces the computational set-up and methods used in this study. §III provides the details of time-averaged flow characteristics over the aerofoils, comparing streamwise and spanwise aerodynamic performance with previous results 26 and analysing WLE amplitude effect on aerodynamic performance. §IV shows the unsteady analysis' results, both in terms of the WLE amplitude effect and the characterisation of unsteady phenomena.…”

“…The present work aims to apply for the first time WLEs to mitigate the shock buffet phenomenon over a supercritical aerofoil in transonic flow. A recent study 26 has shown that wavy aerofoils increase the aerodynamic efficiency at low and moderate angles of attack, mainly because they are able to decrease the pressure drag. The results of this preliminary analysis prove that WLEs in transonic flows should be analysed in further detail.…”