Characteristics of Laminar Separation Bubble with Varying Leading-Edge Shapes and Deflections of the Trailing-Edge Flap

Abstract

Abstract A series of Implicit Large Eddy Simulations (ILES) is carried out to examine the characteristics of a leading edge (LE) separation bubble. The test case comprises a flat plate with an elliptic leading edge which is equipped with a trailing edge flap. Simulations are carried out (a) at three different flap angles (20, 30, 90 degrees) and (b) using two different geometries of Elliptic Leading Edge (ELE) where the ratio of the semi-major to semi-minor axis is set to either 2:1 or 4:1. The flap is modeled using the immersed boundary method which is computationally economical as it avoids regenerating the grid for varying flap angles. The results show that (a) the flow separates at lower flap angles with a decrease in the aspect ratio of the ELE from 4:1 to 2:1 (b) an increase in the flap angle promotes separation at the LE due to an increase in the blockage in the bottom passage and a subsequent increase in the flow incidence at the leading edge. Simulations are carried out using the Gamma-Retheta transition model and comparisons are drawn against ILES and experiments. Although the qualitative trends predicted using both ILES and Reynolds Averaged Navier-Stokes (RANS) agree with experiments, both approaches predict relatively shorter separation bubbles. This is attributed to the excess flow blockage in experiments due to the support plates which are not modeled in the simulations. Nevertheless, the results demonstrate the superior accuracy of ILES over the RANS model.