Effect of Inflow Turbulence on LES of an Airfoil Flow with Laminar Separation Bubble

Abstract

The turbulence intensity of the incoming flow can have a strong impact on the arising flow field around bodies especially when transition to turbulence plays an important role. A classical example is the flow past airfoils at moderate Reynolds numbers (e.g., for micro air vehicles), where laminar separation bubbles are often observed in the boundary layer on the suction side. For such cases experimental investigations carried out in different wind or water tunnels typically show strong variations of the separation, transition and reattachment locations, which to a great extent is caused by different levels of the turbulence intensity of the oncoming flow. Beside these deviations observed due to the natural turbulence level of the facility used, the effect of inflow turbulence becomes even more significant when the turbulence intensity is artificially increased for example by active or passive grids in order to mimic an atmospheric boundary layer. During the last years an increasing interest to simulate these flow phenomena is observed. Eddy-resolving simulations such as LES or hybrid LES-URANS are in principle the right choice for this challenging task. However, the inflow turbulence has to be prescribed in such a manner that it reaches the region of interest. Recently, a source term formulation (Schmidt and Breuer, Comput Fluids, 146:1–22, 2017, [9]) based on a synthetic turbulence inflow generator (STIG) was suggested. It allows to superimpose turbulent fluctuations in finer resolved flow regions, where the damping of small structures due to an inadequate grid resolution is negligible. This technique is applied here to investigate the flow past a SD7003 airfoil at Re$_c$ = 60,000 and an angle of attack $\alpha = 4^{\circ }$ for a wide range of turbulence intensities of the oncoming flow ($0 \le TI \le 11.2\%$). The results of the reference case without inflow turbulence are compared with the predictions with increasing turbulence intensities.