Numerical study of airfoil stall cells using a very wide computational domain

Abstract

The formation of stall cells over a NACA 0012 airfoil at a Reynolds number of one million has been investigated numerically, using unsteady Reynolds-averaged Navier-Stokes (URANS) and delayed detached-eddy simulation (DDES) approaches. The simulations are performed with a very wide computational domain (10 chord length) to minimize the influence of spanwise periodic boundary conditions. For the URANS simulations, four different spanwise mesh resolutions are tested to determine the minimum resolution required to capture the formation of stall cells. Both URANS and DDES results show a sudden decrease in lift and increase in drag between 16°and 17°angle of attack, accompanied by a significant change of separated flow patterns. Stall cell structures are observed clearly in the URANS solutions between 17°and 19°with a spanwise spacing of about 1.4 to 1.8 chord length, which agrees well with a theoretical prediction based on the slope of the lift curve in this angle-of-attack range. The DDES results show much more complex flow patterns over the airfoil at these high angles of attack, although the spectral analysis of wall shear stress suggests the existence of flow structures having a similar spanwise length scale to the stall cells.

Unsteady RANS Simulations of Strong and Weak 3D Stall Cells on a 2D Pitching Aerofoil

“…It is worth noticing that most of the experimental studies used a finite wing while some studies observed stall cell formation on very wide aerofoil (AR = 9-12) to eliminate the impact of the tip vortex. Recently, the authors of the present paper have also conducted a numerical study on this issue [27], further extending an earlier numerical study of Manni et al [28]. In these studies, a NACA 0012 aerofoil with an infinitely long span at stall condition was investigated.…”

“…They can be categorized into two different types. For maximum AoA = 19 • , 20 • , and 21 • , the 3D structures resemble the stall cell structures often observed for a static aerofoil, such as those in [27,28]. For these cases, small 3D structures start to develop near the leading edge when the aerofoil is pitching down to around 17 • and then merge into large counter-rotating vortex pairs dominating the whole span.…”

Transonic buffet instability: From two-dimensional airfoils to three-dimensional swept wings

“…The modes oscillate in the spanwise direction as the buffet cells observed in URANS simulations and DESs. The 3D modes exhibit a flow structure similar to the stall cell phenomenon in the detached boundary layer [36]. Yet, the reasons behind the detachment of the boundary layer are different: At low speed the boundary layer at the suction side detaches because of the smooth and progressive recompression of the flow towards the trailing edge of the airfoil, while in the transonic regime the detachment results from the strong interaction of the boundary layer with the shock wave.…”

Similitude between 3D cellular patterns in transonic buffet and subsonic stall

“…Kamenetskiy et al [39] also encountered this type of flow field while investigating the possibility to obtain multiple fully converged solutions of the discretized RANS equations. Manni et al [40] presented results of stall cells using URANS and DDES simulations. Recent studies point towards an inviscid source for the stall cells.…”