Numerical study of the primary instability in a separated boundary layer transition under elevated free-stream turbulence

Abstract

Numerical studies of laminar-to-turbulent transition in a separation bubble subjected to two free-stream turbulence levels (FST) have been performed using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a plate with a semi-circular leading edge at Re = 3450 based on the plate thickness and the uniform inlet velocity. A numerical trip is used to produce the targeted free-stream turbulence levels and the decay of free-stream turbulence is well predicted. A dynamic sub-grid-scale model is employed in the current study and a good agreement has been obtained between the LES results and the experimental data. Detailed analysis of the LES data has been carried out to investigate the primary instability mechanism. The flow visualisations and spectral analysis of the separated shear layer reveal that the 2D Kelvin-Helmholtz instability mode, well known to occur at low FST levels, is bypassed at higher levels leading to earlier breakdown to turbulence. C 2013 AIP Publishing LLC.[http://dx

Effects of free-stream turbulence intensity on laminar separation bubbles

“…By increasing the level of FSTI, the transition process in the separated shear layer is promoted, leading to earlier mean flow reattachment [53,143] and thus a reduction in the size of the separation bubble. This finding has been confirmed more recently by Lamballais et al [138,139] who simulated the flow over a blunt flat plate with rounded corners using DNS, as well as by Langari and Yang [140] who simulated the flow over a blunt nosed flat plate using Large Eddy Simulations (LES). Lamballais et al [139] showed that the reduction in separation bubble length is more significant for larger corner radii of the plate since a larger separation bubble initially forms at low FSTI, and thus the observed reduction at elevated FSTI is more significant.…”

“…However, at increased levels of FSTI, the spanwise coherence of shear layer rollers at the roll-up location is decreased notably [31,32], as compared to experiments performed in low disturbance environments [18,133] or simulations [16]. At high levels of FSTI (greater than approximately 1-2%), notable streamwise velocity fluctuations are detected upstream of separation [140]. These fluctuations are indicative of bypass transition and, more specifically, are evidence of streamwise oriented streaks forming in the laminar boundary layer upstream of mean separation [16,37,133].…”

“…In some experiments at high levels of FSTI, streamwise oriented streaks are not detected [31,32]. The observation [16,37,133] or absence [31,32,138,140,144] of streamwise oriented streaks at elevated levels of FSTI is curious, but the formation of these streaks is dependent upon the receptivity of the specific flow and is a complex process [12,91,98]. In a study over an airfoil, Butler et al [144] found that at very high FSTI (≈ 10%), boundary layer separation was suppressed as the flow transitioned to turbulence very close to the leading edge of an airfoil.…”

“…At elevated levels of FSTI, the separation bubble is significantly smaller than that formed at lower turbulence intensity levels [37,38], and the roll-up process of the separated shear layer is highly three-dimensional [16,133]. This three-dimensionality is observed as spanwise deformations in Q-critereon [145,146] contours presented by Langari and Yang [140]. Lardeau et al [37] showed, using LES of a separation bubble induced on a flat plate, that at elevated levels of FSTI a spanwise variation in the locations of mean separation and reattachment exists as a result of streamwise streaks formed in the boundary layer upstream of separation.…”

An experimental investigation of a laminar separation bubble on the leading-edge of a modelled aerofoil for different Reynolds numbers

“…LES of Abdalla and Yang 18 on a blunt flat plate have observed that the transition in a separated shear layer occurs through inviscid mechanism and the transformation of the K-H rolls into threedimensional structures is most likely due to helical instability associated with pairing of vortices and breaking down to turbulence. LES of Langari and Yang 19 for a separation bubble produced on the semi-circular leading edge of a flat plate for different levels of free stream turbulence observed that at low free stream turbulence level the separated layer undergoes transition via K-H mechanism, while at high free stream turbulence levels K-H mechanism is bypassed. Recent experimental and computational studies to demonstrate the effect of low Re operation of NACA64A006 and NACA2415 aerofoils has been carried out by Genc et al [20][21][22] Their studies illustrate the formation of separation bubble its transition and reattachment at various angles of attack and associated stall characteristics.…”