Coherent structures in an airfoil boundary layer and wake at low Reynolds numbers

Abstract

Mitigation of preferential concentration of small inertial particles in stationary isotropic turbulence using electrical and gravitational body forces Phys. Fluids 24, 073301 (2012) Axisymmetric intrusions in two-layer and uniformly stratified environments with and without rotation Phys. Fluids 24, 036603 (2012) Wavelet decomposition of forced turbulence: Applicability of the iterative Donoho-Johnstone threshold Phys. Fluids 24, 025102 (2012) Maximizing dissipation in a turbulent shear flow by optimal control of its initial state Phys. Fluids 23, 045105 (2011) A stochastic model of coherent structures for particle deposition in turbulent flows Phys. Fluids 20, 053303 (2008) Additional information on Phys. Fluids Boundary layer and turbulent wake development for a NACA 0025 airfoil at low Reynolds numbers was studied experimentally. Wind tunnel experiments were carried out for a range of Reynolds numbers and three angles of attack. Laminar boundary layer separation occurs on the upper surface of the airfoil for all Reynolds numbers and angles of attack examined. Two flow regimes are investigated ͑i͒ boundary layer separation without reattachment and ͑ii͒ separation bubble formation. The results suggest that coherent structures form in the separated flow region and the wake of the airfoil for both flow regimes. The formation of the roll-up vortices in the separated shear layer is linked to inviscid spatial growth of disturbances and is attributed to the Kelvin-Helmholtz instability. Linear stability theory can be employed to adequately describe the salient characteristics of such vortices and the initial stage of the separated shear layer transition. The development of the roll-up vortices leads to boundary layer transition, and the vortices break down during the transition process. Vortex shedding also occurs in the airfoil wake and vortices form in the near-wake region. It is shown that the boundary layer behavior has a profound effect on the identified coherent structures, and each of the two flow regimes is associated with distinctly different vortex shedding characteristics.

Effect of trailing edge shape on the separated flow characteristics around an airfoil at low Reynolds number: A numerical study

“…The shape of the C p profiles is very similar to that measured experimentally for laminar separation bubbles with transition followed by reattachment [38][39][40] . Following transition, C f reaches a minimum negative value, indicative of the 'reverse flow vortex' 41 .…”

“…This peak is very close to the frequency f 2 = 7.4 of the straight trailing edge case and corresponds to the natural shear layer instability. Indeed, if the appropriate as before length and velocity scales are used, which for the blunt airfoil are ψ 0 ≈ 0.075C and U es = 1.34U ∞ respectively, the corresponding non-dimensional frequency f * = f bl,2 ψ 0 /U es ≈ 0.42, which is again close, albeit slightly below, the range of observed values 40 . Despite the presence of the natural instability, the shedding mode at f bl,1 = 4.5 is significantly more dominant, both in the DMD spectrum ( Figure 11) and in the point spectra in the wake and suction side of the airfoil (Figures 14 and 15 respectively).…”

Flow control over an airfoil using virtual Gurney flaps

“…Yarusevych, Sullivan & Kawall (2009) showed that the length scale of the wake vortices decreased significantly and the vortex pattern became less organized when separation bubbles were formed over the airfoil. Huang & Lin (1995), Yarusevych, Sullivan & Kawall (2006) and Yarusevych & Boutilier (2011) investigated the turbulent wake development of NACA four-digit series airfoils at low Reynolds numbers, and demonstrated that the dimensionless vortex shedding frequency increased linearly with the Reynolds number. Hain, Kähler & Radespiel (2009) showed that the K-H instability led to a spanwise vortex formation in the shear layer above the separation bubble over an SD7003 aerofoil at the Reynolds number of Re = 66 000.…”

Leading and Trailing Edge Effects on the Aerodynamic Performance of Compliant Aerofoils

“…Strong coupling between the membrane oscillation and the vortex shedding at the wake, specially for post-stall incidences has been reported by Song and Breuer, 8 Gordnier 11 and Rojratsirikul et al 9 Yarusevych et al 12 has shown that the vortex shedding frequency for symmetrical aerofoils has a linear dependancy on Reynolds numbers at low Reynolds numbers. They also found that the Strouhal number of the wake vortex shedding may increase significantly in this regime.…”