Post-stall flow control on an airfoil by local unsteady forcing

Abstract

By using a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at post-stall angles of attack, we show that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing-suction near the leading edge with low-level power input. This unsteady forcing can modulate the evolution of the separated shear layer to promote the formation of concentrated lifting vortices, which in turn interact with trailing-edge vortices in a favourable manner and thereby alter the global deep-stall flow field. In a certain range of poststall angles of attack and forcing frequencies, the unforced random separated flow can become periodic or quasi-periodic, associated with a significant lift enhancement. This opens a promising possibility for flight beyond the static stall to a much higher angle of attack. The same local control also leads, in some situations, to a reduction of drag. On a part of the airfoil the pressure fluctuation is suppressed as well, which would be beneficial for high-α buffet control. The computations are in qualitative agreement with several recent post-stall flow control experiments. The physical mechanisms responsible for post-stall flow control, as observed from the numerical data, are explored in terms of nonlinear mode competition and resonance, as well as vortex dynamics. The leading-edge shear layer and vortex shedding from the trailing edge are two basic constituents of unsteady post-stall flow and its control. Since the former has a rich spectrum of response to various disturbances, in a quite wide range the natural frequency of both constituents can shift and lock-in to the forcing frequency or its harmonics. Thus, most of the separated flow becomes resonant, associated with much more organized flow patterns. During this nonlinear process the coalescence of small vortices from the disturbed leading-edge shear layer is enhanced, causing a stronger entrainment and hence a stronger lifting vortex. Meanwhile, the unfavourable trailing-edge vortex is pushed downstream. The wake pattern also has a corresponding change: the shed vortices of alternate signs tend to be aligned, forming a train of close vortex couples with stronger downwash, rather than a Kármán street.

Numerical study of blowing and suction slot geometry optimization on NACA 0012 airfoil

“…Such experiments have demonstrated that suction and blowing can modify the pressure distribution over an airfoil surface and have a substantial effect on lift and drag coefficients. Over the past few decades, various numerical works have been performed on the most common NACA airfoils to measure the lift and drag coefficients under different flow conditions [12][13][14][15][16]. In these studies, the effects of zero net mass flux oscillatory jet (synthetic jet) and leading edge blowing/suction were considered on the vortex flow passing airfoils.…”

Unsteady Aerodynamic Flow Control of Moving Platforms

“…It has been shown that the separating shear layer over stalled 2-and 3-D aerodynamic surfaces is typically dominated by a strong coupling to the instability of the near wake (e.g., Wu et al 1998). Traditional separation control strategy uses actuation coupling to the narrow-band receptivity of the separating flow at the unstable Strouhal numbers of the near wake (e.g., St D ~ O(1), Hsiao et al 1990, Neuberger and Wygnanski 1987, Williams et al 1991, Chang et al 1992, Seifert et al 1993).…”

Separation Control Using ZNMF Devices: Flow Physics and Scaling Laws

“…Closed-loop control can be further classified into quasi-static versus dynamic, the distinction between the two being whether or not the feedback control is performed on a time scale with the dynamical scales of the flow. Since fluid flows are inherently nonlinear (Wu et al 1998), the standard frequency preservation of a linear system does not hold. Consequently, nonlinear feedback control on a very slow time compared to the characteristic times scales of the flow is, in fact, possible and attractive.…”

“…Based on previous studies, Mittal et al (2005) summarize the three situations with regards to separated flow, as shown in Figure 1-5. In post-stall flow (case C in Figure 1-5), leading-edge shear layer rollup and vortex shedding in the wake are two characteristic features (Wu et al 1998). Huerre and Monkewitz (1990) suggest that this type of shear flow (with a pocket of absolute instability of sufficient size) may display intrinsic dynamics of the same nature as in a closed-flow system, in which disturbances can grow upstream (i.e.…”

“…The different frequency scales are indicative of different flow instabilities that may exist in the flow and, if present, may compete with each other (Wu et al 1998). When periodic excitation is introduced, one or more of these instabilities may be energized.…”