Investigations on Quasi-Steady Characteristics for an Airfoil Oscillating at Low Reduced Frequencies

Abstract

Wind tunnel experiments were conducted on NACA-0015 airfoil model to investigate the effect of the reduced frequency (κ=0.0001to 0.5) and that Reynolds number (Re=0.2E06to 0.7E06) on the aerodynamic characteristics and hysteresis behavior associated with the oscillating motion of the airfoil. Pressure measurements were conducted on the midspan of the airfoil for quantitative results. Although dynamic stall study has been conducted to greater extent of reduced frequencies, for current work, emphasis is made on investigating the low reduced frequency regimes up to which the steady conditions prevail under dynamic mode of operation. In present investigations of oscillating sinusoidalα=10∘+15∘sin(ωt)motion, the airfoil was allowed to execute oscillating motion from static to almost steady (quasi-steady) conditions with the incremental increase in the reduced frequency. The limiting conditions ofκandRefor quasi-steady state are carefully extracted and analyzed. Static and quasi-steady conditions are critically assessed in context of the existence of laminar separation bubble within the optedReregime. The flow separation was found to be trailing edge flow separation with existence of LSB upstream towards the leading edge with no evidence of unsteady flow reversals. Normal force defect and pitch damping factor are estimated for varied range ofReandκwithin the quasi-steady conditions. Re effect is dominant at steady and quasi-steady regimes while the effect of reduced frequency is negligible in this domain. Minor hysteresis effect associated with the stalling and reattachment point is due to existence of weak stall flutter which is further restrained by the oscillatory motion of the airfoil. Laminar separation bubble traced during steady condition is conserved in quasi-steady domain as well.

Physics of Dynamic Stall Vortex During Pitching Oscillation of Dynamic Airfoil

“…Gupta and Ansell [9] and Geissler et al [20,22] also found such LEVs in the pressure distributions of the experimental data. Sharma and Poddar [27] found LEVs through the so-called Cn-Cm cross-correlation plots, which were obtained experimentally. In addition, Li et al [28] found LEVs in CFD and PIV results.…”

Understanding and controlling vorticity transport in unsteady, separated flows

“…Laminar separation bubbles (LSB) have been observed to be the incipient vortical structures at the leading edge of pitching NACA airfoils operating at low Reynolds numbers (Re = O(10 4 )-O(10 6 )), and the bursting of these bubbles has been related to the onset of dynamic stall [1,95,110,66]. The effect of an LSB on the surface pressure distribution has been well established, as there exists a well-defined plateau in the surface pressure distribution beneath the LSB.…”

Aerodynamic Response of a Pitching Airfoil with Pulsed Circulation Control for Vertical Axis Wind Turbine Applications

“…(106) concluded for a variety of conditions, that the stall process, with laminar separation bubble, is initiated at the TE of the NACA0015 profile. Considering the TE CC capabilities of the CCNACA0018 airfoil, this is thought to be a favorable result as flow control authority increases as actuation is nearer the point of airfoil surface separation.…”

“…numerous reports. Experiments bySharma, et al (92) (106) reported that Re effects increase at low pitch rates (approaching quasi-steady conditions), causing substantial rise in hysteresis behavior and stall AoA ( stall ) shift. As Re (0.7x10 6 ) increases, such hysteresis effects become negligible and Re effects become limited to the weakening in strength of DSV.…”