Unsteady Separated Flow Characterization on Airfoils Using Time-Resolved Surface Pressure Measurements

Laminar boundary-layer separation and separated shear layer development on a NACA 0025 airfoil at low Reynolds numbers were studied experimentally. Flow visualization, hot-wire velocity measurements, and time-resolved surface pressure measurements were employed in this investigation. The results for two distinct flow regimes, namely, flow separation without subsequent shear layer reattachment and separation bubble formation, are discussed in detail. For both flow regimes, the transition occurs due to the amplification of natural flow disturbances in the separated shear layer. Initially, disturbances within a band of frequencies centered at some fundamental frequency are amplified. Further downstream, nonlinear interactions set in, leading to a breakdown to turbulence. Based on the spectral and correlation analysis of velocity and surface pressure fluctuations, it is demonstrated that the amplification of disturbances and the attendant fluctuations in the flow velocity give rise to distinct surface pressure fluctuations at the fundamental frequency. Thus, time-resolved surface pressure measurements can be employed to estimate important unsteady characteristics of the separated flow region on an airfoil operating in low Reynolds flows.

[1]  Pierre E. Sullivan,et al.  Effect of Acoustic Excitation Amplitude on Airfoil Boundary Layer and Wake Development , 2007 .

[2]  S. Yarusevych,et al.  Coherent structures in an airfoil boundary layer and wake at low Reynolds numbers , 2006 .

[3]  H. Irwin,et al.  Correction of distortion effects caused by tubing systems in measurements of fluctuating pressures , 1979 .

[4]  Israel J Wygnanski,et al.  The control of flow separation by periodic excitation , 2000 .

[5]  Roger W. Ainsworth,et al.  Unsteady Pressure Measurement , 2000 .

[6]  A. A. Bakchinov,et al.  Experiments on a two–dimensional laminar separation bubble , 2000, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[7]  A. Bertelrud,et al.  Experimental Investigation of Laminar Separation Bubbles and Comparison with Theory , 1987 .

[8]  I. Tani.  Low-speed flows involving bubble separations , 1964 .

[9]  Ulrich Rist,et al.  Investigations of time-growing instabilities in laminar separation bubbles , 2002 .

[10]  Victor V. Kozlov,et al.  Laminar boundary layer separation: Instability and associated phenomena , 1994 .

[11]  Othon K. Rediniotis,et al.  Unsteady Calibration of Fast-Response Pressure Probes, Part 1: Theoretical Studies , 2005 .

[12]  Tony Arts,et al.  Measurement techniques for unsteady flows in turbomachines , 2000 .

[13]  J. G. Kawall,et al.  A digital technique for the simultaneous measurement of streamwise and lateral velocities in turbulent flows , 1983, Journal of Fluid Mechanics.

[14]  Laura L. Pauley,et al.  Low-Reynolds-number separation on an airfoil , 1996 .

[15]  Pascal Köppel,et al.  Time-resolved flow measurements with fast-response aerodynamic probes in turbomachines , 2000 .