Unsteady Flow Physics of Airfoil Dynamic Stall

Abstract

A series of wind tunnel experiments were conducted on an NACA 0012 airfoil undergoing a linear pitch ramp maneuver at a fixed dimensionless pitch rate of Ω + = 0.05 and across three transitional Reynolds numbers, Re c = 0.2 × 10 6 , Re c = 0.5 × 10 6 , and Re c = 1.0 × 10 6. The primary objectives of these experiments were to perform a detailed analysis of the flow evolution, with particular emphasis on the underlying physical mechanisms, and to extract the dominant scales associated with the flow perturbations, for a canonical dynamic stall process. A series of unsteady surface pressure measurements, with a high sampling frequency, were acquired in order to investigate the time-dependent behavior of the flow in the immediate vicinity of the airfoil. These surface pressure measurements were used to identify the region of boundary layer transition during the initial stages of the dynamic stall process. A spatially-contracting laminar separation bubble was also identified near the airfoil leading edge from the characteristic pressure plateau in the surface pressure distribution. The dominant frequencies associated with the laminar separation bubble were extracted using a continuous wavelet transform technique. These frequencies were observed to span a wide range of chordbased Strouhal numbers between St = 50 and St = 105, at Re c = 0.5 × 10 6. The off-body flow evolution was inferred and described using a combination of surface pressure measurements and time-resolved particle image velocimetry. For Re c = 0.2 × 10 6 and Re c = 0.5 × 10 6 , the dynamic stall vortex was observed to emerge from a collective interaction of the discrete vortices that were ejected from the leading edge of the airfoil. At Re c = 1.0 × 10 6 , however, the near-wall vortices were observed to amalgamate into two regions, forming a distinct primary and a secondary coherent structure. After formation, these two structures were observed to interact with each other, following a co-rotating vortex merging process and resulting in the emergence of a single, coherent dynamic stall vortex. The process of emergence of the dynamic stall vortex at Re c = 1.0 × 10 6 , observed from the present experiments, is therefore quite distinct from the classical understanding of the dynamic stall vortex formation, which was observed at the lower Reynolds numbers. The time-dependent spectra of the velocity field were calculated using a combination of empirical mode decomposition and Hilbert transformation. From the velocity spectra, the fluctuations in the flow were observed to attain an amplified state during the initial ejection of vorticity from the leading-edge region of

Variation of leading-edge suction during stall for unsteady aerofoil motions

“…The authors wish to gratefully acknowledge the support of the U.S. Army Research Office through grant W911NF-13-1-0061, monitored by Dr Matthew Munson. Professor Phillip Ansell of the University of Illinois at Urbana-Champaign is thanked for sharing the experimental data (Gupta & Ansell 2019) used in this paper. Helpful discussions with Dr Kiran Ramesh of the University of Glasgow are also acknowledged.…”

Modeling the interplay between the shear layer and leading edge suction during dynamic stall

“…However, in most occurrences of dynamic stall, progressive trailing edge separation and flow reversal play a significant role 12,19,20,41 . In this paper, we consider experimental data of deep dynamic stall on a pitching airfoil where stall onset is the result of the roll up of a free shear layer at the interface between the outer flow and a region of flow reversal near the airfoil's suction surface.…”

“…The first term of this equation (12) represents the influence of the geometric camber according to equation 7…”

“…Smoke flow visualisation was used to qualitatively describe the coherent vortex structures developing during dynamic stall. The development of unsteady pressure sensors made it possible to obtain accurate spatially and temporally resolved surface pressure distributions and measure high frequency turbulent flow oscillations [9][10][11][12][13] . The surface pressure measurements revealed the presence and continued growth of a suction peak at the leading edge during dynamic stall development.…”

Influence of Aspect Ratio on Dynamic Stall of a Finite Wing

“…For transitional Reynolds numbers, the DSV was found to consist of an agglomeration of smaller shear-layer vortical structures that convect together 17 . In addition, the time frequency spectra of the pressure measurements related to the formation of these structures has been analyzed to track the progression of the LSB and the formation of the DSV 18 .…”