Dynamic Characteristics of Flow Separation From a Low Reynolds Number Airfoil

Abstract

This study examines the generation of large scale vortices caused by flow separation from a flat wing at various angles of attack. Time-resolved particle image velocimetry is used to determine the evolution and convective characteristics of the large scale structures. A rectangular airfoil with aspect ratio of 0.5 is used and data are collected at a Reynolds number of 23,500, for angles of attack from 0° to 20°. Data consists of two dimensional velocity fields obtained at 500 Hz located at the airfoil centerline. The region of interest is near the separation point but fields of view extend over approximately one half of the chord length from the leading edge to document the downstream progression of the large scale vortical flow elements. The velocity data were processed to identify the vorticity field dynamics in terms of the Kelvin-Helmholtz instability occurring near the leading edge. The vortical structures are identified using vortex detection based on local circulation. The convective nature of the vortex elements are shown to consist of merging, stalling and convecting, with convective velocities on the order of 20% of the freestream velocity with an associated Stouhal number based on chord length and freestream velocity of approximately 1.0.

Experimental Time Resolved Flow Features of Separation Over an Elliptic Leading Edge

“…modes 1 and 2, modes 2 and 3, etc. The primary frequency associated with the cyclic feature found in cross correlation of modes 1-2, and 2-3 is the same as the vortex shedding frequency of approximately 28 Hz discussed previously by Morse and Liburdy [2007].…”

“…This was found by performing autocorrelations as well as Fourier transforms on the u and v component time series as well as a vortex detection criterion. Further explanation is provided by Morse and Liburdy [2007]. The Strouhal number based on freestream velocity and chord length for the leading edge vortex shedding was 3.1.…”

Low Reynolds Number Flow Dynamics of a Thin Airfoil with an Actuated Leading Edge

“…The flow characteristics at the leading edge of a thin, flat, low aspect ratio wing at relatively high angles involving flow separation, vortex generation, and vortex convection have been studied by the authors in the past at various angles of attack (15 • -20 • ) using a three-component, time-resolved PIV. [51][52][53][54][55] This study was performed for a static wing without any actuation (i.e. θ = 0).…”

Research on the Flow Separation Control in Hydrodynamic Coupling Based on Numerical Simulation

“…Bons et al found that the steady Vortex Generator Jets (VGJ) could have the effect of reducing or entirely eliminating the separation zone on the suction surface of a turbine blade at low Reynolds number [1]. The researchers of the Air Force Research Laboratory used a high-order-accurate finite volume code based on the compressible Navier-Stokes equations by direct numerical simulations (DNS) with up to 19.4 million grid points to numerical simulation the flow field of lowpressure turbines with Pulsed VGJs, and found they could effectively mitigate separation [2].…”