Starting vortex strength in an impulsively started airfoil

TL;DR

This study investigates the formation, detachment, and characteristics of the starting vortex and shear layer in an impulsively started NACA 0010 airfoil using PIV measurements, revealing insights into vortex dynamics and flow separation.

Contribution

It provides detailed experimental analysis of vortex detachment, circulation, and secondary vortex formation, linking these phenomena to flow parameters and flow physics.

Findings

Starting vortex detaches during surging at pre-stall angles.

Vortex circulation at detachment aligns with Wagner's function predictions.

Secondary vortex formation depends on Reynolds number, not vortex detachment.

Abstract

This work characterises the starting vortex and unsteady shear layer generated by an impulsively started National Advisory Committee for Aeronautics 0010 (NACA 0010) airfoil at angles of attack ranging from 3 degrees- 7 degrees. Measurements are obtained by using time resolved Particle Image Velocimetry. The net circulation in the field is accurately predicted by Wagner's function. It is observed that the starting vortex detaches from the shear layer whilst the airfoil is surging. Starting vortex circulation at detachment is presented for a range of surge speeds at pre-stall angles of attack for a given surge distance. Kinematic conditions resulting in starting vortex detachment are examined qualitatively through Finite Time Lyapunov Exponents and quantitatively through a non-dimensional velocity ratio. Furthermore, the formation of secondary Kelvin-Helmholtz-type vortices in the shear layer is found to depend on the Reynolds number rather than the detachment of the starting vortex.