Investigation of coherent structures generated by acoustic tube in turbulent flow separation control

Abstract

An acoustic tube was designed and implemented in turbulent backward-facing step (BFS) flow at a Reynolds number of Reh = 2.0×104 with respect to the step height. As an active flow control device the acoustic tube generated periodic actuations at a frequency of fa = 100 Hz which was close to the frequency of the most naturally amplied disturbance of the respective turbulent shear layer. Therefore the acoustic tube educed series of coherent structures in the turbulent shear layer. High resolution 2D-2C- particle image velocimetry (PIV) was used to measure these vortical structures and the reattachment area downstream of the BFS. The flow control result shows that the acoustic tube can suppress recirculation regions behind the step and reduce the reattachment length by 43.0%. Phase-averaged flow fields present eduction of the coherent structures in the linear stage of the turbulent shear layer instabilities and the development of the nonlinear stage further downstream. Entrainments of the coherent structures result in higher Reynolds shear stress and more turbulent kinetic energy and therefore convey kinetic energy from mean flow to random fluctuations. The coherent structures are extracted by Proper Orthogonal Decomposition (POD) and represented by two pairs of modes as 'PODi+i•PODj', of which the coherence are analyzed by corresponding coefficients. Both first and second harmonic series of vortices are reconstructed as travelling waves in the turbulent shear layer, with the fundamental frequency fa and the overtone frequency 2•fa respectively.