Frequency-selection mechanism in incompressible open-cavity flows via reflected instability waves.

We present an alternative perspective on nonharmonic mode coexistence, commonly found in the shear layer spectrum of open-cavity flows. Modes obtained by a local linear stability analysis of perturbations to a two-dimensional, incompressible, and inviscid sheared flow over a cavity of finite length and depth were conditioned by a so-called coincidence condition first proposed by Kulikowskii [J. Appl. Math. Mech. 30, 180 (1966)] which takes into account instability wave reflection within the cavity. The analysis yields a set of discrete, nonharmonic frequencies, which compare well with experimental results [Phys. Fluids 20, 114101 (2008); Exp. Fluids 50, 905 (2010)].

Determining the spectral signature of spatial coherent structures in an open cavity flow.

We applied to an open flow a proper orthogonal decomposition (POD) technique, on two-dimensional (2D) snapshots of the instantaneous velocity field, to reveal the spatial coherent structures responsible for the self-sustained oscillations observed in the spectral distribution of time series. We applied the technique to 2D planes out of three-dimensional (3D) direct numerical simulations on an open cavity flow. The process can easily be implemented on usual personal computers, and might bring deep insights regarding the relation between spatial events and temporal signature in (both numerical or experimental) open flows.