Abstract
Shear-layer driven open cavity flows are known to exhibit strong self-sustained oscillations of the shear-layer. Over some range of the control parameters, a competition between two modes of oscillations of the shear layer can occur. We apply both Proper Orthogonal Decomposition and Dynamic Mode Decomposition to experimental two-dimensional two-components time and spaced velocity fields of an incompressible open cavity flow, in a regime of mode competition. We show that, although proper orthogonal decomposition successes in identifying salient features of the flow, it fails at identifying the spatial coherent structures associated with dominant frequencies of the shear-layer oscillations. On the contrary, we show that, as dynamic mode decomposition is devoted to identify spatial coherent structures associated with clearly defined frequency channels, it is well suited for investigating coherent structures in intermittent regimes. We consider the velocity divergence field, in order to identify spanwise coherent features of the flow. Finally, we show that both coherent structures in the inner-flow and in the shear-layer exhibit strong spanwise velocity gradients, and are therefore three-dimensional.
Highlights
An efficient way for understanding the intrinsic phenomenology of a flow relies on the analysis of its constitutive coherent structures
Proper orthogonal decomposition successes in identifying salient features of the flow, it fails at identifying the spatial coherent structures associated with dominant frequencies of the shear-layer oscillations
We show that, as dynamic mode decomposition is devoted to identify spatial coherent structures associated with clearly defined frequency channels, it is well suited for investigating coherent structures in intermittent regimes
Summary
An efficient way for understanding the intrinsic phenomenology of a flow relies on the analysis of its constitutive coherent structures. Some spatial modes identified by proper orthogonal decomposition are sometimes considered as empirically determined coherent structures of the flow under study. Impingement is a source of perturbation for the pressure field, which, due to incompressibility, triggers new perturbations at the leading edge, closing the loop.[19] On most ranges of the control parameters, only one dominant frequency characterizes the shear-layer oscillations. Dynamic mode decomposition naturally does extract the coherent structures associated with each frequency of oscillation, separately. It is expected that the divergence field is continuous and derivable, if non-vanishing, where the mode is energetic Though it may seem counter-intuitive, we show that both dynamic modes, associated with shear-layer oscillations, are intrinsically three-dimensional structures of the flow. We conclude this paper by putting these results into perspective with respect to what is known from experimental and numerical studies in the literature on open cavity flows
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