Modeling of Transitional Channel Flow Using Balanced Proper Orthogonal Decomposition

TL;DR

This paper develops reduced-order models of three-dimensional channel flow perturbations using balanced proper orthogonal decomposition, which better captures transient growth and localized disturbances than traditional POD, aiding in control design.

Contribution

The paper introduces a BPOD-based modeling approach from physical space simulations, improving accuracy in capturing transient growth and localized effects over standard POD.

Findings

BPOD models accurately capture transient growth at low order.

Standard POD requires more modes and can be less accurate.

BPOD models better reproduce dominant eigenvalues for single-wavenumber perturbations.

Abstract

We study reduced-order models of three-dimensional perturbations in linearized channel flow using balanced proper orthogonal decomposition (BPOD). The models are obtained from three-dimensional simulations in physical space as opposed to the traditional single-wavenumber approach, and are therefore better able to capture the effects of localized disturbances or localized actuators. In order to assess the performance of the models, we consider the impulse response and frequency response, and variation of the Reynolds number as a model parameter. We show that the BPOD procedure yields models that capture the transient growth well at a low order, whereas standard POD does not capture the growth unless a considerably larger number of modes is included, and even then can be inaccurate. In the case of a localized actuator, we show that POD modes which are not energetically significant can be very important for capturing the energy growth. In addition, a comparison of the subspaces resulting from the two methods suggests that the use of a non-orthogonal projection with adjoint modes is most likely the main reason for the superior performance of BPOD. We also demonstrate that for single-wavenumber perturbations, low-order BPOD models reproduce the dominant eigenvalues of the full system better than POD models of the same order. These features indicate that the simple, yet accurate BPOD models are a good candidate for developing model-based controllers for channel flow.