Energy transfer and budget analysis for transient process with operator-driven reduced-order model (2503.20204v4)

Abstract: We present the possibility of energy transfer and budget analysis for transient flow using eigenmodes of the operator from the Navier-Stokes equation. We derive the energy transfer equation, which provides the energy budget for the eigenmodes, through the Galerkin projection of the equation using the bi-orthogonality of the eigenmodes and the adjoint mode. Energy budget and transfer analysis between modes were conducted for two-dimensional flow around a cylinder with eigenmodes growing or decaying from a steady flow. Using the linearized energy transfer equation and eigenmodes from global stability analysis, we identify the energy budget and spatial distribution that determine growth rates of the modes. Moreover, energy budget and transfer analysis are realized by considering the time evolution of the eigenmodes, even during the nonlinear development of the eigenmodes. By introducing time-varying dynamic mode decomposition with a phase-control strategy for multiple time-series datasets from numerical simulations of the phase-controlled initial flow, time-varying eigenmodes are extracted in the transient two-dimensional cylinder flow. With the extracted time-dependent modes and the derived energy transfer equations, the time evolution of the energy budget and spatial distribution of energy transfer can be computed until the eigenmodes developed from the steady field reach the post-transient periodic flow. The temporal evolution of the energy budget and transfer distribution reveals that transient growth processes display different time-dependent characteristics of energy transfer, even in cylinder flows at Reynolds numbers that eventually lead to a periodic state.