Application of energy and exergy balances to drag decomposition in unsteady incompressible flows

Abstract

The accurate evaluation of aerodynamic drag in unsteady flows presents an ongoing challenge for aerodynamic optimisation in tightly coupled engine-airframe configurations. This thesis investigates the use of energy and exergy balance formulations as alternative approaches for drag evaluation, with a focus on their ability to decompose drag power into recoverable and irreversible contributions. Three two-dimensional laminar flow test cases were examined using CFD simulations in ANSYS Fluent. The first, a stationary cylinder at Re = 140 undergoing vortex shedding, served as a benchmark and confirmed that both balances reproduce near-field drag values with high accuracy while revealing the conversion of mechanical energy into viscous dissipation along the wake. The second case, involving an oscillating cylinder in static fluid, demonstrated that the balances remain applicable even in the absence of net aerodynamic drag force, capturing the redistribution of energy through viscous effects in agreement with analytical predictions. The third case combined vortex shedding with azimuthal wall oscillation, producing a more complex flow where the balances showed a modest increase in mean drag but no fundamental changes to dissipation mechanisms. Together, these results establish the robustness of energy and exergy balances for unsteady drag evaluation and highlight their potential for usage in optimisation of aerodynamic performance.