A perspective on unsteady aerodynamic force theories: from the diagnostics of global loads to that of local stress distributions

Abstract This paper reviews field-based diagnostic theories of unsteady aerodynamic forces, ranging from formulations for total force and moment to recent developments in the diagnosis of local stress distributions. We first revisit the vorticity-moment theory (Wu, AIAA J, vol. 19, 1981, 432–441) as a starting point for field-based force analysis, with emphasis on far-field decay requirements and subsequent extensions to finite domains and compressible flows. We then consider the vortex-force theory and force-element theory, which provide more physically interpretable connections between the flow field and aerodynamic loads. Subsequently, we discuss decompositions of the pressure field based on its governing equation and boundary conditions. Finally, we examine the transition from total force and moment analysis to the diagnosis of local surface loading through weighted-integral methods. Overall, unsteady aerodynamic force theories have evolved from asking how the total load can be calculated from the surrounding flow field to asking how individual flow structures and physical processes determine the distribution of stress over the body surface.