High-order optimal mode decomposition analysis of the ground effect on flow past two tandem inclined plates

Abstract

Two-dimensional flow past two tandem near-ground plates with inclination angles of 25° at the Reynolds number of 150 is numerically simulated via the high-order spectral element method. Plate-to-ground gap is varied from G = 0.2L to 1.6L with intervals of 0.2L at two representative inter-plate spacings (i.e., X = 2.5L and 6L). The ground effect on the fluid force, power spectral density, asymmetric gap flow, and wake structure of plates is systematically evaluated. Then, the high-order optimal mode decomposition (HOOMD) method is proposed to synchronously analyze the velocity and pressure fields. The results show that the fluid force and flow structure are closely dependent on G. The presence of the ground inhibits vortex shedding when G < 0.6L; as the gap increases from 0.6 L to 1.4 L, the fluctuating forces are continuously enhanced until the ground effect basically disappears at G > 1.4L. The ground effect exacerbates the asymmetry of the vortex structure near the upper and lower parts of the inclined plates, consequently changing the fluid force. The downstream plate is more sensitive to the ground effect because of impingement from the upward-biased jet flow generated in the narrow gap between the upstream plate and ground. The HOOMD method well captures the spatial morphology and temporal evolution features of different dominant modes at the transition or vortex shedding flow regime. Mode analysis affords a correspondence between the coherent vortex structure and fluid force of plates. Furthermore, the ground effect can simultaneously change the global mode energy and local pressure mode shape, subsequently influencing the fluid force. However, the global mode energy plays the determinant role in the variation of the fluid force of plates with the plate-to-ground distance herein.