Unsteady aerodynamic lateral force on a sphere under a uniform incoming flow

Abstract

Transversal Instabilities due to Longitudinal Winds, affecting overhead power lines with spherical warning beacons, were reported in recent years. Transition modes where intermittency between two quasi-stable states of motion coexist were detected. The lateral force on the sphere plays a fundamental role, but no information about its intermittency has been found in the literature. In this work, results of wind tunnel tests concerning the aerodynamic forces of an isolated sphere as a function of the Reynolds number are reported. The test campaign was carried out with two different mock-ups and measurement systems, obtaining similar results. Intermittency has been found in the aerodynamic lateral force, in both the critical and the supercritical regimes. There are two main states of intermittency, which modify their duration as a function of the Reynolds number. Their nature is intrinsically chaotic, since neither periodicity nor similar duration has been found in consecutive appearances of both states. High-resolution pressure coefficient distribution plots allow to better identify these states, and determine the boundary layer separation line, which is not azimuthally uniform over the sphere, changing its shape over time, thus affecting directly to the direction of the lateral force associated to each state.