Pressure Force Response on a Sphere in Unsteady Flow with Sinusoidally Alternating Crosswind

Abstract

The response forces on a sphere in sinusoidal cross winds with frequencies between 20 Hz and 50 Hz and small amplitudes (<3°) derived from surface pressure measurements have been investigated in wind tunnel experiments. Two different undisturbed inflow velocities were considered, which lead to a sub- and a supercritical flow separation pattern. The amplitude of the side force response was found to depend linearly on the amplitude of the inflow angle and grows non-linearly with increasing reduced frequency. The phase angle between inflow and side force response increases linearly with the reduced frequency. Both the amplitude and the phase angle of the side force response collapse on a single trend for the sub- and supercritical flow separation, hence the trend is only a function of the reduced frequency. Calculating a virtual point of force application, where the force is in phase with the inflow angle, it was shown that this point is independent of the reduced frequency.