Qualitative effect of velocity ratio on the behaviour of a circular synthetic jet flush mounted on a torpedo model in quiescent and cross-flow conditions

Abstract

A synthetic jet on a curved surface and at high velocity ratios is of immense interest for its possible usage in a torpedo or other naval applications. However, despite exhaustive research on synthetic jets on a flat surface in cross-flow, very few studies have been conducted on synthetic jets on a torpedo-like surface. This study experimentally explores a synthetic jet mounted on a torpedo shaped model in quiescent and cross-flow conditions. The synthetic jet actuation mechanism consists of a motor driven eccentric cam, which oscillates the diaphragm and generates synthetic jet. Initially, the synthetic jet is characterized for two different diaphragm displacements and at four distinct actuation frequencies in the range of 1 Hz – 6 Hz in a quiescent flow environment. Subsequently, similar study is carried out in cross-flow for three different cross-flow velocities ranging from 7.2 – 32 cm/s, at a fixed amplitude of diaphragm oscillations. The flow visualization results qualitatively indicate that the vortex rings in synthetic jet can be classified into two major groups: stretched vortex rings and distorted tilted vortex rings. The flow structures primarily depend on the velocity ratio, which is function of actuation frequency and cross-flow velocity. At a low velocity ratio, the leading vortex is followed by a strong trailing jet. Two distinctly different trajectories are observed for the leading and trailing vortices at higher velocity ratios. At a low cross-flow velocity and higher actuation frequency, synthetic jet leads to increase in the velocity within the boundary layer on the upstream side of the orifice.