Harish Ganesh

Effect of Non-Condensable Gas Injection on Cavitation Dynamics of Partial Cavities

Partial cavities can undergo auto-oscillation causing large pressure pulsations, unsteady loading of machinery and generate significant noise. In the current experiments fully shedding cavities forming in the separated flow region downstream of a wedge were investigated. The Reynolds number based on hydraulic diameter was of the order of one million. The cavity dynamics were studied with and without injection of non-condensable gas into the cavity. Gas was injected directly into the cavitation region downstream of the wedges apex, or into the recirculating region at mid cavity so that for the same amount of injected gas less ended up in the shear layer. It was found that relatively miniscule amounts of gas introduced into the shear layer at the cavity interface can reduce vapour production and dampen the auto oscillations, and the same amount of gas injected into the mid cavity would not have the same effect. The authors also examined whether the injected gas can switch the shedding mechanism from one dominated by condensation shock to one dominantly by reentrant jet.

Interaction of a Compressible Bubbly Flow With an Obstacle Placed Within a Shedding Partial Cavity

Bubbly shocks act as the dominant mechanism of shedding of partial cavities under certain conditions. The compressible nature of low void fraction regions in a partial cavity can provide conditions necessary for the existence of such bubbly shocks. Using X-ray densitometry as a flow visualisation mechanism, the dynamics of bubbly shock fronts with and without the presence of an obstacle are presented. Different flow features associated with the shock dynamics are identified and the associated flow physics explained.

Bubbly shock propagation as a mechanism of shedding in separated cavitating flows

Stable attached partial cavitation in separated flows can transition to cloud shedding, and the mechanism of transition has been attributed to the presence of a re-entrant liquid jet. Our findings have revealed the presence of propagating bubbly shock waves as an alternative dominant mechanism of shedding when the compressibility of the bubbly mixture is appreciable. In the present paper, we discuss dynamics associated with these bubbly shock waves, interaction of shock waves with obstacles in their path, and means to manipulate their properties to control the shedding process by non-condensable gas injection.

Acoustic emission due to partial cavity shedding on a hydrofoil

The partial cavity shedding dynamics on a NACA 0015 hydrofoil were examined at angles of attack of 7 and 10 degrees over a range of cavitation numbers. Hydrophone measurements in conjunction with high-speed videos and time resolved X-ray densitometry were used to acquire time synchronous measurements of the void fraction field of the cavity and the emitted noise. Cavitation dynamics changed significantly with the reduction of the inlet cavitation number. At higher cavitation numbers, small stable partial cavities were observed. Upon reduction of cavitation number, the cavities grew in length and pinched off from the rear of the cavity, due to a liquid re-entrant jet. Upon further reduction of inlet cavitation number, the dynamics changed significantly with the cloud collapse of the shed vapor inhibiting the cavity growth. X-ray densitometry measurements revealed the presence of a propagation bubbly shockwave as a mechanism of shedding at the lowest cavitation numbers. This process caused complex, multi-st...

Stationary cavitation bubbles forming on a delta wing vortex

Vortex cavitation forming in the leading-edge vortices of a delta wing was examined to determine how the individual cavitation bubbles incepted, grew, interacted with the underlying vortical flow and produced acoustic tones. The non-cavitating vortical flow over the delta wing was chosen to be similar to those previously reported in the literature. It was found that vortex breakdown was unaffected by the presence of incipient and developed vortex cavitation bubbles in the vortex core. While some cavitation bubbles incepted, grew, and collapsed relatively quickly, others reached an equilibrium position wherein the bubble tip was stationary in the laboratory frame at a particular location along the vortex axis. For a given attack angle, the equilibrium location moved upstream with a reduction in free stream cavitation number. It is shown that the existence of these stationary vortex bubbles is possible when there is a balance between the axial growth of the bubble along the vortex axis and the opposite moti...

Flow-Induced Noise of Shedding Partial Cavitation on a Hydrofoil

The cavitation dynamics on a NACA0015 hydrofoil can have different cavitation shedding behaviors depending upon the attack angle and flow conditions, as discussed in Arndt et al. (Instability of partial cavitation: a numerical/experimental approach, 2000) [1] and Kjeldsen et al. (J Fluids Eng 122(3):481–487, 2000) [8]. Shedding can exhibit varying shedding frequencies (Strouhal numbers) measured based on measurements of the resulting surface pressure and body forces. Moreover, the transition from one regime of shedding to another can be abrupt with changes in cavitation number. In this work, we present an analysis of the acoustic signatures measured by a hydrophone for different shedding conditions using a Morse wavelet analysis. Using measurements of the cavity dynamics based on X-ray densitometry and high-speed video observations (Ganesh et al., 31st symposium on naval hydrodynamics, ONR 2016) [6], we present an explanation for the observed acoustic behavior.