Viscous Effects in the Inception of Cavitation on Axisymmetric Bodies

Cavitation inception and development on two axisymmetric bodies was studied with the aid of a Schlieren flow visualization method developed for that purpose. Both bodies were found to exhibit a laminar boundary layer separation; cavitation inception was observed to occur within this region of separated flow. The incipient cavitation index was found to be closely correlated with the magnitude of the pressure coefficient at the location of flow separation on one of the bodies. There is also experimental evidence that events at the site of turbulent reattachment of the separated flow may also greatly influence cavitation inception

Comparison of moving and stationary clapping bodies

We report differences in the flow dynamics and in the body kinematics between a clapping body that is allowed to propel forward freely (Dynamic) and one that is constrained from moving forward (Stationary). The experiments were done in quiescent water. The body consists of two interconnected plates hinged at one end with a 'torsion' spring. Initially, a thread loop holds the plates apart at an interplate angle of 60 degrees. Cutting of the thread initiates the clapping motion, and if allowed, the body propels forward a certain distance. Experiments have been performed for three values of $d^*$(= depth/length): 1.5, 1.0, and 0.5. In both cases, vortex loops initially develop along the three edges of each plate, which reconnect by the end of the clapping motion resulting in the formation of an elliptical vortex loop in the wake for $d^*$ = 1.5, 1.0 bodies, and multiple connected rings for $d^*$ = 0.5 bodies. Three main and unexpected differences are observed between the 'Dynamic' and 'Stationary' bodies. In the dynamic case, the clapping action is faster compared to the stationary case, with the maximum angular plate velocity being twice as high. The mean thrust coefficient, $\overline{C_T}$, based on plate tip velocity, is higher for the stationary. The value of $\overline{C_T}$ and the circulation in the starting vortices is almost independent of $d^*$ for the dynamic case, whereas it increases with $d^*$ for the stationary case. The core separation of starting vortices closely matches in both stationary and dynamic cases although circulation varies

Some transition and cavitation inception observations on a 1.5 cal ogive

Transition observation on a 1. 5 cal ogive were carried out by Schlieren technique of flow visualization up to Re_D of 1.26 x 10^6. Good agreement is found between computed position of transition by Smith method and those observed by Schlieren technique for tunnel velocities greater than 50 fps (Re_D> 7.85 x 10^5). Cavitation under desinent conditions at tunnel velocities of 30 fps and 40 fps was found to occur within the transition region of the boundary layer. At 50 fps good agreement is found between the present value of inception cavitation index, the value of desinent cavitation index measured by Parkin and the negative value of the pressure coefficient at both predicted and observed positions of transition. These observations strongly suggest that cavitation inception is closely related to transition on smooth bodies at supercritical Reynolds numbers

Sonoluminescence as Quantum Vaccum Radiation

We argue that the available experimental data is not compatible with models of sonoluminescence which invoke dynamical properties of the interface without regard to the compositional properties of the trapped gas inside the bubble.Comment: 2 pages,Revtex,No figures,Submitted to PRL(comments

Instability-induced ordering, universal unfolding and the role of gravity in granular Couette flow

Linear stability theory and bifurcation analysis are used to investigate the role of gravity in shear-band formation in granular Couette flow, considering a kinetic-theory rheological model. We show that the only possible state, at low shear rates, corresponds to a 'plug' near the bottom wall, in which the particles are densely packed and the shear rate is close to zero, and a uniformly sheared dilute region above it. The origin of such plugged states is shown to be tied to the spontaneous symmetry-breaking instabilities of the gravity-free uniform shear flow, leading to the formation of ordered bands of alternating dilute and dense regions in the transverse direction, via an infinite hierarchy of pitchfork bifurcations. Gravity plays the role of an 'imperfection', thus destroying the 'perfect' bifurcation structure of uniform shear. The present bifurcation problem admits universal unfolding of pitchfork bifurcations which subsequently leads to the formation of a sequence of a countably infinite number of 'isolas', with the solution structures being a modulated version of their gravity-free counterpart. While the solution with a plug near the bottom wall looks remarkably similar to the shear-banding phenomenon in dense slow granular Couette flows, a 'floating' plug near the top wall is also a solution of these equations at high shear rates. A two-dimensional linear stability analysis suggests that these floating plugged states are unstable to long-wave travelling disturbances.The unique solution having a bottom plug can also be unstable to long waves, but remains stable at sufficiently low shear rates. The implications and realizability of the present results are discussed in the light of shear-cell experiments under 'microgravity' conditions

The high speed water tunnel facility at the Indian Institute of Science

The present article about the high speed water tunnel facility at the Indian Institute of Science, Bangalore, provides a general description of the tunnel circuit, and brief reports on the performance of the facility and some typical results from investigations carried out in it. A unique aspect of the facility is that it has a horizontal resorber in the form of a large cylindrical tank located in the lower leg of the circuit. The facility has been used, among other things, for flow visualization studies, and investigations on marine propeller hydrodynamics and "synthetic cavitation". The last topic has been primarily developed at the Indian Institute of Science and shows considerable promise for basic work in cavitation inception and noise

A magnetic field diagnostic for sonoluminescence

This study is motivated by the extraordinary process of single bubble sonoluminescence (SBSL), where an acoustically driven spherical shock is thought to power the emitted radiation. We propose new experiments using an external magnetic field which can induce anisotropies in both the shock propagation and radiation pattern. The effects will depend on the temperature, conductivity, and size of the radiating region. Our predictions suggest that such a laboratory experiment could serve as an important diagnostic in placing bounds on these parameters and understanding the physics of sonoluminescence.Comment: Latex File, Two .eps files, 5 pages, submitted to PR