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.
Knowledge of the dynamic performance of p u m p s is essential for the prediction of transient behavior and instabilities in hydraulic systems; the necessary information i s in the form of a transfer function which relates the instantaneous or fluctuating pressure and mass flow rate at inlet to the same quantities in the discharge from the pump. T h e presence of cavitation within the p u m p can have a major e$ect o n this transjer function since dynamical changes in the volume of cavitation contribute to the diflerence in the instantaneous inlet and discharge mass flow rates. T h e preselzt paper utilizes results from free streamline cascade theory to evaluate the elements in the transfer function for a cavitating inducer and shows that the numerical results are consistent with the characteristics observed in some dynamic tests o n rocket engine turbopumps.The development in lecent times of hiqh speed and high performance pumps for hquids and their inclucion in increasingly complex hydraulic systems has created a need for improvement in our unde:standing of these fl2ws. Demands cf space and economy almost invariably lead to pump (or turbine) designs which operate either with cavitation or sufficiently close to that point, so that significant perturbations lead to cavitation. Sometimes the demands of pump size or mass are exceedingly stringent a s in rocket engines (or propulsion devices of other high performance vehicleb) so that :he pumps operate with extensive cavitation at the inlet. In other situations high temperatmes and velocities plus the need to economize on highly expensive equipment draw the designer ever closer if not beyond the point of cavitation inception; such appears to be the case in the boiler feed a~d coolant systems not only of conventional but also of ~mclear generating plants. Another situation occurs in systems such a,. those associated wit,h geothermal generating plants where flashing of two phases can lead to cavitation-like phenomena in the pumps and turbine.. With regard to turb:nes one should remember that cavitation phenomena similar to those at pump inlet a l~o occur on the outlet side of turbines where there exists an analogous set of conditions; though we will speak here only of pumps the complenlcntary problem in turbines should be borne in mind. Major problems remain in connection with the steady-stat operation of such cavitating pumps and turbines. The prediction of the advent of cavitation, the form it takes, its effect upon performance and the material damage it can cause are stil! subjects of intensive research. But it is rapidly becoming apparent that a whole new tet of t,echnological problems are arising w F c h involve the dynamic rather than the steady state operation of such turbomachines. Formerly it was sufficient for the designer to analyze the steadv-state operation of a hydraulic system. With the increasing complexity dynamic and stability analyses are now desirable for pumping sy~tema and are required for hydropower inftallations [I, 21.1 Transient problems als...
This paper describes an experimental investigation of rotor-stator interaction in a centrifugal pump with several vaned diffusers. Steady and unsteady diffuser vane pressure measurements were made for a two-dimensional test impeller. Unsteady impeller blade pressure measurements were made for a second two-dimensional impeller with blade number and blade geometry identical to the two-dimensional impeller used for the diffuser vane pressure measurements. The experiments were conducted for different flow coefficients and different radial gaps between the impeller blade trailing edge and the diffuser vane leading edge (5 and 8 percent of the impeller discharge radius). The largest pressure fluctuations on the diffuser vanes and the impeller blades were found to be of the same order of magnitude as the total pressure rise across the pump. The largest pressure fluctuations on the diffuser vanes were observed to occur on the suction side of the vane near the vane leading edge, whereas on the impeller blades the largest fluctuations were observed to occur at the blade trailing edge. However, the dependence of the fluctuations on the flow coefficient was found to be different for the diffuser vanes and the impeller blades; on the vane suction side, the fluctuations were largest for the maximum flow coefficient and decreased with decreasing flow coefficient, whereas at the blade trailing edge, the fluctuations were smallest for the maximum flow coefficient and increased with decreasing flow coefficient. Increasing the number of the diffuser vanes resulted in a significant decrease of the impeller blade pressure fluctuations. The resulting lift on the diffuser vanes was computed from the vane pressure measurements; the magnitude of the fluctuating lift was found to be larger than the steady lift.
The instantaneous velocity distribution in trailing vortices generated by lifting hydrofoils has been measured in the Low Turbulence Water Tunnel at the California Institute of Technology. Two different rectangular planform hydrofoils with small aspect ratios were tested. Double-pulsed holography of injected microbubbles, which act much as Lagrangian flow tracers, was used to determine instantaneous axial and tangential velocities. Measurements were made at various free-stream velocities, angles of attack, and downstream distances. The vortex core mean axial velocity is consistently greater than the free-stream velocity near the hydrofoil trailing edge, and decreases with downstream distance. The mean axial velocity is strongly Reynolds-number dependent.Axial flow in the trailing vortex is highly unsteady for all the flow conditions studied; peak-to-peak fluctuations on the centreline as large as the free-stream velocity have been observed. The amplitude of these fluctuations falls rapidly with increasing distance from the centreline. For an angle of attack of 10° the fluctuations consist of both ‘fast’ and ‘slow’ components, whereas for α = 5° only ‘fast’ fluctuations have been observed. Peak decelerations of the centreline fluid occur with amplitude comparable to the maximum centripetal acceleration around the centreline. Certain unusual structures of the vortex core - regions in which the flow direction quickly diverges from the free-stream direction, and then equally quickly recovers - have been labelled ‘vortex kinks.’
Holographic and Coulter Counter detection techniques were jointly used to measure the concentration density distribution of cavitation nuclei in the ocean. Comparison of the two techniques indicates that Coulter Counter analysis measures particulate contents up to an order of magnitude smaller than indicated by the holographic method and may also produce a distorted concentration density distribution. Several possible explanations of the observed discrepancies are proposed and discussed, including fundamental differences between the in situ holographic samples and the collected samples examined with the Coulter Counter, differences between the unknown electrical conductivity of the measured particles in the sea water samples and the non-conductive polystyrene spheres used to calibrate the Coulter Counter, the rupture of aggregate particles in the flow through the Coulter Counter orifice, the effect of electronic noise on the Coulter Counter signal, and the influence of statistical sampling error
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