A. J. Acosta

The laminar flow of dilute polymer solutions around circular cylinders

Experimental results are presented for heat transfer by free and forced convection at low velocities from small heated cylinders in dilute solutions of polyethylene oxide in water. The experiments were conducted for a range of velocities (less than 1. 0 ft/sec) and polymer concentrations, with several cylinder diameters, and for several polymer molecular weights. Experimental results are also presented for the drag of a small cylinder in similar liquids and for a comparable range of velocities. The heat transfer and drag results at low velocities were identical to those for a Newtonian liquid; at high velocities, the measured values departed considerably from Newtonian results. These departures result from the viscoelastic nature of the polymer solutions. Visualization studies of the flow around a cylinder and of a minute laminar jet were conducted to determine the gross magnitude of the viscoelastic effects. Due to the liquids elasticity, a significant enlargement of the flow field was observed for both configurations above a critical Reynolds number. An attempt is made to explain the heat transfer and drag results in light of these observations. Photographic materials on pp. 111 - 149 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered

The Dynamic Transfer Function for a Cavitating Inducer

Knowledge of the dynamic performance of pumps is essential for the prediction of transient behavior and instabilities in hydraulic systems; the necessary information is 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. The presence of cavitation within the pump can have a major effect on this transfer function since dynamical changes in the volume of cavitation contribute to the difference in the instantaneous inlet and discharge mass flow rates. The present 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 on rocket engine turbopumps.

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.

Rotor-Stator Interaction in a Diffuser Pump

The interaction between impeller blades and diffuser vanes in a diffuser pump was investigated. Steady and unsteady pressure measurements were taken on the diffuser vanes, and the shroud wall of a vaned and a vane less diffuser. Steady, unsteady, and ensemble averaged unsteady data, as well as frequency spectra are presented. The measurements were made for different flow coefficients, shaft speeds, and radial gaps between impeller blade trailing and diffuser vane leading edge (1.5% and 4.5% based on impeller discharge radius). The resulting lift on the vane, both steady and unsteady, was computed from the pressure measurements at mid vane height. The magnitude of the fluctuating lift was found to be greater than the steady lift. The pressure fluctuations were larger on the suction side than on the pressure side attaining their maximum value, of the same order of magnitude as the total pressure rise across the pump, near the leading edge. Pressure fluctuations were also measured across the span of the vane. and those near the shroud were significantly smaller than those near the hub. The pressure fluctuations on the shroud wall itself were larger for the vaned diffuser than a vaneless diffuser. Lift, vane pressure, and shroud wall pressure fluctuations decreased strongly with increasing radial gap.

Cavitation in Turbopumps -- Part 1

A free-streamline flow through a cascade of semi-infinite flat plates is taken as a simplified model of the cavitation process in a helical inducer pump. The length and thickness of the resulting cavity is determined as a function of blade geometry and cavitation parameter. Loss coefficients resulting from the cavitation are estimated and representative cavity shapes are calculated to aid in designing the leading edge shape of the blades.

Experimental Observations on the Flow Past a Plano-Convex Hydrofoil

Some new measurements and observations on the noncavitating and cavitating flow past a plano-convex hydrofoil are presented. Under some conditions of partial cavitation, strong, periodic oscillations both in the cavity length and forces exerted on the hydrofoil are observed. The reduced frequency of oscillation depends upon the cavitation number and angle of attack; it also depends somewhat on tunnel speed for the lower angles of attack but becomes substantially independent of speed for the highest angle. The peak-to-peak magnitude of the force oscillation can amount to about 20 percent of the average force.

Note on Observations of Cavitation in Different Fluids

Many observations [1, 2, 3] have shown that the performance of a centrifugal pump with different fluids or with the same fluid at different temperatures is not the same at the same cavitation number when the latter is based upon the vapor pressure of the bulk fluid. Various similarity rules have been put forward in these works to account for the observed effect; namely, that lower net positive suction heads are achievable in most cases compared to those observed in cold tap water. This difference is ascribed to the thermal effect associated with evaporating a certain fraction of the bulk fluid and the attendant decrease of vapor pressure. Scaling rules of the vapor-pressure decrease are made by assuming the process static and that all of the fluid in the inlet of the pump is at the same pressure. The measurements of Salemann [4] show that such a simple concept is inadequate, and he offers further speculations about the nature of the cavitation process as do Acosta and Hollander [5]. The purpose of this note is to describe an experiment intended to show the types of cavitation that occur and, where possible, to measure directly the reduction of vapor pressure or net positive suction head observed in pump experiments.

Theoretical, Quasi-Static Analysis of Cavitation Compliance in Turbopumps

The serious POGO instability experienced by many liquid propellant rockets results from a closed loop interaction between the first longitudianl structural mode of vibration and the dynamics of the propulsion system. One of the most important features in the latter is the cavitation compliance of the turbopumps. This report presents calculations of the blade cavitation compliance obtained from free streamline cascade theory and demonstrates the various influences of angle of attack, blade angle, blade thickness and cavitation number. Discrepancies between calculated and experimentally derived values are discussed.

Linearized Dynamics of Two-Dimensional Bubbly and Cavitating Flows Over Slender Surfaces

The present work investigates the dynamics of two-dimensional, steady bubbly flows over a surface and inside a symmetric channel with sinusoidal profiles. Bubble dynamics effects are included. The equations of motion for the average flow and the bubble radius are linearized and a closed-form solution is obtained. Energy dissipation due to viscous, thermal and liquid compressibility effects in the dynamics of the bubbles is included, while the relative motion of the two phases and viscous effects at the flow boundaries are neglected. The results are then generalized by means of Fourier synthesis to the case of surfaces with slender profiles of arbitrary shape. The flows display various flow regimes (subsonic, supersonic and superresonant) with different properties according to the value of the relevant flow parameters. Examples are discussed in order to show the effects of the inclusion of the various energy dissipation mechanisms on the flows subject to harmonic excitation. Finally the results for a flow over a surface with a Gaussian-shaped bump are presented and the most important limitations of the theory are briefly discussed.

Forces On Centrifugal Pump Impellers

Forces are exerted on a centrifugal pump impeller, due to the asymmetry of the flow caused by the volute of diffuser, and to the motion of the center of the impeller whenever the shaft whirls. Recent work in the measurement of these forces as a function of the whirl speed to shaft speed ratio, and the influence of the volute, is reviewed. These forces may be decomposed into a steady force, a static stiffness matrix, a damping matrix and an inertia matrix. It is shown that for centrifugal pumps of the moderate specific speed typical of boiler feed stages, there is a region of potential shaft vibration excitation from the hydrodynamic forces if the operating speed is well above the first flexural critical speed.