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RESEARCH PAPERS

J. Fluids Eng. 1989;111(3):238-242. doi:10.1115/1.3243636.

Instability of axisymmetric jet flows of a fluid having a radius-dependent density is investigated. The necessary condition for the existence of unstable waves depends not only on the velocity profile but also on the density gradient as well. Large density gradients, positive or negative, have stabilizing effects. The semicircle theorem for amplified waves is valid in this case. It is shown by considering the top-hat type velocity profile that the velocity-dependent semicircle bound is the best possible.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):243-247. doi:10.1115/1.3243637.

The paper presents a closed-form analytical solution for the source strength distribution along the circumference of a two-dimensional circular cylinder that is required for producing an arbitrary distribution of normal velocity. Being suitable to be used with flows having arbitrary vorticity distribution, the present formulation can be considered as an alternative and extensive form of the circle theorems. Using the conformal transformation technique, the formulation also serves as a closed-form solution of Laplace’s equation in any two-dimensional flow domain that is reducible to the outer or inner region of a circular cylinder having arbitrary prescribed normal velocity over its boundary.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):248-255. doi:10.1115/1.3243638.

A Reynolds stress transport equation model and the k –ε turbulence model have been applied to the calculation of a confined, strongly swirling flow. A comparison of the results with measurement shows clearly the superiority of the transport equation model. It reproduces the major features of the flow including the strong stabilizing effects of the swirl on the shear stresses and the calculated axial and circumferential components of mean velocity are in reasonable agreement with measured profiles. The corresponding normal stresses are, however, overpredicted but previously suggested modifications to the ε-equation to account for rotation did not bring any improvement. The k –ε model does not contain any mechanism to describe the stabilizing effects of swirling motion and as a consequence it performs poorly; large discrepancies exist between the measured and calculated mean velocity field.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):256-262. doi:10.1115/1.3243639.

This paper reports the results of theoretical and wind-tunnel studies of the effect of pitch location on dynamic stall for an airfoil pitching at constant rate. A modified momentum-integral method was used to predict the effect of pitch location and rate on the delay in quarter-chord separation. The wind-tunnel study involved the collection of time-varying pressure readings from 16 locations on an NACA 0015 airfoil that were subsequently used to determine lift, pressure-drag, and moment coefficients as functions of angle of attack for 140 test cases, covering 28 dynamic airspeed/pitch-rate/pitch-location combinations. Dynamic-stall effects of change (from steady flow) in the angle of attack at which separation occurs at the quarter chord (for comparison with the momentum-integral results), and change in the angle of attack at which stall occurs were extracted from these data and found to collapse best onto a non-dimensional pitch rate given by the chord times the pitch rate divided by two times the free-stream velocity. An adjusted non-dimensional rate formed by replacing one half the chord with the fraction of the chord corresponding to the pitch location was also examined and found not to be the proper non-dimensional variable for collapsing the data. The quarter-chord separation data compared favorably with the theoretical predictions.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):263-270. doi:10.1115/1.3243640.

An experiment was conducted to investigate the effects of sweep angle on hydrofoil loadings in cavitation. Tests were carried out in a water tunnel on constant-chord full-span hydrofoils at four angles of attack for sweep angle 0, 15, 30, and 45 deg, respectively. The results obtained were that the strong, periodic oscillations which appeared under some conditions of partial cavitation decreased their amplitudes with an increase in sweep angle. Moreover, the lift-drag ratio showed the higher value for the highly swept hydrofoil and remained higher down to some lower cavitation numbers than those for the slightly swept model.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):271-277. doi:10.1115/1.3243641.

Experiments are described in which attempts have been made to investigate the fundamental mechanisms of the synergistic effect of cavitation erosion and corrosion. The design of an all-plastic cavitation tunnel is described which allows specimens to be held under potentiostatic control in a flowing seawater system. Experiments were conducted in a 10 × 20 mm working section with a 60° symmetrical wedge cavitation source at an upstream flow velocity of 14.7 m/s. An extensive test programme has been completed comprising three separate tests: Pure Erosion, Pure Corrosion and Combined Erosion and Corrosion, each conducted at two different cavitation intensities. These tests have concentrated on investigating the erosion/corrosion performance of copper in seawater. Preliminary results using cupro-nickel are also reported. It was found that a clearer indication of the synergistic effect was obtained from depth of penetration measurements than from mass loss measurements. The synergistic effect was found to be most marked when cavitation erosion occurs in the presence of mild corrosion. For the worse case studied, 50 percent of the depth of penetration was caused by synergistic mechanisms.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):278-289. doi:10.1115/1.3243642.

An experimental investigation of the noise generated by cavitation in turbulent shear flows produced by confined sharp-edged orifice plates is reported. The acoustic source strength of cavitation was determined by means of reciprocity type measurements. Two experimental checks of the reciprocity method were performed. Proposed scaling relations, derived from dimensional analysis, were empirically adjusted and used to predict prototype acoustic performance based on the results of model tests. The dependence of the acoustic source strength on flow velocity and cavitation number was determined experimentally and compared with similar results reported in the literature.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):290-299. doi:10.1115/1.3243643.

Comparisons are made between a nonlinear method for predicting unsteady sheet cavitation and available experimental data for a pitching foil for the purposes of verifying the calculations and to further analyze the flow. A dynamical approach is employed in which the form of the instantaneous cavity surface is modeled as a semiellipse. The cavity length (major axis), thickness (semiminor axis), and position are determined such that the nonlinear cavity-surface boundary conditions are satisfied approximately. The pressure on the instantaneous cavity surface is prescribed using an unsteady thick-foil potential-flow method based on Green’s second identity. The computational method yields best results in predicting the cavity dynamics, but underpredicts the cavity length. For fixed cavitation number, mean foil angle, and pitch amplitude, the cavity dynamics, such as maximum cavity size and cavity surface behavior, are shown to depend on the ratio of the cavity natural frequency for the foil fixed at the maximum pitch amplitude to the foil reduced frequency. For a certain value of this ratio, the cavitation response is shown to be a minimum. The experimental results confirm the computational trends up to the point that experimental data were obtained.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):300-305. doi:10.1115/1.3243644.

The drag of bodies of revolution at zero degree angle of attack in supercavitating flow has been calculated. The potential flow about the body and cavity is calculated using an axial distribution of source/sink elements and is coupled with laminar and turbulent boundary-layer solutions for the body. Excellent agreement for drag coefficient is demonstrated between theory and a water-tunnel experiment, also between theory and a high-speed water-entry experiment. Results show skin-friction drag is the dominant drag component for high-speed water entry or high-speed underwater travel.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):306-316. doi:10.1115/1.3243645.

A series of experiments has been conducted on a two-dimensional NACA 66 (MOD) foil to examine the effects of viscosity and nuclei on cavitation inception. In this paper the main discussions center on two foil angles having different types of pressure loadings to represent a propeller blade section operating at design and off-design conditions. At one degree design angle of attack the foil experiences a rooftop-type gradually varying pressure distribution. At three degrees off-design angle of attack the foil experiences a sharp suction pressure peak near the leading edge. Cebeci’s viscid/inviscid interactive code is used to compute the viscous scale effects on the development of the boundary layer, lift, drag and pressure distribution on the foil. Chahine’s multibubble interaction code is used to compute the effect of nuclei, test speeds, foil size and foil surface on traveling bubble cavitation. Both computer codes are found to agree satisfactorily with the experimental measurements reported here. Two assumptions commonly used to predict full scale surface cavitation from model tests are examined experimentally and theoretically. The first assumption states that cavitation inception occurs when the static pressure reaches the vapor pressure. On the contrary, the experiments showed that the water flowing over the foil surface sustained significant amounts of tension during inception of midchord bubble cavitation as well as leading edge sheet cavitation. The second assumption states that there is no scale effect on the values of negative minimum pressure coefficient. In the case of a rooftop-type pressure loading, the second assumption is supported by the pressure numerical calculations. However, in the case of a pressure loading with a strong suction peak near the leading edge the value of negative minimum pressure coefficient is as much as 12 to 15 percent lower on a model than at full scale.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):317-323. doi:10.1115/1.3243646.

Using probes responding to changes of slurry electrical resistance with concentration, time spectra of longitudinal concentration fluctuations in turbulent slurry flows have been measured. The sensors, with an effective domain approximately 1 mm in diameter, showed the spectra to be relatively insensitive to location within the pipe cross section. High frequency spectra were found to be relatively insensitive to slurry concentration and particle diameter. Low frequency spectra showed fluctuation amplitudes which increased with solids concentration.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):324-330. doi:10.1115/1.3243647.

A new portable slurry wear test apparatus developed by the Bureau of Mines, U.S. Department of the Interior, makes it possible to gather materials wear and corrosion data at a mineral processing site. The portable wear cell is identical in design to a laboratory cell reported previously. It allows simultaneous evaluation of 16 specimens in a continuous flow of fresh slurry. Data obtained from selected metals and polymers showed high-chromium white cast irons to perform particularly well in tests with an aqueous lead-zinc sulfide ore slurry. However, ultra-high-molecular-weight polyethylene that exhibited superior wear resistance in comparable laboratory tests with an aqueous slurry of silica sand did not perform as well in field tests. Such results show how misleading it can be to use laboratory data to predict relative rates of wear in industrial slurries, even under nominally identical flow conditions. Field testing is therefore needed. In situ electrochemical corrosion measurements on a low-alloy steel showed that the field and laboratory slurries were similarly corrosive.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):331-336. doi:10.1115/1.3243648.

An experimental description of the flow structure of non-Newtonian slurries in the laminar, transitional, and full turbulent pipe flow regimes is the primary objective of this research. Experiments were conducted in a large-scale pipe slurry flow facility with an inside pipe diameter of 51 mm. The transparent slurry formulated for these experiments from silica, mineral oil, and Stoddard solvent exhibited a yield-power-law behavior from concentric-cylinder viscometer measurements. The velocity profile for laminar flow from laser Doppler velocimeter (LDV) measurements had a central plug flow region, and it was in agreement with theory. The range of the transition region was narrower than that for a Newtonian fluid. The mean velocity profile for turbulent flow was close to a 1/7 power-law velocity profile. The rms longitudinal velocity profile was also similar to a classical turbulent pipe flow experiment for a Newtonian fluid; however, the rms tangential velocity profile was significantly different.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):337-341. doi:10.1115/1.3243649.

Pulsed Neutron Activation (PNA) is a means of noninvasive flow velocity measurement based on tagging the flowing medium with a short-lived radioactivity. Previous work with salt or dye-tagging showed poor accuracy in turbulent and failed in laminar flow when conventional data processing was used. However, use of a data acquisition and processing scheme that is based on tag dispersion modelling can produce absolute values over a wide range of flow speeds and regimes with high accuracy. For non-Newtonian/laminar flow, rheological information can also be obtained. The inherently non-intrusive nature of PNA tagging makes this scheme available for slurry measurements. The performance of PNA in slurry flow at up to 60 percent solid content was compared to full-flow diversion and weighing. Errors ranged from less than 0.2 percent at high Reynolds’ numbers to about 2 percent for paste flow. Rheological parameters (yield shear stress or flow behavior index) could be determined with an accuracy that compared to that of a spindle viscometer with grab-samples. The PNA scheme thus offers a unique means of studying slurry flow in a dedicated laboratory facility, or of providing calibration for other flowmeters in an industrial plant through temporary installation by a team of expert consultants.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):342-347. doi:10.1115/1.3243650.

The drop size distributions produced by two geometrically similar fan spray nozzles were measured over an injection pressure range from 1.38 to 10.34 MPa (200 to 1500 psi). The fluids employed were water, water/glycerine mixtures, silicone oils, paint, and paint solvent. These liquids were selected to provide wide ranges in liquid viscosity and surface tension. A nonintrusive Malvern 2600 particle sizer, based on the forward scattering of a 5 mW He-Ne laser beam, was used to yield line-of-sight and cross-sectional drop size distributions. The atomizer was traversed across the laser beam and drop size profiles as a function of distance from the centerline were developed. Analysis of the experimental data showed that the effects of injection pressure and liquid properties on atomization quality are described with good accuracy by the following dimensionally-correct equation.

SMDdh = 2.83σμL2ρAdh3ΔPL20.25
     + 0.26σρLρAdhΔPL0.25

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):348-352. doi:10.1115/1.3243651.

The conditions governing liquid droplet breakup are particularly important to the process of fuel atomization for spark-ignition engines. A proper mathematical description of the problem requires knowledge of the pressure distributions about the droplet surface. This work presents the results of measurements of pressure distributions about the surfaces of certain bluff, axi-symmetric bodies in airflows at ReD = 3 × 104 and ReD = 105 . The results of measurements from a sphere, disk, and two ellipsoids are used to develop a general method for estimating surface pressures. These estimates are compared with the results of pressure measurements about the surfaces of a concave and a nonellipsoidal convex body and with results obtained from a numerical study by other authors.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Fluids Eng. 1989;111(3):353-356. doi:10.1115/1.3243652.

A photoelastic technique using two laser beams and high acquisition rate (20 MHz) has been used to investigate the dynamics of a single bubble in a fluid at rest. The bubbles are spark-generated in the vicinity of a photoelastic wall. The influence of liquid pressure, proximity of the wall and the bubble life-time on the collapse intensity is examined. The results obtained are compared qualitatively with the measurements of pressure obtained by Shima and Tomita for the liquid at atmospheric pressure.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1989;111(3):356-358. doi:10.1115/1.3243653.

The aim of this brief paper is to present a variant of the classical Betz’ theory concerning the flow through a wind turbine. The new model includes losses and gives more realistic maximum values of the characteristic nondimensional coefficients of the machine.

Commentary by Dr. Valentin Fuster

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