J. Fluids Eng. 1984;106(1):5-12. doi:10.1115/1.3242405.

The existence of “sampling bias” in individual-realization laser velocimeter measurements is experimentally verified and shown to be independent of sample rate. The experiments were performed in a simple two-stream mixing shear flow with the standard for comparison being laser-velocimeter results obtained under continuous-wave conditions. It is also demonstrated that the errors resulting from sampling bias can be removed by a proper interpretation of the sampling statistics. In addition, data obtained in a shock-induced separated flow and in the near-wake of airfoils are presented, both bias-corrected and uncorrected, to illustrate the effects of sampling bias in the extreme.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;106(1):13-17. doi:10.1115/1.3242391.

The viscosity-dependence of the velocity coefficient for a free liquid jet, issuing from a sharp-edged orifice, is predicted by computing the dissipation of energy in the boundary layer on the back of the orifice plate. The prediction is upheld by the only known direct measurements of velocity coefficients. The resulting coefficients are much closer to unity for large orifices than they are generally assumed to be. The influence of surface tension on small jets is also explained.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):18-20. doi:10.1115/1.3242392.

An expansion in powers of V /c is derived for the wave number of the fundamental sound mode in a flow conduit, where V is the velocity of fluid in the conduit and c is the local sound speed. Both V and c are assumed to be independent of the longitudinal coordinates and of the time, but may have arbitrary profiles. The calculation applies to frequencies well below the cutoff frequency of the conduit, which may have an arbitrary cross-sectional shape. To lowest order, the wave number depends only on the average of the longitudinal component of V and is independent of its profile.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):21-27. doi:10.1115/1.3242393.

A facility has been constructed to study shear-driven, recirculating flows. In this particular study, the circulation cell structure in the lid-driven cavity is studied as a function of the speed of the lid which provides the shearing force to a constant and uniform density fluid. The flow is three-dimensional and exhibits regions where Taylor-type instabilities and Taylor-Görtler-like vortices are present. One main circulation cell and three secondary cells are present for the Reynolds number (based on cavity width and lid speed) range considered, viz., 1000–10000. The flows becomes turbulent at Reynolds numbers between 6000 to 8000. The transverse fluid motions (in the direction perpendicular to the lid motion) are significant. In spite of this, some key results from two-dimensional numerical simulations agree well with the results of the present cavity experiments.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):30-37. doi:10.1115/1.3242398.

Experimental observations of flow structure and pressure loss have been made for froth flow within 180 deg circular pipe bends. Within the bend the distributions of pressure observed reflected the onset of rotation and phase separation effects, whilst secondary flow effects were apparent in the voidage distributions at outlet. Significant components of the overall pressure drop were found both within the bend itself and in the pipe immediately downstream of the bend. Velocity slip between gas and liquid was found to increase observed loss coefficients by approximately 10 percent. The overall loss coefficients were substantially larger than in single phase flow, particularly for bends with larger radius of centerline curvature where they increased by as much as five times the single phase value. The overall pressure loss coefficients were highest for the sharper radius bends, and it was deduced that flow separation and remixing contributed mainly to the increase over single phase loss coefficients.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):38-44. doi:10.1115/1.3242399.

Experimental results are presented here for laminar flow in a rotating, curved duct of rectangular cross section. The duct geometry is that of the spiral duct aerosol centrifuge designed by Stöber and Flachsbart (1969). Primary velocity was measured by laser Doppler anemometry. Secondary flow velocity was characterized by dye injection. The experiment was done in a dynamically similar Plexiglas mock-up of the centrifuge. Water flow in the mock-up simulated air flow in the aerosol centrifuge. The Reynolds number based on hydraulic diameter was 500. The Rossby number was 0.16. The duct aspect ratio was 3.3. Results are compared for flow in a straight stationary duct, the curved duct with no rotation, the curved duct with rotation in the direction of flow and the curved duct with rotation in the direction opposite of flow. There is agreement between the observed flow and the boundary layer theory of Ludwieg (1951).

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):45-51. doi:10.1115/1.3242400.

A theoretical model is developed to determine the flow characteristics for vertical liquid jets of annular cross section. The fully implicit numerical marching integration procedure is based on the parabolic flow theory of Patankar and Spalding. The method is more accurate and general than previous analytical models, since it accounts for all significant forces acting upon the jet and for complex boundary conditions, such as an ambient pressure difference across the jet or a non-uniform velocity profile at the inlet. Comparison of the results with existing experimental data indicates reasonable agreement, and the predictions correspond closely to those of the earlier theories for simplified flow cases.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):52-53. doi:10.1115/1.3242401.

The equations governing the flow of plane and axisymmetic viscous-gravity jets are shown to have solutions which can be derived from one generalized equation. Solutions for the plane jet for various boundary and flow conditions are given here. Similar solutions for the axisymmetric jet are already in the literature.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):54-59. doi:10.1115/1.3242404.

A perfect flow from a two-dimensional nozzle is considered discharging against a normally placed flat plate. Particular attention is given to the effect of the angle of the nozzle, and results for angles of 15, 30, 45, and 60 deg are compared with those for a parallel-sided orifice. This model can be regarded as a two-dimensional representation of a plate valve or flapper valve in which the flow does not reattach to the land of the nozzle. The variations of the back pressure and the contraction coefficient with valve aperture have been plotted, and the shape of the free streamlines and the pressure distribution on the plate have been found. By considering the effect of fluctuations far upstream, the flow in a tapering orifice is shown to be inherently more unstable than that in a parallel-sided orifice. It is also shown that the control of the valve aperture over the back pressure is better in parallel-sided orifices than in angled ones.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):60-65. doi:10.1115/1.3242406.

A re-evaluation of the skin friction coefficient and equivalent sand roughness data reported by Schlichting in 1936 on fourteen rough surfaces is presented. Several assumptions made during the original data reduction are shown to have significant effects on the final results. Additional data and analytical results published since 1936 allow the use of more precise assumptions and enable a re-evaluation of the original data. Corrected results are presented for seventy-nine runs reported by Schlichting on the fourteen surfaces containing spherical, spherical segment and conical roughness elements with various spacings. The original skin friction coefficients are shown to be higher than the corrected values by amounts ranging from 0.5 to 73 percent, while the original equivalent sand roughness values are higher than the corrected ones by 26 to 555 percent. Roughness Reynolds numbers determined using the corrected data indicate that sixteen of the runs on three surfaces were probably in the transitionally rough regime, not the fully rough regime as originally reported.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):66-73. doi:10.1115/1.3242407.

The paper studies in detail the time history of formation, evolution, and instability of the vortex ring, associated with a family of spheres in the Reynolds number range of 30–2000 and with a blockage ratio of 3–30 percent. The flow visualization results are obtained using the classical dye injection procedure. Simultaneous measurements of pressure distribution on the surface of the sphere help establish correlation between the onset of instability of the vortex ring and the surface loading. The results suggest that the influence of the Reynolds number on the surface pressure distribution is primarily confined to the range Rn < 1000. However, for the model with the highest blockage ratio of 30.6 percent, the pressure continues to show Reynolds number dependency for Rn as high as 2300. In general, effect of the Reynolds number is to increase the minimum as well as the wake pressures. On the other hand, the effect of an increase in the blockage ratio is just the opposite. The wall confinement tends to increase the drag coefficient, however, the classical dependence of skin friction on the Reynolds number Cd,f ∝ R−1/2 , is maintained. The paper also presents useful information concerning location of the separating shear layers as affected by the Reynolds number and blockage. For comparison, available analytical and experimental results by other investigators are also included. Results show that for a given blockage, separation points may move upstream by as much as 20 deg over a Reynolds number range of 100–600. In general, for a given Reynolds number, the wall confinement tends to move the separation position downstream.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):74-78. doi:10.1115/1.3242408.

This paper reports a series of experiments concerning the buckling of a slender fluid layer in a state of longitudinal compression. The experiments consist of floating a layer of highly viscous oil on a pool of water and, manually, compressing the layer from the side. Photographs of the buckled layer show conclusively that the buckling wavelength is largely insensitive to either the rate of compression or the viscosity of the fluid layer. The observations suggest that the buckling wavelength is actually a characteristic length scale (a property) of the fluid layer, in contrast with the buckling theory of purely viscous layers (Buckmaster, Nachman, and Ting, [7]) where the buckling wavelength remains to be determined randomly by initial disturbances.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):79-84. doi:10.1115/1.3242409.

A new viscous-inviscid interaction technique has been developed for computing separated flow in planar diffusers. The method couples a set of integral equations for the boundary layers to a fully elliptic potential core flow. Rapid convergence of the method is demonstrated for planar diffusers with large regions of transitory stall. For these cases, convergence of the new method is an order of magnitude faster than that obtained using the interaction schemes of Carter and Le Balleur. Good agreement between the prediction method and experimental data is obtained for diffusers that are operating near peak pressure recovery. More importantly, the onset of asymmetric detachment is successfully predicted for these cases.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):85-91. doi:10.1115/1.3242410.

A method for the two-dimensional analysis of the separated flow about Savonius rotors is presented. Calculations are performed by combining the singularity method and the discrete vortex method. The method is applied to the simulation of flows about a stationary rotor and a rotating rotor. Moreover, torque and power coefficients are computed and compared with the experimental results presented by Sheldahl et al. Theoretical and experimental results agree well qualitatively.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):92-98. doi:10.1115/1.3242412.

A new concept for detecting cavitation inception has been studied experimentally. In this exploratory study, cavitation is generated by varying the flow velocity and pressure around a circular cylinder. Cavitation inception has been detected by sensing the natural charges and electrification generated during cavitation. The agreement between visual determination and detection using electrostatic probes was quite good. The background and possible mechanisms are reviewed and discussed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):99-104. doi:10.1115/1.3242413.

Measurements are presented of the variation of the acoustic cavitation threshold of water with concentration of the polymer additives polyethylene oxide and guar gum. It was found that small amounts of these additives could significantly increase the cavitation threshold. A theoretical model, based upon nucleation of a gas bubble from a Harvey-type crevice in a mote or solid particle, is developed that gives good agreement with the measurements. The applicability of this approach to an explanation of cavitation index reduction in flow-generated or confined jet cavitation, when polymer additives are introduced, is discussed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 1984;106(1):105-109. doi:10.1115/1.3242383.

The mechanisms responsible for flash-boiling injection were investigated. Using an electromagnetic injector developed for this study, propane, methanol and Indolene were heated and injected into a constant-volume vessel. Two regimes of flash-boiling injection were identified. In the first regime, flash-boiling occurs within the injector nozzle without an increase in spray-cone angle. In the second regime, the nozzle exit pressure is sufficiently low that the two-phase compressible mixture created by flash-boiling within the injector nozzle is underexpanded at the nozzle exit and expands externally to increase the spray-cone angle.

Commentary by Dr. Valentin Fuster


Commentary by Dr. Valentin Fuster


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