The Experts below are selected from a list of 198 Experts worldwide ranked by ideXlab platform

Peter Dalbert - One of the best experts on this subject based on the ideXlab platform.

  • One-Dimensional Performance Prediction of Subsonic Vaned Diffusers
    Volume 1: Aircraft Engine; Marine; Turbomachinery; Microturbines and Small Turbomachinery, 1999
    Co-Authors: Beat Ribi, Peter Dalbert
    Abstract:

    A simple 1-d-theory to predict the performance of a diffuser using as few empirical factors as possible is presented. The prediction method uses two empirical functions to assess both the pressure recovery and the losses. The functions have been calibrated from experimental data from the company’s standard diffusers. The method is, however, adaptable for any type of subsonic vaned diffusers provided that the empirical functions can be calibrated from measurements. The pressure rise in the diffuser is calculated from the continuity equation taking into account the blockage, while the losses are determined by means of displacement and Momentum Thickness. These values are calculated at design point from an integral boundary layer calculation. To take into account the influence of flow separation at off-design the calculated displacement and Momentum Thickness are increased according to empirical functions. When designing a new impeller the method provides a simple way to evaluate the diffuser resulting in the best combination in terms of efficiency and range. It further provides a simple means of estimating the change to be expected in a known stage performance characteristic due to a modification of the diffuser geometry.

  • One-dimensional performance prediction of subsonic vaned diffusers
    Journal of Turbomachinery, 1999
    Co-Authors: Beat Ribi, Peter Dalbert
    Abstract:

    A simple one-dimensional theory to predict the performance of a diffuser using as few empirical factors as possible is presented. The prediction method uses two empirical functions to assess both the pressure recovery and the losses. The functions have been calibrated from experimental data from the company’s standard diffusers. The method is, however, adaptable for any type of subsonic vaned diffusers, provided that the empirical functions can be calibrated from measurements. The pressure rise in the diffuser is calculated from the continuity equation, taking into account the blockage, while the losses are determined by means of displacement and Momentum Thickness. These values are calculated at design point from an integral boundary layer calculation. To take into account the influence of flow separation at off-design, the calculated displacement and Momentum Thickness are increased according to empirical functions. When designing a new impeller, the method provides a simple way to evaluate the diffuser, resulting in the best combination in terms of efficiency and range. It further provides a simple means of estimating the change to be expected in a known stage performance characteristic due to a modification of the diffuser geometry.[S0889-504X(00)01703-7]

Roi Gurka - One of the best experts on this subject based on the ideXlab platform.

  • A stratified wake of a hydrofoil accelerating from rest
    Experimental Thermal and Fluid Science, 2016
    Co-Authors: Hadar Ben-gida, Alex Liberzon, Roi Gurka
    Abstract:

    Wakes of towed and self-propelled bodies in stratified fluids are significantly different from non-stratified wakes. Long time effects of stratification on the development of the wakes of bluff bodies moving at constant speed are well known. In this experimental study we demonstrate how buoyancy affects the initial growth of vortices developing in the wake of a hydrofoil accelerating from rest. Particle image velocimetry measurements were applied to characterize the wake evolution behind a NACA 0015 hydrofoil accelerating in water and for low Reynolds number and relatively strong and stably stratified fluid (Re=5,000, Fr~O(1)). The analysis of velocity and vorticity fields, following vortex identification and an estimate of the circulation, reveal that the vortices in the stratified fluid case are stretched along the streamwise direction in the near wake. The Momentum Thickness profiles show lower Momentum Thickness values for the stratified late wake compared to the non-stratified wake, implying that the drag on an accelerating hydrofoil in a stratified medium is reduced at the acceleration stage. The findings may improve our ability to predict drag due to maneuvering of micro-air/watervehicles in stratified conditions

Ian P. Castro - One of the best experts on this subject based on the ideXlab platform.

  • Rough-wall boundary layers: mean flow universality
    Journal of Fluid Mechanics, 2007
    Co-Authors: Ian P. Castro
    Abstract:

    Mean flow profiles, skin friction, and integral parameters for boundary layers developing naturally over a wide variety of fully aerodynamically rough surfaces are presented and discussed. The Momentum Thickness Reynolds number Reθ extends to values in excess of 47000 and, unlike previous work, a very wide range of the ratio of roughness element height to boundary-layer depth is covered (0.03 < h/δ > 0.5). Comparisons are made with some classical formulations based on the assumption of a universal two-parameter form for the mean velocity profile, and also with other recent measurements. It is shown that appropriately re-written versions of the former can be used to collapse all the data, irrespective of the nature of the roughness, unless the surface is very rough, meaning that the typical roughness element height exceeds some 50% of the boundary-layer Momentum Thickness, corresponding to about $h/\delta\,{\widetilde{>}}\,0.2$.

D. Keith Hollingsworth - One of the best experts on this subject based on the ideXlab platform.

  • Heat Transfer in Turbulent Boundary Layers Subjected to Free-Stream Turbulence—Part I: Experimental Results
    Journal of Turbomachinery, 2003
    Co-Authors: Michael J. Barrett, D. Keith Hollingsworth
    Abstract:

    Turbulent boundary layers were subjected to grid-generated free-stream turbulence to study the effects of length scale and intensity on heat transfer. Relative to conventional boundary layer Thickness measures, test conditions included very small-scale free-stream turbulence. The boundary layers studied ranged from 400–2700 in Momentum-Thickness Reynolds number and from 450–1900 in enthalpy-Thickness Reynolds number. Free-stream turbulence intensities varied from 0.1–8.0%. Ratios of free-stream length scale to boundary-layer Momentum Thickness ranged from 4.4–32.5. The turbulent-to-viscous length-scale ratios presented are the smallest found in the heat-transfer literature; the ratios spanned from 115–1020. The turbulent-to-thermal ratios (using enthalpy Thickness as the thermal scale) are also the smallest reported; the ratios ranged from 3.2–12.3. Relative to clean-free-stream expectations based on the Momentum- and enthalpy-Thickness Reynolds numbers, the skin friction coefficient increased by up to 16%, and the Stanton number increased by up to 46%.

  • Correlating friction velocity in turbulent boundary layers subjected to freestream turbulence
    AIAA Journal, 2003
    Co-Authors: Michael J. Barrett, D. Keith Hollingsworth
    Abstract:

    A turbulent boundary layer was subjected to grid-generated freestream turbulence to study the effects of turbulent length scale and intensity on skin friction. The study emphasized the importance of the turbulent-to-viscous length scale ratio and focused on correlating skin friction without the use of conventional boundary-layer Reynolds numbers. The experimental data were characterized by small-scale freestream turbulence above boundary layers with Momentum-Thickness Reynolds numbers ranging from 225 to 2700. Freestream turbulence intensities varied from 0.1 to 8.0%. The turbulent-to-viscous length scale ratios spanned from 100 to 1000, and ratios of freestream turbulent length scale to boundary-layer Momentum Thickness ranged from 4 to 32. A new boundary-layer model was used to establish a skin-friction correlation that does not require knowledge of the Momentum-Thickness Reynolds number

William L Keith - One of the best experts on this subject based on the ideXlab platform.

  • Features of the turbulent wall pressure field on a long towed cylinder
    Experiments in Fluids, 2010
    Co-Authors: William L Keith, Kimberly M. Cipolla
    Abstract:

    Turbulent wall pressure fluctuation correlation functions were measured in water on a towed cylindrical model of length 129.8 m and diameter 3.8 cm for steady speeds ranging from 6.2 to 15.5 m/s. The drag on the model was measured with a strut-mounted load cell to provide estimates of the Momentum Thickness and friction velocity that are used for scaling the correlation functions. Very high Momentum Thickness Reynolds numbers Reθ were achieved, and varied from 4.8 × 105 to 1.1 × 106. The ratio of boundary layer Thickness to cylinder radius was approximately 24, which is an order of magnitude greater than previous laboratory investigations. The ratio of Momentum Thickness to viscous length scale is significantly greater than for flat plate cases at comparable Reθ. A similarity scaling is shown to be more effective than outer or inner boundary layer scalings for collapsing the correlation functions. Comparisons with the early streamwise and transverse correlation measurements of Willmarth and Yang are favorable, and show consistent trends of a more rapid loss of correlated energy for cylindrical turbulent boundary layers than for flat plate cases. Convection velocities are also presented and shown to collapse well with separation scaled on outer variables. A simple model that relates the peak of the correlation function to the average coherence levels is shown to be valid for spatial separations less than the approximate Momentum Thickness.

  • High Reynolds number thick axisymmetric turbulent boundary layer measurements
    Experiments in Fluids, 2003
    Co-Authors: Kimberly M. Cipolla, William L Keith
    Abstract:

    Experimental measurements of the wall shear stress and Momentum Thickness for thick axisymmetric turbulent boundary layers are presented. The use of a full-scale towing tank allowed zero pressure gradient turbulent boundary layers to be developed on cylinders with diameters of 0.61, 0.89, and 2.5 mm and lengths ranging from 30 m to 150 m. Moderate to high Reynolds numbers (104

  • Momentum Thickness measurements for thick axisymmetric turbulent boundary layers
    Journal of Fluids Engineering-transactions of The Asme, 2003
    Co-Authors: Kimberly Cipolla, William L Keith
    Abstract:

    Experimental measurements of the mean wall shear stress and boundary layer Momentum Thickness on long, thin cylindrical bodies are presented. To date, the spatial growth of the boundary layer and the related boundary layer parameters have not been measured for cases where δ/a (a =cylinder radius) is much greater than one. Moderate Reynolds numbers (10 4 turbulent boundary layer Momentum Thickness at the downstream end of the cylindrical bodies is determined, using a control volume analysis

  • Momentum Thickness Measurements for Thick Axisymmetric Turbulent Boundary Layers
    Journal of Fluids Engineering, 2003
    Co-Authors: Kimberly M. Cipolla, William L Keith
    Abstract:

    Experimental measurements of the mean wall shear stress and boundary layer Momentum Thickness on long, thin cylindrical bodies are presented. To date, the spatial growth of the boundary layer and the related boundary layer parameters have not been measured for cases where δ/a (a =cylinder radius) is much greater than one. Moderate Reynolds numbers (10 4

  • Momentum Thickness Measurements for Thick Axisymmetric Turbulent Boundary Layers
    Volume 1: Fora Parts A and B, 2002
    Co-Authors: Kimberly M. Cipolla, William L Keith
    Abstract:

    Experimental measurements of the mean wall shear stress and boundary layer Momentum Thickness on long, thin cylindrical bodies are presented. To date, the spatial growth of the boundary layer and the related boundary layer parameters have not been measured for cases where δ/a (a = cylinder radius) is of order one or greater. Moderate Reynolds numbers (104 < Reθ < 105 ) encountered in hydrodynamic applications, are considered. Tow tests of cylinders with diameters of 0.89 mm and 2.5 mm and lengths ranging from approximately 30 meters to 150 meters were performed using the High-Speed Seawater Tow Tank at NASA Langley Research Center. The total drag (axial force) was measured at tow speeds ranging from 2.4 to 17.4 m/sec. These data were used to determine the tangential drag coefficients on each test specimen, which were found to be two to three times greater than the values for the corresponding hypothetical flat-plate cases. Using the drag measurements, the turbulent boundary layer Momentum Thickness at the end of the cylindrical bodies is determined, using a control volume analysis. The results show that for the smallest diameter cylinders, there is no indication of relaminarization, and a fully developed turbulent boundary layer exists. In addition, laser measurements showed no large scale transverse motions (snaking) existed during the tows, and the tow angle was less than 1 degree for all cases, confirming that the cylinders were neutrally buoyant.© 2002 ASME