Experimental studies in magneto-fluid dynamics: pressure distribution measurements around a sphere

Measurements of the pressure distribution around a sphere placed in aligned magnetic and velocity fields show that an increase in drag is mainly due to a decrease in the pressure on the base of the body. When magnetic forces are large compared to inertia forces, this decrease is due to a loss in total pressure along streamlines just outside the surface boundary layer and an acceleration of the flow to a velocity much larger than the reference velocity. Separation of a viscous boundary layer takes place behind the equator and still, to a large extent, controls the magnitude of the base pressure and the drag experienced by the sphere. A model consistent with these findings is presented.

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Pressure distribution on a roof in presence of the Moving Surface Boundary-layer Control

Journal of Visualization ◽

10.1007/bf03181406 ◽

1999 ◽

Vol 1 (3) ◽

pp. 255-260 ◽

Cited By ~ 2

Author(s):

V. J. Modi ◽

T. Yokomizo

Keyword(s):

Boundary Layer ◽

Pressure Distribution ◽

Boundary Layer Control ◽

Surface Boundary ◽

Moving Surface ◽

Surface Boundary Layer ◽

Layer Control

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On solitary waves forced by underwater moving objects

Journal of Fluid Mechanics ◽

10.1017/s0022112099004826 ◽

1999 ◽

Vol 389 ◽

pp. 119-135 ◽

Cited By ~ 19

Author(s):

DAOHUA ZHANG ◽

ALLEN T. CHWANG

Keyword(s):

Boundary Layer ◽

Solitary Waves ◽

Body Shape ◽

Moving Objects ◽

The Body ◽

Wave Radiation ◽

Two Dimensional ◽

Viscous Effect ◽

Viscous Boundary Layer ◽

Viscous Boundary

The phenomenon of a succession of upstream-advancing solitary waves generated by underwater disturbances moving steadily with a transcritical velocity in two- dimensional shallow water channels is investigated. The two-dimensional Navier–Stokes (NS) equations with the complete set of viscous boundary conditions are solved numerically by the finite-difference method to simulate the phenomenon. The overall features of the phenomenon illustrated by the present numerical results are unanimous with observations in nature as well as in laboratories. The relations between amplitude and celerity, and between amplitude and period of generation of solitary waves can be accurately simulated by the present numerical method, and are in good agreement with predictions of theoretical formulae. The dependence of solitary wave radiation on the blockage and on the body shape is investigated. It furnishes collateral evidence of the experimental findings that the blockage plays a key role in the generation of solitary waves. The amplitude increases while the period of generation decreases as the blockage coefficient increases. It is found that in a viscous flow the shape of an underwater object has a significant effect on the generation of solitary waves owing to the viscous effect in the boundary layer. If a change in body shape results in increasing the region of the viscous boundary layer, it enhances the viscous effect and so does the disturbance forcing; therefore the amplitudes of solitary waves increase. In addition, detailed information of the flow, such as the pressure distribution, velocity and vorticity fields, are given by the present NS solutions.

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Free-Surface Boundary-Layer Approach to Study the Bow Vortices Generation Ahead of a Semisubmerged Horizontal Circular Cylinder

Journal of Ship Research ◽

10.5957/jsr.1995.39.4.284 ◽

1995 ◽

Vol 39 (04) ◽

pp. 284-296

Author(s):

L. R. Raheja

Keyword(s):

Boundary Layer ◽

Free Surface ◽

Circular Cylinder ◽

The Body ◽

Instability Criterion ◽

Surface Boundary ◽

Surface Shear ◽

Surface Boundary Layer ◽

Surface Separation ◽

Horizontal Circular Cylinder

In the light of experimental observation of a free-surface shear layer, the flow ahead of a semisubmerged horizontal circular cylinder is modeled as a free-surface boundary layer of concentrated vorticity joining the potential flow below it with the aim to study the generation of bow vortices theoretically. The boundary-layer equations are linearized subject to a suitable assumption and are integrated using basically the Kármán Pohlhausen method. It is found that the free surface moves slower than the layer beneath it, but there is more likelihood of bow vortices being generated by instability of velocity profile rather than by separation and backflow. This is confirmed by the nonmonotonicity in the vorticity profile and the fulfillment of the Görtler instability criterion for flow along curved boundaries, near the body upstream. The position of the point of onset of instability, as stipulated from the above observations, compares well with the position of the free-surface separation point as observed in the experiments.

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The axisymmetric Prandtl-Batchelor eddy behind a circular disc in a uniform stream

Journal of Fluid Mechanics ◽

10.1017/s0022112098003097 ◽

1998 ◽

Vol 377 ◽

pp. 253-266

Author(s):

J. F. HARPER

Keyword(s):

Boundary Layer ◽

Reynolds Number ◽

Bluff Body ◽

The Body ◽

Circular Disc ◽

Axially Symmetric ◽

Viscous Boundary Layer ◽

Disc Diameter ◽

Viscous Boundary ◽

Uniform Stream

Analytical support is given to Fornberg's numerical evidence that the steady axially symmetric flow of a uniform stream past a bluff body has a wake eddy which tends towards a large Hill's spherical vortex as the Reynolds number tends to infinity. The viscous boundary layer around the eddy resembles that around a liquid drop rising in a liquid, especially if the body is a circular disc, so that the boundary layer on it does not separate. This makes it possible to show that if the first-order perturbation of the eddy shape from a sphere is small then the eddy diameter is of order R1/5 times the disc diameter, where R is the Reynolds number based on the disc diameter. Previous authors had suggested R1/3 and lnR, but they appear to have made unjustified assumptions.

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Prediction of Atmospheric Turbulence Characteristics for Surface Boundary Layer using Empirical Spectral Methods

Revista Brasileira de Meteorologia ◽

10.1590/0102-77863540083 ◽

2020 ◽

Author(s):

Yagya Dutta Dwivedi ◽

Vasishta Bhargava Nukala ◽

Satya Prasad Maddula ◽

Kiran Nair

Keyword(s):

Boundary Layer ◽

Time Series ◽

Surface Roughness ◽

Wind Speed ◽

Atmospheric Turbulence ◽

Surface Boundary ◽

Low Frequencies ◽

Surface Boundary Layer ◽

Power Spectral ◽

Mean Wind Speed

Abstract Atmospheric turbulence is an unsteady phenomenon found in nature and plays significance role in predicting natural events and life prediction of structures. In this work, turbulence in surface boundary layer has been studied through empirical methods. Computer simulation of Von Karman, Kaimal methods were evaluated for different surface roughness and for low (1%), medium (10%) and high (50%) turbulence intensities. Instantaneous values of one minute time series for longitudinal turbulent wind at mean wind speed of 12 m/s using both spectra showed strong correlation in validation trends. Influence of integral length scales on turbulence kinetic energy production at different heights is illustrated. Time series for mean wind speed of 12 m/s with surface roughness value of 0.05 m have shown that variance for longitudinal, lateral and vertical velocity components were different and found to be anisotropic. Wind speed power spectral density from Davenport and Simiu profiles have also been calculated at surface roughness of 0.05 m and compared with k−1 and k−3 slopes for Kolmogorov k−5/3 law in inertial sub-range and k−7 in viscous dissipation range. At high frequencies, logarithmic slope of Kolmogorov −5/3rd law agreed well with Davenport, Harris, Simiu and Solari spectra than at low frequencies.

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