scholarly journals INITIAL MOTION IN COMBINED WAVE AND CURRENT FLOWS

1988 ◽  
Vol 1 (21) ◽  
pp. 85 ◽  
Author(s):  
J.S. Lee-Young ◽  
J.F.A. Sleath
Keyword(s):  
Steady Flow ◽  
Oscillatory Flow ◽  
Critical Value ◽  
Shear Stresses ◽  
Bed Shear Stress ◽  
Current Flows ◽  
Vector Sum ◽  
Shields Curve ◽  
Initial Motion ◽  
Made In

Measurements are presented of the conditions for the initial motion of sediment under combined steady and oscillatory flow. The measurements were made in a steady flow flume with an oscillating tray set into its bed. The direction of oscillation of the tray was at right angles to the axis of the steady flow flume. Four different grades of sand were tested. It is found that the critical condition for the initiation of motion is reasonably represented by a critical value of the vector sum of the component shear stresses assuming no nonlinear interaction between the steady and oscillatory flows. The resultant bed shear stress was also calculated with the aid of several combined wave-current models. The results of these various approaches are compared with Shields curve.

Gravity Anchors Astride Subsea Pipelines Subject to Oscillatory and Combined Steady and Oscillatory Flows

2012 ◽  
Author(s):  
Xu Zhao ◽  
Liang Cheng ◽  
Ming Zhao ◽  
Hongwei An ◽  
Wei He
Keyword(s):  
Shear Stress ◽  
Oscillatory Flow ◽  
Stokes Equations ◽  
Numerical Study ◽  
Shear Stresses ◽  
Bed Shear Stress ◽  
Oscillatory Flows ◽  
Steady Current ◽  
Stress Amplification ◽  
Subsea Pipelines

This writing presents results of simulating oscillatory and combined steady and oscillatory flows past gravity anchors astride subsea pipelines. It can be considered a companion to a previous numerical study on steady currents past gravity anchors. The gravity anchor system comprises large arch-shaped concrete blocks positioned at intervals astride offshore pipelines, and it is engineered to provide innovative and cost-effective secondary stabilisation for high-capacity gas-transporting pipelines serving in severe metocean conditions, e.g. cyclone-prone offshore areas. A free-settling marine object bottom-seated on the seabed, however, the gravity anchor may subside into scour pits around its base due to locally disturbed flow regimes, imposing integrity risks on the pipe. Also, the effect of gravity anchors on hydrodynamic loading on nearby pipeline lengths is of interest. The present study employed a Petrov-Galerkin finite element method to solve the three-dimensional Navier-Stokes equations in direct numerical simulation. Firstly sinusoidal flow oscillating perpendicularly to the pipe beneath gravity anchors on an immobile bed was simulated at a Keulegan-Carpenter number of 10 and a pipe Reynolds number of 1000. Then, a steady current co-directionally superimposed on the aforementioned oscillatory flow was modelled at a ratio of steady current velocity to oscillatory flow velocity amplitude of 1. With sediment transport capacity related to bed shear stresses, the time-averaged bed shear stress amplification around gravity anchors in oscillatory flow was revealed first, and found to be consistent with laboratory observations of scour patterns. The effect of superimposing steady flow onto oscillatory flow on bed shear stress amplification was then explored. Lastly, hydrodynamic forces on pipelines in the vicinity of gravity anchors were gauged. The present work intends to shed light on the initial seabed responses with regard to the scour process around gravity anchors immersed in the oceanic wave boundary layer, as well as the effect of gravity anchors on hydrodynamic loadings on pipelines.

Efficiency prediction for storage chambers using computational fluid dynamics

1996 ◽  
Vol 33 (9) ◽  
pp. 163-170 ◽  
Author(s):  
Virginia R. Stovin ◽  
Adrian J. Saul
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Shear Stress ◽  
Particle Tracking ◽  
Critical Value ◽  
Complex Geometries ◽  
Bed Shear Stress ◽  
Breadth Ratio ◽  
Computational Fluid Dynamics Cfd ◽  
Simulated Flow

Research was undertaken in order to identify possible methodologies for the prediction of sedimentation in storage chambers based on computational fluid dynamics (CFD). The Fluent CFD software was used to establish a numerical model of the flow field, on which further analysis was undertaken. Sedimentation was estimated from the simulated flow fields by two different methods. The first approach used the simulation to predict the bed shear stress distribution, with deposition being assumed for areas where the bed shear stress fell below a critical value (τcd). The value of τcd had previously been determined in the laboratory. Efficiency was then calculated as a function of the proportion of the chamber bed for which deposition had been predicted. The second method used the particle tracking facility in Fluent and efficiency was calculated from the proportion of particles that remained within the chamber. The results from the two techniques for efficiency are compared to data collected in a laboratory chamber. Three further simulations were then undertaken in order to investigate the influence of length to breadth ratio on chamber performance. The methodology presented here could be applied to complex geometries and full scale installations.

Oscillatory flow past a circular cylinder in a rotating frame

1997 ◽  
Vol 345 ◽  
pp. 101-131
Author(s):  
M. D. KUNKA ◽  
M. R. FOSTER
Keyword(s):  
Circular Cylinder ◽  
Steady Flow ◽  
Stagnation Point ◽  
Oscillatory Flow ◽  
Numerical Solutions ◽  
Initial Conditions ◽  
Temporal Integration ◽  
Oncoming Flow ◽  
Flow Past ◽  
Rear Stagnation Point

Because of the importance of oscillatory components in the oncoming flow at certain oceanic topographic features, we investigate the oscillatory flow past a circular cylinder in an homogeneous rotating fluid. When the oncoming flow is non-reversing, and for relatively low-frequency oscillations, the modifications to the equivalent steady flow arise principally in the ‘quarter layer’ on the surface of the cylinder. An incipient-separation criterion is found as a limitation on the magnitude of the Rossby number, as in the steady-flow case. We present exact solutions for a number of asymptotic cases, at both large frequency and small nonlinearity. We also report numerical solutions of the nonlinear quarter-layer equation for a range of parameters, obtained by a temporal integration. Near the rear stagnation point of the cylinder, we find a generalized velocity ‘plateau’ similar to that of the steady-flow problem, in which all harmonics of the free-stream oscillation may be present. Further, we determine that, for certain initial conditions, the boundary-layer flow develops a finite-time singularity in the neighbourhood of the rear stagnation point.

Resistance and reactance of the excised human larynx, trachea, and main bronchi

1987 ◽  
Vol 63 (5) ◽  
pp. 1788-1795 ◽  
Author(s):  
T. X. Jiang ◽  
M. Cauberghs ◽  
K. P. Van de Woestijne
Keyword(s):  
Steady Flow ◽  
Airway Resistance ◽  
Oscillatory Flow ◽  
Forced Oscillation ◽  
Upper Airway ◽  
Forced Oscillation Technique ◽  
Vocal Cords ◽  
Respiratory Flow ◽  
Human Larynx ◽  
Before And After

We investigated the impedance of excised preparations of the human larynx before and after resection of the vocal cords and of the trachea whether or not in connection with the main bronchi for steady (75–700 ml.s-1) and oscillatory flows (4–64 Hz). To simulate the influence of respiratory flow on oscillatory resistance (Rosc), oscillatory and steady flow were superimposed. This resulted in a marked increase of Rosc, dependent on the value of steady flow, a change of the frequency dependence of Rosc, and a decrease of the reactance. The latter effects were particularly pronounced in the preparations of the larynx, especially with a narrow glottis opening. The influence of steady flow on oscillatory resistances is probably the expression of interactions of steady and oscillatory flow regimes in the larynx. Similar but less pronounced interactions are also met in the trachea. These effects lead to a systematic overestimation of upper airway resistance when measured during spontaneous breathing by means of a forced oscillation technique.

Note on Losses in Cable Sheaths

1928 ◽  
Vol 24 (1) ◽  
pp. 65-73 ◽  
Author(s):  
F. W. Carter
Keyword(s):  
Cross Section ◽  
Eddy Current ◽  
Distribution Systems ◽  
Power Cables ◽  
Circulating Current ◽  
Outer Sheath ◽  
The Cross ◽  
Current Flows ◽  
Made In

In a recent communication to the Society, the author referred to cable-sheath losses, and gave formulae for computing them in certain cases. These appertained to power cables in which were comprised a group of conductors, arranged symmetrically and encased in a single conducting sheath. In some distribution systems, however, the conductors for the several phases are encased in separate lead sheaths, which are either laid in proximity as separate cables, or grouped and comprehended in an outer sheath. The analysis previously given does not include such cases directly. Moreover, it is common practice either to lay the elementary cables with sheaths in contact, or to bond the sheaths together at the ends of suitable sections, in order to prevent differences of potential between them; and, when this is done, a circulating current flows in the circuit of the sheaths and bonds, sufficient to maintain equality of potential between the several sheaths. This current, to which reference was made in the former paper, is additional to the eddy current discussed therein, the integral of which over the cross section of the sheath is zero. It is for convenience here referred to as the “circulating current,” to distinguish it from the “eddy current,” although there is no such distinction between them as the names imply.

Convection from an isolated heated horizontal cylinder rotating about its axis

1953 ◽  
Vol 217 (1131) ◽  
pp. 555-562 ◽  
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
Critical Speed ◽  
Rotating Cylinder ◽  
Critical Value ◽  
Horizontal Cylinder ◽  
Constant Heat ◽  
Rotating Surface ◽  
Experimental Values ◽  
Made In

The heat transfer by convection from an isolated heated horizontal cylinder rotating about its axis in air was measured for varying rotational speeds, cylinder temperatures and diameters; some measurements were also made in air at 4 atm pressure. The results show that the heat transfer is nearly constant at the free convection value for rotational speeds from zero to a critical value, and then increases in proportion to the 2/3 power of the rotational speed. The explanation of the constant heat transfer below the critical speed is shown to be that the heat transfer on the ascending side of the cylinder is higher, and that on the descending side lower, than the free convection at zero speed. The critical speed occurs when the circumferential speed of the rotating surface becomes approximately equal to the upward free convection velocity at the side of the heated stationary cylinder. Theory predicts this to occur at approximately R √( P/G ) = 0.9, which agrees well with the experimental values. Photographs of the flow also confirm these conclusions. Above the critical speed the irregular turbulent motion near the cylinder is shown to be similar to that which occurs in free convection above a heated horizontal surface facing upwards. Starting with known values for the free convection in the latter case, the heat transfer from the rotating cylinder above the critical speed was deduced and agrees well with the experimental results.

Critical bed shear for initial movement of sediments on a combined lateral and longitudinal slope

2004 ◽  
Vol 35 (2) ◽  
pp. 153-164 ◽  
Author(s):  
Subhasish Dey
Keyword(s):  
Shear Stress ◽  
Flow Direction ◽  
Side Wall ◽  
Shear Stresses ◽  
Bed Shear Stress ◽  
Conduit Flow ◽  
Sloping Bed ◽  
Stress Ratios ◽  
Closed Conduit ◽  
Initial Movement

An experimental study on critical bed shear-stress for initial movement of non-cohesive sediment particles under a steady-uniform stream flow on a combined lateral (across the flow direction) and longitudinal (streamwise direction) sloping bed is presented. The aim of this paper is to ascertain that the critical bed shear-stress on a combined lateral and longitudinal sloping bed is adequately represented by the product of critical bed shear-stress ratios for lateral and longitudinal sloping beds. Experiments were carried out with closed-conduit flow, in two ducts having a semicircular invert section, with three sizes of sediments. In laboratory flumes, the uniform flow is a difficult – if not impossible – proposition for a steeply sloping channel, and is impossible to obtain in an adversely sloping channel. To avoid this problem, the experiments were conducted with a closed-conduit flow. The critical bed shear-stresses for experimental runs were estimated from side-wall correction. The experimental data agree satisfactorily with the results obtained from the proposed formula.

scholarly journals A Ringed Non-Uniform Network: How to Raise its Efficiency

2008 ◽  
Vol 45 (6) ◽  
pp. 20-32 ◽  
Author(s):  
J. Survilo
Keyword(s):  
Electric Power ◽  
Power Quality ◽  
Power Lines ◽  
Phase Voltage ◽  
Closed Loops ◽  
Circulating Current ◽  
Current Flows ◽  
Two Sides ◽  
Made In ◽  
Supply Reliability

A Ringed Non-Uniform Network: How to Raise its Efficiency As distinct from radial electric power lines, in closed loops the consumers are fed from two sides. This is advantageous from the viewpoint of supply reliability, power quality and its losses; however, these are the least only when a loop is uniform, which is not always met in practice. In a non-uniform loop a circulating current flows, and the losses increase proportionally to its square. To reduce losses in such a non-uniform loop, the circulating current should be eliminated. For this purpose a booster transformer can be used. The voltage of such a transformer is known to be in quadrature to the phase voltage; the present consideration has shown that such orientation of the opposing voltage gives the best results only when all loads in the loop are active, otherwise the angle of opposing voltage should be regulated. The voltage value should also be regulated depending on the load. Another technique consists in introducing a complementary reactance into the terminal branches. Such reactance should be regulated if loads are changing in time disproportionately with respect to each other. The best results are achieved when all loop node loads have the same cosφ. If the complementary reactance calculated at one end of the loop is positive, then that calculated at the second end of the loop will be negative, and vice versa. The appropriate choice can be made, in particular, involving both loop terminals.

scholarly journals Stereoscopic PIV of Steady Flow Through a Bileaflet Mechanical Heart Valve

2008 ◽  
Author(s):  
C. Hutchison ◽  
P. E. Sullivan ◽  
C. R. Ethier
Keyword(s):  
Steady Flow ◽  
Heart Valve ◽  
Heart Valves ◽  
Mechanical Heart Valve ◽  
Three Dimensional ◽  
Flow Characteristics ◽  
Shear Stresses ◽  
Stereoscopic Piv ◽  
Bileaflet Mechanical Heart Valve ◽  
Component Particle

Each year over 180,000 mechanical heart valves are implanted worldwide, with the bileaflet mechanical heart valve (BiMHV) accounting for approximately 85% of all valve replacements [1,2]. Although much improved from previous valve designs, aortic BiMHV design is far from ideal, and serious complications such as thromboembolism and hemolysis often result. Hemolysis and platelet activation are thought to be caused by turbulent Reynolds shear stresses in the flow [1]. Numerous previous studies have examined aortic BiMHV flow using LDA and two component Particle Image Velocimetry (PIV), and have shown the flow to be complex and three-dimensional [3,4]. Stereoscopic PIV (SPIV) can obtain all three velocity components on a flow plane, and hence has the potential to provide better understanding of three dimensional flow characteristics. The objective of the current study was to use SPIV to measure steady flow, including turbulence properties, downstream of a BiMHV in a modeled aorta. The resulting dataset will be useful for CFD model validation, and the intent is to make it publicly available.

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