Variation Of Velocity Profile Research Articles

Effect of laminar velocity profile variation on mixing in microfluidic devices: The sigma micromixer

The effect of the laminar velocity profile and its variation on mixing phenomena at the reduced scale is studied. It is shown that the diffusive mass flux between two miscible streams, flowing laminar in a microchannel, is enhanced if the velocity at their diffusion interface is increased. Based on this idea, an in-plane passive micromixing concept is proposed and implemented in a working device (sigma micromixer). This mixer shows considerable mixing performance by periodically varying the flow velocity profile, such that the maximum of the profile coincides with the transversely progressing diffusion fronts repeatedly throughout the mixing channel.

Numerical study of the optimization of the pitch angle of an alternative jet fan in a longitudinal tunnel ventilation system

This paper presents the results of a numerical investigation carried out on an alternative jet fan, noted in literature like Banana Jet Fan, equipped with inlet/outlet silencers inclined at a fixed pitch angle ( α) inclined toward the tunnel floor. The aim of this paper is to optimize the pitch angle comparing the different ventilation performances, in terms of reduction of pressure losses and of energy saving, without vehicles and with traffic jam. The average velocity profile variation of the tunnel air jet around the outlet jet fan zone was evaluated with the numerical CFD analysis. This analysis showed that there exists an optimal pitch angle which minimizes the pressure losses, due to the flow attachment by varying the operative conditions (free tunnel and traffic jam).

Influence of a non-uniform velocity field on isochrone geometry along a steady flowline of an ice sheet

The relationship between velocity field and isochrone geometry along a steady flowline of an ice sheet is examined. The method is analytical and based upon the stream function and its vertically normalized form, the normalized stream function (NSF). We show that the slope of the isochrones is the slope of the iso-NSF lines, plus a path term which is the cumulative result of the past trajectory of the ice particles. We illustrate this path term in three different examples: varying basal melting, varying basal sliding (Weertman effect) and varying velocity profile around a divide (Raymond effect). The path term generally counteracts the slope of the iso-NSF lines. In the case of the Raymond effect, it can even lead to depressions surrounding the bumps if the transition from dome to flank velocity profile is sufficiently abrupt.

Computer extended series solution for flow in a narrow channel of varying gap

The problem of steady two-dimensional laminar incompressible flow of a Newtonian fluid in a long narrow channel of varying gap is considered. The similarity transformation reduces the governing equations into nonlinear ordinary differential equation. The resulting equation is solved using long series with polynomial coefficients. Thirty-one effective terms in the perturbation series representing coefficients of pressure gradient for various cases of bearings are obtained. Analytic continuations of series solution unravel the rich spectrum of flow structure which could not be fully revealed in earlier asymptotic as well as pure numerical studies. The qualitative features obtained are comparable to pure numerical results besides this the present analysis enables to extend the study up to αRe = 50 to ∞. The variations of velocity profiles and pressure gradient for different boundary conditions as functions of αRe are analyzed.

NONLINEAR ANALYSIS OF SPATIAL VARIATION OF VELOCITY PROFILE IN A PRESSURIZED LAMINAR FLOW BETWEEN WAVY BOUNDARIES

A nonlinear analysis procedure has been proposed to simulate the spatial variation of velocity profile of pressurized laminar flows between wavy boundaries as an idealized flow model in rock fractures. The main objective of this analysis is to refine the depth averaged flow model for a rock joint, reflecting the results of this analysis. The analysis is based on an assumed general form of velocity profile in the direction of flow, whereby the unknowns are determined through the use of the continuity and momentum equations. Comparison is made between the 2D-vertical model's results and the nonlinear solution through which the applicability of the analysis is verified. The nonlinear solution shows good agreement with the 2D-vertical model's results especially for small amplitude-to-wavelength ratio of the wavy boundaries, but enough to reveal the velocity profile's deformation.

Variation of velocity profile of jet and its effect on interfacial stability

Linear stability theory is used to study a double fluid model for a liquid jet surrounded by a coaxial gas steam. Under the different pressure gradients for liquid and gas flow, the variation of the velocity profile in the model and the thickness of the shear layer were investigated. The effects of such variation on the interfacial stability were discussed with the application of Chebyshev spectral collocation method.

Doppler optical coherence imaging of converging flow

The experimental methods of Doppler optical coherence tomography are applied for two-dimensional flow mapping of highly scattering fluid in flow with complex geometry. Converging flow (die entry) is used to demonstrate non-invasive methods to map varying velocity profiles before and after the entry. Complex geometry flow is scanned with ∼10 × 10 × 10 µm3 spatial resolution. Structural images of the phantom and specific velocity images are demonstrated. A variety of velocity profiles have been obtained before and after the entry. Concave, blunted, parabolic and triangular profiles are obtained at different distances after the entry. Application of the technique to the study of blood circulation is discussed.

A nonuniformly stretched vortex.

A stretched vortex model is proposed which includes a nonuniform stretching in the radial direction that is clearly present in real flows, as well as a slow variation of velocity profiles along the vortex axis. Both features of this boundary layer approximation depart from the classical Burgers solution. This model is shown to be in very good agreement with experimental velocity measurements.

Imaging of non-parabolic velocity profiles in converging flow with optical coherence tomography

The optical coherence tomography method was explored for two-dimensional flow mapping of a highly scattering fluid in flow with complex geometry. Converging flow (capillary entry) with 4:1 constriction was used for demonstration of non-invasive and remote methods of mapping varying velocity profiles. Downstream of the geometry was scanned with ∼10 × 10 × 10 µm3 spatial resolution and structural imaging of the lumen and images of one particular velocity were acquired. Stable concave, blunted and parabolic profiles are obtained at different distances of the inlet length. Application of the technique for the blood circulation is also discussed.

터빈 로터의 익단 간극이 성능에 미치는 영향에 대한 수치해석적 연구

The effect of tip clearance is important part for turbomachinery performance. Tip leakage flow due to tip clearance is mixed with passage vortex. Large amount of loss is generated at the mixing region and overall performance of turbomachinery is reduced. Numerical calculation of the lst stage rotor of GE7FA gas turbine is carried out to investigate tip clearance effect on performance, pitchwise variations of velocity profiles, pressure distributions and loss coefficients. A commercial code, CFX-TascFlow is validated in this study.

Variation of momentum and thermal boundary layers for oscillatory flows in a channel

Zero mean oscillatory flow over a heated plate mounted on the bottom wall of a channel, is analysed by solving two dimensional time dependent governing equations using control volume based pressure correction procedure. A uniform sinusoidally varying velocity profile is applied at the inlet, and temperature of the heated plate is kept constant. The numerical solutions are compared with experimental values and the effects of Reynolds and Wormersly numbers on the velocity and temperature profiles are presented for the same Prandtl number.

On the velocity profile of inclined jets in a boundary layer for vorticity production

The velocity profiles of air-jet vortex generators have been investigated, and the effect of varying velocity profiles on vortex production has been assessed. In the simplest case of a jet generated by fluid effusing through a cylindrical hole drilled into a solid surface, the internal flow in the nozzle is critical to the development of jets to be used for vortex production. A series of Pitot traverses through a range of jet diameters and pitch angles has been made in order to characterize the jet development. In order to produce an axisymmetric jet profile, modifications were made to the jet nozzles, thereby overcoming the critical flow features within the jet nozzle. In order to assess the jets for vortex production, a range of air-jet vortex generator configurations have been used to produce discrete vortices in a turbulent flat plate boundary layer. Velocity vectors were measured over a cross-stream plane through the vortices, from which the vorticity could be derived across the plane. The vorticity at the centre of the vortex (or the peak vorticity) was then used as an indication of the strength of the vortices. It has been shown that the nature of the jet velocity profile is not significant to the strength of the vortex, so long as the average jet velocity is held constant.

Latissimus dorsi muscle: assessment of stimulation after cardiomyoplasty with Doppler US tissue imaging.

To evaluate Doppler ultrasound (US) tissue imaging for assessment of stimulated skeletal muscle contraction. Seven patients were studied after left latissimus dorsi cardiomyoplasty. Myograft contraction programmed on alternate cardiac cycles was assessed with Doppler US tissue imaging. Beat-to-beat variation in inferior wall motion was assessed by examining peak myograft velocities during 10 muscle-assisted and 10 nonassisted cardiac cycles. The temporal relationship between electrostimulation and myograft contraction, changes in cardiac geometry, and the effect of alterations in stimulation voltage and muscle synchronization were assessed. Significant beat-to-beat variation in velocity profile could be detected in the proximal myograft in six patients (P < .05). Potentiation of infero-posterior wall motion was measurable in five patients (mean peak systolic wall velocity: nonassisted, 2.5 cm.sec-1 +/- 0.5 [standard deviation]; assisted, 7.8 cm.sec-1 +/- 6.3). The response between stimulation voltage and inferoposterior wall velocity was sigmoid. Doppler US tissue imaging depicted the effects of myostimulator programming on muscle contraction and ventricular wall motion.

Further studies on effects of thermophoresis on seeding particles in LDV measurements of strained flames

The axial velocity profiles for counterflow premixed and diffusion flames were experimentally measured by laser-doppler velocimetry (LDV) and computationally simulated with detailed reaction mechanism and transport properties. The LDV measurements were found to agree well with the computed values in the cooler, decelerating part of the flow upstream of the flame, but to significantly deviate from the calculated values in the rapidly-accelerating preheat region of the flame in which substantial thermal expansion occurs over a very short distance. An analysis of the motion of the LDV seeding particles under the influence of viscous drag and thermophoresis in these well-characterized counterflow flame environments demonstrates that such deviations are consequences of thermophoresis. Furthermore, since the thermophoretic force is in the direction opposite to that of the temperature gradient, and its influence on the motion of the particle depends upon the local flow velocity, a rich variety of LDV velocity profiles were observed for flames with different temperature profiles and distances to the stagnation surface. The stoichiometric mixture fraction was found to be a useful parameter to characterize the velocity profile variation. The study emphasizes the importance of accounting for the effects of thermophoresis in interpreting LDV as well as PIV (particle image velocimetry) data in flames, both laminar and turbulent. An approach to closely simulate experimental counterflow flames is also presented.

Laser Velocimeter Measurements in a Centrifugal Pump With a Synchronously Orbiting Impeller

Velocity profiles were measured in the impeller of a centrifugal pump with a two-directional laser velocimeter. Blade-to-blade profiles were measured at four circumferential positions and four radii within and one outside the four-bladed impeller. Data are presented herein at two circumferential and three radial locations. The pump was tested in two configurations; with the impeller running centered within the pump, and with the impeller orbiting with a synchronous motion (ε/r2 = 0.016). Variation in velocity profiles among the individual passages in the orbiting impeller were found. At design flow rate, these variations ranged from 30 to 60 percent for the radial component, and 15 to 25 percent for the tangential component. Tangential velocity profiles near the impeller exit (r/r2 = 0.973) were near uniform across each individual passage. Differences in the magnitude of the exit tangential velocities among the passages however, were detected. Systematic differences in the velocity profile shapes of the centered and orbiting impellers were in general not measured, the only exception being at r/r2 = 0.973 at 40 percent of the design flow rate. At this condition, two distinct radial velocity profiles were measured. Two of the impeller passages of the orbiting impeller contained a recirculation region covering 20-30 percent of the blade passage while the other two passages contained no recirculation region. The centered impeller also contained this region of reverse flow. Finally, velocity data were numerically integrated to find the forces and stiffnesses due to momentum fluxes on the impeller for the orbiting condition.

Wall-layer model for the velocity profile in turbulent flows

In the calculation of turbulent flows near walls, a substantial amount of mesh points and computational effort is required to adequately resolve the intense velocity profile variations in the near-wall region. In this study, a profile approximation is obtained for the mean velocity that is based upon consideration of the coherent structure of the time-dependent wall-layer flow. The profile approximation is an analytical formula that satisfies all of the required compatibility conditions at the wall and the logarithmic behavior at the edge of the wall layer. A general method for utilizing the present wall-layer profile model in a prediction method is indicated.

Wing laminar boundary layer in the presence of a propeller slipstream

The effects of a propeller slipstream on the wing laminar boundary layer are being investigated. Hot-wire velocity sensor measurements have been performed in flight and in a wind tunnel. It is shown that the boundary layer cycles between a laminar state and a turbulent state at the propeller blade passage rate. The cyclic length of the turbulent state increases with decreasing laminar stability. Analyses of the time varying velocity profiles show the turbulent state to lie in a transition region between fully laminar and fully turbulent. The observed cyclic boundary layer has characteristics similar to relaminarizing flow and laminar flow with external turbulence.

Experimental and Numerical Investigations of Plane Duct Flows With Sudden Contraction

The present paper reports on experimental and numerical studies of laminar, two-dimensional flow through plane ducts with sudden contractions in cross-sectional area. A laser-Doppler anemometer and a flow computational program were complementarity employed to study details of the flow close to the step. The results reveal details of the velocity profile variations in the vicinity of the contraction. Information is also provided on the separated flow region in the front concave corner of the duct and on the separated flow region just downstream of the lip of the step. The dimensions of the front separation region are shown to agree well with existing data. The recirculating flow region just downstream of the lip is much smaller than occasionally assumed. At higher contraction ratios off-axis velocity maxima occur just downstream of the contraction. It is shown that strong elongational flow fields occur. These are concentrated to a very small region close to the step.

The determination of pipe contraction pressure loss coefficients for incompressible turbulent flow

The accurate prediction of pipe contraction presure loss is important in the design of pipe systems, such as heat exchangers, particularly when close control of the flow distribution in a network of pipes is required. The prediction of the contraction pressure loss depends heavily on experimental data. Large discrepancies in these predictions are evident in the literature. New experimental results giving pressure loss coefficients for range of Reynolds numbers of 4×10 4 to 2×10 5 and area ratios 0.13 to 0.7 are presented and compared with those of other workers and with predictions from a method that allows for velocity profile variation through the contraction. The results show a Reynolds number dependence. The effects of small-bore pipe inlet geometry on the loss coefficients are also examined.

Measurement of instantaneous flow rate through estimation of velocity profiles

It is very important, but difficult to measure instantaneous flow rate in the unsteady flow regime, since the variation of velocity profile limits the dynamic accuracy of flowmeters. Consideration of the velocity profile variation is crucial to improving their dynamic performance. This paper presents a new method of measuring the instantaneous flow rate of unsteady flow in a circular pipe by estimating the velocity profile of the flow. The estimation is formulated as a discrete Kalman filter problem where the dynamics of the velocity profile, which is governed by a linear partial differential equation, is approximated using a finite-dimensional model. An instantaneous flowmeter consisting of a laser Doppler velocimeter (LDV) and an 8-bit microcomputer which implements the solution of the filter problem has been developed. Successful measurements of a pulsating flow and a stepwise changing flow by this flowmeter are presented.