Velocity Profile Parameters Research Articles

Photofragment image analysis via pattern recognition

An algorithm is presented that solves two problems associated with the analysis of velocity map images, which are used, for example, in the study of photofragmentation or photoionization processes. The first part of the algorithm identifies the center, the ring radii, and distortions of circularity without any a priori knowledge about the image. Derived from the Hough transform, it is highly robust with respect to uneven distributions of intensity, background signals, and realistic distortions of circularity. In the second independent part of the algorithm the image parameters are calculated using an analytical description of the image. Here the velocity profile, branching ratios, and spatial anisotropy parameters are obtained directly from the raw image for any form of the velocity broadening function, i.e., without the necessity for “inversion” of the image.

Assessment of turbulence model performance for transonic flow over an axisymmetric bump

AbstractA computational study has been performed to evaluate the predictive capabilities of some existing eddy-viscosity (both linear, LEVM, and non-linear, NLEVM) and Reynolds stress transport turbulence models (RSTM) by reference to a transonic shock-induced separated flow over a 10% axisymmetric bump. The calculations have been carried out during the course of a collaborative research programme including both UK universities and industry. The findings of the project demonstrate that improved results can be obtained for such flows by using more advanced turbulence models. For linear eddy-viscosity models, only the SST approach gave good predictions of shock location, recirculation size and pressure recovery, although this was accompanied by deficiencies in the prediction of post-shock velocity profile shape. Non-linear eddy-viscosity models, particularly at the cubic level, provided a more consistent level of agreement with experiments over the range of shock location, wall pressure and velocity profile parameters. Some improvement was also seen in the prediction of turbulence quantities, although only a move to an RSTM closure model reproduced the measured peak stress levels accurately. It was notable that the use of low-Re variants of the models (instead of wall functions) produced no significant improvement in predictions. There are, however, some shortcomings in all models, particularly in the development of flow after reattachment, which was always predicted to be too slow.

A theory for turbulent pipe and channel flows

A theory for fully developed turbulent pipe and channel flows is proposed which extends the classical analysis to include the effects of finite Reynolds number. The proper scaling for these flows at finite Reynolds number is developed from dimensional and physical considerations using the Reynolds-averaged Navier–Stokes equations. In the limit of infinite Reynolds number, these reduce to the familiar law of the wall and velocity deficit law respectively.The fact that both scaled profiles describe the entire flow for finite values of Reynolds number but reduce to inner and outer profiles is used to determine their functional forms in the ‘overlap’ region which both retain in the limit. This overlap region corresponds to the constant, Reynolds shear stress region (30 < y+ < 0.1R+ approximately, where R+ = u*R/v). The profiles in this overlap region are logarithmic, but in the variable y + a where a is an offset. Unlike the classical theory, the additive parameters, Bi, Bo, and log coefficient, 1/κ, depend on R+. They are asymptotically constant, however, and are linked by a constraint equation. The corresponding friction law is also logarithmic and entirely determined by the velocity profile parameters, or vice versa.It is also argued that there exists a mesolayer near the bottom of the overlap region approximately bounded by 30 < y+ < 300 where there is not the necessary scale separation between the energy and dissipation ranges for inertially dominated turbulence. As a consequence, the Reynolds stress and mean flow retain a Reynolds number dependence, even though the terms explicitly containing the viscosity are negligible in the single-point Reynolds-averaged equations. A simple turbulence model shows that the offset parameter a accounts for the mesolayer, and because of it a logarithmic behaviour in y applies only beyond y+ > 300, well outside where it has commonly been sought.The experimental data from the superpipe experiment and DNS of channel flow are carefully examined and shown to be in excellent agreement with the new theory over the entire range 1.8 × 102 < R+ < 5.3 × 105. The Reynolds number dependence of all the parameters and the friction law can be determined from the single empirical function, H = A/(ln R+)α for α > 0, just as for boundary layers. The Reynolds number dependence of the parameters diminishes very slowly with increasing Reynolds number, and the asymptotic behaviour is reached only when R+ [Gt ] 105.

Scale and Roughness Effects in Ship Performance from the Designer’s Viewpoint

Evaluation of the scale effect in the absence of flow separation is usually based on two independent calculation procedures for the drag and lifting forces. Prediction of the full-scale drag forces is based on modeling of the viscous part of the resistance in accordance with the difference in Reynolds numbers. As a rule, a lifting force coefficient for full-scale bodies is treated as a lift coefficient for a model having the same Froude number. The proposed method is based on the idea that an initiation of the lifting force is associated with the viscous effects in the boundary layer. According to this approach, an estimation of the scale effect is based on a calculation of potential, boundary layer and wake flows with viscous/nonviscous interaction for given Froude and Reynolds numbers. The roughness of a surface is taken into account due to the use of special velocity profile parameters. These parameters depend on type and height roughness (casting, milling grooves, polishing, paint, fouling, acid deposit, etc.). As a result, the effects of viscosity (Reynolds number) and surface roughness are determined as the difference between the drag and lift forces calculated for the model and full-scale conditions. The magnitude of the scale effect may reach 50% to 70% for the drag and 10% to 20% for the lift forces. The unusually large scale effect for the lift may play a significant role in engineering applications. Results of the systematic calculations and experimental evaluations are reported for a broad range of ship types and propellers.

Sound velocity profile estimation: a system theoretic approach

A system-theoretic approach is proposed to investigate the feasibility of reconstructing a sound velocity profile (SVP) from acoustical hydrophone measurements. A state-space representation of the normal-mode propagation model is used. It is shown that this representation can be utilized to investigate the so-called observability of the SVP from noisy measurement data. A model-based processor is developed to extract the required information, and it is shown that even in cases where limited SVP information is available, the SVP can be estimated using this approach. Based on this framework, investigations are made of model-based solutions to the sound velocity profile and related parameter estimation problems. In particular, a processor is designed that allows in situ recursive estimation of the sound velocity profile from simulated data. >

NUMERICAL SIMULATION OF LAMINAR CONVECTION HEAT TRANSFER FROM AN IN-LINE ARRAY OF DISCRETE SOURCES TO A CONFINED RECTANGULAR JET

Conjugate convection/conduction numerical simulations have been performed to determine flow and heat transfer phenomena associated with using a single laminar slot jet to cool a linear array of discrete heat sources. Flow is restricted to a channel formed by an impingement surface, within which the heat sources are flush mounted, and a parallel confinement surface, through which the jet is discharged. Calculations reveal a primary recirculation cell between the jet and the confining surface and, for restricted conditions, a secondary cell on the impingement surface that produces a maximum and a minimum in the distribution of the local convection coefficient. The effects of the jet velocity profile and pertinent dimensionless parameters on flow and heat transfer conditions have been calculated

Non-isothermal masstransport in a circular tube for flow of Non-Newtonian liquid

The non-isothermal mass transport in a circular tube has been investigated for an Ostwald-de-Waele liquid. The influence of the velocity profile parameter, the flow parameter, the thermal diffusion, the diffusive heat transfer as well as the ratio of the Schmidt- to the Prandtl number upon the concentration- and temperature profile along the tube have been determined. The results are compared with the plug flow case and also with the isothermal mass transport. It was found that considerable differences in the mass transport and heat transport occur through their coupling effect.

NONISOTHERMAL MASS TRANSPORT IN A FLAT CONDUIT FOR FLOW OF NON-NEWTONIAN LIQUID

Nonisothermal mass transport in a flat conduit has been investigated for an Ostwald de Waele liquid. The influence of the velocity profile parameter, the flow parameter, the thermal diffusion, the diffusive heat transfer, and the ratio of the Schmidt to the Prandtl number on the concentration and temperature profiles along the conduit has been determined. The results are compared with the plug flow case and also with isothermal mass transport. It is found that considerable differences in the mass transport and heat transport occur through their coupling.

Profile of Suspended Sediment Due to Wave Action

A knowledge of sediment transport due to wave action is required to understand the dynamic process of nearshore morphology; however, these phenomena of sediment transport due to waves are complicated by the unsteady characteristics of the flow, the presence of turbulence and the interactions between the fluids and the sediment. These unsteady phenomena are much more complicated than those of steady uniform flows. Profiles of mean and unsteady concentrations are derived from the mass conservation equation using the diffusion coefficient profile adapted from the eddy viscosity profile, taking into account changes of the friction factor and boundary layer thickness in the presence of suspension. The theoretical profiles are expressed explicitly as functions of the friction factor, velocity and shear stress profile parameters, and the settling velocity of the sediment relative to the fluid velocity. The tabulated data available and the present experimental data of the mean concentration are used to fit the theoretical profile. The analysis shows that the friction factors in the presence of suspension decrease from the sediment‐free values in the same manner and magnitude as the von Karman constant.

Oscillatory Boundary Layer and Eddy Viscosity

An appropriate oscillatory velocity profile is developed from existing experimental data in an oscillating bed, a wave flume, and a water tunnel. The velocity profile is then used to determine the boundary layer thickness, the shear stress profile, the friction factor, and the eddy viscosity profile. It is found that there are differences between an oscillating bed and an oscillating fluid which contradicts the old belief that they are the same. The thickness of the boundary layer depends upon the flow Reynolds number. The friction factor of the three-dimensional bed is about 10 – 20 % higher than that of the two-dimensional bed. The eddy viscosity profile is expressed explicitly as a function of the friction factor, velocity profile parameters, and the phase profile parameter.

Fundamental Studies on the Control of Turbulent Boundary Layers :1st Report, Theoretical and Experimental Analyses of Turbulent Boundary Layer Velocity Profile and Boundary Layer Parameters with or without Adverse Pressure Gradient and Uniform Suction through a Porous Flat Plate

Fundamental Studies on the Control of Turbulent Boundary Layers :1st Report, Theoretical and Experimental Analyses of Turbulent Boundary Layer Velocity Profile and Boundary Layer Parameters with or without Adverse Pressure Gradient and Uniform Suction through a Porous Flat Plate

Wind Profile Parameters and Turbulence Intensity Over Several Roughness Element Geometries

ABSTRACT ESTIMATING equations were developed for the mean velocity profile parameters (ZQ, U^O^ and D) in the logarithmic law and for longitudinal turbulence intensity (ou/uz)- The estimates were based on wind tunnel measurements over several roughness element shapes, sizes, heights, and geo-metrical patterns. All the prediction equations had correlation coefficients > 0.90. These equations have application in wind erosion of soil particles, water evaporation, and transport of gases. They only apply to a particular set of conditions, which indicates a need for more universally applicable equations for the profile parameters and turbulence intensity.

An Investigation of the Forces on Three Dimensional Bluff Bodies in Rough Wall Turbulent Boundary Layers

A study has been made of the influence of grouping parameters on the mean pressure distributions experienced by three dimensional bluff bodies immersed in a turbulent boundary layer. The range of variable parameters has included group density, group pattern and incident flow type and direction for a simple cuboid element form. The three flow regimes associated with increasing group density are reflected in both the mean drag forces acting on the body and their associated pressure distributions. A comparison of both pressure distributions and velocity profile parameters with established work on two dimensional bodies shows close agreement in identifying these flow regime changes. It is considered that the application of these results may enhance our understanding of some common flow phenomena, including turbulent flow over rough surfaces, building ventilation studies and environmental wind around buildings.

Analytical procedure for calculation of attached and separated subsonic diffuser flows

An analytical procedure for computing the static and total pressure distributions in subsonic diffusers with axisymmetric or two-dimension al cross sections is described. It includes velocity profile and shear stress parameters which apply to attached and separated flows and allows for a strong interaction between the inviscid and viscous portions of the duct flow. With this approach, solutions can be obtained for separated regions where conventional boundary-layer methods fail. Analytical and experimental static and total pressure distributions are compared for plane-wall and conical diffusers. An approximate technique is used to apply the analysis to actual aircraft diffusers. A Ae a0 al9...,a7 blfb2,b3 CD D d

Experimental friction factors for turbulent flow with suction in a porous tube

Measurements of pressure gradient have been obtained with air flowing in a porous tube of circular cross-section with fully-developed turbulent profile at the entrance and with uniform mass extraction through the wall; an empirical correlation for the axial gradients with suction is presented. A slight but insignificant gradient in the radial direction was detected. The experiments covered an inlet Reynolds number range of 11000–101000 with a ratio of the transverse velocity at the wall to the mean axial velocity at inlet from zero to about 0.027. The radial distributions of both the temporal-mean axial velocity and the absolute turbulent velocity fluctuation were measured at the tube exit and the velocity profile parameter has been correlated with a suction parameter. The form of the temporal-mean velocity profile is found to depend critically upon the suction rate, becoming more flat at modest rates of suction but more peaked at high rates. The relative turbulence level is found to increase with suction at all radii, save for some reduction in the region of the wall at very low rates of suction. Local and average values of the effective friction factor with suction have been computed and are presented in graphical form. At a fixed inlet Reynolds number average values increased markedly with suction but decreased along the tube. For a given local Reynolds number local values displayed a similar trend.

Heat transfer in non-Newtonian laminar flow in tubes

A theoretical investigation is presented of the heat transfer from fluids in non-Newtonian laminar flow in circular cylindrical tubes. The velocity profile is introduced according to the Prandtl-Eyring formula (2) (6). Eigenvalues and eigenfunctions have been calculated for some values of the velocity profile parameter b (which is proportional to the longitudinal pressure gradient), and graphs of the cup mixing mean temperature of the fluid and of the dimensionless local heat transfer coefficients are given. The results are compared with those of some other workers who carried out calculations assuming the Ostwald profile (1). The influence of the thermal resistance of the tube wall for radial heat flow is also investigated. In an appendix formulae are presented concerning the influence of heat dissipation resulting from internal friction, for the Ostwald profile as well as for the Prandtl-Eyring profile.