Downstream Velocity Profiles Research Articles

A computational hydrodynamics method for horizontal axis turbine – Panel method modeling migration from propulsion to turbine energy

A computational hydrodynamics method was formulated and implemented for horizontal axis tidal turbines. This paper presents a comparative analysis between screw propellers and horizontal axis turbines, in terms of geometry and motion parameters, inflow velocity analysis and the implementation methodologies. Comparison and analysis are given for a marine propeller model and a horizontal axis turbine model that have experimental measurements available in literature. Analysis and comparison are presented in terms of thrust coefficients, shaft torque/power coefficients, blade surface pressure distributions, and downstream velocity profiles. The effect of number of blades from 2 to 5, of a tidal turbine on hydrodynamic efficiency is also obtained and presented. The key implementation techniques and methodologies are provided in detail for the propeller based panel method tool to migrate as a prediction tool for tidal turbine. While the method has been proven to be accurate and robust for many propellers tested in the past, this numerical tool could be validated further for turbines. To further refine and validate the panel method for various turbines, it requires substantial additional experimental measurements. These measurements include downstream velocity profile by using LDV and/or SPIV, which are essential for numerical wake vortices descritization.

Electromagnetic Brake Effects on the Funnel Shape Mold of a Thin Slab Caster Based on a New Type Magnet

In the present work, a new type of magnet specially designed for the thin slab casting funnel shape mold is developed. The unique feature of the new type of magnet lies in the fact that, the inner surfaces of the new magnet facing the mold are irregular to fit the complex funnel shape, while the inner surfaces of the convential magnet are planar. Magnetic fields produced by both the new type and the conventional magnets are computed under the same number of ampere turns for comparison. Keeping the position of the magnet relative to the mold fixed, numerical simulations of the influence of the new and conventional type magnets on the flow field in the mold are made. The commercial software FLUENT is used to predict the behavior of electrically conducting fluid flow under the influence of static magnetic fields. For the computation of magnetic fields, the finite element method (FEM) is adopted, while for fluid flow, the finite volume method (FVM) based on structured body fitted coordinate grids is used. The results show that the new type of magnet can produce larger magnetic flux density in the mold region than the conventional one does with the same current intensity. For the funnel shape mold, the electromagnetic brake (EMBr) due to the new magnet can provide a more uniform downstream velocity profile than that due to the conventional magnet, and almost 40 pct reduction in ampere turns; this fact implies that a lot of electrical energy could be saved.

Perfusate composition is a determinant of the hydrodynamically relevant endothelial glycocalyx thickness

The presence of a hydrodynamically relevant endothelial‐cell surface glycocalyx, ~ 0.5 µm thick, is well established under normal physiological conditions in vivo. It has also been shown that no hydrodynamically relevant layer exists under standard culture conditions in vitro. The endothelial‐cell surface glycocalyx layer is a highly hydrated structure that is constantly bathed in blood in vivo. It is proposed that there is a constant exchange of molecules between the blood and the glycocalyx that is essential to the integrity of the layer. The aim of this study is to test the effect of various perfusate compositions on the structure of the endothelial glycocalyx in vivo. A micromanipulator is used to introduce a micropipette into the lumen of one of the primary feed arterioles to the mouse cremaster‐muscle. The micropipette is then used to steadily perfuse the microvascular network fed by that arteriole. Micro‐particle image velocimetry data are collected from downstream microvessels and velocity profiles extracted from the data are used to estimate the hydrodynamically relevant thickness of the glycocalyx. Preliminary data indicate that vessels perfused with saline in vivo have no detectable hydrodynamically relevant layer. Data will be presented from vessels perfused with saline, cell‐culture media containing 20% serum, or plasma. Supported by NIH grant R01 HL76499.

Secondary flow in sharp open-channel bends

Secondary currents are a characteristic feature of flow in open-channel bends. Besides the classical helical motion (centre-region cell), a weaker and smaller counter-rotating circulation cell (outer-bank cell) is often observed near the outer bank, which is believed to play an important role in bank erosion processes. The mechanisms underlying the circulation cells, especially the outer-bank cell, are still poorly understood, and their numerical simulation still poses problems, not least due to lack of detailed experimental data. The research reported herein provides detailed experimental data on both circulation cells in an open-channel bend such as found in nature. Furthermore, the underlying dynamics are investigated by simultaneously analysing the vorticity equation and the kinetic energy transfer between the mean flow and the turbulence. This shows that turbulence plays a minor role in the generation of the centre-region cell, which is mainly due to the centrifugal force. By accounting for the feedback between the downstream velocity profile and the centre-region cell, a strongly simplified vorticity balance is shown to yield accurate predictions of the velocities in the centre region. For strong curvatures, however, a fully three-dimensional flow description is required. Due to the non-monotonic velocity profiles, the centrifugal force favours the outer-bank cell. Moreover, terms related to the anisotropy of the cross-stream turbulence, induced by boundary proximity, are of the same order of magnitude and mainly enhance the outer-bank cell. Both mechanisms strengthen each other. The occurrence of the outer-bank cell is shown to be not just due to flow instability, like in the case of curved laminar flow, but also to kinetic energy input from turbulence.

Heat transfer in the assimilation of a pre-heated jet into non-uniform streams

The flow assimilation of a pre-heated jet into non-uniform external streams has been examined. Two possible families of downstream velocity and temperature profiles are identified. The formulation of the problem allows for a comprehensive numerical solution over the entire flow and temperature field downstream of the jet source. The role of the respective families of downstream asymptotes is elucidated, and consistency is demonstrated between a spatial stability analysis of the profiles and the numerical results.

Effects of increased geometric complexity on the comparison between computational and experimental simulations

Abstract CFD studies and methods are generally validated against idealized flow configurations [V.C. Patel, W. Rodi, G. Scheuerer, Amer. Inst. Aeronaut. Astronaut. 23 (9) (1984); W. Rodi, The prediction of free turbulent boundary layers by use of a two-equation model of turbulence, Ph.D. Thesis, University of London, 1972]. Whilst it may be argued that most industrial flows consist of a combination of idealized flow types, the interaction between these flow types is not always well understood. Industrial flows are affected by varying degrees of geometric complexity and the present work sets out to quantity the effect of varying geometric complexity on the agreement between industrial standard CFD simulations and experimental results. The experimental basis for the study is the work by Cermak [Physical modelling of the atmospheric boundary layer in long boundary-layer wind tunnels, in: Proc. Int. Workshop on Wind Tunnel Criteria and Techniques in Civil Engineering Applications, Gaithersburg, Md, USA, 1982] investigating the development of a ‘falsely’ thickened atmospheric boundary layer model. The upstream data are utilized as inlet boundary conditions to the computational simulation and results of the simulation compared to experiment at the downstream boundary. The geometric complexity takes the form of the thickening devices which are further upstream again from the inlet boundary. The necessity to manipulate experimental data to prescribe the required inlet boundary conditions to the flow fields are discussed. It is shown that simulations with the standard k - ϵ turbulence model fail to accurately model either the downstream velocity or turbulence intensity profiles. This discrepancy is attributed to the inlet turbulence structure differing from that inherent to the shear velocity profile. This discrepancy increases with the increasing complexity of the flow thickening devices.

Investigation of oblique shock/boundary-layer bleed interaction

The detailed flowfield characteristics in an oblique shock-wave/laminar-boundary-layer interaction with bleed were investigated. The numerical solution for the flowfield was obtained for the strong conservation-law form of the two-dimensional compressible Navier-Stokes equations using an implicit scheme. The computations mod- eled the flow in the interaction region and inside the bleed slot for an impinging oblique shock on a flat-plate boundary layer. The computed results for the streamlines and the pressure and Mach number contours inside the bleed slot indicate that the flow is choked in the slot, with a recirculation zone near the upstream slot corner. The bleed results in the interaction zone demonstrate that flow separation is controlled. The interaction length . is reduced and the downstream velocity profiles are more favorable than the separated flow results at the same shock strength without bleed. HE control of shock/boundary-layer interactions in inlets and nozzles and over vehicle surfaces is accomplished through bleed and/or blowing in the interaction zone. In the case of mixed compression supersonic inlets, the bleed system design is critical to the efficient and stable operation of the system. Hamed and Shang1 reviewed the existing experimen- tal data for shock-wave/boundary-layer interactions in super- sonic inlets and other related configurations. According to this survey, most of the experimental measurements in mixed compression supersonic inlets consisted of total pressure re- covery surveys at the engine face and static pressure distri- butions over the inner surfaces. In the few cases involving velocity profile measurements,2-3 the latter were obtained up- stream and downstream of the interactions. Comparisons of internal flow computational results3'5 with the experimental measurements in supersonic inlets2-3 revealed reasonable agreement between the computed and measured surface pres- sures upstream of the ramp bleed. However, discrepancies in the predicted shock locations and velocity profiles were ob- served downstream of shock/boundary-layer interactions with bleed. There is enough experimental evidence6'11 to indicate that local bleed can control flow separation in shock-wave/bound- ary-layer interactions. There are disagreements,1 however, among the different experimental studies regarding the effects of bleed hole size,7-8 and the location of the bleed holes in relation to the shock.6-9'11 The experimental data in these studies are not sufficient, however, to resolve these discrep- ancies. Strike and Rippy9 measured the surface pressure in the interaction zone of an oblique shock wave impinging a tur- bulent boundary layer over a flat plate, with suction. They determined that less suction is required to control separation, when applied upstream of the shock. Seebaugh and Childs11 investigated experimentally the axisymmetric flow in the in- teraction region of the boundary layer inside a duct. Contrary to the conclusions of Strike and Rippy,9 suction within the

Influence of Jet-Grid Turbulence on Flat Plate Turbulent Boundary Layer Flow and Heat Transfer

The influence of high mainstream turbulence on turbulent boundary layer flow and heat transfer is experimentally investigated for length Reynolds numbers between 4 × 104 and 1.5 × 106. The high mainstream turbulence is produced by a round tube grid with uniform jet injection. Injected air is blown in either an upwind or downwind direction at a controllable flow rate. A flat plate test section instrumented with foil thermocouples is located downstream from the jet grid. The turbulence intensity decay and length scale growth along the test plate, the mean velocity and temperature profiles across the boundary layer, and surface heat transfer distribution are measured. The results show that the grid with downwind injection produces a slightly higher turbulence intensity and a smaller length scale than the grid with upwind injection. A higher turbulence intensity and a smaller length scale further enhance the surface heat transfer coefficient. The jet-induced high turbulence does not alter the downstream velocity and temperature profiles in their logarithmic regions, but the wake regions are lower than the zero turbulence profiles. The Reynolds analogy factor, the augmented friction factor, and the augmented Stanton number are higher than those from existing correlations when the jet grid turbulence intensity is greater than 6 percent.

Investigations of mean and turbulent flow characteristics of a two dimensional plane diffuser

This paper reports the investigation of mean and turbulent flow characteristics of a two-dimensional plane diffuser. Both experimental and theoretical details are considered. The experimental investigation consists of the measurement of mean velocity profiles, wall static pressure and turbulence stresses. Theoretical study involves the prediction of downstream velocity profiles and the distribution of turbulence kinetic energy using a well tested finite difference procedure. Two models, viz., Prandtl's mixing length hypothesis and k-ɛ model of turbulence, have been used and compared. The nondimensional static pressure distribution, the longitudinal pressure gradient, the pressure recovery coefficient, percentage recovery of static pressure, the variation of Umax/Ubar along the length of the diffuser and the blockage factor have been valuated from the predicted results and compared with the experimental data. Further, the predicted and the measured value of kinetic energy of turbulence have also been compared. It is seen that for the prediction of mean flow characteristics and to evaluate the performance of the diffuser, a simple turbulence model like Prandtl's mixing length hypothesis is quite adequate.

Analysis of flow distribution a PWR fuel rod bundle model containng A 90% blockage

An experimental investigation, covering a Reynolds number range from 2 × 10 3 to 3.5 × 10 4 , was conducted to study the velocity and turbulence intensity distributions due to the presence of a blockage in an unheated 7 × 7 rod bundle. The blockage configuration, consisting of a 4 × 4 rod array, created a maximum flow area reduction of 90% in the central nine subchannels. The blockage sleeve length was 38.3 × rod diameter and the 90% blockage zone length extended for 16.4 × rod diameter. The results showed that upstream of the blockage, the flow was not influenced by the blockage until it reached the location where the inlet taper section of the swelling started. At the downstream end, the flow disturbance was extensive and persisted over a distance of about 83 rod diameters. Compared to the downstream velocity profiles, the turbulence intensity measurements however showed a faster recovery from the blockage influence. At the higher Reynolds number, velocity profiles calculated using the COBRA subchannel computer code compared consistently with the experimental data. The general flow behaviour of the various subchannels was reasonably well predicted. However, at low Reynolds number, due mainly to the frictional form loss calculation scheme in COBRA and uncertainty in the flow transition, the flow diversion due to the blockage to the surrounding unblocked subchannels was overestimated. The influence of the degree of recovery from the rod swelling on the flow was also studied using COBRA.

Mean Flow Measurements Near the Plane of an Open Rotor Operating with an Inlet Velocity Gradient

As part of a study on the structure of a trailing vortex, laser doppler anemometer (LDA) measurements were made of the flow field near an open rotor having an inlet velocity gradient. The measurements were made in the 1.22 m dia water tunnel of the Applied Research Laboratory at The Pennsylvania State University. Velocity data were obtained for rotor inlet and outlet flow fields for several different inlet velocity gradients. Velocity data were also obtained downstream of the rotor plane that shows the vortex structure. Flow field measurements show the development of the downstream vortex motion. Small variations in the inlet velocity gradient near the rotor wall caused large differences in the structure of the trailing vortex. In addition, a measured downstream velocity profile is compared with a calculated velocity profile.

Asymptotic behaviour of velocity profiles in the Prandtl boundary layer theory

Consider the Prandtl boundary layer equation for the steady two-dimensional laminar flow of an incompressible viscous fluid past a rigid wall. On the basis of an arbitrary velocity profile at some initial position on the wall, the analysis presented shows that, for a power law streaming speed U ( x ) ═ C ( x + d ) m ( m ≽ 0), the velocity profile which develops downstream is asymptotically given by the well known Falkner-Skan similarity solution. Moreover, for a streaming speed satisfying (6), the velocity profile which develops downstream is asymptotically unique, though of course the particular form of the resulting profile depends on the precise nature of the exterior stream. The rate of convergence for this asymptotic behaviour is estimated, as well as corresponding rates for the convergence of the skin friction coefficient. This result verifies the tacit assumption of a number of writers that the downstream velocity profile is essentially independent of the initial profile, and also supplies a theoretical justification for the role of similar solutions in boundary layer theory. We also prove the existence of concave velocity profiles whenever the pressure gradient is favourable. It follows that, for streaming speeds which correspond to a favourable pressure gradient, concave velocity profiles play somewhat the same role as similarity profiles do for a power law streaming speed.

Turbulent transport coefficients in supersonic flow

Hypersonic ramjets employing supersonic combustion of hydrogen fuel have attractive potentialities for future aircraft or launching systems. The object of the present work §§This work was sponsored by the Office of Aeronautical Research, Office of Advanced Research and Technology, National Aeronautics and Space Administration, Washington, D.C. The Applied Physics Laboratory operates under Contract NOw 62-064c with the Bureau of Naval Weapons, Department of the Navy. was to study quantitatively the effects of fuel injection parameters on the mixing of gaseous hydrogen fuel with a supersonic air stream confined within a cylindrical duct, to provide some of the fundamental background needed for the design of supersonic combustors for high-performance engines. Hydrogen was injected at sonic velocities into Mach 2 and Mach 3 air streams, at overall equivalence ratios of 0.17 to 0.50, in both radial and axial (downstream) directions from circumferential wall slots. Results showed that considerably better mixing occurred in the case of radial injection, although the decrease in stagnation pressure also was greater for this case.The eddy diffusivity of mass, Ed (turbulent diffusion coefficient) and radial velocity, Vr, were determined by differentiating experimental concentration, velocity and density profiles, obtained at various axial distances from the injection station. For the radial injection case, with a 1-in i.d. test section, a simple model in which Ed varied only in the radial direction and Vr varied only in the axial direction, allowed reasonable correlation of the experimental results. The validity of the trends obtained in Ed and Vr were checked by numerical integration of the diffusion equation, and simultaneous solution of the diffusion and momentum equations; computed profiles agreed reasonably well with downstream experimental concentration and velocity profiles. A method for solving turbulent mixing problems by simultaneous solution of the diffusion, momentum and energy equations is presented.