Non-dimensional Streamwise Research Articles

S054081 フリップフロップ・ジェット・ノズルのターゲット長さの影響([S05408]噴流,後流,およびはく離流れの基礎と応用:最前線(8))

In the present study, we investigate a self-excited oscillatory phenomenon of a two-dimensional confined jet with a rectangular-cross-section cylinder as a downstream target. More specifically, in order to reveal the effects of a streamwise size a of the downstream target upon both the occurrence condition for stable jet's oscillation and the jet-oscillation frequency fp, we conduct experiments at Re = 500 in water by an ultrasonic velocity profiler. As a result, we find that the effect the non-dimensional streamwise target size a/b upon the occurrence condition is negligible at alb > 10, where b denotes a length scale of the jet's breadth At alb > 10, we cannot ignore the a/b effect, namely, the jet's oscillation tends to suppress. However, if we can detectfp,fp can be predicted welly by the proposed empirical formula describing the relation of St with the four geometric parameters.

Large eddy simulation of bluff body wake in planar shear flow

AbstractLarge eddy simulation of planar shear flow past a square cylinder has been investigated. Dynamic Smagorinsky model has been used to model subgrid scale stress. The shear parameter, K, namely the nondimensional streamwise velocity gradient in the lateral direction, is 0.0, 0.1 and 0.2. Reynolds number based on the centerline velocity is fixed at Re=21400. The time and span‐averaged velocity components, pressure coefficient, Reynolds stresses for uniform are in good agreement with the literature. In shear flow the calculated flow structure and mean velocity components are shown to be markedly different from those of the uniform flow. With increasing shear parameter, the cylinder wake is dominated by clockwise vortices. Both the velocity components in shear flow are compared with respective components in uniform flow. Comparison of normal and shear stresses between shear and no shear case have also been presented. Copyright © 2009 John Wiley & Sons, Ltd.

Transient nonlinear optically-thick radiative–convective double-diffusive boundary layers in a Darcian porous medium adjacent to an impulsively started surface: Network simulation solutions

A boundary-layer model is described for the two-dimensional nonlinear transient thermal convection heat and mass transfer in an optically-thick fluid in a Darcian porous medium adjacent to an impulsively started vertical surface, in the presence of significant thermal radiation and buoyancy forces in an (X∗,Y∗,t∗) coordinate system. An algebraic approximation is employed to simplify the integro-differential equation of radiative transfer for unidirectional flux normal to the plate into the boundary-layer regime, by incorporating this flux term in the energy conservation equation. The conservation equations are non-dimensionalized into an (X,Y,T) coordinate system and solved using the Network Simulation Method (NSM), a robust numerical technique which demonstrates high efficiency and accuracy. The transient variation of non-dimensional streamwise velocity component (u) and temperature (T) and concentration (C) functions is computed for various selected values of Stark number (radiation–conduction interaction parameter) and Darcy number. Transient velocity (u) and steady-state local skin friction (τX) are also studied for various thermal Grashof number (Gr), species Grashof number (Gm), Schmidt number (Sc) and Stark number (N) values. These computations for the infinite permeability case (Da→∞) are compared with previous finite difference solutions [Prasad et al. Int J Therm Sci 2007;46(12):1251–8] and shown to be in excellent agreement. An increase in Darcy number is seen to accelerate the flow and boost velocity. A decrease in Stark number (corresponding to an increase in thermal radiation heat transfer contribution) is shown to increase the velocity values. Temperature function is observed to fall in value with a rise in Da and increase with decrease in N (corresponding to an increase in thermal radiation heat transfer contribution). Applications of the study include rocket combustion chambers, astrophysical flows, spacecraft thermal fluid dynamics in debris-laden environments (cosmic dust), heat transfer in forest fire spread, geochemical contamination and ceramic materials processing.

End depth computation in inverted semicircular channels using ANNs

The paper presents the application of artificial neural network (ANN) to determine the end-depth-ratio (EDR) for a smooth inverted semicircular channel in all flow regimes (subcritical and supercritical). The experimental data were used to train and validate the network. In subcritical flow, the end depth is related to the critical depth, and the value of EDR is found to be 0.705 for a critical depth–diameter ratio up to 0.40, which agrees closely with the value of 0.695 given by Dey [Flow Meas. Instrum. 12 (4) (2001) 253]. On the other hand, in supercritical flow, the empirical relationships for EDR and non-dimensional discharge with the non-dimensional streamwise slope of the channel are established.

Investigation of the flow field of a highly heated jet of air

Measurements of mean and fluctuating velocity and temperature and their self- and cross-products to the third-order are presented for a heated axisymmetric air jet. Froude numbers in the range of 3500–13,190, Reynolds numbers in the range of 3470–8500 and non-dimensional streamwise distances, X ∗ , from 0.27 to 1.98 are covered by the data. It was found that turbulence intensity decreases for the heated jet in the region between the inertia dominated and the buoyancy dominated regions which is contrary to findings with helium jets mixing with ambient air to produce density fluctuations. The effects of heating on the turbulent kinetic energy budget and the temperature variance budget show small differences for the inertia dominated region and the intermediate region which help to explain the transition process to the far field plume region. Constants are evaluated for the isotropic eddy diffusivity and generalised gradient hypothesis models as well as the scalar variance model. No significant effect of heating on the dissipation time-scale ratio was found. A novel wire array with an inclined cold wire was used. Measurements obtained with this probe are found to lead to asymmetries in some of the higher-order products. Further investigation suggested that the asymmetries are attributable to an as yet unreported interference effect produced by the leading prong of the inclined temperature wire. The effect may also have implications for inclined velocity wires which contain a temperature component when used in heated flows.

Effects of Pressure Gradients on Turbulent Boundary Layer Wave Number Frequency Spectra

pp. 93‐113). The use of ± as an outer length scale is shown to produce distinct trends in the convection velocities. The autospectra for the respective cases are scaled on inner and outer variables, with outer variables collapsing the data over a narrow frequency range. The effects of spatial averaging due to the e nite transducer size are taken into account. Estimations of the wave number frequency spectra are obtained by spatially transforming the cross spectra, and taking into account the variation of convection velocity with spatial separation. A distinct trend in these spectra are displayed, with an asymmetric convective ridge that broadens as the pressure gradient changes from favorable to adverse. Uncertainties in the estimation of the spectra are discussed. The results are applicable to the e ow-induced radiated noise of undersea vehicles and the self-noise of sonar systems. Nomenclature d = wall-pressure sensor diameter d + = nondimensional sensor diameter = dus /m G(n , x o) = cross spectrum k = streamwise wave number Rh = Reynolds number based on momentum thickness= Uoh /m Uo = freestream velocity uc = convection velocity us = friction velocity = p (s w/q ) a = decay constant for the cross spectrum d = boundary-layer thickness d ¤ = boundary-layer displacement thickness h = boundary-layer momentum thickness j = nondimensional streamwise wave number = i kd

Measurements of the convection heat transfer coefficient for a planar wall jet: uniform temperature and uniform heat flux boundary conditions

The local heat transfer coefficients for isothermal and uniform heat flux boundary conditions for a planar wall jet have been determined experimentally. Hot-wire anemometry surveys were used to quantify the velocity field in the wall jet. A micro-thermocouple was used to quantify the temperature field in the wall jet for the isothermal boundary condition. Infrared (IR) imaging was applied to measure the wall temperature for the uniform heat flux boundary condition. The difference in the local convection coefficients due to the different thermal boundary conditions was largest at the leading edge of the heated wall, becoming insignificant for non-dimensional streamwise locations exceeding approximately four slot widths away from the exit plane of the jet. The present results are for non-dimensional streamwise locations that are relevant to automotive windshield defogging/defrosting, which serves as the technological motivation for this study. Specifically, these results are for small non-dimensional streamwise locations compared to those in the published literature.

MIXED CONVECTION FROM HORIZONTAL TUBE BANKS BETWEEN TWO VERTICAL PARALLEL PLATES

The weighted residuals method is applied to analyze mixed convection heal transfer in 3 × 3 in-line horizontal tube bundles placed between two vertical parallel plates. The flow regime of Reynolds numbers up to 500 and Grashof number up to 53,000 is investigated. The local data of different geometries (nondimensional streamwise and transverse cylinder-to-cylinder spacing Sp / d = 2,3 and Sh / d = 2, 3) are reported. Data for both pure forced and mixed convection are presented for purposes of comparison. It is shown that the thermal performance of the front sector of downstream cylinders is strongly affected by the presence of the upstream cylinders, while the backward sector of downstream cylinders remains basically uninfluenced by the upstream cylinders. The average Nusselt number for the array increases 20–30% when the nondimensional streamwise spacing is increased 50%; however, fewer tubes would be contained in a fixed channel length.

Asymptotic Wake Behavior of Swept, Blunt Trailing-Edge Airfoils

The effect of base sweep and the addition of passive flow-control devices at constant base sweep angle (30 deg) on the asymptotic behavior of turbulent wakes produced by flatplate airfoils was experimentally examined. It was determined that values of the nondimensional streamwise velocity defect and wake thickness parameters for the grooved model with 30 deg swept base at fourteen base thicknesses downstream of the base at mid-span were closer to asymptotic values from empirical plane wake predictions than values for the 0, 30, and 45 deg swept baseline models and the 30 deg swept model with Wishbone vortex generators. The grooves apparently inhibited the three-dimensionality of the resulting wake flow.

Mean Streamwise Spacing of Organized Structures in Transitional and Developed Bounded Turbulent Flows

The mean periods of passage of organized structures detected by a wide variety of criteria in all regions of turbulent boundary layers, pipes, and channel flows over a range of two Reynolds number decades are compared to that of the transition 'spike'. When scaled with the local mean velocities and the shear layer thicknesses to form a nondimensional streamwise spacing, they fall within a fairly narrow range of TU/delta = 1.5-3.5. The streamwise spacing shows little variation across most of the shear layer for any particular data set.

SYNOPTIC: Boundary-Layer Computation by an N Parameter Integral Method Using Exponentials

Let X = nondimensional streamwise coordinate; F = nondimensional coordinate normal to X; U = nondimensional velocity in X direction; Ue = velocity at the edge of the boundary layer (prescribed); Uiab(Y) = tabulated experimental or analytical initial velocity profile; V = nondimensional velocity in F direction; F0 = nondimensional wall suction or blowing velocity (prescribed); fk(Y) = weighting functions, finite everywhere with /*(<») = 0; an(X) = unknown parameters in the velocity approximation; and a = prescribed exponent, here a = 1. The nondimensional laminar incompressible boundarylayer momentum equation,