Strong Shear Flow Research Articles

Numerical Study on Heat Transfer of an Impinging Turbulent Plane Jet with Confined Wall.

The present paper describes the numerical results of the fluid flow and heat-transfer characteristics of the impinging turbulent plane jet with the confined wall. Numerical results were obtained in the range of the nondimensional height from H=1.0 to 3.0. The Reynolds number based on the nozzle diameter and mean velocity was set to 8000. According to the experimental results, it had been shown that two peaks of the distribution of local heat-transfer coefficient exist along the impingement wall. To the best of our knowledge, no numerical solution has been obtained for the heat-transfer characteristics of the impinging jet in a very narrow channel. We applied a low Reynolds number version of the anisontropic k-e model of turbulence, i.e., the Myong-Kasagi model, and a WET model for the turbulent heat fluxes to simulate this turbulent impinging flow and heat transfer. The simulation results were in a very good agreement with the experimental one. We could predict the secondary peak of the local heat-transfer distribution on the impingement wall. The reason why the secondary peak appeared was determined. We pointed out that both treatments of low Reynolds number modeling and this anisotropic k-e model of turbulence were very important for this very strong anisotropic turbulent shear flow.

Non-Newtonian Turbulent Lubrication Theory Based on Friction Law of Fluid. Formulation for One-dimensional Lubrication Flow.

This paper describes a new theoretical approach to formulate the non-Newtonian turbulent lubrication theory based on the friction law of fluid. Combining the concept of equivalent pressure flow and resistance formula for pressure flow in the lubrication films, the one-dimensional generalized turbulent lubrication equation considering the non-Newtonian effect of lubrication fluid is derived, in which the equation for non-Newtonian turbulence coefficient is greatly simplified under the assumption of strong shear flow. The non-Newtonian turbulent lubrication equation obtained is applied to the inclined long slider bearing, and the bearing performance such as pressure distribution, load capacity and friction force is analyzed. From the results, the effect of non-Newtonian characteristics on the turbulent lubrication behaviour of long slider bearing is clarified.

Modeling the ionospheric response to artificially produced density enhancements

The motion of plasma density enhancements (barium clouds) artificially introduced into the postsunset equatorial F region is investigated with a two‐dimensional model incorporating flux tube integrated quantities. The temporal development of the ionosphere, in which the density perturbations are imbedded, is derived from a one‐dimensional set of relations modeling plasma transport and the vertical electric field from initial conditions and a prescribed variation of the horizontal electric field as a function of time. The calculations show that the strong horizontal shear flows existing at the nighttime F region ledge (where the perturbations were placed) reduce the growth of polarization fields associated with the enhancements and adjacent relative depletions of plasma for weak perturbations. The reason is that the perturbations develop a strong tilt with respect to the horizontal. More massive density perturbations lead to stronger drop velocities with respect to the rising ambient plasma. At a later time they develop secondary horizontal density perturbations on the side unstable to E × B drift instability due to the motion of neutral constituents. When rising “bubbles” of low density are produced, they (1) form on the steepened eastward side of the enhancement perturbation, (2) have a width comparable to the scale size of the enhancement perturbation, and (3) are most easily produced when the enhancement perturbation size is comparable to the scale height of the integrated density. These simulations show why experimental efforts of initiating rising bubbles and equatorial spread F have not been successful. The experiments require larger‐scale and stronger density perturbations than what can be achieved with conventional sounding rocket releases of barium vapors.

地面効果翼艇マリンスライダー： ミュースカイの模型の低速六分力風洞試験

Six-component measurements of a model of a wing-in-ground effect craft (WIG) “Marine Slider: μ sky” were conducted in a low speed wind tunnel using a running belt as the ground plane. The “μ sky-1” was an experimental marine craft with a simple ram wing. Its wing section was a high cambered thin section to improve its performance in the ground effect. The wing of the test model was a square wing of the aspect ratio 0.625 with tip plates extending downward from both tips. Results obtained show that the ground effect is remarkable for this wing. The drag was reduced not enough. The reason was that there was a strong shear flow in a narrow clearance between the hull bottom and the ground surface. The shear flow increased the drag.

Verification of three-dimensional laser Doppler velocimeter measurements

An experimental comparison method is proposed for the verification of mean flow and turbulence measurements obtained with a three-dimensional laser Doppler velocimeter system. Such measurements can include large errors caused by problems unique to three-dimensional systems. Direct comparisons of laser and hot cross-wire measurements obtained in two-dimensional flows, as is the common practice, will not bear out all the errors associated with three-dimensional laser systems. It is proposed here that the errors may be adequately quantified by making the direct comparisons in a weakly three-dimensional turbulent shear flow. The weak three-dimensional flow ensures high accuracy of the cross-wire data while still generating sufficiently strong secondary mean flow and Reynolds shear stresses so that all the laser measurements may be fully verified. This type of shear flow is easily generated by introducing a weak streamwise vortex into a nominally two-dimensional turbulent shear layer.

Suppression of excluded-volume exponents in shear flow of dilute polymer solutions.

The analogy between polymer and binary critical fluid dynamics in simple shear flow is used to construct a renormalization-group analysis of the scaling exponent \ensuremath{\nu} for the end-to-end distance of an isolated long polymer chain in a good solvent undergoing strong shear flow. The shear flow induces a crossover from the usual fixed point with the good solvent Flory exponent \ensuremath{\nu} to a new, strong shear fixed point with the classical exponent \ensuremath{\nu}=(1/2. Based on the blob picture we argue that the dynamic exponent z is not affected by the flow and remains at its zero shear, z=3 value. Experimentally observable consequences of this result are also discussed.

TRANSPORT MODELS OF THE TURBULENT VELOCITY-TEMPERATURE PRODUCTS FOR COMPUTATIONS OF RECIRCULATING FLOWS

Performance tests of the third-order turbulence closure for predictions of separating and recirculating flows in backward-facing steps were studied. Computations of the momentum and temperature fields in the flow domain being considered entail the solution of time-averaged transport equations containing the second-order turbulent fluctuating products. The triple products, which are responsible for the diffusive transport of the second-order products, attain greater significance in separating and reattaching flows. The computations are compared with several algebraic models and with the experimental data. The prediction was improved considerably, particularly in the separated shear layer. Computations are further made for the temperature-velocity double products and triple products. Finally, several advantages were observed in the usage of the transport equations for the evaluation of the turbulence triple products; one of the most important features is that the transport model can always take the effects of convection and diffusion into account in strong convective shear flows such as reattaching separated layers while conventional algebraic models cannot account for these effects in the evaluation of turbulence variables.

梅雨前線帯のmeso-α-scale convective systemの発達と微細構造

Part I of the present study has shown that a long lived meso-α-scale convective system (MαCS) during 14-15 July 1979, which was accompanied by a weak Baiu frontal depression consisted of meso-β-scale convective systems (MβCSs) and the development/decay and movement of MβCSs had strong influence on the behavior of the MαCS.In Part II, the fine structure of precipitation was studied using dense surface observation data over western Japan (Kyushu). MβCSs consisted of a few meso-γ-scale convective systems (MγCSs), which had the life-time of∼3 hours and brought precipitation of 1-5×108 tonne. Although MγCSs developed in the eastern, central and trailing portions of the depression, the large part of the precipitation over Kyushu was brought about by MγCSs generated around the trailing portion. These MγCSs were generated over the western coast of Kyushu.Under the condition of strong vertical wind shear in the trailing portion, MγCSs showed remarkable band structures generated over the western coast of Kyushu. The intense precipitation (maximum 100mm/hour) in this area was not associated with strong gusts nor with temperature fall.The orographic influence of Kyushu on the generation of MγCSs and the possible role of symmetric instability in the strong vertical shear flow on the formation of precipitation bands were discussed.

On the distribution of included angles for the Rouse chain in strong steady shear flow

The distributions of included angles for Rouse chains in steady shear flow are calculated exactly in the limit of large shear rate. Brownian dynamics simulations confirm the exact results.

Depolarized light scattering from simple liquid under shear flow: Nonequilibrium molecular dynamics study

Shear flow effects were studied on the depolarized light scattering from simple fluids by use of nonequilibrium molecular dynamics calculation in N = 108 Lennard-Jones systems simulating argon-like fluids near the triple point. The four-particle interaction term C 4 xz of the depolarized light scattering was found to be greatly enhanced by strong shear flow in the x − z plane, and strong depolarized light scattering is expected from simple liquids at high shear rates.

Effect of hydrodynamic interactions between the particles on the rheological properties of dilute emulsions

The form of mean stress tensor in monodisperse emulsions is studied in the second-order approximation with respect to the volume density of the particles, for a number of flows which are of rheological interest. It is shown how the particular features of the two-particle interaction between liquid spheres, especially the non-zero differences between the normal stresses in shear flows, give rise to non-Newtonian properties of the emulsion. We know /1, 2/ that in the case of the second-order approximation with respect to the volume concentration of the suspended disperse pháse the mean stress tensor is expressed in terms of two-particle interactions in a linear velocity field, and of the binary correlation function. The binary function is formed under the action of the macroscopic flow. Specific results, however, were obtained only for suspensions and rigid spheres /1, 2/. The present paper deals with the structural model of fluid spheres of equal radius, with hydrodynamic and “contact” interactions. A number of fundamental deviations from /1/ exist in the case of rheologically strong flows, since drop flocculation-deflocculation processes must be considered (i.e. the formation and disruption of aggregates). A strict analysis is given within the framework of the model, of the effect of these processes on the binary correlation function. A connection between the model of “contact” interaction and the result of the D.L.V.O. theory /3–5/ is considered. Numerical values are obtained for the Trouton viscosity in strong rheologically axisymmetric expanding flows. The differences in normal stresses in a strong shear flow are obtained and an approximate estimate is given for the shear viscosity and compared with experimental data /6/. A method given in /2/ is used to compute the effective viscosity of the emulsion in arbitrary, rheologically weak flows in which Brownian motion predomaintes. Considerable use is made of the exact computational methods and asymptotic representations of hydrodynamic functions determining the pairwise interaction of fluid spheres /7–9/.

A comprehensive erosive-burning model for double-base propellants in strong turbulent shear flow

A comprehensive aerothermochemical model of erosive burning of double-base propellants has been developed. The present analysis includes detailed modeling of the combustion process, taking into account both chemical kinetics and diffusion effects in the gas phase. Different reactions simulating the fizz zone and the final flame zone (including dark and luminous zones) are considered in the gas phase. The instantaneous governing partial differential equations are Favre averaged to take into account variable density effects. The turbulence modeling consists of a two-equation ( k-ϵ) turbulence closure model for the final Favre-averaged conservation equations. The set of governing equations is solved numerically. Predicted results compare well with experimental data of Burick and Osborn for a noncatalytic double-base propellant. Predicted temperature profiles in the boundary layer correctly depict the characteristics of double-base propellant combustion. The main mechanisms for augmentation of the burning rate is the increase in heat feedback caused by turbulence-enhanced reactions and transport properties in the fizz zone.

The generalized Lagrangian-mean equations and hydrodynamic stability

The generalized Lagrangian-mean (GLM) formulation of Andrews & McIntyre (1978 a , b ) offers alternative physical concepts and possible saving of effort in calculation, as compared with the more conventional Eulerian-mean approach. Though most existing applications of this theory concern waves on weakly sheared mean flows, it is also suitable for study of waves in strong shear flows. The hydrodynamic stability of parallel shear flows is examined from this point of view. An appreciation is gained of the roles of Stokes drift, pseudomomentum, energy and pseudoenergy in this context, such understanding being a necessary prerequisite for future developments. Several known results of linear stability theory, including the inflexion-point and semicircle theorems, are concisely rederived from the GLM conservation laws.

Dynamics of a Resistive Sheet Pinch

The dynamic evolution and the saturated state of a long sheet pinch subject to growth of resistive tearing modes was investigated by numerical solution of the 2D MHD equations. Both the compressible and the incompressible equations were used, and the difference is found to be negligible. The necessity of considering a resistive equilibrium is stressed. The paper concentrates on a static equilibrium maintained by an external electric field and requiring a special distribution of the resistivity η. In addition the dynamics of the resistivity plays an important part. Assuming η to be time independent, the sheet pinch develops a number of soliton-like magnetic islands, which coalesce. The final state consists of a single soliton, while the generation of further sol-itons is inhibited by a strong shear flow allong the current sheet. When allowance is made for parallel diffusion of the resistivity such that η is essentially a flux function, the final state is quite different. Here the longest wavelength dominates, leading to a single, large island and completely destroying the original sheet pinch

Rayleigh-Taylor and Kelvin-Helmholtz Instabilities in Targets Accelerated by Laser Ablation

With use of the FAST 2D laser-shell model, the acceleration of a 20-\ensuremath{\mu}m-thick plastic foil up to 160 km/s has been simulated. It is possible to follow the Rayleigh-Taylor bubble-and-spike development far into the nonlinear regime and beyond the point of foil fragmentation. Strong shear flow develops which evolves into the Kelvin-Helmholtz instability. The Kelvin-Helmholtz instability causes the tips of the spikes to widen and as a result reduce their rate of fall.

Stellar convection theory. III - Dynamical coupling of the two convection zones in A-type stars by penetrative motions

The thermal convection occurring over many density scale heights in an A-type star outer envelope, encompassing both the hydrogen and helium convectively unstable zones, is examined by means of anelastic modal equations. The single-mode anelastic equations for such compressible convection display strong overshooting of the motions into adjacent radiative zones, which would preclude diffusive separation of elements in the supposedly quiescent region between the two unstable zones. In addition, the anelastic solutions reveal that the two zones of convective instability are dynamically coupled by the overshooting motions. The two solutions that the nonlinear single-mode equations admit for the same horizontal wavelength are distinguished by the sense of the vertical velocity at the center of the three-dimensional cell. It is suggested that strong horizontal shear flows should be present just below the surface of the star, and that the large-scale motions extending into the stable atmosphere would appear mainly as horizontal flows.

Eddy energy in the oceans

Observations of surface drift currents made by merchant ships are used to calculate the kinetic energy of the mean flow as well as the kinetic energy of the fluctuations, which is interpreted as eddy kinetic energy. The distribution of these properties is charted for the North Atlantic Ocean based on 1° squares and for the world oceans based on 5° squares. Both distributions show essentially the same features, namely, high values in the western boundary currents and in the equatorial current system and low values in the subtropical gyres. The ratio between mean energy and eddy energy is high (about 1 to 2) in the strong currents and low (about 1/20 to 1/40) in the central and eastern portions of the gyres. Comparing mean and eddy energies in ocean and atmosphere, it becomes apparent that eddy energies in the two systems are uncorrelated. The results are consistent with the idea that eddy motion in the ocean is generated in areas of strong mean shear flow and is subsequently distributed over the whole ocean.

Wind-wave-current System In Coastal Ocean

Observational discoveries of coastal current structure in Ogata Coast, Japan, were summarized with respect to characteristics of wind and waves in the wave-shoaling region. The observations of flow structure under the storm condition (strong wind & high wave) showed that (1) the coastal longshore current that has a vertically uniform flow profile, was developed in the wide area of coastal zone including the surf zone, and (2) strong shear flow with undertow (offshore-going near-bottom current) was developed in the surf zone. As a generation mechanism of such flow structures, it was designated that sea surface stress was emphasized under storm condition with the energy transfer from wind to waves through whitecap dissipation of shoalkg waves. A mathematical model of whitecaps dissipation stress,z,, ,was proposed as an energy transfer interface between atmosphere and coastal ocean. Using this interface model, numerical Wind-Wave-Current System (WWC System) was established, in which the relation between wind stresses (wind field), whitecap breaker stresses (wave field) and bottom stresses (current field) were integrated. Enhancing effects of whitecap dissipation stress due to wave shoaling was also taken into consideration in the system.