Turbulent Wall Flows Research Articles

Laws of Incompressible-Fluid Turbulent Near-Wall Flows Following from the Limiting Asymptotic Properties

The averaged layered turbulent wall flows of an incompressible fluid are considered for an arbitrary wall roughness and a friction coefficient tending to zero. It is assumed that in the asymptotic limit the flow may be divided into two regions: an outer, only slightly disturbed region, and a thin wall region. A similarity law for the velocity disturbances in the outer region is established for cylindrical tubes with an arbitrary cross section. Using the asymptotically matched solutions in the inner and outer regions, the existence of a logarithmic velocity profile in the matching zone is proved. On the basis of a unique asymptotic approach, the existing semi-empirical solutions for tube flows are brought into consistency. A number of semi-empirical formulas for the velocity profiles, which refine and expand the range of applicability of existing relations, are proposed.

Low-dimensional dynamics of a turbulent wall flow

The low-dimensional dynamics of the structures in a turbulent wall flow are studied by means of numerical simulations. These are made both ‘minimal’, in the sense that they contain a single copy of each relevant structure, and ‘autonomous’ in the sense that there is no outer turbulent flow with which they can interact. The interaction is prevented by a numerical mask that damps the flow above a given wall distance, and the flow behaviour is studied as a function of the mask height. The simplest case found is a streamwise wave that propagates without change. It takes the form of a single wavy low-velocity streak flanked by two counter-rotating staggered quasi-streamwise vortices, and is found when the height of the numerical masking function is less than δ+1 ≈ 50. As the mask height is increased, this solution bifurcates into an almost-perfect limit cycle, a two-frequency torus, weak chaos, and full-edged bursting turbulence. The transition is essentially complete when δ+1 ≈ 70, even if the wall-parallel dimensions of the computational box are small enough for bursting turbulence to be metastable, lasting only for a few bursting cycles. Similar low-dimensional dynamics are found in somewhat larger boxes, containing two copies of the basic structures, in which the bursting turbulence is self-sustaining.

A near-wall two-equation heat transfer model for wall turbulent flows

A proposed near-wall t 2 –ε t two-equation model for turbulent heat transport reproduces the correct near-wall behavior of temperature under various wall thermal boundary conditions. In this model, a mixing timescale is introduced to model the production term of ε t equation, and a more convenient boundary condition for ε t under the uniform wall heat flux is suggested. The present model is tested through application to turbulent heat transfer for channel flow. Predicted results are compared with direct numerical simulation (DNS) data. The near-wall t 2 –ε t two-equation model predicts reasonably well the distributions of the time-mean temperature, normal turbulent heat flux, temperature variance, dissipation rate and their near-wall budgets.

Spectral features of the wall-pressure fluctuations in turbulent wall flows with and without perturbations using LES

The statistical properties of the fluctuating wall-pressure field are studied using a numerical database generated by large eddy simulation of incompressible turbulent flow in a plane channelOResa 640U. Emphasis is given on the study of spectral features in the wavevector space and as a function of frequency, particularly in the region of low wavenumbers that are of major concern in submarine applications. A correction procedure is used to cancel the artificial pseudo-acoustic eAect due to periodic boundary conditions. The influence of superimposed perturbations on one wall of the channel is considered afterwards in comparison with the non-perturbed case. ” 2001 Elsevier Science Inc. All rights reserved.

Direct numerical simulation of wall turbulent flows with microbubbles

The marker-density-function (MDF) method has been developed to conduct direct numerical simulation (DNS) for bubbly flows. The method is applied to turbulent bubbly channel flows to elucidate the interaction between bubbles and wall turbulence. The simulation is designed to clarify the structure of the turbulent boundary layer containing microbubbles and the mechanism of frictional drag reduction. It is deduced from the numerical tests that the interaction between bubbles and wall turbulence depends on the Weber and Froude numbers. The reduction of the frictional resistance on the wall is attained and its mechanism is explained from the modulation of the three-dimensional structure of the turbulent flow. Copyright © 2001 John Wiley & Sons, Ltd.

529 壁乱流の組織構造(流体工学III)

In this paper, wall shear stress fluctuation and simultaneous measurement of three components of the velocity in the turbulent boundary layer were carried out in order to clarify the whole image of ensemble average of flow field in ejection and sweep event which are organized motion of wall turbulent flow. The technique of Furukawa et al was used in order to identify ejection and sweep event from the wall shear stress fluctuation.

A note on “critical roughness height” and “transitional roughness”

An unrigorous but plausible analysis suggests that the concept of a critical roughness height, below which roughness does not affect a turbulent wall flow, is erroneous. The Oseen approximation implies that the effect of roughness on the additive constant in the logarithmic law of the wall should vary as the square of the roughness Reynolds number (specifically the roughness height in “wall units”). This is an important point in determining whether surfaces used in experiments at high unit Reynolds number can be regarded as hydraulically smooth. Attention is also called to the qualitative difference between Nikuradse’s measurements of friction factor in pipe flow with uniform-size sand-grain roughness in the “transitional” range of Reynolds number and the data correlation in the Moody chart of 1944; the latter was derived from tests on miscellaneous real-life rough surfaces in the 1930s. Nearly all textbooks on elementary fluid dynamics present, but practically none discuss, this difference. Nikuradse’s monodisperse roughness is a very rare case with untypical behavior in the transitional range.

Wall Turbulent Flow with Mean Skewed Velocity. 1st Report. Mean Velocity and Statistic Properties of Turbulence.

The skewed wall turbulent flow, driven by sudden span wise wall motion superimposed on plane Poiseulle flow, has been simulated by Direct Numerical Simulation (DNS). The time evolution of the mean velocities and the Reynolds stresses have been studied. Just after the spanwise wall motion starts the flow skewness is confined to the narrow wall region (y+<30) and the flow soon becomes another collateral flow again. During the evolution stage, turbulent kinetic energy first decreases and then increases. The time when q^-2 takes its minimum depends on the Reynolds number of the flow. The difference between shear driven and pressure driven skewed flow is also shown. The production and pressure strain terms in the Reynolds stress transport equation are shown to play an important role in leading to non parallel vectors of velocity gradient and Reynolds shear stresses.

A qualitative assessment of the role of a viscosity depending on the third invariant of the rate-of-deformation tensor upon turbulent non-Newtonian flow

The numerical simulation of some non-Newtonian effects in wall and wall-free turbulent flows, such as drag reduction in pipe flows or the decrease in transverse normal Reynolds stresses, has been attempted in the past with a limited degree of success on the basis of modified wall functions applied to traditional turbulence models ( k– ε), rather than through more realistic rheological constitutive equations. In this work, it is qualitatively shown that if the viscosity function of a generalised Newtonian fluid is assumed to depend on the third invariant of the rate of deformation tensor, there is an increase of the viscous diffusion terms, but especially, of the dissipation of turbulence kinetic energy by a factor equal to the Trouton ratio of the fluid, divided by the Trouton ratio of the solvent, thus indicating a possible way to improve rheological–turbulence modelling.

Turbulence Statistics of CouettePoiseuille Turbulent Flow. 2nd Rerort. Higher Order Turbulence Statistics.

Turbulence statistics in Couette·Poiseuille flow are obtained by measurements. These include correlation coefficient, skewness and flatness factors and four-quadrant analysis of Reynolds shear stress-puv^-. In the region of y+&le;30∼40, the distributions of all these quantities are only affected by non-dimensional parameter μ(≡μ3*/av), as the mean velocity and the turbulence intensities profiles are. The four-quardrant analysis shows that the fractional contribution from 4th-quadrant is affected largely by parameter μ whereas that from 2nd-quadrant remains unaffected. In the case of 0<μ&le;94, the fractional contribution from 4th-quadrant is greater than that from 2nd-quadrant, unlike the conventional wall turbulent flow.

On Reynolds-stress expressions and near-wall scaling parameters for predicting wall and homogeneous turbulent shear flows

Abstract In this paper, a new type of two-equation turbulence model that incorporates some essential characteristics of second-order closure models is proposed. The present model belongs to a nonlinear k−ϵ model taking into account low-Reynolds-number effects originating from the physical requirements, and is applicable to complex turbulent flows with separation and reattachment. The model successfully predicts both wall-turbulent and homogeneous shear flows, the latter of which has been very difficult to simulate with existing two-equation turbulence models. Channel flows with injection and suction at wall surfaces and separated and reattaching flows downstream of a backward-facing step are also calculated. Comparisons of the computational results with the measurements and the direct numerical simulation data indicate that the present model is effective in calculating complex turbulent flows of technological interest. Furthermore, the parameters for scaling the near-wall region in the low-Reynolds-number model functions are re-evaluated, yielding some insights into the near-wall scaling parameters for application to complex turbulent flows.

Turbulent Bursting-Based Sediment Entrainment Function

One of the basic impediments to a clear understanding of a variety of fundamental problems in the context of sediment transport has been the lack of a well-grounded formulation of bed sediment entrainment. This is dealt with herein, physically based on the mechanism that bed sediment particles are actually entrained by the bursting process inherent in wall turbulent flows. A simple theoretical model for sediment entrainment from flat, loose bed is established using the averaged bursting period scaled on inner variables and the spatial scales of turbulent bursts. Sediment entrainment is shown to depend strongly on bed-shear velocity. The theoretical entrainment flux is compared to available laboratory data sets covering both hydraulically smooth and transitional bed situations. Generally good agreement is obtained, representing the best performance of the present model in relation to existing entrainment functions. It appears to characterize a rather encouraging aspect for a new approach to sediment transport, given the enhanced understanding of turbulent bursting.

Diffusion of Matter in the Wall Turbulent Flow. 5th Report. Local Similarity of the Fluctuating Concentration Field of the Plume from a Wall Point Source in a Turbulent Pipe Flow.

In this report, we extend the similarity argument used in the previous report (1) to the mean square concentration fluctuation equation on the symmetrical plane of the wall point source plume in a turbulent pipe flow to find the similar solution for the mean square concentration fluctuation distribution. The main assumptions are the Lagrangean similar hypothesis and an appropriate eddy diffusivity distribution to the triple correlation term and the mixing length theory. It is found that the distribution of the similar solution shows good agreement with experimental data in a pipe flow. In the case of a wall point source in a turbulent boundary layer on a flat plate, the same argument is appliciable, and the similar solution obtained shows good agreement with the experimental data. And the budget of the fluctuating concentration intensity is investigated on the basis of the similar solution and the mean square concentration fluctuation equation.

Diffusion of Matter in Wall Turbulent Flow. (4th Report. Local Similarity of Mean Concentration Field of the Plume from a Wall Point Source in Turbulent Pipe Flow).

The aim of this study is to find similar solutions of the ordinary differential equation for the suitably normalized function in order to describe the mean concentration distribution, the similarities of which were experimentally observed in the some region downstream of a wall point source plume in a turbulent pipe flow. Solutions of these equations are compared with experimental data, which were obtained in an almost fully-developed water turbulent pipe flow (ReD=20000) that has been realized in a horizontal straight pipe of inner diameter 100 mm. It is found that these solutions show good agreement with experimental data, and the widths of similar profiles are strongly affected by the curvature of the boundary of the flow field. This is mainly caused by the diffusion term in a conservation equation for the mean concentration field. Also, the ordinary differential equation for the normalized mean concentration function in the case of a wall point source plume in a turbulent boundary layer on a flat plate is found to be a special case of the corresponding equation for a turbulent pipe flow.

96/06792 A two-equation heat transfer model reflecting second-moment closures for wall and free turbulent flows

96/06792 A two-equation heat transfer model reflecting second-moment closures for wall and free turbulent flows

A two-equation heat transfer model reflecting second-moment closures for wall and free turbulent flows

A new two-equation heat transfer model which incorporates some essential features of second-order modeling is proposed. The present model is applicable to heat transfer problems in both wall and free turbulent flows not unusually deviating from the equilibrium state. Furthermore, by introducing the Kolmogorov velocity scale, the model can appropriately express the low Reynolds number effects in the near-wall region and is also applicable to complex heat transfer fields with flow separation and reattachment. It is shown that the proposed model predicts quite successfully heat transfer in both wall and free turbulent flows; i.e., a homogeneous isotropic decaying flow, a homogeneous shear flow, a boundary-layer flow heated from the origin, and a boundary-layer flow subjected to a sudden change in the wall-heating condition; whereas, such predictions have been almost impossible with existing two-equation heat transfer models.

Scaling of mean square vorticity in turbulent flows

In self-preserving turbulent wall flows, both measurements and direct numerical simulations indicate that the mean square vorticity in the outer region does not depend only on δ and Uτ. It also depends on the Reynolds number.

Diffusion of Matter in Wall Turbulent Flow. 2nd Report. Statistical Properties of the Concentration Fluctuation of a Wall Point Source Plume in a Pipe Flow.

A wall point source plume, that is emitted to a wall turbulent shear flow, will be mixed and convected by the turbulent flow which scale varies with distance from the wall. This feature makes the wall point source plume show the bimodal feature in its concentration fluctuation held. In this paper, the statistical properties of the concentration fluctuation of the wall point source plume in a turbulent pipe flow will be mainly reported. Vertical profiles of the integral scale in several cross sections downstream from a wall point source show self-similarity and bimodal distributions like as in a point source plume in a turbulent boundary layer on a flat plate. In addition, vertical profiles of the dissipation of the concentration fluctuation show also self-similarity. Kullback-Leibler divergence of PDFs of the concentration will also be discussed.

Diffusion of Matter in the Wall Turbulent Flow. 3rd Report. Conditional Statistics of the Concentration Fluctuation of a Wall Point Source Plume in a Pipe Flow.

A plume released into a turbulent flow shows an intermittent structure owing to the transport process definitely regulated by the flow field. Conditional statistics for a wall point source plume released into a turbulent pipe flow will be discussed. The distributions of the intermittency factor, conditional mean and r. m. s. fluctuation concentration show self-similarity. In particular, the slope of the distribution of the intermittency factor is more moderate than the one in the turbulent boundary layer on a flat plate. Spectra of intermittency functions reveal the property of detection Patterns of concentration in a wall point source plume. The KL-divergence between PDFs of the fluctuating concentration and a log-normal distribution is almost zero in the near wall region, and the distributions of the KL-divergence value are also self-similar in the downstream direction.

Coherent motions and heat transfer in a wall turbulent shear flow

In wall turbulence, it is widely accepted that the coherent motions determine the essential features of turbulent transport phenomena. In the present study, we have refined a trajectory-based detection algorithm for coherent motions and have investigated the relationship between coherent motions and scalar (heat) transfer from a structural point of view, i. e. trajectory analysis of the VITA heat transfer events, extraction of key flow modules and the relevant heat transport, and the prediction of passive scalar transfer by means of an autoregressive (AR) model. As a result, it is shown that the phase relationship of fluctuating velocity components dominates the essential characteristics of the transport processes of heat and momentum in wall turbulence and there exist distinct differences in individual correspondence between the coherent motions and heat transport processes, neither of which can be revealed by the widely used VITA technique. Also, the AR model is shown to provide good time-series predictions for turbulent heat transfer associated with coherent structures near the wall.