Structures In Turbulent Channel Flow Research Articles

220 チャンネル乱流場における壁面圧力に基づく組織構造の解析(OS13-2 乱流現象の実験と数値シミュレーション(2),OS13 乱流現象の実験と数値シミュレーション)

220 チャンネル乱流場における壁面圧力に基づく組織構造の解析(OS13-2 乱流現象の実験と数値シミュレーション(2),OS13 乱流現象の実験と数値シミュレーション)

Heat transfer and flow structure in turbulent channel flow over protrusions

In this study, heat transfer characteristics and flow structures over protrusions in a turbulent channel flow were systematically investigated by DES numerically. Densely arranged protrusions with different depth ratios (h/D) are considered for Reynolds number Re2H (based on bulk velocity and full channel height) between 3000 and 6000 while Prandtl number Pr is fixed at 0.7. It is found that larger height ratio induces higher friction factor and heat transfer. However the performance factor, on the other hand, first increases then reaches its asymptotic limit and even decreases with increasing height ratio. It is also observed that the highest skin friction, form drag and localized Nusselt number are found at the upstream portion of protrusion. Additionally, the distributions of friction factors and Nusselt number exhibit symmetrical features for the low protrusion configuration and asymmetric characteristics for the high protrusion arrangement. The asymmetric distribution of localized Nusselt number at large height ratio (h/D⩾15%) is found to be closely linked to the asymmetric vortex shedding from the back ridge of protrusions.

Geometric study of Lagrangian and Eulerian structures in turbulent channel flow

We report the detailed multi-scale and multi-directional geometric study of both evolving Lagrangian and instantaneous Eulerian structures in turbulent channel flow at low and moderate Reynolds numbers. The Lagrangian structures (material surfaces) are obtained by tracking the Lagrangian scalar field, and Eulerian structures are extracted from the swirling strength field at a time instant. The multi-scale and multi-directional geometric analysis, based on the mirror-extended curvelet transform, is developed to quantify the geometry, including the averaged inclination and sweep angles, of both structures at up to eight scales ranging from the half-height δ of the channel to several viscous length scales δν. Here, the inclination angle is on the plane of the streamwise and wall-normal directions, and the sweep angle is on the plane of streamwise and spanwise directions. The results show that coherent quasi-streamwise structures in the near-wall region are composed of inclined objects with averaged inclination angle 35°–45°, averaged sweep angle 30°–40° and characteristic scale 20δν, and ‘curved legs’ with averaged inclination angle 20°–30°, averaged sweep angle 15°–30° and length scale 5δν–10δν. The temporal evolution of Lagrangian structures shows increasing inclination and sweep angles with time, which may correspond to the lifting process of near-wall quasi-streamwise vortices. The large-scale structures that appear to be composed of a number of individual small-scale objects are detected using cross-correlations between Eulerian structures with large and small scales. These packets are located at the near-wall region with the typical height 0.25δ and may extend over 10δ in the streamwise direction in moderate-Reynolds-number, long channel flows. In addition, the effects of the Reynolds number and comparisons between Lagrangian and Eulerian structures are discussed.

Structure of Turbulent Channel Flow under Spanwise System Rotation

Direct numerical simulation of turbulent channel flow under spanwise system rotation is performed. When system rotation is imposed on turbulent Poiseuille flow, the Reynolds number decreases in one side of a channel, while it increases in the other side. Hence, the effects of the Reynolds number as well as spanwise system rotation cannot be negligible in turbulent Poiseuille flow. To exclude the Reynolds-number effects, we performed the numerical simulation of open channel flow where the free-slip boundary condition is imposed on one wall, and the Reynolds number defined by the friction velocity and channel width can be always constant. When system rotation is imposed to enhance the spanwise vorticity of the mean shear, large-scale turbulence is attenuated and the mean velocity gradient increases over an entire channel. In contrast, when system rotation is imposed in the opposite sense to the mean spanwise vorticity, the mean velocity is leveled and small-scale turbulence is markedly decreased. We also investigate the effects of system rotation on the longitudinal vortical structure typically observed in near-wall turbulence.

Structure of Turbulent Channel Flow under Spanwise System Rotation

Direct numerical simulations of a turbulent channel flow under spanwise system rotation are performed. When the system rotation is imposed on the turbulent Poiseuille flow, the Reynolds number decreases in one side of a channel, while it increases in the other side. Hence, the effects of the Reynolds number as well as spanwise system rotation cannot be negligible in the turbulent Poiseuille flow. To exclude the Reynolds-number effects, we performed the numerical simulations of the open channel flow where the free-slip-boundary condition is imposed on a wall, and the Reynolds number can be set constant. When the spanwise system rotation is imposed to enhance mean spanwise vorticity associated with the mean shear, turbulence at the large scale is attenuated and the mean velocity gradient increases over the entire channel. In contrast, when the system rotation is imposed in the opposite sense to the mean spanwise vorticity, the mean velocity is leveled and turbulence at the higher wavenumbers is markedly decreased. We also investigate the effects of spanwise system rotation on the longitudinal vortical structure typically observed in the near-wall turbulence.

Viscoelastic effects on higher order statistics and on coherent structures in turbulent channel flow

In this work we study, using the results of direct numerical simulations [Housiadas and Beris, “Polymer-induced drag reduction: Viscoelastic and inertia effects of the variations in viscoelasticity and inertia,” Phys. Fluids 15, 2369 (2003)], the effects of changes in the flow viscoelasticity and the friction Reynolds number on several higher order statistics of turbulence, such as the Reynolds stress, the enstrophy, the averaged equations for the conformation tensor, as well as on the coherent structures through a Karhunen–Loeve (K-L) analysis and selected flow and conformation visualizations. In particular, it is shown that, as the zero friction Weissenberg number Weτ0 increases (for a constant zero friction Reynolds number Reτ0) dramatic reductions take place in many terms in the averaged equations for the Reynolds stresses and in all terms of the averaged enstrophy equations. From a Karhunen–Loeve analysis of the eigenmodes of the flow we saw that the presence of viscoelasticity increases significantly the coherence and energy content of the first few modes. The K-L dimension of the flow at Weτ0=125 is fully one order of magnitude lower than its Newtonian counterpart. As far as the effect of viscoelasticity is concerned, it is manifested primarily by changes in the boundary layer, which are mostly accomplished by Weτ0=50–62.5. In comparison, increasing the Reτ0, from 125 to 590, induces significant changes in various terms in the budgets, despite the fact that the drag reduction remains practically the same over that range. However, the near the wall region seems to change significantly only up to Reτ0=395, with few changes observed upon a further increase to Reτ0=590.

Structure of turbulent channel flow with square bars on one wall

The organised motion in a turbulent channel flow with a succession of square bars on the bottom wall has been investigated using direct numerical simulations. Several values of the ratio w/ k, where k is the bar height and w is the longitudinal separation between consecutive bars have been examined in detail. Relative to a smooth surface, the streamwise extent of the near-wall structures is decreased while their spanwise extent is increased. As w/ k increases, the coherence decreases in the streamwise direction having a minimum for w/ k=7. This is due to the outward motion occurring, most of all, near the leading edge of the elements. The Reynolds stress anisotropy tensor and its invariants show a closer approach to isotropy over the rough wall than over a smooth wall.

Model Simulations of Large-Scale Structures in Turbulent Channel Flow.

Model Simulations of Large-Scale Structures in Turbulent Channel Flow.

Coherent Fine Scale Structure in Turbulent Channel Flows.

Fine scale structure in near-wall turbulence is investigated by using DNS data bases of turbulent channel flows. Fine scale eddies in turbulent channel flows are educed by a new identification scheme. Characteristics of fine scale vortical structures in turbulent channel flow coincide with those in homogeneous isotropic turbulence and turbulent mixing layers. The most expected diameter and maximum azimuthal velocity of the fine scale eddy are about 10 times of the Kolmogorov microscale and about a half of the r.m.s. velocity fluctuation, respectively. These results show that streamwise vortices and hairpin-type vortices near the wall can be classified into the coherent fine scale structure in turbulence. Near the wall, probability density functions of inclination angles and tilting angles of the coherent fine scale eddy show regularity in their spatial distribution, which corresponds to the anisotophy of the near-wall turbulence.

Analysis of Organized Structure in Turbulent Channel Flow with Injection Using Conditional Sampling.

An organized structure in a fully developed turbulent channel flow with uniform injection is investigated using conditional sampling techniques. Mean frequency and duration time of bursting are obtained using the VITA method. The contributions to uv in the four quadrants of the u-v plane are examined by means of the quadrant analysis technique. The frequency and duration time of bursting increase with increasing injection rate. The contribution of the sweep to Reynolds shear stress increases near the wall region. In the core region of the channel, the contribution of the ejection increases. These results are consistent with the finding that the Reynolds shear stress increases due to injection.

Plane waves and structures in turbulent channel flow

A direct simulation of turbulent flow in a channel is analyzed by the method of empirical eigenfunctions (Karhunen–Loève procedure, proper orthogonal decomposition). This analysis reveals the presence of propagating plane waves in the turbulent flow. The velocity of propagation is determined by the flow velocity at the location of maximal Reynolds stress. The analysis further suggests that the interaction of these waves appears to be essential to the local production of turbulence via bursting or sweeping events in the turbulent boundary layer, with the additional suggestion that the fast acting plane waves act as triggers.

On the shape and dynamics of wall structures in turbulent channel flow

Turbulence-producing events in turbulent channel flow were found to be predominantly associated with asymmetric vortical structures rather than pairs of counter-rotating structures. An asymmetry-preserving averaging scheme was devised, allowing a picture of the ‘‘average’’ structure that more closely resembles the instantaneous one to be obtained. In addition, these structures were found to persist for long distances with little change while convecting downstream.

開水路乱流における秩序構造の階層性

Coherent structures in turbulent channel flow are investigated using a visualization technique that utilizes fluorescent dye excited by a sheet of laser light. The results indicate the existance of the hierarchy of the coherent structures and the interaction of coherent motions in the inner and outer layer of turbulent channel flow. To further understand the characteristics of the coherent motions, three dimensinal structures of the streamwise vortex in the inner layer and large-scale structure in the outer layer are illustrated based on Taylors hypothesis, and discussed in detail.

On the structure of turbulent channel flow

Hot-film measurements of the streamwise velocity component were carried out in a fully developed turbulent water-channel flow for three different Reynolds numbers (13800, 34600 and 48900). The results for the first four statistical moments complement and extend the results from previous studies of turbulent channel flow. The VITA variance technique waa employed to detect deterministic events in the streamwise velocity. It waa demonstrated that the VITA technique has a band-pass-filter character. The number of events detected was found to decrerrae exponentially with the threshold level and the events occupy a wide range of timescales. This makes it impossible to define one unique frequency of occurrence or one unique duration of the events. However, by using this technique information was obtained on the amplitude and timescale distributions of the events. The chmacteristic features of the conditional iverages were found to be related to the skewness and flatness factors.