Large Eddy Motion Research Articles

ウェーブレット自己相関法を用いた拘束噴流の流れ構造の解析

To investigate the unsteady structure of secondary flow of a turbulent bounded jet in both frequency and physical spaces, the wavelet auto-correlation analysis was used to analyze the fluctuating velocities at various positions in abounded jet. The wavelet auto-correlation analysis provides the auto-correlation characteristics of signals in terms of period and time delay. From the wavelet auto-correlation analysis, the branching structures in WR(a, τ) reveal that a periodic large eddy motion contains the periodic small eddy motions. In the shear layer near the wall at X/b0=8.0, Y/b0 =1.0 and Z/b0=1.9, the wavelet auto-correlation analysis may determine the period and the location of the large-scale structure and the secondary flows.

238 感潮域の河床凹部を遡上する下層密度流のPTV可視化計測

Experiments on a salt-water flow running up over a trench are carried out on the basis of a flow-visualization technique to consider the local hydraulic characteristics of flows around the trench, which is necessary for maintaining or reproducing the function of trench as a habitat. The flow regime inside and outside the trench significantly depends on the aspect ratio of a opening length to a step height of trench. In case that the opening length of trench is shorter than the reattachment length, large eddy motions induced periodically inside the trench by the vortex shedding from the step. The eddies have direct effects upon momentum exchange through the trench-mouth, and reduce the magnitude of velocity inside the trench.

A144 ウェーブレット自己相関による拘束噴流内の二次流れの解析

To investigate the unsteady structure of secondary flow of a turbulent bounded jet in both Fourier and physical spaces, the wavelet auto-correlation analysis was used to analyze the fluctuating velocities at various positions in a bounded jet. The wavelet auto-correlation analysis provides the auto-correlation characteristics of signals in terms of period and time delay. From the wavelet auto-correlation analysis, the branching structures in WR(a, τ) reveal that a periodic large eddy motion contains the periodic small eddy motions. In the shear layer near the wall at x/b0=8, y/b0=1 and z/b0=1.87, the wavelet Auto-correlation analysis may determine the period and the location of the large-scale structure and the secondary flows.

Numerical Simulation of Dispersed Two-Phase Turbulent Flow with GAL-LES Model. Application to a Plane Bubble Plume.

The authors have recently developed a new framework for multiphase LES. The framework is based on a new formulation of discrete-particle motion, referred to as GAL model. For extending the applicability of the new multiphase LES (GAL-LES model), a numerical simulation is performed on a plane bubble plume, as a typical gas-liquid phase turbulent flow. The present model has succeeded in reproducing the large-eddy motion of the fluid phase, which is found to be closely related to the discharge of the bubble at the inlet slot. The comparison between the computational and experimental results for the patterns of the bubble concentration demonstrates the validity of the GAL-LES model.

Concerning the effects of mean shear on the fine-scale structure of concentration fluctuations in a plume dispersing in the atmospheric surface layer

The effect of mean shear (large-eddy motion) on the small-scale plume concentration structure is investigated experimentally using high-resolution plume concentration fluctuation measurements made simultaneously at two different heights in a plume dispersing in the very high-Reynolds number turbulent flow of the atmospheric surface layer (where the Taylor microscale based Reynolds number Re λ ≈ 5200). The generalized dimension spectra of a one-dimensional surrogate of the concentration variance dissipation rate are presented as a function of the mean shear. It is shown that the mean shear (due to the large-eddy motions in the flow) have a persistent effect on the small-scale plume concentration structure even at very high Reynolds numbers. In particular, it is demonstrated that increasing mean shear tends to reduce small-scale spatial intermittency in the concentration fluctuations (viz., the higher the mean shear, the more space-filling and less singular the associated local dissipation rate of concentration variance becomes).

115 WAVELET ANALYSIS OF A BOUNDED JET

In thispaper, in order to reveal the coherent structure of a turbulent bounded jet in both Fourier and physical spaces, the wavelet transform and local wavelet Reynolds stress are used to analyze the large eddy motions and the secondary flow structure over a measured frequency-physical plane. The experimental fluctuating velocities at various positions in the near field are used for the analyses. From wavelet analysis, the frequency and the time of the large-scale structure inthe shear layer and of the secondary flow near the wall can be determined. The branching structure of multi-scale eddies in the wall shear layer becomes more complex than that in the free shear layer. Due to the effect of the secondary flow the scales of eddy and entrainment process become small.

Wavelet Auto-Correlation Analysis Applied to Eddy Structure Identification of Free Turbulent Shear Flow.

To investigate the eddy structure of turbulent shear flow in both physical-space and Fourier-space, a new combination of conventional techniques of signal processing, called wavelet auto-correlation analysis, has been developed based on wavelet transform. The wavelet auto-correlation analysis provides the unique capability for describing the auto-correlation characteristics of arbitrary signals in terms of period and time delay. The wavelet auto-correlation technique was used to analyze the fluctuating velocities at various positions in a plane turbulent jet. It was found that the peaks of WR(α, τ) in the high period region correspond to the large peaks of R(τ), and the peaks of WR(α, τ) in the low period region are equivalent to the small peaks of R(τ). The branching structures in WR(α, τ) reveal that a periodic large eddy motion contains the periodic small eddy motions. In the shear layer at x/d=10, the periodic eddy motions with α=15, 40, 75, 100 and 150 ms are clearly observed.

Boundary-layer relaxation after a separated region

With reference particularly to the work of Peter Bradshaw and his associates, some remarks are made about the recovery of previously distorted shear flows. It is emphasized that such recovery is usually extremely slow, and this is further illustrated by new measurements of the velocity field and turbulence structure in the relaxing flow downstream of a separated region. Data have been obtained for downstream distances ( x) up to about 20 times the length of the separated region ( x r), or about 75 times the flow thickness at reattachment. This is a significantly more extensive region than has been previously studied, and the data are more comprehensive than any previously available. It is shown that the recovery is even slower than previously surmized. Furthermore, the measurements demonstrate that the turbulence stresses eventually fall below standard boundary-layer values (at the same Reynolds number), although around reattachment they are very much higher, having values more akin to those in plane mixing layers. This undershoot is apparently a new finding and is argued to be a result of the influence of the outer part of the flow on the growing inner region. The usual log-law only begins to appear beyond x/ x r = 2.5. It effectively “sees” a turbulent outer region that recovers even more slowly than itself, and the response of the inner region therefore has similarities to the response of an ordinary boundary layer to free-stream turbulence. It is concluded that even current second-order (i.e., Reynolds stress) models may not capture the exquisitely slow decay of the strong, large eddy motions in the outer part of the flow and the subtleties of their influence on the inner region.

124 相関法を用いたラグランジュ的計測の乱流場への適用

To investigate the evolving process of the large structures in turbulence, imageprocessing technique for traversing camera at mean flow velocity along the axis of the flow was developed by using cross-correlation technique and applied to the axisymmetric jet. In order to obtain a complete velocity profiles, a traversing unit for CCD was controlled by a personal computer to synchronize its traversing rate with the mean flow. Video signals from the CCD were recorded continuously on VTR with two binary barcode; one for identify the video field, the other for specify the camera position and the traversing rate. From recorded images, the cross-correlation technique was employed to obtain two-dimensional velocity distributions. From successive maps of instantaneous velocity and vorticity distribution, the motion of large eddies (vortex pairing, entrainment) can be dynamically revealed which was unable to measure by fixed camera measuring techniques.

Experiments on the turbulent shear flow in a turn-around duct(II)—The structure of turbulence

Detailed measurements including two-component mean velocities (U andV), RMS of turbulent fluctuations (u′ andv′) and turbulent cross-correlation (uv) were made throughout a turn-around duct. Two-component velocity data were obtained using a Laser Doppler Velocimeter. With the aid of a digitized data acquisition system, energy spectra were estimated using a Fast Fourier Transform. Along the outer wall, the flow is affected more than the half across the channel height due to the centrifugal instability. The measured results are consistent with the turbulence production mechanism for stabilizing and destabilizing curved flows. Energy production and dissipation are reduced along the convex wall and amplified along the concave wall. In the quasi-laminar region, turbulent fluctuations and cross-correlations are damped. The mean flow and turbulence structure in this region are influenced mainly by the streamwise pressure gradient rather than curvature. The flow in the downstream part of the turn is dominated by the inertial effect. The turbulent large eddy motions along the concave wall are strongly anisotropic.

Turbulent gas flux measurements below the air-water interface of a grid-stirred tank

Experiments have been conducted in an effort to elucidate the details of the interfacial gas transfer mechanism under liquid-side control. Near-surface liquid turbulence and oxygen concentration fluctuations have been measured in a laboratory grid-stirred tank for a range of turbulent Reynolds number R L from 80 to 660. Our observations show that the thickness of the gas boundary layer closely agrees in trend and magnitude with the Lewis-Whitman's film thickness as determined from the bulk concentration change. Furthermore, the functional dependence on turbulent Reynolds number is consistent with a large eddy domination. Additional simultaneous measurements of turbulent velocity and oxygen concentration fluctuations, including direct flux measurements, give further indication that the large scale eddy motions are more effective in the interfacial mass transfer process.

Heat Transfer Characteristics in Horizontal Fluid Layer with Honeycomb.

Natural convection in a heated horizontal fluid layer have been studied experimentally and theoretically for many years. In industrial components such as boilers or reactors, heat and mass are sometimes transferred through a horizontal fluid layer, including the internal structures. The heat transfer characteristics was experimantally studied in the horizontal water layer, including honeycombs of different flow area reduction ratios. Large eddy motion was reduced with the honeycomb but the heat transfer rate through the layer could not be affected, even at the flow area reduction rate of 0.09. Further reduction of the flow area resulted in the nonuniform temperature distribution of the central fluid layer and depression of the heat transfer rate.

Direct Numerical Simulation of Three-Dimensional Navier Stokes Equations for a Slit Nozzle Free Jet and Experimental Verification. (2nd Report. Turbulent Characteristics).

The third-order upwind finite difference scheme is applied to the convective terms in three-dimensional Navier-Stokes equations to simulate directly the free jet flow emitted from a slit nozzle. The calculated flow field ranges from the nozzle exit to the fully developed turbulent region. The calculated results of turbulent characteristics (turbulent intensity distributions, Reynolds stresses, power spectra, and velocity autocorrelations) are compared with the experimental data. The mesh size is roughly ten times the kolmogorov microscale; however, the calculated turbulent characteristics fairly well represent the experimental ones, except that the data strongly depend upon small-scale eddies. This means that the simulation well documents the motion of large scale eddies which play an important role in a flow-field formation. The neglect of the small-scale eddies in the third-order upwind finite difference scheme does not greatly affect the calculated results of large-scale eddy motions.

Three-dimensional large-eddy motions and fine-scale activity in a plane turbulent wake

A pattern recognition technique has been applied to simultaneously sampled multipoint hot-wire anemometry data obtained in the far wake of a circular cylinder. Data from both the streamwise fluctuating velocity field and the temperature field have been analysed employing a computer code that uses a correlation approach to automatically detect and ensemble average flow patterns and patterns for mean-square fluctuations. Statistical tests then allow the significance and contribution to the turbulence intensity of the detected structures to be evaluated. This procedure has been used to infer the three-dimensional topology of the double-roller eddies previously identified in the far-wake region and to relate these to the motions responsible for entrainment. It appears that the two types of motion are not independent, but are linked together, forming parts of horseshoe vortex structures which account for at least 40% of the total turbulence energy. These structures originate near the centre of the flow, may extend across the centreline and typically occur in groups of about three. The resulting picture of the flow dynamics is related to the conclusions drawn from similar data by other workers and a possible regeneration mechanism is presented. The addition to the code of a fine-scale activity indicator, the choice of which is discussed in some detail, has allowed the relationship between these energetic large-scale motions and smaller eddies to be investigated. It seems that the most intense fine-scale activity is associated with the vortical cores of the double-roller eddies. It is shown that this observation is consistent with the concepts of ‘isotropy’ and ‘spotiness’ of the dissipative scales. It also suggests that the horseshoe vortices loose energy both to their own secondary instabilities and to smaller scales resulting from the breakup of other highly strained large eddies.

Some topological features of the entrainment process in a heated turbulent wake

The temperature signals measured in a fully turbulent, thermally contaminated wake behind a heated cylinder at x/D = 140, are analysed using a pattern-recognition procedure. The temperature patterns detected show that the wake flow is dominated by shear-aligned structures with a strong three-dimensional character. The temperature footprints have a limited extent in the spanwise and streamwise directions, while their dimension is of the order of the wake width in the vertical coordinate. The footprints are characterized by a steep temperature gradient at their back edge, which appears as a sharp hot-to-cold transition. The topological features inferred from the temperature signals suggest that the entrainment process is accomplished mainly by engulfing external fluid. The large-scale organized structures are interpreted as the temperature footprints of the double-rollers characterizing the large-eddy motion in far wakes. The analysis of present results within the context of previous studies about intermittency in heated wakes, brings forward the idea that the intermittent bulges are the emerging double rollers at the three-dimensional turbulent/non-turbulent interface.

A numerical analysis of mass transfer in a plane shear layer.

The two-dimensional unsteady motion of a plane shear layer and its mass transfer process were numerically simulated by using the discrete vortex method, combined with the third-order upwind finite difference method. A visualized distribution of the mass concentrations showed that the occurrence of entrainment corresponded to large eddy motion. The distribution profiles of the average concentration and the fluctuation intensity showed self-similarities. The distribution profile of the average concentration had a stepwise shape, and that of the fluctuation had two peaks. The entrainment of the fluid of the high-speed side proved to be larger than that of the low-speed side.

A numerical analysis of mass transfer in a plane shear layer.

The unsteady motion of a two-dimensional incompressible shear layer and its mass transfer process were numerically simulated by using the Discrete Vortex Method combined with the third-order Upwind Finite Difference Method. The visualized distribution of mass concentration showed that the occurrence of entrainment corresponded to the large eddy motion. The distribution profiles of the average concentration and the fluctuation intensity showed similarities. The distribution profile of the average concentration had a stepwise shape and that of the fluctuation had two peaks. The entrainment of the fluid of the high speed side proved to be larger than that of the low speed side.

Numerical analysis of the reaction process in a plane shear layer.

The unsteady reaction process in an incompressible plane shear layer was numerically simulated by using the discrete vortex method combined with the third-order upwind finite difference method. The average product concentration increased and the thickness of the reaction zone decreased as the Damkohler number increased. The distribution profiles of the average concentrations and the fluctuation intensities showed similarities. The two-dimensional large eddy motion produced the stepwise shape of the average concentrations and the two-peak profiles of the fluctuation intensities. The asymmetrical entrainment of the highspeed fluid in a plane shear layer resulted in variations in the reaction rate and in the product thickness, as the stoichiometric ratio was changed.

An experimental study of the ignition and burning mechanisms in a pulse combustor (Combustion characteristics supplied with a propane-air mixture)

The ignition and burning mechanisms of pulse combustion are experimentally investigated using a basic two-dimensional combustor supplied with a propane-air mixture. The forced mixture supply widens the pulsation limit to a practically usable one. the burning process is so rapid that there exists no flame in the combustor at the time of the mixture inflow in the next combustion cycle. The back-flow from the tail-pipe also contains no flame. Then the ignition of the mixture is found to be caused by a direct contact and rapid mixing with residual hot burned gases, which can be regarded as a spontaneous ignition. High-speed schlieren movies reveal that a pair of large scale eddy motions, formed by the in flowing mixture jet, promote reveal that a pair of large scale eddy motions, formed by the inflow mixture jet, promote the mixing of the mixture and hot nurned gases, resulting in a distributed reaction zone.

A turbulent transport model for free-shear flows

An attempt is made to formulate a more accurate and physically plausible diffusion model, which will include the effects of large eddy motion on turbulent transport, by incorporating bulk convection and gradient diffusion. The two-equation model of turbulence thus obtained is used in self-similar form to calculate axisymmetric and plane two-dimensional jets.