Local Turbulent Kinetic Energy Research Articles

Influence of fluid motion on growth and vertical distribution of cyanobacterium Microcystis aeruginosa

Cyanobacterium Microcystis aeruginosa (M. aeruginosa) is one of the dominant algae in lakes surrounded by urban and agriculture-dominated landscapes. Microcystis blooms pose ecological, economical, and human health risk, and there is a need to identify the forcing factors that influence Microcystis growth and blooms. We hypothesize that fluid motion has an influence on the growth and the vertical variability of M. aeruginosa, and its influence can be quantified by measuring the corresponding rate of energy dissipation levels of fluid motion. We conducted laboratory experiment measuring the population growth and vertical distribution of M. aeruginosa in a dual tower bioreactor with one tower used as experimental control. Fluid velocities were measured using a two-dimensional particle image velocimetry allowing the estimate of turbulence statistics that were related to measured M. aeruginosa growth rates and vertical cell concentration profiles. The effects of turbulence on M. aeruginosa physiology was evident by a 22 % increase in growth rate at depth-averaged energy dissipation rate ( $$\left\langle \varepsilon \right\rangle$$ ) of 8.5 × 10−5 m2 s−3, suppressed growth rate at $$\left\langle \varepsilon \right\rangle$$ > 2.8 × 10−4 m2 s−3, and no impact for $$\left\langle \varepsilon \right\rangle$$ < 4 × 10−6 m2 s−3. A similar trend was observed by analyzing the depth-dependent M. aeruginosa concentration versus the corresponding time-averaged local turbulent kinetic energy dissipation rate. A maximum cell biomass was discovered at e ~ 8.0 × 10−5 m2 s−3. Findings from this study can facilitate a greater understanding of physiology and spatial distribution of M. aeruginosa in aquatic ecosystems. The results will be instrumental in developing mechanistic models of the spatial and temporal distribution of M. aeruginosa that can improve management of aquatic ecosystems.

Numerical description of a pressure-swirl nozzle spray

Abstract Pressure-swirl nozzles are widely used in both industrial and experimental processes. However, the spray flow fields are difficult to predict due to the complexities of the dense break-up region. Herein, an inflow boundary condition approach is employed whereby droplets are computationally injected at 9 mm downstream of the orifice based on experimentally derived descriptions of the mean droplet and gas velocities, combined with empirical relations for turbulent kinetic energy and dissipation. The gas flow is then predicted with a Reynolds-Averaged Navier-Stokes solution while the particle trajectories are computed with a discrete random walk model. This approach was found to reasonably describe downstream spatial distribution of gas and droplet velocity, with moderate mesh resolution and CPU time. In addition, the evolution of the droplet velocity fluctuations was found to be consistent with turbulent diffusion theory based on the local Stokes numbers and turbulent kinetic energy.

Sensitivity of PBL schemes of the WRF-ARW model in simulating the boundary layer flow parameters for their application to air pollution dispersion modeling over a tropical station

Mesoscale atmospheric circulations play an important role in the transport of air pollution and local air quality issues. The planetary boundary layer (PBL), the thermo-dymamical structure and the flow field play an important role in air pollution dispersion. Hence, the PBL parameters over Nagpur, India are simulated using the ARW v. 3.6.1 mesoscale model. High-resolution simulations are conducted with triple nested domains having a horizontal resolution of 27, 9 and 3km, as well as 27 vertical levels by using the 1×1° NCEP Final Analysis meteorological fields for initial and boundary conditions. Eight fair-weather days in winter and summer (January and April 2009) with no significant synoptic activity were chosen for the study. Sensitivity experiments of the ARW model were conducted with two non-local (Yonsei University [YSU], and Asymmetric Convective Model v. 2 [ACM2]) and three local turbulence kinetic energy (TKE) closure (Mellor-Yamada Nakanishi and Niino Level 2.5 PBL [MYNN2], Mellor-Yamada-Janjic [MYJ], and quasi-normal scale elimination [QNSE]) turbulence diffusion parameterizations, to study the evolution of PBL parameters and the thermodynamical structure during the study period. After validation of the simulated parameters with the available in situ data, it was revealed that the non-local PBL scheme YSU, followed by local scheme MYNN2, could be able to capture the characteristic variations of surface meteorological variables and the thermodynamical structure of the atmosphere. The present results suggest that PBL schemes, namely YSU and MYNN2, performed better in representing the boundary-layer parameters and are useful for air pollution dispersion studies.

Performance Evaluation of PBL Schemes of ARW Model in Simulating Thermo-Dynamical Structure of Pre-Monsoon Convective Episodes over Kharagpur Using STORM Data Sets

In the present study, advanced research WRF (ARW) model is employed to simulate convective thunderstorm episodes over Kharagpur (22°30′N, 87°20′E) region of Gangetic West Bengal, India. High-resolution simulations are conducted using 1 × 1 degree NCEP final analysis meteorological fields for initial and boundary conditions for events. The performance of two non-local [Yonsei University (YSU), Asymmetric Convective Model version 2 (ACM2)] and two local turbulence kinetic energy closures [Mellor–Yamada–Janjic (MYJ), Bougeault–Lacarrere (BouLac)] are evaluated in simulating planetary boundary layer (PBL) parameters and thermodynamic structure of the atmosphere. The model-simulated parameters are validated with available in situ meteorological observations obtained from micro-meteorological tower as well has high-resolution DigiCORA radiosonde ascents during STORM-2007 field experiment at the study location and Doppler Weather Radar (DWR) imageries. It has been found that the PBL structure simulated with the TKE closures MYJ and BouLac are in better agreement with observations than the non-local closures. The model simulations with these schemes also captured the reflectivity, surface pressure patterns such as wake-low, meso-high, pre-squall low and the convective updrafts and downdrafts reasonably well. Qualitative and quantitative comparisons reveal that the MYJ followed by BouLac schemes better simulated various features of the thunderstorm events over Kharagpur region. The better performance of MYJ followed by BouLac is evident in the lesser mean bias, mean absolute error, root mean square error and good correlation coefficient for various surface meteorological variables as well as thermo-dynamical structure of the atmosphere relative to other PBL schemes. The better performance of the TKE closures may be attributed to their higher mixing efficiency, larger convective energy and better simulation of humidity promoting moist convection relative to non-local schemes.

Anisotropic turbulent model based on V2f model for natural and mixed convection in enclosed spaces

In previous work (Zhang et al. 2007) several Reynolds-averaged Navier–Stokes turbulent models were evaluated by the classic natural convection experiment. It was demonstrated that the V2f turbulent model shows the best overall performance compared to other models in terms of accuracy of temperature, velocity, and turbulent kinetic energy. However, all compared turbulent models cannot simulate the pseudo-laminar phenomenon in the center zone of the classic natural convection experiment. Moreover, the V2f model needs to solve four equations, and the transport equation for the wall normal stress () and the elliptic equation for the relaxation function (f) make the model numerically unstable. Based on the idea of the V2f model, an anisotropic model (BV2fAM) was developed. In the BV2fAM model, as the turbulent kinetic intensity normal to streamlines was adopted and computed by local turbulent kinetic energy and local flow characteristic variable instead of the transport equation. The validity of the developed BV2fAM model was confirmed by two benchmark cases (classic natural convection in a tall cavity and mixed convection in a square cavity). It can be seen that the developed BV2fAM model can yield better simulated results of temperature, velocity, and turbulent kinetic energy than other compared models. Moreover, it can simulate the pseudo-laminar phenomenon in the center zone of the classic natural convection experiment.

Mesoscale atmospheric flow-field simulations for air quality modeling over complex terrain region of Ranchi in eastern India using WRF

In the present study mesoscale meteorological flow and planetary boundary layer (PBL) parameters over the complex topographic region of Jharkhand state of tropical India are simulated using high resolution Advanced Research WRF (ARW) mesoscale model to study their role in the air pollution dispersion characteristics using a Lagrangian Particle Dispersion Model (FLEXPART). Eight fair weather days in different seasons (winter, pre-monsoon, monsoon, and post-monsoon) are chosen. Sensitivity experiments are conducted with two non-local [Yonsei University (YSU), Asymmetric Convective Model version 2 (ACM2)] and three local turbulence kinetic energy (TKE) closure [Mellor- Yamada Nakanishi and Niino Level 2.5 PBL (MYNN2), Mellor-Yamada-Janjic (MYJ), and quasi-normal scale elimination (QNSE)] PBL parameterisations to study the evolution of PBL parameters and thermodynamic structure. Simulated parameters are validated with available in situ meteorological observations at three stations in the study region. Results indicate that the low-level flow field is highly influenced by the topography and widely varies in different seasons. Simulated vertical PBL structure varied across seasons with shallow boundary layers (1–1.7 km) during winter, monsoon and post-monsoon seasons, deep mixed layers (2–2.7 km) in pre-monsoon season. Simulations in various seasons revealed that ACM2 followed by MYNN2 and YSU reproduced various PBL features such as topographic flows, surface layer fluxes, meteorological variables and the thermo-dynamical structure reasonably well indicating the suitability of the above PBL schemes for air quality simulations over the region. This is corroborated with the error statistics as well. Simulations with FLEXPART using ARW derived meteorology revealed higher dilution potential of the atmosphere in monsoon and pre-monsoon compared to post monsoon and winter seasons over the region. Results also indicate the ACM2, MYNN2 and YSU produce relatively larger dispersion than the QNSE and MYJ.

On the Micrometeorology of the Southern Great Plains. 2: Turbulence Statistics

Fast-response micrometeorological data obtained from an instrumented 32-m tower at an arid site near Ocotillo, Texas are used to examine the daily time evolution of the lower atmosphere. Correlation coefficients between turbulence properties (fast response wind-speed components and temperature) confirm that over this sparsely vegetated site the effects of convection are observed soon after sunrise, well ahead of the morning transition from stable to unstable stratification. Details of this kind are obscured when results are considered as functions of conventional stability parameters, since such standard analytical methods combine features of the morning and evening transitions into a single presentation. Partial correlation coefficients and semi-partials indicate that the local turbulent kinetic energy is mainly associated with local fluxes of heat and momentum near neutral and in most stable conditions, but decreases substantially during the times of strongest instability (possibly reflecting the scatter introduced by sampling infrequent convective episodes using a single tower). For many of the variables considered here, the standard deviations are about the same as the linear averages, indicating that the distributions are close to log-normal. The present data indicate that if the intent is to address some specific situation then \(\pm \)10 % error bounds on turbulence quantities (e.g. fluxes) correspond to averaging over a distance scale of the order of 10 km and a time scale of about 3 h. As the distance and time scales become smaller, the uncertainties due to factors external to the local surface increase.

Number density of turbulent vortices in the entire energy spectrum

In coalescence and break‐up modeling, vortex number density and size distributions of turbulent vortices are required to calculate the rate of interaction between continuous and dispersed phases. Existing number density models are only valid for the inertial subrange of the energy spectrum and no model of the vortex number density, valid for the entire energy spectrum, is available. The number density of the turbulent vortices were studied and modeled for the entire energy spectrum including the dissipative, inertial, and energy containing subranges. It was observed that the new number density model depends on vortex size, local turbulent kinetic energy, and dissipation rate. Moreover, the new number density model was validated by the number density distributions quantified in a turbulent pipe flow. The turbulent vortices of the pipe were identified and labeled using a vortex‐tracking algorithm that was developed recently by the authors. © 2014 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 60: 3989–3995, 2014

Impact of Steam-Dilution on the Flame Shape and Coherent Structures in Swirl-Stabilized Combustors

Humidified gas turbines and steam-injected gas turbines are promising technologies to lower the emissions and increase the efficiency and fuel flexibility of gas turbines. In the current study, the influence of steam-dilution on swirl-stabilized methane and hydrogen-fired flames is experimentally investigated at Reynolds numbers in the range of 22,000 to 32,000. Velocity fields and flame positions were measured using high-speed particle image velocimetry and OH* chemiluminescence. An extension of the quantitative light sheet technique was employed to estimate the temperature fields. The combined results reveal strong changes in the flame position, the velocity field, and the temperature field with increasing rates of steam dilution. In particular, three different flow and flame patterns are encountered: At dry conditions, a V-shaped flame stabilizes in a broad inner recirculation zone with low local turbulent kinetic energy; at moderate steam content, the flame changes into a trumpet-like shape; and at very high rates of steam-dilution, the flame detaches and shows an annular shape. The associated coherent flow structures are extracted from the particle image velocimetry data employing proper orthogonal decomposition. The isothermal flow is dominated by a helical instability arising near the combustor inlet. This structure is completely suppressed for the dry flame and reappears for the heavily steam-diluted detached flame with a similar shape and frequency as for the isothermal case. The flow field of the trumpet-like flame at intermediate to high steam dilution rates features a helical instability of lower frequency that is located further downstream than in the isothermal and very wet case. A conceptional explanation is presented that relates the suppression of the helical instability to the specific encountered temperature fields and flame shapes.

Scaling of pressure spectrum in turbulent boundary layers

Scaling of pressure spectrum in zero-pressure-gradient turbulent boundary layers is discussed. Spatial DNS data of boundary layer at one time instant (Reθ = 4500) are used for the analysis. It is observed that in the outer regions the pressure spectra tends towards the -7/3 law predicted by Kolmogorov's theory of small-scale turbulence. The slope in the pressure spectra varies from -1 close to the wall to a value close to -7/3 in the outer region. The streamwise velocity spectra also show a -5/3 trend in the outer region of the flow. The exercise carried out to study the amplitude modulation effect of the large scales on the smaller ones in the near-wall region reveals a strong modulation effect for the streamwise velocity, but not for the pressure fluctuations. The skewness of the pressure follows the same trend as the amplitude modulation coefficient, as is the case for the velocity. In the inner region, pressure spectra were seen to collapse better when normalized with the local Reynolds stress than when scaled with the local turbulent kinetic energy

Impacts of boundary layer parameterization schemes and air-sea coupling on WRF simulation of the East Asian summer monsoon

The planetary boundary layer (PBL) scheme in the regional climate model (RCM) has a significant impact on the interactions and exchanges of moisture, momentum, and energy between land, ocean, and atmosphere; however, its uncertainty will cause large systematic biases of RCM. Based on the four different PBL schemes (YSU, ACM2, Boulac, and MYJ) in Weather Research and Forecasting (WRF) model, the impacts of these schemes on the simulation of circulation and precipitation during the East Asian summer monsoon (EASM) are investigated. The simulated results of the two local turbulent kinetic energy (TKE) schemes, Boulac and MYJ, are more consistent with the observations than those in the two nonlocal closure schemes, YSU and ACM2. The former simulate more reasonable low-level southwesterly flow over East China and west pacific subtropical high (WPSH) than the latter. As to the modeling of summer monsoon precipitation, both the spatial distributions and temporal evolutions from Boulac and MYJ are also better than those in YSU and ACM2 schemes. In addition, through the comparison between YSU and Boulac experiments, the differences from the results of EASM simulation are more obvious over the oceanic area. In the experiments with the nonlocal schemes YSU and ACM2, the boundary layer mixing processes are much stronger, which lead to produce more sea surface latent heat flux and enhanced convection, and finally induce the overestimated precipitation and corresponding deviation of monsoon circulation. With the further study, it is found that the absence of air-sea interaction in WRF may amplify the biases caused by PBL scheme over the ocean. Consequently, there is a reduced latent heat flux over the sea surface and even more reasonable EASM simulation, if an ocean model coupled into WRF.

The turbulent/non-turbulent interface and entrainment in a boundary layer

AbstractThe turbulent/non-turbulent interface in a zero-pressure-gradient turbulent boundary layer at high Reynolds number ($\mathit{Re}_\tau =14\, 500$) is examined using particle image velocimetry. An experimental set-up is utilized that employs multiple high-resolution cameras to capture a large field of view that extends $2\delta \times 1.1\delta $ in the streamwise/wall-normal plane with an unprecedented dynamic range. The interface is detected using a criteria of local turbulent kinetic energy and proves to be an effective method for boundary layers. The presence of a turbulent/non-turbulent superlayer is corroborated by the presence of a jump for the conditionally averaged streamwise velocity across the interface. The steep change in velocity is accompanied by a discontinuity in vorticity and a sharp rise in the Reynolds shear stress. The conditional statistics at the interface are in quantitative agreement with the superlayer equations outlined by Reynolds (J. Fluid Mech., vol. 54, 1972, pp. 481–488). Further analysis introduces the mass flux as a physically relevant parameter that provides a direct quantitative insight into the entrainment. Consistency of this approach is first established via the equality of mean entrainment calculations obtained using three different methods, namely, conditional, instantaneous and mean equations of motion. By means of ‘mass-flux spectra’ it is shown that the boundary-layer entrainment is characterized by two distinctive length scales which appear to be associated with a two-stage entrainment process and have a substantial scale separation.

Laser-Induced Spark Ignition of Premixed Confined Swirled Flames

Optimization of the ignition location is of crucial importance for many combustion systems and requires advanced knowledge of the ignition process for both fundamental and applied configurations. In this article, a premixed swirl burner was designed to experimentally study the impact of the spark location on successful ignition and to detail the scenario from ignition to flame stabilization. Two swirl numbers were investigated to evaluate their impact on the ignition process. Particular attention was paid to providing accurate data on cold flow velocity field statistics (obtained by stereoscopic particle image velocimetry) as well as on ignition conditions. Ignition probability maps were obtained for a constant level of deposited energy. Contrary to previous studies, no correlation between local turbulent kinetic energy and ignition probability was observed, and a deeper analysis of the temporal evolution of the flame kernel within the combustion chamber is required. Coupling fast flame visualization with the corresponding pressure signal demonstrated that the efficiency of the ignition location was not only controlled by the local flow properties, but also by the early flame kernel development, linked by its typical trajectories within the combustion chamber.

Detached direct numerical simulations of turbulent two-phase bubbly channel flow

DNS simulations of two-phase turbulent bubbly channel flow at Re τ = 180 (Reynolds number based on friction velocity and channel half-width) were performed using a stabilized finite element method (FEM) and a level set approach to track the air/water interfaces. Fully developed turbulent single-phase solutions obtained previously using the same stabilized FEM code were used as the initial flow field, and an appropriate level-set distance field was introduced to represent the air bubbles. Surface tension and gravity forces were used in the simulations to physically represent the behavior of a bubbly air/water two-phase flow having a liquid/gas density ratio of 858.3. The simulation results were averaged to obtain the liquid and gas mean velocity distributions, the local void fractions as well as the local turbulent kinetic energy and dissipation rate of the liquid phase. The liquid phase parameters were compared with the corresponding single-phase turbulent channel flow to determine the bubbles’ influence on the turbulence field.

Hybrid Reynolds-Averaged Navier-Stokes/Kinetic-Eddy Simulation of Stall Inception in Axial Compressors

A hybrid Reynolds-averaged Navier-Stokes/kinetic-eddy simulation turbulence model is used for the stall predictions in a transonic axial compressor stage. This hybrid Reynolds-averaged Navier-Stokes/kinetic-eddy simulation model solves Menter's k-ω-shear-stress-transport model near walls and switches to the kinetic-eddy simulation model away from walls. The kinetic-eddy simulation model solves directly for local turbulent kinetic energy and local turbulent length scales, thus alleviating the grid spacing dependency found in other detached-eddy simulation and hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation models. The current methodology is used in the prediction of the performance map and the stall inception for the NASA stage 35 compressor configuration as a representative of a modern compressor stage. The present approach is found to satisfactorily predict the onset of stall. It is found that the rotor blade-tip leakage vortex and its interaction with the shock wave is the main reason behind the stall inception in this compressor stage.

Evolution of the turbulence structure in the surf and swash zones

The velocity field and turbulence structure within the surf and swash zones forced by a laboratory-generated plunging breaking wave were investigated using a particle image velocimetry measurement technique. Two-dimensional velocity fields in the vertical plane from 200 consecutive monochromatic waves were measured at four cross-shore locations, shoreward of the breaker line. The phase-averaged mean flow fields indicate that a shear layer occurs when the uprush of the bore front interacts with the downwash flow. The turbulence characteristics were examined via spectral analysis. The larger-scale turbulence structure is closely associated with the breaking-wave- and the bore-generated turbulence in the surf zone; then, the large-scale turbulence energy cascades to smaller scales, as the turbulent kinetic energy (TKE) evolves from the outer surf zone to the swash zone. Smaller-scale energy injection during the latter stage of the downwash phase is associated with the bed-generated turbulence, yielding a −1 slope in the upper inertial range in the spatial spectra. Depth-integrated TKE budget components indicate that a local TKE equilibrium exists during the bore-front phases and the latter stage of the downwash phases in the outer surf zone. The TKE decay resembles the decay of grid turbulence during the latter stage of the uprush and the early stage of the downwash, as the production rate is small because of the absence of strong mean shear during this stage of the wave cycle as well as the relatively short time available for the growth of the bed boundary layer.

Analysis of flow structures and energy spectra in chemical process equipment

Studies have been carried out to extract relevant information on flow structures, their break-up distribution and the associated local turbulence phenomena using the 1D and 2D energy spectra. The 1D analysis uses the hot film anemometry (HFA) and large eddy simulation (LES) data sets, while the 2D analysis uses the PIV results. The chemical process equipment considered for the study are jet loop reactor (JLR), channel flow (CHA) and bubble column reactor (BCR). The work involves relative comparison of spectral methods, namely, fast Fourier transform (FFT), discrete wavelet transform (DWT), continuous wavelet transform (CWT), eddy isolation methodology (EIM) and proper orthogonal decomposition (POD) in evaluating the 1D and 2D spectra for the three equipment. FFT studies are used to identify the dominant frequencies, but they do not reveal the time dependent changes in the flow structure. This limitation is overcome by using the EIM, DWT and CWT and methodologies for identification of flow structures are discussed. Using DWT, a novel intermittency-based quantifier, namely, the scale relative local intermittency measure (SRLIM) is found to be particularly useful for identification of intermittent flow structures. For example, it has been used to track the intermittent bursts in the CHA, the vortex tube related intermittencies in the JLR and the bubble induced eddies in the BCR. The advantages of CWT for obtaining the eddy time-scale and length-scale distribution from the velocity time series is shown. The evaluation of distributions has been carried out using wavelet transform modulus maxima (WTMM) along with a symbolic analysis of the loci of the maxima lines. The structure break-up distribution and the energy break-up distribution that are obtained show a generalization to the known L and P models for intermittencies. The turbulence parameters, i.e., local turbulent kinetic energy, energy dissipation rate and turbulent viscosity, have been calculated and compared using the results obtained from the FFT, DWT, CWT and EIM. For the different approaches these parameters lie in reasonable ranges.

Supersonic Virtual Valve Design for Numerical Simulation of a Large-Bore Natural Gas Engine

In many applications of supersonic injection devices, three-dimensional computation that can model a complex supersonic jet has become critical. However, in spite of its increasing necessity, it is computationally costly to capture the details of supersonic structures in intricate three-dimensional geometries with moving boundaries. In large-bore stationary natural gas fueled engine research, one of the most promising mixing enhancement technologies currently used for natural gas engines is high-pressure fuel injection. Consequently, this creates considerable interest in three-dimensional computational simulations that can examine the entire injection and mixing process in engines using high-pressure injection and can determine the impact of injector design on engine performance. However, the cost of three-dimensional engine simulations—including a moving piston and the kinetics of combustion and pollutant production—quickly becomes considerable in terms of simulation time requirements. One limiting factor is the modeling of the small length scales of the poppet valve flow. Such length scales can be three orders of magnitude smaller than cylinder length scales. The objective of this paper is to describe the development of a methodology for the design of a simple geometry supersonic virtual valve that can be substituted in three-dimensional numerical models for the complex shrouded poppet valve injection system actually installed in the engine to be simulated. Downstream flow characteristics of the jets from an actual valve and various virtual valves are compared. Relevant mixing parameters, such as local equivalent ratio and turbulence kinetic energy, are evaluated in full-scale moving piston simulations that include the effect of the jet-piston interaction. A comparison of the results has indicated that it is possible to design a simple converging-diverging fuel nozzle that will produce the same jet and, subsequently, the same large-scale and turbulent-scale mixing patterns in the engine cylinder as a real poppet valve.

Direct determination of energy dissipation in stirred vessels with two‐point LDA

AbstractThe ensemble‐averaged kinetic energy viscous dissipation rate was determined with a multipoint LDA technique in a vessel stirred by a Rushton turbine. Nine out of the total of 12 mean squared velocity gradients constituting the dissipation rate were directly measured in the impeller stream, and the deviation of the flow from local isotropy was investigated. The dissipation rate normalized with N3D2, where N and D are the impeller rotational speed and the impeller diameter, respectively, was found to be approximately constant for Reynolds numbers of 20,000–40,000, whereas the fluctuating gradients were found to vary significantly, by up to three times the ensemble‐averaged values, with blade angular position. The ensemble‐averaged dissipation rate values were also assessed through a local turbulence kinetic energy balance. © 2005 American Institute of Chemical Engineers AIChE J, 2005

422 一様等方性乱流における微細渦クラスターの統計的性質(S35-1 乱流渦構造(1),S35 乱流渦構造)

To investigate the statistics of the clusters of coherent fine scale eddies, direct numerical simulation database of homogeneous isotropic turbulence up to Re_λ=256.1 have been analyzed. The cluster of the strong coherent fine scale eddies tends to exist in the region with high local turbulent kinetic energy, which is defined as local average in the cube of integral length scale, and with high strain rate in large scales. Even in the Kolmogorov length unit, the most expected diameter of the coherent fine scale eddies depend on the characteristics of the integral length box. The diameter in each integral length boxes can be scaled by the local Kolmogorov scale (ηl). The most expected diameter is about 10_<ηl>. These characteristics of coherent fine scale eddies are similar to those in the near-wall turbulence. Moreover, the large scale eddies tend to exist in the region around the cluster where the energy transfer from GS to SGS is strong.