Spanwise Turbulence Intensities Research Articles

Experimental and numerical characterization of the airflow in the wake of a heavy truck

The wake flow of a heavy truck model is investigated at Re=8.5×104 using particle image velocimetry measurements combined with computational fluids dynamics-simulations. Experimental measurements are carried out on a 1:28-scale model, focusing exclusively on the central longitudinal plane, in the rear of the truck model. Numerical simulations are performed based on the URANS (unsteady Reynolds averaged Navier–Stokes) approach using two statistical turbulence models, i.e., the shear stress transport k–ω and the baseline Reynolds stress (BSL-RSM) models. A comparison between the numerical and experimental results of the mean velocity profiles in the wake of the heavy truck is found to be relatively consistent. The BSL-RSM model, however, gives a better prediction of experiments, with a deviation of 6% in the near wake, against 13% for the SST k–ω. Both URANS models undervalue the streamwise and spanwise turbulence intensity components with a deviation around 24%, compared with the experimental results. The characteristic feature of the wake flow topology is the formation of a recirculation bubble resulting from the shear layers separated from the truck surfaces. Different identification methods, including visualization of closed streamlines, vorticity magnitude, and the Q-invariant criterion, are considered and highlight the existence of two particular vortex regions in the mean flow: a vortex-shedding area in the upper recirculation region and a back-truck attached vortical structure. It is found that the Q criterion-based technique is a relevant indicator of the vortex cores regions.

Modulation of turbulence by saltating particles on erodible bed surface

Abstract

Influence of atmospheric stability on wind-turbine wakes with a certain hub-height turbulence intensity

Atmospheric stability can significantly influence the spreading of wind-turbine wakes. The previous studies often coupled atmospheric stability with the turbulence intensity and attributed the influence of atmospheric stability to the enhancement or suppression of turbulence due to the buoyancy effect. In this study, we decouple atmospheric stability with the ambient turbulence intensity, and the pure influence of atmospheric stability on the turbine wakes is investigated at a certain hub-height turbulence intensity via large-eddy simulation. We find that the spanwise turbulence transport plays a crucial role in wake recovery, and atmospheric stability influences this transport by redistributing the turbulence intensity between the three components and altering the spatial scales of the atmospheric motion. Under the convection condition, the spanwise turbulence intensity is greatly enhanced with enlarged flow scales. Hence, more Reynolds shear stress is generated under the shear effect between the ambient flow and the wake flow, which thus enhances spanwise turbulence transport, resulting in a faster recovery of turbine wakes. While for the stable condition, although the spanwise turbulence intensity is slightly enhanced, the flow scales are obviously reduced, resulting in a decrease in wake meandering, which leads to a decrease in turbulence transport in the wake region and a slower recovery of wind-turbine wakes.

Attached eddy model revisited using a minimal quasi-linear approximation

Townsend’s model of attached eddies for boundary layers is revisited within a quasi-linear approximation. The velocity field is decomposed into a mean profile and fluctuations. While the mean is obtained from the nonlinear equations, the fluctuations are modelled by replacing the nonlinear self-interaction terms with an eddy-viscosity-based turbulent diffusion and stochastic forcing. Under this particular approximation, the resulting fluctuation equations remain linear, enabling solutions to be superposed, the same theoretical idea used in the original attached eddy model. By leveraging this feature, the stochastic forcing is determined self-consistently by solving an optimisation problem which minimises the difference between the Reynolds shear stresses from the mean and fluctuation equations, subject to a constraint that the averaged Reynolds shear-stress spectrum is sufficiently smooth in the spatial wavenumber space. The proposed quasi-linear approximation is subsequently applied to channel flow for Reynolds number ranging from 500 to 20 000. The best result is obtained when the Reynolds stress is calculated by retaining only the two leading proper orthogonal decomposition modes, which further filters out the modelling artefact caused by the unphysical stochastic forcing. In this case, the resulting turbulence intensity profile and energy spectra exhibit the same qualitative behaviour as direct numerical simulation (DNS) data throughout the entire wall-normal domain, while reproducing the early theoretical predictions of the original attached eddy model within a controlled approximation to the Navier–Stokes equations. Finally, the proposed quasi-linear approximation reveals that the peak streamwise and spanwise turbulence intensities may deviate slightly from the logarithmic scaling with the Reynolds number for , and the supporting evidence is presented using the existing DNS data.

The amplification of large-scale motion in a supersonic concave turbulent boundary layer and its impact on the mean and statistical properties

Direct numerical simulation is conducted to uncover the response of a supersonic turbulent boundary layer to streamwise concave curvature and the related physical mechanisms at a Mach number of 2.95. Streamwise variations of mean flow properties, turbulence statistics and turbulent structures are analysed. A method to define the boundary layer thickness based on the principal strain rate is proposed, which is applicable for boundary layers subjected to wall-normal pressure and velocity gradients. While the wall friction grows with the wall turning, the friction velocity decreases. A logarithmic region with constant slope exists in the concave boundary layer. However, with smaller slope, it is located lower than that of the flat boundary layer. Streamwise varying trends of the velocity and the principal strain rate within different wall-normal regions are different. The turbulence level is promoted by the concave curvature. Due to the increased turbulence generation in the outer layer, secondary bumps are noted in the profiles of streamwise and spanwise turbulence intensity. Peak positions in profiles of wall-normal turbulence intensity and Reynolds shear stress are pushed outward because of the same reason. Attributed to the Görtler instability, the streamwise extended vortices within the hairpin packets are intensified and more vortices are generated. Through accumulations of these vortices with a similar sense of rotation, large-scale streamwise roll cells are formed. Originated from the very large-scale motions and by promoting the ejection, sweep and spanwise events, the formation of large-scale streamwise roll cells is the physical cause of the alterations of the mean properties and turbulence statistics. The roll cells further give rise to the vortex generation. The large number of hairpin vortices formed in the near-wall region lead to the improved wall-normal correlation of turbulence in the concave boundary layer.

Comparison of large- and very-large-scale motions in turbulent pipe and channel flows

Statistical measures of turbulence intensities in turbulent pipe and channel flows at a friction Reynolds number of Reτ ≈ 930 were explored by a population of large-scale motions (LSMs) and very-large-scale motions (VLSMs). Although the statistical measures characterizing these internal turbulent flows were similar in the near-wall region, the extents of the mean streamwise velocities and cross-stream components of the turbulence intensities differed in the core region. The population density of VLSMs/LSMs decreased/increased significantly in the core region of the pipe flow. The survival time of VLSMs of the pipe flow was shorter than that of the channel flow. The area fractions of the VLSMs displayed similar trends to the population density. The wall-normal and spanwise turbulence intensities in the pipe flow increased in the core regions due to the high-speed large-scale structures and associated motions above the structures. The large-scale structures increased the streamwise intensity and the Reynolds shear stress in the pipe and channel flows, whereas the effective streamwise intensities and the Reynolds shear stress were equivalent in both flows.

Two- and Four-Way Coupled Euler–Lagrangian Large-Eddy Simulation of Turbulent Particle-Laden Channel Flow

Large-eddy simulations (LES) of a vertical turbulent channel flow laden with a very large number of solid particles are performed. The motivation for this research is to get insight into fundamental aspects of co-current turbulent gas-particle flows, as encountered in riser reactors. The particle volume fraction equals about 1.3%, which is relatively high in the context of modern LES of two-phase flows. The channel flow simulations are based on large-eddy approximations of the compressible Navier–Stokes equations in a porous medium. The Euler–Lagrangian method is adopted, which means that for each individual particle an equation of motion is solved. The method incorporates four-way coupling, i.e., both the particle-fluid and particle–particle interactions are taken into account. The results are compared to single-phase channel flow in order to investigate the effect of the particles on turbulent statistics. The present results show that due to particle–fluid interactions the mean fluid profile is flattened and the boundary layer is thinner. Compared to single-phase turbulent flow, the streamwise turbulence intensity of the gas phase is increased, while the normal and spanwise turbulence intensities are reduced. This finding is generally consistent with existing experimental data. The four-way coupled simulations are also compared with two-way coupled simulations, in which the inelastic collisions between particles are neglected. The latter comparison clearly demonstrates that the collisions have a large influence on the main statistics of both phases. In addition, the four-way coupled simulations contain stronger coherent particle structures. It is thus essential to include the particle–particle interactions in numerical simulations of two-phase flow with volume fractions around one percent.

Numerical simulation of compressible plane jets

Two- and three-dimensional spatial direct numerical simulations of a compressible plane jet exhausting into a parallel stream are described. These simulations reveal the inadequacy of a two-dimensional model in capturing the totality of the flow physics. In two dimensions, instabilities evolve into highly organized large-scale mixing events; two-dimensional time-averaged turbulence quantities also suffer from artificial vortex organization. Mean normal velocity profiles show a significant reduction in entrainment with increased compressibility, while the effect is much less pronounced in three dimensions. While streamwise and spanwise turbulence intensities exhibit no change with increased compressibility, normal intensity and shear stress are significantly reduced.

Sensitivity analysis of freestream turbulence parameters on stagnation region heat transfer using a neural network

A neural network has been used to predict stagnation region heat transfer in the presence of freestream turbulence. The neural network was trained using data from an experimental study to investigate the influence of freestream turbulence on stagnation region heat transfer. The integral length scale, Reynolds number, all three components of velocity fluctuations and the vorticity field were used to characterize the freestream turbulence. The neural network is able to predict 50% of the test data within ±1%, while the maximum error of any data point is under 3%. A sensitivity analysis of the freestream turbulence parameters on stagnation region heat transfer was performed using the trained neural network. The integral length scale is found to have the least influence on the stagnation line heat transfer, while the normal and spanwise turbulence intensities have the highest influence.

Effects of Periodic Blowing from Spanwise Slot on a Turbulent Boundary Layer

(A + =0:25;0:50, and 1:00) at a e xed blowing frequency of f + =0:017. The effect of steady blowing (SB) was also examined, and the SB results were compared with those for periodic blowing (PB). PB reduced the skin friction near the slot, although to a slightly lesser extent than SB. PB was found to generate a spanwise vortical structure in the downstream of the slot. This vortex generates a reverse e ow near the wall, thereby reducing the wall shear stress. The wall-normal and spanwise turbulence intensities under PB are increased as compared to those under SB, whereas the streamwise turbulent intensity under PB is weaker than that under SB. PB enhances more energy redistribution than SB. Theperiodicresponseof thestreamwiseturbulence intensity to PBispropagated to a lesser extent than that of the other components of the turbulence intensities and the Reynolds shear stress.

Difference of Statistics between Compressible and Incompressible Turbulent Channel Flows.

The difference of turbulent statistics between compressible and incompressible turbulent channel flows is investigated using the DNS data of the compressible turbulent channel flow between isothermal walls and the DNS data of the incompressible turbulent channel flow. For comparison of compressible and incompressible flows, the dependence of turbulent statistics on the semi-local friction Reynolds number Re*τ and the wall friction Reynolds number Reτ is clarified in the compressible and incompressible turbulent flows, respectively. The semi-local friction Reynolds number is based on the local density and viscosity, semi-local friction velocity, and channel half-width. The main results of this paper are as follows: (1) The dependence of the turbulence intensities of the wall-bounded compressible turbulent flow on Re*τ is almost the same as that of the incompressible turbulent flow on Reτ except for the streamwise turbulence intensity near the wall. The streamwise intensity depends on the Mach number. (2) The reduction in the pressure-strain correlation term suppresses the energy transfer from the streamwise component to the wall-normal and spanwise components. As a result, the streamwise turbulence intensity becomes larger, and the wall-normal and spanwise turbulence intensities become smaller in the compressible turbulent channel flow, comparing with the incompressible one. (3) The reduction in the pressure-strain correlation term is due to the reductions of the RMS velocity derivative fluctuations and the absolute values of correlation coefficients between pressure and velocity derivative fluctuations.

3-D MEASUREMENTS OF OPEN-CHANNEL FLOWS WITH TWO SETS OF LASER DOPPLER ANEMOMETERS

Three-dimensional measurements of open-channel flows were conducted by making use of two sets of laser Doppler anemometers. One is able to measure the u-v components, and the other is able to measure the u-w components. The Reynolds stress (-vw) could be first measured with high accuracy. The streamwise velocities which were measured by u-v measurement and u-w measurements were in a good agreement with each other. The third moments of the velocity fluctuations were agree well with the results of DNS (Direct Numerical Simulation). A new empirical formula, involving the van Driest damping function, expresses the spanwise turbulence intensity in the inner region.

The influence of temperature on the measurements of Reynolds stresses in shear free turbulence near a wall

Temperature changes have a significant influence on the measurements of Reynolds stresses in turbulent boundary layers. As compared to the spanwise velocity fluctuations the streamwise turbulence intensity is especially sensitive to temperature deviations. Although this is a general statement its importance is clearly elucidated in a shear-free turbulence near a solid wall, since the mixing due to turbulence production is minimized in this flow. A consequence of temperature influence on hot-wire measurements is that frictional heating from the wall has produced contradictory results in different experiments on shear-free turbulence. In the current paper, measurements of streamwise and spanwise turbulence intensities have been conducted at different wall temperatures, thereby simulating the contradictory results mentioned above. A simple model has been developed showing that the turbulence intensities are affected by both the rms. value of the temperature fluctuations and the correlation between fluctuating temperature and velocity. These correlations are measured and the developed model is used to explain deviations in earlier measurements on shear-free turbulence. Moreover, the individual magnitudes of the two correlations in the temperature correction are estimated and their individual importance is discussed.

Three-dimensional velocity field in a compressible mixing layer

A turbulent, compressible mixing layer with a relative Mach number of 1.59 has been investigated experimentally using a two-component laser Doppler velocimeter system. Two sets of profiles were obtained at each streamwise measurement location to compile the streamwise, transverse, and spanwise turbulence statistics. Results from the fully developed region of the mixing layer showed similar peak values of streamwise and spanwise turbulence intensities along with reduced peak values of transverse turbulence intensity, normalized primary Reynolds shear stress, and normalized turbulent kinetic energy in comparison to the respective quantities from incompressible shear layers

Three-Dimensional Simulations of Compressible Mixing Layers: Visualizations and Statistical Analysis

The effects of compressibility on the planar mixing layer are investigated by means of visualization and statistical analysis of the flowfield quantities computed using three-dimensional temporally evolving inviscid simulations. The levels of compressibility studied range from relative Mach numbers of Mr — 0.2-2.4. The objectives of this research are to identify large-scale structures present in the mixing layer at different levels of compressibility as well as to examine the statistical description of the flowfield, in order to gain understanding of the physical entrainment and mixing processes. Three-dimensional simulation visualizations of both the passive scalar and pressure fields show the nature of the large-scale structure present in the planar mixing layer to change from nearly two dimensional and spanwise at low compressibility to highly three dimensional and oblique at increased compressibility. Statistical analysis of the flowfield quantities shows that the shear layer width, Reynolds shear stress, and transverse turbulence intensity decrease with increasing compressibility levels, whereas the streamwise turbulence intensity remains nearly constant and the spanwise turbulence intensity increases. These statistics support the increasingly three-dimensional nature of the large-scale motion of the mixing layer with increasing Mr.