Near-wall Turbulence Characteristics Research Articles

On the role of inertia in channel flows of finite-size neutrally buoyant particles

We consider suspensions of finite-size neutrally buoyant rigid spherical particles in channel flow and investigate the relevance of different momentum transfer mechanisms and the relation between the local particle dynamics and the bulk flow properties in the highly inertial regime. Interface-resolved simulations are performed in the range of Reynolds numbers $3000 \leq Re \leq 15\ 000$ and solid volume fractions $0 \leq \phi \leq 0.3$ . The Lagrangian particle statistics show that pair interactions are highly inhomogeneous and dependent on the distance from the wall: in their vicinity, the underlying mean shear drives the pair interactions, while a high degree of isotropy, dictated by more frequent collisions, characterizes the core region. Analysis of the momentum balance reveals that while the particle-induced stresses govern the dynamics in dense conditions, $\phi =0.3$ , and moderate Reynolds numbers, $Re <10\ 000$ , the turbulent stresses take over at higher Reynolds numbers. This behaviour is associated with a reduced particle migration toward the channel core, which decreases the importance of the particle-induced stress and increases the turbulent activity. Our results indicate that Reynolds stresses and the associated velocity fluctuations, characteristics of near-wall turbulence, prevail at high inertia over the resistance to deformation presented by the particles for volume fractions lower than 30 %.

Effect of micro rib on aerothermal dynamic in channel flow

In the present development of gas turbine, there is a main contradiction between the limited amount of cold air, the pressure head in the gas supply system and the large flow loss caused by the cooling unit inside the turbine blade. It leads to demands for the cooling techniques to provide efficient heat transfer with lower pressure loss. Surface micro turbulator is considered as a potential cooling technique. In this study, numerical simulation and experimental research were undertaken to evaluate the turbulent flow heat transfer and pressure loss in channel with a typical 2-D micro rib on one wall with different rib heights. Investigated rib heights range from 0.0013~0.053 of the channel hydraulic diameter ratio(h/D). The Reynolds number is kept at 20,000 to control the inlet boundary layer thickness, which makes the height of all cases located between the viscous layer and the log-law region in the boundary layer with a dimensionless height h+=2~80. Reynolds-Averaged Navier–Stokes(RANS) and Large Eddy Simulation(LES) results were validated by experiment. The results show that the pressure loss and heat transfer show different trends with the h+, which is mainly due to the enhancement of near-wall turbulence caused by the small rib height and the form drag caused by the large rib height. Furthermore, the analysis of near-wall turbulence characteristics given by LES indicates the enhancement of the normal turbulence fluctuation by the small rib within the turbulent boundary layer transition zone dominates the heat transfer enhancement downstream of the rib. the heat transfer is enhanced and the pressure loss is small when the rib height changes in the boundary layer, Which leads to a better overall thermal performance(OTP). The conclusion affirms that micro rib is a potential cooling technique to attain high heat transfer performance with low pressure loss. which is of great significance to enhance the overall efficiency of gas turbine.

Studies of combustion effects on near-wall turbulence in supersonic flows by large eddy simulation

In scramjet combustors, combustion occurring inside supersonic boundary layers can affect the near-wall turbulence, which can further reduce wall skin friction substantially. To reveal the mechanisms of the skin-friction reduction by boundary-layer combustion, high-accuracy large eddy simulations (LES) of the Burrows-Kurkov supersonic reacting wall-jet experiment are conducted in the present research. The LES results are then employed to investigate combustion effects on characteristics of near-wall turbulence, including flow structures, intensity of turbulent fluctuations, wall skin friction and heat flux, et al. The studies show that ignition generates turbulent kinetic energy and Reynolds shear stress dramatically but they will be depressed quickly by heat release at the downstream stable combustion region. The combustion chemical reactions affect the characteristics of the near-wall turbulence to further influence wall skin friction and heat flux. Due to volumetric expansion and turbulence-generation effects of ignition, distributions of wall skin friction at the ignition region exhibit the feature of a sudden decrease following by a drastic increase. Furthermore, the changes of wall skin friction and heat flux at the ignition region and downstream stable combustion region are mainly due to the changes of wall normal gradients of mean velocity and temperature. It is also found that ignition is a dominant factor for combustion effects on near-wall turbulence, which was generally ignored in the previous studies. Therefore, ignition reaction should be taken into consideration seriously in future application of the skin-friction reduction techniques by boundary-layer combustion.

Effect of the Particle Size on the Near-Wall Turbulence Characteristics of the Polymer Fluid Flow and the Critical Velocity Required for Particle Removal from the Sand Bed Deposited in Horizontal Wells

Water-based polymer drilling fluids are commonly used for drilling long horizontal wells where eliminating the drilling fluid-related formation damage and minimizing the environmental impact of the drilling fluids are the main concerns. An experimental study was conducted to investigate the turbulent flow of a polymer fluid over a stationary sand bed deposited in a horizontal pipeline. The main objectives of the study were to determine the effects of sand particle size on the critical velocity required for the onset of the bed erosion and the near-wall turbulence characteristics of the polymer fluid flow over the sand bed. Industrial sand particles having three different size ranges (20/40, 30/50, 40/70) were used for the experiments. The particle image velocimetry (PIV) technique was used to determine instantaneous local velocity distributions and near-wall turbulence characteristics (such as Reynolds stress, axial and turbulence intensity profiles) of the polymer fluid flow over the stationary sand bed under turbulent flow conditions. The critical velocity for the onset of the particle removal from a stationary sand bed using a polymer fluid flow was affected by the sand particle size. The critical velocity required for the particle removal from the bed deposits did not change monotonously with the changing particle size. When polymer fluids were used for hole cleaning, the particle size effect on the critical velocity varied (i.e., critical velocity increased or decreased) depending on the relative comparison of the sand particle size with respect to the thickness of the viscous sublayer under turbulent flow condition.

Characteristics of quasistationary near-wall turbulence subjected to strong stable stratification in open-channel flows

Turbulent open channel-flow under strongly stable thermal stratification is investigated. It is shown that the dominant effects of strong stable stratification on the characteristics of near-wall turbulence are transient. The budget of turbulent kinetic energy, the budget for the tangential Reynolds stress, and the relevant length scales are discussed. It is shown that near-wall turbulence at quasistationarity is approximately insensitive to the choice of upper thermal boundary condition.

Elliptic Blending with One-Equation Model

A wall-distance-free modification to Wray-Agarwal (WA) one-equation turbulence model is convoked using an elliptic relaxation approach to accurately accounting for non-local characteristics of near-wall turbulence. Model coefficients/functions are parameterized with the elliptic relaxation function to preserve the combined effects of near-wall turbulence and nonequilibrium. The characteristic length scale associated with the elliptic relaxation equation is formulated in terms of viscous and turbulent length scales in conjunction with the invariant of strain-rate tensor. Consequently, non-local effects are explicitly influenced by the mean flow and turbulent variables. A near-wall damping function is introduced to relax the viscous length-scale coefficient adhering to the elliptic relaxation model. Comparisons indicate that the new model improves the accuracy of flow simulations compared to the widely used Spalart-Allmar as model and remains competitive with the SST model.A good correlation is obtained between the current model and DNS/experimental data.

Effect of Particle Size and Surface Properties on the Sandbed Erosion with Water Flow in a Horizontal Pipe

Summary An experimental study was conducted to investigate the transport of sand particles over the sand bed deposited in a horizontal conduit by using turbulent flow of water. The main objectives were to determine the near-wall turbulence characteristics at the onset of bed erosion (i.e., near-wall velocity profile, Reynolds shear stresses, and axial-turbulent intensity); to determine critical velocity required for particle removal from the bed deposits; and more specifically, to determine how the sand-particle size and surface characteristics would influence the critical velocity required for the onset of bed erosion and the near-wall turbulence characteristics. A large-scale horizontal flow loop equipped with a nonintrusive laser-based particle-image velocimetry (PIV) system has been used for the experiments. The effect of sand-particle surface characteristics (i.e., wettability) on the critical velocity and the near-wall turbulence characteristics were investigated by using treated and untreated industrial sands of four different mesh sizes (i.e., 20/40, 30/50, 40/70, 100). The PIV technique was used to determine instantaneous local velocity distribution near the stationary sandbed fluid interface under subcritical and critical flow conditions. The near-wall velocity distribution measured directly at the sand bed/fluid interface together with the measured frictional pressure-loss values were then used for the evaluation of the Reynolds shear stresses and axial turbulent intensities acting at the bed/fluid interface. The results indicated that critical velocity for the onset of particle removal from sand beds increased with the increasing particle size. When sands with special surface treatment were used, it was observed that the critical velocity required for the onset of the bed erosion was significantly lower than that of required for the untreated sands. The degree of reduction in critical velocity varied between 14 and 40% depending on the particle size. In this study, by conducting experiments under controlled conditions, we provided much-needed fundamental data that can be used for the development of improved solid-transport design criteria and suitable mitigation technologies. In particular, we have shown the proof of concept that the surface-treated sand particles might have great potential for improving the transport efficiency of proppants used for hydraulic-fracturing operations.

A large-scale filter for applications of QSQH theory of scale interactions in near-wall turbulence

An outlook on the recently proposed quasi-steady quasi-homogeneous (QSQH) theory of the effect of large-scale structures on the near-wall turbulence is provided. The paper focuses on the selection of the filter, which defines the large-scale structures. It gives a brief overview of the QSQH theory, discusses the filter needed to distinguish between large and small scales, and the related issues of the accuracy of the QSQH theory, describes the probe needed for using the QSQH theory, and outlines the procedure of extrapolating the characteristics of near-wall turbulence from medium to high Reynolds numbers.

Roughness effects on near-wall turbulence modelling for open-channel flows

ABSTRACTAccurate modelling of near-wall turbulence influenced by roughness is crucial in hydraulic engineering. This study presents modifications on the k-ϵ turbulence model for open-channel flows over rough bed based on reliable particle image velocimetry data. Experiments have been conducted for different roughness heights. Some near-wall turbulence characteristics (e.g. bursting events) are identified and the profiles of mean streamwise velocity, Reynolds shear stress and turbulence intensities are validated with direct numerical simulation data. Experimental results confirm that the coefficient cμ involved in the k-ϵ model varies with roughness Reynolds number in outer region, and with both and y+ (normalized distance from the wall) in near-wall region. A new damping function is reconstructed, and the profile of turbulent kinetic energy close to the wall is analytically obtained. The present modifications accounting for wall roughness effects are further implemented into the k-ϵ model and validation shows that the prediction accuracy for the near-wall flow has been improved.

Unsteady characteristics of near-wall turbulence using high repetition stereoscopic particle image velocimetry (PIV)

This study is part of a project that is aimed at building dynamic boundary conditions near a solid wall, in order to reduce the large eddy simulation spatial resolution that is necessary in this region. The objective is to build a low-order dynamical system in a plane parallel to the wall, which will mimic the unsteady behaviour of turbulence. This dynamical system should be derived from a POD decomposition of the velocity field. The POD decomposition is to be applied on an experimental database of time-resolved velocity fields. In order to obtain the experimental database, a specific experiment of high-speed stereoscopic particle image velocimetry (PIV) has been performed. This experiment was carried out in the turbulent boundary layer of the LML wind tunnel. The plane under study was parallel to the wall located at 100 wall units. This database is validated via comparison with hot-wire anemometry (HWA). Despite some peak locking observed on the streamwise velocity component, the PDF and the power spectra are in very good agreement with the HWA results. The two-point spatial correlations are also in good agreement with the results from the literature. As the flow is time-resolved, space–time correlations are also computed. The convection of the flow structure is observed to be the most important effect at this wall distance. The next step is to compute the dynamical system and to couple it to a large eddy simulation.

Rotation effect on near-wall turbulence statistics and flow structures

Large eddy simulation (LES) of turbulent flow through an axially rotating pipe, coupled with nonlinear dynamic subgrid-scale (SGS) model, is carried out to investigate rotation effect on the near-wall turbulence characteristics and flow structures. In the rotating turbulent pipe flow, it is found that the tendency towards a relaminarized flow appears and the axial velocity fluctuation is suppressed; however, the azimuthal fluctuation is enhanced due to the presence of the pipe wall rotation. The joint probability density function (joint PDF) of the velocity fluctuations and the probability density function (PDF) of the helicity fluctuation are analyzed in detail. It is revealed that the resolved Reynolds stress and helicity fluctuation in the wall region are closely related to the correlation between the velocity and vorticity fluctuations and affected significantly by the rotation-induced azimuthal mean flow. Further, the budgets of resolved Reynolds stresses indicate that the rotation effect is responsible for the more active turbulent energy redistribution and the production of the azimuthal turbulence fluctuation. The near-wall inclined streaky structures with respect to the axial direction are ascribed to the spiral motion of the fluid induced by the rotating pipe. The turbulence characteristics revealed in this study are of great help for the understanding of physical fundamentals in the rotating turbulent flows and for the development of reliable turbulence model.

Active control of near-wall turbulence by local oscillating blowing

The effect of time-periodical blowing through a spanwise slot on the near-wall turbulence characteristics is investigated. The blowing velocity changes in a cyclic manner from 0 to 5 wall units. The frequency of the oscillations is nearly equal to the median frequency of the near-wall turbulence. The measurements of the wall shear stress and the streamwise velocity are reported and discussed. The flow field near the blowing slot is partly relaminarized during the acceleration phase of the injection velocity which extends 40 wall units downstream. The imposed unsteadiness is confined to the buffer layer, and the time-mean structural parameters under unsteady blowing are found to be close to those of isotropic turbulence in this region. The relaminarized phase is unstable and gives way to a coherent spanwise structure that increases the shear from 80 to 300 wall units downstream of the slot in a predictable way. This phenomenon is strongly imposed-frequency dependent.

Direct numerical simulation of ‘short’ laminar separation bubbles with turbulent reattachment

Direct numerical simulation of the incompressible Navier–Stokes equations is used to study flows where laminar boundary-layer separation is followed by turbulent reattachment forming a closed region known as a laminar separation bubble. In the simulations a laminar boundary layer is forced to separate by the action of a suction profile applied as the upper boundary condition. The separated shear layer undergoes transition via oblique modes and Λ-vortex-induced breakdown and reattaches as turbulent flow, slowly recovering to an equilibrium turbulent boundary layer. Compared with classical experiments the computed bubbles may be classified as ‘short’, as the external potential flow is only affected in the immediate vicinity of the bubble. Near reattachment budgets of turbulence kinetic energy are dominated by turbulence events away from the wall. Characteristics of near-wall turbulence only develop several bubble lengths downstream of reattachment. Comparisons are made with two-dimensional simulations which fail to capture many of the detailed features of the full three-dimensional simulations. Stability characteristics of mean flow profiles are computed in the separated flow region for a family of velocity profiles generated using simulation data. Absolute instability is shown to require reverse flows of the order of 15–20%. The three-dimensional bubbles with turbulent reattachment have maximum reverse flows of less than 8% and it is concluded that for these bubbles the basic instability is convective in nature.

A two-scale low-Reynolds number turbulence model

A two-scale low-Reynolds number turbulence model is proposed. The Kolmogorov turbulence time scale, based on fluid kinematic viscosity and the dissipation rate of turbulent kinetic energy (ν, e), is adopted to address the viscous effects and the rapid increasing of dissipation rate in the near-wall region. As a wall is approached, the turbulence time scale transits smoothly from a turbulent kinetic energy based (κ, e) scale to a (ν, e) scale. The damping functions of the low-Reynolds number models can thus be simplified and the near-wall turbulence characteristics, such as the e distribution, are correctly reproduced. The proposed two-scale low-Reynolds number turbulence model is first examined in detail by predicting a two-dimensional channel flow, and then it is applied to predict a backward-facing step flow. Numerical results are compared with the direct numerical simulation (DNS) budgets, experimental data and the model results of Chien, and Lam and Bremhorst respectively. It is proved that the proposed two-scale model indeed improves the predictions of the turbulent flows considered

Simulation of the Effect of the Freestream Turbulence Parameters on the Boundary Layer of a Curvilinear Airfoil

Modified variants of differential turbulence models which make it possible continuously to calculate both the entire flow region with laminar, transition and turbulent regimes and local low Reynolds number zones are proposed for investigating the flow and heat transfer in the boundary layers developing in compressible gas flow past curvilinear airfoils. The effect of the intensity and scale of free-stream turbulence and their variability along the outer boundary layer edge, as well as the combined action of the turbulence intensity and the streamwise pressure gradient in flow past blade profiles, on the heat transfer and near-wall turbulence characteristics is analyzed. The numerical results are compared with experimental and theoretical data.

On near-wall turbulent flow modelling

The characteristics of near-wall turbulence are examined and the result is used to assess the behaviour of the various terms in the Reynolds-stress transport equations. It is found that all components of the velocity-pressure-gradient correlation vanish at the wall. Conventional splitting of this second-order tensor into a pressure diffusion part and a pressure redistribution part and subsequent neglect of the pressure diffusion term in the modelled Reynolds-stress equations leads to finite near-wall values for two components of the redistribution tensor. This, therefore, suggests that, in near-wall turbulent flow modelling, the velocity-pressure-gradient correlation rather than pressure redistribution should be modelled. Based on this understanding, a methodology to derive an asymptotically correct model for the velocity-pressure-gradient correlation is proposed. A model that has the property of approaching the high-Reynolds-number model for pressure redistribution far away from the wall is derived. A similar analysis is carried out on the viscous dissipation term and asymptotically correct near-wall modifications are proposed. The near-wall closure based on the Reynolds-stress equations and a conventional low-Reynolds-number dissipation-rate equation is used to calculate fully-developed turbulent channel and pipe flows at different Reynolds numbers. A careful parametric study of the model constants introduced by the near-wall closure reveals that one constant in the dissipation-rate equation is Reynolds-number dependent, and a preliminary expression is proposed for this constant. With this modification, excellent agreement with near-wall turbulence statistics, measured and simulated, is obtained, especially the anisotropic behaviour of the normal stresses. On the other hand, it is found that the dissipation-rate equation has a significant effect on the calculated Reynolds-stress budgets. Possible improvements could be obtained by using available direct simulation data to help formulate a more realistic dissipation-rate equation. When such an equation is available, the present approach can again be used to derive a near-wall closure for the Reynolds-stress equations. The resultant closure could give improved predictions of the turbulence statistics and the Reynolds-stress budgets.