Nonlinear analysis of near-wall turbulence time series

We present the results of the nonlinear analysis of a near-wall turbulence signal, measured using a Laser Doppler Anemometer. Despite being preliminary in part, the results highlight some interesting aspects of the dynamics of coherent structures. Careful reconstruction of the dynamics from the scalar time series highlight particular phases of the bursting cycle, corresponding to the VITA detections, immersed in the overall high-dimension dynamics: the possibility to distinguish these trajectories in the embedding space provides indications for conditional sampling techniques on the reconstructed attractor. The application of nonlinear prediction to a system of high-dimension produces forecasts of rapidly decreasing quality over time, with no consequences for practical applications. Much more interesting instead is the analysis of the forecast errors for very short forecast intervals, where we observed a greater difficulty to predict the beginning of coherent structure instability probably due to the quasi-random forcing of the outer flow.     

基于颗粒湍流和颗粒碰撞相互作用规律的湍流模型的研究

颗粒湍流和颗粒碰撞的相互作用规律是两相流动中的核心问题。用颗粒湍流模型和颗粒碰撞的动力论模型叠加的方法在研究两相湍流流动方面取得了一定的成效,但是还有待改进。本文基于颗粒湍流形成大尺度脉动和颗粒间碰撞引起小尺度脉动的概念,从双流体模型出发,建立了两相流动的双尺度kp-pε两相湍流模型。利用该模型对下行床和突扩室内的气固...     

Effect of particle clustering on the gravitational settling velocity in homogeneous turbulence

Astatistical model of the gravitational settling velocity of clusters in homogeneous isotropic turbulence is developed. The effects of particle hydrodynamic interaction, inertia, size, and volume fraction on the particle settling velocity component attributable to particle clustering are analyzed.     

Effects of imperfect spatial resolution on turbulence measurements in the very near-wall viscous sublayer region

 Experiments were carried out to study the effects of imperfect spatial resolution on turbulence measurements in the very near-wall region using hot wires of different lengths, l ⁺ (in wall units). Previous works have indicated that the distributions of the longitudinal velocity rms value, skewness and flatness factors are independent of l ⁺ in the buffer region and beyond provided l ⁺<20–25. Our results obtained using l ⁺=3, 6, and 22 in the viscous sublayer region show that generally the said distributions are dependent on l ⁺ and attentuate in magnitude with increasing l ⁺. Further experiments were also carried out at different Reynolds numbers (Re _c , based on centerline velocity and channel’s height) but with measurements made using hot wire of the same l ⁺. The latter shows that the rms value and other higher order moments of longitudinal velocity fluctuations are independent of Re _c , thereby extending similar findings by Johansson and Alfredsson (1983), valid in the buffer region into the viscous sublayer region. Received: 29 January 1996 / Accepted: 10 August 1996     

Comparative analysis of turbulence models for calculating a near-wall boundary layer

The integral relations, the algebraic model, and the family of differential turbulence models widely used in theoretical and practical boundary layers studies are considered. The differential turbulence models are analyzed for fully developed incompressible-fluid flow in three regions: the viscous sublayer, the logarithmic layer, and the flow core. Numerical results are compared with analytical and experimental dependences. The numerical results for the boundary layer in a liquid rocket engine (LRE) nozzle, where compressibility, the temperature factor, and flow acceleration are significant, are presented. Recommendations on the applicability of the turbulence models considered are given. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 44–58, January–February, 1998. The work received financial support from the Russian Foundation for Fundamental Research (project No.96-01-00260).     

Second-moment closure for modelling the near-wall turbulence in 3D mean flows

A second-moment closure for the near-wall turbulence is proposed. The limiting behaviour of this closure near a wall is consistent with that of the exact Reynolds-stress transport equations, and it converts asymptotically into a high-Reynolds-number closure remote from the wall. The closure is applied to a pressure-driven 3D transient channel flow. The predicted results are in fair agreement with the DNS data. The project supported by the National Natural Science Foundation of China     

Modification of near-wall turbulence structure in a shear-driven three-dimensional turbulent boundary layer

 Most high Reynolds number flows of engineering interest are three-dimensional in nature. Key features of three-dimensional turbulent boundary layers (3DTBLs) include: non-colateral shear stress and strain rate vectors, and decreasing ratio of the shear stresses to the turbulent kinetic energy with increasing three-dimensionality. These are indicators that the skewing has a significant effect on the structure of turbulence. In order to further investigate the flow physics and turbulence structure of these complex flows, an innovative method for generating a planar shear-driven 3DTBL was developed. A specialized facility incorporating a relatively simple geometry and allowing for varying strengths of crossflow was constructed to facilitate studies where the skewing is decoupled from the confounding effects of streamwise pressure gradient and curvature. On-line planar particle image velocimetry (PIV) measurements and flow visualization results indicate that the experimental configuration generates the desired complex flow, which exhibits typical characteristics associated with 3DTBLs. Furthermore, spanwise shear results in modification of the near-wall turbulence structure. Analysis of near-wall flow visualization photographs revealed a reduction of mean streak length with increasing spanwise shear, while streak spacing remained relatively constant. In the most strongly sheared case, where the belt velocity is twice that of the freestream velocity, the mean streak length was reduced by approximately 50%. Received: 28 October 1997/Accepted: 4 February 1998     

An experimental study on turbulence modification in the near-wall boundary layer of a dilute gas-particle channel flow

Turbulence modifications of a dilute gas-particle flow are experimentally investigated in the lower boundary layer of a horizontal channel by means of a simultaneous two-phase PIV measurement technique. The measurements are conducted in the near-wall region with y ⁺?<?250 at Re _τ (based on the wall friction velocity u _τ and half channel height h)?=?430. High spatial resolution and small interrogation window are used to minimize the PIV measurement uncertainty due to the velocity gradient near the wall. Polythene beads with the diameter of 60?μm (d _p ⁺ ?=?1.71, normalized by the fluid kinematic viscosity ν and u _τ) are used as dispersed phase, and three low mass loading ratios (Φ _m ) ranging from 10^?4 to 10^?3 are tested. It is found that the addition of the particles noticeably modifies the mean velocity and turbulent intensities of the gas-phase, as well as the turbulence coherent structures, even at Φ _m ?=?0.025?%. Particle inertia changes the viscous sublayer of the gas turbulence with a smaller thickness and a larger streamwise velocity gradient, which increases the peak value of the streamwise fluctuation velocity ( $ u_{\text{rms}}^{ + } $ ) of the gas-phase with its location shifting to the wall. Particle sedimentation increases the roughness of the bottom wall, which significantly increases the wall-normal fluctuation velocity ( $ v_{\text{rms}}^{ + } $ ) and Reynolds shear stress ( $ - \langle u^{ \prime } v^{\prime } \rangle^{ + } $ ) of the gas-phase in the inner region of the boundary layer (y ⁺?<?10). Under effect of particle–wall collision, the Q2 events (ejections) of the gas-phase are slightly increased by particles, while the Q4 events (sweeps) are obviously decreased. The spatial scale of the coherent structures near the wall shrinks remarkably with the presence of the particles, which may be attributed to the intensified crossing-trajectory effects due to particle saltation near the bottom wall. Meanwhile, the $ v_{\text{rms}}^{ + } $ and $ - \langle u^{ \prime } v^{\prime } \rangle^{ + } $ of the gas-phase are significantly reduced in the outer region of the boundary layer (y ⁺?>?20).     

Effect of thermophoresis and other parameters on the particle deposition on a tilted surface

In this study a modified version of v2-f turbulence model (φ-α), is applied to simulate a non-isothermal air-flow. The φ-α model and a two-phase Eulerian approach complement each other to predict the rate of particle deposition on a tilted surface. The φ-α model can accurately calculate the normal fluctuations, which mainly represent the non-isotropic nature of turbulence regime near the wall. The Eulerian model was modified considering the most important mechanism in the particle deposition rate when compared to the experimental data. The model performance is examined by comparing the rate of particle deposition on a vertical surface with the experimental data in a turbulent channel flow available in the literature. The effects of lift force, turbophoretic force, thermophoreric force, electrostatic force, gravitational force and Brownian/turbulent diffusion were examined on the particle deposition rate. The results show that, using the φ-α model predicts the rate of deposition with reasonable accuracy. The results of modified particle model are in good agreement with the experimental data. This study highlights the paramount effect of thermophoretic force on the particle deposition rate and clearly shows that when the temperature difference exceeds a certain limit, the electrostatic force has insignificant effect on the particle deposition rate. Furthermore, it is indicated that even at small temperature differences, the effect of tilt angle on the particle deposition rate for intermediate-size particles is negligible.     

An experimental and theoretical study of particle deposition due to thermophoresis and turbulence in an annular flow

The paper describes an experimental and theoretical study of the deposition of small particles from a turbulent annular-flow with cross-stream temperature variation, focusing on the effects of thermophoresis. Various expressions for the thermophoretic force on a spherical particle are critically discussed. The well-known composite formula of Talbot et al. (1980) does not include the ‘second mechanism of thermophoresis’ and it is concluded that the more recent theoretical approach of Beresnev and Chernyak (1995) is probably more reliable. New experimental measurements of particle deposition from a turbulent flow with cross-stream temperature gradients are then presented. The measurement technique is similar to the method of Liu and Agarwal (1974) but in the test section the aerosol flows vertically downwards in an annular gap between two concentric pipes. By heating the outer pipe and cooling the inner it is possible to establish a substantial, near-constant temperature difference between the two walls and hence a thermophoretic force which varies only with radius. Numerical calculations provide a comparison of theory with experiment. The theory is based on the turbulent deposition models of Young and Leeming (1997) and Slater et al. (2003) modified to include thermophoresis and the annular geometry. The theory of Beresnev and Chernyak gives good agreement with the experimental measurements.     

On the capability of the curvilinear immersed boundary method in predicting near-wall turbulence of turbulent channel flows

The immersed boundary method has been widely used for simulating flows over complex geometries.However, its accuracy in predicting the statistics of near-wall turbulence has not been fully tested. In this work, we evaluate the capability of the curvilinear immersed boundary(CURVIB) method in predicting near-wall velocity and pressure fluctuations in turbulent channel flows. Simulation results show that quantities including the time-averaged streamwise velocity, the rms(root-mean-square) of velocity fluctuations, the rms of vorticity fluctuations, the shear stresses, and the correlation coefficients of u and v computed from the CURVIB simulations are in good agreement with those from the body-fitted simulations. More importantly, it is found that the time-averaged pressure, the rms and wavenumber-frequency spectra of pressure fluctuations computed using the CURVIB method agree well with the body-fitted results.     

Environmental conditions in near-wall plasmas generated by impact of energetic particle fluxes

Directional flows of energetic ions produced by laser-exploded foils were used to investigate transient phenomena accompanying the plasma interaction with surfaces of solid targets (walls). In experiments carried out on the iodine laser system PALS, the formation of energetic plasma jets from burn-through foils of Al and Ta was optimized using the three-frame interferometry and applied to a design of alternate experimental configurations. The interaction of the directional plasma flows with secondary targets was studied via X-ray imaging, optical and high-resolution X-ray spectroscopy. The environmental conditions in near-wall plasmas created at surfaces of plasma-exposed solids, in particular the velocity distribution of impinging and back-scattered ions, were determined via analysis of the observed spatially-resolved spectra of Al Lyα and Heα groups. The validity of the ion velocity gradients derived from the Doppler effect induced shifts and splitting of the spectral lines was supported by theoretical modeling based on a combination of hydrodynamic, atomic and collisional-radiative codes.     

Quantitative characterization of coherent structures in the buffer layer of near-wall turbulence. Part 1: streaks

Streaks play a major role in the process of turbulent generation. Numerous studies have been performed to characterize them, most of which used only single point measurements and only a few characteristics were studied. To investigate the streaks in more detail, a stereoscopic particle image velocimetry (SPIV) experiment was conducted to record 2D3C velocity fields in ten planes parallel to the wall from y ⁺ = 14.5 to y ⁺ = 48 at Reynolds number Re _θ = 7,800 in a fully developed turbulent boundary layer along a flat plate. This study develops a method based on pattern recognition to detect streaks from velocity fields obtained by SPIV and characteristizes them in depth. The results are in good agreement with the previous studies and expand significantly the information about the characteristics of the streaks.     

迷宫密封湍流增阻的数值研究

迷宫密封是依靠节流间隙中的节流过程和迷宫空腔内的动能耗散过程来实现密封的,这两个流体流动过程从本质上规定了迷宫密封的密封性能。通过密封内部流动本质的研究,合理利用湍流流动的规律和特点,能够发挥密封内部湍流的增阻作用,提高密封性能。本文数值计算了一种锯齿型径向迷宫密封的内部流动,得到了密封内部的流动形态,揭示了密封内部流动与密封性能的关系。结果表示,密封存在一个最佳的齿插入长度。在最佳齿插入长度时,尽管节流间隙较大,但由于节流间隙内流动收缩和偏转,间隙的有效通流面积却很小,泄漏介质可以在间隙中获得较大的速度,在迷宫空腔内形成正确的耗散涡流,密封具有最佳的密封效果。所以,充分发挥湍流增阻的作用,可以突破节流间隙的微小尺寸限制,实现较大间隙下的良好密封。这不仅会给迷官密封的制造、安装及运行等带来很大的方便,而且可以为发展新的迷宫密封技术奠定理论基础。     

Wall-shear-stress and near-wall turbulence measurements up to single pixel resolution by means of long-distance micro-PIV

A digital large-format long-distance micro-particle image velocimetry system (μ-PIV) was developed to measure the wall-shear-stress and the near-wall flow properties in a laminar, transitional and turbulent boundary layer flow along a flat plate, non-intrusively with high accuracy and spatial resolution. To achieve the desired measurement accuracy and spatial resolution, all experimental limitations associated with the seeding, light-sheet, out-of-focus particles, optical aberrations and distortions were successfully solved and various spatial correlation image analysis approaches based on the two-point or single-pixel ensemble correlation were developed, analyzed and compared with the state-of-the-art spatial correlation techniques. The instrument is well suited to prove fundamental fluid mechanical hypotheses such as the universality of the constants κ and B of the logarithmic law. However, for the analysis of flows at large Reynolds and Mach numbers, where small spatial dimensions and strong flow gradients prevent accurate measurements, this technique can be applied as well.     

Effect of correcting near-wall forces on nanoparticle transport in a microchannel

This paper studies the importance of corrections that account for the presence of walls on the forces acting on nanoparticles during their transport in microchannels. Theoretical and experimental investigations have reported anisotropic and hindered motion of nanoparticles near a microchannel wall. To investigate the influence of the near-wall effects, various conditions were examined. In particular, computer simulations were performed with and without the near-wall correction of forces. The corresponding capture efficiency and the average penetration of the captured nanoparticles were compared, and the importance of the near-wall corrections was assessed. Effects were evaluated for the nanoparticle diameter, the channel width, the channel length, and the pressure gradient. The results indicate that the inclusion of wall effects is crucial for the analysis of nanoparticle transport in microchannels.     

Numerical study on turbulence modulation in gas–particle flows

A mathematical model is proposed based on the Eulerian/Lagrangian approach to account for both the particle crossing trajectory effect and the extra turbulence production due to particle wake effects. The resulting model, together with existing models from the literature, is applied to two different particle-laden flow configurations, namely a vertical pipe flow and axisymmetric downward jet flow. The results show that the proposed model is able to provide improved predictions of the experimental results.     

Effect of correcting near-wall forces on nanoparticle transport in a microchannel

This paper studies the importance of corrections that account for the presence of walls on the forces act- ing on nanoparticles during their transport in microchannels.Theoretical and experimental investigations have reported anisotropic and hindered motion of nanoparticles near a microchannel wall. To investigate the influence of the near-wall effects, various conditions were examined. In particular, computer simu- lations were performed with and without the near-wall correction of forces. The corresponding capture efficiency and the average penetration of the captured nanoparticles were compared, and the importance of the near-wall corrections was assessed. Effects were evaluated for the nanoparticle diameter, the chan- nel width, the channel length, and the pressure gradient. The results indicate that the inclusion of wall effects is crucial for the analysis of nanoparticle transport in microchannels.