Turbulence Modulations in the Boundary Layer of a Horizontal Particle-Laden Channel Flow

Turbulence modulations are experimentally investigated using particle image velocimetry （PIV） in the lower boundary layer of a fully developed horizontal channel flow. A simultaneous two-phase PIV measurement technique is adopted to acquire the turbulent statistics quantities and to examine the coherent structures in the near-wall region. Polythene beads with diameters of 60 μm are used as dispersed phases, and the PIV measurements have been performed at three mass loadings varying from 2.5 ×10^-4 to 5 × 10^-3. All the experiments are performed at a wall shear Reynolds number of Reτ = 430. The results show that the presence of the particles suppresses the coherent structures, with shorter streamwise extent of the quasistrearnwise structures, and then, the wall-normal velocity fluctuations and shear Reynolds stresses are both decreased in the near-core region. In addition, as a result of the particle wake, the turbulence intensity and shear Reynolds stress both increase in the vicinity of the wall. Due to the drag effects of the particles on the gas, the streamwise velocity gradients decrease in the outer region and increase in the viscous sublayer, meanwhile the thickness of the viscous sublayer also decreases. These results cause the peak values of the streamwise velocity fluctuations adjacent to the wall to increase, and the peak positions shift to the wall. This is the reason for decreasing the near-wall region and increasing the near-core region of the streamwise velocity fluctuations in appearance.     

Influence of uniform momentum zones on frictional drag within the turbulent boundary layer

Based on a set of experimental databases of turbulent boundary layers obtained from particle image velocimetry in the streamwise-wall-normal plane at friction-velocity-based Reynolds number Reτ=612,the influence of uniform momentum zones(UMZs)on the skin-friction drag is investigated.The skin-friction drag is measured by the single-pixel ensemble correlation method.The results show that the velocity fields with the number of UMZs larger than the mean value have a relatively low skin-friction drag,while the velocity fields with the number of UMZs less than the mean value have a relatively high skin-friction drag.By analyzing the statistical characteristics of UMZs,the dynamic correlation between the UMZs and skin-friction drag is explored.The velocity fields with a low number of UMZs present a sweep event.These sweep motions intensify the small-scale Reynolds shear stress in the near-wall region by modulation effects.The enhancement of small-scale Reynolds shear stress is the direct reason for the high skin-friction drag.Increasing the proportion of velocity fields with high UMZs amount may be a direction to reduce the skin-friction drag within the TBL.     

Turbulent Boundary Layer Control via a Streamwise Travelling Wave Induced by an External Force

Turbulent boundary layer control via a streamwise travelling wave is investigated based on direct numerical simulation of an incompressible turbulent channel flow. The streamwise travelling wave is induced on one side wall of the channel by a spanwise external force, e.g., Lorenz force, which is con~ned in the viscous sublayer. As the control strategy used in this study has never been examined, we pay our attention to its efficiency of drag control. It is revealed that the propagating direction of the travelling wave, i.e., the downstream or upstream propagating direction with respect to the streamwise flow, has an important role on the drag control, leading to a significant drag reduction or enhancement for the parameters considered. The coherent structures of turbulent boundary layer are altered and the underlying mechanisms are analysed. The results obtained provide physical insight into the understanding of turbulent boundary layer control.     

Investigation on very large scale motions(VLSMs) and their influence in a dust storm

Based on the real-time synchronous measurements of the wind velocity,temperature,the PM10 concentration at 16 m and 47 m during a dust storm event,in which Reynolds number Re exceeds 6×106,this study reveals the existence of the very large scale motions(VLSMs) during the stable stage both in the stream velocity and the temperature field at the two heights,whose streamwise scales reach up to 10 times the thickness of the boundary layer.The streamwise velocity and the PM10 concentration display a similar frequency corresponding to the peaks of their energy spectra,which implies that the VLSMs of streamwise flow have a significant role in dust transportation.In contrast,the salient deviations of the PM10 concentration at 47 m from the Gaussian distribution are revealed,which means that 47 m is not in the dust transportation layer,but is a region where the dust transportation layer and the outer flow intersect each other.Analysis demonstrates that the energy spectra of the PM10 concentrations at 16 m and 47 m display the "-1" scaling law feature,which has the same frequency range(0.001-0.1 Hz) as that of the wind velocity.This provides a new paradigm for the existence of the self-similarity scaling region in turbulent flow.     

Phonon relaxation and heat conduction in one-dimensional Fermi-Pasta-Ulam β lattices by molecular dynamics simulations

The phonon relaxation and heat conduction in one-dimensional Fermi-Pasta-Ulam (FPU) β lattices are studied by using molecular dynamics simulations.The phonon relaxation rate,which dominates the length dependence of the FPU β lattice,is first calculated from the energy autocorrelation function for different modes at various temperatures through equilibrium molecular dynamics simulations.We find that the relaxation rate as a function of wave number k is proportional to k 1.688,which leads to a N 0.41 divergence of the thermal conductivity in the framework of Green-Kubo relation.This is also in good agreement with the data obtained by non-equilibrium molecular dynamics simulations which estimate the length dependence exponent of the thermal conductivity as 0.415.Our results confirm the N 2/5 divergence in one-dimensional FPU β lattices.The effects of the heat flux on the thermal conductivity are also studied by imposing different temperature differences on the two ends of the lattices.We find that the thermal conductivity is insensitive to the heat flux under our simulation conditions.It implies that the linear response theory is applicable towards the heat conduction in one-dimensional FPU β lattices.     

Analytic prediction for planar turbulent boundary layers

Analytic predictions of mean velocity profile(MVP) and streamwise(x) development of related integral quantities are presented for flows in channel and turbulent boundary layer(TBL), based on a symmetry analysis of eddy length and total stress. Specific predictions include the relations for momentum Reynolds number(Reθ) with friction Reτ and streamwise Re_x: Re_θ≈ 3.27Re_τ,and Re_x/Re_θ = 4.94(lnRe_θ + 1.88)~2 + 1; the streamwise development of the friction velocity u_τ: U_e/u_τ≈ 2.22 lnRe_x + 2.86. 3.83ln(lnRe_x), and of the boundary layer thickness δ_e: x/δ_e ≈ 7.27 lnRe_x.5.18.12.52ln(lnRe_x), which are fully validated by recent reliable data.     

Detecting Multi-Scale Coherent Eddy Structures and Intermittency in Turbulent Boundary Layer by Wavelet Analysis

Multi-scale decomposition by wavelet transform has been performed to velocity time sequences obtained by fine measurements of turbulent boundary layer flow. A conditional sampling technique for detecting multi-scale coherent eddy structures in turbulent field is proposed by using multi-scale instantaneous intensity factor and flatness factor of wavelet coefficients. Although the number of coherent eddy structures in the turbulent boundary layer is very small, their energy percentage with respect to the turbulence kinetic energy is high. Especially in buffer layer, the energy percentages of coherent structures are significantly higher than those in the logarithmic layer, indicating that the buffer layer is the most active region in the turbulent boundary layer. These multi-scale coherent eddy structures share some common dynamical characteristics and are responsible for the anomalous scaling law in the turbulent boundary layer.     

Aerothermal characteristics of bleed slot in hypersonic flows

Two types of flow configurations with bleed in two-dimensional hypersonic flows are numerically examined to investigate their aerodynamic thermal loads and related flow structures at choked conditions. One is a turbulent boundary layer flow without shock impingement where the effects of the slot angle are discussed, and the other is shock wave boundary layer interactions where the effects of slot angle and slot location relative to shock impingement point are surveyed. A key separation is induced by bleed barrier shock on the upstream slot wall, resulting in a localized maximum heat flux at the reattachment point. For slanted slots, the dominating flow patterns are not much affected by the change in slot angle, but vary dramatically with slot location relative to the shock impingement point. Different flow structures are found in the case of normal slot, such as a flow pattern similar to typical Laval nozzle flow, the largest separation bubble which is almost independent of the shock position. Its larger detached distance results in 20% lower stagnation heat flux on the downstream slot corner, but with much wider area suffering from severe thermal loads. In spite of the complexity of the flow patterns, it is clearly revealed that the heat flux generally rises with the slot location moving downstream, and an increase in slot angle from 20° to 40° reduces 50% the heat flux peak at the reattachment point in the slot passage. The results further indicate that the bleed does not raise the heat flux around the slot for all cases except for the area around the downstream slot corner. Among all bleed configurations, the slot angle of 40° located slightly upstream of the incident shock is regarded as the best.     

Mean and fluctuating velocity fields of a diamond turbulent jet

The present paper reports the first investigation on a turbulent jet issuing from a diamond orifice(hereafter termed a "diamond jet") with an aspect ratio of 1.7.Velocity measurements were conducted in the transitional region,and the exit Reynolds number of the jet was 50000.For comparison,a round jet with identical normalized boundary conditions was also measured.It is shown that the diamond jet decays and spreads faster than the round jet does over the measured flow region.The axis-switching phenomenon is observed in the diamond jet.Although both jets display primary coherent structures in the near field,these structures are found to break down more rapidly in the diamond jet,due to the higher three-dimensionality of the flow.Moreover,the streamwise components of the Reynolds normal stress and all the shear stresses reach their maxima around the location of the maximal mean shear while the maxima of the lateral components of the Reynolds normal stresses occur around the centreline of the jet.     

Large Eddy Simulation of Turbulent Channel Flow with 3D Roughness Using a Roughness Element Model

Large eddy simulation of turbulent channel flow with dense and small 3D roughness elements is carried out using a roughness element model Profiles of mean Reynolds stress, mean velocity and rms velocity as well as turbulent structures near the wall are obtained. The shear stress in the rough wall is larger than that in the smooth wall side and the rough wall has a larger influence on the channel flow. Profiles of mean streamwise velocity near the wall have logarithmic velocity distributions for both smooth and roughness walls, while there is a velocity decrease for the rough wall due to larger fractional drag. All the three components of rms velocities in the rough wall region are larger than that in the smooth wall region, and the roughness elements on the wall increase turbulent intensity in all directions. The s~reak spacing and average diameter of near wall quasi-s~reamwise vortices increase with the presence of roughness elements on the wall and it is shown that the rough wall induces complex and strong streamwise vortices. Results of dense and small 3D roughness elements in both turbulent statistics and structure, obtained with a relatively simple method, are found to be comparable to related experiments.     

Effects of Prandtl number in two-dimensional turbulent convection

We report a numerical study of the Prandtl-number(Pr) effects in two-dimensional turbulent Rayleigh-Benard convection.The simulations were conducted in a square box over the Pr range from 0.25 to 100 and over the Rayleigh number(Ra) range from 107 to 1010.We find that both the strength and the stability of the large-scale flow decrease with the increasing of Pr,and the flow pattern becomes plume-dominated at high Pr.The evolution in flow pattern is quantified by the Reynolds number(Re),with the Ra and the Pr scaling exponents varying from 0.54 to 0.67 and-0.87 to-0.93,respectively.It is further found that the non-dimensional heat flux at small Ra diverges strongly for different Pr,but their difference becomes marginal as Ra increases.For the thermal boundary layer,the spatially averaged thicknesses for all the Pr numbers can be described by δ_θ～Ra~(-0.30) approximately,but the local values vary a lot for different Pr,which become more uniform with Pr increasing.     

Correlating dynamic relaxation and viscoelasticity in metallic glasses

Relaxation dynamics,essential for the structural evolution of non-equilibrium systems like glassy materials,remain enigmatic.Here,we explore relaxation dynamics and viscoelastic properties in three types of metallic glasses with distinct β relaxation behavior.In systems with significant β relaxation,stress relaxation and creep experiments reveal a transition from two-step to one-step relaxation with rising temperature.However,such a phenomenon is absent in systems with weaker β relaxation.We mod...     

The effect of the Taylor-Grtler vortex on Reynolds stress transport in the rotating turbulent channel flow

We investigates the effect of Taylor-Grtler vortex on the Reynolds stress transport in the rotating turbulent channel flow by direct numerical simulation. The Taylor-Grtler vortex is detected by longitudinal average of velocity fluctuation in the channel and defined as TG fluctuation. It has been found that turbulent diffusion is significant in the Reynolds stress transportation at the suction side of rotating turbulent channel in contrast with the turbulent channel flow without rotation and Taylor-Grtler vortex plays an important role in the turbulent diffusion in Reynolds stress transport. The paper focuses on the low and moderate rotation number, but the effect of the rotation number on the Reynolds stress transport is also reported.     

Coherent structures in wall turbulence and mechanism for drag reduction control

Herein is introduced the mechanism for active control influencing the generation of the near-wall streamwise vortices,which are closely related to the production of high skin friction in wall-bounded turbulent flows.A new opposition control scheme with adjusting control amplitude is proposed and evaluated in turbulent channel flow by direct numerical simulations.The maximum drag reduction rate can be greatly enhanced by the strengthened control.Finally the effectiveness of the control to the coherent structures at high Reynolds numbers is investigated by using a linear transient growth model.     

The investigation of coherent structures in the wall region of a supersonic turbulent boundary layer based on DNS database

Through temporal mode direct numerical simulation, flow field database of a fully developed turbulent boundary layer on a flat plate with Mach number 4.5 and Reynolds number Reθ =1094 has been obtained. Commonly used detection meth- ods in experiments are applied to detecting coherent structures in the flow field, and it is found that coherent structures do exist in the wall region of a supersonic turbulent boundary layer. The detected results show that a low-speed streak is de- tected by using the Mu-level method, the rising parts of this streak are detected by using the second quadrant method, and the crossing regions from a low-speed streak to the high-speed one are detected by using the VITA method respectively. Notwithstanding that different regions are detected by different methods, they are all accompanied by quasi-stream-wise vortex structures.     

Scaling of heat transfer in gas-gas injector combustor

The scaling of heat transfer in gas-gas injector combustor is investigated theoretically, numerically and experimentally based on the previous study on the scaling of gas-gas combustion flowfield. The similarity condition of the gas-gas injector combustor heat transfer is obtained by conducting a formulation analysis of the boundary layer Navier-Stokes equations and a dimensional analysis of the corresponding heat transfer phenomenon. Then, a practicable engineering scaling criterion of the gas-gas injector combustor heat transfer is put forward. The criterion implies that when the similarity conditions of inner flowfield are satisfied, the size and the pressure of gas-gas combustion chamber can be changed, while the heat transfer can still be qualitatively similar to the distribution trend and quantitatively correlates well with the size and pressure as q ∝ pc0 .8d t-0.2. Based on the criterion, single-element injector chambers with different geometric sizes and at different chamber pressures ranging from 1 MPa to 20 MPa are numerically simulated. A single-element injector chamber is designed and hot-fire tested at seven chamber pressures from 0.92 MPa to 6.1 MPa. The inner wall heat flux are obtained and analysed. The numerical and experimental results both verified the scaling criterion in gas-gas injector combustion chambers under different chamber pressures and geometries.     

Direct numerical simulation on relevance of fluctuating velocities and drag reduction in turbulent channel flow with spanwise space-dependent electromagnetic force

Based on the Fourier–Chebyshev spectral method, the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation(DNS) methods for different Reynolds numbers. A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient. With the application of electromagnetic force, the in-depth relations among the fluctuating velocities near the wall, Reynolds stress, and the effect of drag reduction for different Reynolds numbers are discussed. The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers. The fluctuating velocities along the streamwise and normal directions are suppressed significantly,while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force. However, the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction. Therefore, the significant effect of drag reduction is obtained. Moreover,the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.     

Model predictive inverse method for recovering boundary conditions of two-dimensional ablation

A model predictive inverse method (MPIM) is presented to estimate the time- and space-dependent heat flux onthe ablated boundary and the ablation velocity of the two-dimensional ablation system. For the method, first of all, therelationship between the heat flux and the temperatures of the measurement points inside the ablation material is establishedby the predictive model based on an influence relationship matrix. Meanwhile, the estimation task is formulated as aninverse heat transfer problem (IHTP) with consideration of ablation, which is described by an objective function of thetemperatures at the measurement point. Then, the rolling optimization is used to solve the IHTP to online estimate theunknown heat flux on the ablated boundary. Furthermore, the movement law of the ablated boundary is reconstructedaccording to the estimation of the boundary heat flux. The effects of the temperature measurement errors, the numberof future time steps, and the arrangement of the measurement points on the estimation results are analyzed in numericalexperiments. On the basis of the numerical results, the effectiveness of the presented method is clarified.     

Analytical study of Cattaneo–Christov heat flux model for a boundary layer flow of Oldroyd-B fluid

We investigate the Cattaneo–Christov heat flux model for a two-dimensional laminar boundary layer flow of an incompressible Oldroyd-B fluid over a linearly stretching sheet. Mathematical formulation of the boundary layer problems is given. The nonlinear partial differential equations are converted into the ordinary differential equations using similarity transformations. The dimensionless velocity and temperature profiles are obtained through optimal homotopy analysis method(OHAM). The influences of the physical parameters on the velocity and the temperature are pointed out. The results show that the temperature and the thermal boundary layer thickness are smaller in the Cattaneo–Christov heat flux model than those in the Fourier's law of heat conduction.     

Different effects of grain boundary scattering on chargeand heat transport in polycrystalline platinum and goldnanofilms

The in-plane electrical and thermal conductivities of several polycrystalline platinum and gold nanofilms with different thicknesses are measured in a temperature range between the boiling point of liquid nitrogen (77K) and room temperature by using the direct current heating method. The result shows that both the electrical and thermal conductivities of the nanofilms reduce greatly compared with their corresponding bulk values. However, the electrical conductivity drop is considerably greater than the thermal conductivity drop, which indicates that the influence of the internal grain boundary on heat transport is different from that of charge transport, hence leading to the violation of the Wiedemann--Franz law. We build an electron relaxation model based on Matthiessen's rule to analyse the thermal conductivity and employ the Mayadas & Shatzkes theory to analyse the electrical conductivity. Moreover, a modified Wiedemann--Franz law is provided in this paper, the obtained results from which are in good agreement with the experimental data.