Heat Transfer Enhancement from A Rectangular Flat Plate With Constant Heat Flux in Pulsating Flows

Experiments have been carried out to study heat transfer enhancement from a heated rectangular flat plate in pulsating flows. A heat transfer empirical formula of the heated rectangular flat plate in pulsating flows was developed that correlates the heat transfer enhancement factor to the Womersley number (α = 3.3–23.8), the Reynolds number (Re = 527–4,217), and the pressure coefficient (C _p  = 41.3–31,644.6). The results demonstrate that heat transfer from the rectangular flat plate was enhanced significantly under proper conditions. In addition, the influence of the Reynolds number on the heat transfer enhancement factor increases as the pressure amplitude increases.     

Strengthened opposition control for skin-friction reduction in wall-bounded turbulent flows

An opposition control scheme with strengthened control input is proposed and tested in turbulent channel flows at friction Reynolds number Re_τ = 180 by direct numerical simulations. When the detection plane is located at less than 20 wall units, the drag reduction rate can be greatly enhanced by increasing the control amplitude parameter. The maximum drag reduction rate achieved in the present study is around 33%, which is much higher than the best value of 25% reported in literature. The strengthened control can be more efficient to attain a given drag reduction rate. Based on the total shear stress at the virtual wall established between the real wall and the detection plane by the control, a new friction velocity is proposed and the corresponding coordinate transform is made. Scaled by the proposed friction velocity, the wall-normal velocity fluctuation and the Reynolds shear stress of the controlled flows are collapsed well with those of the uncontrolled flow in the new coordinate. Based on the similarity, a relation between drag reduction rate and the effectiveness of the virtual wall is deduced, which disclosed that the elevation and residual Reynolds shear stress at the virtual wall are the key parameters to determine the drag reduction rate. The conclusion are also validated at Re_τ = 395 and 590. The decrease of the drag reduction rate with the increase of the Reynolds number is attributed to the enhanced residual Reynolds shear stress at the virtual wall.     

Stagnation Region Heat Transfer for Circular Jets Impinging on a Flat Plate

The heat transfer characteristics in a stagnation region were investigated experimentally for five circular free jets impinging into a heated flat plate. The local temperature distributions are estimated from the thermal images obtained from an infrared camera. To get a precise heat transfer data over the plate, fully developed straight pipe jets were used in this study. Mean jet Reynolds number varied from 1,000 to 45,000, jet-to-plate vertical non-dimensional distance H/D varied from 2 to 6, and the spacing distance jet-to-jet S/D varied from 2 to 8. A geometrical arrangement of one jet surrounded by four jets an in-line array was tested. The results show that the stagnation point Nusselt number is correlated to a jet Reynolds number as Nu_st∝Re^0.61. The average Nusselt number is higher at a separation distance of 2D for three cases of spacing distances, S/D = 2, 4, and 6.     

Two- and three-dimensional waves in falling film flow in the nonlaminar flow regime: an NMR study

Nuclear magnetic resonance (NMR) velocity-encoded imaging results as well as propagators are presented for the nonlaminar flow regime of falling films. The film is generated by a continuous flow of silicon oil along a vertical poly(methyl methacrylate) plate. While the film remains purely laminar for a film Reynolds number Re_f=0.5, it exhibits laminar-wavy behavior for 1.0≤Re_f≤2.5. In this range, a laminar residual film can be distinguished from averaged waves near the surface of the film from measurements of the flow velocity ν₂ along the direction of gravity as a function of the coordinatex normal to the plate. The perpendicular velocity components, ν _x and ν _y are zero within the accuracy of the measurement, indicating that the wave motion is two-dimensional in the laminar-wavy case. For higher Re_f the waves are found to be three-dimensional, a straightforward division into a residual film and waves in ν _z is lost and the waves extend over the whole thickness of the film.     

On the vertical large eddy breakup device capability to decrease the turbulent drag

The possibility of decreasing the turbulent friction with the use of streamwise-aligned vertical large eddy breakup devices installed normal to the surface of a flat plate in an incompressible equilibrium turbulent boundary layer with a nominally gradientless flow past this plate is studied experimentally. The Reynolds number based on the boundary-layer momentum thickness is 1099 in the cross section where these vertical devices are mounted. It is shown that elements of this geometry are effective tools for modification of the turbulent boundary layer from the viewpoint of both the gain in friction drag and the suppression of turbulent fluctuations of velocity and, hence, can be considered as one of the most important factors of controlling the structure of the near-wall turbulence in the flow past a flat surface.     

Proposed stochastic parameterisations of subgrid turbulence in large eddy simulations of turbulent channel flow

Stochastic and deterministic subgrid parameterisations are developed for the large eddy simulation (LES) of a turbulent channel flow with friction-velocity-based Reynolds number of Re_τ = 950 and centreline-based Reynolds number of Re₀ = 20,580. The subgrid model coefficients (eddy viscosities) are determined from the statistics of truncated reference direct numerical simulations (DNSs). The stochastic subgrid model consists of a mean-field shift, a drain eddy viscosity acting on the resolved field and a stochastic backscatter force of variance proportional to the backscatter eddy viscosity. The deterministic variant consists of a net eddy viscosity acting on the resolved field, which represents the net effect of the drain and backscatter. LES adopting the stochastic and deterministic models is shown to reproduce the time-averaged kinetic energy spectra of the DNS within the resolved scales.     

Experimental investigation on the flow over normal flat plates with various corner shapes

The influence of corner modification on the flow over normal flat plates is experimentally investigated in a water tunnel. Particle image velocimetry measurement is performed at Re = 2240 based on the width of the plate. Besides the flat plate with sharp edges, others with chamfered corners, rounded corners and stepped corners at the windward side are studied. Although the flat plate is very thin compared with square cylinders, corner modification can also achieve a significant drag reduction. The mechanism for the drag reduction is explored based on the solid analysis of the vortical structures and the statistical parameters. With corner modification, the strength and the scale of the vortex are decreased, while the formation of the vortex is postponed, resulting in a weakened vortex-interaction process. Thus, the fluctuation intensity of the wake is weakened, the wake width is narrowed down with the recirculation region elongated, and the vortex shedding frequency becomes higher. These modifications interpret well how the drag of a normal plate could be reduced, which is confirmed by the drag estimation from the wake profiles.     

Visualization of flow past a square prism with cut-corners at the front-edge

Flow past a square prism with cut-corners at the front-edge is numerically and experimentally visualized to investigate a mechanism of drag reduction. An adaptive numerical scheme based on the vortex method is implemented for two values of the Reynolds number between 200 and 1,250, and the results are compared with experiments. Experimental visualization techniques include the hydrogen-bubble technique atRe=4,000 and the oil-flow technique atRe=10,000 for a global wake formation, and the aluminum-flake technique for transient flow at the early stage of motion atRe=1,250. A similar reattachment flow pattern is shown in a wide range of the Reynolds number between 200 and 10,000, which implies a possibility of the drag reduction in the Reynolds number being approximately lower than 8,000 unlike the previous findings.     

Experimental study of the evolution of periodic controlled disturbances in a hypersonic viscous shock layer on a flat plate

Characteristics of the fields of mean density and density fluctuations measured with introduction of periodic disturbances into a hypersonic viscous boundary layer on a flat plate are presented. The experiments are performed for a flow Mach number M_∞ = 21, Reynolds number per meter Re_1∞ = 6·10⁵ m⁻¹, and temperature factor of the surface T _w /T ₀ = 0.26. The disturbances are introduced into the shock layer by an oblique gasdynamic whistle. The work was financially supported by the Russian Foundation for Basic Research (Grants Nos. 04-01-00474 and 05-08-33436).     

Vorticity,backscatter and counter-gradient transport predictions using two-level simulation of turbulent flows

The two-level simulation (TLS) method evolves both the large-and the small-scale fields in a two-scale approach and has shown good predictive capabilities in both isotropic and wall-bounded high Reynolds number (Re) turbulent flows in the past. Sensitivity and ability of this modelling approach to predict fundamental features (such as backscatter, counter-gradient turbulent transport, small-scale vorticity, etc.) seen in high Re turbulent flows is assessed here by using two direct numerical simulation (DNS) datasets corresponding to a forced isotropic turbulence at Taylor’s microscale-based Reynolds number Re_λ ≈ 433 and a fully developed turbulent flow in a periodic channel at friction Reynolds number Re_τ ≈ 1000. It is shown that TLS captures the dynamics of local co-/counter-gradient transport and backscatter at the requisite scales of interest. These observations are further confirmed through a posteriori investigation of the flow in a periodic channel at Re_τ = 2000. The results reveal that the TLS method can capture both the large- and the small-scale flow physics in a consistent manner, and at a reduced overall cost when compared to the estimated DNS or wall-resolved LES cost.     

Generation of vortices by a streamwise oscillating cylinder

The wake of a streamwise oscillating cylinder is presently investigated. The Reynolds number investigated is 300, based on the cylinder diameterd. The cylinder oscillates at an amplitude of 0.5d and a frequencyf _e/f_s=1.8, wheref _e is the cylinder oscillating frequency andf _s is the natural vortex shedding frequency of a stationary cylinder. Under these conditions the flow is essentially two dimensional. A two-dimensional direct numerical simulation (DNS) scheme has been developed to calculate the flow. The DNS results display a street of binary vortices, each containing two counter-rotating vortical structures, symmetrical about the centerline, which is in excellent agreement with measurements. The drag and lift on the cylinder have been examined. The time averaged drag and lift are 1.4 and 0, respectively, which are the same as those on a stationary cylinder at the sameRe. However, the fluctuating drag was high, about 2.68. It has been found that, being symmetrically formed about the centerline, the binary vortices induce an essentially zero fluctuating lift, which may have a profound implication in flow control and engineering.     

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.     

Effect of Microbubble Gas Saturation on Near-Wall Turbulence and Drag Reduction

The article presents results of an experimental study of the effect of gravitational orientation of the flow along its lower/upper solid boundaries on reduction of turbulent drag and void fraction profiles with injection of gas through a porous channel wall. The shear stress on the wall was measured in the Reynolds number range Re_x = (0.23–1.1) × 10⁷ by floating element transducers; the void fraction profile was determined using a fiber-optic sensor. The void fraction in the inner (near-wall) region of the boundary layer was shown to be a key parameter for turbulent drag reduction. The size of the inner region depends on the gas flow rate, the fluid velocity, the distance downstream of the gas generator, and the gravitational orientation of the wall.     

POD-based wall boundary conditions for the numerical simulation of turbulent channel flows

Simulation of turbulent wall-bounded flows requires a high spatial resolution in the wall region, which limits the range of Reynolds numbers which can be effectively reached. In previous work, we proposed proper orthogonal decomposition (POD) based wall boundary conditions to bypass the simulation of the inner wall region. Tests were carried out for direct numerical simulation at a low Reynolds number Re_τ = 180. The boundary condition is based on the POD spatial eigenfunctions which are determined a priori in the full channel. It consists of a three-component velocity field on the plane y⁺ = 50 which is reconstructed at each instant from a combination of selected eigenfunctions. The coefficients of the combination are estimated from the simulation in the reduced domain using the threshold-based reconstruction method described in Podvin et al. The study is now extended to large-eddy simulation at higher Reynolds numbers Re_τ = 295 and Re_τ = 590. Two versions of the reconstruction method are considered. In the first version, both the phases and the moduli of the coefficients are allowed to vary. In the second version, only the phases are adjusted. We find that the latter method is associated with improved statistics and is relatively robust with respect to the reconstruction threshold. However, it is sensitive to the details of the numerical simulation, unlike the former method, which is associated with less accurate statistics and is more dependent on the reconstruction threshold.     

几种三元过渡金属碳化物弹性及电子结构的第一性原理研究

基于密度泛函理论平面波方法研究了六方WC型Re_xW_1－xC(x=1, 0.25, 0.75, 0),Re_0.5Os_0.5C和Os_0.5W_0.5C的晶体结构、弹性和电子结构性质.研究发现Re_0.25W_0.75C晶体具有优异的弹性性能及稳定性,其剪切模量(312 GPa)超过了所有其他实验合成和     

Investigating asymptotic suction boundary layers using a one-dimensional stochastic turbulence model

The turbulent asymptotic suction boundary layer is studied using a one-dimensional turbulence (ODT) model. ODT is a fully resolved, unsteady stochastic simulation technique. While flow properties reside on a one-dimensional domain, turbulent advection is represented using mapping events whose occurrences are governed by a random process. Due to its reduced spatial dimensionality, ODT achieves major cost reductions compared to three-dimensional (3D) simulations. A comparison to recent direct numerical simulation (DNS) data at moderate Reynolds number (Re = u_∞ / v₀ = 333, where u_∞ and v₀ are the free stream and suction velocity, respectively) suggests that the ODT model is capable of reproducing several velocity statistics, i.e. mean velocity and turbulent kinetic energy budgets, while peak turbulent stresses are under-estimated by ODT. Variation of the Reynolds number in the range Re ∈ [333,400,500,1000] shows that ODT can reproduce various trends observed as a result of increased suction in turbulent asymptotic suction boundary layers, i.e. the reduction of Reynolds stresses and enhanced skin friction. While up to Re = 500 our results can be directly compared to recent LES data, the simulation at Re = 1000 is currently not feasible through full 3D simulations, hence ODT may assist the design of future DNS or LES simulations at larger Reynolds numbers.     

Fluctuation measurements in a supersonic boundary layer implying in-situ calibration of the hot-wire anemometer

A calibration technique for the constant-temperature hot-wire anemometer is presented, which is based on traversing the probe through the boundary layer of a flat plate while simultaneously performing fluctuation measurements. The free stream Mach number was M = 2.54, and the Reynolds number Re_d, based on wire diameter, ranged from 9 to 23. A comparison of the sensitivity values obtained with the aid of such a calibration procedure — under the condition of neglecting low temperature loadings (t<0.6) — agrees well with sensitivities determined with free-stream data-The use of a modified transfer function for correcting the power spectra of flow perturbations revealed a conformity of wide parts of the corrected spectra with the Kolmogorov decay. The fluctuation levels of total temperature and mass flux were computed for the boundary layer of a flat plate. This work was presented at the International Conference on the Methods of Aerophysical Research ICMAR 2007, which was held in Novosibirsk on 5–10 February 2007.     

Effects of upstream roughness and Reynolds number on separated and reattached turbulent flow

This paper presents experimental investigation of upstream roughness and Reynolds number effects on the recirculation region over a smooth forward facing step. The upstream rough wall was produced from 1.5 mm sand grains and the Reynolds number based on step height, Re_h, was varied from 2040 to 9130 for both the upstream smooth and rough walls. For the smooth wall, the reattachment length increased monotonically with Re_h to an asymptotic value of 2.2 step heights for Re_h ≥ 6380. Upstream roughness reduced the reattachment length by 44% because of larger momentum deficit and higher turbulence level in the rough wall boundary layer. The mean velocities and Reynolds stresses were also reduced by roughness. The Reynolds shear stress and production of turbulent kinetic energy showed high negative values at the leading edge of the step indicating counter-gradient diffusion. The implications of these results for standard eddy viscosity models are discussed.     

Anisotropic Reynolds stress tensor representation in shear flows using DNS and experimental data

Tensorial decompositions and projections are used to study the performance of algebraic non-linear models and predict the anisotropy of the Reynolds stresses. Direct numerical simulation (DNS) data for plane channel flows at friction Reynolds number (Re_τ = 180, 395, 590, 1000), and for the boundary layer using both DNS (Re_τ = 359, 830, 1271) and experimental data (Re_τ = 2680, 3891, 4941, 7164) are used to build and evaluate the models. These data are projected into tensorial basis formed from the symmetric part of mean velocity gradient and non-persistence-of-straining tensor. Six models are proposed and their performances are investigated. The scalar coefficients for these six different levels of approximations of the Reynolds stress tensor are derived, and made dimensionless using the classical turbulent scales, the kinetic turbulent energy (κ) and its dissipation rate (ε). The dimensionless coefficients are then coupled with classical wall functions. One model is selected by comparing the predicted Reynolds stress components with experimental and DNS data, presenting a good prediction for the shear and normal Reynolds stresses.     

A high-resolution code for turbulent boundary layers

A new high-resolution code for the direct simulation of incompressible boundary layers over a flat plate is described. It can accommodate a wide range of pressure gradients, and general time-dependent boundary conditions such as incoming wakes or wall forcing. The consistency orders of the advective and pressure-correction steps are different, but it is shown that the overall resolution is controlled by the higher-order advection step. The formulation of boundary conditions to ensure global mass conservation in the presence of arbitrary forcing is carefully analyzed. Two validation boundary layers with and without a strong adverse pressure gradient are presented, with maximum Reynolds numbers Re_θ≈2000${\mathit{Re}}_{\theta }\approx 2000$. They agree well with the available experiments. Turbulent inflow conditions for the zero-pressure case are implemented by a recycling method, and it is shown that at least the initial 300 momentum thicknesses have to be discarded before the effect of the artificial inflow is forgotten. It is argued that this is not a defect of the method used to generate the inflow, but a property of the boundary layer.