Numerical simulation of radiative-convective heat transfer at streamlining of the semitransparent film of melt

Nonstationary radiative-convective heat transfer was simulated at turbulent streamlining of a semitransparent film of melt on a flat plate by the gas mixture and solid particles. The moving film is subjected to intensive radiation heating by an external source radiating within a limited spectral range. The temperature and velocity fields are calculated in the boundary layer and in the film. Calculation results allow determination of the effect of source temperature and film initial heating on dynamics of temperature and velocity fields of the medium in the boundary layer-film system.     

Radiative-Convective Heat Transfer in Discharge Chamber at Turbulent Gas Flow

In the paper radiative-convective heat transfer in electric arc flowed through by Ar in the discharge chamber is studied. The equation system for heat, mass and momentum transfer in boundary layer approximation is considered, and the turbulence model with radiation pulsation and electrodynamic values is suggested. By solution of the equation system with the help of finite differences method distributions of temperature, heat flows and turbulent transfer coefficients over chamber length were obtained. The radiation and turbulence influence on the electric arc characteristics is analysed.     

Unsteady coupling of Navier-Stokes and radiative heat transfer solvers applied to an anisothermal multicomponent turbulent channel flow

Direct numerical simulations (DNS) of an anisothermal reacting turbulent channel flow with and without radiative source terms have been performed to study the influence of the radiative heat transfer on the optically non-homogeneous boundary layer structure. A methodology for the study of the emitting/absorbing turbulent boundary layer (TBL) is presented. Details on the coupling strategy and the parallelization techniques are exposed. An analysis of the first order statistics is then carried out. It is shown that, in the studied configuration, the global structure of the thermal boundary layer is not significantly modified by radiation. However, the radiative transfer mechanism is not negligible and contributes to the heat losses at the walls. The classical law-of-the-wall for temperature can thus be improved for RANS/LES simulations taking into account the radiative contribution.     

Direct numerical simulation of turbulence/radiation interaction in premixed combustion systems

An important fundamental issue in chemically reacting turbulent flows is turbulence/radiation interaction (TRI); TRI arises from highly nonlinear coupling between temperature and composition fluctuations. Here, a photon Monte Carlo method for the solution of the radiative transfer equation has been integrated into a turbulent combustion direct numerical simulation (DNS) code. DNS has been used to investigate TRI in a canonical configuration with systematic variations in optical thickness. The formulation allows for nongray gas properties, scattering, and general boundary treatments, although in this study, attention has been limited to gray radiation properties, no scattering, and black boundaries. Individual contributions to emission and absorption TRI have been isolated and quantified. Of particular interest are intermediate values of optical thickness where, for example, the smallest hydrodynamic and chemical scales are optically thin while the largest turbulence scales approach an optically thick behavior. In the configuration investigated, the temperature self-correlation contribution (emission) is primarily a function of the ratio of burned-gas temperature to unburned-gas temperature, and is the dominant contribution to TRI only in the optically thin limit. Even in the most optically thin case considered, the absorption coefficient–Planck function correlation and absorption coefficient–intensity correlation are not negligible. At intermediate values of optical thickness, contributions from all three correlations are significant.     

Experimental characterization of the convective heat transfer in a vortex-wall interaction

The development of turbulence models and wall laws for the numerical simulation of flows in complex geometries requires a detailed experimental analysis of turbulence and of the phenomena that appear in turbulent boundary layers. There is a strong need to develop new measurement systems allowing the determination of unsteady wall heat transfer coefficients. In order to improve the knowledge of the unsteady phenomena occurring in perturbed boundary layers, a fundamental study is conducted on the interaction of a single vortex with a flat plate. An experimental methodology using a specific thermal sensor whose surface temperature is measured by an infrared thermography system is presented. It allows the characterization of the unsteady convective heat transfer coefficient whose evolution is compared with the fluctuations of the wall friction coefficient, calculated from velocity profiles measured by laser Doppler velocimetry.     

Direct numerical simulation of turbulence-radiation interactions in a statistically one-dimensional nonpremixed system

An important issue in chemically reacting turbulent flows is the interaction between turbulence and radiation (TRI), which arises from highly nonlinear coupling between fluctuations in temperature and species composition of the flow field with the fluctuations of radiative intensity. Here direct numerical simulation (DNS) has been employed to investigate TRI in canonical nonpremixed systems in three-dimensional geometries. A photon Monte Carlo method has been used to solve the radiative transfer equation (RTE), which has been coupled with the flow solver. Radiation properties employed here correspond to a nonscattering fictitious gray gas with a Planck-mean absorption coefficient, which mimics that of typical hydrocarbon-air combustion products. Individual contributions of emission and absorption TRI have been isolated and quantified. The temperature self-correlation, the absorption coefficient-Planck function correlation, and the absorption coefficient-intensity correlation have been examined for intermediate-to-large values of the optical thickness, and contributions from all three correlations were found significant but the relative importance of their contribution varies with optical thickness.     

Generalized wall functions for turbulent flows with strong adverse pressure gradient

The generalized wall functions for turbulent flows with strong adverse pressure gradients are derived on the basis of the asymptotic theory of near-wall turbulence. The generalized wall functions have a correct asymptotic behavior in the limit of zero friction velocity and can be applied to computations of flows under a strong adverse pressure gradient and with separation or reattachment. Calculations of a turbulent boundary layer in a strong adverse pressure gradient with the aid of the developed modified k-ɛ model of turbulence and comparison with the experimental data validate the advantages of the generalized wall functions over traditional wall functions based on the logarithmic law of the wall.     

Velocity,temperature, and Reynolds-stress scaling in the wall region of turbulent boundary layer on a permeable surface

A finite total number of flow parameters in the wall region of a turbulent boundary layer points to universal behavior of turbulent shear stress as a function of mean-velocity gradient and turbulent heat flux as a function of both mean-velocity and mean-temperature gradients. Combined with dimensional arguments, this fact is used to reduce the momentum and heat equations to first-order ordinary differential equations for temperature and velocity profiles amenable to general analysis. Scaling laws for velocity and temperature in boundary layer flows with transpiration are obtained as generalizations of well-known logarithmic laws. Scaling relations are also established for shear stress and rms transverse velocity fluctuation. The proposed method has substantial advantages as compared to the classical approach (which does not rely on fluid-dynamics equations [1–3]). It can be applied to establish scaling laws for a broader class of near-wall turbulence problems without invoking closure hypotheses.     

A Modeling Study on Particle Dispersion in Wall-Bounded Turbulent Flows

Three physical mechanisms which may affect dispersion of particle's motion in wall-bounded turbulent flows, including the effects of turbulence, wall roughness in particle-wall collisions, and inter-particle collisions, are numerically investigated in this study. Parametric studies with different wall roughness extents and with different mass loading ratios of particles are performed in fully developed channel flows with the Eulerian-Lagrangian approach. A low-Reynolds-number $k-\epsilon$ turbulence model is applied for the solution of the carrier-flow field, while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. It is shown that the mechanism of inter-particle collisions should be taken into account in the modeling except for the flows laden with sufficiently low mass loading ratios of particles. Influences of wall roughness on particle dispersion due to particle-wall collisions are found to be considerable in the bounded particle-laden flow. Since the investigated particles are associated with large Stokes numbers, i.e., larger than $\mathcal{O}(1)$, in the test problem, the effects of turbulence on particle dispersion are much less considerable, as expected, in comparison with another two physical mechanisms investigated in the study.     

Fluctuations de temperature et flux de chaleur en aval d'une source linéaire placée dans une couche limite turbulente

The purpose of the present communication is to present and discuss some recent experimental results concerning the dispersion downstream of a heated line source located, in a turbulent boundary layer, successively at two distances from an adiabatic wall. Information on the mean and fluctuating temperature fields and associated heat fluxes are presented and analysed by testing closure assumptions for model dispersion at first and second orders. This model, which only requires the use of temporal velocity scales of the flow, leads to a rather good agreement with measurements.     

Inversion of combustion gas temperature/concentration profile with radiation/turbulence interaction using SRS

The spectral remote sensing (SRS) method is applied to the combustion gas with radiation/turbulence interaction to invert the temperature and concentration profiles. The flame is made and controlled to be optically thin per each fluctuation length so that spatially fluctuating gas layer can be treated equivalently as timewisely fluctuating one sharing identical expression in the radiative transfer equation. Then, the spectral intensity, temperature and concentration distributions are measured for the inversion and as the reference solutions. From the inversion results, we find that SRS can successfully invert the coupled temperature/concentration fluctuation amplitudes and mean values. For the two cases of experiment, inverted values are in good agreement with measured ones. However, SRS cannot find the detailed local fluctuation parameter such as pattern or phase, etc. as far as they do not affect the resulting radiative intensity. So, it is deduced that the pattern or phase is not so influential, whereas the mean value and the fluctuation amplitude of temperature/concentration profile are important parameters for the radiative intensity in the case of the optically thin condition. Further, the radiation/turbulence interaction is verified to play an important role in the radiation.     

Direct numerical simulations of second-order Stokes wave driven smooth-walled oscillatory channel: investigation of net current formation

In wall-bounded time-periodic flows, nonlinearity, associated with higher harmonic term(s) in velocity and/or acceleration outside the boundary layer, can significantly change the wall turbulence compared with that in the linear Stokes Boundary Layer. A significant feature of a nonlinear wall-bounded turbulent time-periodic flow is the formation of a net current which has not yet been mechanistically explained. This study investigates the effects of asymmetric velocity outside the boundary layer on wall turbulence and net current formation through Direct Numerical Simulations of a smooth-walled planar channel driven by the Second-order Stokes Wave. Simulation results suggest that net current characteristics depend on whether developed turbulence is present. When turbulence is developed, asymmetric viscous length scale is found to be the primary reason of the net current whereby a vertical offset between negative and positive Reynolds shear stress profiles, associated with forward and reverse flows, respectively, is created in a cycle. After averaging over a cycle, residual Reynolds shear stress, which drives the net current, is observed to be within the offset layer.     

Numerical study of heat transfer in an optically thick semi-transparent spherical porous medium

In this paper, heat transfer by simultaneous convection, conduction and radiation in a semi-transparent spherical porous medium is investigated. The ROSSELAND approximation is adopted to take account of radiation in the heat transfer rate. The routine used here to solve the set of differential equations is taken from the IMSL MATH/LIBRARY. Various results are obtained for the dimensionless temperature profiles in the solid and fluid phases, the radiative, conductive, convective and total heat fluxes. The effects of some radiative properties of the medium on the heat transfer rate are examined.     

Reply to the note on the flow behaviour of combustion gas in the boundary layer of an MHD generator

The temperature profile in Fig. 3 of Ref. 1 is better approximated by a relation for a turbulent boundary layer than a laminar boundary layer. The critical Reynolds number is expected to be lowered in the flow train of MHD combustion gas because of large turbulence in the combustor and by roughness along the duct.     

The effect of high temperature induced variation of specific heat on the hypersonic turbulent boundary layer and its computation

For hypersonic vehicles,as the temperature in its boundary layer usually exceeds 600 K,for which the molecular vibrational degree of freedom is excited,the perfect gas model is no longer valid.In this paper,the effect of high temperature induced variation of specific heat on the hypersonic turbulent boundary layer of flat plates is investigated by direct numerical simulations with the perfect gas model,i.e.with constant specific heat,as well as with a variable specific heat gas model.The comparison of the results from the two gas models has found that the effect of the variation of specific heat on the velocity of the turbulent boundary layers is relatively small,while its effect on temperature,such as the mean temperature,the temperature fluctuations,is appreciable.It is also found that the mean specific heat is quite close to the specific heat calculated by using the mean temperature,indicating that it is possible to do turbulence modeling.The modeling is done under the variable specific heat gas model with the mean temperature as the variable.The feasibility of such consideration is verified by applying the SST model for variable specific heat turbulence computation.     

Extensive characterisation of a high Reynolds number decelerating boundary layer using advanced optical metrology

Over the last years, the observation of large-scale structures in turbulent boundary layer flows has stimulated intense experimental and numerical investigations. Nevertheless, partly due to the lack of comprehensive experimental data at sufficiently high Reynolds number, our understanding of turbulence near walls, especially in decelerating situations, is still quite limited. The aim of the present contribution is to combine the equipment and skills of several teams to perform a detailed characterisation of a large-scale turbulent boundary layer under adverse pressure gradient. Extensive particle image velocimetry (PIV) measurements are performed, including a set-up with 16 sCMOS cameras allowing the characterisation of the boundary layer on 3.5 m, stereo PIV and high resolution near wall measurements. In this paper, detailed statistics are presented and discussed, boundary conditions are carefully characterised, making this experiment a challenging test case for numerical simulation.     

Effect of particles on the flow structure and dispersion of solid impurities in a two-phase axisymmetric jet

The Euler approach is used for studying the structure of a flow and the propagation of a disperse impurity in a submerged two-phase jet for small values of the mass concentration of particles (M _L1 = 0 to 0.5) upon a variation of the size and material of particles in a wide range. The effect of particles on the propagation of a two-phase jet, gas turbulence, and solid phase dispersion is analyzed. The addition of particles decreases the jet opening angle, increases the jet range, suppresses turbulence, and deteriorates turbulent mixing with the surrounding submerged space. It is shown that at the first stage, particle accumulation effects (pinching) in the axial region of the jet appear upon an increase in the particle size and the density of the particle material. Then, upon an increase in the inertia of particles, pinching changes to intense scattering of the disperse phase in the initial cross sections of the jet. The results are compared with the results of measurements for mono- and polydisperse two-phase jet flows.     

高超声速飞行器气体辐射噪声计算方法

高超声速飞行器周围的激波层内高温气体会发生剧烈的物理化学变化,伴随强烈的光辐射过程,直接影响红外导引头的光学成像效果。采用流体力学Navier-Stokes方程和热化学非平衡模型模拟高温非平衡流动,考虑电子跃迁和振转跃迁以窄带法求解气体辐射特性参数,基于有限体积法离散辐射传输方程,在分别验证流场解算、辐射参数求解和辐射传输计算的基础上,进行了飞行器高速飞行的流动和辐射模拟。数值求解得到了飞行器流场特征和粒子数空间分布。计算的选定波长范围内的气体辐射发射系数空间分布显示其与激波形状和波后气体温度分布相似。通过传输得到的飞行器光学窗口视线路径上的气体辐射噪声成轴对称分布,发现辐射噪声和飞行速度、气体成分等密切相关,马赫数增加时气体辐射噪声显著增强。     

Turbulent thermal boundary layer on a permeable flat plate

Scaling laws are established for the profiles of temperature, turbulent heat flux, rms temperature fluctuation, and wall heat transfer in the turbulent boundary layer on a flat plate with transpiration. In the case of blowing, the temperature distribution represented in scaling variables outside the viscous sublayer has a universal form known from experimental data for flows over impermeable flat plates. In the case of suction, the temperature distribution is described by a one-parameter family of curves. A universal law of heat transfer having the form of a generalized Reynolds analogy provides a basis for representation of the heat flux distributions corresponding to different Reynolds numbers and transpiration velocities in terms of a function of one variable. The results are obtained without invoking any special closure hypotheses.     

DNS of compressible turbulent boundary layer around a sharp cone

Direct numerical simulation of the turbulent boundary layer over a sharp cone with 20° cone angle (or 10° half-cone angle) is performed by using the mixed seventh-order up-wind biased finite difference scheme and sixth-order central difference scheme. The free stream Mach number is 0.7 and free stream unit Reynolds number is 250000/inch. The characteristics of transition and turbulence of the sharp cone boundary layer are compared with those of the flat plate boundary layer. Statistics of fully developed turbulent flow agree well with the experimental and theoretical data for the turbulent flat-plate boundary layer flow. The near wall streak-like structure is shown and the average space between streaks (normalized by the local wall unit) keeps approximately invariable at different streamwise locations. The turbulent energy equation in the cylindrical coordinate is given and turbulent energy budget is studied. The computed results show that the effect of circumferential curvature on turbulence characteristics is not obvious.