Turbulence modelling of the shear layers in the trailing edge region of a modern rear-loaded airfoil at incidence

After carefull analysis in a turbulent zero-pressure gradient flow, various simple algebraic turbulence models were applied to the almost separated flow on the upperside of an airfoil at incidence. The Johnson-King and Horton non-equilibrium (or rate equation) models give clearly improved results.     

Advanced Turbulence Modelling of Separated Flow in a Diffuser

The paper describes an investigation into the predictive performance of linear and non-linear eddy-viscosity models and differential stress-transport closures for separated flow in a nominally two-dimensional, asymmetric diffuser. The test case forms part of a broader collaborative exercise between academic and industrial partners. It is demonstrated that advanced turbulence models using strain-dependent coefficients and anisotropy-resolving closure offer tangible advantages in predictive capability, although the quality of their performance can vary significantly, depending on the details of closure approximations adopted. Certain features of the flow defy resolution by any of the closures investigated. In particular, no model resolves correctly the flow near the diffuser's inclined wall immediately downstream of the inlet corner, which may reflect the presence of a “flapping” motion associated with a highly-localised process of unsteady separation and reattachment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamics and radiation of a single bubble under conditions of abnormal compressibility of a bubbly liquid

An equation is proposed for the pulsation of a single cavity in an abnormally compressible bubbly liquid which is in pressure equilibrium and whose state is described by the Lyakhov equation. In the equilibrium case, this equation is significantly simplified. Numerical analysis is performed of the bubble dynamics and acoustic losses (the profile and amplitude of the radiation wave generated on the bubble wall from the side of the liquid). It is shown that as the volumetric gas concentration k₀ in the equilibrium bubbly medium increases, the degree of compression of the cavity by stationary shock wave decreases and its pulsations decrease considerably and disappear already at k₀ = 3%. In the compression process, the cavity asymptotically reaches an equilibrium state that does not depend on the value of k₀ and is determined only by the shock-wave amplitude. The radiation wave takes the shape of a soliton whose amplitude is much smaller and whose width is considerably greater than the corresponding parameters in a single-phase liquid. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 3, pp. 51–57, May–June, 2007.     

Analysis of a discontinuous Galerkin approximation of the Stokes problem based on an artificial compressibility flux

In this work, we propose and analyse a discontinuous Galerkin (DG) method for the Stokes problem based on an artificial compressibility numerical flux. A crucial step in the definition of a DG method is the choice of the numerical fluxes, which affect both the accuracy and the order of convergence of the method. We propose here to treat the viscous and the inviscid terms separately. The former is discretized using the well‐known BRMPS method. For the latter, the problem is locally modified by adding an artificial compressibility term of the form (1/c²)(?p/?t) for the sole purpose of interface flux computation. The flux is obtained as the exact solution of a local Riemann problem. The analysis of the method extends the well‐established strategies for the DG discretization of the Laplacian to the resulting partially coercive problem. Copyright © 2007 John Wiley & Sons, Ltd.     

Reynolds stress modelling of jet and swirl interaction inside a gas turbine combustor

Influences of the inlet swirl levels on the interaction between the dilution air jets and the swirling cross‐flow to the interior flow field inside a gas turbine combustor were investigated numerically by Reynolds stress transport model (RSTM). Due to the intense swirl and jet interaction, a high level of swirl momentum is transported to the centreline and hence, an intense vortex core is formed. The strength of the centreline vortex core was found to depend on the inlet swirl levels. For the higher swirling inlet, the decay of the swirling motion causes strong streamline variation of pressure; and consequently leads to an elevated level of deceleration of its axial velocity. Predictions contrasted with measurements indicate that the stress model reproduces the flow correctly and is able to reflect the influences of inlet swirl levels on the interior flow structure. Copyright © 1999 John Wiley & Sons, Ltd.     

Turbulence enhancement of stagnation point heat transfer on a body of revolution

Enhancement, by free stream turbulence, of convective heat transfer to the stagnation region of a hemispherical-nosed cylinder has been studied. Increases in heat transfer were found to depend primarily on the Reynolds number and turbulence intensity of the free stream, experimental results being most successfully correlated on a NuRe^?0.5 versus TuRe^0.5 basis. Flow visualization studies have demonstrated the validity of a phenomenological model of the enhancement process, predictions of this theory showing good agreement with experimental results. The effect of free stream turbulence on the stagnation point velocity gradient has also been evaluated.     

Turbulence in a skewed three‐dimensional wall‐bounded shear flow: effect of mean vorticity on structure modification

Mean‐flow three‐dimensionalities affect both the turbulence level and the coherent flow structures in wall‐bounded shear flows. A tailor‐made flow configuration was designed to enable a thorough investigation of moderately and severely skewed channel flows. A unidirectional shear‐driven plane Couette flow was skewed by means of an imposed spanwise pressure gradient. Three different cases with 8°, 34°and 52°skewing were simulated numerically and the results compared with data from a purely two‐dimensional plane Couette flow. The resulting three‐dimensional flow field became statistically stationary and homogeneous in the streamwise and spanwise directions while the mean velocity vector V and the mean vorticity vector Ω remained parallel with the walls. Mean flow profiles were presented together with all components of the Reynolds stress tensor. The mean shear rate in the core region gradually increased with increasing skewing whereas the velocity fluctuations were enhanced in the spanwise direction and reduced in the streamwise direction. The Reynolds shear stress is known to be closely related to the coherent flow structures in the near‐wall region. The instantaneous and ensemble‐averaged flow structures were turned by the skewed mean flow. We demonstrated for the medium‐skewed case that the coherent structures should be examined in a coordinate system aligned with V to enable a sound interpretation of 3D effects. The conventional symmetry between Case 1 and Case 2 vortices was broken and Case 1 vortices turned out to be stronger than Case 2. This observation is in conflict with the common understanding on the basis of the spanwise (secondary) mean shear rate. A refined model was proposed to interpret the structure modifications in three‐dimensional wall‐flows. What matters is the orientation of the mean vorticity vector Ω relative to the vortex vorticity vector ω _v, that is, the sign of Ω · ω _v. In the present situation, Ω · ω _v > 0 for the Case 1 vortices causing a strengthening relative to the Case 2 vortices. Copyright © 2011 John Wiley & Sons, Ltd.     

Resuspension and Sedimentation of Particles from a Sediment Bed by Turbulent Jets

An experimental study was carried out to investigate the resuspension of heavy particles of a sediment bed by axi-symmetric turbulent jets issuing from below. The case of point jets and loosely-held monodispersed particles was considered, in particular, the cases of a single jet and an array of jets located in the intersection points of a square grid. The aim was to map the flow field and the nature of sediment distribution as a function of governing parameters and to parameterize salient observables such as the maximum height of rise of sediments. The results were extrapolated to study sediment resuspension in karstic lakes, especially lake Banyoles located in Catalonia, northeastern Spain.     

Effect of flows between formations and capillary forces on the process of oil displacement in a stratified inhomogeneous bed

The results of a numerical investigation of the process of oil displacement in a stratified inhomogeneous formation on the basis of the two-phase flow model with account for capillary forces are presented. It is shown that in many cases the vertical inhomogeneity of oil reservoirs may not be a cause of nonuniform displacement and the non-recovery of large oil reserves by the time of water breakthrough to the extraction surface. The action of the capillary forces is an additional factor leading to equalization of the water propagation front in the inhomogeneous formation, water breakthrough delay, and intensification of the mass transfer between the layers with different permeabilities. Analysis of the contribution of the interlayer flows to the water flooding of low-permeability formation intervals calls into question the practicability of blocking high-permeability inclusions in the neighborhood of pumping wells.     

Effect of the Turbulent Energy Gradient and the Presence of a Wall on the Turbulent Process of Momentum Transport

A mathematical model of turbulent transport processes is modified to make allowance for the turbulent energy gradient and the presence of walls. The modification consists in making the variance tensor in the Gaussian probability density distribution for the initial mole velocities anisotropic for nonzero turbulent energy gradient and a ratio of the turbulence scale to the distance from the wall of the order of unity. Formulas for the variance tensor components are derived and the empirical coefficients of these formulas are determined. The expression for the dimensionless turbulent friction stress is compared with experimental data for three boundary-layer-type flows, namely, in the wake of a cylinder, in the boundary layer on a flat plate, and in a channel with parallel walls.     

Experimental study of extreme thrust on a tidal stream rotor due to turbulent flow and with opposing waves

Time-varying thrust has been measured on a rotor in shallow turbulent flow at laboratory scale. The onset flow has a turbulence intensity of 12% at mid depth and a longitudinal turbulence length scale of half the depth, about 5 times the vertical scale, typical of shallow flows. The rotor is designed to have thrust and power coefficient variations with tip speed ratio close to that of a full-scale turbine. Three extreme probability distributions give similar thrust exceedance values with the Type 1 Pareto in mid range which gives 1:100, 1:1000 and 1:10 000 exceedance thrust forces of 1.38, 1.5 and 1.59 times the mean value. With opposing waves superimposed the extreme thrust distribution has a very similar distribution to the turbulent flow only. Exceedance forces are predicted by superposition of a drag force with drag coefficient of 2.0 based on the wave particle velocity only and with an unchanged mean thrust coefficient of 0.89. These values are relevant for the design of support structures for marine turbines.     

Experimental investigation of convective flows in a plane horizontal fluid layer heated from below and rotating about the vertical axis

The convective coherent structures in a plane horizontal fluid layer, heated from below and capable of rotation about the vertical axis, are experimentally investigated. It is shown that with increase in the supercriticality the time it takes for the convective structures to be formed decreases sharply. Rotation and an increase in the layer thickness-to-diameter ratio lead to an increase in the steady-state attainment time.     

爆轰波通过扩张喷管的双曝光全息实验和数值研究

结合实验和数值模拟方法,对以脉冲爆轰发动机为背景的爆轰波通过扩张喷管的流动进行了系列研究。实验采用双曝光全息干涉方法对爆轰波绕射流场进行测量,得到了比传统的纹影法更清晰和可定量化的照片。发展了基于非结构四边形网格自适应有限体积程序,结合基元化学反应模型对扩张喷管中爆轰化学反应流场进行了数值模拟,模拟结果与实验照片吻合较好。实验和数值模拟结果表明,爆轰波绕射具有许多和激波绕射不同的流场特征,其中包括二次起爆现象、化学反应面与前导激波相脱离而引起的复杂流场等,同时初始压力和扩张角度变化也对爆轰波绕射过程产生较大影响,初始压力越低,化学反应区和前导激波分离现象越明显,且前导激波的曲率越大。     

一对钢－钢摩擦副摩擦磨损的原位动态研究

摩擦磨损的扫描电子显微镜原位动态研究，可以跟踪观察磨屑形成和磨痕演变的全过程，以及材料表面在摩擦磨损中变化的真实情况．在由国产台式扫描电子显微镜改装成的滑动摩擦磨损试验装置上，对１５＃钢－４５＃钢摩擦磨损过程所作的原位动态研究表明：１５＃钢在对摩过程中始终存在粘－滑现象，微切削是其主要磨损机理；１５＃钢磨痕形貌变化可分５个阶段——第１阶段出现与滑动方向垂直的横向裂纹，第２阶段产生大片状剥离，第３阶段产生小片状疲劳剥离，第４阶段产生块状磨屑并形成表面凹坑，第５阶段磨痕趋于平坦．这种滑动摩擦磨损试验装置不仅具有对摩擦磨损过程作动态跟踪观察的功能，而且可用于对摩擦学过程作准动态观察，这两种观察方法对研究摩擦磨损过程同等重要     

The role of macroscopic hardening and individual length-scales on crack tip stress elevation from phenomenological strain gradient plasticity

This paper quantifies the effect of strain gradient plasticity (SGP) on crack tip stress elevation for a broad range of applied loading conditions and constitutive model parameters, including both macroscopic hardening parameters and individual material length-scales controlling gradient effects. Finite element simulations incorporating the Fleck-Hutchinson SGP theory are presented for an asymptotically sharp stationary crack. Results identify fundamental scaling relationships describing (i) the physical length-scales over which strain gradients are prominent, and (ii) the degree of stress elevation over conventional Hutchinson-Rice-Rosengren (HRR) fields. Results illustrate that the three length-scale theory predicts much larger SGP effects than the single length-scale theory. Critically, the first length-scale parameter dominates SGP stress elevation: this suggests that SGP effects in fracture can be predicted using the length-scales extracted from nanoindentation, which exhibits similar behavior. Transitional loading/material parameters are identified that establish regimes of SGP relevance: this provides the foundation for the rational application of SGP when developing new micromechanical models of crack tip damage mechanisms and associated subcritical crack propagation behavior in structural alloys.     

The effect of surface condition and rotation on film / transition boiling from a sphere

The effects of liquid subcooling and surface condition on transition boiling heat transfer from a rotating surface were investigated in a series of experiments in which Freon 113 was boiled on a spinning sphere. In addition to the usual thermal measurements, the boiling behavior in the transition regime was studied by visual means. It was found that the shape of the system's boiling curve was very dependent on the parameters in question. Hydrodynamic instabilities created in the film boiling regime by the rotation of the sphere were carried on to the transition and nucleate boiling regimes of the boiling curve. Boiling curves exhibiting more than one peak flux were obtained.     

Effect of decomposition of a gas hydrate on the gas recovery from a reservoir containing hydrate and gas in the free state

The problem of gas recovery in the process of methane hydrate dissociation in a reservoir saturated with gas-hydrate mixture is considered. The mathematical model of hydrate decomposition into gas and water is generalized to include the negative temperature interval and takes ice formation into account. The solution of the problem is represented in the self-similar approximation. It is shown that there exists a transition hydrate decomposition regime in which water and ice are formed simultaneously. A comparative analysis of the recovery is carried out on the basis of relations derived for the masses of recovered gas in different hydrate dissociation regimes. It is shown that an anomalous increase in the recovered gas volumes is observed in the transition hydrate dissociation regime.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, 2005, pp. 132–142. Original Russian Text Copyright © 2005 by Tsypkin.