两个角区湍流场及其尾迹的实验研究

绕两个翼型－平面的角区流动及其尾迹的实验是在低湍流度风洞中完成的．在零攻角条件下，对翼型－平面的角区流场内诸参数，如翼型表面和平板面上的压力分布、绕翼型及尾迹区内的平均速度、脉动速度、湍动能、二阶关联量ｕ′ｖ′及ｕ′ｗ′进行了广泛的测量．通过对比，分析了这两种模型与平面所构成的角区及其尾迹区内的流动特性     

Velocity–pressure integrated versus penalty finite element methods for high-Reynolds-number flows

Velocity–pressure integrated and consistent penalty finite element computations of high-Reynolds-number laminar flows are presented. In both methods the pressure has been interpolated using linear shape functions for a triangular element which is contained inside the biquadratic flow element. It has been shown previously that the pressure interpolation method, when used in conjunction with the velocity-pressure integrated method, yields accurate computational results for high-Reynolds-number flows. It is shown in this paper that use of the same pressure interpolation method in the consistent penalty finite element method yields computational results which are comparable to those of the velocity–pressure integrated method for both the velocity and the pressure fields. Accuracy of the two finite element methods has been demonstrated by comparing the computational results with available experimental data and/or fine grid finite difference computational results. Advantages and disadvantages of the two finite element methods are discussed on the basis of accuracy and convergence nature. Example problems considered include a lid-driven cavity flow of Reynolds number 10 000, a laminar backward-facing step flow and a laminar flow through a nest of cylinders.     

Effects of an upstream inclined rod on the circular cylinder–flat plate junction flow

An inclined rod is installed upstream of a circular cylinder mounted on a flat plate to mitigate the horseshoe vortices in the junction flow. Smoke-wire visualization, hot-wire velocity measurement and surface pressure measurement are employed to study the effects of the inclined rod on the laminar and turbulent junction flows. The results show a properly placed inclined rod can significantly weaken the horseshoe vortices in front of the cylinder, depress the unsteady oscillation of the vortex system, change the separation position on the flat plate and narrow the wake of the cylinder. The inclined rod method provides a promising way to suppress the horseshoe vortices in the junction flow because of its effectiveness and its easiness to implement and adjust to fit different flow conditions.     

Time-resolved velocity field measurements of separated flow in front of a vertical fence

 An experimental investigation of the velocity fields of the turbulent shear flow in front of a vertical fence was performed using a hybrid particle tracking velocimetry technique. The vertical fence, with an aspect ratio of 10, was embedded in a thin laminar boundary layer. The Reynolds number of the flow based on the fence height was about 5,000. The measured instantaneous velocity fields and flow visualization results revealed a complex but organized flow structure in front of the fence. The stagnation point detached from the surface of the fence, increasing the streamline curvature near the fence. Time resolved turbulence statistics were obtained and compared with those of a 3D junction flow. Received: 23 August 2000/Accepted: 1 February 2001     

Method of Lines for Transient Flow Fields

A computational fluid dynamics (CFD) code based on the method of lines (MOL) approach was developed for the solution of transient, two-dimensional Navier-Stokes equations for incompressible separated internal flows in complex rectangular geometries. The predictive accuracy of the code was tested by applying it to the prediction of flow fields in both laminar and turbulent channel flows with and without sudden expansion, and comparing its predictions with either measured data or numerical results available in the literature. The predicted flow fields were found to be in favorable agreement with those available in the literature for laminar channel flow with sudden expansion and turbulent channel flow with Re=6600. The code was then applied to the prediction of the highly turbulent flow field in the inlet flue of a heat recovery steam generator (HRSG). The predicted flow field was found to display the same trend with the experimental findings and numerical solutions reported previously for a turbulent diverging duct. As the code uses the MOL approach in conjunction with (i) an intelligent higher-order spatial discretization scheme, (ii) a parabolic algorithm for pressure, and (iii) an elliptic grid generator using a body-fitted coordinate system for complex geometries, it provides an efficient algorithm for future direct numerical simulation (DNS) applications in complex rectangular geometries.     

Numerical simulation of forced convection of nanofluid in a confined jet

In this paper two-dimensional incompressible water–Al₂O₃ nanofluid flow in a confined jet in the laminar flow regime is numerically investigated. A finite volume technique on a collocated grid is employed for discretizing the governing equations by applying the SIMPLE algorithm to link the pressure and velocity fields. The present computations are in a very good agreement with experimental results in open literature.     

Microscopic particle image velocimetry measurements of transition to turbulence in microscale capillaries

The character of transitional capillary flow is investigated using pressure-drop measurements and instantaneous velocity fields acquired by microscopic PIV in the streamwise–wall-normal plane of a 536 μm capillary over the Reynolds-number range 1,800 ≤ Re ≤ 3,400 in increments of 100. The pressure-drop measurements reveal a deviation from laminar behavior at Re = 1,900 with the differences between the measured and the predicted laminar-flow pressure drop increasing with increasing Re. These observations are consistent with the characteristics of the mean velocity profiles which begin to deviate from the parabolic laminar profile at Re = 1,900, interpreted as the onset of transition, by becoming increasingly flatter and fuller with increasing Re. A fully-turbulent state is attained at Re ≅ 3,400 where the mean velocity profile collapses onto the mean profile of fully-developed turbulent pipe flow from an existing direct numerical simulation at Re = 5,300. Examination of the instantaneous velocity fields acquired by micro-PIV in the range 1,900 ≤ Re < 3,400 reveal that transitional flows at the microscale are composed of a subset of velocity fields illustrating a purely laminar behavior and a subset of fields that capture significant departure from laminar behavior. The fraction of velocity fields displaying non-laminar behavior increases with increasing Re, consistent with past observations of a growing number of intermittent turbulent spots bounded by nominally laminar flow in macroscale pipe flow with increasing Re. Instantaneous velocity fields that are non-laminar in character consistently contain multiple spanwise vortices that appear to streamwise-align to form larger-scale interfaces that incline slightly away from the wall. The characteristics of these “trains” of vortices are reminiscent of the spatial features of hairpin-like vortices and hairpin vortex packets often observed in fully-turbulent wall-bounded flow at both the macro- and micro-scales. Finally, single-point statistics computed from the non-laminar subsets at each transitional Re, including root-mean-square velocities and the Reynolds shear stress, reveal a gradual and smooth maturation of the patches of disordered motion toward a fully-turbulent state with increasing Re.     

Premixed flame flashback in wall boundary layers studied by long-distance micro-PIV

Premixed flame propagation against the main flow direction in wall boundary layers, a situation known as wall flashback, has been studied by long-distance particle image velocimetry with spatial micron resolution (μ-PIV) and simultaneous flame luminescence recordings. Numerical simulations of laminar wall flashback assist the interpretation of the experimental results. Inside a turbulent boundary layer, the flame propagates in discrete flame cusps pointing in upstream direction and showing a well-defined pattern of formation and break-up. In a laminar boundary layer, the leading flame region is smooth and exhibits low curvature. The instantaneous velocity fields reveal a backflow region upstream of the flame during flashback, which is constrained to the leading flame zone. The backflow is caused by an interaction between the pressure increase upstream of the flame and the boundary layer. The flashback limit is controlled by thermal quenching of the flame tip in the backflow region.     

Flow development upstream of a fence

The effect of Reynolds number on the flow development upstream of a rigid, non-porous, static fence is investigated experimentally. The flow field is measured using time-resolved, two-component particle image velocimetry at Reynolds numbers based on fence height of 18000, 36000, and 54000. The results show that a laminar separation bubble forms upstream of the junction vortex at the base of the fence. The mean extent of the bubble decreases with increasing Reynolds number, with mean separation moving downstream and mean reattachment moving upstream. In the aft portion of the bubble, shear layer vortices form and are shed at scaled frequencies and wavelengths that are comparable to laminar separation bubble shedding in low Reynolds number airfoils and flat plates with an imposed adverse pressure gradient. The strong periodicity of the associated coherent structures and the proximity of shear layer roll-up relative to the fence should be taken into consideration in the relevant designs due to potential implications to structural loading. A simple flow separation prediction model combining inviscid fence flow solution with Thwaites’ method is introduced and shows good agreement with the experimental results for the Reynolds number range considered.     

Analysis of DNS and LES of Flow in a Low Pressure Turbine Cascade with Incoming Wakes and Comparison with Experiments

The flow around a low-pressure turbine rotor blade with incoming periodic wakes is computed by means of DNS and LES. The latter adopts a dynamic sub-grid-scale model. The computed results are compared with time-averaged and instantaneous measured quantities. The simulation sreveal the presence of elongated flow structures, stemming from the incoming wake vorticity, which interact with the pressure side boundary layer. As the wake approaches the upstream half of the suction side, its vortical structures are stretched and align with the main flow, resulting in an impingement at virtually zero angle of attack. Periodically, in the absence of impinging wakes, the laminar suction side boundary layer separates in the adverse pressure gradient region. Flow in the laminar separation bubble is found to undergo transition via a Kelvin–Helmholtz instability. Subsequent impingement of the wake inhibits separation and thus promotes boundary layer reattachment. LES provides a fair reproduction of the DNS results both in terms of instantaneous, phase-averaged, and time-averaged flow fields with a considerable reduction in computational effort. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental investigation of ice slurry flow pressure drop in horizontal tubes

Pressure drop behaviour of ice slurry based on ethanol–water mixture in circular horizontal tubes has been experimentally investigated. The secondary fluid was prepared by mixing ethyl alcohol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature ?4.4 °C). The pressure drop tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 30% depending on test conditions. Results from flow tests reveal much higher pressure drop for higher ice concentrations and higher velocities in comparison to the single phase flow. However for ice concentrations of 15% and higher, certain velocity exists at which ice slurry pressure drop is same or even lower than for single phase flow. It seems that higher ice concentration delay flow pattern transition moment (from laminar to turbulent) toward higher velocities. In addition experimental results for pressure drop were compared to the analytical results, based on Poiseulle and Buckingham–Reiner models for laminar flow, Blasius, Darby and Melson, Dodge and Metzner, Steffe and Tomita for turbulent region and general correlation of Kitanovski which is valid for both flow regimes. For laminar flow and low buoyancy numbers Buckingham–Reiner method gives good agreement with experimental results while for turbulent flow best fit is provided with Dodge–Metzner and Tomita methods.Furthermore, for transport purposes it has been shown that ice mass fraction of 20% offers best ratio of ice slurry transport capability and required pumping power.     

Development of an artificial compressibility methodology using flux vector splitting

An implicit, upwind arithmetic scheme that is efficient for the solution of laminar, steady, incompressible, two-dimensional flow fields in a generalised co-ordinate system is presented in this paper. The developed algorithm is based on the extended flux-vector-splitting (FVS) method for solving incompressible flow fields. As in the case of compressible flows, the FVS method consists of the decomposition of the convective fluxes into positive and negative parts that transmit information from the upstream and downstream flow field respectively. The extension of this method to the solution of incompressible flows is achieved by the method of artificial compressibility, whereby an artificial time derivative of the pressure is added to the continuity equation. In this way the incompressible equations take on a hyperbolic character with pseudopressure waves propagating with finite speed. In such problems the ‘information’ inside the field is transmitted along its characteristic curves. In this sense, we can use upwind schemes to represent the finite volume scheme of the problem's governing equations. For the representation of the problem variables at the cell faces, upwind schemes up to third order of accuracy are used, while for the development of a time-iterative procedure a first-order-accurate Euler backward-time difference scheme is used and a second-order central differencing for the shear stresses is presented. The discretized Navier–Stokes equations are solved by an implicit unfactored method using Newton iterations and Gauss–Siedel relaxation. To validate the derived arithmetical results against experimental data and other numerical solutions, various laminar flows with known behaviour from the literature are examined. © 1997 John Wiley & Sons, Ltd.     

Study of laminar–turbulent flow transition under pulsatile conditions in a constricted channel

In this paper, direct numerical simulation is performed to investigate a pulsatile flow in a constricted channel to gain physical insights into laminar–turbulent–laminar flow transitions. An in-house computer code is used to conduct numerical simulations based on available high-performance shared memory parallel computing facilities. The Womersley number tested is fixed to 10.5 and the Reynolds number varies from 500 to 2000. The influences of the degree of stenosis and pulsatile conditions on flow transitions and structures are investigated. In the region upstream of the stenosis, the flow pattern is primarily laminar. Immediately after the stenosis, the flow recirculates under an adverse streamwise pressure gradient, and the flow pattern transitions from laminar to turbulent. In the region far downstream of the stenosis, the flow becomes re-laminarised. The physical characteristics of the flow field have been thoroughly analysed in terms of the mean streamwise velocity, turbulence kinetic energy, viscous wall shear stresses, wall pressure and turbulence kinetic energy spectra.     

Finite element solutions of laminar flow and heat transfer of air in a staggered and an in-line tube bank

A finite element method is used to solve the full Navier-Stokes and energy equations for the problems of laminar flow and heat transfer characteristics of air around three isothermal heated horizontal cylinders in a staggered tube bank and around four isothermal heated horizontal cylinders in an in line tube bank. The variations of surface shear stress, pressure and Nusselt number are obtained over the entire cylinder surface, including the zone beyond the separation point. The predicted values of total drag, pressure drag and friction drag coefficients, average Nusselt number, and the plots of velocity flow fields and isotherms are also presented.     

A study of the asymptotic behaviour of the external fringes of compressible, laminar boundary layers of a dissociating gas

The structure of the hypersonic laminar boundary layer of an ideal dissociating gas far from the wall surface is analysed analytically for the case of (i) frozen flow and (ii) equilibrium flow corresponding to the local pressure and temperature, respectively. For the equilibrium flow also numerical results are given. It is observed that the transition zone between the viscous and the outer fringe of the boundary layer, which exists in the perfect gas case, is smeared out here due to the coupling between the imperfect nature of the gas and the viscosity of the gas.On leave from N.A.L., Bangalore, India.     

Couche limite sur une antenne de pression statique

Following a theoretical study of the incompressible flow past a static probe in uniform stream, the calculation of the laminar boundary layer on this body of revolution without incidence is presented.With the two Pohlhausen and Saljnikov one-parameter methods, occure a local separation at sphere-cylinder junction. This separation was noted in a flow visualisation. Transition point, depending on Reynolds number, is determined with this two methods by criterion Walz.     

SCALE ADAPTIVE SIMULATION OF FLOWS AROUND A CIRCULAR CYLINDER AT HIGH SUB-CRITICAL REYNOLDS NUMBER

借助γ-Re_θ转捩模型,实现了高亚临界雷诺数（Re=1.4×10⁵）下圆柱层流分离流动的尺度自适应模拟.统计平均结果表明数值计算和实验测量较为接近,尤其在圆柱后半段的分离区中,压力系数和实验符合得很好,误差主要源于分离点预测的不准确. 瞬态流动则显示,层流分离的剪切层中出现了展向不稳定,且在向下游的输运过程中不断增强,最后转捩为完全湍流. 在湍流分离模拟中,由于缺乏剪切层失稳的非定常性,SST-SAS 模型的尺度分辨能力变弱,因此在分离区以及下游尾迹中求解出的湍流尺度要明显较层流分离时大.     

高亚临界雷诺数圆柱绕流的尺度自适应模拟

借助γ-Re_θ转捩模型,实现了高亚临界雷诺数（Re=1.4×105）下圆柱层流分离流动的尺度自适应模拟.统计平均结果表明数值计算和实验测量较为接近,尤其在圆柱后半段的分离区中,压力系数和实验符合得很好,误差主要源于分离点预测的不准确. 瞬态流动则显示,层流分离的剪切层中出现了展向不稳定,且在向下游的输运过程中不断增强,最后转捩为完全湍流. 在湍流分离模拟中,由于缺乏剪切层失稳的非定常性,SST-SAS 模型的尺度分辨能力变弱,因此在分离区以及下游尾迹中求解出的湍流尺度要明显较层流分离时大.      

Numerical modeling of a viscous incompressible unsteady separated flow past a rotating cylinder

For studying unsteady flow past a rotating circular cylinder the Navier-Stokes equations are used. The numerical algorithm is based on an artificial-compressibility method, an implicit three-layer second-order scheme with subiterations with respect to time, a third-order scheme with splitting of the flux vectors for the convective terms, and a central-difference scheme for integrating the viscous terms. The calculated velocity profiles, the vorticity fields, the Strouhal numbers, the distribution of the pressure and friction coefficients over the cylinder surface, and the coefficients of the drag and lift forces for the laminar flow regime are analyzed.