Three-dimensional computations of the impulsive wave discharged from a duct

A sudden discharge of mass flow from the exit of a duct can generate an impulsive wave, generally leading to undesirable noise and vibration problems The present study develops an understanding of unsteady flow physics with regard to the impulsive wave discharged from a duct, using a numerical method A second order total variation diminishing scheme ts employed to solve three-dimensional, unsteady, compressible Euler equations Computations are performed for several exit conditions with and without ground and wall effects under a change in the Mach number of an initial shock wave from 11 to 15 The results obtained show that the directivity and magnitude of the impulsive wave discharged from the duct are significantly influenced by the initial shock Mach number and by the presence of the ground and walls     

Numerical Study of Flow Characteristics due to Interaction Between a Pair of Vortices in a Turbulent Boundary Layer

This paper represents a numerical study of the flow field due to the interactions between a pair of vortices produced by vortex generators in a rectangular channel flow. In order to analyze longitudinal vortices induced by the vortex generators, the pseudo-compressibility method is introduced into the Reynolds-averaged Navier-Strokes equations of a 3-dimensional unsteady, incompressible viscous flow. A two-layer κ-ε turbulence model is applied to a flat plate 3-dimensional turbulence boundary to predict the flow structure and turbulence characteristics of the vortices. The computational results predict accurately the vortex characteristics related to the flow field, the Reynolds shear stresses and turbulent kinetic energy. Also, in the prediction of skin friction characteristics the computational results are reasonably close to those of the experiment obtained from other researchers.     

A gas-kinetic discontinuous Galerkin method for viscous flow equations

This paper presents a Runge-Kutta discontinuous Galerkin (RKDG) method for viscous flow computation. The construction of the RKDG method is based on a gas-kinetic formulation, which not only couples the convective and dissipative terms together, but also includes both discontinuous and continuous representation in the flux evaluation at the cell interface through a simple hybrid gas distribution function. Due to the intrinsic connection between the gaskinetic BGK model and the Navier-Stokes equations, the Navier-Stokes flux is automatically obtained by the present method. Numerical examples for both one dimensional (1D) and two dimensional (2D) compressible viscous flows are presented to demonstrate the accuracy and shock capturing capability of the current RKDG method.     

Simulations of the transient flow generated from a started flat plate

Transient operations are commonly founded in fluid machineries such as the starting, stopping, and variations of rotor speeds, etc. Flow generated from a started flat plate is of fundamental importance. Experiments have been done to observe the flow evolution in current researches. And in order to explore the flow in more detailed scale, some vortex methods with high resolution and other numerical methods were developed to solve various related problems by some researchers. But the promotion of vortex method to engineering application is rare due to its complexity and difficulty in specifying the boundary conditions. In order to build up a method of numerical study for such problems, a simplified model is built up with a flat plate. The development of two-dimensional viscous incompressible flow generated from an impulsively started and uniformly accelerated infinitesimally thin flat plate is simulated numerically. A dynamic mesh(DM) method based on the spring analogue and local remeshing is applied to realize the mesh motion caused by the started plate. Researches show that the mesh quality will decline under large grid shear force during the updating process. To conquer this problem, a region near the plate is separated to guarantee the mesh quality at location of interest which is the innovation of the present paper. All computations at least cover a period during which the plate translates 6 times its length. The simulated instantaneous velocity profiles, flow structures and drag coefficients under several Reynolds numbers (20 Re 126) and accelerations (20 m/s2 a 152 m/s2) are presented and compared with existing results in literatures. Comparisons are found to be satisfactory, confirming the validity of the current proposed method(region separated DM). The proposed DM method is firstly used to study the transient flow generated from a started flat plate and can be used in further study of transient characteristics during transient operations of turbo machineries.     

Nodeless variables finite element method and adaptive meshing teghnique for viscous flow analysis

A nodeless variables finite element method for analysis of two-dimensional, steady-state viscous incompressible flow is presented. The finite element equations are derived from the governing Navier-Stokes differential equations and a corresponding computer program is developed. The proposed method is evaluated by solving the examples of the lubricant flow in journal bearing and the flow in the lid-driven cavity. An adaptive meshing technique is incorporated to improve the solution accuracy and, at the same time, to reduce the analysis computational time. The efficiency of the combined adaptive meshing technique and the nodeless variables finite element method is illustrated by using the example of the flow past two fences in a channel.     

高压下环面节流静压气体轴承适用的润滑方程探讨

采用纯黏性润滑方程和基于层流模式、SST k-ω湍流模式的N-S方程,对环面节流静压气体润滑推力轴承内的压力分布进行了研究,分析了随着气膜厚度的变化,轴承流场内压力的变化及其变化机理。实验证明,在轴承厚度很小时两种方程求得的压力值与实验结果一致,然而随着气膜厚度的增大,采用纯黏性润滑方程计算所得结果的偏差很大,而采用N-S方程计算所得结果与实验结果基本一致,但在逆压力梯度段存在偏差;SST k-ω 湍流模式能较好地处理湍流剪切应力在逆压梯度边界层内的输运和激波与边界层的相互作用,准确模拟出气膜入口附近复杂的流动状态。      

Scaling of turbulence intensity for low-speed flow in smooth pipes

In this paper, we compare measured, modelled, and simulated mean velocity profiles. Smooth pipe flow simulations are performed for both incompressible (below Mach 0.2) and compressible (below Mach 0.1) fluids. The compressible simulations align most closely with the measurements. The simulations are subsequently used to make scaling formulae of the turbulence intensity as a function of the Reynolds number. These scaling expressions are compared to scaling derived from measurements. Finally, the found compressible scaling laws are used as an example to show how the flow noise in a flowmeter is expected to scale with the mean flow velocity.     

无动量亏损尾迹三维数值模拟

利用计算流体力学软件,对粘性不可压缩流体的平板翼型绕流进行了三维数值模拟,采用有限体积法计算程式,求解不可压缩Navier-Stokes方程,模拟平板翼型在尾缘有无吹气时的流动情况,得到了平板翼型尾迹的传播特性。并使用四种常用湍流模型在相同的网格条件下进行数值模拟,模拟结果与相应的试验数据对比,分析各种湍流模型对带尾缘吹气的三维模型数值模拟结果的影响,得出了不同湍流模型对带尾缘吹气的模型的适用性以及精度的评估性结论。数值模拟的结果表明SST湍流模拟对模拟尾迹区流动具有很好的适应性,预测得到的轴向速度分布、尾迹特征长度变化以及流动相似性和试验结果进行了对比,和试验结果吻合较好。     

Analysis of the Hydrodynamic Effects in a Surface Textured Circumferential Gas Seal

A theoretical finite-element model was developed to study the hydrodynamic effect of micro-pores generated by laser surface texturing (LST) in a circumferential gas seal. The seal is represented by two non-contacting annular surfaces of a rotating shaft and a stationary ring. The micro-pores of spherical segment shape are distributed uniformly over one of the annular surfaces. The hydrodynamic dimensionless pressure distribution in the uniform clearance between the annular surfaces is obtained from a solution of the Reynolds equation for compressible viscous gas in a laminar flow.

Results of a parametric study along with a numerical example for a specific circumferential seal demonstrate a substantial hydrodynamic effect that can raise the opening average pressure in the seal clearance above the ambient one by up to 50 percent.     

A Theoretical Analysis of Non-Isentropic Flow of a Compressible,Viscous Gas in Narrow Passages

A theoretical solution to the problem of two-dimensional flow of a compressible gas in a thin passage is presented. The solution accounts for the effects of viscosity, inertia, compressibility and irreversible entropy production due to viscous dissipation. It represents the first complete two-dimensional solution which lakes all these factors into consideration. The final equations developed are easy to solve and give good agreement with experiment and previous theory.     

Characteristics of the inlet with the pressure perturbation in the ramjet engine

Flows in a ramjet inlet is simulated for the study of the rocket-ramjet transition. The flow is unsteady, two-dimensional axisymmetric, compressible and turbulent. Double time marching method is used for the unsteady calculation and HLLC method is used as a higher order MUSCL method. As for turbulent calculation, k — ω SST model is used for more accurate viscous calculations. Sinusoidal pressure perturbation is given at the exit and the flow fields at the inlet is studied. The cruise condition as well as the ground test condition are considered. The pressure level for the ground test condition is relatively low and the effect of the pressure perturbation at the combustion chamber is small. The normal shock at the cruise condition is very sensitive to the pressure perturbation and can be easily detached from the cowl when the exit pressure is relatively high. The sudden decrease in the mass flux is observed when the inlet flow becomes subcriticai, which can make the inlet incapable. The amplitude of travelling pressure waves becomes larger as the downstream pressure increases, and the wavelength becomes shorter as Mach number increases. The phase difference of the travelling perturbed pressure wave in space is 180 degree.     

Stability analysis of the confined mixing layer

This paper investigates the linear stability of confined mixing layers with special emphasis on effects of heat release and compressibility. Velocity and density profiles for laminar flows are obtained by solving the compressible boundary-layer equations. The results show that reflection of supersonic disturbances by the walls makes the confined supersonic mixing layer more unstable than the unconfined free shear layer. Decreasing the distance between the walls makes the flow more unstable. However, subsonic disturbances are relatively unaffected by the walls. Mach number hardly changes the growth rates of supersonic disturbances. The most unstable supersonic disturbances are three-dimensional in confined flows.     

Pressure sensitive paint measurements at high Mach numbers

Depending on the case study examined, different PSPs may be used, each applied using a different method onto the model. For polymer PSP the paint is sprayed on. In contrast, the model may first be anodised or covered with a thin-layer chromatography plate and then dipped in PSP. The objective of the present study is to analyse the characteristics of different PSP substrates at high Mach numbers which use two well-known PSP molecules: (i) tris-Bathophenanthroline Ruthenium (II) Perchlorate and (ii) Platinum-tetrakis (pentafluorophenyl) Porphyrin. Using a double ramp geometry under a Mach 5 hypersonic flow the feasibility of applying each of the aforementioned PSP methods is investigated and compared to discrete pressure measurements. The flow over a 3D bump under a Mach 1.3 flow is also studied to give a broader Mach number range. In the hypersonic tunnel, all PSP techniques and formulations were able to capture the complex flowfield with the results quantitatively agreeing with the discrete measurements. For the transonic bumps however, it was found that the polymer based Platinum PSP could map the flowfield more accurately.     

Three-dimensional effects of supersonic cavity flow due to the variation of cavity aspect and width ratios

Unlike the steady closed-type supersonic cavity flow, open-type cavity flow is divided into internal and external flows by turbulent shear layer. The cavity flow may cause resonance phenomena due to pressure oscillation, depending on the cavity geometry and the flow conditions. These phenomena may induce noise generation, structural damage, and aerodynamic instability. In this research, the flow characteristics of three-dimensional supersonic cavity flow of Mach number 1.5 were analyzed with the variations of aspect ratio and width ratio. Three-dimensional unsteady compressible Reynolds-averaged Navier-Stokes (RANS) equations were used with a turbulence model. For numerical calculations, the 4th-order Runge-Kutta method and the FVS method with van Leer’s flux limiter were applied. The numerical calculations were performed by using a parallel processing program with 16 CPUs. The sound pressure level (SPL) spectra of pressure variations were analyzed at the point of cavity leading edge. The correlation of pressure distribution (CPD) was also analyzed for the propagation of dominant oscillation pressure waves with respect to the reference point of the cavity leading edge. The dominant oscillation frequency was compared with the oscillation modes of Rossiter’s formula. Oscillation Mode 2 appeared as a dominant oscillation frequency regardless of the aspect ratio of cavity in the two-dimensional flow. Oscillation Modes1 and 2 appeared in three-dimensional cavities of small aspect ratios. However, as the aspect or the width ratio increases, only the mode 2 or 3 frequency appeared as a dominant oscillation frequency.     

Effect of thermal non-equilibrium on transient hydromagnetic flow over a moving surface in a nanofluid saturated porous media

This work is made to study the effect of local thermal non-equilibrium (LTNE) on transient MHD laminar boundary layer flow of viscous, incompressible nanofluid over a vertical stretching plate embedded in a sparsely packed porous medium. The flow in the porous medium is governed by simple Darcy model. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. Three temperature model is used to represent the local thermal non-equilibrium among the particle, fluid, and solid-matrix phases. By applying similarity analysis, the governing partial differential equations are transformed into a set of time dependent nonlinear coupled ordinary differential equations and they are solved by Runge-Kutta Fehlberg Method along with shooting technique. Numerical results of the boundary layer flow characteristics for the fluid, particle and solid phases are obtained for various combinations of the physical parameters. It is found that the thermal non-equilibrium effects are strongest when the fluid/particle, fluid/solid Nield numbers and thermal capacity ratios are small. Moreover, the amount of heat transfer is maximum in nanoparticles than that of fluid and solid phases because of enhancement of thermal conductivity in nanofluids.     

Consideration of basic equations,and their application,in the forming of metal powders and porous metals

Yield criteria and stress—incremental strain relations for compressible materials, such as metal powders and sintered porous metals, are described and experimental verification of these basic equations is then given. The application of slip-line field theory, upper-bound theory, the finite-element method and the visio-plasticity method based on the above equations is described and some results are presented.When powders are compacted, or sintered metals are plastically deformed, they undergo a change in density and also density variations occur within the product. Since the change in density and the density variation have a great effect upon the properties of the product, it is important to be able to evaluate them. Loads to cause densification or plastic deformation, and pressures exerted on the die-walls, are also important in tool design. Using the above theories or methods, it is possible to predict the working load, the pressures exerted on the die walls and the density variation within the product. Whereas these important factors have not been treated theoretically in the past, a rational basis can now be introduced into the forming of metal powders and sintered metals.     

A hybrid time-stepping method of local preconditioning on high aspect ratio grids

Time-derivative preconditioning methods have provided robust convergences and accurate solutions for low-speed inviscid flows by using local flow properties as the reference scaling parameter in the preconditioning matrix. During viscous or turbulent flow calculations, the use of high aspect ratio grids to obtain accurate solutions near the wall often results in stiff convergence and nonphysical solutions in the singular regions. In this paper, a switching method is proposed as a remedy to that problem by combining the advantages of the typical local time-stepping and min-CFL/max-VNN methods. The proposed method is based on results of our study of the inviscid bump flow and flat plate boundary layer flows and also proved to be effective to a turbulent flow simulation around the NACA 4412 airfoil.     

A computational analysis of unsteady transonic/supersonic flows over backward facing step in air jet nozzle

A transonic/supersonic axisymmetric backward facing step nozzle flow in an air-jet loom has been analyzed numerically by using a time accurate characteristic based upwind flux difference splitting compressible Navier-Stokes method. The unsteady pressure and Mach number behavior along the center line of the main nozzle were analyzed by periodic inlet condition changes to simulate the intermittent flow inside main nozzle of an air-jet loom.     

Three-dimensional instability in compressible reacting mixing layers with heat release

This paper investigates the linear stability of compressible reacting mixing layers with special emphasis on three-dimensionality. The governing equations for laminar flows are from two-dimensional compressible boundary-layer equations. The chemistry is a finite rate single step irreversible reaction with Arrhenius kinetics. For incompressible reacting mixing layers, two-dimensional outer modes are more amplified than three-dimensional ones. For compressible non-reacting mixing layers at M _c > 0.6, the most unstable modes are oblique center modes that are subsonic relative to both free streams. For compressible reacting mixing layers with T _ad > 3, the most unstable modes are two-dimensional outer modes even at high Mach numbers. Three-dimensional modes agree well with experimental data compared to two-dimensional modes.     

Solar radiation assisted mixed convection MHD flow of nanofluids over an inclined transparent plate embedded in a porous medium

The present paper investigates analytically and numerically the magneto-hydrodynamic (MHD) mixed convection flow of nanofluid over a nonlinear stretching inclined transparent plate embedded in a porous medium under the solar radiation. The two-dimensional governing equations are obtained considering the dominant effect of boundary layer and also in presence of the effects of viscous dissipation and variable magnetic field. These equations are transformed by the similarity transformation to two coupled nonlinear transformed equations and then solved using a numerical implicit method called Keller-Box. The effect of various parameters such as nanofluid volume fraction, magnetic parameter, porosity, effective extinction coefficient of porous medium, solar radiation flux, plate inclination angle, diameter of porous medium solid particles and dimensionless Eckert, Richardson and Prandtl numbers have been studied on the dimensionless temperature and velocity profiles. Also the results are presented based on Nusselt number and Skin friction coefficient.