Numerical simulation of flows around axisymmetric inlet with bleed regions

A numerical simulation of flows in an axisymmetric supersonic inlet with bleed regions is performed. An existing code which solves the Reynolds Averaged Navier-Stokes equations and the two-equation turbulence model equations is converted into an axisymmetric code. In addition, a bleed boundary condition model has been applied to the code. In this paper, the modified code is validated by comparing numerical results against experimental data and other computational results for flows on a bump and over an oblique shock with bleed region. Using the code, numerical simulation is performed for the flows in an inlet with multiple bleed regions.     

Numerical study on the low density gas flows in a plasma etch reactor

The direct simulation Monte Carlo method is employed to predict the etch rate distribution on Al wafer for a chlorine feed gas flow. The etching process of an Al wafer in a plasma etch reactor is examined by simulating molecular collisions of reactant and product. The surface reaction on the Al wafer is simply modelled by one-step reaction: 3Cl₂+2Al → 2AlCl₃. The gas flow inside the reactor is compared for six different nozzle locations. The present numerical results show that the etch rate increases with the mass flow rate of source gas Cl₂. It is also shown that the flow field inside the reactor is significantly affected by the nozzle locations.     

Experimental and numerical research on cavitating flows around axisymmetric bodies

We investigated the cavitating flows around different axisymmetric bodies based on experiments and numerical simulation. In the numerical simulation, the multiphase Reynolds averaged Navier Stokes equations (RANS) were solved via the commercial computational fluid dynamics code CFX. The modified k-ω SST turbulence model was used along with the transport equation-based cavitation model. In the experiments, a high-speed video technique was used to observe the unsteady cavitating flow patterns, and the dynamic force measurement system was used to measure the hydrodynamics of the axisymmetric bodies under different cavitation conditions. Results are shown for the hemisphere bodies, conical bodies and blunt bodies. Reasonable agreements were obtained between the computational and experimental results. The results show that for the hemispherical body, the cavity consists of quasi-steady transparent region and unsteady foggy water-vapor mixture region, which contains small-scale vortices and is dominated by bubble clusters, causing irregular disturbances at the cavity interfaces. The curvature at the front of the conical body is larger, resulting in that the flow separates at the shoulder of the axisymmetric body. The cavity stretches downstream and reaches to a fixed cavity length and shape. For blunt bodies, the incipient cavitation number is larger than that for the hemispherical body. A large cloud cavity is formed at the shoulder of the blunt body in the cores of vortices in high shear separation regions and the re-entrant jet does not significantly interact with the cavity interface when it moves upstream. As to the dynamic characteristics of unsteady cavitating flows around the axisymmetric bodies, the pulsation frequency for the hemispherical body is larger than that for the blunt body. For the hemispherical body, the pulsation is mainly caused by the high-frequency, small-scale shedding at the rear end of the cavity, while for the blunt body, the main factor for the pulsation frequency is the periodically shedding of large-scale vortex cavities.     

Numerical investigation of suction vortices behavior in centrifugal pump

A numerical simulation on suction vortices behavior in a centrifugal pump was carried out to investigate their influence on the internal flow through impellers including formation and shedding of cavitation by using a finite-volume method and k-ω Shear Stress Transport turbulence model. For cavitating flow, a two phase homogeneous cavitation model was used. A full three-dimensional flow in a single-section centrifugal pump consisting of a six blade impeller and shroud ring was computed with structured mesh. A constant suction vortex is imposed as a boundary condition. Vortices behavior was investigated according to the variation of flow rates of two pump systems with and without suction vortices. From the results, suction vortices induced biased flow structure and more cavitations, especially at the low flow rate condition. Complicated internal flow phenomena through impellers such as formation of cavitations, growing and shedding of the vortex, flow separation and flow unsteadiness due to the suction vortices are investigated and discussed.     

Numerical study of ventilated cavitating flows with free surface effects

Cavitating flow is usually formed on the surface of a high-speed underwater object. When the object moves near the free surface at high speed, the cavitation signature becomes a main factor to be overcome by the sensors of a military satellite. This paper studies the free surface effect on the ventilated cavitation process. The governing equations are Navier-Stokes equations based on a homogeneous mixture model. The multiphase flow solver used here relies on an implicit preconditioning scheme in curvilinear coordinates. The cavitation model used is a new cavitation model developed by Merkle et al. (2006). Computations of free surface effects were carried out with a NACA0012 hydrofoil to enable comparisons with experimental data presented in the literature. Calculations were then performed considering the ventilated cavitation process, including the effect of a noncondensable gas with free surface effect.     

Numerical study on energy transmission for rotating hard disk systems by structural intensity technique

The main purpose of this paper is to apply structural intensity technique to indicate the magnitude and direction of vibration energy flow for a rotating system, so as to modify and control the energy flow path to reduce the vibrational problems. Numerical simulations are carried out for a rotating flexible disk-spindle system supported by ball bearing and flexible shaft by the finite element method, and the vibrational energy transmission caused by an eccentric mass is analyzed by using structural intensity technique. The structural intensities in hybrid 1D, 2D and 3D elements, which are used to model different components of HDD, are obtained. Three different damping effects on the diversion and dissipation of energy flow are investigated. The calculated results show that different types of damping or dampers have different effect on energy flow path in HDD. Through numerical simulation, a more reasonable design scheme may be explored by scientifically arranging damping components for HDD to further suppress its vibration and noise problems.     

Numerical study of droplet motion in a microchannel with different contact angles

The droplet motion in a microchannel with different contact angles, which is applicable to a typical proton exchange membrane fuel cell (PEMFC), was studied numerically by solving the equations governing the conservation of mass and momentum. The gas-liquid interface or droplet shape was determined by a level set method which was modified to treat the static and dynamic contact angles. The matching conditions at the interface were accurately imposed by incorporating the ghost fluid approach based on a sharp-interface representation. Based on the numerical results, the droplet dynamics including the sliding and detachment of droplets was found to depend significantly on the contact angle. Also, the effects of inlet flow velocity, droplet size and side wall on the droplet motion were investigated. This paper was recommended for publication in revised form by Associate Editor Haecheon Choi Gihun Son received a B.S. and M.S. degree in Mechanical Engineering from Seoul National University in 1986 and 1988, respectively. He then went on to receive his Ph.D. degrees from UCLA in 1996. Dr. Son is currently a Professor of Mechanical Engineering at Sogang University in Seoul, Korea. Dr. Son’s research interests are in the area of multiphase dynamics, heat transfer, and power system simulation. Jiyoung Choi received a B.S. degree in Mechanical Engineering from Sogang University in 2005. He is a graduate student of Mechanical Engineering at Sogang University in Seoul, Korea. Choi’s research interests are in the area of PEM fuel cell and microfluidics.     

Experimental investigation of inception cavitating flows around axisymmetric bodies with different headforms

Experiments have been performed to study the inception cavitation phenomena in the separated flows adjacent to two axisymmetric bodies whose forebodies are blunt and conical, respectively. A high-speed video camera was used to visualize the dynamic process of incipient cavitation, and the PIV (Particle imaging velocimetry) technology was applied to measure the velocity field, the vorticity and turbulence fluctuations under non-cavitation and inception cavitation conditions. Observations suggest that incipient cavities around the two axisymmetric bodies have the similar onset appearance but different development patterns and cycles with different inception cavitation index. Also, it is found that incipient cavities are always located within the separated vortex upstream the reattachment point during the whole dynamic process, and the scale of separation vortex greatly influences the positions where incipient cavities generate and collapse. Measures of the turbulence fields show that the distribution of vorticity can be changed by unsteady incipient cavities, and inception cavitation causes slight uniformity of velocity in shear layer and conspicuous increase of turbulent fluctuations. Compared to that of the conical headform, inception cavitation around the blunt headform presents more vortical traits and turbulence fluctuations.

     

Numerical simulation of flying phenomena of the running tape on rotating drum

This paper analyzes the running mechanism of flexible and thin tape above rotating protrusion through a numerical simulation. The scope of analysis is confined to the phenomena of elastohydrodynamic lubrication between the rotating drum with a protrusion and the running tape. This model is based on the modified Reynolds equation and the equation of plate considering the effect of geometric nonlinearity and geometry of protrusion. Finite element method of Bubnov-Galerkin type is adopted as a numerical simulation technique to solve the above two coupled nonlinear equations. In numerical simulation, the influences of tape tension and protrusion velocity are evaluated in simple model. In complex models, the reciprocal action of two protrusions is simulated.     

基于转镜运动补偿的弹丸同步摄影技术研究

弹丸的空间飞行姿态是武器性能的一项重要指标。为了使数字摄影相机实现对高速运动弹丸运行轨迹的成像,设计了一种基于转镜运动补偿的弹丸同步摄影装置。在高速数字相机前设置一面反射转镜,当弹丸进入跟踪视场时,利用转镜的旋转补偿弹丸的空间运动,使得由转镜反射到相机像面的弹丸图像偏移在许可范围内,实现同步摄影过程。实验证明该方法能够满足常规弹丸同步摄影的需求。     

轴对称矢量喷管内外流场的数值模拟

在同一偏转角度下,随着喷管压比的增大,推力系数先减后增再减小,而矢量角先增大再减小;在相同喷管压比下,矢量角与喷管偏转角呈线性关系.利用数值计算结果和人工神经网络,建立该轴对称矢量喷管的性能预测模型,喷管性能预测结果与计算值吻合较好,从而验证了该模型的有效性,为减少计算和试验点数节省经费和时间提供了有效的途径.     

回转机械自同期运动机理的建模与仿真

提出了回转机械自同期运动的概念,通过实验测试分析了其自动平衡效果。通过建模与仿真,揭示出回转机械的自同期运动机理和滚动摩擦因数、粘性阻尼系数、加速度等参数的影响规律。     

不同轴向间隙时多级叶轮机械内部流动分析

应用数值模拟技术,计算了不同轴向间隙轴流风扇级内部三维流动和总性能,详细分析了该风扇级分别在最大流量状态和近失速点状态下内部流动细节,分析不同轴向间隙对风扇级堵塞和失速机理的影响,并比较不同轴向间隙对风扇级总性能的影响.结果表明,轴向间隙的大小对风扇级性能影响很大,反压相同时,轴向间隙减小,流过风扇级质量流量、压比、效率均减小.     

Numerical calculation of the viscous flow around a rotating marine propeller

Reliable and accurate prediction of the viscous flow around a marine propeller operating at the stern is of practical importance for design and performance prediction of propellers. A computer code was developed in the present study for the full viscous flow simulation around the marine propeller at the stern and its performance was investigated using the available data. The continuity and Navier-Stokes equations with a standardk-ε model in the rotating coordinate fixed on the propeller were numerically solved using FVM. The predicted profiles of circumferentially averaged velocity and turbulent kinetic energy were in accordance with the measured data at the downstream of the propeller, while there were significant discrepancies in the near wake. The value of velocity was small in a core associated with the formation of the tip vortex and the location of maximum axial velocity on the suction side of the blade was observed in the simulation. The flow structure observed in the experiment was confirmed in the simulation, however not quantitatively. The blade wake was diffused more in comparison with the measured one.     

Numerical simulation of flows of non-Newtonian fluids in the stenotic and bifurcated tubes

Steady flows of Newtonian and non-Newtonian fluids in the stenotic and bifurcated tubes are numerically simulated. Four rheologically different fluids such as water, aqueous sugar solution, aqueous Carbopol solution and blood are selected for the namerical simulation and the modified power-law model is used for the numerical simulation of non-Newtonian fluids in the stenotic and bifurcated tubes. Apparent viscosity of a non-Newtonian fluid in the modified power-law model is expressed as a function of the shear rate. Flows in the circular tube with sudden contraction-sudden expansion and gradual contraction-gradual expansion are studied numerically. Analyses in the stenotic tubes are concentrated on the effects of rheological properties, the stenotic geometry and Reynolds number. Flow characteristics of Carbopol solution in the stenotic tubes are compared with those of blood. Effects of the bifurcation geometry on the flow behaviors of Newtonian and non-Newtonian fluids are numerically investigated. Numerical analyses are focused on the flow patterns in the branch tubes of which angles are 30°, 60° and 90° and on the diameter ratios for Newtonian and non-Newtonian fluids. Variations of the axial velocity and pressure drop along the bifurcated tubes for various flow parameters are presented for Newtonian and non-Newtonian fluids.     

Numerical investigations on cavitating flows with thermodynamic effects in a diffuser-type centrifugal pump

A cavitation model with thermodynamic effects for cavitating flows in a diffuser-type centrifugal pump is developed based on the bubble two-phase flow model. The proposed cavitation model includes mass, momentum, and energy transportations according to the thermodynamic mechanism of cavitation. Numerical simulations are conducted inside the entire passage of the centrifugal pump by using the proposed cavitation model and the renormalization group-based k-? turbulent model coupled with the energy transportation equation. By using the commercial computational fluid dynamics software FLUENT 6.3, we have shown that the predicted performance characteristics of the pump, as well as the pressure, vapor, and density distributions in the impeller, agree well with that calculated by the full cavitation model. Simulation results show that cavitation initially occurs slightly behind the inlet of the blade suction surface, i.e., the area with maximum vapor concentration and minimum pressure. The predicted temperature field shows that the reduction in temperature restrains the growth of cavitating bubbles. Therefore, the thermodynamic effect should be treated as a necessary factor in cavitation models. Comparison results validate the efficiency and accuracy of the numerical technique in simulating cavitation flows in centrifugal pumps.     

A comparative study on the use of calibrated and rainbow schlieren techniques in axisymmetric supersonic jets

Detailed experimental comparisons had been conducted between calibrated and rainbow schlieren on perfectly- and under-expanded axisymmetric supersonic jets through a modified Z-type schlieren system. The techniques were implemented by using a weak lens in the field-of-view to provide calibration information for the extraction of quantitative density gradients from the experimental schlieren images. Sixth-order polynomial curve fits were obtained for both calibrated and rainbow schlieren respectively. The effects of light inhomogeneity caused by the mirrors and system diaphragm aperture had been evaluated for the colour images and results indicate that averaging the background hue is an acceptable approach for minimizing light variations with less than 2% experimental error. Density gradients as calculated via Abel transform have also been evaluated to validate the two different set-ups. Additionally, experimental results have been compared to validated numerical results and they show that calibrated schlieren is able to predict density gradients within 2% of the numerical results. This is significantly more superior to rainbow schlieren, where errors in the estimated density gradients are closer to 20%. It is shown here that rainbow schlieren results are more adversely impacted by the system diaphragm aperture, especially for vertical light cut-off configuration. This is partly due to the loss of sensitivity of the schlieren system, as well as potential light diffusion caused by the filter.     

Numerical analysis of nozzle shape effect on fluid motion in LPCVD batch-type furnace

Journal of Mechanical Science and Technology - The importance of equipment technology for semiconductor mass production is growing as well as the exponential increase in demand for semiconductors....     

Analytical and numerical study on plastic instabilities for axisymmetric tube bulging

In this paper, plastic instabilities of elasto–plastic tubes subject to internal pressure are discussed. For diffuse necking prediction, the classical intrinsic criteria for diffuse necking are accurate for long cylindrical tubes. However, for short tubes, geometric changes are important, and the intrinsic criteria become insufficient. For this purpose, a new diffuse necking criteria is proposed including geometric effects in the prediction.On the other hand, for the local necking prediction, the Hill's criterion is not accurate for short tubes, due to the biaxial stretching. As an alternative, a local necking criterion based on a modified Hill's assumption for localized necking is proposed. The numerical calculations carried out for different tube dimensions, explains the geometrical effects on the localization of deformations for pressurized tubes, and improves the accuracy of the proposed criteria.