Adaptive panel representation for oblique collision of two vortex rings

In this article, we use a hierarchical panel method for representing vortex sheet surface motion in 3D flow to investigate the oblique collision of two vortex rings. The particles representing the sheet are advected by a regularized Biot-Savart integral with smoothed Rosenhead-Moore kernel. The particle velocities are evaluated by an adaptive treecode algorithm based on Taylor expansions in Cartesian coordinates. The method allowed us to consider late stages of a vortex rings collision, producing a funnel region. Vorticity iso-surfaces evolution is also investigated.     

Numerical study on magneto-convection of cold water in an open cavity with variable fluid properties

The aim of the present study is to understand the problem of buoyancy and thermocapillary induced convection of cold water near its density maximum in an open cavity with temperature dependent properties in the presence of uniform external magnetic field. The governing equations are solved by the finite volume method. The results are discussed for various values of reference temperature parameter, density inversion parameter, Rayleigh, Hartmann and Marangoni numbers. It is observed that the temperature of maximum density leaves strong effects on fluid flow and heat transfer due to the formation of bi-cellular structure. Convection heat transfer is enhanced by thermocapillary force when buoyancy force is weakened.     

Collision efficiency of two nanoparticles with different diameters in Brownian coagulation

The collision efficiency of two nanoparticles with different diameters in the Brownian coagulation is investigated. The collision equations are solved to obtain the collision efficiency for the dioctyl phthalate nanoparticle with the diameter changing from 100 nm to 750 nm in the presence of the van der Waals force and the elastic deformation force. It is found that the collision efficiency decreases as a whole with the increase of both the particle diameter and the radius ratio of two particles. There exists an abrupt increase in the collision efficiency when the particle diameter is equal to 550 nm. Finally, a new expression is presented for the collision efficiency of two nanoparticles with different diameters.     

Numerical simulation of viscous oscillatory flow past four cylinders in square arrangement

The results of a numerical study of the viscous oscillating flow past four circular cylinders, for a constant frequency parameter equal to 50 and KC ranging between 0.2 and 10, are presented. The cylinders were placed on the vertices of a square, two sides of which were perpendicular and two parallel to the oncoming flow, for pitch ratios, P/D, ranging between 2 and 5. The finite-element method was employed for the solution of the Navier-Stokes equations, in the formulation where the stream function and the vorticity are the field variables, whereas the pressure distribution throughout the computational domain was obtained from the solution of Poisson’s equation. When the Keulegan-Carpenter number is lower than 4, the flow remains symmetrical with respect to the horizontal axis of symmetry of the solution domain and periodic at consecutive cycles. As KC increases to 4, the flow becomes aperiodic in different cycles, although symmetry with respect to the horizontal central line of the domain is preserved. For KC equal to 5, asymmetries appear intermittently in the flow, which are eventually amplified as KC increases still further. These asymmetries, in association with the aperiodicity of flow in different cycles, lead to an almost chaotic configuration as KC grows larger. For characteristic cases the flow pattern and the time histories of the in-line and transverse forces exerted on the cylinders are presented. The mean transverse forces acting on the cylinders, the r.m.s. values of the in-line and transverse forces, together with the drag and inertia coefficients of the in-line force, were evaluated for each pitch ratio in the range of Keulegan-Carpenter numbers examined and are presented in diagrams.     

Flow field characteristics study of a flapping airfoil using computational fluid dynamics

The flow field of a flapping airfoil in Low Reynolds Number (LRN) flow regime is associated with complex nonlinear vortex shedding and viscous phenomena. The respective fluid dynamics of such a flow is investigated here through Computational Fluid Dynamics (CFD) based on the Finite Volume Method (FVM). The governing equations are the unsteady, incompressible two-dimensional Navier-Stokes (N-S) equations. The airfoil is a thin ellipsoidal geometry performing a modified figure-of-eight-like flapping pattern. The flow field and vortical patterns around the airfoil are examined in detail, and the effects of several unsteady flow and system parameters on the flow characteristics are explored. The investigated parameters are the amplitude of pitching oscillations, phase angle between pitching and plunging motions, mean angle of attack, Reynolds number (Re), Strouhal number (St) based on the translational amplitudes of oscillations, and the pitching axis location (x/c). It is shown that these parameters change the instantaneous force coefficients quantitatively and qualitatively. It is also observed that the strength, interaction, and convection of the vortical structures surrounding the airfoil are significantly affected by the variations of these parameters.     

Observations of acoustically generated cavitation bubbles within typical fluids applied to a scroll expander lubrication system

An experimental study to evaluate the dynamic performance of three different types of cavitation bubbles is conducted. An ultrasonic transducer submerged into the working fluids of a scroll expander is utilised to produce cavitation bubbles and a high speed camera device is used to capture their behaviour. Three critical regions around the ultrasonic source, between the source and the solid boundary, and across the solid boundary were observed. Experimental results revealed that refrigerant bubbles sustain a continuous oscillatory movement, referenced as “wobbling effect”, without regularly collapsing. Analytical results indicate the influence of several factors such as surface tension/viscosity ratio, Reynolds number and Weber number which interpret that particular behaviour of the refrigerant bubbles. Within the refrigerant environment the bubbles obtain large Reynolds numbers and low Weber numbers. In contrast, within the lubricant and the water environment Weber number is significantly higher and Reynolds number substantially lower. The bubble radius and velocity alterations are accurately calculated during the cavitation process. Lubricant bubbles achieve the highest jet velocity while refrigerant bubbles having the lowest jet velocity are not considered as a destructive mean of cavitation for scroll expander systems.     

Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube

An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and helically dimpled tube under turbulent flow with constant heat flux is presented in this work using CuO/water nanofluid as working fluid. The effects of the dimples and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with a fully developed turbulent flow for the Reynolds number in the range between 2500 and 6000. The height of the dimple/protrusion was 0.6 mm. The effect of the inclusion of nanoparticles on heat transfer enhancement, thermal conductivity, viscosity, and pressure loss in the turbulent flow region were investigated. The experiments were performed using helically dimpled tube with CuO/water nanofluid having 0.1%, 0.2% and 0.3% volume concentrations of nanoparticles as working fluid. The experimental results reveal that the use of nanofluids in a helically dimpled tube increases the heat transfer rate with negligible increase in friction factor compared to plain tube. The experimental results showed that the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% (for 0.1%, 0.2% and 0.3% volume concentrations respectively) higher than the Nusselt number obtained with plain tube and water. The experimental results of isothermal pressure drop for turbulent flow showed that the dimpled tube friction factors were about 2-10% higher than the plain tube. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds number, pitch ratio and volume concentration fits with the experimental data within ±15%.     

基于能量原理确定系泊碰撞载荷的有限元法

准确地确定出船舶系泊时的撞击载荷, 对船舶及海洋平台的强度研究都是极其重要的. 提出了通过能量原理和有限元分析确定系泊撞击载荷的方法. 碰撞时, 动能和变形能满足能量守恒, 动能是包括附连水质量在内的船体总动能, 此动能在撞击时转化为变形能, 根据变形能与载荷的关系, 求出碰撞载荷. 通过具有碰撞载荷理论解的梁的验证, 表明该方法是完全可行的. 为实际工程提供了确定护舷撞击力的理论依据和计算方法.     

抗阵风载荷的小型无人飞行器设计及相关风洞舵效比较

本文目的在于探索验证微小型飞行器的气动布局与抗突风载荷能力的关系,结合风洞试验验证了结果并进行了数据分析。在低雷诺数条件下采用大边条、小展弦比非常规的气动布局设计,根据性能指标要求对飞行器的机翼面积、翼展、翼型等相关参数进行了分析设计;运用数值模拟等手段对全机简化模型进行了气动力计算,对结果进了行初步的定性分析,为初始的设计提供了验证;结合实际的靶机制作得到了实际飞机的风洞验证缩比模型,对其进行了风洞的常规测力实验。根据风洞试验结果对飞机升降舵舵面与副翼舵面偏转0°、±15°、±30°的气动性能进行了评估,为进一步的理论分析提供依据。结合实际外场飞行实验情况得出飞行器的实际气动性能,并对大量相关风洞试验数据进行了分析;探索性地给出了一条适用于微小型飞行器进行的新设计方法和设计流程,为设计适应于阵风载荷等恶劣气候条件下微小的型飞行器提供了参考新思路。     

随机参数超空泡射弹弹道随机特性分析

超空泡射弹的发射装置及射弹在制造,安装过程中存在各种不确定性.本文采用最大熵法来分析这些不确定性对射弹弹道的影响.介绍了超空泡射弹的动力学模型,对射弹的受力特性和随机参数进行了分析.根据给出的理论编制了基于最大熵法的随机参数超空泡射弹弹道随机特性分析的仿真程序,并对一个模型弹仿真计算.计算表明,与发射装置有关的随机参数...