Shihua Yuan

Theoretical and Experimental Study of Cavitation Effects on the Dynamic Characteristic of Spiral-Groove Rotary Seals (SGRSs)

A theoretical model is developed to analyze oil-film stiffness and damping coefficients of SGRSs using two different cavitation models, i.e., the Reynolds and JFO boundary conditions. By applying the small perturbation method, the steady and perturbed Reynolds equations could be obtained. The control volume method was used for the spiral grooves, and the upwind scheme was applied to improve the convergence of the solution. The performance of the computation algorithm was studied by analyzing the mesh dependencies and computing power. A new test setup was built to measure oil-film stiffness, to verify the theoretical models, and to study the validity of the Reynolds and JFO boundary conditions. A representative SGRS was analyzed at different operating speeds and inlet pressures to investigate the occurrence of cavitation and the dynamic characteristic. The effect of the spiral-groove parameters on the dynamic characteristics is also discussed.

On–off motion of a hydraulic free-piston engine

The current paper presents the on–off characteristics of a compression ignition single-piston hydraulic free-piston engine. A pilot control circuit and a control method for on–off control of the engine are introduced. Detailed piston motion under on–off control of the engine is investigated. The results indicate that the piston on–off motion of 1 cycle can be divided into a pilot phase and a wave phase. The piston motion in the pilot phase is determined by the flow capacity of the frequency control valve. The piston acceleration during the accelerated compression process peaks in the pilot phase. The wave phase consists of a rebound stage, a decaying stage and a creep stage. The rebound stage determines the initial conditions of the other two stages and affects the piston frequency control. The control method proposed is immune from the bottom-dead-centre cycle variation. A high piston frequency results in a better engine fuel economy.

Flow dynamical behavior and performance of a micro viscous pump with unequal inlet and outlet areas

ABSTRACT The micro viscous pump is an important type of fluidic device. Optimizing the working performance of the pump is crucial for its wider application. A micro viscous pump design with unequal inlet and outlet areas is proposed in this paper. The flow field of the viscous pump is investigated using 2D laminar simulations. The mass flow rate and driving power are studied with different opening angles. The effects of the Reynolds number and the pressure load on the working performance are discussed in detail. Flow structures and vortex evolution are analyzed. With larger inlet and outlet areas, a higher mass flow rate is obtained and less driving power is achieved. A high pressure load results in a reduction in mass flow rate and an increase in driving power. Pumps with large opening angles are more susceptive to the Reynolds number and the pressure load. The adverse impact of the pressure load can be reduced by increasing the rotor speed. The vortex structure is affected by the geometric and operating parameters in the flow field. The flow dynamical behavior of the viscous pump exerts significant influence on its pumping ability. The present work gives rise to performance improvements for the micro viscous pump.

Modelling the vertical dynamics of unmanned ground vehicle with rocker suspension

In this paper, the vertical dynamic characteristics of unmanned ground vehicle (UGV) with rocker suspension are studied. A mathematical model is established to investigate the vertical dynamic characteristics of three-axle UGV with rocker suspension. This mathematical model is validated by the multi-body dynamics software Adams. The pitch angle acceleration of vehicle is selected as the evaluation index of vehicle ride comfort because of its terrible influence on detection equipment such as the camera. The influence of the stiffness and damping of the front, middle and rear axles on the ride comfort of UGV is analysed by changing the stiffness and damping value of each axle respectively. The simulation results show that the mathematical model can effectively reflect the vertical dynamic characteristics of the UGV with rocker suspension. The modelling method is valid. When the change is the same, the variation of the stiffness and damping in front or rear axle has great influence on the ride comfort of the vehicle. Based on this, to improve the ride comfort of the three-axle UGV with rocker suspension, the stiffness and damping in front and rear axle should be optimized first.

Functional Analysis Method for Dynamic Speed Ratio Control of CVT-Based Vehicles

Speed ratio control of CVT-based vehicles is critical to vehicle dynamic performance. With vehicle accelerating, transmission efficiency and rolling resistance coefficient vary at real-time. Considering real-time variables into speed ratio control, functional analysis method is proposed. Choosing speed ratios as functional argument, a general functional form of acceleration time is established. The performance index is strongly nonlinear, which makes it difficult to obtain theoretical optimal solutions. So hybrid function of genetic algorithm with fmincon is adopted to obtain the numerical solutions. Continuously variable transmission model is established to compare the dynamic performance in different strategies. The simulation results show that accelerating time of 0–90 km/h decreased by 2.31 s after optimization,realizing the application of Functional Analysis Method on CVT ratio control problems.

Modeling bubble evolution in air-oil mixture with a simplified method

This work addresses the problem of bubble evolution arising from gas cavitation in hydraulic oils. Two significant aspects, including the interphase mass transfer represented by air release and absorption phenomena and different thermodynamic considerations, are currently taken into account using a simplified method. In particular, three new models in progressive relationship are proposed on the basis of Rayleigh–Plesset equation which describes bubble dynamics. They are Model A in which air content is assumed to be constant, Model B in which the interphase mass transfer is introduced with the air undergoing an isothermal transformation, and Model C assuming an adiabatic process for the bubble evolution. With the goal of investigating the effects of these aspects, comparisons of the three models for two typical cases are presented with regard to the practical circumstances in which the oil pressure is set to increase linearly or oscillate sinusoidally. Results show a consistent trend for both cases concerning Model B and Model C compared to Model A. Although its speed relates to many factors, air release and absorption has a relevant impact on gas bubble radius. By the reason of adiabatic assumption, Model C provides a slower response regarding the oil pressure change. However, Model B and Model C may be both inaccurate if considering the actual interfacial heat transfer. In this viewpoint, the oil temperature in fluid power system could be affected.

Architecture of a Hydraulic Hybrid Vehicle with Pressure Cross-Feedback Control

A hydraulic hybrid vehicle equipped with the hydraulic transformer is proposed. The hybrid system features in rotary swash plate type hydraulic transformer and pressure cross-feedback control. The full numerical model has been built. Extensive simulated and tested results are given. The contributions of the hydraulic transformer dynamics to the hydraulic hybrid vehicle performance are investigated. The proposed hydraulic hybrid vehicle can achieve the ideal vehicle dynamic performance by adjusting the hydraulic transformer controlled angle. The switching between the driving and the regenerative breaking of the hydraulic hybrid vehicle is realized by the pressure cross-feedback control. The propulsion mode of the hydraulic hybrid vehicle can be changed automatically. The larger hydraulic transformer controlled angle realises a higher driving pressure while the smaller one achieves a higher driving flow. Due to the small inertia of the hydraulic transformer, the hybrid system is able to satisfy the speed requirement of the driver.

Numerical study of turbulent drag reduction over non-smooth surfaces of rotating-stationary disk system

Turbulent drag reduction has an important significance for energy conservation and emission reduction of the engineering fields, such as mechanical transmission and longdistance transport pipeline transportation. The air-oil two-phase flow model of non-smooth surfaces of rotating-stationary disk system was established based on the finite volume method, the volume of fluid method and RNG k-ε turbulence model. The flow field distribution of lubricant oil is obtained through the numerical analysis of three-dimensional Navier-Stokes equations of the two-phase of lubricating medium inside the rotating-stationary disk system. The turbulent boundary layer flow and stickiness resistance of the rotating-stationary disk system, as well as, the turbulent drag reduction capability by numerical calculation is investigated. The flow field of the smooth surfaces and non-smooth surfaces of rotating-stationary disk system are analyzed and the mechanism of turbulent drag reduction is discussed. The factors of influencing turbulent drag reduction are discussed by changing groove numbers, depth, and area ratio of grooves. The results indicate the grooves make it easier for air to enter the rotating disk system, and the film more easily broken, thus inhibiting the rise in the turbulent drag torque; the turbulent drag reduction efficiency will enhance with the number of grooves increase; will enhance with the depth of grooves increase and will enhance with the area ratio of grooves increase. The results can be used for the flow field analysis and optimization of the rotating-stationary disk system, especially supply a new method to the energy conservation and emission reduction.

Numerical investigation of turbulent flow in an open rotor-stator system

The physics of fluid flow between two parallel rotating disks is the theoretical basis of the analysis and design of rotating machinery such as gas turbine. Rotor-stator system is one of the most pervasive types among rotating disk configurations. In this paper, the flow field in a rotor-stator system with its periphery totally open to atmosphere was numerically studied. The SST k-m model was employed to simulate the turbulent flow and the computational procedure was validated by comparing the computed results with experimental data in previous work. Velocity distribution in the disk cavity was obtained. Both Batchelor-type and Stewartson-type flow can be found in the cavity, and a transition zone exists for the transformation of the two flow structures. Effect on the development of flow of two governing parameters in the present configuration, the gap-ratio and the rotational Reynolds number were discussed. With the increase of the gap-ratio, the Batchelor-type region shrinks radically inward, meanwhile, the rotational Reynolds number plays a less obvious impact on the velocity profile at two radial positions we study. The present work is expected to provide profound insights for better understanding of the flow in rotating disk system.

Dynamics of hydraulic transformer controlled motor system

The hydraulic transformer controlled motor system is the key component of the hydraulic common pressure rail system. The hydraulic transformer controlled motor system can realize energy bidirectional transmission and pressure regulatory continuously without throttling loss theoretically. Based on the working principle and the single piston kinematic model, the dynamic model of the hydraulic transformer controlled motor system is established. The pulsation characteristic of pressure is analyzed. The inherent characteristics of the system are investigated. The results indicate that the pressure pulsation has a decrease trend when the hydraulic transformer controlled angle becomes large. In the common variation ranges, the step change of the hydraulic transformer controlled angle will cause the negative response of the load pressure. And also, because of the operation condition of hydraulic transformer is easy affected by load pressure, the system stiffness is relatively low. The negative response and pulsation of the pressure cause the motor torque fluctuation. The system is not suitable for the situation with high torque control precision. The novel system structure and control method are needed.