Unsteady Numerical Simulation of Flow around 2-D Circular Cylinder for High Reynolds Numbers

In this paper, 2-D computational analyses were conducted for unsteady high Reynolds number flows around a smooth circular cylinder in the supercritical and upper-transition flow regimes, i.e. 8.21×104〈Re〈1.54×106. The calculations were performed by means of solving the 2-D Unsteady Reynolds-Averaged Navier-Stokes （URANS） equations with a k-ε turbulence model. The calculated results, produced flow structure drag and lift coefficients, as well as Strouhal numbers. The findings were in good agreement with previous published data, which also supplied us with a good understanding of the flow across cylinders of different high Reynolds numbers. Meanwhile, an effective measure was presented to control the lift force on a cylinder, which points the way to decrease the vortex induced vibration of marine structure in future.

Unsteady Numerical Simulation of Steam-Solid Two-Phase Flow in the Governing Stage of a Steam Turbine

Solid particle erosion (SPE) in an ultra-supercritical steam turbine control stage with block configuration is investigatednumerically, based on the finite volume method and the fluid-particle coupling solver. We apply the particlediscrete phase model to model the solid particles flow, and use the Euler conservation equations to solve thecontinuous phase. The investigation is focused on the influence of the solid particle parameters (such as particlediameter, particle velocity and particle trajectory) on the erosion rate of the stator and rotor blade surface in unsteadycondition. The distributions of the highly eroded zone on the stator and rotor blade surfaces are shown anddiscussed in detail according to the mechanism of solid particle/blade wall interaction. We obtain that the erosionrate of the vane blade is sensitive to the fluctuation of the potential flow field, and the smaller particle has agreater impact on the erosion distribution of rotor blade. The erosion rate does not entirely depend on the diametersize of the solid particle.

Unsteady numerical simulation on helium cooldown process for the 650 MHz two-cell superconducting cavity

Background Superconducting cavity is usually needed to be gradually cooled from room temperature to the superconducting temperature zone(4.2 K and below)in the testing and sophisticated operation process of superconducting cavity.Purpose The purpose of this paper is to study the cooling law on the helium cooldown process for the 650 MHz two-cell superconducting cavity with the unsteady numerical simulation.Method A three-dimensional coupled heat-flow model of 650 MHz two-cell superconducting cavity was established.The unsteady numerical simulation of different inlet temperatures,flow rates and pressure conditions was carried out.The equiva-lent convective heat transfer coefficient and temperature distribution of 650 MHz two-cell superconducting cavity during cooldown process were obtained.The effects of cooling time and entrance parameters on the cooldown process were analyzed.Results The temperature distribution of the lower intersection lines has a large drop in the initial stage of cooldown process(120 s),while the temperature near the flanges at the both ends is still higher(remaining at the initial temperature of 300 K).With the passage of time,the temperature of the upper and lower intersection lines decreases.The maximum temperature difference on the lower intersections is within 2 K in the final stage of cooldown process(3600 s).The maximum temperature difference increases by 180%,and the difference between the maximum temperature and the minimum temperature(dT)at the end of a cooldown stage increases by 130%after 1 h,respectively,when the inlet temperature drops from 290 to 270 K(under the condition of the initial temperature of 300 K).Conclusions The maximum temperature difference and the dT at the end of a cooldown stage increase with the decrease in the inlet temperature.The maximum temperature difference increases with the increase in the inlet flow rate,while the dT at the end of a cooldown stage decreases with the increase in the inlet flow rate.The effect of changing the inlet flow rate on the cooling rate is not as obvious as changing the inlet temperature.Once there is a certain flow rate,the advantage of further increasing the flow rate to reduce the temperature of the superconducting cavity is not so great.

A numerical simulation of unsteady flow in small diameter helical grafts

The application of small diameter arterial grafts is limited due to the fact of relatively poor long-time patency which is caused by thrombosis formation in the short term and intimal hyperplasia(IH) in the medium and long term.Thrombosis,obstructing the flow of blood

NUMERICAL SIMULATION OF UNSTEADY FLOW AROUND A CIRCULAR CYLINDER AT HIGH REYNOLDS NUMBERS

A Lagrangian-Eulerian hybrid scheme to solve unsteady N-S equation in two-dimensional incompressible fluid at high Reynolds numbers is presented in this paper. A random walk is imposed to simulate the viscous diffusion, the vortex-in-cell method is used to obtain the convection velocity, and nascent vortices are created on a cylinder to satisfy the zero-slip condition. The impulsively started flow around a circular cylinder and the separation induced by a pair of incident vortices symmetrically approaching a circular cylinder have been successfully simulated by the hybrid scheme. The impulsively started flow from rest has been computed at Reynolds numbers 3000 and 9500. Comparisons are made with those results of finite-difference method, vortex method and flow visualization. Agreement is good. The particular attention has been paid to the evolutions of flow pattern. A topological analysis has been proposed in the region of the near wake. The bulge, isolated secondary vortex, a pair of secondary vortices, ' forewake phenomenon and other patterns are simulated numerically. The separation induced by a pair of incident vortices approaching a circular cylinder has been investigated by using the same scheme. The rebounding phenomenon of the incident vortex is observed and is attributed to the effect of the secondary vortex. In particular, we have found that a tertiary vortex can be formed near the surface; this phenomenon has been verified by flow visualization reported recently.

Numerical Simulation for Effect of Inlet Cooling Rate on Fluid Flow and Temperature Distribution in Tundish

The fluid flow in tundish is a non-isothermal process and the temperature variation of stream from teeming ladle dominates the fluid flow and thermal distribution in tundish. A numerical model was established to investigate the effect of inlet cooling rate on fluid flow and temperature distribution in tundish based on a FTSC （Flexible Thin Slab Casting） tundish. The inlet cooling rate varies from 0. 5 to 0. 25 ~C/rain. Under the present calculation conditions, the following conclusions were made. When the stream temperature from teeming ladle drops seriously （for inlet cooling rate of 0.5℃/min）, there is a ＂backward flow＂ at the coming end of casting. The horizontal flow along the free surface turns to flow along the bottom of tundish. The bottom flow shortens the fluid flow route in tundish and deteriorates the removal effect of nonmetallic inclusions from molten steel. Nevertheless, when the inlet cooling rate decreases to 0.25℃/min, the horizontal flow is sustained during the whole casting period. The present research provides theoretical directions for temperature control in teeming ladle and continuous casting tundish during production of advanced steels.

Unsteady flow characteristic analysis of turbine based combined cycle(TBCC)inlet mode transition10.1016/j.jppr.2015.07.006

A turbine based combined cycle(TBCC)propulsion system uses a turbine-based engine to accelerate the vehicle from takeoff to the mode transition flight condition,at which point,the propulsion system performs a“mode transition”from the turbine to ramjet engine.Smooth inlet mode transition is accomplished when flow is diverted from one flowpath to the other,without experiencing unstart or buzz.The smooth inlet mode transition is a complex unsteady process and it is one of the enabling technologies for combined cycle engine to become a functional reality.In order to unveil the unsteady process of inlet mode transition,the research of over/under TBCC inlet mode transition was conducted through a numerical simulation.It shows that during the mode transition the terminal shock oscillates in the inlet.During the process of inlet mode transition mass flow rate and Mach number of turbojet flowpath reduce with oscillation.While in ramjet flowpath the flow field is non-uniform at the beginning of inlet mode transition.The speed of mode transition and the operation states of the turbojet and ramjet engines will affect the motion of terminal shock.The result obtained in present paper can help us realize the unsteady flow characteristic during the mode transition and provide some suggestions for TBCC inlet mode transition based on the smooth transition of thrust.

Unsteady Flow Variability Driven by Rotor-stator Interaction at Rotor Exit

Numerical investigation of the unsteady flow variability driven by rotorstator interaction in a transonic axial compressor is performed. Two models with close and far axial gap between rotor and stator rows are studied in the simulation. Particular attention is attached to the analysis of mechanisms involved in driving rotor wake oscillation, rotor wake skewing and flow angle fluctuation at rotor exit. The results show that smaller axial gap is favorable to enhance the interaction in the region between two adjacent rows, and the fluctuation of the static pressure difference between two sides of rotor wake is improved by potential field from down stator, which is the driving force for rotor wake oscillation. The interaction between rotor and stator is weakened by increasing axial distance, rotor wake shifts to suction side of rotor blade with 5%-10% of rotor pitch, the absolute value of flow angle at rotor exit is less than that in the case of close interspace for every time step, and the fluctuation amplitude is also decreased.

Time-delay effect and design of closed-loop control system of circulation control airfoil

In this paper,unsteady numerical simulation of jet Circulation Control(CC)is carried out with the NACA0012-CC airfoil as the research object.The dynamic process from the opening of jet slot and adjustment of jet intensity to the stable state of jet control effect is explored.The time-delay effect and flow mechanism of jet are analyzed.The mechanism of jet momentum coefficient and moment coefficient fluctuating with time is revealed.The fluctuation of jet momentum coefficient is caused by the change of the pressure coefficient distribution on the Coanda surface or the structure of the wave system inside the jet,and the oscillation frequency of the wave system structure of the under-expansion supersonic jet reaches 1481 Hz at the opening moment.Based on the aerodynamic model and Proportional-Integral-Derivative(PID)control theory,the closed-loop control system of CC airfoil is designed.The parameters of PID control system are adjusted by the Genetic Algorithm(GA),which significantly improves the response ability of the control system to step,ramp and sine signals,and improves the dynamic performance of the system.Aimed at the special time-delay effect of jet control,Long Short-Term Memory(LSTM)neural network module is added to the control system to predict the target input signal,which strengthens the prediction ability of GA-PID control system to the target signal at the next time moment.By using LSTM neural network correction,the control hysteresis caused by jet time-delay effect is alleviated,and the response ability of the control system is effectively improved.Finally,the designed LSTM-GA-PID control system is applied to the NACA0012-CC airfoil for the pitch control simulation test.The test results show that the control system designed in this paper has good dynamic performance and can respond quickly and accurately to complex input signals,which confirms the effectiveness of the control system.

Flow response hysteresis of throat regulation process of a two-dimensional mixed-compression supersonic inlet

The variable geometry supersonic inlet tends to decrease the throat area to reduce the Mach number upstream of the terminal shock,so as to reduce the flow loss.However,excessive Internal Contraction Ratio(ICR)exposes the inlet to a greater risk of unstart,which inevitably results in a process of increasing the throat area to aid the inlet restart.In the above throat regulation process,the inlet undergoes the start,unstart,and restart states in turn.In order to reveal the flow structure and mechanism of this process,a two-dimensional unsteady numerical simulation combined with a dynamic mesh technique were employed.The shock-on-lip Mach number of the studied inlet is 4.0 and the flight angle of attack is+6°.Analysis was focused on the state with a freestream Mach number of 3.0.The results clearly show that the flow response hysteresis appears,and restart is only realized when the throat area is obviously increased as compared to that of unstart due to the historical unstart flow structure.In addition,three typical flow fields were analyzed,and it is found that the separation ahead of the inlet was the key factor affecting the hysteresis.Finally,unstart and restart boundaries of the inlet were discussed,and the factors influencing its deviation from the typical boundaries of dual-solution area were analyzed.The newly predicted unstart and restart boundaries are much closer to the CFD results.