A review on fluid dynamics of flapping foils

The aero-optical effects of an optical seeker with a supersonic jet for hypersonic vehicles in near space were investigated by three suites of cases, in which the altitude, angle of attack, and Mach number were varied in a large range. The direct simulation Monte Carlo based on the Boltzmann equation was used for flow computations and the ray-tracing method was used to simulate beam transmission through the nonuniform flow field over the optical window. Both imaging displacement and phase deviation were proposed as evaluation parameters, and along with Strehl ratio they were used to quantitatively evaluate aero-optical effects. The results show that aero-optical effects are quite weak when the altitude is greater than 30 km, the imaging displacement is related to the incident angle of a beam, and it is minimal when the incident angle is approximately 15°. For reducing the aero-optical effects, the optimal location of an aperture should be in the middle of the optical window.

Aero-optical effects of an optical seeker with a supersonic jet for hypersonic vehicles in near space.

Unified gas-kinetic wave-particle methods I: Continuum and rarefied gas flow

In this paper, we present a unified gas-kinetic wave-particle (UGKWP) method on unstructured mesh for multiscale simulation of continuum and rarefied flow. Inheriting from the multicale transport in the unified gas-kinetic scheme (UGKS), the integral solution of kinetic model equation is employed in the construction of UGKWP method to model the flow physics in the cell size and time step scales. A novel wave-particle adaptive formulation is introduced in the UGKWP method to describe the flow dynamics in each control volume. The local gas evolution is constructed through the dynamical interaction of the deterministic hydrodynamic wave and the stochastic kinetic particle. Within the resolution of cell size and time step, the decomposition, interaction, and evolution of the hydrodynamic wave and the kinetic particle depend on the ratio of the time step to the local particle collision time. In the rarefied flow regime, the flow physics is mainly recovered by the discrete particles and the UGKWP method performs as a stochastic particle method. In the continuum flow regime, the flow behavior is solely followed by macroscopic variable evolution and the UGKWP method becomes a gas-kinetic hydrodynamic flow solver for the viscous and heat-conducting Navier--Stokes solutions. In different flow regimes, many numerical test cases are computed to validate the UGKWP method on unstructured mesh. The UGKWP method can get the same UGKS solutions in all Knudsen regimes without the requirement of the time step and mesh size being less than than the particle collision time and mean free path. With an automatic wave-particle decomposition, the UGKWP method becomes very efficient. For example, at Mach number 30 and Knudsen number 0.1, in comparison with UGKS several-order-of-magnitude reductions in computational cost and memory requirement have been achieved by UGKWP.

Unified Gas-kinetic Wave-Particle Methods II: Multiscale Simulation on Unstructured Mesh

The unified gas-kinetic scheme (UGKS) provides a framework for simulating multiscale transport with the updates of both gas distribution function and macroscopic flow variables on the cell size and time step scales. The multiscale dynamics in UGKS is achieved through the coupled particle transport and collision in the particle evolution process within a time step. In this paper, under the UGKS framework, we propose an efficient multiscale unified gas-kinetic wave-particle (UGKWP) method. The gas dynamics in UGKWP method is described by the individual particle movement coupled with the evolution of the probability density function (PDF). During a time step, the trajectories of simulation particles are tracked until collision happens, and the post-collision particles are evolved collectively through the evolution of the corresponding distribution function. The evolution of simulation particles and distribution function is guided by evolution of macroscopic variables. The two descriptions on a gas particle, i.e. wave and particle, switch dynamically with time. A new concept of multiscale multi-efficiency preserving (MMP) method is introduced, and the UGKWP method is shown to be an MMP scheme. The UGKWP method is specially efficient for hypersonic flow simulation in all regimes in comparison with the wave-type discrete ordinate methods, and presents a much lower stochastic noise in the continuum flow regime in comparison with the particle-based Monte Carlo methods. Numerical tests for flows over a wide range of Mach and Knudsen numbers are presented. The examples include mainly the hypersonic flow passing a circular cylinder at Mach numbers $20$ and $30$ and Knudsen numbers $1$ and $10^{-4}$, low speed lid-driven cavity flow, and laminar boundary layer. These results validate the accuracy, efficiency, and multiscale property of UGKWP method.

Unified Gas-kinetic Wave-Particle methods I: Continuum and Rarefied Gas Flow

https://www.ki-net.umd.edu/pubs/files/Ren.pdf

An asymptotic-preserving Monte Carlo method for the Boltzmann equation ☆

Bio-inspired aerodynamic designs have been mutually promoted by the studies on flapping wings in the past decades. Among the tons of researches topics, the wake formation/structure of an oscillating airfoil is more attractive, and it results in the development of both numerical methods and flow diagnostics techniques. In this paper, wake formation behind a sinusoidally piching NACA 0012 has been studied with PIV measurements. The evolution of wake structures with increasing Strouhal number was reproduced successfully. With further experiments, the effect of Strouhal number, the amplitude and mean value of AOA on the asymmetric wake formation were also investigated. Results showed that the distance between vortex street became larger with increasing amplitude, while mean strength of vortices declined. Besides, the asymmetric wake formation was strongly dependent on the mean. Specifically, based on our experiments, the direction of wake asymmetry was changed at a ̅ and .

An Experimental Investigation on the Asymmetric Wake Formation of an Oscillating Airfoil

In this paper, the effect of compressibility and Knudsen number on optical distortions in supersonic/hypersonic flows were investigated. The flowfields were resolved by the DSMC method, regarding the non-equilibrium state of hypersonic flows. The code used in this work was validated, and the results were in good agreement with current recognized DSMC codes. Besides, a new ray tracing scheme was proposed to compute the phase difference from the index of refraction fields. Two dimensional supersonic/hypersonic flows around a cylinder (radius = 152.4 mm) were investigated. Results showed that (1) A smaller Knudsen number would lead to larger

Study on the Effect of Compressibility and Knudsen number on Aero Optics in Supersonic/Hypersonic Flows

Bio-inspired aerodynamic designs have been mutually promoted by the studies on flapping wings in the past decades. Among the tons of researches topics, the wake formation/structure of an oscillating airfoil is more attractive, and it results in the development of both numerical methods and flow diagnostics techniques. In this paper, wake formation behind a sinusoidally piching NACA 0012 has been studied with PIV measurements. The evolution of wake structures with increasing Strouhal number was reproduced successfully. With further experiments, the effect of Strouhal number, the amplitude and mean value of AOA on the asymmetric wake formation were also investigated. Results showed that the distance between vortex street became larger with increasing amplitude, while mean strength of vortices declined. Besides, the asymmetric wake formation was strongly dependent on the mean. Specifically, based on our experiments, the direction of wake asymmetry was changed at a and .

https://www.aere.iastate.edu/~huhui/paper/2013/AIAA-2013-0794-Ren-Wei-flapping.pdf

An Experimental Investigation on the Asymmetric Wake of an Oscillating Airfoil

As an important department of railway transportation and production, large freight train depot is responsible for the regular overhaul and maintenance of railway trucks. The shunting operation of freight train depot covers the whole process of railway trucks entering, storing, overhauling and leaving the depot. It is an important step in the implemen-tation of the maintenance operation. Usually, shunting personnel in the depot transport the trucks to be overhauled to the maintenance line by relying on the shunting operation plan, which is the key to determine the shunting operation plan according to the distribution relationship between vehicles and maintenance. Firstly, this paper analyzes the process of centralized shunting operation in the freight train depot and the factors affecting the time-consuming based on the research idea of flexible workshop scheduling problem. Then, on the premise that the proportion of the weight coefficient will have an impact on the time-consuming of truck busy and shunting in the shunting process, and with the goal of minimizing the time-consuming of truck maintenance busy and shunting, the allocation model between trucks and maintenance lines is established; In addition, an improved genetic algorithm is proposed to solve the established model; Finally, combined with the maintenance of railway trucks in a large freight train depot, an example analysis is carried out on this basis. The results demonstrate that using simulated annealing genetic algorithm to solve the model can obtain the allocation scheme between railway trucks and maintenance operation line. Under the influence of three different coefficients, compared with genetic algorithm, simulated annealing genetic algorithm can reduce the deten-tion time of railway trucks in depot by 0.21%, 0.09% and 0.12% respectively, which is beneficial to reducing the deten-tion time of maintenance vehicles in depot, It plays a positive role in improving the maintenance efficiency of trucks in the depot, and also provides new ideas for the research of railway truck shunting operation.



Wei Ren

Papers

An asymptotic-preserving Monte Carlo method for the Boltzmann equation ☆

An Experimental Investigation on the Asymmetric Wake Formation of an Oscillating Airfoil

Study on the Effect of Compressibility and Knudsen number on Aero Optics in Supersonic/Hypersonic Flows

An Experimental Investigation on the Asymmetric Wake of an Oscillating Airfoil

Determination of Truck Maintenance Allocation Scheme Based on SA-GA