Yichuan Fang

Heat transfer in microchannel devices using DSMC

The heat transfer characteristics of supersonic flows in microchannels is studied using direct simulation Monte Carlo (DSMC) method. The velocity components and the spatial coordinates of the simulated particles are calculated and recorded by using a variable-hard-sphere (VHS) collision model. The effects of Knudsen number (Kn) on the heat transfer of the microchannel flows are examined. The results show that the magnitude of the temperature jump at the wall increases with increasing Kn. The heat transfer to the isothermal wall is found to increase significantly with Kn. The possible causes for the increase of wall heat transfer are discussed.

Computations of the Flow and Heat Transfer in Microdevices Using DSMC With Implicit Boundary Conditions

The heat transfer and the fluid dynamics characteristics of subsonic gas flows through microchannels are examined using the direct simulation Monte Carlo (DSMC) method. A simple implicit treatment for the low-speed inflow and outflow boundaries for the DSMC of the flows in microelectromechanical systems (MEMS) is used. Micro-Couette flows and micro-Poiseuille flows are simulated with the value of the Knudsen numbers ranging between 0.06 and 0.72. Where appropriate, the calculated velocity slip and temperature distribution are compared with analytical solutions derived from the Navier-Stokes equations with slip-boundary conditions. A patterned microstructure with nonuniform surface temperature is also simulated

Skin Friction Prediction for High-Speed Turbulent Boundary Layers with Ablation

A new computational methodology is presented in this paper for the prediction of the skin friction coefficient for high -speed turbul ent boundary layers with ablation. The method considers the coupled effects of roughness and blowing on skin friction. For the roughness correlation, an implicit formula has been developed based on the roughness correlation for adiabatic 1 and cooling walls . 2 With the critical roughness Reynolds number included, the new correlation successfully models the roughness -augmented skin friction for high -speed flows with Mach number up to 12. For the effects of blowing, the correlation of Lees 3 is used. The method calculates the roughness correlation first and the blowing correlation is made based on the calculated roughness correction. The proposed formulation of the method is universal and can be applied to flat plate or cone geometries. Results presented for the supersonic boundary layer over a flat plate with a wide range of blowing agree well with the experimental data.

The development of a Burnett equations solver for microfluid flow and heat transfer simulations

A three-dimensional numerical solver for the Burnett equations for microfluid flows is developed based on a Navier-Stokes code. The second order stresses and heat fluxes in the conventional Burnet equations are implemented into the code. Velocity-slip/temperature-jump conditions of Maxwell at the wall are also used. The new solver is validated using flows through microchannels. Results are compared with those obtained by using the Navier-Stokes equations with and without the slip-wall conditions. For the cases considered, with the reference Knudsen number up to 0.02, there is a good agreement between the Navier-Stokes solutions and the Burnett solutions when the same type of wall boundary conditions is applied. The effects of Knudsen number on the flow and the heat transfer characteristics of the microchannel are also discussed.

Forced Couette flow simulations using direct simulation Monte Carlo method

Three-dimensional unsteady flows between two infinite walls are simulated by using the direct simulation Monte Carlo (DSMC) method. An artificial forcing that mimics the centrifugal force in the Taylor problem has been applied to the flow. The sampled behaviors of the resulting flow, including the long time average and the disturbance components, are studied. The computations have been preformed using parallel computer clusters. The results presented are for two different channel heights with various values for the forcing coefficient. The change in the channel height, which also results in changes in the flow Knudsen number and Reynolds number, affects the development of both the mean flows and the disturbances. Spatially coherent mean flow patterns, which are dominated by a hierarchy of harmonic modes, can be identified in the DSMC solutions. Temporally, the evolution of the Fourier amplitudes of the harmonic modes shows that these modes grow in a sequential manner. Disturbances with energy spectra that are significantly higher than the statistical noises are resolved. Their pathline patterns indicate that the disturbance flow fields are three dimensional and spatially coherent. These results suggest that the discrete DSMC approach is capable of capturing unsteady, three-dimensional flow disturbances that evolve around a stationary mean flow.

Global numerical prediction of bursting frequency in turbulent boundary layers

The frequencies of the bursting events associated with the streamwise coherent structures of spatially developing incompressible turbulent boundary layers were predicted. The structures were modeled as wavelike disturbances associated with the turbulent mean flow using a direct‐resonance theory. Global numerical solutions for the resonant eigenmodes of the Orr‐Sommerfeld and the vertical vorticity equations were developed. The global method involves the use of second and fourth order accurate finite difference formulae for the differential equations as well as the boundary conditions. The predicted resonance frequencies were found to agree very well with previous results using a local shooting technique and measured data.

Toward the Development of Information Preservation and Its Statistical Scattering for DSMC

The development of a general information preservation technique has been introduced for the DSMC method. The derivation of general governing equations for the preserved information starts from the Boltzmann equation. Following the DSMC procedures, the information preservation equations are obtained by reducing the general governing equations. The physics and theoretical requirement for the model closure have been discussed. The information preservation technique uses the DSMC procedures as its carrier to calculate its own information variables based on the conservative transport of preserved mass, preserved momentum, and preserved energy, without affecting the solution of DSMC method. The statistical scatter of IP and DSMC has been analyzed and compared theoretically. The information preservation has been applied to micro-Couette flows. Its numerical solutions have been validated and compared with those of NS and DSMC methods. Numerical results show that the IP technique improves the statistical scatter of DSMC significantly.

Direct Numerical Simulation of A Forced Micro Couette Flow using DSMC

The direct simulation Monte Carlo (DSMC) method is used in a time‐dependent manner to simulate three‐dimensional micro Couette flows. An artificial forcing that mimics the centrifugal force in the Taylor problem has been applied to the flow. The sampled behaviors of the resulting flow, including the averaged properties and disturbances, are studied. The computations have been performed using a parallel computer cluster. The results presented include those with various channel heights, plate speeds, and the forcing level. These changes also result in changes in the flow Reynolds number and Knudsen number. Spatially coherent flow patterns can be identified in the averaged flow and the disturbance flow fields. The results indicate that the discrete approach can capture unsteady, three‐dimensional vortical flow structures. In cases with strong forcing, the disturbance energy spectra show significant content above statistical scatter.

DSMC Collision Separation Distance Effects on the Pattern Formation of Stationary Microflows

The direct simulation Monte Carlo (DSMC) method is applied to simulate forced micro Couette flows in a time-dependent manner. An artificial forcing is imposed on the monatomic molecules in the direction normal to the solid surfaces. The forcing is proportional to the square of the velocity deficit and is therefore analogous to the centrifugal force in Taylor-Gortler problems. Vortical flow patterns, similar to the Taylor-Gortler vortices, have been observed reported in the previous works. In this paper, the collision separation distance (CSD) effects on the simulated microflows are studied. Two and three- dimensional forced micro Couette flows are simulated with different number of particles. In simulated flows, CSD does not significantly affect the formation of vortical flow pattern with a value less than 0.74 of the mean free path in two-dimensional flow simulations and 1.54 of the mean free path in three-dimensional flow simulations. A reduction of the sub-cell number by a factor of two in a 2D case shows that, within this range, appropriate CSDs can still be obtained by using the large number of sub-cells per cell without increasing the number of simulated particles.

FORCED COUETTE FLOW SIMULATIONS USING DSMC

computational molecules are allowed to move and collide in the physical space. Macroscale flow properties can be obtained by sample averaging. Figure 1 shows a sketch describing the physical reasoning of the discrete modeling approach of fluid motion. Three-dimensional unsteady flows between two infinite walls are simulated by using the direct simulation Monte Carlo method (DSMC). An artificial forcing that mimics the centrifugal force in the Taylor problem has been applied to the flow. The sampled behaviors of the resulting flow, including the averaged properties and disturbances, are studied. The computations have been preformed using parallel computer clusters. The results presented are for two different channel heights. The change in channel height, which also results in changes in the flow Reynolds number and Knudsen number, has significant effects on both the average flows and the disturbances. Spatially coherent flow patterns, which are dominated by a hierarchy of harmonic modes, can be identified in the DSMC solutions. Temporally, the evolution of the Fourier amplitudes of the harmonic modes shows that these modes grow in a sequential manner. The disturbance flow patterns and the energy spectra also show significant content above statistical scatter. These results indicate the discrete DSMC approach can capture unsteady vortical flow structures.