Dr. Tafreshi is a Professor of Mechanical Engineering at Virginia Commonwealth University, and has been with the Mechanical and Nuclear Engineering Department since 2007, as Assistant Professor (2007–2012) and Associate Professor (2012–2015). Dr. Tafreshi’s research is in the areas of computational and experimental multiphase flows. His main focus is on single/multiphase fluid, heat, and particle transport through fibrous and porous media. He is also active in modeling interfacial phenomena and superhydrophobicity. He has more than 100 journal publications in the above and other areas of thermo-fluid sciences, including liquid jets and nozzle flows.
Industry Expertise (2)
Areas of Expertise (3)
Lappeenranta University of Technology: Ph.D., Mechanical Engineering 2000
University of Tehran: M.S., Mechanical Engineering 1997
K.N.T. University of Technology: B.S., Mechanical Engineering 1995
Selected Articles (3)
A mathematical framework developed to calculate the shape of the air–water interface and predict the stability of a microfabricated superhydrophobicsurface with randomly distributed posts of dissimilar diameters and heights is presented. Using the Young–Laplace equation, a second-order partial differential equation is derived and solved numerically to obtain the shape of the interface, and to predict the critical hydrostatic pressure at which the superhydrophobicity vanishes in a submersed surface. Two examples are given for demonstration of the method’s capabilities and accuracy.
This work reports on a Monte Carlo Ray Tracing (MCRT) simulation technique devised to study steady-state radiative heat transfer in fibrous insulation materials. The media consist of specular opaque fibers having unimodal/bimodal fiber diameter distributions. The simulations are conducted in 2-D ordered geometries, and the role of lateral symmetric or periodic boundary conditions are discussed in detail. Our results indicate that with the symmetric or periodic boundary condition, view factor Fi,i should be excluded from the calculations leading to temperature prediction. This is especially important when the media are made of fibers arranged in ordered configurations. In agreement with our previous 3-D MCRT simulations, the 2-D MCRT simulations presented here reveal that heat flux through a fibrous medium decreases by increasing packing fraction of the fibers, when fiber diameter is kept constant. Moreover, increasing fibers’ absorptivity was found to decrease the radiation transmittance through the media. In this work, we have also studied radiative heat transfer through bimodal fibrous media, and concluded that increasing fibers’ dissimilarity increases energy transmittance through the media, if porosity and number of fibers are kept constant. It was also found that temperature of the fibers is almost independent of the media’s porosity or diameter ratios.
Previous studies dedicated to modeling drag reduction and stability of the air-water interface on superhydrophobicsurfaces were conducted for microfabricated coatings produced by placing hydrophobic microposts/microridges arranged on a flat surface in aligned or staggered configurations. In this paper, we model the performance of superhydrophobicsurfaces comprised of randomly distributed roughness (e.g., particles or microposts) that resembles natural superhydrophobicsurfaces, or those produced via random deposition of hydrophobic particles. Such fabrication method is far less expensive than microfabrication, making the technology more practical for large submerged bodies such as submarines and ships. The present numerical simulations are aimed at improving our understanding of the drag reduction effect and the stability of the air-water interface in terms of the microstructure parameters. For comparison and validation, we have also simulated the flow over superhydrophobicsurfaces made up of aligned or staggered microposts for channel flows as well as streamwise or spanwise ridges configurations for pipe flows. The present results are compared with theoretical and experimental studies reported in the literature. In particular, our simulation results are compared with work of Sbragaglia and Prosperetti, and good agreement has been observed for gas fractions up to about 0.9. The numerical simulations indicate that the random distribution of surface roughness has a favorable effect on drag reduction, as long as the gas fraction is kept the same. This effect peaks at about 30% as the gas fraction increases to 0.98. The stability of the meniscus, however, is strongly influenced by the average spacing between the roughness peaks, which needs to be carefully examined before a surface can be recommended for fabrication. It was found that at a given maximum allowable pressure, surfaces with random post distribution produce less drag reduction than those made up of staggered posts.