Articles written in Sadhana

    • Determination of tangential momentum accommodation coefficient and slip coefficients for rarefied gas flow in a microchannel


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      This paper presents an experimental study of rarefied gas flow in a trapezoidal microchannel with a constant depth of 103 μm, top width of 1143 μm, bottom width of 998 μm and length of 2 cm. The aim of the study is to verify the upper limit of the validity of the second-order slip boundary condition to model rarefied gas flows. The slip coefficients and the tangential momentum accommodation coefficient (TMAC) are determined for three different gases, viz. argon, nitrogen and oxygen, and it is observed that they compare well to theliterature values. The range of mean Knudsen number (Knm) investigated is 0.007–1.2. The non-dimensional mass flow rate exhibits the well-known Knudsen minimum in the transition regime (Knm ~ 1). It is seen that the Navier–Stokes equation with a second-order boundary condition fits the data satisfactorily with a high value of correlation coefficient (r² >99.95%) in the entire range of Knm investigated. This work contributes by extending the range of Knudsen number studied in the context of validity of the second-order slip boundary condition.

    • Cut-cell-based Direct Simulation Monte Carlo method on a Cartesian grid for rarefied gas flow around complex geometries


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      The present work proposes a cut-cell-based Direct Simulation Monte Carlo (DSMC) solver, for computing rarefied flows around complex geometries on Cartesian grids, wherein analytical expression for the surface of the immersed boundary (IB) is considered to evaluate cut-cell volume as well as to implement the particle–boundary interactions. Consequently the proposed DSMC solver models an accurate collision rate in the cut cells and ensures an analytically expressed IB-based implementation of the boundary conditions at thesurface of the immersed geometry, as in the IB methods for the continuum flows. Performance of the present Cartesian cut-cell-based DSMC solver is tested on a variety of rarefied gas flows around three complex geometries (cylinder, NACA 0012 airfoil and double-wedge airfoil) for various flow speeds (ranging from Ma = 2 to 10) and degrees of rarefication (varying from Kn = 0:25 to around 0.0032). Results of our computations on Cartesian grids show a very good agreement with the corresponding DSMC results in literature computed on body-fitted grids. Furthermore, the present results show a good agreement with the corresponding experimental data in the literature. Straightforward and analytically expressed IB-based implementation in the proposed DSMC solver can make it a natural choice for its coupling with an immersed boundary method (IBM)- based continuum solver for a novel coupled IBM–DSMC method for continuum–rarefied gas flows.

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