Accommodation coefficient normal momentum

The pressure dependence of effective viscosity obviously depends upon the value of the momentum accommodation coefficient. Momentum accommodation data are relatively rare, but some representative data are given in Table 1. Note that all values are relatively close to unity. Because of this observation, momentum accommodation coefficients are normally assumed to be unity in applications... 

The accommodation coefficient or represents the fraction of the gas molecules that leave the surface in equilibrium with the surface. The fraction I — cr is specularly reflected such that the velocity normal to the surface is reversed. As in the case of Stokes law, the drag is proportional to the velocity of the spheres. However, for the free molecule range, the friction coefficient is proportional to dj whereas in the continuum regime dp ip), it is proportional to dp. The coefficient a must, in general, be evaluated experimentally but is usually near 0.9 for momentum transfer (values differ for heat and mass transfer). The friction coefficient calculated from (2.19) is only 1% of that from Stokes law for a 20-A particle. 

Knudsen accommodation coefficients of normal momentum, tangential momentum, energy, and thermal creep. The additional coefficients a y, a y, and ayy are... 

A mathematical model for the scattering indicatrix that meets the normalizing condition over final velocities and satisfies reciprocality principle has been proposed by Cercignani (Cercignani 1975). It contains accommodation coefficients of taingen-tial momentum (ar) aird normal energy (a ) and has the form... 

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