In order to carry out the averaging process in (22), a pair probability density
function is needed. A probability density function will be used which is
factorized in a velocity distribution function and a configurational distribution
function. With regard to the velocity distribution it will be assumed that all
bubbles move with the average gas velocity U,. Since w, = ( wi) = w = U, - U,
the calculated mass coefficient of the kinetic energy density, in frame moving
with U,, and the effective added mass density will be identical.