Numerically solving the Boltzmann kinetic equations with the small Knudsen number ischallenging due to the stiff nonlinear collision terms. A class of asymptotic-preservingschemes was introduced in [F. Filbet and S. Jin,J. Comput. Phys. 229 (2010)7625–7648] to handle this kind of problems. The idea is to penalize the stiff collisionterm by a BGK type operator. This method, however, encounters its own difficulty whenapplied to the quantum Boltzmann equation. To define the quantum Maxwellian (Bose-Einsteinor Fermi-Dirac distribution) at each time step and every mesh point, one has to invert anonlinear equation that connects the macroscopic quantity fugacity with density andinternal energy. Setting a good initial guess for the iterative method is troublesome inmost cases because of the complexity of the quantum functions (Bose-Einstein orFermi-Dirac function). In this paper, we propose to penalize the quantum collision term bya ‘classical’ BGK operator instead of the quantum one. This is based on the observationthat the classical Maxwellian, with the temperature replaced by the internal energy, hasthe same first five moments as the quantum Maxwellian. The scheme so designed avoids theaforementioned difficulty, and one can show that the density distribution is still driventoward the quantum equilibrium. Numerical results are presented to illustrate theefficiency of the new scheme in both the hydrodynamic and kinetic regimes. We also developa spectral method for the quantum collision operator.
