动网格设置问题

最近在做动网格,对 dynamicMeshDict 里面的各项参数意思不是很了解?请问各位大神,diffusivity 的参数分别是什么意思呢?

dynamicFvMesh   dynamicMotionSolverFvMesh;

motionSolverLibs   ("libfvMotionSolver.so");

solver velocityComponentLaplacian z;

diffusivity  uniform;
diffusivity  directional (0 0 1);
diffusivity  motionDirectional (1 1000 0);
diffusivity  inverseDistance 1(movingWall);
diffusivity  file motionDiffusivity;
diffusivity  quadratic inverseDistance 1(movingWall);
diffusivity  exponential 2000 inverseDistance 1(movingWall);