Numerical diffusion is an effect arising from the numerical discretization scheme. Its scale depends on the angle at which the flow hits the cell faces. Thereby, its magnitude is dependent on the mesh type.
Consider two inviscid streams aligned to each other. Ideally they should not interact with each other since there ought to be no diffusion of momentum (viscosity). But in finite volume numerics there unfortunately is. A velocity magnitude scalar scene clearly shows diffusion of momentum:
Since for this case, the exact solution is known, you can quantify the error.
L2-Norm of the error vs. flow angle reads:
Trimmed meshes show a vanishing numerical diffusion when the flow is aligned exactly with the grid. If the flow is just slightly missaligned, the numerical diffusion in a trimmed mesh is identical to the numerical diffusion of a polyhedral mesh.
Numerical diffusion of the polyhedral mesher is always isotropic.