About Preconditioning Euler eqs. convergence

Rake

Hi, I use a 2d Euler code to learn some Preconditioner. The first preconditioner is a gloable preconditioner,as discribed in "High-resolution Viscous flow simulations at arbitrary Mach number" ,W.R.Briley,L.K. Taylor,D.L Whitfield,Jounal of Computational Physics. The second is a local preconditioner following AIAA 99-2209.

Because the 2d Euler code is solved implicit, I modified the right and left eigenvectors, the local time-step, and the ROE dissipation terms according fifferent preconditioner.Although the farfield BC should be modified for accuracy if using preconditioning, in order to reduce the effect of far field,the mesh around NACA0012 I choose far enough way from the solid,about 25 chord of the airfoil. for subsonic flow the results are similar as the origennal. When i reduce the Mach number to 0.05,all these two preconditioner have poor convengence for continuty residul. and for very low Mach 0.001 or lower both these 2 preconditioner failed. I don't know why. WHo can give me some advice? thx!