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 January 11, 2000, 11:51 Computation through radial impeller #1 Ursenbacher Guest   Posts: n/a Hello, I'm doing a computation through a radial impeller but I have some convergence problems. The numerical scheme I use is a 2nd order central sheme with 2nd and 4th order dissipation for spatial discretisation and a 4th order Runge Kutta for time discretisation. The problem is the following: The massflow, the efficiency, the torque,etc.. oscillate. If I reduce the CFL, the wave length of theses oscillations increases. What can I do to avoid theses oscillations and to have a converged solution? What are the effect of the 2nd and 4th order dissipation? Can I suppress the oscillation by varrying theses two parameters? Thanks for your attention, F.Ursenbacher

 January 11, 2000, 12:35 Re: Computation through radial impeller #2 Joern Beilke Guest   Posts: n/a Are you sure that the flow is steady-state in reality ? What about your mesh (2d/3d/y+/??) and the boundary conditions. Does the impeller rotates somehow?

 January 11, 2000, 13:13 Re: Computation through radial impeller #3 John C. Chien Guest   Posts: n/a (1). The physics of the problem is rather complex, because it always involve 3-D flow and flow separations. So, I am not going to get into that area. (2). The method you are using is Jameson's type formulation, that is transient, compressible formulation. The transient, compressible formulation is sensitive to the low Mach number effect. So, try to stay away from the low Mach number regime. (3). It is essential to mention your problem's Mach number and Reynolds number, along with any turbulence model and treatment used. So, remember that next time.

 January 11, 2000, 13:35 Re: Computation through radial impeller #4 Dan Hinch Guest   Posts: n/a The instability could be due to a large number of causes (even if your numerical scheme is appropriate) including the grid (both spacing, and the size/shape of the inlet and exit extensions), the Mach number level (as John has mentioned), the initial guess (how far does the solution converge before going unstable?), and even the design being analyzed (is the flow truely steady state?).

 January 11, 2000, 23:57 Re: Computation through radial impeller #5 Mohammad Kermani Guest   Posts: n/a Hi there: > Dan Hinch, Tue, 11 Jan 2000, 10:35 a.m. WROTE: > and even the design being analyzed (is the flow truely steady state?). when you mention about the unsteadiness of the flow, do you mean flow is physically unsteady in reallity? If so, how such a thing could be invetigated before doing an experiment? Just by cfd simulations? Is that somehow related to the transion from laminar to turbulent? Does these kinds of unsteadinesses happen only in viscous flow simulations? Does the inlet buzz also fall into these categories? Thanks.