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high rotational speed centrifugal pump

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Old   January 9, 2020, 07:41
Default high rotational speed centrifugal pump
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Marek
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Hello everyone,

I am simulating the flow in centrifugal water pump. This is quite a special case because the pump has a very low specific speed of 12, a high speed of 30,000 rpm and very low flow rate. For the purposes of the simulation, I prepared the structural rotor mesh in Turbogrid and the unstructured volute grid (maximum skewness is 0.712). The grids have 283320 and 1205389 elements, respectively. The geometry consists of a static inlet part (to avoid effect of boundary conditions), impeller, volute, and pipe (to avoid backflow). The boundary conditions are total pressure inlet and mass flow outlet. The turbulence model is SST, and high resolution shemes. I also used the mixing plane and rotor segment approach to save computing power.
However, my results are unsatisfactory, high RMS residuals, on the order of 10 ^ -3, and a head rise 36% higher than the design one. I think that the hydraulic efficiency is also too high. I tried to reduce the timescale factor from 3 * 10 ^ -5 (auto) to 5 * 10 ^ -7 but this does not improve convergence and even causes divergence. I think that the key issue in this case is the very high rotational speed pump, because at lower speeds (15,000 rpm) the RMS residuals go below 10 ^ -4.
Do you have any advice or suggestions on what I can do wrong? Whether mixing-plane approach is appropriate in this case. Should a special approach be used for a pump with such a high rotational speed?

I also have a question related to transient simulation. Should the Couranat number always be less than 1 in the entire computational area to get reliable results?
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Last edited by marekpl; January 10, 2020 at 13:28.
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Old   January 19, 2020, 04:40
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Glenn Horrocks
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Some FAQs:
Non-convergence - https://www.cfd-online.com/Wiki/Ansy...gence_criteria

Accuracy - https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F

Courant Number < 1? No. CFX is an implicit solver and can handle Courant Numbers > 1. But this simulation is a steady state one anyway, so Courant Number is not relevant. Courant Numbers calculated on psuedo-time steps from steady state simulations are not meaningful.
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