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Mesh configuration in Omnis/turbomachinery

sriluta

Hey, together in Omnis iam using the structured mesh for turbo machinery. I have two Problems, first iam investigating a blade with a wavy leading edge. In the geometry context the blade looks good. In the Mesh context the leading edge is unfortunately no longer wavy but very shape, as you can see in the Picture. 

Is the reason because I chose a structured mesh, if not how can I optimize the leading edge to make it wavy again. The 2nd point is my solution unfortunately does not converge, this is certainly because my max asp. ratio is 50,000 and is far from the recommended range as in the manual. My maximum spanwise angular deviation is also far from the recommended range at 57 degrees. Under which settings could I improve these parameters ?

thank you very much greetings Hussein

  • sriluta
    0 in reply to Colinda

    Hello Colinda and domen, thank you for your advice. By increasing the first cell size at the walls by a factor of 10 changed my max asp. ratio to 4900. How can I improve maximum spanwise angular deviation ? Its still with 59 degree to high, the other values are in the recommended range. As you can see from the residuals, I am far from the convergence criterion of 10-^6. The mass flow in and out looks good, is converging with a mass flow error of -0.007101%

  • sriluta
    0 in reply to sriluta

    But what I find strange is that when I select in the outlet  subsonic/cylindrical/pressure imposed/ static pressure imposed with a value of 1.52 bar, the residuals are in the displayed range. But if I set the static pressure to 0.95 bar, which corresponds to an outlet outlet mach number of  1.2, my residuals are between 0.70 and 1, a big difference from the previous residuals which are around 23.

  • Colinda
    0 in reply to sriluta

    The residuals are not absolute values but relative to the residual in the first iteration. In addition it is in logarithmic scale. It may happen though that the initial solution and boundary condition are such that the residual in next iterations is much higher than the initial residual. In such case the residuals in the plot can be high. Changing the initial solution and boundary condition can indeed have a significant impact.
    It is important to check not only the residuals but also the evolution of other flow field quantities.

  • domen
    +1 in reply to sriluta

    The spanwise angular deviation is a parameter that allows to identify if a few cells deviate from the adjacent ones, because of failure in the mesh generation. We measure the angles between adjacent spanwise cells to find if something went wrong. In your case, considering the "teeth" of the blade, the deviation may be normal and somehow inevitable. Check the flow around the region with high deviation to see if you see something unusual.

    W.r.t. the residuals: as Colinda mentioned in the post below, the residuals are just one check to assess convergence. If the other quantities are stable (no oscillations, no changes in the last hundred(s) iterations), then the computation is likely converged.

    Decreasing the static pressure at the outlet means that you have a new flow field, perhaps more unstable (you mention high Mach number), possibly with shocks in the domain that make the computation more difficult to converge. 

    Check the results, it's the best way to understand what's going on.

  • sriluta
    0 in reply to domen

    thx a lot domen and colinda, you helped me a lot :)

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