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Thread: Computational Fluid Dynamics (CFD) Q&A

  1. #16
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    Are you planning to do an unsteady flow sim with moving piston?

    In Post4T I have an option to output the inlet, exhaust and cylinder pressures in a format as used by Siemens' Simcenter STAR-CCM+ for in-cylinder simulation, do you need something like that in Post2T?

  2. #17
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    That would be the ultimate goal, but quite a ways off I think.

    For now I've been working from the files that post2T reads in to pull out the pressure, temp, mach, and mass flow rate into a single sheet. the data is there to do 1 steps, but for now I'm stepping in 5. Current aims are two fold: 1. get the work flow down so I can run a batch of cases and collect the outcomes (half way there on this one) 2. get a feel for how things flow at different points in the cycle (5 step and 2D models inform, but are not the whole picture).

    With my current flow, I think I could do a test of unsteady as each mesh is a morph of the prior with the same number of nodes. When I finish this round of cleanup on the scripts I'll give that a try.

    Another goal would be to determine a Cd value for the exhaust at the given crank positions after optimization. But not sure if or how that could be rolled back into the engmod2T run.
    Patrick Owens
    www.OopsClunkThud.com

  3. #18
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    Quote Originally Posted by OopsClunkThud View Post
    Another goal would be to determine a Cd value for the exhaust at the given crank positions after optimization.
    That's about the top item on my wish list. It would allow a great extension of the angle-area approach.

  4. #19
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    I'm following your efforts closely. I need to get more flow information for my opposed piston design. I'll use your Fusion 360 script and see if I understand the method.

    Lohring Miller

  5. #20
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    Here's a demo fusion model and the CSV file. This generates 280 step files as the output, so if you want to just test it, remove lines from the CSV.

    demo.zip
    Last edited by OopsClunkThud; 8th May 2021 at 11:53. Reason: error in attachment
    Patrick Owens
    www.OopsClunkThud.com

  6. #21
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    Exhaust Gas Properties

    Been tracking down the exhaust gas constants called for in the simulations:

    From Blair for AFR 13:
    Gas % by Volume:
    CO 5.85%
    CO2 8.02%
    H2O 15.6%
    O2 0.0%
    N2 70.52%

    Ratio of Specific Heats:
    500K gamma=1.362
    1000K gamma=1.317

    Gas Constant:
    293K-1000K R=299.8

    NASA gas tables is a likely a better source for these values:
    https://cearun.grc.nasa.gov/ThermoBuild/):

    For Dynamic Viscosity what I have here is likely better than just using air, but I don't fully trust that it's right:
    simscale (openFOAM) and SU2 both use Sutherland law.

    https://www.cfd-online.com/Wiki/Sutherland's_law
    https://www.grc.nasa.gov/www/BGH/viscosity.html

    Sutherlands law allows us to use the viscosity at a reference temperature and compute the viscosity at another. Just need to come up with the Sutherland constant for our mixture of exhaust gas and the reference temp/viscosity. Seems the norm is to back into the Sutherland constant by curve fitting.

    found two ways to get viscosity values for the exhaust gas mixture:
    1. use someone else's model and hope it's correct: https://www.firecad.net/engineering-.../GasProperties
    2. Calculate the mixed value using the Herning and Zipperer approximation (have not tried this yet)

    By curve fitting I came up with the following values to use in Sutherland's law:

    Sutherland Viscosity Ref (1.716E-5 default value for AIR SI)
    MU_REF= 2.400E-05

    Sutherland Temperature Ref (273.15 K default value for AIR SI)
    MU_T_REF= 500

    Sutherland constant (110.4 default value for AIR SI)
    SUTHERLAND_CONSTANT= 217.5

    Had to use a higher reference temp as the water vapor makes things go wonky at the lower temp.
    Patrick Owens
    www.OopsClunkThud.com

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