Computational Fluid Dynamics (CFD) Q&A

[Vannik]

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?

[OopsClunkThud]

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.

[Frits Overmars]

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.

[lohring]

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

[OopsClunkThud]

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

[OopsClunkThud]

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.