Definition of parameters gamma and eps

XimenaOdio

Hi,

I would like to ask what is the physical meaning of these parameters and their effect in the simulations? In other words, what would change if I increase or decrease each parameter.




If you can show me the equation where they are used I would appreciate it.

SLongshaw

Hi XimenaOdio,

"Gamma" refers to the gamma value used in the equation of state used to correlate density to pressure. Effectively the higher this is, the stiffer the equation of state.
"eps" refers to the epsilon used in the X-SPH formulation employed in DualSPhysics. The higher this is (between 0 and 0.5) the greater the damping effect of the X-SPH formulation.

For details about both of these, I suggest looking through the details in the SPHysics documentation: https://wiki.manchester.ac.uk/sphysics/images/SPHysics_v2.2.000_GUIDE.pdf

Hope that helps.

XimenaOdio

Thank you very much, this was very helpful.

Alex

For these kind of quetions, please spend some time reading the guide of SPHysics: https://wiki.manchester.ac.uk/sphysics/images/SPHysics_v2.2.000_GUIDE.pdf

or the paper:
Gómez-Gesteira M, Crespo AJC, Rogers BD, Dalrymple RA, Domínguez JM and Barreiro A. 2012. SPHysics - development of a free-surface fluid solver- Part 2: Efficiency and test cases. Computers & Geosciences, 48: 300-307. doi:10.1016/j.cageo.2012.02.028.

Mojtaba

Hi SLongshaw

In the paper written by Gomez 2010 (State-of-the-art of classical SPH for free-surface flows-JHR) "eps" is defined between 0 and 1.
How could we find out the right amount for this parameter?
Is it an empirical variable in XSPH equation?
I didn't find the answer in Monaghan(1989) paper which is the reference for the moving particle equation(XSPH) in DualSPHysics guide.

Thanks for your helps.
Regards

SLongshaw

Hi Mojtaba,

I think it is fair to say that "eps" is empirical and defined on a per case basis.

Personally, I tend to stick to a value between 0 and 0.5 and tend towards 0 as far as possible. This is because higher values of "eps" can introduce a greater drag effect than expected according to the chosen viscosity formulation and associated variables.

The best method is probably to pick the test scenario that best fits your problem and utilise the value for "eps" found there.

Regards.

Mojtaba

Hi SLongshaw

Thank you very much for you help!
I was trying to utilize the value by changing it and running the model to get best answer for my problem. I think it's the best way too.
The range you mentioned above for the eps value and its effects on the simulation might help me!

I have the same question about gamma value. It should be between 1 and 7 which I tend to pick a value for it like eps term as you said.

Thanks again.
Regards
Mojtaba Jandaghian

SLongshaw

With regards to gamma, I think most of the guys who work on DualSPhysics would agree, it is best to stick with a value of 7, as with all of the example test cases.

This is a number which has found to be most successful by the weakly-compressible SPH community when simulating a water like fluid. It is difficult to comment on the efficacy of WC-SPH at other values.

Regards.

Mojtaba

Hi Dr. Stephen,

I found out from the equation of state and the relation between Mach number and density difference, that value of 7 gives us more incompressibility than other smaller values of gamma parameter. In fact value of 7 for gamma decrease pressure oscillation due to density deferences. Actually as you mentioned above for weakly-compressible SPH value of 7 for simulating a water with constant density, is the best!

Thanks for your helps.
Regards.
Mojtaba Jandaghian