# How to find density without having dynamic viscosity?

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 January 20, 2014, 10:07 How to find density without having dynamic viscosity? #1 Member   Join Date: Feb 2012 Posts: 49 Rep Power: 14 For a particular purpose, I need to know the density in my flow simulation. But the only parameter can be set, is the kinematic viscosity. For example consider a simulation with the nu=0.01. If i don't have the dynamic viscosity, how can i find the density?!

 January 20, 2014, 10:47 #2 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,938 Rep Power: 38 Hi, I guess you are doing your incompressible flow simulation. So density is constant everywhere. So if you take random simulation with random value of kinematic viscosity there's no way to get density out of it. ooo likes this.

 January 20, 2014, 13:42 #3 Senior Member     Marco A. Turcios Join Date: Mar 2009 Location: Vancouver, BC, Canada Posts: 740 Rep Power: 28 As long as you have set your kinematic viscocity correctly (in transportProperties) for the fluid you are simulating, the density is whatever you need it to be. Just remember that the pressure for incompressible cases is the kinematic pressure (p/rho_0). So if you have the kinematic viscocity of say 7.4E-7 m^2/s, then you could either have methanol at 20 C or Isobutane at around -60 C (just a table I have in front of me at the moment). So depending on the fluid you want to be simulating, all the relevant properties should be scaled by the proper density. In the cases I said earlier, densitites are 788.4 and around 650 kg/m^3 respectively, and you would have to multiply your kinematic viscocity, kinematic pressure and any similarly scaled property accordingly. ooo likes this.

January 27, 2014, 06:38
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 Originally Posted by mturcios777 As long as you have set your kinematic viscocity correctly (in transportProperties) for the fluid you are simulating, the density is whatever you need it to be. Just remember that the pressure for incompressible cases is the kinematic pressure (p/rho_0). So if you have the kinematic viscocity of say 7.4E-7 m^2/s, then you could either have methanol at 20 C or Isobutane at around -60 C (just a table I have in front of me at the moment). So depending on the fluid you want to be simulating, all the relevant properties should be scaled by the proper density. In the cases I said earlier, densitites are 788.4 and around 650 kg/m^3 respectively, and you would have to multiply your kinematic viscocity, kinematic pressure and any similarly scaled property accordingly.
I've implemented a method to simulate the flow around a 2d cylinder without having the geometry of the cylinder.But i have forces on the coordinates of the cylinder.
So i would like to use these forces to compute drag coefficient!
The formula is : Cd = Fd / (0.5 * rho * u^2 * D ) . the points is that the dimension of Fd in my algorithm is m/s^2 and its not divided by rho !

 January 27, 2014, 16:49 #5 Senior Member     Marco A. Turcios Join Date: Mar 2009 Location: Vancouver, BC, Canada Posts: 740 Rep Power: 28 How are you calculating Fd?

January 27, 2014, 19:55
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 Originally Posted by mturcios777 How are you calculating Fd?
The velocity is interpolated to a lagrangian grid, then a force in lagrangian grid computes by : F=U/dt (then this force is distributed to the eulerian grid and ....)
But the points is that the dimension of the force is m/s2 ...

 January 28, 2014, 12:46 #7 Senior Member     Marco A. Turcios Join Date: Mar 2009 Location: Vancouver, BC, Canada Posts: 740 Rep Power: 28 Well that makes sense, as Force is actually d(mU)/dt: http://en.wikipedia.org/wiki/Newton'...27s_second_law The units are consistent with an incompressible fluid, as rho is not changing, and m=rho*V (where V is volume). Because your rho is already been divided out in the incompressible formulation, Cd = Fd/(1/2 u^2 D) ooo likes this.