
[Sponsors] 
How to find density without having dynamic viscosity? 

LinkBack  Thread Tools  Search this Thread  Display Modes 
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

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. 

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 7.4E7 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.


January 27, 2014, 06:38 

#4  
Member
Join Date: Feb 2012
Posts: 49
Rep Power: 14 
Quote:
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 ! I would appreciate if you have some idea about this. 

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 

#6 
Member
Join Date: Feb 2012
Posts: 49
Rep Power: 14 

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) 

Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Problem with divergence  TDK  FLUENT  13  December 14, 2018 06:00 
calculation of laminar dynamic viscosity in BasicThermo.H  hz283  OpenFOAM  0  December 28, 2012 12:06 
dynamic viscosity setup  fat bug  CFX  5  March 20, 2008 11:01 
TwoPhase Buoyant Flow Issue  Miguel Baritto  CFX  4  August 31, 2006 12:02 
Where can I find air density  Matthew Bliss  Main CFD Forum  2  December 9, 1998 03:28 