Pressure units in incompressible solvers
Posted February 23, 2017 at 04:17 by kindle
Quote:
Originally Posted by Per
Hi
I am quite new to OpenFOAM, and have some basic questions about units. From the tutorial cases I see that the units for pressure in incompressible solvers (e.g. simpleFoam) are m^2/s^2. Which make sense since pressure is constant. I guess I then have to scale (divide) my pressure initial and boundary conditions with rho in order to get a correct solution? My real question is: can I define my pressure units to be kg/ms^2 and define density rho in the transportProperties file and get the same result? I want to be able to do this in order to avoid having to scale my pressure.
Thanks in advance for replies.
Per
I am quite new to OpenFOAM, and have some basic questions about units. From the tutorial cases I see that the units for pressure in incompressible solvers (e.g. simpleFoam) are m^2/s^2. Which make sense since pressure is constant. I guess I then have to scale (divide) my pressure initial and boundary conditions with rho in order to get a correct solution? My real question is: can I define my pressure units to be kg/ms^2 and define density rho in the transportProperties file and get the same result? I want to be able to do this in order to avoid having to scale my pressure.
Thanks in advance for replies.
Per
nu = mu / rho must be specified in /constant/transportproperties because Re= U * D / nu will determine the flow pattern in the no-dimensional sense.
Solving adimensional N-S and rescale with U_ref, D_ref chosen and rho (phsical property, dimensionally independant from U_ref and D_ref [demanded by Pi-theorem], only that rho do not adimensionalise rho. but in the derivation of Froude number [https://en.wikipedia.org/wiki/Froude...in_application cauchy momentum equation], we use a rho_ref to adimensional rho, rho is then used to get rid of pressure), we get the U_ref to adimensionalize U, D_ref/U_ref to adimensionalize t and rho*U_ref^2, rho*U_ref^2 for pressure, D_ref for variables of position (x y and z).
Substitution these to N-S and divide by U_ref^2/D_ref, we get the non-dimensionalized N-S. With only one no-dimensional nu/(U_ref*D_ref) number resides with viscous term (1/rho before pressure gradient is eliminated by choosing rho*U_ref^2 for pressure).
If we choosed p0 for p. There will be something before grad(p) which will be - p0/(rho*U_ref**2). And since these are reference parameter,
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