Total Pressure or Total Pressure in Rel Frame
 

Hello,


I must compute frictional losses in an hydraulic runner. I am not sure if I must take total pressure or total pressure in Rel Frame. In theory user’s guide of CFX, there is little information on total pressure in Rel Frame. I really don't understand the definition of total pressure (ptot = pstat + 0.5*rho*(Urel^2-(wR)^2)). Can anybody help me and explain me ?


Thank you !

i hope this will answer your questions:

http://www.cfd-online.com/Forums/cfx...-pressure.html

Yes, I saw this subject but it isn't clear …

Are you familiar with the Bernoulli equation? I believe understanding Bernoulli equation is a good way to come to grips with the meaning of total pressure. Once you understand total pressure, just realize that you are in a rotating reference frame so 'v' in the dynamic head (1/2 rho v^2) is now zero if the particles are rotating with the frame, not if they are stationary relative to the stator. i.e.

Total pressure in relative frame: Pressure that fluid would be at if isentropically decelerated to zero velocity relative to the current reference frame

Total pressure in stationary frame: Pressure that fluid would be at if isentropically decelerated to zero velocity relative to the stationary frame

in a simple way, the total pressure represents the external energy of the fluid, internal energy is the temperature.

so external energy is composed by pressure and velocity but the two can be exchanged, for example:
in a nozzle with free slip wall pressure is converted in velocity
the stop point (I don't know the translation) at a profile supposing reversible transformation convert velocity in pressure...

now supposing your fluid will decelerate to an imaginary stop point, the static pressure will be the total pressure...
the question is where do you want your fluid to stop, in which frame ?