4 Replies Latest reply on Mar 27, 2012 9:17 AM by David Maxham

    How Do I Change the Density of the Project Fluid?

    David Maxham

      I'm running a Flow Simulation model where I want to change the density of the project fluid, which is air.   It appears that in the Engineering database I can modify the molecular mass, the specific heat, dynamic viscosity and thermal conductivity, but I don't see how to enter a different density for example, simulating air a 30,000 ft.   Maybe I'm missing something very obvious, or too accustomed to using Flotherm where the density of the working fluid is very easy to change.

       

      Thanks,

       

      Dave

        • Re: How Do I Change the Density of the Project Fluid?
          Timothy Holman

          I am by no means an expert in Flow but isn't air density calcualted using pressure and temperature.  These two things can be defined when you set up the study under initial and ambient conditions in the thermodynamic paremeters.

            • Re: How Do I Change the Density of the Project Fluid?
              David Maxham

              Timothy,

               

              Thanks for your answer.  Since I've been using Flotherm for so many years, I'm used to entering a specific value for density, especially for our high-altitude simulations.  Your suggestion agrees with an alternate method in Flotherm, where you can use the perfect gas law, which, after assigning the pressure and temperature, the density is calculated.  So, if this is how it works with Flow Simulation, where density is derived using the perfect gas law and from the user supplied values for pressure and temperature,  I'll use that method - can anyone else confirm?

               

              I also notice that under Flow Simulation/Results there is a selection for Default Reference Parameters where you can adjust reference pressure and reference velocity and density.  I'm wondering if changing the reference density at this location, the density being used during the solution will also change.  Seems somewhat confusing to put such a critical element, density, under the results - like the cart before the horse?

               

              Anyway, I'll keep searching for the definite answer, and appreciate the help.

               

              Dave

                • Re: How Do I Change the Density of the Project Fluid?
                  Bill McEachern

                  Yes the program uses the equation of state for properties for ideal gases.

                    • Re: How Do I Change the Density of the Project Fluid?
                      David Maxham

                      Bill - thanks for confirming Timothy's remarks.  The next question I had was - what about liquids, like water - which I may have found the answer.  I changed the project fluid to water and then took a look at what parameters can be varied, and density is one of them.  So, thanks to both your and Timothy's help, I'm starting to understand how the fluid properties can be adjusted.

                       

                      So, just to confirm, which values does the program use for the calculating the density - the ones found under Input Data/General Settings/Initial Conditions/Thermodynamic Properties, where the pressure and temperature can also be entered? 

                       

                      Do the values found under Flow Simulation/Results/Default Reference Parameters have any effect on the determination of the density being used for the project fluid?  Or are the values found there just for comparison purposes, say, like ultimate stresses are in FEA?

                       

                      I know I have some more digging to do in the manuals, etc, to better understand how Flow Simulation calculates the derived properties like density, but now I'm off to a good start.

                       

                      Dave