9 Replies Latest reply on Mar 10, 2015 2:48 PM by Jared Conway

    solidworks flow simulation volume flow rate

    Roche Boedhoe

      Hi,

       

      My goal: calculate the volume flow rate through a gap with a predefined pressure difference. This gap is currently just a hole in the wall, but later on I want to put in elements which will limit the flow, and I want to see the difference in percentage volume flow rate with restrictions or not.

       

      I started with modelling a wind tunnel with a gap of a certain area. I choose an internal analysis, air, and input 2 static pressures (difference of 10 Pa), 101315 at one side and 101325 Pa at the other side. You can see 2 gaps in the middle (I have excluded one in the analysis, I am now just focussing on one gap). I made sure that there is a high local mesh at the gap section. I have checked this with a cut plot.

      model.png

      I calculate the volume flow rate by putting an extruded boss in the gap and select the surface to calculate the volume flow rate, but excluding this boss in the simulation so it doesn't block the flow. After calculation the flow looks OK visually:

       

      model2.png

      model3.png

      As you can see, the air only goes through one gap as the other gap is excluded from the analysis.

      The interesting part is for me the result. It now gives me a volume flow rate of 2066 m3/h. I was pretty confident about this number, till I started playing with the length of the tunnel; the numbers changed!

       

      I don't understand. Our company hired an external specialist who checked my model and input and said it was OK. He even imported my model in his own CFD software with the same input parameters but he got another value. However, his value is consistent. He afterwards calculated it by hand and his hand calculation was pretty close to the solution he got from his software.

       

      So what am I doing wrong? The volume flow rate of a surface area with prescribed pressure difference shouldn't be that difficult, and shouldn't change if I change the length of the wind tunnel.

       

      Can someone help me in the right direction? Thank you.

        • Re: solidworks flow simulation volume flow rate
          Jared Conway

          what type of BC are you adding on both sides?

           

          if it is an environmental pressure your inlet is total pressure and outlet is static pressure

           

          make sure you're comparing total pressure or static pressure difference when comparing to other software, many only use total pressure differences

           

          that being said, changing the length of the tunnel shouldn't affect things except that now you've increased the length of where the pressure drop happens, which should be minimal in this case.

            • Re: solidworks flow simulation volume flow rate
              Roche Boedhoe

              what is BC? I use static pressure for both pressures.

                • Re: solidworks flow simulation volume flow rate
                  Jared Conway

                  BC = Boundary condition.

                   

                  I ran into a case recently like yours where 2 software programs were being compared. In flow it was setup as static > static. It did not match the other software because it worked as total pressure always. This is something I'd recommend checking.

                    • Re: solidworks flow simulation volume flow rate
                      Roche Boedhoe

                      Hi Jared,

                       

                      unfortunatelly I don't have access to his software but when we compared the software we literally sat next to each other with the laptop and compared all input parameters, and he also chose static pressure on both sides.

                       

                      I am not really interested in (comparing) his software, but I want solidworks to give me the same results because I know his result is 100% correct. Is my SW model correct? Is the BC correct or should I use a different pressure? Static seemed logic to me.

                        • Re: solidworks flow simulation volume flow rate
                          Jared Conway

                          If you're concerned that the software is not solving the problem properly, you'll need something with a known solution that you can test. The tutorials and verification problems might be a good first step.

                           

                          "The interesting part is for me the result. It now gives me a volume flow rate of 2066 m3/h. I was pretty confident about this number, till I started playing with the length of the tunnel; the numbers changed!

                           

                          I don't understand. Our company hired an external specialist who checked my model and input and said it was OK. He even imported my model in his own CFD software with the same input parameters but he got another value. However, his value is consistent. He afterwards calculated it by hand and his hand calculation was pretty close to the solution he got from his software."

                           

                          But regarding your comments here. You said that 2066 is a good number in flow simulation.

                           

                          Then you say that the numbers changed with a change. What was the change? How much was the change? Were all parameters affected? How did you validate that the solution was mesh independent?

                           

                          Moving to the other software, how much was the difference between flow and the hand calculation and the other software. Are we talking % or are we talking a huge amount? In this case we also cannot know for sure or confirm that the problems were setup the same. Do we have the hand calculations and can we setup that in flow simulation to compare?

                           

                          In general, when the software does not match expectations you have to check if the software is solving the problem correctly or if you're missing a condition or that it doesn't match. My guess is the issue is the latter. This may be something you want to speak with your reseller about but they will ask for the same information to be able to start an investigation.

                            • Re: solidworks flow simulation volume flow rate
                              Roche Boedhoe

                              Hi Jared,

                               

                              Sorry for the late reply, got caught up in another project.

                               

                              What you are saying is correct. The external flow consultant looked at the BC in SolidWorks and he said it is correct, but... he is not specialized in SolidWorks but in a different software program so he didn't know for sure if he was missing something. I know 100% sure that the value calculated by the external flow consultant is correct. He actually tested different CFD programs by comparing the outcomes and their background calculation model with his own hand-calculation and based on these outcomes he chose the software he thought was best (this is not about my model but he did this for himself he explained to me). He is currently being hired by top international firms and has publised many papers, so that's why I believe his value is 100% correct. Also, his value is coherent with the "general value" in the industry I am working in. I first thought 2066 m3/h was a good value, because we are all "proud" of the product we are selling we were a little biased (2066 is a very good value in our industry!), the difference is around 30%.

                               

                              So I have at least one result as a verification. I now need to find that missing piece, do I need to fix the BC or is SolidWorks not capable of calculating this?