7 Replies Latest reply on Aug 21, 2013 1:06 AM by Jared Conway

    Inlet Velocity icm Outlet Velocity as boundery conditions doesn't work. Why?

    Luc Spruyt

      Hello there,

       

      Again I have a small issue with Flow Simulation.

      It's probably due to the fact that I'm a little unexperienced.

       

      Here's the question.

       

      The job is to engineer a ducting system with the use of Fabric (e.g. FabricAir ) in order to fill a room with air going appr. 0.05 m/s.

      The first try out is as shown on the picture.

      The blue vertical tubes go outside the room.

      The red ones represent the fabric channels that blow air inside.

      We intend to create horizontal flowing air from the inlets to the outlets.

      There is no temperature delta so convection is negligible.

       

      We can asume that each clilinder produces air at a constant velocity of 0.05 m/s.

      Therefore the boundery condition is " inlet velocity: 0.05 m/s" for all red cilinder surfaces.

       

      When I put a "Static Pressure" boundery condition in the blue cilinders, I get realistic results.

      But in real life the blue cilinders will have a pressure a little less then atmospheric because they are connected to the inlet channel of Fans.

      They will suck air in.

      So I thought, lets change the "Static Pressure" into "Outlet Velocity 0.05 m/s".

      Because that the only parameter we have. The fabric is designed for this speed.

      That combination didn't work.

       

      So the question is:

       

      Why can't we have a Outlet Velocity in combination with Inlet Velocity for a study.

       

       

      When I change it back to Static Pressure of lets say 1 bar (atm = 1.03 ) so 3000 Pa under pressure then there is no flow.

       

      What am I doing wrong.

       

      ps sorry for the poor englisch writing here and there.

       

      Thanks again.

      Luc Spruyt

        • Re: Inlet Velocity icm Outlet Velocity as boundery conditions doesn't work. Why?
          Jared Conway

          your combinations are volume flow/mass/velocity in and a pressure out OR 2 pressures for a valid flow simulation setup.

           

          based on the way that you've described the problem, why not place outlet fans on the outlet side and pressure inlets on the other side?

            • Re: Inlet Velocity icm Outlet Velocity as boundery conditions doesn't work. Why?
              Luc Spruyt

              Hello Jared,

               

              Thanks for the quick respons.

              That makes this a valuable platform.

               

              OK, so if I understand you correctly you're saying that the inlet and outlet can not be defined by velocities both.

              Wich is quit resonable when you think of it..

               

              So I should define a pressure for the outlet.

              That would be the ideal situation.

              I can edit the inlet velocity and outlet pressure until I get a nice horizonal flow of appr. 0.05 m/s.

              With these two values we can enter the next engineerings phase.

              But why isn't there any flow when I work with a static pressure value of 1 bar.

              Resume. In what way can I define a suction line???

               

              The reason why I don't use fans in combination with a pressure value is because there is fabric in between.

              To be alble to simulate this I would have to have more spec's of the fabric.

              FabricAir can advise my customer when he desires a certain inblowing velocity over a cilindrical surface as wel as the outgoing spec's.

               

              The image shows the velocity in the chamber with (default)Static Pressure on the outlet.

               

              Have a nice day.

              Thanks again.

              Vertical Tubes 2.JPG


                • Re: Inlet Velocity icm Outlet Velocity as boundery conditions doesn't work. Why?
                  Jared Conway

                  a suction only exists from a pressure difference

                   

                  your velocity inlet doesn't know what pressure that it is at. that is an output. so the pressure difference regardless of what you enter will be the same.

                   

                  so you can put a pressure difference in a pipe for example, your output is velocity, flow rate, mass flow rate

                   

                  or you can put a volume flow rate and a pressure opening, your output is the pressure at the inlet

                    • Re: Inlet Velocity icm Outlet Velocity as boundery conditions doesn't work. Why?
                      Luc Spruyt

                      I'm sorry but I don't get it.

                       

                      A suction line olny exists from a pressure difference. OK clear.

                       

                      Why does my velocity inlet doesn't know it's pressure.

                      It calculates with the values under the Thermodynamic Parameters or doesn't it.

                      And why does it solve the analysis without problems using a velocity inlet in combination with a Static Pressure (default 1.01325 bar) outlet.

                      The inlet velocity had the unchanged default pressure under Thermodynamic Parameters.

                       

                      Thanks for the patience..

                        • Re: Inlet Velocity icm Outlet Velocity as boundery conditions doesn't work. Why?
                          Jared Conway

                          hi luc, here's what i would suggest

                           

                          setup a very simple model that shows the basic concept of what you are trying to do. a box with an inlet and an outlet.

                           

                          setup the 2 cases.

                           

                          identify what you expect the results to be and post them up here for everyone to take a look at.

                           

                          something you might want to check is what the thermodynamic properties are used for and their defition in the help for a volume flow rate or velocity flow rate. these are just place holders. at the end of your simulation, they will likely be different. this can be seen by using a pressure surface parameter or goal.

                            • Re: Inlet Velocity icm Outlet Velocity as boundery conditions doesn't work. Why?
                              Luc Spruyt

                              Hi Jared,

                               

                              I'm sorry for the late answer.

                              It's not because I'm not interrested, on the contrary.

                               

                              OK, so the Thermoodynamic Properties are just place holders as a starting point.

                              Off course the simulation causes there parameters to change and stabilize.

                              I think I overestimated these Thermodynamic Prop. values. Is it correct for me to think that changing these values only matter when you're running a time-dependent study.?? Because when the solver finishes on a fully balanced situation, then changing these Thermodynamic Prop's dont't mind really do they?. This only applies for the pressure value in the Thermod.Prop. Changing the temperature will have more effect.

                               

                              I recently discovered why the solver did run the Velocity In - Static Pressure out (default) correctly and wouldn't solve when changing these Thermd.Prop. It seemed that my cilinders where touching the roof of the computational domain. After I stretched the top of the cilinders a bit further, the problem was gone. From that moment I was able to solve the simulation also when the Thermod.Prop.'s were changed.

                              Because still it should be possible to change the Thermod. Prop. and run a study. (now knowing that it wouldn't affect my results.)

                               

                              Anyway, I did learn and that's nice.

                              Thanks Jared for your comments.

                               

                              I'm now running a study using poreus material.

                              Interesting..

                              Is it correct for me to conclude that when a perforated plate is in between an channel (so between the in and outlet), I should define this as a poreus material (unidirectional)?? Perforated plates can only be on Fan / enviromental boundery conditions.

                               

                              Again learning..

                               

                              Have a nice day.

                              Regards Luc