7 Replies Latest reply on Mar 21, 2016 8:14 AM by Amit Katz

    Proper set for time-depended simulation of automotive intake manifold

    Anatoly Pupkin

      Hi, I'm relatively new to Flow Simulation and I have task to run simulation of intake manifold. In case if anybody doesn't know, it's reservoir with 9 (in my particular case it's 9) holes: 1 for receiving air volume flow and 8 for distributing this volume air flow between them when they opens at certain time for equal time and they have to receive equal volume flow (in perfect case mass, but at this point I need volume flow). Everything runs ok with static settings of constant incoming flow and constantly opened all 8 ports. and I can see flow visualization, which is important for manual corrections of some parts of testing model.


      Here's image for better understanding (top is hidden, it's internal simulation):



      And as next step I decided to run proper analysis with realistic loading using timings of opening and closing ports. Frequency is pretty high, at some point ports open and close just for 0.5 ms. I have timing table which I'm importing for each of 8 lids. It's time and environmental pressure which drops to 92700 Pa when lid is "open" and 101325 Pa when it's closed.


      In this sample last port closes at 0.015 s. This is duration of test that I set in solving settings as Physical time. It's 1 engine/manifold cycle.

      As goals I set Volume Flow per each of 8 lids. And Full Pressure (or total pressure, IDK how to translate it accurately).

      So here's pressure chart per time, which looks ok:


      And here's volume flow chart:


      As you can see, 5 port doesn't get as much volume flow as others. I thought it could be timing settings mistake. But I checked and everything is right. And also pressure drop on pressure chart shows that 5 port opens and closes properly. So I think, port 5 may not get enough volume flow due to internal geometry, which is questionable. So this is my first issue.


      My second question is why volume flow keeps passing the lids when they should be closed? I'm new at CFD, and I just found pressures in the internet, so I'd be glad if somebody explained it to me.


      Third issue is that with the same settings but with other timings I'm getting charts like this:


      Looks like ports are almost all the time open, but pressure chart shows that they work fine:


      And, which is more important, I'm not getting 100% solving at this speed. Everything shows 0%. Can it be a reason of that flow charts and how to fix that?


      Sorry for that amount of pictures, I'm new here and I didn't see spoiler button. I hope that somebody here will help me with that. I'm not asking to do the job instead of me, I'm just learning how this flow simulation works. Thanks for your attention.

        • Re: Proper set for time-depended simulation of automotive intake manifold
          Amit Katz

          I have seen a similar question here before about the same application, the problem is that you can't really close off a pressure boundary condition. The way you're doing it is shutting off your vacuum by setting it to environmental pressure, but that won't stop flow from going through the opening. If backpressure is being built up in your manifold then air can freely flow out through those surfaces. Worse yet, you may have additional air coming INTO the manifold from those surfaces when you enable the vacuum on another surface.


          Unfortunately I do not know of a good way to turn off boundary conditions in time-dependant fashion, I don't think there was ever a consensus here last time we took a look at a problem like this.


          Finally, something else to consider is that your simulation is only operating for one cycle. You're not giving the domain enough time to form a stable solution, there are always start-up formations that eventually dissipate with time.

          • Re: Proper set for time-depended simulation of automotive intake manifold
            Amit Katz

            You can move the mesh plot dynamically when you have it selected, try to move it over the hole for port 5 to make sure there isn't any unresolved geometry.


            I hate to sound pessimistic, but I really don't think Flow Sim is the right tool for solving dynamic problems like this. There just isn't a way to simulate opening and closing valves, short of stopping the sim altogether and turn off your boundary conditions. But doing that may destabilize your entire simulation since you are forcing the fluid to abruptly change directions.


            What you can do is to simulate each port individually on a steady-state condition and compare how much air you can move through the manifold. You can compare results from all 8 ports and see where you need to change your dimensions to make the intakes more equal. This isn't an elegant or efficient solution, but I can't think of a way to do it in one go.