Fluids

Fluids

Supersonic Overexpanded Impinging jet – Convergence issue

    • GuillaumeCRENN
      Subscriber

      Hello,


      I am working on impinging jets and to get confidence with Fluent, I am trying to get similar results from a template report. This report has been written by Xu and al. (2011), « Particle image velocimetry study of the impinging height effect on an overexpanded supersonic impinging free jet at Ma=1.754. Part I: global coherent structure and Part II : detailed velocity distribution ».


      Here are the specifications :

      • Ma (exit)= 1.754
      • Nozzle Pressure Ratio (NPR) = 4.7625
      • T = 300K
    • RK
      Ansys Employee
      Hello, nCan you please give us details of the mesh? Also, What solution methods are you using? I would suggest AUSM with Roe flux difference (Density based approach). Also reduce the courant number. In order to monitor convergence, I would also suggest to monitor a value at a particular location using report definition. You can also refer to our free courses (found on Ansys forum) where you will find simulation examples with best practices for nozzle expansion. n
    • GuillaumeCRENN
      Subscriber
      Hello,nAbout the details of the mesh : 0.71 million elements, 0.4 million nodes, Triangles except for inflationnnnnAspect Ratio : (moyenne = average)nnSkewness :nnOrthogonal Quality :nnHere are the Mesh quality given by Fluent : nnThese are the General options :nnA clarification concerning the boundary conditions, I used a 'Pressure-Inlet' condition for the 'Outlet 1' and a 'Pressure outlet' condition for 'Outlet'. To be noted, I used the identical backflow Turbulent length scale for every boundary condition. (0.006m : diameter of the nozzle exit). This might be something to be improve. Regarding the Thermal component, I used for every boundary condition 300K. nnnAbout the Solution Methods you asked :nnAbout the Solution Controls choices. I can not increase or decrease the Courant Number with this set-up (Density-based + Steady). During my simulation, I decreased them until 0.1.nnI used a hybrid-initilization. I did not use the command /solve/initialize/fmg yes. This could be helping. nnAbout the Free Courses, I found this one :n and this one : nBut you are probably talking about this one ?nnNone of them are dealing with a Supersonic Impinging jet but I will work on it!nnThank you for your help!nnGuillaumen
    • GuillaumeCRENN
      Subscriber
      Hello, nFollowing the turorials, I have made some changes :nI used a Hydraulic diameter for Pressure Inlet (Turbulence Specification Method). I also reduced the turbulent viscosity % for Pressure Inlet and Pressure Outlet. More, I used 101 325 Pa as a Supersonic/Initial Gauge Pressure. nI used /solve/initialize/fmg yesnThe result remains the same, the solution is not convergent...nnWhere can my error come from?nThank you
    • RK
      Ansys Employee
      Hello, nThank you for all the information. Did you use AUSM? Can you also give me some information on the monitors that you have setup as that would give me a better understanding of how the simulation is behaving?n
    • GuillaumeCRENN
      Subscriber
      Hello,nI used AUSM but the result is not better.nFor the monitors, I used an Area-Weighted Average of Mach number at the outlet nozzle which is constant (1.5767) and I made a monitor video to see where the flow can change and the flow change a little bit in the stagnation zone. nI have heard that given my case, the proximity between the exit and the area impinged, L/D =3, with a supersonic flow, the residuals could not go below 10e-2 or 10e-3.Thank you,nGuillaumen
    • RK
      Ansys Employee
      Guillaume, nLooking at residuals alone does not determine the convergence of solution. Monitoring a value and also confirming the flux balance from the reports will also be other factors to take into consideration for convergence. nAnd your observation is right too! n
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