# reverse flows

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 February 1, 2010, 14:40 reverse flows #1 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Sponsored Links hello friends, Iam simulating a CI engine manifold. For this manifolds there are one inlet and two outlets.Iam using velocity inlet and presssure outlet boundary conditions. For outlets the pressures are below atmp pressures means suction pressures. (I tried by taking guage and absolute pressures but) When iam running the calculation one of the outlet is becoming the inlet and reverse flows are occureing in the so many faces. Please tell me what is the reson for this problem i tried in so many ways but iam unable to find out the root of the problem.

 February 2, 2010, 03:02 #2 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 Let pressure outlet to 0. The pressure distribution will be calculated from your geometry and your masflow (velocity inlet). Once it has converged, check the pressure at your inlet, it will be your pressure drop (dp = pressure_inlet-pressure-outlet) __________________ In memory of my friend Hervé: CFD engineer & freerider

 February 2, 2010, 07:02 #3 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Thanks for u r replay max , But i didn't get u ,can u please tell me how we control the reverse flows at manifold outlets ( in these outlets one of the outlet is changing in to inlet i think the reason for this is reverse flows ). can u please give me the suggestion to avoid my problem.

 February 2, 2010, 07:41 #4 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 extend your domain by extruding your outlet. Your outlet cannot change to inlet from itself. What is the computed pressure at your inlet? __________________ In memory of my friend Hervé: CFD engineer & freerider

 February 2, 2010, 09:39 #5 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Thanks for u r replay, The inlet pressure is around 2bar and at the outlets the pressures are changing from suction to atmospheric (like 89456 to 101325 pascals). I think due to these suction pressures at outlets the atmospheric air entering in to outlets (reverse flows are occurring at outlet faces) and that outlet is becoming in to inlet. Is this thought is correct. How can avoid these reverse flows on outlet faces.

 February 2, 2010, 09:53 #6 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 Are you working with transient solver? __________________ In memory of my friend Hervé: CFD engineer & freerider

 February 2, 2010, 10:06 #7 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Yes i am working with transient solver

 February 2, 2010, 10:35 #8 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 And I assume you are varying the value of your pressure outlet with time. If your pressure of 2 bar at your inlet is in accordance with what you are expecting, you can try followings: *switch velocity-inlet to pressure inlet *fix the pressure outlet to 0Pa *Create the profile for your inlet, with the right values, ie: 2e5 Pa - the fluctuations you wanted at your outlet *Check the massflow, it should be in accordance with your velocity inlet __________________ In memory of my friend Hervé: CFD engineer & freerider Last edited by -mAx-; February 2, 2010 at 13:52.

 February 3, 2010, 04:04 #9 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Thanks for u r replay, And i think u r suggestions will definitely work for one outlet but , I have two outlets, and there pressures are changing with respect to time and two outlets have different pressure values. Then how would i Create the profile for inlet, with the right values, ie: 2e5 Pa - the fluctuations you wanted at your outlet Last edited by uday; February 3, 2010 at 04:25.

 February 3, 2010, 05:30 #10 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 ah.. your outlets don't have the same value. So it won't work. Did you try to compute a steady state scenario, to see if your backflow occures? __________________ In memory of my friend Hervé: CFD engineer & freerider

 February 3, 2010, 06:24 #11 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Thank u for u r replay, yes i tried steady state at that time there is no reserve flow problem. when am using transient conditions in this some pressure are below atmp(suction) i think thats why these reverse flows are occurring is this is right.

 February 3, 2010, 07:06 #12 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 How is your convergence? Try to plot the massflow rates at each outlet depending with iterations __________________ In memory of my friend Hervé: CFD engineer & freerider

 February 3, 2010, 07:45 #13 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 mass imbalance is 3.45*10^-7 OK i will try to plot the mass flow rates at each outlet depending with iterations but How we will avoid this reverse flow problem by mass flow rates.

 February 3, 2010, 09:25 #14 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 This is not a method for avoiding backflows... but just to be sure that the massflow rates converged between each time step __________________ In memory of my friend Hervé: CFD engineer & freerider

 February 4, 2010, 06:19 #15 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Thank u for u r replay max, I am sure that mass flow rates are converging in every timesteps because my simulation time is small 0.069 and time step size is 0.000236. I am using pressure based solver, For tubulence K-epsilon standard ,material is air, for inlet velocity inlet and for outlets pressure outlets BC are using. back flow specification direction is normal to boundary. For turbulence specification i am using turbulence intensity and hydraulic dia are using. is there any thing wrong in my procedure.

 February 4, 2010, 06:34 #16 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 I assume all is ok. If your mass imbalance is around 1e-7, then what flows in... flows out. And you only have reversed flow at some faces. YOu can extrude your inlets to set the outlets far from the suctions area. __________________ In memory of my friend Hervé: CFD engineer & freerider

 February 4, 2010, 09:08 #17 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Actually i didn't get u, If i extrude inlets to set the outlets far from the suction area .how reverse flows are reduced.means suction pressure are outlets of the geometry. and if i extrude inlets the manifold design will change.

 February 5, 2010, 01:58 #18 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,230 Rep Power: 34 sorry I wanted to say: "extrude your outlets" __________________ In memory of my friend Hervé: CFD engineer & freerider

 February 5, 2010, 11:40 #19 Member   uday Join Date: Sep 2009 Posts: 38 Rep Power: 9 Thanks for u replay, I will try with u r suggestion.

 April 12, 2012, 12:37 I also need help #20 New Member   Vijay Join Date: Mar 2009 Posts: 20 Rep Power: 10 I am also faced with same issue. Did you solve it ?

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