3D simulation in Fluent

 Register Blogs Members List Search Today's Posts Mark Forums Read

 May 6, 2007, 16:51 3D simulation in Fluent #1 Amer Guest   Posts: n/a Hi, I did a 3D simulation for a marine propeller to study cavitation. I used a single phase, steady, and turbulent flow (J=0.2 and a cavitation number of 2). I considered the flow to be rotating and the propeller to be fixed. Could you please answer the following: 1- My solution is converging with only 200 iterations to 0.0001. (I heard that since it's 3D then it should take at least 2000 iterations for the solution to converge). what could be the problem (if there is any)? 2- The value of Y-star that I got is 10^4 and a professor told me that it shouldn't be more than 500. So what should I do (given I've tried the simulation with normalizing and then without normalizing in my solution and it gave me almost the same value for Y-star). how can I decrease the Y-star value)? Please help me in both or any of the above questions. Thank you

 May 6, 2007, 17:47 Re: 3D simulation in Fluent #2 Phil Guest   Posts: n/a It's the number of cells that determines the number of iterations - and 3d meshes generally have alot more cells than 2d. How many cells do you have. Without combustion a 3d mesh with under 100000 cells could converge in 200 iterations.

 May 7, 2007, 06:20 Re: 3D simulation in Fluent #3 Amer Guest   Posts: n/a thx alot! the number of cells I have is 450000 cells. does it make sense to converge in 200 iterations! and do u anything about the Y-star value?

 May 7, 2007, 07:32 Re: 3D simulation in Fluent #4 Phil Guest   Posts: n/a 450000 is alot to converge in that number of iterations. Are you sure Fluent read the full 450000 cells? Sorry, don't know anything about y-star. YOU SHOULD CHECK THE CONVERGED RESULTS AND SEE IF THEY LOOK SENSIBLE.

 May 7, 2007, 16:43 Re: 3D simulation in Fluent #5 Amer Guest   Posts: n/a thanks alot! I've been told that maybe it's because I'm checking the box scale (I'm activating scale) near normalize in my solution. So, do u think it's a good practice to do that or my results will not be accurate? should I use normalize instead or is it better not to use any of them?

 May 7, 2007, 17:55 Re: 3D simulation in Fluent #6 Phil Guest   Posts: n/a If you mean in the residual monitors window, the default in the normalization box is to have scale checked and normalize unchecked. I wouldn't have expected it to affect convergence that way. One thing may be that you have the convergence criterion too low. it should usually be 0.001 (0.000001 for energy). Make sure all boundary conditions and models are correct and if necessary re-read the mesh into a fresh run of fluent(with defaults) and try again to run the simulation.

May 16, 2012, 16:13
#7
New Member

david
Join Date: May 2012
Posts: 7
Rep Power: 6
Quote:
 Originally Posted by Amer ;141347 Hi, I did a 3D simulation for a marine propeller to study cavitation. I used a single phase, steady, and turbulent flow (J=0.2 and a cavitation number of 2). I considered the flow to be rotating and the propeller to be fixed. Could you please answer the following: 1- My solution is converging with only 200 iterations to 0.0001. (I heard that since it's 3D then it should take at least 2000 iterations for the solution to converge). what could be the problem (if there is any)? 2- The value of Y-star that I got is 10^4 and a professor told me that it shouldn't be more than 500. So what should I do (given I've tried the simulation with normalizing and then without normalizing in my solution and it gave me almost the same value for Y-star). how can I decrease the Y-star value)? Please help me in both or any of the above questions. Thank you
hi
perhaps your professor told you y-plus must be below 500 that you should decreaseing the first layer of wall in the computational domain(e.g. first layer on the hydrofoil, ...)

 May 17, 2012, 07:22 #8 New Member   Yogesh Patil Join Date: Nov 2009 Posts: 20 Rep Power: 8 Your professor is right.Ystar should be less than 500 and more than 15 if you are using standard wall function. Fluent uses Ystar instead of Yplus values for their own reasons. Refine the mesh near wall to get Y plus around 50-60 this should fix your problem. Please read fluent help on Near wall treatment.

May 18, 2012, 07:13
#9
New Member

david
Join Date: May 2012
Posts: 7
Rep Power: 6
Quote:
 Originally Posted by foamcfd Your professor is right.Ystar should be less than 500 and more than 15 if you are using standard wall function. Fluent uses Ystar instead of Yplus values for their own reasons. Refine the mesh near wall to get Y plus around 50-60 this should fix your problem. Please read fluent help on Near wall treatment.
hi
i think Yplus is right no Ystar. of coure you know in about propeller this parameter isnot more important and pressure coeffienet and thrust and torque coeffient are important that more discussed in literature. wall function isn't more important than mesh near the wall if you see appropriated articles and books.

all the best
david

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Steve FLUENT 9 April 20, 2012 06:42 santhosh1987 FLUENT 0 May 14, 2010 10:32 yash Main CFD Forum 1 April 21, 2009 05:05 Ted Chu FLUENT 10 November 26, 2008 22:06 Lam FLUENT 3 June 10, 2003 12:00

All times are GMT -4. The time now is 16:39.