CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

1st or 2nd order gradients approach?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 13, 2014, 09:26
Default 1st or 2nd order gradients approach?
  #1
New Member
 
huang yu
Join Date: Jul 2012
Posts: 2
Rep Power: 0
fireflies is on a distinguished road
hi every one
i have written a 3d unstructured(hybrid) NS solver using a cell-centered finite volume method. three methods of green, unweighted-LSQ and weighted LSQ have implemented to calculate the gradients.after a series of tests on a field which i know analytic solutions of gradients, i found

1)Compare the three methods result with analytic one on regular grid, the relatively error were about 1e-13(machine zero is 1e-14 on my system); and on iregualr grid, the relatively error were about 1e-3 to 1e-5. dose this mean my program works right?

2) on regular grid, all the three methods cannot achieve 2nd order accuracy, the green and unweighted-LSQ achieved 1st order and weighted LSQ was more than 1st; and on iregualr grid the green and unweighted-LSQ cannot achieved 1st order and weighted LSQ was 1st order. i am confused because some people said the three method are 1st order and some said they are 2nd order method, and i want to know dose the gradient accuracy less than 2nd order would have any affect on accuracy of spatial discretization, if i use a linear reconstruction of Barth
fireflies is offline   Reply With Quote

Old   September 17, 2014, 15:30
Default
  #2
Senior Member
 
duri
Join Date: May 2010
Posts: 130
Rep Power: 7
duri is on a distinguished road
There is nothing called first order gradient and second order gradient. Gradient is always first order term. Accuracy of gradient calculation would affect the solution.
duri is offline   Reply With Quote

Old   September 18, 2014, 08:07
Default
  #3
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 531
Blog Entries: 14
Rep Power: 17
sbaffini will become famous soon enough
The gradient accuracy order for an overall second order FV cell-centered code (linear reconstruction) just need to be first. However, it is important that it is first order on any grid. Still, for fully cartesian, uniform, meshes i expect some methods to return a second order accuracy.

How are you handling your boundary conditions? Is it possible that they are not treated consistently for the analytic solution?
sbaffini is offline   Reply With Quote

Reply

Tags
gradient accuracy

Thread Tools
Display Modes

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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Limiters for 2nd order solvers Heinz Wilkening Main CFD Forum 1 March 14, 2013 04:40
Solution scheme: 1st, 2nd order, etc in momentum eqs nottern Main CFD Forum 1 May 11, 2012 00:30
2nd order boundary conditions for 2nd order discretization? quarkz Main CFD Forum 30 December 26, 2011 08:12
1st, 2nd order or other Ivan FLUENT 3 January 28, 2008 10:45
Multicomponent fluid Andrea CFX 2 October 11, 2004 05:12


All times are GMT -4. The time now is 10:46.