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

Relationship between element height and Reynolds number

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 16, 2013, 10:29
Default Relationship between element height and Reynolds number
  #1
Member
 
Join Date: Apr 2011
Posts: 36
Rep Power: 6
amanbearpig is on a distinguished road
Hello, I have a question regarding the relationship between the height of an element (dy) and the Reynolds number of a flow (Re).

From any of the standard element height estimating scripts, such as here (http://www.cfd-online.com/Tools/yplus.php), or even just estimating dy by solving the y+ equation, I see that my dy is smaller for smaller Re. For flow along a flat plate, my dy would be smaller at an earlier section of a plate than it would be farther along the plate, as the Re increases with the length of the plate.

Does this mean then, that in generating a mesh for a turbulent boundary layer flow over a flat plate, that I am under-meshed if I take the dy from the end of the plate, with the highest Re? I can see perhaps growing the size of the elements along with the growth of the boundary layer, so my dy grows naturally with increasing Re, however when I look at other studies (such as here: http://turbmodels.larc.nasa.gov/flatplate.html ) I see that they use a constant dy height along the wall.

I am looking at LES of flow over a flat plate (so my dy and y+ are quite important), what seems a standard computation. Do I base my dy on the Re at the end of the plate, as I would think? If I do, am I not resolving turbulence at the earlier sections of the plate? Am I misunderstanding the issue completely?
amanbearpig is offline   Reply With Quote

Old   April 16, 2013, 11:25
Default
  #2
agd
Senior Member
 
Join Date: Jul 2009
Posts: 191
Rep Power: 7
agd is on a distinguished road
For a fixed y+ value, the required dy value will decrease as the Re is increased. For something like a flat plate, standard practice is to use the largest value of Re (typically at the end of the plate) and grid based on the dy for that value. This typically leads to grids that have more than enough resolution near the leading edge of the plate.
agd is offline   Reply With Quote

Old   April 16, 2013, 11:52
Default
  #3
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,588
Rep Power: 20
FMDenaro will become famous soon enough
Quote:
Originally Posted by agd View Post
For a fixed y+ value, the required dy value will decrease as the Re is increased. For something like a flat plate, standard practice is to use the largest value of Re (typically at the end of the plate) and grid based on the dy for that value. This typically leads to grids that have more than enough resolution near the leading edge of the plate.
I agree, if you are constrained by a structured grid is better to evaluate your minimum dy based on the most critical situation, which happens at the end of the flat plate.
However, a lot of grid points are used without necessity, therefore would be better to use a multi-block grid ...
FMDenaro is offline   Reply With Quote

Old   April 16, 2013, 12:59
Default
  #4
Member
 
Join Date: Apr 2011
Posts: 36
Rep Power: 6
amanbearpig is on a distinguished road
Quote:
Originally Posted by agd View Post
For a fixed y+ value, the required dy value will decrease as the Re is increased. For something like a flat plate, standard practice is to use the largest value of Re (typically at the end of the plate) and grid based on the dy for that value. This typically leads to grids that have more than enough resolution near the leading edge of the plate.

Forgive me if I'm wrong, but calculating it out, or even just using the dy calculator in the OP, it appears that dy increases with Re, not decreases. Thus, with a higher Re you have a larger element size near the wall. Which seems odd to me.

Actually I wrote it out, and keeping everything else the same (viscosity, density and free-stream velocity), the element height dy grows with length to the 1/14 power. Is this correct?
amanbearpig is offline   Reply With Quote

Old   April 16, 2013, 14:47
Default
  #5
agd
Senior Member
 
Join Date: Jul 2009
Posts: 191
Rep Power: 7
agd is on a distinguished road
Just using the calculator you linked to, simply dropping the viscosity by an order of magnitude results in a larger Re and a smaller dy. I'm not sure what you are calculating, but the requisite wall spacing drops as Re increases.

To FM Denaro - I agree with your assessment, although most people don't want to put much effort into grid generation. Of course, good grids beget good results, while bad grids beget Powerpoint presentations...
agd is offline   Reply With Quote

Old   April 17, 2013, 01:22
Default
  #6
Senior Member
 
Martin Hegedus
Join Date: Feb 2011
Posts: 497
Rep Power: 9
Martin Hegedus is on a distinguished road
dy is a function of both x and Re_x. dy will get smaller as x gets smaller (assuming a constant y+, constant freestream velocity, constant kinematic viscosity, and turbulent flow over the entire surface). Also, as Re_x gets larger at a given x, due to larger velocity or lower kinematic viscosity, dy decreases. It is important to get an appropriate dy over a good portion of the surface so the skin friction is calculated accurately. In general, dy, (for a given y+, velocity, and kinematic viscosity), is mildly sensitive to x.
Martin Hegedus is offline   Reply With Quote

Reply

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
Low Reynolds Number Flow over a Flat Plate Go FLUENT 4 August 28, 2013 05:19
SimpleFoam: High residuals after increase in Reynolds Number JasonG OpenFOAM Running, Solving & CFD 20 January 10, 2013 14:09
Weird velocity vector shape after step height (BFS) warlocklw Main CFD Forum 5 May 24, 2012 05:51
Reynolds for real world problems... Renato. Main CFD Forum 13 August 2, 2006 09:37


All times are GMT -4. The time now is 18:08.