# hemisphere in channel flow

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

 September 12, 2004, 08:21 hemisphere in channel flow #1 kirill Guest   Posts: n/a Hi dear all, I would like to ask for your ideas: My problem: I need to model a hemisphere lying on the bottom of the channel with aim to obtain drag and lift coeficients for the Re-number range from 1e+4 to 2e+5. The flow is fully developed, so as inflow boundary condition i give the log velocity profile. Here it comes: in reality the bottom of the channel is covered with big roughness elements (their size is comparable with the hemisphere size), the hemisphere is lying on the top of them. I give the modified velosity profile as inflow and consider the top of roughness elements as fictive bottom (moving wall boundary condition). Do you think I can get a reasonable estimation of forces acting on the hemisphere and is the bottom boundary condition appropriate? Thanks a lot for any help Kirill

 September 13, 2004, 06:58 Re: hemisphere in channel flow #2 Titiksh Patel Guest   Posts: n/a Hi I think the B.C. at the inlet is alright. But you can change the wall roughness in the Wall B.C. panel. Why are you using moving wall. I cant understand that..?? Is your bottom wall moving? I think u can estimate the forces even if you dont use moving wall. Regards, Titiksh

 September 14, 2004, 17:29 Re: hemisphere in channel flow #3 kirill Guest   Posts: n/a Hallo Titiksh, thanks a lot for Your response. I want to use moving wall because the roughness is to big and according to the log law (I use modified one for such big roughness) on the level of hemisphere bottom the mean velocity component has still a rather big value. In Fluent user manual states: "Note that it is not physically meaningful to have a mesh size such that the wall-adjacent cell is smaller than the roughness height. For best results, make sure that the distance from the wall to the centroid of the wall-adjacent cell is greater than Ks". In my case the hemisphere radius is 4 cm and roughness Ks is also of the same range, the hemisphere is lying directly on the top of roughness elements. It is mean that my mesh have to be at least 4 cm in direction normal to the wall, or am I wrong? I tried to simulate with moving and stationary wall - I get the same drag coefficient but different lift coeffs. What do You think? Kirill

 September 16, 2004, 11:51 Re: hemisphere in channel flow #4 Titiksh Patel Guest   Posts: n/a Hi Kirill Well I understand your point that yr mesh should be at minimum of 4 cm from the wall, because the scale of yr roughness is 4 cm or whatever. But the roughness doesn't remain constant along the wall then how are you calculating effective roughness. What is the velocity of the wall you are provinding? Acc. to me the drag coefficient may change if you change the wall velocity, then how will you make sure that which wall velocity will give accurate results and on what basis..? As in practical there is no movement of wall. Secondaly kindly provide me how are you calculating drag coefficient, which formulae are you using. Regards, Titiksh

 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 Weihua Main CFD Forum 5 January 24, 2012 06:16 dryhill Main CFD Forum 0 June 24, 2009 03:33 svens OpenFOAM Post-Processing 0 June 20, 2009 20:33 al0Vera FLUENT 0 June 15, 2009 10:20 Jung Main CFD Forum 2 November 21, 2007 07:51

All times are GMT -4. The time now is 10:23.