CFD Online URL
[Sponsors]
Home > Forums > Main CFD Forum

Oral Qualifying Exam Problems!

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 1, 2003, 04:55
Default Re: Oral Qualifying Exam Problems!
  #21
Prasad
Guest
 
Posts: n/a
What is the basic difference between Finite Volume Method (FVM), Finite Difference Method (FDM) and Finite Element Method(FEM)? Why is FVM & FDM preferred for CFD analysis where as FEM for Structural analysis?
  Reply With Quote

Old   April 2, 2003, 04:55
Default Re: Oral Qualifying Exam Problems!
  #22
Peter J.
Guest
 
Posts: n/a
A bunch of interesting unanswered questions here... I think that it is time that we all start answering them. It seems like Wen Long has given up and needs some help
  Reply With Quote

Old   April 3, 2003, 12:03
Default Re: Oral Qualifying Exam Problems!
  #23
Wen Long
Guest
 
Posts: n/a
First, thanks to everybody. I have just passed the qualify exams last week, and had a trip to Univesity of Virginia, good place to go.

I found the answer of some questions, some others, I don't.

1) Peter's question of blunt body flow:

I'm not very sure, but I guess it's due to transition from to supersonic to subsonic flow near the body head. The transition tends to be unstable, right? I'm studying hydrodynamics, seems samething happened in hydraulic jumps, when the Fr number near 1, the flow is very unstable, and sensitive to noises.

2) John Bradley's question of Chimney spiraling pattern:

Sorry, I don't understand the question.

3) John Bradley's question of tea rotation:

Yes, I got it, as also pointed out by Jan Rusas. Mainly due to oblique pressure gradient, and secondary flow circulation.

4) Jasen's question:

a)laminar separation from a curved surface

This one is relatively easier, but not after the separation. The curvature of the surface gives rise of adverse pressure gradient.

b)turbulent separation from a curved surface

This one is harder than the upper one, first, outer flow is turbulent, second, we need adress turbulence closures in the boundary layer. Log law may not work for curved surface.

c)by-pass transition:

d)natural transition

I don't know what's by-pass transition and natural transition. But I guess, by pass transition means putting some obstacle upsteam to cause transition to turbulence. and natural transition is just the wall and keep increasing velocity. then c) is harder than d). For we have to resolve the by-pass obstacle for details.

e)skin friction:

This depends on whether you want the total friction or detailed friction. For total friction on wall, can use Karman Boundary Layer Integration Method. For detailed distribution skin, friction, it's very hard. I really want to know more about simulation of turbulent boundary layers, especially under unsteady outer flow condition. I will appreciate very much, if you can provide more guideline or information for me.

d)heat transfer:

we have to use more equations, i.e. equation of state , and first thermodynmaic law. so it's not a trival problem at all. For perfect gas, it's well established. For liquid or two-phase flow, I don't know if there is much advance.

e)Pressure distribution:

f)total pressure loss

e) is harder than f). As. total pressure loss can be predicted by integral equations, but distribution requires detailed simuation. for irrotational flow, can use Bernoulli equation, for rotational flow, viscous flow, it's not easy at all, especially for unsteady cases.

5)Prasad's question about FEM, FVM, FDM:

FEM and FVM are more popular is because it can use unstructured grids to fit complex geometry. Also FVM is based on local balance conservation of mass + momentum of cells, it's straightforward.

FEM needs assumption of shape functions, I think it's more suitable for elliptic problems, for unsteady strong convection flow, it will sort of fail, if anyone can point me more about it, I'll appreciate you very much.

FDM: It's simple, but everything based on Taylor expansion, also linear stability analysis is well established. Lots of schemes are proposed, and I'm sort of confused with which one to use. I think there is a disgusting trade off between stability and accuracy. It's very hard to achieve both. Also very hard to deal shock problems such as breaking waves.

I appreciate everybody's problems, and pls explain to me you think I missed your points,

thanks!

wen

  Reply With Quote

Old   April 3, 2003, 12:45
Default Re: Oral Qualifying Exam Problems!
  #24
Praveen
Guest
 
Posts: n/a
1) Peter's question of blunt body flow: The questions refers to space-marching methods. You cannot use them for blunt bodies because of the presence of a subsonic region at the nose. Space marching requires that the equations be hyperbolic.

2) John Bradley's question of Chimney spiraling pattern: The spirals are put in to prevent periodic vortex shedding which can lead to unstable oscillations of the chimney, and final destruction.
  Reply With Quote

Old   April 4, 2003, 09:13
Default Re: Oral Qualifying Exam Problems!
  #25
Peter Attar
Guest
 
Posts: n/a
Very good!!! You pass : )
  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
[ICEM] Problems with coedge curves and surfaces tommymoose ANSYS Meshing & Geometry 0 August 5, 2011 17:02
Needed Benchmark Problems for FSI Mechstud Main CFD Forum 4 July 26, 2011 13:13
Some problems with Star CD Micha CD-adapco 0 August 6, 2003 14:55
PhD Oral Exam Questions FVSuppe Main CFD Forum 5 July 8, 2002 03:31
Using Fluent for Geophysical Problems Hassid Samuel FLUENT 0 February 23, 2001 07:04


All times are GMT -4. The time now is 03:21.