Drag and Lift in 3D flow around a cylinder...

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 September 25, 2005, 22:54 Drag and Lift in 3D flow around a cylinder... #1 Renato N. Elias Guest   Posts: n/a Sponsored Links Hello all, It's my first time running the benchmark problem of the flow around a cylinder in my PhD's program. The program is based on the finite element method using tetrahedra elements (hence three dimentional) and I'm getting confusing about how do I go about computing the drag and lift forces. How could I compute shear stresses (due the drag forces) on the cylinder's wall if this wall is assumed to have no slip boundary conditions (ux=uy=uz=0.0)? The shear stresses would be null but it seems wrong for me... any hint? thanks in advance Renato N. Elias

 September 26, 2005, 00:55 Re: Drag and Lift in 3D flow around a cylinder... #2 Praveen. C Guest   Posts: n/a Note that the gradient of velocity is zero only along the wall but is not zero in any other direction.

 September 26, 2005, 04:36 Re: Drag and Lift in 3D flow around a cylinder... #3 jean-marie Guest   Posts: n/a The drag is due to the normal gradient of the tangential velocities at the wall (wall shear stress), but mostly to the lower pressure on the downstream face in the detached flow region (integration of the pressure on the closed cylinder surface). This is very dependent on the angular location of the separation (this point oscillates with vortex shedding).

 September 26, 2005, 13:01 Re: Drag and Lift in 3D flow around a cylinder... #4 Mani Guest   Posts: n/a I don't want to appear too critical, but if you pursue a PhD in fluid dynamics, this is really high time to review some of the basics. I would expect my undergraduate students to know this, not to mention a PhD student.

 September 26, 2005, 13:36 Re: Drag and Lift in 3D flow around a cylinder... #5 Renato N. Elias Guest   Posts: n/a "Thanks a lot for your help"... I think, I should ask your under graduate students and they could be more helpful for me than you. I was thinking that this forum should be used to discuss topics related with CFD... If my question don't worth your atention please call one of the your students to help me. Renato. ps.: Where have I told you that I had a PhD in Fluid Dynamics? I'm a PhD student in HIGH PERFORMANCE COMPUTING, so I'm still learning about CFD, parallel computing, solvers, preconditioners, etc...

 September 26, 2005, 13:41 Re: Drag and Lift in 3D flow around a cylinder... #6 Mani Guest   Posts: n/a Nevertheless, take my friendly advice and read a textbook to get an introduction to fluid dynamics, if you will be dealing with fluid dynamic problems. It will not hurt you.

 September 26, 2005, 13:50 Re: Drag and Lift in 3D flow around a cylinder... #7 Renato N. Elias Guest   Posts: n/a What do I have doing here? I was just reading about the subjetc (or anybody that could take me a short cut) when you have posted your "helpful" message. We are not struggling to know who knows more about anything. We are trying to help ourselves. I think so. regards Renato N. Elias

 September 26, 2005, 16:23 Re: Drag and Lift in 3D flow around a cylinder... #8 Mani Guest   Posts: n/a Dear Renato, have you not been helped by Praveen's and Jean-Marie's posts? If not, I cannot add anything except to suggest that you consult any fluid dynamics textbook for more detailed information on the subject, and that's what I suggested. No need to take offense as none was intended.

 September 26, 2005, 17:00 Re: Drag and Lift in 3D flow around a cylinder... #9 Renato N. Elias Guest   Posts: n/a Dear Mani, you should think more before writing what you have written. Yes, I've taken your answer as an offense because I think that there isn't any basic question that doesnīt worth attention or a polite answer. BSc, MSc, PhD or any other "God of the knowledge" has your basic doubts about many things. We're always learning anything... and itīs impossible to know everything about a so huge topic as CFD is. Or do you know every basic detail about, compressible/incompressible flows, laminar/turbulent flows, Newtonian/non-Newtonian, Finite/diference/volume finites, etc..., etc..., etc... !? Even so, I must be very grateful to Praveen's and Jean-Maries's answers. They've respected my basic doubt. I know what equations are employed to compute the drag and lift forces, there are a bunch of books and papers with these equations. I was only in doubt about how to compute shear stresses in a surface with no-slip boundary condition, just it. Best regards Renato.

 September 26, 2005, 20:07 Re: Drag and Lift in 3D flow around a cylinder... #10 zxaar Guest   Posts: n/a well said

 September 26, 2005, 20:22 Re: Drag and Lift in 3D flow around a cylinder... #11 zxaar Guest   Posts: n/a if you have access to peric's book, in its chapter complex geomteries it is nicely explained. if you do not have access to book please let me know.

 September 26, 2005, 22:09 Re: Drag and Lift in 3D flow around a cylinder... #13 Renato N. Elias Guest   Posts: n/a Thanks a lot for your hint zxaar! I have this book and I've just found what I was looking for all day long. Best regards Renato.

 September 26, 2005, 22:29 Re: Drag and Lift in 3D flow around a cylinder... #14 zxaar Guest   Posts: n/a i have a phd thesis i wish to send it to you, please give me your email id where i can post it. Its very much relevant to your field of interest, just have a look.

 September 26, 2005, 22:40 Re: Drag and Lift in 3D flow around a cylinder... #15 Renato N. Elias Guest   Posts: n/a Thanks again for your interest in helping me with this question being it basic or not. You may send me messages through rnelias@gmail.com. Sincerely grateful Renato.

 September 27, 2005, 06:05 Re: Drag and Lift in 3D flow around a cylinder... #16 Renato N. Elias Guest   Posts: n/a For whom that could be interested in this topic I've found 2 nice papers discussing the subject: L. Baranyi, "Computation of Unteady Momentum and Heat Transfer from a Fixed Circular Cylinder in Laminar Flow", J. Comput. and Appl. Mech., 4(1):13-25, 2003 L. Baranyi, "Lift and Drag Evaluation in Translating and Rotating Non-Inertial Systems", J. Fluids and Struct. 20:25-34, 2005 The reference suggested by zxaar is very good too. Regards Renato.

 October 4, 2005, 11:32 Re: Drag and Lift in 3D flow around a cylinder... #17 Renato Guest   Posts: n/a I've got so disappointed and astonished by the answer that I've got for my question that I've been asking myself what happened... I think I was not so well understood as I was expecting because some of the answered arguments were about that my question was not related with CFD but with the basics of fluid dynamics. After researched a bit about the subject I've found several articles talking about the computation of drag and lift forces in unstructured discretized schemes (I think it's really related with CFD!!!) and I was feeling obligated to explain my question better. let me try to explain it with an example: imagine a very thin plate immersed in a fluid flow. If this plate is parallel to the fluid flow we will have drag force only due shear forces. Suppose that you are using triangles in a finite element scheme to discretize and solve your problem, thus you will have triangles with 2 or 1 nodes touching the plate and these nodes will have null velocities due the no-slip boundary condition. You will not get non null velocity derivatives if you compute the derivative at the baricentre of the element, thus the derivatives will be "averaged" with the node out of the boundary (MY QUESTION WAS ABOUT IT AND NOT ABOUT WHERE THE DRAG AND LIFT FORCES COME FROM). Of course this problem becomes much more complicated when you're dealing with complex geometries in 3D computations employing tetrahedra. In fact, I've found two ways of computing drag and lift forces: one by approximating the forces with the equations easily found in CFD textbooks and another one using the discretization scheme itself (I'm still reading about this...) where the former is less accurate. For those interested by this topic I suggest the following articles where this subject is nicely explained: Tabata, M. and Itakura, K., A precise computation of drag coefficients of a sphere, The International Journal of Computational Fluid Dynamics, Vol. 9, pp. 303-311, 1998 http://www.math.kyushu-u.ac.jp/~taba...der/WR0068.pdf and Tabata, M. and Tagami, D., Error estimates for finite element approximations of drag and lift in nonstationary Navier-Stokes flows, Japan Journal of Industrial and Applied Mathematics, Vol. 17, pp. 371-389,2000. http://www.math.kyushu-u.ac.jp/~taba...der/WR0091.pdf "...I think that some answers shouldn't be posted here because they discourage the participation of those people that are looking for any kind of CFD related help..." Furthermore, if you think that the question you're reading is not well posed, please ask for the poster to explain it again and better before posting useless messages. remember that you are in an open and international forum where not everybody is fluent in english. Regards Renato

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