Parameter of drag coefficient of flow over cylinder

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 March 9, 2017, 15:22 Parameter of drag coefficient of flow over cylinder #1 New Member   Frontull Matthias Join Date: Mar 2017 Location: Italy Posts: 6 Rep Power: 8 Hello all, I am currently working on an assignment for university. I have to perform a parameter study of the drag coefficient versus Reynolds number for a flow around a cylinder. Basically i have to recreate the following diagram as best as possible http://www.esru.strath.ac.uk/EandE/W...efficientC.jpg Problem specs: diameter = 1 density fluid = 1 VISCOSTY = 10^-5 (seems to me to be the problem) Reynolds numberrange : [0.1 to 10^6]; I have a circular flow domain with radius 64meter (according to tutorial of cornell university, https://confluence.cornell.edu/displ...ast+a+Cylinder) MESH For the mesh i tried a face mesching and edge sizing on the cylinder wall and a global face sizing getting almost 100000 cells. MODEL and BOUNDARY I tried to simulate the creeping flow region (Re = 0.1 Re = 1, u_inlet = 0.0001), using the laminar model, and calculate the drag coefficient. MY PROBLEM: Instead of values around 40 as in the experiment I get basically a drag coefficient equal to zero! And I dont really undertand why? the viscosity is very low, could that be the problem, that i get a ill conditioned system of equations or smt similar? I would be glad for any hind or suggestion, thx in advance!!

 March 9, 2017, 15:38 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,290 Rep Power: 67 how do you compute the stresses around the body?

 March 9, 2017, 15:54 #3 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 736 Rep Power: 14 I worked a similar (or may be even the same) geometry without any problems. But I finished with Re al low as 2. I made everything to 1 except the viscosity, which I use for controling the Re number: U=1m/s, d=1m. I tried a different setup with the same Re, and that gives virtual the same results. For very low Re numbers the computed region around the cylinder should be rather large: The stream lines get widened even far away from the cylinder. I set it to 10m = 20 radii at each side. For Re=2 I got a drag coefficient at around 10. ~ > how do you compute the stresses around the body? There are two ways: 1) Simply use the libforces.so library which needs to be included in the controlDict 2) More complicated: Use "wallShearStress -latestTime" for calculation of the stress, use the calculated pressure for the resistance part of the drag. The results for the drag coefficient are identical, if you extrapolate the pressure to the patch centers. If you use the element centered value , they differ slightly. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

 March 9, 2017, 16:39 #4 New Member   Frontull Matthias Join Date: Mar 2017 Location: Italy Posts: 6 Rep Power: 8 to FMDenaro: I dont compute the stresses, i directly compute the drag force in Fluent with Report/Forces/ and than selecting the cylinderwall and the x direction! Or did you smt else to piu58: My flow region has a diameter of 64 meter, so 32 times larger, should be enough? Thats somehow the same for me, if i try to compute the coefficient by controlling the viscosity i get mor reasonable values for Re = 1, but on the other side if i increase Re to 20 50 or 80 the coefficient rises instead of falling as expected from the diagrams! Which somehow would even make sense, that if all the rest remains equal and i increse the viscosity also the friction and drag rises or am i wrong?

 March 9, 2017, 18:51 #5 New Member   Frontull Matthias Join Date: Mar 2017 Location: Italy Posts: 6 Rep Power: 8 i just tried to simulate another time controlling the reynolds number by the velocity. rho = 1, viscosity = 1, velocity = [0.1 1 10 100] I get the results for cd = [22.66 70.024 156.65 8962.2], so actually completely the opposite behaviour as expected from the experimental curve! And i mistold, by flow domain diameter is 32meter, not 64meter!

 March 10, 2017, 01:21 #6 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 736 Rep Power: 14 > if i increase Re to 20 50 or 80 the coefficient rises instead of falling May be you calculate the force coefficient instead of the drag coefficient. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

 March 10, 2017, 02:57 #7 New Member   Frontull Matthias Join Date: Mar 2017 Location: Italy Posts: 6 Rep Power: 8 May be you calculate the force coefficient instead of the drag coefficient.[/QUOTE] But the drag coefficient is calculated from the dragforce?

 March 10, 2017, 04:51 #8 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 736 Rep Power: 14 > But the drag coefficient is calculated from the dragforce? Yes, drag coefficient divided by dynamic pressure. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

 March 19, 2017, 07:13 #9 New Member   Frontull Matthias Join Date: Mar 2017 Location: Italy Posts: 6 Rep Power: 8 Still could no find a solution, for small viscosities I don't get any dragforce! Could it be that due to the low viscosity I have to define very very small elements on the cylinder surface? or exactly the opposite? If i computed forces from stresses, and stresses from larger CV-surfaces I should get larger drag value for each CV but just with lower resolution?

 March 19, 2017, 07:19 #10 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,290 Rep Power: 67 what is relevant in the size of the cell is the Re number, not the viscosity value alone

 March 19, 2017, 09:40 #11 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 736 Rep Power: 14 > for small viscosities I don't get any dragforce! In such cases take a look at the flow in the near of the cylinder cyrface. Is that, what the program calculates, physical plausible? In some cases I did not get s smoth flow but an oscillationg one. Drag forces form such a solution cannot be correct, of course. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

 March 21, 2017, 22:18 #12 New Member     Patcharawat Join Date: Jan 2014 Posts: 12 Rep Power: 11 If you are using Fluent, Cd can be obtained directly from the software. You should compare those values with yours that whether it is same. Sent from my iPhone using CFD Online Forum mobile app

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