# help!problems in simulationg von karman vortex street.Really simple.Thank you.

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 July 17, 2013, 07:41 help!problems in simulationg von karman vortex street.Really simple.Thank you. #1 New Member   Join Date: Jul 2013 Posts: 25 Rep Power: 6 Sponsored Links hello guys, i'm studying CFX and i'm a beginner. sorry to bother you if you think my questions are stupid, but please give me a hand. I'm trying to simulate von karman vortex street, the results are great, the animation is perfect. but when i try to plot the Drag Coefficient Cd Vs Time, i got two questions. 1. what's the exact definition of Cd in this case, i mean, since the vortex is generating and "going away" repeatedly, it makes sense that the value is always fluctuating(see the table exported below). so how do they give an exact number in those literature? 2. the distance between my inlet boundry and the "leading edge" of my cylinder is 550mm, and the initial velocity at the inlet is 0.031m/s, Re=200, it should have taken the air flow 0.55/0.031=17.7s to arrive at the leading edge, so why does the table below show that at Time=0.4s, the air flow seems to take effect on the cylinder?(Total time 100s, timestep=0.1s, one hour for computation.) Thank you so much. Cd Vs Time: Time [ s ],Cd 0, 0 0.1, 51.7202 0.2, -21.3084 0.3, 0.436604 0.4, 0.989016 0.5, 0.988177 0.6, 0.937793 0.7, 0.895956 0.8, 0.856551 0.9, 0.832134 1, 0.819006 1.1, 0.814777 1.2, 0.8173 1.3, 0.823203 1.4, 0.82998 1.5, 0.836536 1.6, 0.843096 1.7, 0.849809 1.8, 0.85688 1.9, 0.864708 2, 0.873446 2.1, 0.882944 2.2, 0.89275 2.3, 0.902558 2.4, 0.9123 2.5, 0.92184 2.6, 0.931046 2.7, 0.939953 2.8, 0.948684 2.9, 0.957309 3, 0.965738 3.1, 0.973848 3.2, 0.981768 3.3, 0.989439 3.4, 0.992386 3.5, 1.00113 3.6, 1.00469 3.7, 1.01338 3.8, 1.0165 3.9, 1.01991 4, 1.02399 4.1, 1.02838 4.2, 1.03265 4.3, 1.03659 4.4, 1.04009 4.5, 1.04317 4.6, 1.04591 4.7, 1.0484 4.8, 1.05069 4.9, 1.05279 5, 1.05469 5.1, 1.0564 5.2, 1.05791 5.3, 1.05925 5.4, 1.0604 5.5, 1.06139 5.6, 1.06221 5.7, 1.06287 5.8, 1.0634 5.9, 1.06379 6, 1.06407 6.1, 1.06426 6.2, 1.06435 6.3, 1.06434 6.4, 1.06424 6.5, 1.06404 6.6, 1.06378 6.7, 1.06346 6.8, 1.06307 6.9, 1.06264 7, 1.06216 7.1, 1.06162 7.2, 1.06102 7.3, 1.06038 7.4, 1.0597 7.5, 1.05898 7.6, 1.05822 7.7, 1.05743 7.79999, 1.05662 7.89999, 1.0558 7.99999, 1.05496 8.09999, 1.05409 8.2, 1.0532 8.3, 1.05228 8.4, 1.05133 8.5, 1.05036 8.6, 1.04938 8.7, 1.04837 8.8, 1.04735 8.9, 1.04633 9, 1.04529 9.1, 1.04424 9.2, 1.04318 9.3, 1.0421 9.4, 1.04101 9.5, 1.0399 9.6, 1.03878 9.7, 1.03765 9.8, 1.03652 9.9, 1.03539 10, 1.03426 10.1, 1.03312 10.2, 1.03199 10.3, 1.03085 10.4, 1.02972 10.5, 1.02858 10.6, 1.02744 10.7, 1.0263 10.8, 1.02516 10.9, 1.02404 11, 1.02292 11.1, 1.0218 11.2, 1.02069 11.3, 1.01958 11.4, 1.01848 11.5, 1.01738 11.6, 1.01628 11.7, 1.01519 11.8, 1.01411 11.9, 1.01303 12, 1.01194 12.1, 1.01086 12.2, 1.00979 12.3, 1.00873 12.4, 1.00768 12.5, 1.00664 12.6, 1.00561 12.7, 1.00458 12.8, 1.00356 12.9, 1.00255 13, 1.00154 13.1, 1.00055 13.2, 0.999558 13.3, 0.99858 13.4, 0.997606 13.5, 0.996638 13.6, 0.995675 13.7, 0.994717 13.8, 0.993765 13.9, 0.992818 14, 0.991878 14.1, 0.990944 14.2, 0.990019 14.3, 0.989099 14.4, 0.988183 14.5, 0.987279 14.6, 0.986379 14.7, 0.98549 14.8, 0.984608 14.9, 0.983735 15, 0.982867 15.1, 0.982003 15.2, 0.981146 15.3, 0.980297 15.4, 0.979456 15.5, 0.978621 15.6, 0.977793 15.7, 0.976974 15.8, 0.976161 15.9, 0.975353 16, 0.974553 16.1, 0.973759 16.2, 0.97297 16.3, 0.972187 16.4, 0.971408 16.5, 0.970781 16.6, 0.969975 16.7, 0.96917 16.8, 0.968344 16.9, 0.967579 17, 0.966809 17.1, 0.966102 17.2, 0.96536 17.3, 0.964652 17.4, 0.963914 17.5, 0.963196 17.6, 0.962468 17.7, 0.96175 17.8, 0.961034 17.9, 0.960321 18, 0.95961 .............. ..............

 July 17, 2013, 18:52 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,737 Rep Power: 106 Yes, absolutely the Cd value fluctuates. That's what the vortex street is all about. So the Cd value reported in the literature is usually the time average value. I presume this fluid was modelled as incompressible. This means that pressure is transmitted instantly across the entire flow domain. So when the inlet boundary starts flowing at the rate you specify the whole fluid domain starts moving instantly with it. KevinW likes this.

July 17, 2013, 21:08
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 Yes, absolutely the Cd value fluctuates. That's what the vortex street is all about. So the Cd value reported in the literature is usually the time average value. I presume this fluid was modelled as incompressible. This means that pressure is transmitted instantly across the entire flow domain. So when the inlet boundary starts flowing at the rate you specify the whole fluid domain starts moving instantly with it.
Dear Glenn Horrocks.
Yep, it's incompressible flow, you are correct.
So many thanks for your reply, i see what you mean, so happy that probelms solved.
The first thread in CFD-online, i like this place.Thank you again!

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