[KevinW]

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
..............
..............

[ghorrocks]

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]

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!