Drag coefficient for cluster of steel work

Using SimScale Fluid Flow / CFD
1

I am a structural engineer mainly working on telecom towers. As usual I use codes to predict the wind loads on structures but recently I am curious to learn how CFD can be effectively used to get the more realistic drag coefficients.

I tried Simscale to compare between drag coefficients from codes so I created a simple circular pipe section(114mm OD x 4.5mm thk.). Most of the code suggest Cd as 1.2 for smooth circular section.

below results shows the variation of drag coefficient with time. Initially for up to150s the variation is too much (25.19 to -6.9). from 150s to 250s the variation is small (-1.17 to -1.06) after250s it is average of -0.93.

My question is how can we determine correct drag coefficient and why there is too much of fluctuation at beginning?

How can we determine a over all drag coefficient for cluster of steel work for example a portion of lattice tower?

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2

Dear @Kamesh, welcome to the forums.

I could not load the pictures. Also, it is important to attach your case link.

You need to understand the nature of the solver you are using ( I suppose you are using a steady-state solver). As @ggiraldof mentioned here: CFD Result Inconsistency, “Please bear in mind that at the first iterations it is normal to have big variations, and usually the convergence is evaluated on the last iterations.”.

So if you are indeed using a steady-state solver, the “s” does not represent physical seconds but # of iterations.

After a large number of iterations, fluctuations likely indicate transient behaviour (check out this post): Convergence issue - #4 by SBlock

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