Drag coefficient issues

Project Support
1

Hi all,

I am trying to quantify the drag coefficient on an immobile obstacle (30cm in length) in a moving flow (0.05 or 0.1 m/s) with three different orientations with respect to the flow (0º, 35º, and 90º). I expected to find higher drag coefficients as I went from a parallel position (0º) to a perpendicular position (90º). However, I am getting the opposite: I am getting lower drag coefficients when the obstacle is perpendicular to the current (90º) than when it’s parallel (0º).

Here is the link to the project:

Oddly enough, I tried the same with the obstacle being 15 cm (half the size) and the trend was reversed. Meaning higher drag coefficients in perpendicular (90º) positions compared to parallel (0º). It doesn’t make sense to me.

Here is the other one:

Regarding the settings of “Force moments and coefficients” on Simscale I calculated the reference length as the length along the x axis (decreasing as the obstacle gets closer to 90º), and the reference area roughly estimated as the frontal area of an ellipse covering the obstacle seen from the x axis (increasing as the obstacle gets closer to 90º).

Could anyone help me quantify the drag in this situation properly or explain how to interpret these counterintuitive results (at least to me :joy:).

Thanks everyone for your time.

Daniel

2

Hello Daniel,

Having a look at the runs, although they are progressing in a rather stable fashion, it does not appear that any of them has reached convergence at this point. Make sure to check this article for more insights.

Cheers

1 Like
3

Thank you very much! I just have a question, why does it appear that any of them have reached convergence? I see the trends in the parapets becoming more stable. Am I reading it wrong?

Would the solution just be running the simulation longer?
Cheers,
Daniel

4

Correct, the simulations seem to be progressing in a stable way, but the coefficients are still changing (e.g. you can see a very clear upwards trend for Cd).

PS: just to clarify: just because the Cd value for ‘run A’ is larger than for ‘run B’, it does not mean that the actual drag force in A is greater than the drag force in ‘B’. See the formulations here.

5

I see that thanks. I take that it would make more sense to calculate drag force instead, right? Is there a way to calculate it on Simscale

6

Yes Forces and Moments | Result Control | SimScale

7

Thank you, again. One final question. Which output is the drag force? Pressure force X?

Maybe also knowing the Cd i can just calculate it myself as Fd = 1/2 p(v^2) CdA?

Thank you for all the help. It’s been very helpful.

Daniel

8

Yes, if you know Cd you can calculate drag via the formula.

Drag is by definition parallel to the flow direction, so you just have to check what is the flow direction in your simulation.

9

Flow in my simulation goes along the X axis so must be pressure force X then!
Thank you!

1 Like