Ridiculously low drag coefficient from OpenFOAM CFD

In summary, the individual is using SimFlow 3.1 with OpenFOAM 5 on Linux to evaluate the drag coefficient of a water rocket body traveling at 77 m/s. They have created a wedge mesh and set it up for incompressible flow with RANS k-ω SST turbulence, but are getting a drag coefficient of 0.00058, which is significantly lower than expected. They are seeking advice on potential adjustments to their setup and questioning the accuracy of the results. Suggestions include checking boundary conditions, using a finer mesh, researching expected drag coefficients for their specific case, and validating results with experimental data or expert opinions.
  • #1

Anachronist

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TL;DR Summary
Need to know how to adjust drag coefficient from OpenFOAM calculation with wedge mesh.
(New thread following up to a conversation in another thread).

I'm using SimFlow 3.1 with OpenFOAM 5 on Linux to try to evaluate the drag coefficient of a water rocket body, constructed from a 2-liter Coke bottle with a conical nose, traveling through air at 77 m/s.

This should be a simple axisymmetric problem. So I created a wedge mesh, set it up for incompressible flow with RANS k-ω SST turbulence, force monitoring in the x direction. Here's a picture of my model with my wedge mesh:

Screenshot from 2019-09-05 21-27-09.png


I have a fair idea of what the drag coefficient should be: somewhere between 0.1 and 0.2. But OpenFoam reports orders of magnitude lower: 0.00058.

Why is this?

I'm a newbie when it comes to CFD. When I was taking college physics 35 years ago, I don't believe CFD even existed then; at least it wasn't taught. I chose SimFlow as an OpenFOAM front-end mostly to have a GUI and to shield me from drowning in the OpenFOAM documentation, which isn't written for newbies and assumes a high level of expertise already. SimFlow also has good tutorials on their website.

As far as I can tell from the OpenFOAM documentation, the wedge angle is 5 degrees by default, or 1/72 of a circle. Is there some adjustment I need to do, to account for this wedge size and the reference area? I'm using the circular area projected into the airflow as Aref and the overall length as Iref. Should I be using the projected area just inside the wedge, or the whole wetted area from nose to tail inside the wedge, and then multiply the result by 72 or something?

Another thing I don't understand is the absence of flow separation at the bottle taper. I'm pretty certain that a 2-liter soda bottle traveling at 77 m/s through air will have flow separation there. This is no airfoil! I've seen separation on small airfoils in actual wind tunnels at lower velocities than that. The separation at the nozzle collar and nozzle opening is expected, but I can't bring myself to believe that there's laminar flow over the bottle shoulder. Here's what I get:

Screenshot from 2019-09-05 21-56-26.png


This has been a long slogging learning curve so far. Not just for SimFlow, but for the FreeCAD software I had to learn to create my model. I'm happy that I got ANY result at all... but I am not believing what OpenFOAM is telling me. The drag coefficient is unrealistically low, and the flow lines look too smooth.

Does anyone have any advice on how to proceed?
 
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  • #2


Hello,

Thank you for sharing your progress with us. It seems like you have put a lot of effort into your simulation and it is great that you are questioning the results and seeking advice.

Firstly, I would suggest checking your boundary conditions and making sure they are appropriate for your problem. For example, are you using the correct velocity and pressure values for the inlet and outlet boundaries? Are the walls of your wedge mesh properly defined as no-slip walls?

Secondly, it may be helpful to use a finer mesh to capture the flow around the bottle more accurately. This could potentially improve the results and show a more realistic flow separation.

In terms of the drag coefficient, it is important to note that it is affected by many factors such as the shape and size of the object, the velocity of the flow, and the properties of the fluid. It is possible that the drag coefficient for your specific setup is lower than the expected range of 0.1-0.2. I would suggest doing some research or consulting with experts in the field to get a better understanding of what to expect for your particular case.

Lastly, it is always a good idea to validate your simulation results with experimental data or other reliable sources. This can help you verify the accuracy of your simulation and make any necessary adjustments.

I hope this helps and good luck with your simulation!
 

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