March 28, 2021 at 1:14 amshehab GamrahSubscriber
I have a dynamic mesh simulation with a vibrating inner sphere moved by a udf. I can get the drag coefficient at each timestep, however the problem is the pressure and cd are calculated based on the velocity, and the drag is calculated based on the velocity in the reference value which is constant however my sphere's velocity is a function of time, so how can I get accurate drag results based on the variable velocity and not the one in reference valuesMarch 28, 2021 at 4:17 amYasserSelimaSubscriberYou can create a report definition for the drag force , and a report definition for the velocity ... then write them in the same file. Now you can calculate the drag coefficientnMarch 28, 2021 at 3:44 pmshehab GamrahSubscriberThanks, I have reported the drag force and calculating the cd by using the velocity of the inner sphere from the function. However, I keep getting unreasonable values of drag force and coefficient. For instance at time 0.001 (1st time step) I had a drag force of 7164 N, which means the cd is 519, however this is an unreasonable cd value for a half sphere. The properties of the fluid are density 1000.567 and viscosity 0.0009. Is there a reason for these drastically high valuesnMarch 28, 2021 at 4:06 pmYasserSelimaSubscriberHello,nFirst, I remember you were using 2D model. If you are still using it, all your calculations are for cylinders, not spheres ... nSecond, depending in your turbulence model, the values of the forces at the wall are very sensitive to the mesh size. Your first cell height should be equal to Y+ in case of k-omega and you can increase this by an order of magnitude with k-epsilon. In your case I recommend k-omega as the flow domain is small with no outle and you you will have a transition flow. nThird, As I can see from your above photo that the reference length is 1 [m] ... So this force is for 1 meter long cylinder if you are still using 2D model.nMarch 28, 2021 at 4:19 pmshehab GamrahSubscriberNah it is a 3d model, and yes I used a y+ value with a kepsilon model and the results converged, however I forgot to change the reference length to length of inner spherenMarch 28, 2021 at 6:28 pmYasserSelimaSubscriberIf this is 3D model, the reference area is the important one and I can see you set this right.nBut now you are actually getting very high force. ... how large is the gab between the two spheres? nAlso if your cell height is Y+ , try to use smaller time step ... 0.001 is large ... try making this 0.0001 .. Also the first few timesteps would not be accurate ... if you plot the force you will see it oscillates around a value at the beginning until it diverges to a certain value. nMarch 28, 2021 at 8:42 pmMarch 28, 2021 at 9:59 pmYasserSelimaSubscriberTry smaller time stepnViewing 7 reply threads
Ansys Innovation Space
- You must be logged in to reply to this topic.
Earth Rescue – An Ansys Online Series
The climate crisis is here. But so is the human ingenuity to fight it. Earth Rescue reveals what visionary companies are doing today to engineer radical new ideas in the fight against climate change. Click here to watch the first episode.
Subscribe to the Ansys Blog to get great new content about the power of simulation delivered right to your email on a weekly basis. With content from Ansys experts, partners and customers you will learn about product development advances, thought leadership and trends and tips to better use Ansys tools. Sign up here.Trending discussions
- Suppress Fluent to open with GUI while performing in journal file
- Floating point exception in Fluent
- What are the differences between CFX and Fluent?
- Heat transfer coefficient
- Getting graph and tabular data from result in workbench mechanical
- The solver failed with a non-zero exit code of : 2
- Difference between K-epsilon and K-omega Turbulence Model
- Time Step Size and Courant Number
- Mesh Interfaces in ANSYS FLUENT
- error in cfd post
© 2023 Copyright ANSYS, Inc. All rights reserved.Ansys does not support the usage of unauthorized Ansys software. Please visit www.ansys.com to obtain an official distribution.