# wiggling in the Cd value

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April 26, 2013, 04:25
wiggling in the Cd value
#1
Member

Kai Zhang
Join Date: Jan 2013
Location: yokohama, Japan
Posts: 83
Rep Power: 4
Dear all,
I am simulating a 2D kepsilon case of flow around cylinder, Re=100.
A rough look at the drag coefficient plot looks good. But when I zoom in, it seems there are two parallel lines forming the plot, and the value of Cd is wiggling in a small range.
any idea why?
Attached Images
 overview.jpg (69.5 KB, 10 views) zoomin.jpg (78.8 KB, 10 views)

 June 16, 2013, 12:02 #2 Super Moderator   Bruno Santos Join Date: Mar 2009 Location: Lisbon, Portugal Posts: 8,301 Blog Entries: 34 Rep Power: 84 Hi Kai, Since you mentioned this thread on your other thread: wiggles in the Cd and Cl - here's what I can figure out: The mesh is very likely to have the exact size, plus the "deltaT" ahve the right value, for this phenomenon to occur. This has to do with the effect of discretization... for which I can never find a good example on the web But imagine the following scenario: you have a plot of a sine with a period of 1 second and amplitude of 1. If your sampling rate is exactly 0.5 s. What do you see for each period? 3 points with 0 value. You change the sampling rate to 0.25s. What do you see? 5 values, where 3 are zeros, a "+1" and a "-1". This is basically what you're seeing in this plot. This is why I mentioned in the other thread about the "deltaT". If it were small enough, you would see the complete wobbling plot. And maybe even the flow would change considerably, because it had more iterations for resolving the cumulative effect of the kinetic energy blobs moving around the mesh Best regards, Bruno __________________ OpenFOAM: Frequently Asked Questions | Useful links for building and using Forum: How to ask for help | Posting code and output with [CODE] My to-do list and when I'll be able to come to the forum: http://wyldckat.github.io And please: Read this before sending private messages to me

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