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Savithri July 9, 1998 17:09

Viscous Damping
 
Most books that deal with vibration, shock etc. deal with the viscous damping term broadly. Is there a source that tells you how to go about calculating the coefficient of viscous damping. I have come across numerous articles that plot various parameters as a function of eta (viscous damping/critical viscous damping, i.e. C/Cc). For example, if a plate is hinged at two points and is vibrating in a package that has still air, is it possible to calculate the contibtion of damping due to the air?

Any suggestions would be greatly appreciated.

Regards. Savithri

Rick July 10, 1998 10:36

Re: Viscous Damping
 
Yes, you can compute the damping force on the plate from the air. It will however require a CFD engineer, an expensive code, an expensive computer and time... so unless this is a very critical application you are probably better of using empirical correlations that have been developed for the specific application. There is not general formula to compute aerodynamic forces on vibrating objects. Fortunately, otherwise many of us here would be out of a job ;-)

Savithri July 10, 1998 10:51

Re: Viscous Damping
 
Thanks Rick for the response. Actually I do have three of the 4 things you mentioned. CFD engineer, computer and expensive CFD code :), but TIME. The project is at its infancy and I was hoping to do a rough estimate using empirical correlations to get an approximate figure. Eventually I do hope to do the CFD analysis (which in my estimate would take me at least a couple of weeks to set up the moving boundary problem etc. and get it to converge etc.). However in relation to all the other numerous programs which I support, I don't have two weeks to spare. So I was looking for a quick "engineering guestimate".

Regards. Savithri

Rick July 10, 1998 11:08

Re: Viscous Damping
 
Ah, great, I guess you mean business then :). How does this thing look? Dimensions? Vibration frequencies? Geometry? ... I think it is difficult for anyone to say anything without details aboutthe specific application.

Savithri July 10, 1998 12:57

Re: Viscous Damping
 
Haven't you heard this before :) "Proprietary information"? Anyways, here are some details to get some feedback. I am not in a position to give out fine details.

The object is diamond "shaped" in the horizontal plane, except that the lines don't meet at a point, but rather it is like a squished octagon. Two of the opposite sides (+/-Y) sides are hinged and the object is about 1mm thick, 5mm by 3.5mm (x an y). The first four modes are about 105, 115, 1610,2550 hz. resp. The max. deflection is 5.75 and 7 degrees in the x and Y resp.

Is that enough to go by? All I am looking for is some reference where they deal with viscous damping calculations for any geometry. I can derive the necessary terms for my case.

Regards. Savithri




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