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javorpanev
Posts: 6
Registered: ‎10-18-2012

Gravitational accelaration

238 Views, 1 Replies
02-17-2013 10:17 AM

I am modelling the drop of a wineglass from a height of 70cm using MES. Analysis went ok but I noticed that the glass fell faster than I expected. When plotting the 'second derivative' of a node displacement it gave a constant value of 18.2m/s^2 while it was supposed to be just around 9.81m/s2. The only load acting on the system is gravity set under standart conditions equaling to 9.81  as suggested by the solver.

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Valued Mentor
AstroJohnPE
Posts: 494
Registered: ‎08-30-2012

Re: Gravitational accelaration

02-17-2013 03:10 PM in reply to: javorpanev

Hi javorpanev,

 

I sure hope that MES can handle gravity correctly! I cannot think of any combination that would result in gravity being 1.85 times too strong. So there must be something else wrong with the input. Please check the following:

 

  • Gravity constant correct for the units.
  • X,Y,Z gravity multipliers form a unit vector. In other words, if  you are dropping the glass at an angle relative to one of the global directions, you would not enter a Y multiplier of 1.56 and a Z multiplier of -1 because that would result in a total gravity of 1.85.
  • The load curve multiplier must be 1 at time 0 and at the end (or beyond) of the analysis.
  • The graph has a multiplier that can be entered. It's on the same dialog as the 2nd derivative. Make sure it is 1.00
  • If the glass is not being dropped in a global direction (either X, Y, or Z) but dropped in some combination of the three directions, use the graph to compute the second derivative of the displacement in each of the directions. Then calculate the resultant for those three results.

That's about all that I can think off.

 

P.S. I predict the glass will break. :smileywink:

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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