Register to reply 
Impulse on a water bag being dropped 
Share this thread: 
#1
Aug2313, 04:07 PM

P: 36

I have been trying to derive an equation to represent how a bag full of fluid would react when dropped dependent on the fluids viscosity. Basically i have the general impulse equation: F∂t=∂P but the time is dependent on the fluids viscosity (higher viscosity the longer it takes for the impact to finish) so I tried using the equation of viscosity: η=F/A*t but in this case of the equation I have a changing area that is dependent on the time as well as the dimensions of the bag. I don't have the slightest idea how to model this varying area since it is an nonuniform shape. Anyone have any ideas how to model this?



#2
Aug2313, 04:17 PM

Sci Advisor
HW Helper
Thanks
PF Gold
P: 5,256

This is a difficult fluid mechanics problem, and would probably require solution of the partial differential fluid mechanics equations (Navier Stokes equations) or turbulent flow approximations, even if the bag were assumed to be spherical and the deformation were assumed to be axisymmetric. You would also probably have to model the deformational mechanics of the bag, and its interaction with the fluid. I'm guessing that modeling this problem would require use of finite element fluid mechanics software coupled with finite element solid mechanics software. The moving boundaries would add even more complexity to the analysis. 


#3
Aug2613, 11:32 AM

P: 36

I was afraid of something like that although I already knew that was probably the answer anyway. I will probably just make a lot of assumptions for something like this to save time during the calculations to get an estimate that makes sense and just work from there.



#4
Aug2613, 12:58 PM

Sci Advisor
Thanks
P: 1,923

Impulse on a water bag being dropped
There may be a model already available for this type of problem. I see two situations that might have similarities. Firstly the impulse sound of rain drops falling on a hard surface, and secondly, flood modelling.
In flood modelling a sphere of water equal to a reservoir's volume is released at one point on the landscape to model the catastrophic failure of the containment structure. For the real world, finite element analysis is used for flood modelling, but the time scale needs to be checked against a simple theoretical model. That model would include viscosity. You might google { dam burst flood modelling } and trace references back to verification of the model. 


#5
Aug2613, 05:17 PM

Sci Advisor
HW Helper
Thanks
PF Gold
P: 5,256

Chet 


Register to reply 
Related Discussions  
Dropped some Water in my Laptop. What to do?  Computers  6  
Calculating force on object being lowered/dropped into water  General Physics  3  
A 50kg object was dropped from a height of 2 meters from water level  Introductory Physics Homework  1  
Distance traveled of a ball dropped in water.  Introductory Physics Homework  4  
Pressure gain from water dropped from a height  General Physics  4 