|
[Sponsors] |
May 29, 2014, 12:19 |
Gravity Fed system (Conceptual Question)
|
#1 |
New Member
OR
Join Date: Mar 2014
Posts: 8
Rep Power: 12 |
Hello all,
I'm working on a team to design an Air Tanker plane (plane which drops retardant on fires). We're coming into the project half way through so the plane exist but it isn't performing as expected. Its a gravity fed system with roughly 107" of head, during a drop, the flow rate of the first 1000 gallons of retardant is slow and then speeds up. So in the first 2 seconds the flow rate is slow and then it actually increases, then naturally as we loose head it decreases. I'm thinking its because it takes time for gravity to act on the the water. However, using Bernoulli's equation thsi time for the fluid to accelerate is not represented... I'm wondering if i'm on the right track with my explanation, and if so, how would i mathematically represent the time for the fluid to accelerate? Any thoughts or opinions are appreciated! |
|
May 29, 2014, 14:15 |
|
#2 |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,769
Rep Power: 71 |
well, your flow problem is governed by the equations of the fluid motion, therefore mass, momentum, energy and chemical species are the equations you should solve. In particular, the acceleration of the fluid can be determined from the momentum quantity.
Integral estimation could be done but I don't know enough about your problem ... |
|
May 30, 2014, 10:54 |
|
#3 |
New Member
Harshad Joshi
Join Date: Nov 2010
Location: UK
Posts: 9
Rep Power: 15 |
Wouldn't the initial two seconds be spent in achieving the expected velocity i.e. accelerating?
Also, how accessible is it for air to reach the space vacated by the falling fluid. Possibly the path of air is restricted initially causing the fluid not be released as expected. Remember the old trick where the water does not spill out of the tumbler when it's mouth is covered by a plate, even when held upside down. |
|
May 30, 2014, 12:27 |
|
#4 | |
New Member
OR
Join Date: Mar 2014
Posts: 8
Rep Power: 12 |
Quote:
V = sqrt(2*g*h) where h is height of the fluid in the container. So the tiem for acceleration isn't represented... I'm also suspicious of the venting, however before i tackle that i was hoping to get a working model to explain how the fluid accelerates.. |
||
May 30, 2014, 12:36 |
|
#5 |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,769
Rep Power: 71 |
well, first I am not sure from your problem that you can apply Bernouilli! Perhaps it would be suitable to be applied in the time-dependent form...
then, you have a chemical reaction, isn't it?? |
|
May 30, 2014, 12:41 |
|
#6 |
New Member
OR
Join Date: Mar 2014
Posts: 8
Rep Power: 12 |
the retardant does complicate the situation, however we can't even get it to perform with water.
What i did was basically solve a bunch of steady state problems with a for loop in matlab to predict the flow rate per time. I believe it meets all of the Bernoulli assumptions... lets say for the sake of discussion, that it does meet all assumptions, i just dont see where the time for fluid acceleration is taken into account... |
|
May 30, 2014, 12:48 |
|
#7 |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,769
Rep Power: 71 |
well, you can use the unsteady form of the Bernouilli equation...
http://web.mit.edu/2.016/www/handout...ion_050921.pdf Again, you should ensure that it can be applied.... |
|
May 30, 2014, 18:12 |
|
#8 | |
New Member
OR
Join Date: Mar 2014
Posts: 8
Rep Power: 12 |
Quote:
Thank you for teh Ref. Document. It put me on the right track that's for sure. I finally have the acceleration represented. Now I'm thinking it might be a venting issue, I'd like to use openfoam to simulate the problem. I've made a couple of attempts at it using teh interfoam solver... It keeps on crashing midway through the analysis. One general question I have, do i need to actually mesh volumes of air to replace the water leaving the tank? Or can i just use some kind of boundary condition ? I attached an image of the case I have been trying to run... The red represents water in the tank before the drop chute opens. (keep in mind the tank is very simplified) |
||
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
[blockMesh] Spherical coordinate system | ehsan_vaghefi | OpenFOAM Meshing & Mesh Conversion | 0 | September 3, 2008 22:31 |
help!! Question about activating gravity | mahe | Phoenics | 0 | June 7, 2008 05:03 |
Gravity question | zhou | FLUENT | 0 | February 25, 2004 13:31 |
Simulating Water Splash Using a Particle System | Zi-Wei Chiou | Main CFD Forum | 0 | March 4, 2002 06:33 |
another question about engine cooling system | marivani | Main CFD Forum | 0 | February 2, 2000 11:55 |