# Rate of flow under gravity

1. Jan 23, 2007

### dug_rose

Hi !

I need to find the rate of flow of water coming out of the bottom of a can, under gravity to put in the equation:

r. of f. = speed x area

How do I calculate the speed of which the water drops, taking into account height etc.

Many thanks,

Doug

2. Jan 23, 2007

### hover

For that part you would need to use calculus i think. Give me the height of where the water drop starts to fall and i'll reply back.

3. Jan 23, 2007

### dug_rose

Hi,

To give you a bit of background we need to drain a can of beer from the bottom of a can into a glass , 25 cm below. What factors effect the speed in which the liquid will drop so I can find the speed and use it to find the flow rate? The can will be on the edge of a table, so 125 cm high say. We are using a 440ml can if that makes any difference, through a machined mild steel pipe. I assuming 9.81 will be a mulitplier somewhere in there?

Thanks for any help you can give me, Doug

4. Jan 23, 2007

### hover

The factors that will affect the speed the drop falls is the acceleration of gravity (9.8m/s) and air resistance.

5. Jan 23, 2007

### dug_rose

Thanks a lot for your help hover

6. Jan 23, 2007

### hover

It will take about half a second for a drop to reach the ground. Using distance/time you will get the average speed of the fall. So 125/.5= 250cm/sec if i am correct. Although i got a answer i don't know if it is totally correct.

hope this helps

7. Jan 23, 2007

### Gokul43201

Staff Emeritus
dug: the speed with which the beer starts to flow out the opening in the bottom is proportional to the square root of the level of beer in the can. So, it flows out fastest when the can is full, and the flow rate falls to zero, as the can empties.

A reasonable approximation to the mean outflow speed is given by an application of Bernoulli:

$$v_{out} = \sqrt{2gh}$$
g : acceleration due to gravity = 9.8m/s2
h : beer level in can, in meters (some number from 0 to about 0.15m, for a typical can)

It's just one step further to calculate the time it takes to drain the can, for a given hole size.

8. Aug 8, 2008

### John Hee

Hi, how are you?

Actually i have something to ask also.

I have a adhesive tank 1250kg ( dimension: 1400mm x 1100mm x 900mm) connect to a 11/2"

stainless steel pipe flow to another tank. So, may i know how to calculate the gravity flow

rate at maximum & minimum fluid level and also what is the pipe angle?

Thank you