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General Science => General Science => Topic started by: santanu on 04/10/2012 10:28:59

Title: How does resistance affect water flowing vertically through a pipe?
Post by: santanu on 04/10/2012 10:28:59

Hi,

Assume we have a water tank with an outlet at the bottom with fixed hole size. The water flows at certain rate say 5l/min.
If we attach a downward pipe to the hole, one logic is that the the resistance will increase and hence the flow will decrease, the other is that the pressure increases in the lower point and since the flow at the bottom of the pipe will be same as that at the tank outlet, the flow will increase.
What would actually happen?

Santanu

Title: Re: How does resistance affect water flowing vertically through a pipe?
Post by: damocles on 06/10/2012 02:11:39

I think that the latter part of the argument for a faster or equal flow is an unsound one. Why? Because if the higher hydrostatic pressure that would apply at the bottom of the pipe is somehow going to control the flow all the way up the pipe, why can't we use the same argument to say that the lower hydrostatic pressure at the top of the pipe is going to hinder the flow all the way down the pipe?

I am fairly sure that the water will flow slower through the pipe because of the viscous drag all the way along the pipe. This is quite a large force -- it has drastic effects if you try to connect garden hoses together for a really long run in a small channel. In my younger days I had a 0.3 ha property, and tried to run a standard half inch hose for about 60 m. It was not long before the first length (closest to the tap) began to bulge and bubble from a back pressure way above the hose specification.

Title: Re: How does resistance affect water flowing vertically through a pipe?
Post by: santanu on 16/10/2012 14:56:46

I still feel that the flow will increase. think of a pipe connected to a water tank vertically downward to three floors. The lowermost floor seems to have more flowrate and fills up a bucket faster.

Santanu

Title: Re: How does resistance affect water flowing vertically through a pipe?
Post by: bizerl on 17/10/2012 01:06:19

I think that if you measured the speed of the flow a metre below the hole, it would be the same whether the tank had a 1 metre pipe attached or not. What may change is that the entire volume of water at that point would be concentrated into a smaller area with a pipe, and therefore more water would be passing through a smaller area. This would increase the pressure but not necessarily the speed.

The test would be to set up multiple tanks with varying lengths of pipe coming out of them, then see if there is a difference in how fast they empty.

Another factor would be that if it was a sealed tank, the air may be able to get in more easily if it doesn't have to "glug" or funnel up a huge pipe, therefore no pipe may improve the consistency of the flow.