Poor Guy

Simple question really.

A cylinder of diameter .6m is filled with water to a depth of 1.5m. work out the pressure max and thrust on the cylinder wall.

ro G h = 1000 x 9.81 x 1.5 = 14.715kN/m2


Thrust = P x area

so for the bottom is 14715 x (pi x 0.6squared/4)
= 4160.6kN


Now, I think the thurst on the walls is average pressure (14.715/2) x contact area (pi x 0.6 x 1.5)

which is 20.83kN



but my lecturer says it should be on the projected area. am stuck as to why!
help?

j4ckos mate

peasant the answer is simple, pi x 0.6 x 1.5 -14.715/2) x 1.5

FlightMan

There's a ( missing there somewhere!

HTH

Poor Guy

any why, may i ask, is this so?

Poor Guy

got me pressure x me net area of the wall


why the extra 1.5?

j4ckos mate

you dont need it for navier stokes equations

Poor Guy

pi x 0.6 x 1.5 - (14.715/2) x 1.5

so thrust on the cylinder walls = Contact Wall Area - Average pressure x depth


?

Poor Guy

this doesnt work, its gives a negative answer.

Leslie

Did he mean at the top or the bottom of the cylinder?

Les

j4ckos mate

i dont know mate im having you over

windyboy

Another question;

1 bottle of beer holds 330ml of 5% alcohol chilled to 3'c, the mouth of the bottle is 20mm wide and the alcohol is poured out at and angle of 60' from the horizontal.

How many bottles will I need to drink before I fall off the garden wall ??


answers on a post card...

windyboy

Poor Guy

32

j4ckos mate

3 windy boy

jono_pryor11

Is your average pressure correct?

The centre of mass would be at the half way point, but the hydrostatic pressure at a given point is a function of the mass of water above it. In this case the pressure at a lower point in the cyclinder will be considerably greater that that at a shallow depth. With this relationship youd end up with the centre of pressure being 2/3 of the depth down or something?

The hydrostatic force (thrust) occurs from the mass of water above the point pressing down and reacting in all directions. The mass of the water in the cylinder is the volume x density. The volume of the cylinder is its projected area x the height. Using the wetted area is giving you a volume not related to how much water is in the cyclinder.

speedking

Pressure is 0 at the surface and 14.715kN/m² at the bottom. It varies linearly so the average is the value at half the depth. The centre of pressure is at 2/3 the depth.

So the thrust from the sea is much greater than the thrust in a bucket of the same depth ?

Perhaps the question means the force in the cylinder wall? The system must be in equilibrium so the force of water in one direction is balanced by tension in the cylinder wall. It is projected area that you must use for this calculation.

Force from water = .6*1.5*14.715/2 = 6.62kN
There are two cylinder walls so the force in each is 3.31kN.
Say it was 10mm thick then the stress would be 3310/(1500x10)=0.22N/mm².

HTH

ash002004

iTrader: (1)

what are u studying civil engineering? this sounds like the stuff im given to rack my brain on. which uni u at, sheffield or cov?

jono_pryor11

Looks like i need to think harder about things!

Leslie

If you were concerned about the ability of the cylinder wall to withstand the pressure, wouldn't you have to be concerned about the maximum which as stated above is at the bottom of the cylinder. Its the same as dam walls being thicker at the bottom then at the top. That was why I asked the above question of course.

Les

speedking

True. But the original question spoke about thrust, which is a total force, not a stress.

Anyway, most cylinders have a base contributing to strength there, so the weakest point is somewhere above the base

Leslie

Yes you are right of course.

Les

Poor Guy

Cheers guys, this has helped a lot!!!

Im doing hnd mech eng at doncaster college.

ash002004

iTrader: (1)

ah nice one...ill be doing civil engineering next year @ cov or sheffield, alot of the equasions are like that. got one yr left @ college starting in sept doing btec n we also have stuff like that 2 do. my scenario working on at the min.....a drainage system for a rural village, versus a drainage system for a busy town, working out the stress on all the underground grids, the gate pressures and failsafe options if another fails etc. annoys me jus looking at it rather explaining it on here, lol