# Fluid Mechanics HELPPPPP!!!!!

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Hi,
I have a deadLine untill Thursday to find out the solutions to the following Questions.

1) A cylinder with Relative Density=0.5 and height=10m is holding vertical with the bottom base touching the surface of water.
If we leave the cylinder to move vertical towards water, Find the maximum depth in which the upper base of cylinder will reach.

2) Flow of Water in an experimental tube with diametre 100mm has pressure fall 10.000 Pascal with n(a coefficient)=0.01. Find the length of the tube when Turbulent appears with Re=1000000

3) Water is flowing in a Venturi Meter of 400mm X 100mm with discharge 72000lit/h and pipe difference 250mm.
If the 'narrowing' is 200mm X 50 mm,
find the coefficient z of the meter.

4) An Open reservoir fill with water has at it's side walls and in 2meters depth from the surface, Orifice(hole) with diametre=25mm.
a) Find the Velocity of Water at the Exit of the Orifice
b) At the Orifice we Adjust a Pipe with diametr=25mm and length=9meters. Find the velocity at the exit of the Pipe.
c) How much % has the Velocity increase or Decrease??
Generally, take the water flow normal.

5) A cube with edge of 10 cm and Relative Density p=1.2 is equalising in 2 non-mixable fluids of relative density a=0.6 and b=1.6.
Find the parts of the Cube's Edge that are Immersed for each case.

This looks like a test. I will be happy to critique your thinking.

First of All, I ask for help cause I lost all my notes and books in a Fire!
Plus, I must find the answers till Thursday.

For All I know [Let's say for quetion number 1],

I guess it can be solved by using the following equation :

Weight of Cylinder = Weight of Moved Water =>
9.81(gravity) x 0.5 (Relative Density) x 1000 = 9810 x(10m X h) [h is what we're looking for].

so solving this :

h=0.05m

But again, this number must be WRONG. If you asked me, I'd say that the Cylinder would go down to water vertically for about 50 meters. I think the 0.05 m is too small as a number, OR i've made a mistake somewhere.

Generally, I try as HELL to remember What method to use for my problems. All my friends are in Vacation and The books are too Expensive for me to Buy right now.

1) A cylinder with Relative Density=0.5 and height=10m is holding vertical with the bottom base touching the surface of water.
If we leave the cylinder to move vertical towards water, Find the maximum depth in which the upper base of cylinder will reach.
I guess it can be solved by using the following equation :

Weight of Cylinder = Weight of Moved Water =>
9.81(gravity) x 0.5 (Relative Density) x 1000 = 9810 x(10m X h) [h is what we're looking for].

so solving this :

h=0.05m

But again, this number must be WRONG. If you asked me, I'd say that the Cylinder would go down to water vertically for about 50 meters. I think the 0.05 m is too small as a number, OR i've made a mistake somewhere

****Response: Archemedes principle...weight of cylinder = weight of displaced water correct so far
Weight of cylinder= density*volume= .5*densitywater* g*PI*radius^2*10

equate these two, solve for h. Height ought to be 5 meters. Think on that, the cylinder is 1/2 as dense as water.

Ok, I'll check it out.

As for the Rest questions now,

For subject 3)The only thing I remember from the Venturi meter problems I've deal with, is that it has 2 coefidience, a 'c' and a 'K'!!. I think the K is for the flow but I don't remember what 'c' is for!.

I have no memory of how to solve this problem!. Maybe 'c' is 'z'. If that is a fact, then it could be put in the following Type :

K = c / [Root 1-(D2/D1)^4] or
K = c / [Root 1-(A2/A1)^2]

So I guess :

c = K * [Root 1-(D2/D1)^4] or
c = K * [Root 1-(A2-A1)^2]

but what A1,A2 stand for??
And, Where will I put the 'pipe difference wich is given 250mm' ???

Can you help with this (then we will proceed to the next question)

Thanx

3) Water is flowing in a Venturi Meter of 400mm X 100mm with discharge 72000lit/h and pipe difference 250mm.
If the 'narrowing' is 200mm X 50 mm,
find the coefficient z of the meter.

I assume pipe difference is the difference between diameters inlet/outlet. I assume again your coefficent z is the Coefficent of Discharge, if it is not that, I have no idea.

See http://flow.netfirms.com/venturiflow/theory.htm

well,

The 'pipe difference' is not the best translation I can make (excuse my English).
What I mean is 'Hydraulic grade pipes/tubes'.

The question Asks for the coeficient of the meter. Since we have the discharge (72000 lit/h) , I don't know if a discharge coeficient is what we need? (I don't remember how this can be solved, but if you can show me how to find the discharge coeficient, i will think it over)

Thank you for your help so far. You were right for the first question (stupid of me! I first put (pi)x2^2 cause I thought we have 2 cylinder bases. Thanks again for Letting me out of the Dark!

If you Can help with the Rest of the Questions it would be perfect.

Well, I suppose we could skip the Venturi meter question,
Tommorow I have to deliver the project so If you can help me with the other subjects please, do. There's no time left. I am All day in the internet downloading Manuals and Guides but I am lost.

If I could have at least 3 subjects solved, my year's study won't go waisted.

A disk of radius R rotates at angular velocity inside another hollow disk filled with oil of viscosity, as shown. Assuming a linear velocity profile and neglecting shear on the outer disk edges and corner effects, derive an expression for the viscous torque on the disk.

how many cups in a fluid ounce

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