Answers - hydro.pdf

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TOPICS FOR THE HYDRAULICS EXAM
1. Give the explanations of the fundamental physical properties of water
Density – is a ratio of mass of a given quantity of a substance to the volume occupied
by that quantity.
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In engineering we often use weight density (specific weight)
Viscosity – represents the susceptibility of a given fluid to shear deformation, and is
defined by the ratio of the applied shear stress to rate of shear strain. It is the friction
that exists within the fluid as it flows.
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Surface tension – phenomenon caused by strong bonds created between molecules at
the surface of the liquid
Capillarity – in the vertical tube water level increases due to adhesive forces between
liquid and glass surface.
Elasticity of water – ability of water to change volume under pressure.
Compressibility of water is inversely proportional to volume modulus of elasticity
2. Give the specification of fluid forces, and give their examples
Fluid forces:
Body forces – they act on all particles in the body of water as a result of some
external body or effect (for example gravitational force, inertial force, forces
resulting from elastic effects).
Surface forces- act on the surface of the water body by direct contact. Can be
external (pressure or friction forces) or internal (viscous forces )
Line forces – act on a liquid surface normal to a line drawn in the surface (for
example surface tension)
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3. What is the meaning of absolute and gauge pressures
Absolute pressure - is the pressure of a system relative to the pressure of an absolute
vacuum
Gauge pressure – is equal to absolute pressure minus atmospheric pressure.
4. Show procedure of calculation of hydrostatic forces acting on flat surfaces
The total hydrostatic force on any submerged surface plane surface is equal to the
product of the surface area and the pressure acting at the centroid of the plane surface.
Pressure forces are distributed all over the surface and act in the direction normal to
the surface.
5. Give the procedure of calculation of hydrostatic forces on curved surfaces
The horizontal component of the total hydrostatic pressure on any surface is
always equal to the total pressure acting on the vertical projection of the
surface. The resultant force of the horizontal component can be located
through the center of pressure of this projection.
The vertical component of the total hydrostatic pressure force on any surface is
always equal to the weight of the entire water column above the surface
extending vertically to the free surface. The resultant force of the vertical
component can be located through the centroid of this column.
6. Give the water pressure definition, and procedure of its calculation
Pressure – surface force expressed as the ratio of force to the area over which that
force is distributed. Water pressure is calculated according to the following formula
7. On the simple example show Archimede's law.
Archimede's law – the weight of s submerged body is reduced by an amount equal to
the weight of the liquid displaced by the body
8. What is the meaning of Newton's law in water movement phenomenon.
In order to apply Newton’s law for water movement we need to introduce term:
control volume of water – imaginary region within a body of flowing fluid. We are
interested only in forces acting on external surface of control volume, due to the fact
that all internal forces cancel each other.
Newton’s law states that momentum entering the system is equal to energy leaving the
system, so
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9. Give the classification of fluid movement for pressured and open flow, give
explanation of every kind of movement.
Flow
Timescale
- Steady- parameters describing flow do not vary with time
- Unsteady- parameters describing flow vary with time
Scale of distance
- Uniform – magnitude of parameters describing flow do not vary with
distance
- Non- uniform- magnitude of parameters describing flow vary along the
flow path
Combinations: steady-uniform, unsteady non-uniform and so on)
10. What is the meaning of Bernoulli’s equation
Bernoulli’s equation describes energy of unit weight of water entering and leaving the
system. If there are no energy loses caused by friction or, for example, sudden contraction, the
energy entering the system is equal to energy leaving the system. There are three heads:
pressure, elevation and velocity head.
11. Give the properties of laminar and turbulent flow
Laminar flow - occurs when a fluid flows in parallel layers, with no disruption
between the layers
Turbulent flow - flow regime that is characterized by eddies or small packets of fluid
particles which result in lateral mixing
In laminar flow value of velocity is the biggest in the center of the pipe and decreases
closer to the pipe wall due to roughness of pipe. In turbulent flow distribution of water
velocity in pipe cross-section is almost uniform.
12. Give minimum two reasons of fundamental meanings of Reynolds’ experiment
Reynolds found the transition between laminar to turbulent flow depends not only on
water velocity but also on pipe diameter and fluid viscosity
Onset of turbulence is related to particular index number
13. Try to explain the meaning – loss of head from pipe friction, and what is the procedure
of its calculation
Energy of unit weight of water travelling through the system decreases along the flow
path. The head loss is caused by pipe wall friction and the viscous dissipation in
flowing water.
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14. What was the meaning of Nikuradse’s experiment
Nikuradse was examining the influence of pipe roughness for friction factor . He
discovered that:
For laminar flow
equation can be derived by balancing pressure forces and
friction forces
For turbulent flow – there are two zones, in first depens on Re and relative
roughness of the wall(then can be calculate from Colebrook equation), in the second
it depends solely on relative roughness
15. Give the explanation of friction factor for laminar and turbulent flow
For laminar flow
equation can be derived by balancing pressure forces and
friction forces. Friction factor is independent of pipe walls roughness.
For turbulent flow – there are two zones, in first depens on Re and relative
roughness of the wall(then can be calculate from Colebrook equation), in the second
it depends solely on relative roughness
16. What is the procedure of calculation of pipe friction head losses in laminar and
turbulent flow
Equation
For laminar and turbulent flow we use different friction coefficient
Laminar flow
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Turbulent flow
One equation for hydraulically smooth pipes (lambda independent of the
pipe roughness)
One equation for hydraulically rough pipes (lambda independent of )
Colebrook equation for neither smooth nor rough pipes (transition zone)
Hydraulically smooth pipes – roughness height is smaller than the thickness of the layer of
laminar flow that exists near the pipe wall even in turbulent water motion.
17. Give practical usefulness of Darcy-Weisbach equation
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Calculation of major loses in the systems of pipes, so that we can take them
into account during designing water systems and provide needed pressure.
In Darcy-Weisbach equation frictional loses are conveniently expessed in
terms of velocity head so it can be put into Bernoulli’s equation
18. What is the reason of local losses of head in pressured flow, what is procedure of their
calculation
There are four main reasons:
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Loses due to change of the pipe cross-section (sudden contraction)
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Entrance head loss
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Loss of head in pipe bends
Loss of head in pipe valves
19. What does it mean partially full pipes, give procedure of such devices calculation
partially full pipes –system designed to flow full, but not under pressure. Water
surface in pipe is parallel to pipe invert (pipe floor). Examples of such pipes system
are: drainage and sewerage. In comparison, in main pipes, flow is full and under
pressure.
For partially full pipes we have following parameters
Instead of diameter, we have hydraulic radius
In order to determine friction loss coefficient we can use White-Colebrook
formula (instead of pipe diameter we put radius)
We can use HRS (Hydraulics Research Standard chart), which presents
correlation between water velocity, discharge, hydraulic gradient and pipe
diameter.
(given gradient of pipe and discharge for full pipe)-> (diameter, water velocity
for full pipe)
We can use figure showing proportional discharge and velocity for pipes
flowing partially full
(given proportional discharge )->(proportional height ) -> (proportional
water velocity )->(real water velocity)
It is important that water velocity in such pipes should be big enough so that pipe clean itself
(self-cleansing velocity)
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