TOPICS FOR THE HYDRAULICS EXAM
1. Give the explanations of the fundamental physical properties of water
żð Density  is a ratio of mass of a given quantity of a substance to the volume occupied
by that quantity.
[ ]
In engineering we often use weight density (specific weight)
[ ]
żð 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.
( )
[ ]
[ ]
żð Surface tension  phenomenon caused by strong bonds created between molecules at
the surface of the liquid
żð Capillarity  in the vertical tube water level increases due to adhesive forces between
liquid and glass surface.
żð 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:
żð 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).
żð Surface forces- act on the surface of the water body by direct contact. Can be
external (pressure or friction forces) or internal (viscous forces )
żð 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
żð 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.
żð 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
żð Timescale
- Steady- parameters describing flow do not vary with time
- Unsteady- parameters describing flow vary with time
żð 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
żð Laminar flow
żð 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
- 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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żð Loses due to change of the pipe cross-section (sudden contraction)
( )
żð Entrance head loss
żð Loss of head in pipe bends
żð 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
żð Instead of diameter, we have hydraulic radius
żð In order to determine friction loss coefficient we can use White-Colebrook
formula (instead of pipe diameter we put radius)
żð 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)
żð 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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20. What is the fundamental of hydrodynamics equation, give some examples of its use
żð Conservation of mass (continuity equation)  Mass cannot be
created neither destroyed, it can be only transformed.
żð conservation of energy - energy cannot be created neither destroyed.
żð conservation of momentum  we can evaluate the force that causes the change
of momentum in fluid (for example determining forces in pipe bends). We can
also use it to solve problem in which we cannot evaluate energy loses in direct
way, and when the flow is unsteady.
21. Give the definitions and properties of the most hydraulically effective open-channels.
Hydraulically effective open-channels  open channels with the biggest possible
hydraulic radius ( ). In channels with the biggest hydraulic radius
water velocity and discharge are the biggest, according to following formula
The best shapes: half-square, half-hexagonal, semi-circular
22. Show some differences in calculation procedure of artificial and natural channels
Difference in calculation results from different parameters.
Artificial/Natural
Regular cross-section/ Irregular cross-section
Regular slope/Irregular slope
Know-roughness/surface roughness changes with time, distance and water surface
elevation
Easy to define geometric properties for calculation/ we need to use graphs and tables
Usually accurate results/more difficult to apply hydraulics theories and obtain
satisfactory results
23. What is and where we meet gradually varied flow in artificial and natural channels
Gradually varied flow- flow regime changes due to changing slope of the channel.
Flow regime is in transition from sub-to supercritical.
24. Give the specific energy definition, what is the energy efficiency of open-channel flow
Specific energy in a channel section is defined as the energy head measured with
respect to the channel bottom at the section.
In open channel specific energy is equal to
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(velocity head+depth)
energy efficiency of open-channel flow  depth of water in the channel is equal to critical
depth so that specific discharge can be delivered through the cross-section with minimal
possible energy (specific energy of the flow takes a minimum value)
25. What is a hydraulic jump, and what is the procedure of its calculation
Hydraulic jump-type of rapidly rapid flow and also an example of a stationary surge wave.
Occurs when supercritical flow meets subcritical flow. The resulting flow transition is rapid
and involves a large energy loss due to turbulence. Under these circumstances we cannot use
specific energy diagram we need to use the momentum equation.
26. What do you understand in control gates meanings
Control gates- we use them in order to regulate the discharge
Different kinds of gates
żð Radial gate
żð Vertical gate
żð Drum gate
Choice of the gate type depends on application. They can be designed for underflow and
overflow operation
27. Give the procedure of calculation of water outflow through large orifices
Look at 28.
28. Show calculation procedure for water outflow through orifices  a small and a big
ones
żð Small orifice -
We can calculate water velocity from Bernoulli s equation
"
Then
But due to contraction and energy losses we introduce two coefficients:
"
żð Large orifice  due to significant variation of h we can no longer use solution
for small orifice. The best solution is to subdivide the orifice into smaller ones
with constant h, for every element calculate v, Q, and then sum up discharges
of all subdivisions.
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29. Show the overflow classification, and give principles of such devices calculations
Overflow classification
żð Thin plate (sharp crested weir)
-rectangular weir (full width, contracted)
"
-vee weir/
żð Long based weirs (broadcrested)
żð Over flow spillways
30. What does it mean critical, subcritical and supercritical motion of water?
żð Critical motion of water  depth of water has critical value, it means that a
specific discharge can be delivered through the section at minimum energy
żð Subcritical  depth of water in cross-section is larger than value of critical
depth, the specific discharge of water can be delivered through the section with
smaller velocity, but bigger specific energy than at critical depth.
żð Supercritical  depth of water in cross-section is smaller than value of critical
depth, the specific discharge of water can be delivered through the section with
bigger velocity and bigger specific energy than at critical depth.
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