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Hydraulic comes from the Greek word hydraulikos = water.
Hydraulics is the science of studying the mechanical behavior of water at rest or in motion.
Hydraulic Engineering is the application of fundamental principles of fluid mechanics on water.
Hydraulic systems
Systems which are designed to accommodate water at rest and in motion.
Hydraulic Engineering Systems:
Involve the application of engineering principles and methods to :
Examples of Hydraulic Projects
- Water distribution systems,
- Dams and water control structures,
- Rivers and manmade canals,
- Coastal and Harbour structures,
- Irrigation and Drainage Projects,
HYDRAULICS ENGINEERING:
Water resource engineering:
Water resources engineering usually deals with the application of fluid mechanics principles to water flow problems, but may also include fluids ranging from blood to magma. Engineering hydrology quantifies the distribution and movement of water in the environment. Some problems encountered in water resources engineering include: floods, sediment transport, water supply, wave forces, hydromachinery, and the protection or restoration of surface and ground water resources.
Engineers in the hydraulics/hydrology area may spend their time with applied mathematics, laboratory experimentation, or field construction and testing. The skills necessary range from imagination and common sense to sophisticated analytical and computer modeling ability.
First we will discuss about water properties and fluid statistics and dynamic properties before starts hydraulics applications in water resources engineering:
Surface tension variation
- Directly affects the evaporation loss from a large water body in storage;
- Variation of water viscosity with temperature is important to all problems involving water in motion.
The Earth's Atmosphere and Stratosphere
- The earth's atmosphere layer thickness is approximately 1500 km of mixed gases.
- Nitrogen makes up ~ 78% of the atmosphere,
- Oxygen makes up ~ 21%,
- The remaining 1 % consists mainly of water vapor, argon, and trace amounts of other gases.
- The Stratosphere is the second layer from the Earth.
- The jet stream is in the Stratosphere, which is where jets fly, because of high wind speeds.
- Stratosphere is about 28 km.
- The ozone layer is also in the Stratosphere
Atmospheric Pressure
- The total weight of the atmospheric column exerts a pressure on every surface with which it comes in contact.
- At sea level, under normal conditions, the atmospheric pressure is 1.014 • l05 N/m2 or 1 bar. (1 Pascal)
- In the atmosphere, each gas exerts a partial pressure independent of the other gases.
- The partial pressure exerted by the water vapor in the atmosphere is called the vapor pressure.
Phases of Water
- The amount of energy holding the molecules together depends on the temperature and pressure.
- Depending on its energy content, different forms of water are called three phases:
1. Solid (snow and ice)
2. Liquid (the most commonly recognized form)
3. Gaseous form in air (Moisture, water vapor)
Change of water from one phase to another phase
- Energy must either be added or taken away from the water.
- Latent energy : the amount of energy required to change water from one phase to another.
- To Melt ice requires a latent heat (heat effusion) of 79.71 cal/g.
- 79.71 cal of heat energy must be taken out of each gram of water to freeze.
- Evaporation requires a latent heat (heat of vaporization) of 597 cal/g.
- Under standard atm.P, water boils at 100°C
Properties of water
Understand the physical properties of water to solve problems in hydraulic engineering systems.
Main Water properties:
1- Density (r),
2- Surface tension
3- Viscosity (n)
Density and Specific Weight of water
Density (r): mass per unit volume (kg/m3).
Density depends on size and weight of the molecules and the mechanisms by which these molecules are bonded together.
Water expands when it freezes. The expansion of freezing water causes stresses on the container walls. These stresses are responsible for the bursting of frozen water pipes, chuck holes in pavement, and for the weathering of rocks in nature.
Water reaches a maximum density at 4°C. It becomes less dense when heated.
Density of sea water about 4% more than that of fresh water. Thus, when fresh water meets sea water without sufficient mixing, salinity increases with depth.
Variation of Density and Specific Weight of Water with temperature
Temperature (°C) | Density ( r , kg/m3) | Specific Weight (g, N/m3) |
0°(ice) | 917 | 8996 |
0° (water) | 999 | 9800 |
4 ° | 1000 | 9810 |
10° | 999 | 9800 |
20° | 998 | 9790 |
30° | 996 | 9771 |
40° | 992 | 9732 |
50° | 988 | 9692 |
60° | 983 | 9643 |
70° | 978 | 9594 |
80° | 972 | 9535 |
90° | 965 | 9467 |
100° | 958 | 9398 |
Variation of Density in a Large Reservoir
Change of density with T causes water in a lake to stratify:
- During summer, water tends to stratify, with warmer water on the surface.
2. During the fall, the surface water drops rapidly and sinks toward the lake bottom. The warmer water near the bottom rises to the surface, resulting in fall overturn of the lake.
3. In the winter (water temperature falls below 4°C, with highest water density ), the lake surface freezes while warmer water remains at the bottom. The winter stratification is followed by spring overturn of the lake.
Specific Weight of Water
The weight W = m.g
- m: mass of project (m, in grams, kilograms, etc.),
- g : the gravitational acceleration (g = 9.81 m/sec2).
Weight is expressed in the force units of newton (N) = the force required to accelerate 1 kg of mass at a rate of 1 m/sec2.
The specific weight (g) = weight per unit volume of water (N/m3)
Specific gravity (S): the ratio of the specific weight of any liquid to that of water at 4°C.
Example 1
Viscosity of Water
Consider that water fills the space between two parallel plates at a distance y a part. A horizontal force T is applied to the upper plate and moves it to the right at velocity V while the lower plate remains stationary. The shear force T is applied to overcome the water resistance R, and it must be equal to R because there is no acceleration involved in the process.
Example 2
Newtonian fluids and non-Newtonian fluids
Equation (1.2) is commonly known at Newton's law of viscosity. Most liquids abide by this relationship and are called Newtonian fluids. Liquids that do not abide by this linear relationship are known as non-Newtonian fluids. These include most house paints and blood.
Kinematic Viscosity
Surface Tension and Capillarity
Even at a small distance below the surface of a liquid body, liquid molecules are attracted to each other by equal forces in all directions.
The molecules on the surface, however, are not able to bond in all directions and therefore form stronger bonds with adjacent water molecules. This causes the liquid surface to seek a minimum possible area by exerting surface tension (s) tangent to the surface over the entire surface area.
The rise or fall of liquid in capillary tubes are the results of surface tension.
- Most liquids adhere to solid surfaces.
- The adhesive force varies depending on the nature of the liquid and of the solid surface.
- If the adhesive force between the liquid and the solid surface is greater than the cohesion in the liquid molecules, the liquid tends to spread over and wet the surface, as shown in Figure 1.3(a).
- If the cohesion is greater, a small drop forms, as shown in Figure 1.3(b).
- Water wets the surface of glass, but mercury does not. If we place a small vertical glass tube into the free surface of water, the water surface in the tube rises (capillary rise ). The same experiment performed with mercury will show that the mercury falls. The phenomenon is commonly known as capillary action.
Capillary effect is the rise or fall of a liquid in a small-diameter tube. It is caused by surface tension.
The magnitude of the capillary rise (or depression), h, is determined by the balance of adhesive force and the weight of the liquid column above (or below) the liquid-free surface.
The angle (q) at which the liquid film meets the glass depends on the nature of the liquid and the solid surface.
The upward (or downward) motion in the tube will stop when the vertical component of the surface tension force around the edge of the film equals the weight of the raised (or lowered) liquid column.
- The negative sign means that a positive change in pressure will cause the volume to decrease.
- The bulk modulus of elasticity (Eb) of water varies both with temperature and pressure.
- Typical value: Eb = 2.2 x 109 N/m2 (300,000 psi)
- Large values of the bulk modulus indicate incompressibility
- Incompressibility indicates large pressures are needed to compress the volume slightly
Forces in a Fluid Field
Various types of forces may be exerted on a body of water at rest or in motion. These forces usually include:
- the effects of gravity,
- inertia, elasticity,
- friction,
- pressure, and
- surface tension.
These forces may be classified into three basic categories according to their physical characteristics:
1. body forces
- force per unit mass (N/kg) or force per unit volume (N/m3).
- act on all particles in a body of water as a result of some external body or effect but not due to direct contact.
- an example …gravitational force and Inertial forces and forces due to elastic effects.
2. Surface forces
- force per unit area (N/m2)
- act on the surface of the water body by direct contact.
- may be either external (Pressure forces and friction forces) or internal (viscous force inside a fluid body).
3. line forces.
- force per unit length (N/m).
- Surface tension is thought of as the force in the liquid surface normal to a line drawn in the surface. Thus, it may be considered as a line force.
