Water management

• Hydraulics is a science dealing with the physical properties and movement phenomena of liquids (water)
• Hydrostatics


  The part of hydraulics dealing with the laws of quiescent liquids
• Hydrodynamics


  The part of hydraulics that examines the laws of flowing fluids (water movements)
• The most important physical properties of water

  • 3 states (liquid, solid, gaseous)
  • Boiling and melting points depend on pressure and solute content
• Density


  Ratio of mass (m) to volume (V), sign: ρ (Greek letter), temperature and pressure dependent
• The maximum density of water at standard atmospheric pressure is the highest at 4 oC
• At atmospheric pressure, in a normal environment, the change in density of water is negligible, so we consider the density of water as 1000 kg/m3
• Specific gravity/weight


  Sign: γ (gamma), unit of measure: N/m³, weight per volume unit, expressed as density: γ=ρ∙g, where g is gravity acceleration
• Specific weight of water, at 5 ºC, at sea level: 9.807 kN/m³
• Surface tension


  A fundamental property of liquids, they strive to create the smallest specific surface area (sphere) if no external force field acts on them, due to cohesive force
• Wetting


  The degree of wetting can be described by the wetting edge angle (Θ, theta), if <90 degrees it is partial wetting, if >90 degrees it is partial non-wetting
• Water


  Due to adhesion exceeding cohesive force, the resultant points outside the wall, so the water surface shows a concave shape from above
• Mercury


  Due to higher cohesion, the resultant points inside the wall, so the mercury surface is convex from above
• Capillarity


  The phenomenon related to surface tension, where adhesion affects the liquid level against gravity, resulting in capillary rise or capillary sinking
• Capillary rise of water: hcap=30/d (mm), where d is the pipe diameter
• Capillary descent of mercury: hcap=10/d (mm)
• Capillarity is strongly, but reversely correlating with the grain size of the sediment
• Detergent washes due to reducing the surface tension of water, forming a "bridge" between water and fat particles
• Viscosity


  Internal friction, characterized by sliding stress, directly proportional to the velocity gradient, dynamic viscosity: η (Pa∙s), kinematic viscosity: ν=η/ρ (m2/s)
• Kinematic viscosity of water: at 10 °C ν10 = 0.013 cm2/s; At 20 °C ν20 = 0.01 cm2/s = 10-6 m2/s
• Ideal fluid


  A fluid that fills the space and has zero viscosity, homogeneous, incompressible and frictionless
• Quiescent fluid space


  Absolute rest, when the fluid parts are stationary in the terrestrial coordinate system, or relative rest, when the liquid parts are stationary in the coordinate system fixed to the wall of the container
• Basic theorems of hydrostatics

  • Stationary liquid only exerts a perpendicular effect on the wall of the containment vessel
  • At any internal point of the quiescent liquid, the pressure acts in all directions and at one point has the same magnitude regardless of direction
  • Pascal's law: the effect (pressure) of external forces acting on the fluid space at rest propagates and prevails in the fluid space without loss
• Hydrostatic pressure


  Pressure due to the weight of the liquid, described by Pascal's theorem: P= hγ [N/m2=Pa] where h is liquid column height, γ is specific gravity/weight of the liquid, and ρ is density of the liquid
• Absolute pressure


  pabs=p0+hγ, where p0 is air pressure (≈100kPa)
• Buoyancy


  The resultant of the vertical pressure components acting on a body immersed in water points upwards, and its magnitude is the weight of water of the same volume as the body (Archimedes' law)
• The line of action of the buoyant force passes through the center of gravity of the displaced water volume
• Inertia


  The tendency of an object to resist changes in its motion. It's described by Newton's first law of motion, which states that an object at rest will stay at rest, and an object in motion will stay in motion, unless acted upon by an external force.
• Buoyant force


  The force that makes objects float in a fluid (like water or air). It's equal to the weight of the fluid that the object displaces. The line of action of the buoyant force always passes through the center of gravity of the displaced fluid volume.