Fluid is defined as in formal state, is a substance which deforms continuously, or flows, under the application of a shear stress. The fluid embraces in either a fluid or a gas. A given mass of liquid will occupy a definite volume whereas a gas will fills its container.
Mechanics is essentially the application of the laws for force and motion where it is divided into two branches of fluid mechanics; dynamics and statics. This subject relates mostly to water (H20) and studies of its behavior in more advanced approach.
In fluid mechanics, there are two branches:
- Fluid dynamics is the study of fluids in motion state. The main equation required is from Newton’s second law for accelerating bodies, Σ F = ma
- Fluid statics or hydrostatics is the study of fluids at rest state. The main equation required is from Newton’s second law for non-accelerating bodies, Σ F = 0
Density
Mass Density is a mass per unit volume of substance,
Density = Mass / Volume [SI unit in kg/m3]
Mass Density is a mass per unit volume of substance,
Density = Mass / Volume [SI unit in kg/m3]
Relative Density is a ratio of mass density of a substance to that of water at a standard temperature and pressure (atmospheric). Also known as Specific Gravity; [no unit]
For water: ρ = 1000 kg/m3, and Relative Density, σ = 1
Example; ρ of oil / ρ of water = (850 kg/m3 at 20°C) / (1,000 kg/m3) = 0.85
Typical value: Water = 1, Mercury = 13.5, Paraffin oil = 0.85
Example; ρ of oil / ρ of water = (850 kg/m3 at 20°C) / (1,000 kg/m3) = 0.85
Typical value: Water = 1, Mercury = 13.5, Paraffin oil = 0.85
Specific Weight is a force exerted by the Earth’s gravity upon a unit volume of the substance,
Specific Weight = Density x Gravity [SI unit in N/m3]
Typical value: Water = 9814 N/m3, Mercury = 132943 N/m3, Air = 12.07 N/m3, Paraffin oil = 7851 N/m3
Specific Weight = Density x Gravity [SI unit in N/m3]
Typical value: Water = 9814 N/m3, Mercury = 132943 N/m3, Air = 12.07 N/m3, Paraffin oil = 7851 N/m3
Viscosity
Viscosity is the property of fluid which, by virtue of cohesion and interaction between fluid molecules.
Viscosity is the property of fluid which, by virtue of cohesion and interaction between fluid molecules.
Dynamic Viscosity or coefficient of viscosity, [SI unit in Ns/m2 , kg/ms or Pa-s]
Dynamic Viscosity = (Force x Time) / Time -or- Mass / (Length x Time)
Typical value: Water = 1.14 x 10-3 kg/ms, Mercury = 1.552 kg/ms, Air = 1.78 x 10-5 kg/ms, Paraffin oil = 1.9 kg/ms
Dynamic Viscosity = (Force x Time) / Time -or- Mass / (Length x Time)
Typical value: Water = 1.14 x 10-3 kg/ms, Mercury = 1.552 kg/ms, Air = 1.78 x 10-5 kg/ms, Paraffin oil = 1.9 kg/ms
Kinematic Viscosity is the ration of dynamic viscosity to mass density,
Kinematic Viscosity = Dynamic Viscosity / Mass Density [SI unit in m2/s]
Typical value:
Water = 1.14 x 10-6 m2/s, Mercury = 1.145 x 10-4 m2/s, Air = 1.46 x 10-5 m2/s, Paraffin oil = 2.375 x 10-3 m2/s
Kinematic Viscosity = Dynamic Viscosity / Mass Density [SI unit in m2/s]
Typical value:
Water = 1.14 x 10-6 m2/s, Mercury = 1.145 x 10-4 m2/s, Air = 1.46 x 10-5 m2/s, Paraffin oil = 2.375 x 10-3 m2/s
Bulk Modulus of Elasticity
The bulk modulus of elasticity is expressing the compressibility of fluid under the application of external forces.
Bulk Modulus of Elasticity = Pressure change / Volumetric strain [SI unit in N/m2]
The bulk modulus of elasticity is expressing the compressibility of fluid under the application of external forces.
Bulk Modulus of Elasticity = Pressure change / Volumetric strain [SI unit in N/m2]
Gas Law
Very often we found the fluid flows of gases at or near the atmospheric pressure. In these cases, the changes in pressure (p), density (ρ) and absolute temperature (T) of a gas particle may related to accurately to each other by Perfect Gas Law.
Very often we found the fluid flows of gases at or near the atmospheric pressure. In these cases, the changes in pressure (p), density (ρ) and absolute temperature (T) of a gas particle may related to accurately to each other by Perfect Gas Law.
Formulas: Mass Density, Specific Weight, Dynamic Viscosity, Kinematic Viscosity, Bulk modulus of elasticity, Perfect Gas Law
Surface Tension
Meniscus Fluid of Water
Surface tension (σ) is a property of liquids (cohesion and adhesion) which is felt at the interface between the liquid and another liquid (typically a gas). It has the dimension of force per unit length, N/m in SI unit (as accordance to the 3rd Newton’s Law). It is also important at the interface between a liquid, a solid, and a gas.
Vapor Pressure
Vapor pressure is defined as the pressure at which a liquid will boil (vaporized) and the temperature rises as the pressure rises. If the pressure above a liquid reaches the vapor pressure of the liquid, boiling will occurs.
Vapor pressure is defined as the pressure at which a liquid will boil (vaporized) and the temperature rises as the pressure rises. If the pressure above a liquid reaches the vapor pressure of the liquid, boiling will occurs.
Additional notes:
- Water is a Newtonian fluid having a dynamic viscosity of 10-3 Ns/m2 or kinematic viscosity of 10-3 m2/s at 20°C.
- Bulk modulus of water is 2.1 x 109 N/m2 at 20°C.
- Universal gas constant, Rg is 8.31 J/mol.K.
- Surface tension for water is 73 x 10-3 N/m at 20°C.
- Saturated vapor pressure of water is 2.45 x 103 N/m2 at 20°C.
Fluid Mechanics in Civil Engineering
This particular subject is essential as it involved in nearly all areas of Civil engineering with some of the examples below which concerning the manipulation of fluid:
This particular subject is essential as it involved in nearly all areas of Civil engineering with some of the examples below which concerning the manipulation of fluid:
- River and sea defenses against flood,
- Water distribution and sewerage networks,
- Hydraulics design of water as well as sewerage treatment works,
- Dams, irrigation, pumps and turbines, and water retaining structures.
Fluid mechanics represents that branch of applied mechanics dealing with the behavior of fluids at rest (static) and in motion state. In fluid statics, weight is the important property, whereas in fluid flow, density and viscosity are predominant properties.
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