PROPERTIES OF FLUID

Properties of fluid

• Density - Mass per unit volume.

        Density ρ = m/v

        Unit- Kg/m³

• Specific weight or Weight density -Weight per unit volume of a liquid at a standard temp. and pr.

        γ = ρg= mg/v

        ρ = density     and g = acceleration due to gravity

        Unit - N/m³

        Weight Density of water is 9810 N/m³ or 1000 kgf/m³ at 4⁰C and normal pressure.

        Sp. weight of fluid varies due to - a. change of gravity and b. Effect of pr. and temp.

• Specific Gravity - Ratio of weight density (or density)  of  a fluid to the weight density  (or density) of a standard fluid at STP.

 SP. Gravity = γ_g/ γ_s

        It is dimensionless or unit less. 

        Sp. gravity of water is 1.

        SP. Gravity of HG = 13.6

        value of density of HG will be = 13.6 x 1000 = 13600 Kg/m³

        value of weight density of HG will be = 13.6 x 9810 = 133416 N/m³

• Specific Volume - volume per unit mass of  a liquid.

        v =V/m=1/ρ

• Viscosity ( Also known as absolute or dynamic viscosity): - Property of a fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of fluid.

        Viscosity of a liquid is due to cohesion mainly.

• Newton’s law: -shear stress is proportional to velocity gradient.

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        i.e. t µ du/dy  => t = µ(du/dy)

        µ = coefficient of viscosity called dynamic viscosity

•  Dynamic Viscosity: Unit of dynamic viscosity is N-S/m² or Pa-S or poise

        1 Poise =  0.1 Pa-S

        Dimension of dynamic viscosity is [ML^-1T^-1]

• Kinematic viscosity: It is the ratio between dynamic viscosity and density of fluid and is denoted by v

        So, v = µ/ ρ  [µ= Dynamic Viscosity]

        Unit - m²/s or stoke

        Dimension  [L²T^-1]

        1 stoke = 10^-4 m²/S

• Temperature effect on viscosity: viscosity of liquid decreases with increase of temperature while the viscosity of gases increases with the increase of temperature.

    Types of fluid:

• Ideal fluid: It is incompressible and has no viscosity. It is an imaginary fluid.

• Real fluid: A fluid which posses viscosity called real fluid all fluids in practice are real fluid.

• Newtonian fluid: A real fluid which follows Newton’s law of viscosity

·       For a perfect gas P/ ρ = RT

            R = g as constant = 287 J/kg⁰K

·       For isothermal process P/ ρ = constant and for adiabatic process P/ρ^k = constant

·       Bulk modulus of elasticity is given as K = -dp/(dv/v)

            Reciprocal of bulk modules of elasticity is called compressibility.

• Surface tension: - Surface tension is the tendency of liquid surfaces to minimize surface area as the tensile force acting on the surface of a liquid in contact with a gas or on the surface between two immiscible liquids such that contact surface behaves like a membrane under tension.

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            Mathematically Surface tension = Force/Length.

            Unit of surface tension is N/m 

            Dimension is MT^-2

• Surface tension on hollow bubble : P = 8σ/d
• Surface tension on liquid droplet : P =4σ/d
• Surface tension on liquid jet : P = 2σ/d

 (P = pressure intensity inside droplet σ = surface tension, d = dia of drop let)

• Capillarity: It is a phenomenon of rise or fall of liquid in a small tube relative to the adjacent general level of liquid when the tube is held vertically in the liquid.

        Height of capillary rise, h = 4σcosθ/γd

        θ = angle of contact between liquid and glass tube

        σ = surface tension, γ = unit weight, d = dia of tube

• Cavitation: If pressure at any point in a flowing fluid is less than or equal to vapor pressure, vaporization of liquid starts. The bubbles of these vapors are carried by flowing fluid into the region of high pressure where they collapse, giving rise to high impact pressure. This phenomenon is called cavitation.