Fluid Mechanics Units and Properties

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Questions and Answers

What occurs when the pressure above a liquid surface is reduced to equal or below its vapor pressure?

  • The liquid freezes instantly
  • The boiling point of the liquid increases
  • Vaporization ceases completely
  • The liquid begins to boil at its current temperature (correct)

Which type of pressure is measured above absolute zero or vacuum?

  • Absolute pressure (correct)
  • Vacuum pressure
  • Gauge pressure
  • Differential pressure

What is the primary cause of cavitation in a flowing liquid?

  • Chemical reactions within the liquid
  • High temperatures in the liquid
  • Presence of solid particles in the liquid
  • Pressure falling below vapor pressure (correct)

What distinguishes gauge pressure from absolute pressure?

<p>Gauge pressure accounts for atmospheric pressure in its measurement (C)</p> Signup and view all the answers

What device is considered a simple manometer for measuring fluid pressure?

<p>U-tube manometer (D)</p> Signup and view all the answers

In the context of cavitation, what happens to the vapor bubbles when they enter a region of higher pressure?

<p>They collapse, creating high pressure (A)</p> Signup and view all the answers

What is a defining feature of vacuum pressure?

<p>It is the difference between atmospheric pressure and absolute pressure (A)</p> Signup and view all the answers

Which mechanical gauge type uses a spring or dead weight in measurement?

<p>Bourdon tube pressure gauge (A)</p> Signup and view all the answers

Why is it significant that the vapor pressure changes with temperature?

<p>It influences the boiling point relative to the surrounding pressure (B)</p> Signup and view all the answers

What is the primary characteristic of an ideal fluid?

<p>It is compressible and has no viscosity. (B)</p> Signup and view all the answers

Which type of fluid is characterized by shear stress being directly proportional to the rate of shear strain?

<p>Newtonian fluid (A)</p> Signup and view all the answers

What defines kinematic viscosity?

<p>The ratio between dynamic viscosity and density of fluid. (C)</p> Signup and view all the answers

In the equation $ au = \mu (du/dy)$, what does $ au$ represent?

<p>The shear stress (D)</p> Signup and view all the answers

Which equation describes the pressure inside a droplet based on surface tension?

<p>P = $ rac{4 \sigma}{d}$ (C)</p> Signup and view all the answers

How does capillary rise occur in a thin tube?

<p>Because the adhesive forces exceed the cohesive forces. (C)</p> Signup and view all the answers

What condition does the equilibrium state describe when discussing liquid height in capillarity?

<p>The weight of the liquid is balanced by surface tension forces. (B)</p> Signup and view all the answers

How is the capillary rise in a tube expressed mathematically?

<p>h = $ rac{4 \sigma}{\rho g d}$ (A)</p> Signup and view all the answers

What phenomenon is described by the change from the liquid state to the gaseous state?

<p>Vaporization (A)</p> Signup and view all the answers

What is the effect of surface tension on a hollow bubble?

<p>Two surfaces experience surface tension simultaneously. (B)</p> Signup and view all the answers

Which type of fluid would most likely not demonstrate Newton's law of viscosity?

<p>Non-Newtonian fluid (A)</p> Signup and view all the answers

What occurs to the pressure intensity when a liquid jet is examined?

<p>It combines surface tension and pressure area forces. (C)</p> Signup and view all the answers

What is the relationship between shear stress and shear strain in Newtonian fluids?

<p>They are directly proportional. (A)</p> Signup and view all the answers

What principle defines the relationship of shearing motion in fluids?

<p>Newton's law of viscosity (B)</p> Signup and view all the answers

What is the unit of measurement for specific weight or specific density?

<p>N/m3 (D)</p> Signup and view all the answers

Which statement correctly defines specific gravity?

<p>It is the ratio of the density of a fluid to the density of a standard fluid. (A)</p> Signup and view all the answers

How is mass density calculated?

<p>Mass divided by volume (C)</p> Signup and view all the answers

Which of the following units is equivalent to 1 Pascal?

<p>1 N/m² (B)</p> Signup and view all the answers

What is the relationship between a Joule and a Watt?

<p>1 Watt is equivalent to 1 Joule per second. (C)</p> Signup and view all the answers

If a fluid has a density of 800 kg/m³, what would its specific gravity be when compared to water?

<p>0.8 (D)</p> Signup and view all the answers

According to Newton's second law of motion, what is the force in Newtons for a mass of 2 kg that is accelerated at 3 m/s²?

<p>6 N (D)</p> Signup and view all the answers

What does the term 'specific volume' refer to?

<p>Both B and C (B)</p> Signup and view all the answers

Flashcards

Density

Mass per unit volume of a fluid (e.g., kg/m³).

Newton

Unit of force; force required to accelerate 1 kg of mass by 1 m/s².

Pascal (Pa)

Unit of pressure; force per unit area (N/m²).

Joule (J)

Unit of work or energy; force multiplied by distance (N⋅m).

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Watt (W)

Unit of power; rate of energy transfer or work done per unit time (J/s).

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Specific Gravity

Ratio of a fluid's density to the density of a reference substance (like water).

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Specific Volume

Volume per unit mass of a fluid (m³/kg).

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Mass Density

Mass per unit volume of a fluid.

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Vapor Pressure

The pressure exerted by the vapor of a liquid in equilibrium with its liquid phase at a given temperature.

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Boiling Point

The temperature at which the vapor pressure of a liquid equals the surrounding atmospheric pressure, causing the liquid to change into a vapor.

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Cavitation

The formation of vapor bubbles within a flowing liquid due to a decrease in pressure below its vapor pressure, followed by the sudden collapse of these bubbles in a region of higher pressure.

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Absolute Pressure

Pressure measured relative to a perfect vacuum (zero pressure).

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Gauge Pressure

Pressure measured relative to atmospheric pressure.

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Vacuum Pressure

Pressure below atmospheric pressure.

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Manometer

A device that measures pressure by balancing a column of liquid against another liquid column or a known pressure.

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Simple Manometer

A manometer with one end open to the atmosphere and the other end connected to the pressure point.

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Piezometer

A simple manometer consisting of a vertical tube connected to the pressure point, used to measure the pressure head.

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U-tube Manometer

A simple manometer with a U-shaped tube, used to measure pressure differences by balancing liquid columns in the two arms.

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Specific Gravity (S)

The ratio of the weight density of a liquid or gas to the weight density of water or air, respectively.

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Weight Density

Weight per unit volume of a substance. It describes how dense a substance is in terms of weight.

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Density of Liquid

Mass per unit volume of a liquid. It tells us how tightly packed the molecules are in the liquid.

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Viscosity

A fluid's resistance to flow, the internal friction between layers of the fluid.

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Kinematic Viscosity

Ratio of dynamic viscosity to density of a fluid. Represents how easily a fluid flows under certain conditions.

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Newtonian Fluid

A fluid where shear stress is directly proportional to the rate of shear strain. It flows smoothly under constant conditions.

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Non-Newtonian Fluid

A fluid where shear stress is not directly proportional to the rate of shear strain, viscosity changes depending on applied force.

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Shear Stress (τ)

Force per unit area acting parallel to the surface of a fluid. Caused by layers of fluid moving at different speeds.

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Surface Tension (σ)

Tensile force acting on the surface of a liquid, causing it to behave like a stretched membrane.

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Capillary Rise

The phenomenon of a liquid rising in a narrow tube due to surface tension and adhesive forces.

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Capillary Depression

The phenomenon of a liquid falling in a narrow tube due to surface tension and cohesive forces.

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Vaporization

The change from liquid to gaseous state.

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Study Notes

Fluid Mechanics Units and Properties

  • Dimensions and Units: Dimensions describe measurable characteristics (mass, length, temperature). Units are standardized measurements for dimensions. Fundamental dimensions are mass (kg), length (m), time (s), and temperature (Kelvin or °C).

  • Density (ρ): Mass per unit volume (kg/m³). Water's density is 1000 kg/m³. ρ = mass of fluid / volume of fluid

  • Specific Weight (w): Weight per unit volume (N/m³). w = weight of fluid / volume of fluid

  • Specific Volume: Volume per unit mass (m³/kg). Specific volume = volume of fluid / mass of fluid.

  • Specific Gravity (S): Ratio of a fluid's density to a reference fluid's density (dimensionless). For liquids, water is the reference. For gases, air is the reference. Specific Gravity = weight density of liquid/ weight density of water

  • Viscosity (μ): Resistance to fluid layer movement. Shear stress (τ) is proportional to the velocity gradient (du/dy). τ = μ (du/dy). Newtonian fluids obey this relationship; Non-Newtonian fluids do not.

  • Kinematic Viscosity (ν): Ratio of dynamic viscosity to density. ν = μ/ρ

Types of Fluids

  • Ideal Fluid: Imaginary fluid with zero viscosity and incompressibility.

  • Real Fluids: Possess viscosity, found in practice.

  • Newtonian Fluids: Stress is directly proportional to the rate of shear strain.

  • Non-Newtonian Fluids: Stress isn't proportional to the rate of shear strain.

  • Ideal Plastic Fluids: Shear stress exceeds a yield value, and then is proportional to the shear strain rate.

Surface Tension

  • Surface Tension (σ): Tensile force on a liquid surface, acting between the liquid and a surrounding gas, or behaves as a stretched membrane. Measured in N/m.

  • Surface Tension on a Liquid Droplet: Internal pressure (p) is related to surface tension and droplet diameter (d): p = 4σ/d.

  • Surface Tension on a Hallow Bubble: Internal pressure of a bubble is related to surface tension and diameter: p = 8σ/d.

  • Surface Tension on a Liquid Jet: Internal pressure (p) of a liquid jet is related to surface tension and diameter: p = 2σ/d

Capillarity

  • Capillarity: Rise or fall of a liquid in a small tube due to surface tension.

  • Capillary Rise (h): Height of liquid rise in a tube. h = 4σ cosθ / (ρgd) where σ is surface tension, θ is contact angle, ρ is density, g is acceleration due to gravity, and d is tube diameter

  • Capillary Fall: A fall of liquid level in a tube, happens with materials where the contact angle between liquid and tube is greater than 90°

Vapor Pressure and Cavitation

  • Vapor Pressure: Pressure exerted by a liquid's vapor when vaporization occurs

  • Cavitation: Formation and collapse of vapor bubbles in a liquid due to low pressure. Leads to erosion.

Pressure Measurement

  • Absolute Pressure: Pressure relative to a complete vacuum.

  • Gauge Pressure: Pressure relative to atmospheric pressure.

  • Vacuum Pressure: Pressure below atmospheric pressure.

  • Manometers: Devices measuring pressure by balancing fluid columns.

  • Mechanical Gauges: Measuring pressure by springs or dead weights e.g., diaphragm gauges, Bourdon tube gauges.

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