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Fluids Chapter 14 Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.
14-1 Fluid Density and Pressure Distinguish fluids from solids When mass is uniformly distributed, relate density to mass and volume Apply the relationship between hydrostatic pressure, force, and the surface area over which that force acts. Learning Objectives © 2014 John Wiley & Sons, Inc. All rights reserved.
14-1 Fluid Density and Pressure Physics of fluids is the basis of hydraulic engineering A fluid is a substance that can flow, like water or air, and conform to a container This occurs because fluids cannot sustain a shearing force (tangential to the fluid surface) They can however apply a force perpendicular to the fluid surface Some materials (pitch) take a long time to conform to a container, but are still fluids The essential identifier is that fluids do not have a crystalline structure © 2014 John Wiley & Sons, Inc. All rights reserved.
14-1 Fluid Density and Pressure The density, ρ, is defined as: In theory the density at a point is the limit for an infinitesimal volume, but we assume a fluid sample is large relative to atomic dimensions and has uniform density. Then Density is a scalar quantity Units kg/m 3 Eq. (14-1) Eq. (14-2) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-1 Fluid Density and Pressure The pressure, force acting on an area, is defined as: We could take the limit of this for infinitesimal area, but if the force is uniform over a flat area A we write We can measure pressure with a sensor Eq. (14-3) Eq. (14-4) Figure 14-1 © 2014 John Wiley & Sons, Inc. All rights reserved.
14-1 Fluid Density and Pressure We find by experiment that for a fluid at rest, pressure has the same value at a point regardless of sensor orientation Therefore static pressure is scalar, even though force is not Only the magnitude of the force is involved Units: the pascal (1 Pa = 1 N/m 2 ) the atmosphere (atm) the torr (1 torr = 1 mm Hg) the pound per square inch (psi) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-2 Fluids at Rest Apply the relationship between the hydrostatic pressure, fluid density, and the height above or below a reference level Distinguish between total pressure (absolute pressure) and gauge pressure. Learning Objectives © 2014 John Wiley & Sons, Inc. All rights reserved.
14-2 Fluids at Rest Hydrostatic pressures are those caused by fluids at rest (air in the atmosphere, water in a tank) Figure 14-2 © 2014 John Wiley & Sons, Inc. All rights reserved.
14-2 Fluids at Rest Write the balance of forces: Rewrite: forces with pressures, mass with density For a depth h below the surface in a liquid this becomes: Eq. (14-5) Eq. (14-7) Eq. (14-8) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-2 Fluids at Rest The pressure in 14-8 is the absolute pressure Consists of p 0, the pressure due to the atmosphere, and the additional pressure from the fluid The difference between absolute pressure and atmospheric pressure is called the gauge pressure because we use a gauge to measure this pressure difference The equation can be turned around to calculate the atmospheric pressure at a given height above ground: © 2014 John Wiley & Sons, Inc. All rights reserved.
14-2 Fluids at Rest Answer: All the pressures will be the same. All that matters is the distance h, from the surface to the location of interest, and h is the same in all cases. © 2014 John Wiley & Sons, Inc. All rights reserved.
14-3 Measuring Pressure Describe how a barometer can measure atmospheric pressure Describe how an open- tube manometer can measure the gauge pressure of a gas. Learning Objectives © 2014 John Wiley & Sons, Inc. All rights reserved.
14-3 Measuring Pressure Figure 14-5 shows mercury barometers The height difference between the air-mercury interface and the mercury level is h: Eq. (14-9) Figure 14-5 © 2014 John Wiley & Sons, Inc. All rights reserved.
14-3 Measuring Pressure Only the height matters, not the cross-sectional area Height of mercury column is numerically equal to torr pressure only if: o Barometer is at a place where g has its standard value Temperature of mercury is 0°C © 2014 John Wiley & Sons, Inc. All rights reserved.
14-3 Measuring Pressure The height difference between the two interfaces, h, is related to the gauge pressure: The gauge pressure can be positive or negative, depending on whether the pressure being measured is greater or less than atmospheric pressure Eq. (14-10) Figure 14-6 shows an open-tube manometer Fig © 2014 John Wiley & Sons, Inc. All rights reserved.
11-4 Pascal's Principle Identify Pascal's principle For a hydraulic lift, apply the relationship between the input area and displacement and the output area and displacement. Learning Objectives © 2014 John Wiley & Sons, Inc. All rights reserved.
14-4 Pascal's Principle Eq. (14-12) Figure 14-7 Pascal's principle governs the transmission of pressure through an incompressible fluid: Consider a cylinder of fluid (Figure 14-7) Increase p ext, and p at any point must change Independent of h © 2014 John Wiley & Sons, Inc. All rights reserved.
14-4 Pascal's Principle Describes the basis for a hydraulic lever Input and output forces related by: The distances of movement are related by: Figure 14-8 Eq. (14-13) Eq. (14-14) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-4 Pascal's Principle So the work done on the input piston equals the work output The advantage of the hydraulic lever is that: Eq. (14-15) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-5 Archimedes' Principle Describe Archimedes' principle Apply the relationship between the buoyant force on a body and the mass of the fluid displaced by the body For a floating body, relate the buoyant force to the gravitational force For a floating body, relate the gravitational force to the mass of the fluid displaced by the body Distinguish between apparent weight and actual weight Calculate the apparent weight of a body that is fully or partially submerged. Learning Objectives © 2014 John Wiley & Sons, Inc. All rights reserved.
14-5 Archimedes' Principle The buoyant force is the net upward force on a submerged object by the fluid in which it is submerged This force opposes the weight of the object It comes from the increase in pressure with depth Figure © 2014 John Wiley & Sons, Inc. All rights reserved.
14-5 Archimedes' Principle The stone and piece of wood displace the water that would otherwise occupy that space Archimedes' Principle states that: The buoyant force has magnitude Where m f is the mass of displaced fluid Eq. (14-16) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-5 Archimedes' Principle A block of wood in static equilibrium is floating: This is expressed: Because of Eq we know: Which means: Eq. (14-18) Eq. (14-17) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-5 Archimedes' Principle The apparent weight of a body in a fluid is related to the actual weight of the body by: (apparent weight) = (actual weight) – (buoyant force) We write this as: Eq. (14-19) Answer: (a) all the same (b) 0.95ρ 0, 1ρ 0, 1.1ρ 0 © 2014 John Wiley & Sons, Inc. All rights reserved.
14-6 The Equation of Continuity Describe steady flow, incompressible flow, nonviscous flow, and irrotational flow Explain the term streamline Apply the equation of continuity to relate the cross- sectional area and flow speed at one point in a tube to those quantities at a different point Identify and calculate volume flow rate Identify and calculate mass flow rate. Learning Objectives © 2014 John Wiley & Sons, Inc. All rights reserved.
14-6 The Equation of Continuity Motion of real fluids is complicated and poorly understood (e.g., turbulence) We discuss motion of an ideal fluid 1. Steady flow: Laminar flow, the velocity of the moving fluid at any fixed point does not change with time 2. Incompressible flow: The ideal fluid density has a constant, uniform value 3. Nonviscous flow: Viscosity is, roughly, resistance to flow, fluid analog of friction. No resistive force here 4. Irrotational flow: May flow in a circle, but a dust grain suspended in the fluid will not rotate about com © 2014 John Wiley & Sons, Inc. All rights reserved.
14-6 The Equation of Continuity Visualize fluid flow by adding a tracer Each bit of tracer (see figure 14-13) follows a streamline A streamline is the path a tiny element of fluid follows Velocity is tangent to streamlines, so they can never intersect (then 1 point would experience 2 velocities) Figure Figure © 2014 John Wiley & Sons, Inc. All rights reserved.
14-6 The Equation of Continuity Fluid speed depends on cross-sectional area Because of incompressibility, the volume flow rate through any cross-section must be constant We write the equation of continuity: Figure Eq. (14-23) Holds for any tube of flow whose boundaries consist of streamlines Fluid elements cannot cross streamlines © 2014 John Wiley & Sons, Inc. All rights reserved.
14-6 The Equation of Continuity We can rewrite the equation as: Where RV is the volume flow rate of the fluid (volume passing a point per unit time) If the fluid density is uniform, we can multiply by the density to get the mass flow rate: Eq. (14-24) Eq. (14-25) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-6 The Equation of Continuity Answer: 13, out © 2014 John Wiley & Sons, Inc. All rights reserved.
14-7 Bernoulli's Equation Calculate the kinetic energy density in terms of a fluid's density and flow speed Identify the fluid pressure as being a type of energy density Calculate the gravitational potential energy density Apply Bernoulli's equation to relate the total energy density at one point on a streamline to the value at another point Identify that Bernoulli's equation is a statement of the conservation of energy. Learning Objectives © 2014 John Wiley & Sons, Inc. All rights reserved.
14-7 Bernoulli's Equation Figure represents a tube through which an ideal fluid flows Applying the conservation of energy to the equal volumes of input and output fluid: The ½ρv 2 term is called the fluid's kinetic energy density Eq. (14-28) Figure © 2014 John Wiley & Sons, Inc. All rights reserved.
14-7 Bernoulli's Equation Equivalent to Eq , we can write: These are both forms of Bernoulli's Equation Applying this for a fluid at rest we find Eq Applying this for flow through a horizontal pipe: Eq. (14-29) Eq. (14-30) © 2014 John Wiley & Sons, Inc. All rights reserved.
14-7 Bernoulli's Equation Answer: (a) all the same volume flow rate (b) 1, 2 & 3, 4 (c) 4, 3, 2, 1 © 2014 John Wiley & Sons, Inc. All rights reserved.
Density Fluid Pressure A substance that can flow Can exert a force perpendicular to its surface 14 Summary Eq. (14-8) Eq. (14-4) Pressure Variation with Height and Depth Eq. (14-2) Pascal's Principle A change in pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and to the walls of the containing vessel © 2014 John Wiley & Sons, Inc. All rights reserved.
Archimedes' PrincipleFlow of Ideal Fluids 14 Summary Bernoulli's Equation Eq. (14-29) Eq. (14-16) Eq. (14-19) Eq. (14-24) Eq. (14-25) © 2014 John Wiley & Sons, Inc. All rights reserved.
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