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Unit 3 - FLUID MECHANICS

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FLUIDS A fluid is any substance that flows and conforms to the boundaries of its container. A fluid could be a gas or a liquid. An ideal fluid is assumed to be incompressible (so that its density does not change), to flow at a steady rate, to be non-viscous (no friction between the fluid and the container through which it is flowing), and to flow without rotation (no swirls or eddies).

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**DENSITY AND SPECIFIC GRAVITY**

The density (ρ) of a substance is defined as the quantity of mass (m) per unit volume (V): For solids and liquids, the density is usually expressed in (g/cm3) or (kg/m3). The density of gases is usually expressed in (g/l). The specific gravity (SG) of a substance is the ratio of the density of the substance to the density of another substance that is taken as a standard. SG = density of substance/density of water The density of pure water at 4°C is usually taken as the standard (1 g/cm3 or 1x103 kg/m3).

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PRESSURE Any fluid can exert a force perpendicular to its surface on the walls of its container. The force is described in terms of the pressure it exerts, or force per unit area: Units: N/m2 or Pascal (Pa) One atmosphere (atm) is the average pressure exerted by the earth’s atmosphere at sea level 1.00 atm = 1.01 x105 N/m2 = kPa

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PRESSURE IN FLUIDS A static (non-moving) fluid produces a pressure within itself due to its own weight. This pressure increases with depth below the surface of the fluid. Consider a container of water with the surface exposed to the earth’s atmosphere:

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PRESSURE IN FLUIDS The pressure P1 on the surface of the water is 1 atm. If we go down to a depth from the surface, the pressure becomes greater by the product of the density of the water ρ the acceleration due to gravity g, and the depth h. Thus the pressure P2 at this depth is: P2 = P1 + ρ g h

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Note that the pressure at any depth does not depend on the shape of the container, but rather only on the pressure at some reference level and the vertical distance below that level.

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**GAUGE PRESSURE AND ABSOLUTE PRESSURE**

Ordinary pressure gauges measure the difference in pressure between an unknown pressure and atmospheric pressure. The pressure measured is called the gauge pressure and the unknown pressure is referred to as the absolute pressure. Pabs = Pgauge + Patm ΔP = Pabs - Patm

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PRESSURE GAUGES All these gauges work by measuring the force exerted on a known area by comparing it to either a spring of some sort or a known pressure.

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PASCAL'S PRINCIPLE Pascal’s Principle states that pressure applied to a confined fluid is transmitted throughout the fluid and acts in all directions.

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The principle means that if the pressure on any part of a confined fluid is changed, then the pressure on every other part of the fluid must be changed by the same amount. This principle is basic to all hydraulic systems. Pout = Pin

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**BUOYANCY AND ARCHIMEDES' PRINCIPLE**

Archimedes’ Principle states that a body wholly or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid it displaces. An object lowered into a fluid “appears” to lose weight. The force that causes this apparent loss of weight is referred to as the buoyant force. The buoyant force is considered to be acting upward through the center of gravity of the displaced fluid. FB = mF g = ρF g VF

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In general, objects floats on a fluid if its density is less than that of the fluid. At equilibrium (when floating) the buoyant force on an object has a magnitude equal to the weight of the object.

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The fraction of the floating object that is submerged is equal to the ratio of the density of the object to that of the fluid.

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**1. An aluminum object has a mass of 27. 0-kg and a density of 2**

1. An aluminum object has a mass of 27.0-kg and a density of 2.70x103 kg/m3. The object is attached to a string and immersed in a tank of water. a. Determine the volume of the object m = 27 kg ρAl = 2700 kg/m3 ρwater = 1000 kg/m3 = 0.01 m3

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**FT Vwater = VAl Fgwater = ρwVwg FB = 1000(0.01)(9.8) = 98 N**

b. Determine the tension in the string when it is completely immersed. FT Vwater = VAl Fgwater = ρwVwg = 1000(0.01)(9.8) = 98 N FB = Fgwater FB Fg ΣF = FT + FB - Fg = 0 FT = Fg - FB = 27(9.8) - 98 = N

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**FB = Fg - Fgapparent = 0.086(9.8) - (0.073)(9.8) = 0.1274 N FB = ρwVwg**

2. A piece of alloy has a mass of 86 g in air and 73 g when immersed in water. Find its volume and its density. mair = kg mwater = kg ρwater = 1000 kg/m3 FB = Fg - Fgapparent = 0.086(9.8) - (0.073)(9.8) = N FB = ρwVwg = 1.3x10-5 m3 = 6.6x103 kg/m3

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**FB Fgbox FB = Fgbox = ρgV = ρgAh’ = 0.075 m**

3. A 60-kg rectangular box, open at the top, has base dimensions 1.0 m by 0.80 m and a depth of 0.50 m. a. How deep will it sink in fresh water? m = 60 kg A = 1 x 0.8 m2 w = 0.5m FB Fgbox FB = Fgbox = ρgV = ρgAh’ = m

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**ΣF = FB - Fgbox - Fgballast = 0 Fgballast = FB - Fgbox **

b. What weight of ballast will cause it to sink to a depth of 30 cm? depth = 0.3 m FB ΣF = FB - Fgbox - Fgballast = 0 Fgballast = FB - Fgbox = ρgV - mg = ρgAh - mg = 1000(9.8) (1)(0.8)(0.3) - 60(9.8) = 1764 N Fgbox Fgballast

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FLUIDS IN MOTION The equations that follow are applied when a moving fluid exhibits streamline flow. Streamline flow assumes that as each particle in the fluid passes a certain point it follows the same path as the particles that preceded it. There is no loss of energy due to internal friction (viscosity) in the fluid. Figure a on next slide. In reality, particles in a fluid exhibit turbulent flow, which is the irregular movement of particles in a fluid and results in loss of energy due to internal friction in the fluid. Turbulent flow tends to increase as the velocity of a fluid increases. Figure b on next slide.

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FLOW RATE Consider a fluid flowing through a tapered pipe:

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**The flow rate is the mass of fluid that passes a point**

per unit time: m/t = ρ A v Units: kg/s Where ρ is the density of the fluid, A is the cross-sectional area of the tube of fluid at the particular point in question, and v is the velocity of the fluid at the point in question.

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**Since fluid cannot accumulate at any point, the flow rate is constant**

Since fluid cannot accumulate at any point, the flow rate is constant. This is expressed as the equation of continuity. ρ A v = constant In streamline flow, the fluid is considered to be incompressible and the density is the same throughout. The equation of continuity can then be written in terms of the volume rate of flow (R) that is constant throughout the fluid: R = Av = constant Units: m3/s or: ρ1 A1 v1 = ρ2 A2 v2

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**A = πr2 = π(0.04)2 = 0.005 m2 R = Av = 0.005(4) = 0.02 m3/s**

4. Oil flows through a pipe 8.0 cm in diameter, at an average speed of 4 m/s. What is the flow in m3/s and m3/h? r = 8/2 = 0.04 m v = 4 m/s A = πr2 = π(0.04)2 = m2 R = Av = 0.005(4) = 0.02 m3/s R = 0.02 m3/s (3600 s/h) = 72.4 m3/h

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BERNOULLI’S EQUATION In the absence of friction or other non- conservative forces,the total mechanical energy of a system remains constant, that is, PE1 + K1 = PE2 + K2 There is a similar law in the study of fluid flow, called Bernoulli’s principle, which states that the total pressure of a fluid along any tube of flow remains constant.

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**Assume incompressible fluid in a laminar flow**

with no viscosity. Bernoulli’s equation is a form of the conservation of energy law.

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**Bernoulli’s principle:**

Where the velocity of fluid is high the pressure is low. Where the velocity of fluid is low the pressure is high.

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**Find the velocity of a liquid flowing out of a spigot:**

The pressure is the same P1=P2. The velocity v2 » 0. Bernoulli’s equation is: This is called Torricelli’s Theorem.

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**Fluid Pressure in a Pipe of Varying Elevation**

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Another special case: Fluid flow with no change in height: y1 = y2. Bernoulli’s equation: When pressure is low (high), velocity is high (low).

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**= 9.9 m/s R (flow rate)= v2A2 = 9.9 (π(1.5x10-2)2) = 7x10-3 m3/s**

5. What volume of water will escape per minute from an open top tank through an opening 3.0 cm in diameter that is 5.0 m below the water level in the tank? r = 1.5x10-2 m Top (1) Bottom (2) h1 = 5 m h2 = 0 If tank is large v1 = 0 = 9.9 m/s (Torricelli’s equation) R (flow rate)= v2A2 = 9.9 (π(1.5x10-2)2) = 7x10-3 m3/s

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6. A water tank springs a leak at position 2 in the previous figure, where the water pressure is 500 kPa. What is the velocity of escape of the water through the hole? P1 - P2 = 5x105 N/m2 h1 = h2 v1 = 0 =31.6 m/s

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**7. A pipe has a diameter of 16 cm at section 1 and 10 cm at section 2**

7. A pipe has a diameter of 16 cm at section 1 and 10 cm at section 2. At section 1 the pressure is 200 kPa. Point 2 is 6.0 m higher than point 1. When oil of density 800 kg/m3 flows at a rate of m3/s, find the pressure at point 2 if viscous effects are negligible. ρ = 800 kg/m^3 r1 = 0.08 m = (.16/2) r2 = 0.05 m P1 = 2x10^5 Pa (kilo = 10^3) h1 = 0 m h2 = 6 m V1 A1 = V2 A2 = R = 1.49 m/s = 3.82 m/s

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= 2x105 + ½ (800)( ) (9.8) (0-6) = 1.48x105 kPa

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