CE 230-Engineering Fluid Mechanics

Week 1

Introduction

Policy

Get to know each other

Things to know about instructor policy

Grading

Basic terminology

What is fluid mechanics

What is a fluid

Difference between solid, liquid and gas

Applications

Applications of fluid mechanics in daily life

Fluid Properties

Units

Basic Units versus derived units

SI versus Traditional units

prefixes

What are the units of?

Mass Length Time Temperature Weight Pressure Energy Power

Intensive versus Extensive properties

Density Specific weight Specific gravity

Tables A.2-A.4 at the end of the text

Gases, air, water, and some liquids

Example

What is the weight of a 1 cubic meter of water if the temperature is 5 C?

What if the temperature was 90 C? or 0 C?

terminology

Incompressible fluid Compressible fluid Ideal fluid Perfect gas?

Perfect gas law

P=ρRT

R is the gas constant with units of m.N/kg K or ft lb/slug R

See appendix A.2 for R values

Problem 2.6

What is the weight of a 10 cubic feet tank containing oxygen if it is pressurized to 400 psi (absolute pressure) and at a temperature of 70 F?

answer 22.5 lb

Elasticity & Bulk modulus

The needed pressure change needed to reduce the volume of a fluid

Ev = -Δp/(ΔV/V)

What does it mean if Ev is very big?

For water Ev = 2.2 GN/m2

Problem 2.46

What pressure increase must be applied to water to reduce its volume by 1%?

Surface tension

Due to unbalanced molecular forces at interface of two fluids. Force /length

For water-air surface σ=0.073 N/m

This accounts for capillary rise

Estimate the capillary rise of water in a tube of diameter d.

Applications of surface tension

Pressure in a droplet Pressure in a soap bubble Cylinder supported by surface tension Ring being pulled out of liquid Capillary rise between two plates

Vapor pressure

The pressure at which a liquid boils Function of T (direct proportion)

At what pressure does water boil?

Table A.5

Example

Consider two cases of water boiling in a tea kettle one near the sea and the other on top of a very high mountain.

a) compare the value of vapor pressure for both cases

b) compare the temperature for both cases

does this make sense?

Fluid Properties (2)

Examine velocity distribution in pipe

Viscosity

Consider two plates separated be a thin film of fluid. The lower plate is fixed and the upper is moving with a constant velocity.

Investigate what is happening in the fluid in terms of flow or stresses.

Viscosity

No slip boundary condition

Relation between shear stress and velocity gradient

Classification of fluids

Mechanisms of fluid resistance to shear stress

Particle cohesion

Transfer of molecular momentum

Influence of temperature on viscosity

Units of viscosity

Dynamic viscosity

absolute viscosity

viscosity

Kinematic viscosity

Influence of temperature on viscosity

Classification of fluids according to relation of shear to strain rate

Problems on viscosity

Board sliding on an inclined surface separated by a thin film of oil

Is there a shear stress in the film?

let’s calculate the terminal velocity

Problem # 34

Problem # 39

Calculate the weight of piston given: Terminal velocity μ Spacing between piston and cylinder Piston dimensions

Problem 2.39

Problem 2.41