Lecture- 2 Fluid Properties

(Part A)

Dr. Dhafer Manea Hachim AL-HASNAWI Assist Proof

Al-Furat Al-Awsat Technical University Engineering Technical College / Najaf

email:coj.dfr@atu.edu.iq

Fluid Mechanics: Fundamentals of Fluid Mechanics, 7th Edition, Bruce R. Munson. Theodore H. Okiishi. Alric P. Rothmayer

John Wiley & Sons, Inc.l, 2013

Learning Objectives

• After completing this Lecture, you should be able to:

• identify the key fluid properties used in the analysis of fluid behavior.

• calculate common fluid properties given appropriate information.

Outline

• Fluid Mechanics Overview

• Measures of Fluid Mass and Weight: Density

• Specific Weight

• Specific Gravity

• Ideal Gas Law

• Viscosity

• Kinematic Viscosity

Fluid Mechanics Overview

Gas Liquids Statics Dynamics

Air, He, Ar,

N2, etc.

Water, Oils,

Alcohols,

etc.

0 iF

Viscous/Inviscid

Steady/Unsteady

Compressible/

Incompressible

0 iF

Laminar/

Turbulent

, Flows

Compressibility Viscosity Vapor

Pressure

Density

Pressure Buoyancy

Stability

Chapter 1: Introduction Chapter 2: Fluid Statics Fluid Dynamics:

Rest of Course

Surface

Tension

Fluid Mechanics

Measures of Fluid Mass and Weight: Density

v

m

The density of a fluid is defined as mass per unit volume.

•Different fluids can vary greatly in density

•Liquids densities do not vary much with pressure and temperature

•Gas densities can vary quite a bit with pressure and temperature

•Density of water at 4° C : 1000 kg/m3

•Density of Air at 4° C : 1.20 kg/m3

Alternatively, Specific Volume:

1

m = mass, and v = volume.

In the BG system, has units of 𝑠𝑙𝑢𝑔 𝑓𝑡3 and in SI the units are 𝑘𝑔 𝑚3 .

Measures of Fluid Mass and Weight: Specific Weight

g The specific weight of fluid is its weight per unit volume.

•Specific weight characterizes the weight of the fluid system

•Specific weight of water at 4° C : 9.80 kN/m3

•Specific weight of air at 4° C : 11.9 N/m3

g = local acceleration of gravity, 9.807 m/s2

In the BG system, term 𝛾 has units of 𝐼𝑏 𝑓𝑡3 and in SI the units are

𝑁 𝑚3 Under conditions of standard gravity (𝑔 = 32.174 𝑓𝑡 𝑠2 ,

𝑔 = 9.807 𝑚 𝑠2 ).

Measures of Fluid Mass and Weight: Specific Gravity

OH

SG

2

The specific gravity of fluid is the ratio of the density of the fluid

to the density of water @ 4° C.

•Gases have low specific gravities

•A liquid such as Mercury has a high specific gravity, 13.2

•The ratio is unitless.

•Density of water at 4° C : 1000 kg/m3

Example: Determine the density, specific volume and specific weight of the liquid whose specific gravity is 0.85 (take 𝜌𝑤 = 1000 𝑘𝑔 𝑚3 𝑎𝑛𝑑 𝑔 = 9.81 𝑚 𝑠2 )

Ideal Gas Law

• Gases are highly compressible in comparison to liquids, with changes in gas density directly related to changes in pressure and temperature

through the equation: 𝜌 =𝑃

𝑅𝑇

• where p is the absolute pressure, ρ the density, T the absolute temperature, and R is a gas constant.

Example: Determine the density, specific weight, and specific volume of Co2 contained in a vessel at a pressure of 600 KN/m3 absolute and temperature 30oC (take 𝑅𝑐𝑜2 = 189 𝑗 𝑘𝑔. 𝐾 , 𝑎𝑛𝑑 𝑔 = 9.81 𝑚 𝑠2 )

Viscosity: Introduction

dy

du

The viscosity is measure of the “fluidity” of the fluid which is not

captured simply by density or specific weight. A fluid can not resist a

shear and under shear begins to flow. The shearing stress and

shearing strain can be related with a relationship of the following form

for common fluids such as water, air, oil, and gasoline:

is the absolute viscosity or dynamics viscosity of the fluid, u is the

velocity of the fluid and y is the vertical coordinate as shown in the

schematic below:

“No Slip

Condition”

Viscosity: Measurements

A Capillary Tube Viscosimeter is one method of measuring

the viscosity of the fluid.

Viscosity Varies from Fluid to Fluid and is dependent on

temperature, thus temperature is measured as well.

Units of Viscosity are N·s/m2 or lb·s/ft2

Movie Example using a Viscosimeter:

Viscosity: Newtonian vs. Non-Newtonian

Newtonian Fluids are Linear Relationships between stress and

strain: Most common fluids are Newtonian.

Non-Newtonian Fluids are Non-Linear between stress and strain

Corn

Starch

Latex

Paint

Toothpaste

Viscosity: Kinematic Viscosity

•Kinematic viscosity is another way of representing

viscosity

•Used in the flow equations

•The units are of L2/T or m2/s and ft2/s

Example: A square metal plate 1.8 m side and 1.8 mm weight 60 N is to be lifted through a vertical gap of 30 mm of infinite extent. The oil in the gap has specific gravity of 0.96 and viscosity of 3 N.s/m2 as shown in Fig. 4.A. If the metal plate is to be lifted at a constant speed of 0.12 m/s. Determine: (1)-Contact area of plate with oil, (2)-Thickness of the oil film, (3)-Shear stress, and (4)-Force required F