Fluid Properties

Question 1

What are the properties of a fluid (4)

Answer 1

Density
Viscousity: coefficient of stress-strain relation in fluid
Surface tension: interface force on fluid
Specific weight
Specific gravity

Question 2

Define density (2)

Answer 2

-defined as the fluids mass per unit volume
-the density of a fluid varies with pressure and temperature

Question 3

State the equation of state and what does it express (4)

Answer 3

PV = nRT

No. Of moles = mass of gas/molar mass of gas

Pressure = density x R x T

-it expresses the Changes in gas density is directly related to changes in pressure and changes in temprature

Question 4

Define specific weight and state its equation (2)

Answer 4

-defined as the fluids weight per unit volume

Specific weight = density x gravity

Question 5

Define specific gravity (2)

Answer 5

-defined as the ratio of the density of the fluid to the density of water at some specified temperature typically 4 Celsius

Specific gravity = density of the fluid / density of water

Question 6

What is the non slip condition in fluid mechanics and state and example for better understanding (3)

Answer 6

-refers to the phenomenon where a fluid in contacts with a solid surface has zero relative velocity to the surface
-this means that the fluid sticks to the surface and the fluid particles touching the surface do not move relative to it

Example=
-in a pipe flow, he fluid at the inner surface of the pipe has zero velocity relative to the pipe, while the fluid further from the surface moves faster.

Question 7

Define and explain viscosity (5)

Answer 7

-sometimes referred to as the dynamic viscosity
-it is a fundamental property of fluids that describes their internal resistance to flow and deformation.
-it can be thought of as the thickness or stickiness of a fluid.
-for example honey has a higher viscosity than water meaning it flows more slowly under the same applied force
-viscosity variations with pressure are generally negligible but viscosity is very sensitive to temperature

Question 8

Define and explain dynamic viscosity and mention and equation to explain it (5)

Answer 8

-denoted by the Greek symbol Miu (u)
-is the measure of a fluids internal resistance to shear force
-in simpler terms, it quantifies how much force is needed to make a liquid flow
-ratio between applied shear stress and the resulting rate of strain in a fluid

Shear force = dynamic viscosity x rate of change of velocity in the direction perpendicular to the flow

Question 9

Explain Newtonian fluids and give examples (5)

Answer 9

-for Newtonian fluids the dynamic viscosity is constant and independent of the applied shear stress.
-in these fluids, the relationship between shear stress and strain rate is linear
-meaning the fluid deforms proportionally to the applied force

Example = water air oil

Shear force = dynamic viscosity x rate of change of velocity in the direction perpendicular to the flow

Question 10

Define and explain non Newtonian fluids and give examples of non Newtonian fluids (3)

Answer 10

-non Newtonian have a viscosity that changes with the rate of deformation (shear stress)
-their behavior maybe non linear meaning the relationship between the stress and the strain is more complex

Examples= blood,paint and certain plastics

Question 11

State the SI unit of viscosity (1)

Answer 11

N . s . m-2. Or kg . m-1 . s-1

Question 12

State and equation of viscosity in relation to density and explain it (3)

Answer 12

Kinematic viscosity = viscosity / density

-kinematic viscosity measures how fast momentum diffuses through a fluid
-SI units of m2/s

Question 13

Define and explain surface tension (3)

Answer 13

-surface tension is a physical property that arises at the interface between 2 phases such has liquid and a gas
-it causes the surface of a liquid to behave as if it is covered by a stretched elastic membrane
-this phenomenon results from the cohesive forces between molecules in the liquid which are stronger at the surface than in the bulk of a liquid

-in simple words, surface tension is like an invisible skin that forms on the surface of a liquid where it meets air or another liquid that doesn’t mix with it.
-this happens because the molecules at the surface of the liquid are pulled more strongly towards the liquids interior than towards the air or other liquid

-inside the liquid molecules are pulled equally in all direction by their neighbors so they don’t feel a strong force in any One Direction
-But at the surface, they don’t have molecules above them so they get pulled inward more strongly
-this makes the surface molecules stick together more tightly almost like the liquid is wrapped in a thin stretched elastic layer

Question 14

State the factors that affect the surface tension and explain (4)

Answer 14

-temperature:
-surface tension decreases as temperature increases
-this is because increasing temperature reduces the cohesive forces between the molecules causing them to spread out
-making it easier for the surface to expand

-Impurities:
-the presence of surfactants or contaminants can significantly reduce surface tension
-example adding soap to water decreases its surface tension allowing it to spread more easily

Question 15

State and explain uniform flow (2)

Answer 15

-in uniform flow ,the velocity of the fluid is the same at every point in the flow
-this means if you look at different spots in the fluid, the speed and direction are exactly the same

Question 16

State and explain non uniform flow (2)

Answer 16

-in non uniform flow, the velocity varies from one point to another.
-the fluid might move faster in some areas and slow in others

Question 17

State and explain steady flow (2)

Answer 17

-steady flow means that the properties of the flow (velocity and pressure) do not change over time at any given point.
-the fluid might have different speeds at different points in space,
-but those speeds remain constant over time

Question 18

State and explain unsteady flow (2)

Answer 18

-in unsteady flow, the fluids properties (velocity and pressure) change overtime at a given point.
-the speed or direction of the flow might increase, decrease or fluctuate overtime at a

Question 19

What is pathline and state its example and uses (6)

Answer 19

-a path line is the actual path traced by a single fluid particles as it moves through the flow over time
-it shows the trajectory that one specific particle follows

Example:
-Imagine dropping a small, visible particle (like a leaf) into a river and watching where it travels.
-the path it follows as it moves downstream is its path line

Use:
-path line help track the movement of individual particles in unsteady flow where the flow changes over time.
-it shows where a particle started and where it’s going

Question 20

What is streaklines and state its example and uses (6)

Answer 20

-a streakline is the line formed by all the fluid particles that have passed through a particular point in space overtime.
-it represents the current location of particles that passed through the same spot at different times

Example:
-if you inject dye into a flowing river from a fixed point, the streamline is the line of dyed fluid formed as all the particles that passed through the injection point spread out downstream

Use:
-streaklines are useful for visualizing steady and unsteady flow.
-they are often what you see when visualizing flow using smoke or dye

Question 21

What is streamline and state its example and uses (6)

Answer 21

-a streamline is a line that is tangent to the velocity vector of the flow at every point
-it represents the direction that the fluid is moving at every point at a particular instant in time
-there is no flow across a streamline, only along it

Example:
-in steady flow around an airplane wing, you can imagine the air moving smoothly along the streamlines
-the streamlines will bend around the wing
-showing how the fair is flowing at that moment

Uses:
-steamlines are helpful in understanding flow direction and patterns
-especially in steady flow, the indicate how fluid moves around objects and help in analyzing aerodynamic properties

Question 22

What are the key differences between pathlines, streakline, streamline (3)

Answer 22

Pathline: the actual trajectory of one fluid over time

Streakline: the location of all particles that have passed through a specific point

Streamlines: a line that is always tangent to he velocity of the flow at a given moment

Question 23

Imagine a smoke trail, explain how pathline, streakline and streamline in relation to this scenario (3)

Answer 23

-the path line would be the actual route a smoke particles takes,
-it shows the motion of individual particles overtime

-the streak line would be the entire trail of smoke formed by particles passing through the same release point
-it shows a pattern formed by particles passing through a point

-the stream line would be the direction of the flow at every point, showing the overall flow structure
-Shows the instantaneous flow direction