Sediments and Fluids

Question 1

What is the relationship between fluid velocity and pressure?

Answer 1

Along any stream line (path of fluid) fluid velocity varies inversely with fluid pressure.

Question 2

What is Bernoulli’s equation?

Answer 2

Question 3

What is Newton’s first law of motion?

Answer 3

Unless a force acts on a body, its velocity will not change.

Question 4

What is Newton’s second law of motion?

Answer 4

The rate of change of momentum of a body is proportional to the force that acts upon it. F=ma.

Question 5

What is Newtons third law of motion?

Answer 5

For any force exerted, there is an equal and opposite force.

Question 6

What is viscosity?

Answer 6

A measure of the resistance of a fluid to deformation.

Question 7

What is shear stress?

Answer 7

Force per unit area.

Question 8

What is the rate of shear (velocity gradient)?

Answer 8

For a given fluid subject to a given shear force, the rate of shear (i.e. the gradient of the line of flow between the stationary surface and the moving surface) is constant.

Question 9

How do you calculate shear stress?

Answer 9

t=μ(du/dz), where μ is viscosity and du/dz is the rate of change of local fluid velocity (u) in the direction z normal to the shearing surfaces at any point.

Question 10

What are low velocity flows called?

Answer 10

Laminar flow, where the fluid particles travel in regular lines along planes parallel to the base or boundary of the flow.

Question 11

What is high velocity flow called?

Answer 11

Turbulent flow, irregular turbulent flow.

Question 12

What is the equation for the Reynolds number?

Answer 12

Re=ρud/μ, where u=fluid velocity, ρ=fluid density, μ=fluid viscosity, d=diameter of pipe.

Question 13

When does flow become turbulent?

Answer 13

When Re > 2000.

Question 14

How is laminar flow visualised?

Answer 14

• In linear paths generally parallel to the boundary of the flow.
• At constant velocity.
• Fluid motion is predictable.
• May be analysed by mathematical theory.

Question 15

How is turbulent flow visualised?

Answer 15

• In irregular sinuous paths.
• With velocity changing (subject to accelerations/deceleration).
• Fluid motion is unpredictable.
• Cannot be analysed by mathematical theory.
• Requires empirical/experimental study.

Question 16

What are streamlines?

Answer 16

A line that is tangiental to the instantaneous velocity direction. Fluid doesn’t cross the line, and streamlines can’t cross.

Question 17

What is a pathline?

Answer 17

The trajectory or path of a given fluid particle, may be thought of as the record of a path a fluid particle has taken. They can intersect as different particles may follow different paths at different times.

Question 18

What is a streakline?

Answer 18

The instantaneous locus or trace or all the fluid particles that pass through a given fixed point in space. If we injected dye continuously from a fixed point - the dye would form a streakline as it passed downstream. Streaklines cannot intersect betcause two particles cannot occupy the same space at the same instant.

Question 19

What is the characteristics of a steady flow?

Answer 19

When velocity does not change with time, streamlines, pathlines and streaklines are identical.

Question 20

What are time lines?

Answer 20

The instantaneous location of a line of fluid particles in the flow, usually the initial formed line is perpendicular to the flow boundary. Looking at changes in the shape of a line tells us aboutthe relative changes in flow velocity, e.g. away from a boundary.

Question 21

What is a streamtube?

Answer 21

A streamtube is an imaginary tube whose talls are streamlines, it may be engisaged as a bundle of streamlines. Just as streamlines cannot cross, so the boundary of the streamtube cannot be crossed. Pipes or channels are essentially stream tubes.

Question 22

What is the dimensionless form of fluid drag?

Answer 22

C_D=f(ρud/μ), where f=fluid drag.

Question 23

Which forces dominate in the high Re number world?

Answer 23

Inertial forces, or, ρud.

Question 24

Which forces dominate in the low Re number world?

Answer 24

Viscous forces, or μ

Question 25

How do the velocity profiles of laminar and turbulent flow differ?

Answer 25

In laminar flow, viscous shear stress is transmitted through the fluid - the upper part of the flow procedes first. In turbulent flow, turbulent shear stress dominates - the bottom part of the flow procedes first.

Question 26

What is the boundary layer?

Answer 26

The region of fluid next to the boundary across which the velocity of the fluid grades from that of the boundary to that of the unaffected part of the flow “free stream”, i.e. a gradient of velocity.

Question 27

How does a boundary layer expand downstream?

Answer 27

Continuously until they pass an obstruction and viscous forcces degrage shearing in boundary, or it meets another one growing from another surface.

Question 28

What is the flow separation?

Answer 28

The overall pattern of flow at fairly high Re past blunt bodies or through sharply expanding channels.

Question 29

What is the wake?

Answer 29

The separated boundary layer.

Question 30

How does lift occur?

Answer 30

When lift velocity is higher than the pressure at the bottom edge of the airfoil (vice versa when descending).

Question 31

What is the stall angle?

Answer 31

16 degrees, after that the separation point expands and reduces liftk and the turbulent wake reduces lift and increases the pressure drag.

Question 32

What is the froude number?

Answer 32

A measure of the ratio of inertial forces over gravitational forces. u/√gh.

Question 33

What occurs when Fr > 1?

Answer 33

Supercritical or shooting flow.

Question 34

What occurs when Fr < 1?

Answer 34

Tranquil flow, where there is an increase in flow depth, a loss of flow power and the generation of turbulence.

Question 35

What is the hydraulic jump?

Answer 35

When liquid of high velocity discharges into a zone of low velocity there is a sudden rise in height due to turbulent energy.

Question 36

What is √gh?

Answer 36

Celerity or the group velocity of gravity waves in shallow water.

Question 37

What is a standing wave?

Answer 37

A wave that occurs when the velocity of the wave is equal to the velocity of the current, and the Froude number is 1 (though they can occur between 0.8 and 1.77)

Question 38

What is the angle of stability?

Answer 38

32 degrees.

Question 39

How are ripples formed?

Answer 39

A sweep of high velocity turbulent fluid suspends grains. The grains settle to form a pile, and flow separation occurs causing further erosion, forming ripples.

Question 40

What is the hydrostatic equation?

Answer 40

dp/dz=gp where dp/dz is the vertical pressure gradient, p is the density and g is acceleration due to gravity.