Waves - Lecture 3

Question 1

How does wind generation influence waves?

Answer 1

Tiny ripples first
Bulldozed forward
Pile onto others - growth

Question 2

What are the pressure gradients of waves?

Answer 2

Always HP -> LP

Question 3

How does the wind vary the size of waves?

Answer 3

Short crests - messy and chaotic (sea waves)

Longer crests - swell, big/larger and organised

Question 4

Bigger waves are generated from..?

Answer 4

Stronger winds
Longer fetch (distance over which wind is blowing)
Longer duration
Limited by depth

Question 5

What are a wave’s elements?

Answer 5

Crest - top of wave
Wavelength - distance between crests
Height - between crest and trough
Trough - bottom of wave

Question 6

What are the processes acting within a wave?

Answer 6

Hill of water - gravity (pressure) gradient
Top of hill - (high) pressure gradient extends below surface (depth) = means propagation of shape extends below surface
Middle of hill - (inflection between crest and trough)
Bottom of hill (Low pressure) - squeezing - water has to go up (surface elevation)

Question 7

Water is a compressible or incompressible fluid?

Answer 7

Incompressible

Question 8

What does the pressure of water represent

Answer 8

Weight of water

Question 9

Where and how do horizontal velocities and vertical velocities occur in hill of water?

Answer 9

Horizontal velocity maximum at top of hill and 0 in middle (inflection point)
Vertical velocity maximum at inflection point and 0 at top of hill

Question 10

What is the process of movement in the right hand side of a hill of water?

Answer 10

Hill moves itself forward - horizontal squeezing (converging) out front makes surface come up

Question 11

What is the process of movement in the left hand side of a hill of water?

Answer 11

Surface must go down, vertical velocity maximum lowest and horizontal velocity = 0 (
Down (stretch - diverge) -> Up (squeeze - converge)

Question 12

What occurs within a water parcel with a net displacement of 0?

Answer 12

crest moves forward, half of wavelength slows down, hits trough, going in opposite direction
(no current, doesn’t drift anywhere) - perfect circle in profile and moves backwards and forwards from above

Question 13

How does propagation velocity differ from orbital motion?

Answer 13

Each orbital equal in diameter to wave height, but cycling independent from speed of wave (will pass much slower and wave passes quickly)
wave motion (passes @ m/s) and water motion (within orbital @ cm/s) are different

Question 14

How proportional is orbital velocity in relation to wave height and at what depth?

Answer 14

Is proportional to wave height at all depths

Question 15

What is propagation velocity and what does it depend on?

Answer 15

Speed of wave and depends on depth of squeezing - falls off to some proportion of surface motion (wavelength / 2)

Question 16

At what proportional rate does the movement in a hill of water?

Answer 16

Moves at a rate directly proportional to how deep convergence/divergence goes

Question 17

How do wavelength depths differ?

Answer 17

Large wavelength felt with depth, short wavelength not felt (duck diving)
The energy of the wave itself will diminish with depth

Question 18

Long wavelengths characteristics

Answer 18

Slower motion = lose pressure gradient with depth more slowly
Travels faster than short wavelength = more total squeezing

Question 19

Short wavelength characteristics

Answer 19

Faster motion = lose pressure gradient with depth more quickly
Travel slowly = less total squeezing (and lose a lot more energy to friction) so don’t propagate as quick

Question 20

Water parcel reality characteristics

Answer 20

Wave drift (asymmetry - moving much further at the top - moves more forward than backward) 
Have complex shapes, wave drift never really 0 and drift increases with wave height

Question 21

What happens to a wave parcel in a realistic setting

Answer 21

Water velocity at bottom of orbit < velocity at top of orbit
More time moving with crest than trough
Parcel moves in direction of wave propagation relative to reverse direction

Question 22

What is wave superposition?

Answer 22

Waves added to waves
Adding up different sinusoids
Constructive interference (two different wavelengths, cancel each other out)
Destructive interference
Mixed = groups, sets.

Question 23

What are groups/sets of waves?

Answer 23

Wave groups with small range in wavelengths, narrower the range, the more distinct the grouping
Addition of two waves with nearly same frequency - add/cancel