Waves - Lecture 3 Flashcards
How does wind generation influence waves?
Tiny ripples first
Bulldozed forward
Pile onto others - growth
What are the pressure gradients of waves?
Always HP -> LP
How does the wind vary the size of waves?
Short crests - messy and chaotic (sea waves)
Longer crests - swell, big/larger and organised
Bigger waves are generated from..?
Stronger winds
Longer fetch (distance over which wind is blowing)
Longer duration
Limited by depth
What are a wave’s elements?
Crest - top of wave
Wavelength - distance between crests
Height - between crest and trough
Trough - bottom of wave
What are the processes acting within a wave?
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)
Water is a compressible or incompressible fluid?
Incompressible
What does the pressure of water represent
Weight of water
Where and how do horizontal velocities and vertical velocities occur in hill of water?
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
What is the process of movement in the right hand side of a hill of water?
Hill moves itself forward - horizontal squeezing (converging) out front makes surface come up
What is the process of movement in the left hand side of a hill of water?
Surface must go down, vertical velocity maximum lowest and horizontal velocity = 0 (
Down (stretch - diverge) -> Up (squeeze - converge)
What occurs within a water parcel with a net displacement of 0?
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
How does propagation velocity differ from orbital motion?
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
How proportional is orbital velocity in relation to wave height and at what depth?
Is proportional to wave height at all depths
What is propagation velocity and what does it depend on?
Speed of wave and depends on depth of squeezing - falls off to some proportion of surface motion (wavelength / 2)
At what proportional rate does the movement in a hill of water?
Moves at a rate directly proportional to how deep convergence/divergence goes
How do wavelength depths differ?
Large wavelength felt with depth, short wavelength not felt (duck diving)
The energy of the wave itself will diminish with depth
Long wavelengths characteristics
Slower motion = lose pressure gradient with depth more slowly
Travels faster than short wavelength = more total squeezing
Short wavelength characteristics
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
Water parcel reality characteristics
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
What happens to a wave parcel in a realistic setting
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
What is wave superposition?
Waves added to waves Adding up different sinusoids Constructive interference (two different wavelengths, cancel each other out) Destructive interference Mixed = groups, sets.
What are groups/sets of waves?
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
