Waves Flashcards
What is the energy contained in a wave proportional to?
wave height squared
What is the ‘significant’ wave height (H_1/3)?
the average height of the highest third of all waves occurring in a particular time period
What is SHM?
When the restoring force is proportional to displacement and in the opposite direction
What the fuck is an amphidromic system……
system of tidal action in which the tide wave progresses around a point or centre of little or no tide. Result from the combined effects of ocean basin geometry constraint, non-linear friction and the coriolis force.
Amphidromic point….
where the crest of the tidal wave circulates round once a tidal cycle. Have zero tidal amplitude for a tidal constituent. Co-tidal lines
emanate from the amphidromic point and link all the points where the tide is at the same
stage (phase) of its cycle. Co-range lines cut across co-tidal lines approx. at right angles and
join up locations having the same tidal range (or amplitude) Note that amphidromic systems
tend to rotate anticlockwise in the N hemisphere (clockwise in Southern).
What is Stokes drift?
The small net forward displacement caused by orbits moving slightly further forward in crests than they do backwards in troughs
What defines a shallow wave?
lambda>20*d
What defines a deep wave?
lambda<2*d
What is g in c=sqrt(gλ/ 2π)
???
5 ways of wave energy dissipation
- White-capping
- Viscous attenuation
- Air resistance
- Non-linear wave-wave interaction
- Shoaling
What is fetch?
The unobstructed distance of sea/ a wave that wind blows
In deep water limit how does group speed relate to the phase speed?
group speed is half the phase speed
Shallow water limit
Speed dependent on depth but not wavelength
Deep water limit
Speed dependent on wavelength but not depth
Particle velocity
the velocity of individual particles or parcels of water
Group velocity
the velocity of a collection of waves … or the velocity with which the “energy”
of the wave propagates.
Phase velocity
the velocity of an individual wave (of given wavelength)
Define non-dispersive waves and give an example
For a non-dispersive wave the phase velocity is independent of wavelength, and thus the group
velocity is the same as the phase velocity (e.g. tides).
Define dispersive waves and give an example
For dispersive waves the phase velocity
is dependent on wavelength and the group velocity is different to the phase velocity (e.g. wind
waves or tsunamis generated by short wavelength disturbances).
provide the relationship between phase and group velocities for waves that are in the deep limit
Towards the deep water limit both slow (with longer wave lengths travelling faster),
with the group velocity converging to half the value of the phase velocity.
provide the relationship between phase and group velocities for waves that are in the shallow limit
In the shallow water limit the phase and group velocity reach a maximum with the same value
of √(gH).
describe typical particle trajectories in the water column
particle trajectories are near circular near surface, and come more elliptical with depth until the ellipse flattens near the bed, but the extent of the to and fro motion in the horizontal is not reduced with depth.
Explain centrifugal force in the context of the Earth-Moon system
The Earth-Moon system rotates about a common centre of mass. This creates a (fictitious)
centrifugal force for every point on Earth. Every point traces the same path and hence this
force is the same for every point on Earth and is parallel to the line connecting the Earth and
Moon centres of mass, and in the opposite direction to the Moon
explain the tide generating force and the presence of semi-diurnal tides
Every point on Earth experiences a gravitational attraction towards the moon, with varying magnitudes based on an inverse square law.
Summing these two forces yields the variable tide generating force, the horizontal component
of which is responsible for the tides.
This creates two “bulges”, one under the moon and another on the opposite side of the Earth.
As the Earth rotates over approx a day an observer will find themselves under high water
levels and low water levels twice in that period – hence a semi-diurnal tidal signal.