sensagent's content

• definitions
• synonyms
• antonyms
• encyclopedia

Dictionary and translator for handheld

New : sensagent is now available on your handheld

sensagent's office

Shortkey or widget. Free.

Windows Shortkey: . Free.

Vista Widget : . Free.

Webmaster Solution

Alexandria

A windows (pop-into) of information (full-content of Sensagent) triggered by double-clicking any word on your webpage. Give contextual explanation and translation from your sites !

Try here  or   get the code

SensagentBox

With a SensagentBox, visitors to your site can access reliable information on over 5 million pages provided by Sensagent.com. Choose the design that fits your site.

WordGame

The English word games are:
○   Anagrams
○   Wildcard, crossword
○   Lettris
○   Boggle.

Lettris

Lettris is a curious tetris-clone game where all the bricks have the same square shape but different content. Each square carries a letter. To make squares disappear and save space for other squares you have to assemble English words (left, right, up, down) from the falling squares.

boggle

Boggle gives you 3 minutes to find as many words (3 letters or more) as you can in a grid of 16 letters. You can also try the grid of 16 letters. Letters must be adjacent and longer words score better. See if you can get into the grid Hall of Fame !

English dictionary
Main references

Most English definitions are provided by WordNet .
English thesaurus is mainly derived from The Integral Dictionary (TID).
English Encyclopedia is licensed by Wikipedia (GNU).

The wordgames anagrams, crossword, Lettris and Boggle are provided by Memodata.
The web service Alexandria is granted from Memodata for the Ebay search.
The SensagentBox are offered by sensAgent.

Translation

Change the target language to find translations.
Tips: browse the semantic fields (see From ideas to words) in two languages to learn more.

last searches on the dictionary :

6179 online visitors

computed in 0.062s

 Gravity Bamboo Pintail Complete Waves Cruiser Pin Longboard Skateboard Brand New (169.95 USD)Commercial use of this term The Search for Gravity Waves, Davies, P. C. W. Cambridge University Press Hardco (6.0 USD)Commercial use of this term The Everything Kids' Nature Book: Create Clouds, Make Waves, Defy Gravity and Mu (1.99 USD)Commercial use of this term Internal Gravity Waves (Hardback) (123.39 AUD)Commercial use of this term NEW Gravity's Shadow: The Search for Gravitational Waves by Harry Collins Paperb (60.25 USD)Commercial use of this term INTERNAL GRAVITY WAVES - B. R. SUTHERLAND (HARDCOVER) NEW (130.99 USD)Commercial use of this term The Everything Kids' Nature Book : Create Clouds, Make Waves, Defy Gravity... (9.11 USD)Commercial use of this term Modulation of Wind Generated Waves by Long Gravity Waves (24.6 USD)Commercial use of this term NEW Gravity Waves - National Bureau of Standards |National Bureau of (19.77 USD)Commercial use of this term Gravity Waves (27.74 USD)Commercial use of this term Gravity Waves NEW by National Bureau of Standards (27.58 USD)Commercial use of this term NEW Internal Gravity Waves - Sutherland, B. R. (132.96 USD)Commercial use of this term NEW A Numerical Study of Convectively Generated Gravity Waves Atop Thunderstorms (77.09 USD)Commercial use of this term NEW Estimation of Intrinsic Gravity Wave Parameters from Multiple, Ground-Based (77.09 USD)Commercial use of this term Skinit My First Wave Skin for Samsung Gravity SGH-T459 (11.24 USD)Commercial use of this term NEW Gravity Wave Effects on the Occurrence and Brightness of Polar Mesospheric C (77.09 USD)Commercial use of this term Gravity Waves in a Heterogeneous Incompressible Fluid (20.4 USD)Commercial use of this term NEW An Introduction to Atmospheric Gravity Waves by Carmen J. Nappo Hardcover Bo (115.38 USD)Commercial use of this term Model equations for gravity-capillary waves. NEW (93.71 USD)Commercial use of this term

»

# definitions

## gravity wave(n.)

1.(physics) a wave that is hypothesized to propagate gravity and to travel at the speed of light

gravitation wave

# Gravity wave

Surface gravity wave, breaking on an ocean beach.
Wave clouds over Theresa, Wisconsin, United States.
Atmospheric gravity waves as seen from space.
NASA satellite image of a gravity wave cloud pattern formed in the wake of the Île Amsterdam, a volcanic island in the southern Indian Ocean.

In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media (e.g., the atmosphere and the ocean) which has the restoring force of gravity or buoyancy.

When a fluid element is displaced on an interface or internally to a region with a different density, gravity tries to restore the parcel toward equilibrium resulting in an oscillation about the equilibrium state or wave orbit.[1] Gravity waves on an air–sea interface are called surface gravity waves or surface waves while internal gravity waves are called internal waves. Wind-generated waves on the water surface are examples of gravity waves, and tsunamis and ocean tides are others.

Wind-generated gravity waves on the free surface of the Earth's ponds, lakes, seas and oceans have a period of between 0.3 and 30 seconds (3 Hz to 0.03 Hz). Shorter waves are also affected by surface tension and are called gravity–capillary waves and (if hardly influenced by gravity) capillary waves. Alternatively, so-called infragravity waves, which are due to subharmonic nonlinear wave interaction with the wind waves, have periods longer than the accompanying wind-generated waves.[2]

## Atmosphere dynamics on Earth

In the Earth's atmosphere, gravity waves are a mechanism for the transfer of momentum from the troposphere to the stratosphere. Gravity waves are generated in the troposphere by frontal systems or by airflow over mountains. At first, waves propagate through the atmosphere without appreciable change in mean velocity. But as the waves reach more rarefied air at higher altitudes, their amplitude increases, and nonlinear effects cause the waves to break, transferring their momentum to the mean flow.

This process plays a key role in controlling the dynamics of the middle atmosphere.[citation needed]

The clouds in gravity waves can look like altostratus undulatus clouds, and are sometimes confused with them, but the formation mechanism is different.[citation needed]

## Quantitative description

The phase speed $\scriptstyle c$ of a linear gravity wave with wavenumber $\scriptstyle k$ is given by the formula

$c=\sqrt{\frac{g}{k}},$

where g is the acceleration due to gravity. When surface tension is important, this is modified to

$c=\sqrt{\frac{g}{k}+\frac{\sigma k}{\rho}},$

where σ is the surface tension coefficient, ρ is the density, and k is the wavenumber (spatial frequency) of the disturbance.

Since $\scriptstyle c=\omega/k$ is the phase speed in terms of the angular frequency $\scriptstyle\omega$ and the wavenumber, the gravity wave angular frequency can be expressed as

$\omega=\sqrt{gk}.$

The group velocity of a wave (that is, the speed at which a wave packet travels) is given by

$c_g=\frac{d\omega}{dk},$

and thus for a gravity wave,

$c_g=\frac{1}{2}\sqrt{\frac{g}{k}}=\frac{1}{2}c.$

The group velocity is one half the phase velocity. A wave in which the group and phase velocities differ is called dispersive.

## Lunitidal Waves

High tide does not occur when the moon is at its zenith, but about 6 hours later, which means the moon is always maximally out of phase with the tides that it causes. This delay is called the lunitidal interval. The main cause is that the extremely long wavelength gravity waves that transport the water don't travel fast enough to keep up with the earth's rotation. The oceans have a depth $h < L / 20$, where $L$ is the length of these waves; hence the case of propagation in shallow water applies and their speed is proportional to $\sqrt{h}$. These lunitidal waves travel at over 500 mph, but they would need to travel at slightly more than 1000 mph to keep up with the earth's rotation, since the equator moves 25,000 miles in 24 hours. If the oceans were four times as deep the lunitidal interval would vanish and the tides would be in phase with the moon.

## The generation of waves by wind

Wind waves, as their name suggests, are generated by wind transferring energy from the atmosphere to the ocean's surface, and capillary-gravity waves play an essential role in this effect. There are two distinct mechanisms involved, called after their proponents, Phillips and Miles.

In the work of Phillips,[3] the ocean surface is imagined to be initially flat (glassy), and a turbulent wind blows over the surface. When a flow is turbulent, one observes a randomly fluctuating velocity field superimposed on a mean flow (contrast with a laminar flow, in which the fluid motion is ordered and smooth). The fluctuating velocity field gives rise to fluctuating stresses (both tangential and normal) that act on the air-water interface. The normal stress, or fluctuating pressure acts as a forcing term (much like pushing a swing introduces a forcing term). If the frequency and wavenumber $\scriptstyle\left(\omega,k\right)$ of this forcing term match a mode of vibration of the capillary-gravity wave (as derived above), then there is a resonance, and the wave grows in amplitude. As with other resonance effects, the amplitude of this wave grows linearly with time.

The air-water interface is now endowed with a surface roughness due to the capillary-gravity waves, and a second phase of wave growth takes place. A wave established on the surface either spontaneously as described above, or in laboratory conditions, interacts with the turbulent mean flow in a manner described by Miles.[4] This is the so-called critical-layer mechanism. A critical layer forms at a height where the wave speed c equals the mean turbulent flow U. As the flow is turbulent, its mean profile is logarithmic, and its second derivative is thus negative. This is precisely the condition for the mean flow to impart its energy to the interface through the critical layer. This supply of energy to the interface is destabilizing and causes the amplitude of the wave on the interface to grow in time. As in other examples of linear instability, the growth rate of the disturbance in this phase is exponential in time.

This Miles–Phillips Mechanism process can continue until an equilibrium is reached, or until the wind stops transferring energy to the waves (i.e., blowing them along) or when they run out of ocean distance, also known as fetch length.

## Rogue waves

When the wavelength of a gravity wave $L$ is less than twice the depth of the ocean or lake its behavior is governed by the nonlinear Schrodinger equation and its speed is proportional to $\sqrt{L}$. This means that different waves can travel at different speeds. Just by chance it can happen that several peaks, traveling at different speeds arrive simultaneously at the location of an unfortunate ship. If the combined height of the different peaks is a little more than twice that of normal storm waves it is considered a rogue wave and will usually sink the ship. This is one way in which rogue waves can arise. By contrast, the more familiar sound and light waves satisfy linear differential equations, so their speed is independent of wavelength. Experiments in large tanks with wave generating machines are used with models of ships to study rogue waves. The wave machines can generate waves of several different wavelengths simultaneously, and when these wavelengths are less than twice the depth of the tank the above phenomenon can be observed.

## Notes

1. ^ Lighthill, James (2001), Waves in fluids, Cambridge University Press, p. 205, ISBN 9780521010450
2. ^ Bromirski, Peter D.; Sergienko, Olga V.; MacAyeal, Douglas R. (2010), "Transoceanic infragravity waves impacting Antarctic ice shelves", Geophysical Research Letters (American Geophysical Union) 37 (L02502), Bibcode 2010GeoRL..3702502B, DOI:10.1029/2009GL041488.
3. ^ Phillips, O. M. (1957), "On the generation of waves by turbulent wind", J. Fluid Mech. 2 (5): 417–445, Bibcode 1957JFM.....2..417P, DOI:10.1017/S0022112057000233
4. ^ Miles, J. W. (1957), "On the generation of surface waves by shear flows", J. Fluid Mech. 3 (2): 185–204, Bibcode 1957JFM.....3..185M, DOI:10.1017/S0022112057000567

## References

• Gill, A. E., "Gravity wave". Atmosphere Ocean Dynamics, Academic Press, 1982.