Glossary
Glossary of Terms for Fluid Dynamics
Add as you feel necessary. When needed, provide a link to a reference page or other terms.
Purpose: Many of the terms on this list have multiple definitions depending on context. The context for these definitions is geophysical and environmental fluid dynamics.
Disclaimer: this list is mostly the result of googling, and as such should not be referenced directly.
Note: The AMS Glossary is a good source for definitions, should the definition that you seek not be available below.
A-D
- Barotropic fluid
- A fluid in which . This means that surfaces of constant pressure and constant density are parallel. Fluids in which or the density is constant () are necessarily barotropic.
- Baroclinic motion
- Motion caused by the misalignment of the surfaces of constant pressure and constant density (i.e. ).
- Boundary Layer
- Region near a surface in which friction becomes important.
- - plane
- The - plane approximation assumes that the Coriolis frequency varies linearly with latitude i.e. . and where is the period of Earth's rotation, is the reference latitude, and is the mean radius of the Earth. Wikipedia's entry on this is a good one. cf. -plane
- Capillary Wave
- Waves in which the dominant restoring force is due to surface tension. Typical length scales are under 7cm (Kundu, 4th ed.).
- Chaotic Advection
- The advection of particles under a chaotic flow map or dynamical system.
- Characteristic Scale
- This scale is context dependent. In an engineering situation like a jet out of a small hole one scale is given by the size of the hole, and another, less easily quantifiable scale will be the length over which the jet mixes with the ambient fluid.
- Circulation
- The circulation of a flow is the integral of the vorticity over a surface, , and represents the vorticity flux through the surface . By Stokes' theorem, .
- Correlation Time
- The time it takes for the auto correlation function of a process to decrease by a given amount.
- Direct Numerical Simulation (DNS)
- Simulation in which you make no assumption on turbulence, and typically attempt to resolve as much as possible. cf. LES
- Dispersion Relation
- For a given wave, the dispersion relation is the relationship between wavenumber, , and (angular) wave frequency, ; typically written as . See also Phase Velocity, Group Velocity, and Dispersive Waves.
- Dispersive Wave
- A wave for which the group velocity does not equal the phase velocity, so .
E-J
- Energy Cascade
- When the coherent structures of the continuum move to smaller and smaller scales until viscosity causes dissipation.
- Enstrophy
- , the norm squared of the vorticity.
- - plane
- The - plane approximation assumes that the Coriolis frequency is constant in latitude i.e. . Where , is the period of Earth's rotation, and is the reference latitude. cf. -plane
- Internal Solitary Wave
- A single, internal, travelling wave, often located at a pycnocline.
- Internal Tide
- Internal Waves generated at a tidal frequency. See Internal Wave.
- Internal Wave
- Waves in which the displaced quantity is an isopycnal. These can include gravity waves and Rossby waves.
- Isentropic Surface
- A surface of constant entropy.
- Gravity Wave
- A wave in which the dominant restoring force is due to gravity acting to restore displaced mass.
- Group Velocity
- The velocity with which the energy of a wave packet propagates. Mathematically, the group velocity , where is the wavenumber. The group velocity is closely related to the velocity of a wave envelope .
- Gyre
- A vortex, a region dominated by a coherent rotating structure.
- Halocline
- Region with a high gradient in salinity. See also pycnocline and thermocline
K-Q
- Large Scale Flow
- In geophysical fluid dynamics this refers to the flow dominated by the Earth's rotation, so almost geostrophic flow.
- Large Eddy Simulation
- A simulation in which a turbulence model has been included to approximate small scale motion and thus reduce the complexity of the problem.
- Meridional
- Along a north-south direction; or along a meridian.
- Nepheloid Layer
- A layer of water with a high concentration of suspended sediment which occurs near the deep ocean bottom, close to the base of the continental slope.
- Normal Mode
- For a linear PDE, the normal modes are the functions which describe the spatial structure of the standing waves that solve that PDE. We can then approximate any wave that solves the PDE, including non-standing waves, by using the normal modes as a basis.
- Particle Image Velocimetry (PIV)
- A non-invasive method used to measure whole velocity fields by taking two images shortly after each other, and calculating the distance individual tracer particles have travelled within the time interval. From the known time interval and the measured displacement, the velocity is calculated.
- Phase Velocity
- The velocity at which a wave crest or trough propagates. Mathematically, where is the phase velocity, is the magnitude of the wavenumber, and is the unit vector corresponding to . cf. group velocity.
- Poincaré Wave
- Gravity waves that are slow/large enough to feel the effects of rotation, but for which gravity remains the dominant restoring force.
- Pycnocline
- Region with a high gradient in density. See also halocline and thermocline.
R-Z
- Rossby Wave
- Waves in which the dominant restoring force is to due the conservation of potential vorticity.
- Soliton
- a single wave crest or trough that propagates at a constant speed, without changing its shape, even after interacting with other waves.
- Stratification
- The way in which a fluids density varies with depth.
- Surface Wave
- Waves in which the displaced quantity is a water-air interface. These can include gravity waves, Rossby waves, and capillary waves.
- Thermocline
- Region with a high gradient in temperature. See also halocline and pycnocline
- Thermohaline Flow
- Flow in the ocean due to density gradients which are caused by surface heat (thermal fluxes) and freshwater or saline fluxes.
- Wave Envelope
- The curve which outlines the extremes of a Wave Packet. See Wave Packet
- Wave Packet
- Composition of waves in a localized region acting in matrimony.
- Zonal
- Along an east-west direction; or along a latitude circle.