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List of dimensionless quantities : ウィキペディア英語版
List of dimensionless quantities
A dimensionless quantity is any number or defined expression which is either unitless or has composite dimensions which cancel out to unity. These are found in numerous fields such as mathematics, physics, engineering, and economics. In contrast, quantities containing dimensions include length, time, mass, luminosity, and electric charge, and products of these fundamental quantities such as volume, speed, and voltage.
In mathematics, numbers do not have a dimension; this includes 1, , , , and others. Physics often uses dimensionless quantities to simplify the characterization of systems with multiple interacting physical phenomena. These may be found by applying the Buckingham π theorem or otherwise may emerge from making partial differential equations unitless by the process of nondimensionalization. Furthermore, dimensionless physical constants allow systems of units such as natural units to be constructed based on the known fundamental physical constants. Engineering, economics, and other fields often extend these ideas in design and analysis of the relevant systems.
== List of dimensionless quantities ==

All pure numbers are dimensionless quantities by definition. The list below shows many of the well known dimensionless quantities to illustrate their variety of forms and applications. The table does not include dimensionless units such as percentage (%) and dozen or dimensionless physical constants such as the proton-to-electron mass ratio; these topics are discussed in the article on dimensionless quantities.
|| optics (dispersion in optical materials)
|-
| Activity coefficient || \gamma || \gamma= \frac || chemistry (Proportion of "active" molecules or atoms)
|-
| Albedo || \alpha ||\alpha= (1-D) \bar \alpha(\theta_i) + D \bar|| climatology, astronomy (reflectivity of surfaces or bodies)
|-
| Archimedes number || Ar || \mathrm = \frac|| fluid mechanics (motion of fluids due to density differences)
|-
| Arrhenius number || \alpha || \alpha = \frac || chemistry (ratio of activation energy to thermal energy)〔
|-
| Atomic weight || ''M'' || || chemistry (mass of atom over one atomic mass unit, u, where carbon-12 is exactly 12 u)
|-
| Atwood number || A || \mathrm = \frac || fluid mechanics (onset of instabilities in fluid mixtures due to density differences)
|-
| Bagnold number || Ba ||\mathrm = \frac|| fluid mechanics, geology (ratio of grain collision stresses to viscous fluid stresses in flow of a granular material such as grain and sand)〔(Bagnold number )〕
|-
| Bejan number
(fluid mechanics)|| Be || \mathrm = \frac || fluid mechanics (dimensionless pressure drop along a channel)
|-
| Bejan number
(thermodynamics)|| Be || \mathrm = \frac}+ \dot S'_} || thermodynamics (ratio of heat transfer irreversibility to total irreversibility due to heat transfer and fluid friction)
|-
| Bingham number || Bm ||\mathrm = \frac|| fluid mechanics, rheology (ratio of yield stress to viscous stress)〔
|-
| Biot number || Bi ||\mathrm = \frac|| heat transfer (surface vs. volume conductivity of solids)
|-
| Blake number || Bl or B ||\mathrm = \frac || geology, fluid mechanics, porous media (inertial over viscous forces in fluid flow through porous media)
|-
| Bodenstein number || Bo or Bd || \mathrm = vL/\mathcal = \mathrm\, \mathrm || chemistry (residence-time distribution; similar to the axial mass transfer Peclet number)
|-
| Bond number || Bo ||\mathrm = \frac|| geology, fluid mechanics, porous media (buoyant versus capillary forces, similar to the Eötvös number) 〔(Bond number )〕
|-
| Brinkman number || Br || \mathrm = \frac || heat transfer, fluid mechanics (conduction from a wall to a viscous fluid)
|-
| Brownell–Katz number || NBK || \mathrm_\mathrm = \frac || fluid mechanics (combination of capillary number and Bond number)
|-
| Capillary number || Ca || \mathrm = \frac || porous media, fluid mechanics (viscous forces versus surface tension)
|-
| Chandrasekhar number || Q || \mathrm = \frac = \dfrac\, , || aeronautics, fluid dynamics (resistance to fluid motion)
|-
| Dukhin number || Du || \mathrm = \frac || optics (slit diffraction)〔(Fresnel number )〕
|-
| Froude number || Fr || \mathrm = \frac = \frac || fluid mechanics (gravitational over viscous forces)
|-
| Golden ratio || \varphi || \varphi = \frac \approx 1.61803 || mathematics, aesthetics (long side length of self-similar rectangle)
|-
| Görtler number || G || \mathrm = \frac \left( \frac \right)^ || fluid dynamics (boundary layer flow along a concave wall)
|-
| Graetz number || Gz || \mathrm = \mathrm\, \mathrm || heat transfer, fluid mechanics (laminar flow through a conduit; also used in mass transfer)
|-
| Grashof number || Gr || \mathrm_L = \frac || heat transfer, natural convection (ratio of the buoyancy to viscous force)
|-
| Gravitational coupling constant || \alpha_G ||\alpha_G=\frac|| gravitation (attraction between two massy elementary particles; analogous to the Fine structure constant)
|-
| Hatta number || Ha || \mathrm = \frac}^ || chemical engineering (adsorption enhancement due to chemical reaction)
|-
| Hagen number || Hg || \mathrm = -\frac\frac\frac || heat transfer (ratio of the buoyancy to viscous force in forced convection)
|-
| Hydraulic gradient || ''i'' || i = \frac = \frac = \frac = \frac) }|| chemistry (ratio of sensible to latent energy absorbed during liquid-vapor phase change)
|-
| Karlovitz number || Ka || \mathrm = k t_c || turbulent combustion (characteristic flow time times flame stretch rate)
|-
| Keulegan–Carpenter number || KC || \mathrm = \frac || fluid dynamics (ratio of drag force to inertia for a bluff object in oscillatory fluid flow)
|-
| Knudsen number || Kn || \mathrm = \frac || gas dynamics (ratio of the molecular mean free path length to a representative physical length scale)
|-
| Kt/V || ''Kt''/''V'' || || medicine (hemodialysis and peritoneal dialysis treatment; dimensionless time)
|-
| Kutateladze number || Ku || \mathrm = \frac\right)^} || fluid mechanics (counter-current two-phase flow)
|-
| Laplace number || La || \mathrm = \frac || fluid dynamics (free convection within immiscible fluids; ratio of surface tension to momentum-transport)
|-
| Lewis number || Le || \mathrm = \frac = \frac} || heat and mass transfer (ratio of thermal to mass diffusivity)
|-
| Lift coefficient || ''C''L || C_\mathrm = \frac || aerodynamics (lift available from an airfoil at a given angle of attack)
|-
| Lockhart–Martinelli parameter || \chi || \chi = \frac \sqrt} || two-phase flow (flow of wet gases; liquid fraction)〔(Lockhart–Martinelli parameter )〕
|-
| Love numbers || ''h'', ''k'', ''l'' || || geophysics (solidity of earth and other planets)
|-
| Lundquist number || ''S'' || S = \frac || plasma physics (ratio of a resistive time to an Alfvén wave crossing time in a plasma)
|-
| Mach number || M or Ma || \mathrm = \frac} || gas dynamics (compressible flow; dimensionless velocity)
|-
| Magnetic Reynolds number || Rm || \mathrm_\mathrm = \frac || magnetohydrodynamics (ratio of magnetic advection to magnetic diffusion)
|-
| Manning roughness coefficient || ''n'' || || open channel flow (flow driven by gravity)
|-
| Marangoni number || Mg || \mathrm = - }\frac || fluid mechanics (Marangoni flow; thermal surface tension forces over viscous forces)
|-
| Morton number || Mo || \mathrm = \frac || fluid dynamics (determination of bubble/drop shape)
|-
| Nusselt number || Nu ||\mathrm =\frac || heat transfer (forced convection; ratio of convective to conductive heat transfer)
|-
| Ohnesorge number || Oh || \mathrm = \frac}} = \frac = \mathrm\, \mathrm|| heat transfer (advectiondiffusion problems; total momentum transfer to molecular heat transfer)
|-
| Peel number || ''N''P || N_\mathrm = \frac} || coating (adhesion of microstructures with substrate)
|-
| Perveance || ''K'' || = \frac\,\frac} || elasticity (load in transverse and longitudinal direction)
|-
| Porosity || \phi || \phi = \frac} || geology, porous media (void fraction of the medium)
|-
| Power factor || ''P''/''S'' || || electronics (real power to apparent power)
|-
| Power number || ''N''''p'' || N_p = || electronics (power consumption by agitators; resistance force versus inertia force)
|-
| Prandtl number || Pr ||\mathrm = \frac = \frac|| heat transfer (ratio of viscous diffusion rate over thermal diffusion rate)
|-
| Prater number || ''β'' || \beta = \frac} || reaction engineering (ratio of heat evolution to heat conduction within a catalyst pellet)
|-
| Pressure coefficient || ''CP'' || C_p = \rho_\infty V_\infty^2} || aerodynamics, hydrodynamics (pressure experienced at a point on an airfoil; dimensionless pressure variable)
|-
| Q factor || ''Q'' || Q = 2 \pi f_r \frac} || physics, engineering (damping of oscillator or resonator; energy stored versus energy lost)
|-
| Radian measure || rad ||\text/\text || mathematics (measurement of planar angles, 1 radian = 180/π degrees)
|-
| Rayleigh number || Ra || \mathrm_ = \frac (T_s - T_\infin) x^3 || heat transfer (buoyancy versus viscous forces in free convection)
|-
| Refractive index || ''n'' || n=\frac || electromagnetism, optics (speed of light in a vacuum over speed of light in a material)
|-
| Relative density || ''RD'' || RD = \frac} || hydrometers, material comparisons (ratio of density of a material to a reference material—usually water)
|-
| Relative permeability || \mu_r || \mu_r = \frac || magnetostatics (ratio of the permeability of a specific medium to free space)
|-
| Relative permittivity || \varepsilon_r || \varepsilon_ = \frac} || electrostatics (ratio of capacitance of test capacitor with dielectric material versus vacuum)
|-
| Reynolds number || Re || \mathrm = \frac || fluid mechanics (ratio of fluid inertial and viscous forces)〔(【引用サイトリンク】format=PDF )
|-
| Richardson number || Ri || \mathrm = \frac = \frac || fluid dynamics (effect of buoyancy on flow stability; ratio of potential over kinetic energy)〔(Richardson number )〕
|-
| Rockwell scale || – || || mechanical hardness (indentation hardness of a material)
|-
| Rolling resistance coefficient || ''Crr'' ||C_ = \frac || vehicle dynamics (ratio of force needed for motion of a wheel over the normal force)
|-
| Roshko number || Ro || \mathrm = =\mathrm\,\mathrm || fluid dynamics (oscillating flow, vortex shedding)
|-
| Rossby number || Ro || \mathrm=\frac || geophysics (ratio of inertial to Coriolis force)
|-
| Rouse number || P or Z || \mathrm = \frac || sediment transport (ratio of the sediment fall velocity and the upwards velocity of grain)
|-
| Schmidt number || Sc || \mathrm = \frac || mass transfer (viscous over molecular diffusion rate)〔(Schmidt number )〕
|-
| Shape factor || ''H'' || H = \frac || boundary layer flow (ratio of displacement thickness to momentum thickness)
|-
| Sherwood number || Sh || \mathrm = \frac || mass transfer (forced convection; ratio of convective to diffusive mass transport)
|-
| Shields parameter || \tau_
* or \theta || \tau_ = \frac || sediment transport (threshold of sediment movement due to fluid motion; dimensionless shear stress)
|-
| Sommerfeld number || S || \mathrm = \left( \frac \right)^2 \frac || hydrodynamic lubrication (boundary lubrication)〔(Sommerfeld number )〕
|-
| Specific gravity || ''SG'' || || (same as Relative density)
|-
| Stanton number || St || \mathrm = \frac = \frac\,\mathrm} || heat transfer and fluid dynamics (forced convection)
|-
| Stefan number || Ste ||\mathrm = \frac|| phase change, thermodynamics (ratio of sensible heat to latent heat)
|-
| Stokes number || Stk or Sk ||\mathrm = \frac|| particles suspensions (ratio of characteristic time of particle to time of flow)
|-
| Strain || \epsilon ||\epsilon = \cfrac} - 1|| materials science, elasticity (displacement between particles in the body relative to a reference length)
|-
| Strouhal number || St or Sr ||\mathrm = || fluid dynamics (continuous and pulsating flow; nondimensional frequency)〔(Strouhal number ), Engineering Toolbox〕
|-
| Stuart number || N || \mathrm = \frac = \frac = \frac|| fluid dynamics (rotating fluid flows; inertial forces due to rotation of a fluid versus viscous forces)
|-
| Transmittance || ''T'' || T = \frac|| optics, spectroscopy (the ratio of the intensities of radiation exiting through and incident on a sample)
|-
| Ursell number || U ||\mathrm = \frac|| wave mechanics (nonlinearity of surface gravity waves on a shallow fluid layer)
|-
| Vadasz number || Va ||\mathrm = \frac}|| porous media (governs the effects of porosity \phi, the Prandtl number and the Darcy number on flow in a porous medium)
|-
| van 't Hoff factor || ''i'' || i = 1 + \alpha (n - 1)|| quantitative analysis (''K''f and ''K''b)
|-
| Wallis parameter || ''j''
*
||j^
* = R \left( \frac \right)^\frac|| multiphase flows (nondimensional superficial velocity)
|-
| Weaver flame speed number || Wea ||\mathrm = \frac = \frac|| multiphase flow (strongly curved surfaces; ratio of inertia to surface tension)
|-
| Weissenberg number || Wi ||\mathrm = \dot \lambda || viscoelastic flows (shear rate times the relaxation time)〔(Weissenberg number )〕
|-
| Womersley number || \alpha ||\alpha = R \left( \frac \right)^\frac|| biofluid mechanics (continuous and pulsating flows; ratio of pulsatile flow frequency to viscous effects)〔(Womersley number )〕
|}

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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