Name  Standard symbol  Definition  Field of application 
Abbe number In physics and optics, the Abbe number, also known as the Vnumber or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index... 
V 

opticsOptics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light... (dispersionIn optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media... in optical materials) 
Activity coefficientAn activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture, the interactions between each pair of chemical species are the same and, as a result, properties of the mixtures can be expressed... 
γ 

chemistryChemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds.... (Proportion of "active" molecules or atoms) 
AlbedoAlbedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it... 


climatologyClimatology is the study of climate, scientifically defined as weather conditions averaged over a period of time, and is a branch of the atmospheric sciences... , astronomyAstronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth... (reflectivityIn optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength... of surfaces or bodies) 
Archimedes number 
Ar 

motion of fluid In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.... s due to densityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight... differences 
Arrhenius number 


Ratio of activation energy In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction... to thermal energyThermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

Atomic weight Atomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon12... 
M 

chemistryChemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

Bagnold number The Bagnold number is the ratio of grain collision stresses to viscous fluid stresses in a granular flow with interstitial Newtonian fluid, first identified by Ralph Alger Bagnold.The Bagnold number is defined by... 
Ba 

flow of bulk solids such as grain GRAIN is a small international nonprofit organisation that works to support small farmers and social movements in their struggles for communitycontrolled and biodiversitybased food systems. Our support takes the form of independent research and analysis, networking at local, regional and... and sandSand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and nontropical coastal... . 
Blowdown circulation number 
BC 

deviation from isothermal flow in blowdown (rapid depressurization) of a pressure vessel 
Bejan number There are two Bejan numbers in use, named after Duke University professor Adrian Bejan in two scientific domains: thermodynamics and fluid mechanics.Thermodynamics:...
(thermodynamics) 
Be 

the ratio of heat transfer irreversibility to total irreversibility due to heat transfer and fluid friction 
Bejan number There are two Bejan numbers in use, named after Duke University professor Adrian Bejan in two scientific domains: thermodynamics and fluid mechanics.Thermodynamics:...
(fluid mechanics) 
Be 

dimensionless pressure drop along a channel 
Bingham number 
Bm 

Ratio of yield stress to viscous stress 
Bingham capillary number 
Bm.Ca 

Ratio of yield stress to capillary pressure 
Biot number The Biot number is a dimensionless number used in nonsteadystate heat transfer calculations. It is named after the French physicist JeanBaptiste Biot , and gives a simple index of the ratio of the heat transfer resistances inside of and at the surface of a body... 
Bi 

surface vs. volume conductivity of solids 
Blake number The Blake number in fluid mechanics is a nondimensional number showing the ratio of inertial force to viscous force.It is used in momentum transfer in general and in particular for flow of a fluid through beds of solids... 
Bl or B 

relative importance of inertia compared to viscous forces in fluid flow through porous media 
Bodenstein number 
Bo 

residencetime Residence time is the average amount of time that a particle spends in a particular system. This measurement varies directly with the amount of substance that is present in the system.... distribution 
Bond number 
Bo 

capillary actionCapillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paintbrush, in a thin tube, or in porous material such as paper or in some nonporous material such as liquified carbon fiber, or in a... driven by buoyancyIn physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the... 
Brinkman number The Brinkman number is a dimensionless number related to heat conduction from a wall to a flowing viscous fluid, commonly used in polymer processing... 
Br 

heat transfer by conduction from the wall to a viscous fluid 
Brownell–Katz number 


combination of capillary number In fluid dynamics, the capillary number represents the relative effect of viscous forces versus surface tension acting across an interface between a liquid and a gas, or between two immiscible liquids... and Bond number 
Capillary number In fluid dynamics, the capillary number represents the relative effect of viscous forces versus surface tension acting across an interface between a liquid and a gas, or between two immiscible liquids... 
Ca 

fluid flow influenced by surface tensionSurface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

Coefficient of static friction



friction of solid bodies at rest 
Coefficient of kinetic friction



friction of solid bodies in translational motion 
Colburn j factor Chilton–Colburn Jfactor analogy is a successful and widely used analogy from heat, momentum, and mass transfer analogies. The basic mechanisms and mathematics of heat, mass, and momentum transport are essentially the same...



dimensionless heat transfer coefficient 
Courant–Friedrich–Levy number



numerical solutions of hyperbolic PDEs 
Damkohler number 
Da 

reaction time scales vs. resonance time 
Damping ratio [[Image:Damped spring.gifrightframeUnderdamped [[spring–mass system]] with ζ 1 , and is referred to as overdamped.*Underdamped:If s is a complex number, then the solution is a decaying exponential combined with an oscillatory portion that looks like \exp... 


the level of damping In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect... in a system 
Darcy friction factor 
or 

fluid flow 
Dean number The Dean number is a dimensionless group in fluid mechanics, which occurs in the study of flow in curved pipes and channels. It is named after the British scientist W. R. Dean, who studied such flows in the 1920s . Definition :... 
D 

vortices in curved ducts 
Deborah number The Deborah number is a dimensionless number, often used in rheology to characterize the fluidity of materials under specific flow conditions. It was originally proposed by Markus Reiner, a professor at Technion in Israel, inspired by a verse in the Bible, stating "The mountains flowed before the... 
De 

rheology Rheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force.... of viscoelastic fluids 
DecibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities... 
dB 

ratio of two intensities, often sound 
Drag coefficient In fluid dynamics, the drag coefficient is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment such as air or water. It is used in the drag equation, where a lower drag coefficient indicates the object will have less aerodynamic or... 


flow resistance 
Dukhin number Dukhin number is a dimensionless quantity that characterizes the contribution of the surface conductivity to various electrokinetic and electroacoustic effects, as well as to electrical conductivity and permittivity of fluid heterogeneous systems.... 
Du 

ratio of electric surface conductivitySurface conductivity is an additional conductivity of an electrolyte in the vicinity of charged surfaces. Close to charged surfaces a layer of counter ions of opposite polarity exists which is attracted by the surface charges. This layer of higher ionic concentration is a part of the interfacial ... to the electric bulk conductivity in heterogeneous systems 
Euler's number 
e 

mathematics Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

Eckert number The Eckert number is a dimentionless number used in fluid dynamics. It expresses the relationship between a flow's kinetic energy and enthalpy, and is used to characterize dissipation. It is named after Ernst R. G... 
Ec 

convective heat transfer 
Ekman number The Ekman number is a dimensionless number used in describing geophysical phenomena in the oceans and atmosphere. It characterises the ratio of viscous forces in a fluid to the fictitious forces arising from planetary rotation... 
Ek 

geophysics Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and... (frictional (viscousViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity... ) forces) 
Elasticity (economics) In economics, elasticity is the measurement of how changing one economic variable affects others. For example:* "If I lower the price of my product, how much more will I sell?"* "If I raise the price, how much less will I sell?"... 
E 

widely used to measure how demand or supply responds to price changes 
Eötvös number In fluid dynamics the Eötvös number is a dimensionless number named after Hungarian physicist Loránd Eötvös . It is also known as the Bond number... 
Eo 

determination of bubble/drop shape 
Ericksen number 
Er 

liquid crystal flow behavior 
Euler number The Euler number is a dimensionless number used in fluid flow calculations. It expresses the relationship between a local pressure drop e.g. over a restriction and the kinetic energy per volume, and is used to characterize losses in the flow, where a perfect frictionless flow corresponds to an...

Eu 

hydrodynamics (pressure forces vs. inertia forces) 
Fanning friction factor 
f 

fluid flow in pipes 
Feigenbaum constants The Feigenbaum constants are two mathematical constants named after the mathematician Mitchell Feigenbaum. Both express ratios in a bifurcation diagram.The first Feigenbaum constant ,... 


chaos theoryChaos theory is a field of study in mathematics, with applications in several disciplines including physics, economics, biology, and philosophy. Chaos theory studies the behavior of dynamical systems that are highly sensitive to initial conditions, an effect which is popularly referred to as the... (period doubling) 
Fine structure constant 


quantum electrodynamics Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved... (QED) 
fnumberIn optics, the fnumber of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the fnumber is the focal length divided by the "effective" aperture diameter... 


opticsOptics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light... , photographyPhotography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a lightsensitive material such as photographic film...

Foppl–von Karman number 


thinshell buckling 
Fourier number In physics and engineering, the Fourier number or Fourier modulus, named after Joseph Fourier, is a dimensionless number that characterizes heat conduction. Conceptually, it is the ratio of the heat conduction rate to the rate of thermal energy storage. Together with the Biot number, it... 
Fo 

heatIn physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between... transfer 
Fresnel number The Fresnel number F, named after the physicist AugustinJean Fresnel, is a dimensionless number occurring in optics, in particular in diffraction theory.... 
F 

slit diffraction Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665... 
Froude numberThe Froude number is a dimensionless number defined as the ratio of a characteristic velocity to a gravitational wave velocity. It may equivalently be defined as the ratio of a body's inertia to gravitational forces. In fluid mechanics, the Froude number is used to determine the resistance of an... 
Fr 

waveIn physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass... and surface behaviour 
Gain In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,... 


electronicsElectronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies... (signal output to signal input) 
Gain Ratio 


system of representing bicycle A bicycle, also known as a bike, pushbike or cycle, is a humanpowered, pedaldriven, singletrack vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist.... gearing 
Galilei number In fluid dynamics, the Galilei number , sometimes also referred to as Galileo number , is a dimensionless number named after Italian scientist Galileo Galilei .... 
Ga 

gravitydriven viscous flow 
Golden ratio In mathematics and the arts, two quantities are in the golden ratio if the ratio of the sum of the quantities to the larger quantity is equal to the ratio of the larger quantity to the smaller one. The golden ratio is an irrational mathematical constant, approximately 1.61803398874989... 


mathematics Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity... and aestheticsAesthetics is a branch of philosophy dealing with the nature of beauty, art, and taste, and with the creation and appreciation of beauty. It is more scientifically defined as the study of sensory or sensoriemotional values, sometimes called judgments of sentiment and taste...

Graetz number 
Gz 

heatIn physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between... flow 
Grashof number 
Gr 

free convectionConvection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

Gravitational coupling constant In physics, the gravitational coupling constant, αG, is the coupling constant characterizing the gravitational attraction between two charged elementary particles having nonzero mass... 


GravitationGravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...

Hatta number 
Ha 

adsorption enhancement due to chemical reaction 
Hagen number The Hagen number is a dimensionless number used in forced flow calculations. It is the forced flow equivalent of the Grashof number and was named after the German hydraulic engineer G. H. L... 
Hg 

forced convection 
Hydraulic gradient 
i 

groundwater Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock... flow 
Jakob Number 
Ja 

Ratio of sensible to latent energy absorbed during liquidvapor phase change 
Karlovitz number 


turbulent combustion turbulent combustion 
Keulegan–Carpenter numberIn fluid dynamics, the Keulegan–Carpenter number, also called the period number, is a dimensionless quantity describing the relative importance of the drag forces over inertia forces for bluff objects in an oscillatory fluid flow. Or similarly, for objects that oscillate in a fluid at rest... 


ratio of drag force to inertia Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to... for a bluff object in oscillatoryOscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes... fluid flow 
Knudsen number The Knudsen number is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid... 
Kn 

ratio of the molecular mean free pathIn physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.Derivation:... length to a representative physical length scale 
Kt/V In medicine, Kt/V is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy.*K  dialyzer clearance of urea*t  dialysis time*V  volume of distribution of urea, approximately equal to patient's total body water... 


medicineMedicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

Kutateladze number 
K 

countercurrent twophase flow 
Laplace number The Laplace number , also known as the Suratman number , is a dimensionless number used in the characterization of free surface fluid dynamics... 
La 

free convection within immiscible Miscibility is the property of liquids to mix in all proportions, forming a homogeneous solution. In principle, the term applies also to other phases , but the main focus is usually on the solubility of one liquid in another... fluids 
Lewis number 
Le 

ratio of mass diffusivity and thermal diffusivity 
Lift coefficientThe lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body... 


liftA fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction... available from an airfoilAn airfoil or aerofoil is the shape of a wing or blade or sail as seen in crosssection.... at a given angle of attackAngle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...

Lockhart–Martinelli parameter



flow of wet gas A wet gas is any gas with a small amount of liquid present. The term "wet gas" has been used to describe a range of conditions ranging from a humid gas which is gas saturated with liquid vapour to a multiphase flow with a 90% volume of gas... es 
Love number The Love numbers h, k, and l are dimensionless parameters that measure the rigidity of a planetary body and the susceptibility of its shape to change in response to a tidal potential.... 


measuring the solidity of the earth 
Lundquist number In plasma physics, the Lundquist number is the dimensionless ratio of an Alfvén wave crossing timescale to a resistive diffusion timescale... 


ratio of a resistive time to an Alfvén wave An Alfvén wave, named after Hannes Alfvén, is a type of magnetohydrodynamic wave.Definition:An Alfvén wave in a plasma is a lowfrequency travelling oscillation of the ions and the magnetic field... crossing time in a plasma 
Mach numberMach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure... 
M 
Ratio of current speed to the speed of sound, i.e. Mach 1 is the speed of sound, Mach 0.5 is half the speed of sound, Mach 2 is twice the speed of sound. 
gas dynamics Gas dynamics is a branch of fluid dynamics concerned with studying the motion of gases and its consequent effects. Gas dynamics combines the principles of fluid mechanics and thermodynamics...

Magnetic Reynolds number The Magnetic Reynolds number is a dimensionless group thatoccurs in magnetohydrodynamics. It gives an estimate of the effects of magnetic advection to magnetic diffusion, and is typically defined by:where* U is a typical velocity scale of the flow... 


magnetohydrodynamics Magnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...

Manning roughness coefficient The Manning formula, known also as the Gauckler–Manning formula, or Gauckler–Manning–Strickler formula in Europe, is an empirical formula for open channel flow, or freesurface flow driven by gravity...

n 

open channel flow The Manning formula, known also as the Gauckler–Manning formula, or Gauckler–Manning–Strickler formula in Europe, is an empirical formula for open channel flow, or freesurface flow driven by gravity... (flow driven by gravity) 
Marangoni number The Marangoni number is a dimensionless number named after Italian scientist Carlo Marangoni.The Marangoni number may be regarded as proportional to surface tension forces divided by viscous forces... 
Mg 

Marangoni flow due to thermal surface tension deviations 
Morton number In fluid dynamics, the Morton number is a dimensionless number used together with the Eötvös number to characterize the shape of bubbles or drops moving in a surrounding fluid or continuous phase, c.The Morton number is defined as... 
Mo 

determination of bubble/drop shape 
Mpemba number 


thermal conduction and diffusion in freezing of a solution 
Nusselt number In heat transfer at a boundary within a fluid, the Nusselt number is the ratio of convective to conductive heat transfer across the boundary. Named after Wilhelm Nusselt, it is a dimensionless number... 
Nu 

heat transferHeat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phasechange transfer... with forced convectionConvection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

Ohnesorge number 
Oh 

atomization of liquids, Marangoni flow 
Péclet number The Péclet number is a dimensionless number relevant in the study of transport phenomena in fluid flows. It is named after the French physicist Jean Claude Eugène Péclet. It is defined to be the ratio of the rate of advection of a physical quantity by the flow to the rate of diffusion of the same... 
Pe 

advection Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries... –diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles... problems; relates total momentun transfer to molecular heat transfer. 
Peel number 


adhesion of microstructures with substrate 
Perveance Perveance is a notion used in the description of charged particle beams. The value of perveance indicates how significant the space charge effect is on the beam’s motion... 
K 

measure of the strength of space charge in a charged particle beam 
Pi' is a mathematical constant that is the ratio of any circle's circumference to its diameter. is approximately equal to 3.14. Many formulae in mathematics, science, and engineering involve , which makes it one of the most important mathematical constants... 


mathematics Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity... (ratio of a circle's circumference to its diameter) 
Poisson's ratioPoisson's ratio , named after Siméon Poisson, is the ratio, when a sample object is stretched, of the contraction or transverse strain , to the extension or axial strain .... 


elasticity In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain.... (load in transverse and longitudinal direction) 
PorosityPorosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%... 


geologyGeology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

Power factor The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time... 


electronicsElectronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies... (real power to apparent power) 
Power number The power number Np is a commonlyused dimensionless number relating the resistance force to the inertia force.The powernumber has different specifications according to the field of application... 


power consumption by agitators 
Prandtl number 
Pr 

convectionConvection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids.... heat transferHeat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phasechange transfer... (thickness of thermal and momentum boundary layers) 
also known as thermicity factor 
ratio of heat evolution to heat conduction within a catalyst pellet 
Pressure coefficient The pressure coefficient is a dimensionless number which describes the relative pressures throughout a flow field in fluid dynamics. The pressure coefficient is used in aerodynamics and hydrodynamics... 


pressure experienced at a point on an airfoil 
Q factorIn physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency.... 


describes how underdamped In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect... an oscillatorOscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes... or resonatorA resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical... is 
Radian Radian is the ratio between the length of an arc and its radius. The radian is the standard unit of angular measure, used in many areas of mathematics. The unit was formerly a SI supplementary unit, but this category was abolished in 1995 and the radian is now considered a SI derived unit... 
rad 

measurement of angles 
Rayleigh number In fluid mechanics, the Rayleigh number for a fluid is a dimensionless number associated with buoyancy driven flow... 
Ra 

buoyancy and viscous forces in free convection 
Refractive indexIn optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium.... 
n 

electromagnetism, optics 
Reynolds number 
Re 

Ratio of fluid inertial and viscous forces 
Relative densityRelative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water... 
RD 

hydrometerA hydrometer is an instrument used to measure the specific gravity of liquids; that is, the ratio of the density of the liquid to the density of water.... s, material comparisons 
Richardson number 
Ri 

effect of buoyancy on flow stability 
Rockwell scale The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different scales, denoted by a single... 


mechanical hardness 
Rolling resistance coefficient 
C_{rr} 

vehicle dynamics Vehicle dynamics refers to the dynamics of vehicles, here assumed to be ground vehicles. Vehicle dynamics is a part of engineering primarily based on classical mechanics but it may also involve chemistry, solid state physics, electrical engineering, communications, psychology, control theory,...

Rossby number The Rossby number, named for CarlGustav Arvid Rossby, is a dimensionless number used in describing fluid flow. The Rossby number is the ratio of inertial to Coriolis force, terms v\cdot\nabla v\sim U^2 / L and \Omega\times v\sim U\Omega in the Navier–Stokes equations, respectively... 


inertial forces in geophysics Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...

Rouse number The Rouse number is a nondimensional number in fluid dynamics which is used to define a concentration profile of suspended sediment and which also determines how sediment will be transported in a flowing fluid... 
Z or P 

sediment transportSediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

Schmidt number Schmidt number is a dimensionless number defined as the ratio of momentum diffusivity and mass diffusivity, and is used to characterize fluid flows in which there are simultaneous momentum and mass diffusion convection processes... 
Sc 

fluid dynamics (mass transfer and diffusion Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles... ) 
Shape factor 
H 

ratio of displacement thickness to momentum thickness in boundary layer flow 
Sherwood number 
Sh 

mass transfer with forced convection 
Shields parameter The Shields parameter, also called the Shields criterion or Shields number, is a nondimensional number used to calculate the initiation of motion of sediment in a fluid flow. It is a nondimensionalization of a shear stress, and is typically denoted \tau_\ast or \theta... 
τ_{∗} or θ 

threshold of sediment movementSediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained... due to fluid motion 
Sommerfeld number In the design of fluid bearings, the Sommerfeld number, or bearing characteristic number, is a dimensionless quantity used extensively in hydrodynamic lubrication analysis... 


boundary lubrication Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed... 
Stanton number The Stanton number, St or CH, is a dimensionless number that measures the ratio of heat transferred into a fluid to the thermal capacity of fluid... 
St 

heat transfer in forced convectionConvection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

Stefan number 
Ste 

heat transfer during phase change 
Stokes number The Stokes number, named after Irish mathematician George Gabriel Stokes, is a dimensionless number corresponding to the behavior of particles suspended in a fluid flow... 
Stk or 

particle dynamics in a fluid stream 
StrainIn continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacementgradient theory, deals with infinitesimal deformations of a continuum body...



materials science Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates... , elasticityIn physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....

Strouhal number In dimensional analysis, the Strouhal number is a dimensionless number describing oscillating flow mechanisms. The parameter is named after Vincenc Strouhal, a Czech physicist who experimented in 1878 with wires experiencing vortex shedding and singing in the wind... 
St or Sr 

nondimensional frequency, continuous and pulsating flow 
Taylor number In fluid dynamics, the Taylor number is a dimensionless quantity that characterizes the importance of centrifugal "forces" or socalled inertial forces due to rotation of a fluid about an axis, relative to viscous forces. The typical context of the Taylor number is in characterization of the... 
Ta 

rotating fluid flows 
Ursell number In fluid dynamics, the Ursell number indicates the nonlinearity of long surface gravity waves on a fluid layer. This dimensionless parameter is named after Fritz Ursell, who discussed its significance in 1953.... 
U 

nonlinearity of surface gravity waves In fluid dynamics, wind waves or, more precisely, windgenerated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Waves in the... on a shallow fluid layer 
Vadasz number 
Va 

governs the effects of porosity , the Prandtl number and the Darcy number on flow in a porous medium 
van 't Hoff factor The van 't Hoff factor i is a measure of the effect of a solute upon colligative properties, such as vapor pressure, osmotic pressure and freezing point depression. The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved, and the... 
i 

quantitative analysis In chemistry, quantitative analysis is the determination of the absolute or relative abundance of one, several or all particular substance present in a sample.... (K_{f}Freezingpoint depression describes the phenomenon in which the freezing point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a nonvolatile solute is added to a pure solvent, such as water... and K_{b}) 
Wallis parameter 
J^{*} 

nondimensional superficial velocity Superficial velocity , in engineering of multiphase flows and flows in porous media, is an hypothetical fluid velocity calculated as if the given phase or fluid were the only one flowing or present in a given cross sectional area. Other phases, particles, the skeleton of the porous medium, etc... in multiphase flows 
Weaver flame speed number 


laminar burning velocity relative to hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly... gas 
Weber number The Weber number is a dimensionless number in fluid mechanics that is often useful in analysing fluid flows where there is an interface between two different fluids, especially for multiphase flows with strongly curved surfaces. It can be thought of as a measure of the relative importance of the... 
We 

multiphase flow with strongly curved surfaces 
Weissenberg number The Weissenberg number is a dimensionless number used in the study of viscoelastic flows. It is named after Karl Weissenberg. The dimensionless number is the ratio of the relaxation time of the fluid and a specific process time... 
Wi 

viscoelastic flows 
Womersley number The Womersley number is a dimensionless number in biofluid mechanics. It is a dimensionless expression of the pulsatile flow frequency in relation to viscous effects. It is named after John R. Womersley . The Womersley number is important in keeping dynamic similarity when scaling an experiment. An... 


continuous and pulsating flows 