Dimensionless numbers, abbreviated as DN, are nondimensional quantity of dimension one and can not be measured on a scale of physical units.  All quantities of dimension one are said to have the SI coherent derived unit "one" with the symbol "1".  It is a pure number, thus always having a dimension of 1.  The number does not change even if the number system you are working in does.  Dimensionless numbers or quantities are used in many disiplines such as chemistry, economics, engineering, mathematics, and physics.

Dimensionless numbers Glossary

• A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

A

• Abbe number  -  The measure of material's dispersion (variation of refractive index versus wavelength) with high values of V indicating dispersion.  It is used to classify glass and other optically transparent materials.

• Aeration number  -  Is used for the agitated mixing of gas and liquids.

• Archimedes number  -  Analyzes flow as it relates to a system of density differences.  It is used when dealing with gravitational settling of particles in fluid.

• Arrhenius equation  -  Where the temperature dependance of the reaction rate constant which is the rate of chemical reaction.

• Atomic number  -  Equal to the number of protons in an atom's nucleus. The atomic number determines which element an atom is.

• Atwood number  -  Describes density difference between two adjacent fluids with a common interface.

B

• Bagnold number  -  The ratio of grain collision stresses to various fluid stresses in a granular flow with interstitial Newtonian fluid.

• Bejan number - fluid mechanics  -  The pressure drop along a length of channel.

• Bejan number - heat transfer  -  The pressure drop along a channel of length.

• Bejan number - mass transfer  -  The pressure drop along a channel of length.

• Bingham number  -  The ratio of yield stress to visvous stress.

• Biot number  -  The ratio of internal thermal resistance of solid to fluid thermal resistance.  This is used for heat transfer between fluids and solids.

• Blake number  -  Is proportional to inertial force and viscous force and is used in momentum transfer and flow through beds of solids.

• Bodenstein number  -  Used in mass transfer and diffusion in reactors calculations in particular.

• Bond number  -  Represents the relationship of gravitational force to surface tension force.

• Buckling coefficient  -  Is an instability that leads to a failure mode.

C

• Capillary number  -  Representing the relative effect of viscous forces against the surface tension between a liquid/gas or liquid/liquid interface.

• Carnot efficiency  -  The theroetical maximum efficiency of any heat engine depending only on the temperatures it operates between.

• Cauchy number  -  The ratio of inertial force to compressibility force in a flow.  When the compressibility is important the elastic forces must be considered along with inertial forces.

• Cavitation number  -  Expresses the relationship between the difference of a local absolute pressure from the vapor pressure and the kinetic energy per volume.

• Chezy coefficient  -  Dependent on surface roughness and the hydraulic mean depth.

• Coefficient of discharge  -  See discharge coefficient

• Coefficient of friction  -  See friction coefficient

• Coefficient of kinetic friction  -  See kinetic friction coefficient

• Compound machines efficiency  -  A simple machine is a mechanical device that changes the direction or magnitude of a force.  A compound machine is machines connected in series.

• Compression ratio  -  The ratio of the maximum volume to the minimum volume in a cylinder.

• Courant number  -  The measure of how much information traverses a computational grid cell in a given time interval.

D

• Damping ratio  -  Describes how oscillations in a system decay after a disturbance.

• Darcy friction factor  -  Formulas are equations that help calculate the friction losses in pipe flow and open channel flow.  This coefficient is used in the Darcy-Weisbach equation.

• Darcy friction factor - Brkić  -  Equations that allow you to calculate the friction losses in pipe flow and open channel flow.

• Darcy friction factor - free surface flow  -  Equations that allow you to calculate the friction losses in pipe flow and open channel flow.

• Darcy friction factor - laminar flow  -  Equations that allow you to calculate the friction losses in pipe flow and open channel flow.

• Darcy-Weisbach friction factor  -  See Darcy friction factor

• Darcy-Weisbach equation  -  The most common way of expressing the pressure drop of a piped fluid.  The equation is valid for fully developed, steady state and incompressible flow.

• Dean number  -  Used in momentum transfer for the flow in curved pipes and channels.

• Deborah number  -  The ratio of fluidity of materials under specific flow conditions, primarly used in rheology.

• Degree of saturation  -  The ratio of volume of water to the volume of voids.

• Discharge coefficient  -  The ratio of actual discharge to the theoretical discharge.

• Drag coefficient  -  Used to quantify the amount of force or resistance an object experienced as it moves through the fluid stream.

E

• Eckert number  -  Used to calculate the ratio of the kinetic energy to enthalpy.

• Efficiency  -  Expressed in percentage and always less than 100%.

• Elasticity number  -  The ratio of elastic force to internal force in viscoelastic flow.

• Energy efficiency rating  -  Measure the efficiency with which a product uses energy to function.  It is calculated by dividing a product's BTU output by its wattage.

• Ericksen number  -  The ratio of the viscous to elastic forces.

• Euler number  -  Used for analyzing flow where the differential pressure between two points is important.

F

• Factor of safety  -  The ability of a system's structural capacity to be usable beyond it's expected or acrual loads.

• Fan efficiency  -  The ratio of input and output power.

• Fanning friction factor  -  The ratio of shear stress acting on the surface of a solid.

• Flow coefficient  -  Described as the volume (in US gallons) of water at 60°F that will flow per minute through a valve with a pressure drop of 1 psi across the valve. 

• Fourier modulus  -  See Fourier number

• Fourier number  -  The ratio of heat conduction rate to the rate of thermal energy storage in a solid.

• Friction coefficient  -  The ratio between two contacting surfaces and the frictional force that resists the normal force of the object.

• Friction factor  -  Used in internal flow calculations with the Darcy-Weisbach equation.  Depending on the Reynolds Number, the friction factor may be calculated one of several ways.

• Friction loss  -  How much loss of flow through a pipe is due to the viscosity, the measure of the internal friction/resistance to the flow of a liquid near the surface of the pipe.

• Froude number  -  The ratio of inertial force to gravitational force.  It is used for wave and surface behavior for mixed natural and forced convection.

G

• Galilei number  -  See Galileo number

• Galileo number  -  Used in fluid dynamics to describe fluid film flow over walls.  It is a ratio of forces present in the flow of viscous fluids.

• Glide ratio  -  Heavier-than-air flight without the use of thrust.

• Golden ratio (divine porportion, golden mean, golden section)  -  A mathematical ratio commonly found in nature and design.

• Graetz number  -  Characterizes laminar flow with heat transfer in a conduit.  This number is used to determine the thermal development on the entrance to ducts.

• Grashof number  -  The ratio of buoyant to viscous forces.

H

• Hagen number  -  Used in forced flow calculations.  The equation is similar to the Grashof Number but for forced flow rather than natural convection.

• Hartmann number  -  The ratio of electromagnetic force to the viscous.

• Hazen-Williams coefficient  -  Used in the Hazen-Williams Equation.  The lower the coefficient, the smoother the pipe is.  The higher the coefficient, the less fluid flow is restricted.

• Hazen-Williams friction coefficient  -  See Hazen-Williams coefficient

• Heat capacity ratio  -  See specific heat ratio

• Hedstrom number  -  A dimensionless quantity used in fluid dynamics.

• Hydraulic gradient  -  The change in height (pressure) to length between any two points.

• Hydraulic efficiency  -  Is the ratio of hydrodynamic energy in the form of fluid to the amount of mechanical energy delivered to the rotor.

I

• Injury frequency rate  -  The number of injuries based on the number of hours worked on an annual basis.

• Injury severity rate  -  How critical or serious the injuries and illnesses sustained in a period of time by using the number of lost days per accident.

J

• Jakob number  -  The ratio of sensible latent heat absorbed or released during liquid vapor phase change.

K

• Kinetic friction coefficient  -  The amount of force that resists motion at a constant velocity.

• Knudsen number  -  Used in fluid dynamics to determine what type of analysis should be used to model an object moving through a fluid.  It is defined as the ratio of the molecular mean free path length to a representative physical length scale.

L

• Laplace number  -  Used in the charcaterization of free surface fluid dynamics.  It is a ratio of surface tension to the momentum transport inside a fluid.

• Lewis number  -  The ratio of thermal diffusivity to mass diffusivity.  It is used to characterize fluid flows where there is simultaneous heat and mass transfer.
  

• Lift coefficient  -  Represents the lift generated by a lifting body to the fluid density around the body, the density of the fluid around the body, velocity of the fulid and an associated reference area.

• Liquid critical pressure ratio factor FF  -  The ratio between pressure in vena contracta and the pressure at the maximum effective choke pressure drop across the valve.

• Liquid index  -  Scaling the natural moisture content of a soil sample to the liquid limit and plastic limit.

• Liquid pressure recovery factor  -  The ratio of pressure drop that occures between the vena contracta and the upstream pressure.

• Loss coefficient  -  Measures the minor loss to the change in velocity due to friction thru pipes, fittings, and valves.

• Lost time injury frequency rate  -  The number of lost time injuries that have occured within a given accounting period, relative to the total number of hours worked in that period.

• Lost time injury incident rate  -  The number of incidents that result in time away from work.

• Lost workday incident rate  -  The total number of working days lost within a workplace due to occupational injury or illness.

M

• Mach number  -  The ratio of the velocity of flow to the velocity of sound.  The speed of sound in this equation is dependent on the density of the medium that the sound is traveling through.

• Machinability index  -  Used to compare the machinability of different materials in the various cutting process.

• Manning's roughness coefficient  -  Measures the frictional resistance exerted by a channel, culvert or pipe on the flow.

• Margin of safety  -  The ratio of the system's structural capacity to the requirements or how much excess capacity.

• Margin of safety for a failure load  -  The ratio of the system's structural capacity to the requirements or how much excess capacity.

• Mechanical efficiency  -  The ratio of the work output to work inpit.

• Mole fraction  -  The number of molecules (moles) of a solution in a mixture divided by the total number of moles present in the solution.

• Morton number  -  The shape of bubbles or drops moving in a surrounding fluid or continuous phase.

• Motor efficiency  -  The ratio of shaft power power out and electric power input of a motor.

N

• Nusselt number  -  Describes the ratio of convective heat transfer to conductive heat across a boundary.

O

• Ohnesorge number  -  Relates the viscous force to inertial forces and surface tension forces.

• Overall efficiency  -  Measures the ratio of the output of actual power to the input of actual power.  It determines the amount of energy lost overall.

P

• Péclet number  -  Defined as a ratio of heat transport by convection to heat transport by conduction.

• Pi  -  Archimedes' constant number that never ends and never repeats. The ratio of the circumference of a circle to the diameter.

• Pigging efficiency  -  Expressed in percentage and always less than 100%.

• Pipeline parameter  -  Proportional to maximum water hammer pressure rise and static pressure.

• Piping geometry factor  -  The pressure and velocity changes caused by fittings such as bends, expanders, reducers, tees, and Y's if directly conected to the valve.

• Porosity  -  The ratio between the pore volume of a substance and its total volume.  Porosity of a measure of a rocks capacity to store fluids.

• Power number  -  Represents the proportional drag force and inertial force. It is used in momentum transfer and power consumption by fans, mixers and pump impellers at a specified rotational speed.

• Prandtl number  -  In fluid dynamics is used to calculate force by the ratio of momentum diffusivity (kinematic viscosity) and thermal diffusivities.

• Pressure coefficient  -  The relative pressures throughout a flow field in fluid dynamics.

• Pressure gradient  -  Describing which direction and at what rate the pressure changes the most at a specific location.

• Pump efficiency  -  The ratio of total horsepower output to shaft horsepower input of the pump or power output to power input.

Q

R

• Ratio of specific heat  -  See specific heat ratio

• Rayleigh number  -  Associated with free or natural convection.  It is a modified Grashof number used for natural convection calculations.

• Relative density  -  The density or ratio of any substance relative to another substance.

• Relative humidity  -  A measure of the amount of moisture in the air with respect to the temperature.

• Relative roughness  -  Of a pipe is a ratio of the surface roughness to the diameter of the pipe.

• Resistance coefficient  -  How much resistance to the flow an obstacle has.

• Reynolds number  -  Measures the ratio of inertial forces to viscosity forces.

• Richardson number  -  The ratio of the buoyancy term to the flow shear term.

• Roshko number  -  Describing oscillating flow mechanisms.

• Rossby number  -  Used in describing fluid flow.  It is the ratio of internal to Coriolis force.

• Rouse number  -  Used in fluid dynamics showing how a concentration of suspended sediment will be transported in a flowing fluid.

• Runoff curve number  -  The water flow that occurs when the soil is infiltrated to full capacity and excess water or rain.

S

• Safety factor  -  See factor of safety

• Schmidt number  -  Used in analyzing fluid flows where there is an interface between two different fluids. 

• Shape factor  -  Used in structural it is the strength of the shape no matter the scale for a given load, bending, torsion, or twisting.

• Sherwood number  -  The ratio between convective mass transfer and diffusive mass transfer.

• Skin friction coefficient  -  The ratio of shear stress and dynamic pressure of a free stream.

• Sommerfield number  -  Used extensively in hydrodynamic lubrication analysis.

• Specific gravity  -  The density or ratio of any substance to another substance.

• Specific gravity of gas  -  The ratio of the density of the gas to the density of air at a standard pressure and temperature.

• Specific gravity of soil  -  The mass of solids in the soil compared to the mass of water at the same volume.

• Specific heat ratio  -  The ratio of two specific heats or the ratio of the heat capacity at constant pressure to heat capacity at constant volume.

• Stanton number  -  The heat transfer into a fluid to the thermal capacity of fluid.

• Static efficiency  -  A measure of an air mover's efficiency based on its air horsepower in terms of flow and static pressure vs. required shaft input power.

• Static friction coefficient  -  The amount of force that resists motion that is on the verge of motion.

• Strouhal number  -  Used to describe oscillating flow.  It is used in fluid dynamics and is the ratio of inertial forces due to the unsteadiness of the flow or acceleration of inertial forces due to the changes in velocity between points in a flow field.

• Suction head coefficient  -  The angular velocity of the turbomachine shaft and the turbomachine impeller diameter.

• Suratman number  -  See Laplace number

T

• Tayor number  -  The quantity that characterizes the importance of centrifugal forces due to the rotation of a fluid about an axis, relative to viscous forces.

• Thermal efficiency  -  The fraction of heat that is converted to work or desired output divided by required input.

• Total case incident rate  -  The number of work-related injuries per 100 full-time workers during a one year period.

• Total pump efficency  -  The conversion of one form of energy into another form of energy.

U

• Ursell number  -  Indicates the nonlinearity of long surface gravity waves on a fluid layer.

V

• Vadasz number  -  Governs the effect of porosity on flow in a porous media.

• Valve coefficient  -  See flow coefficient

• Valve flow coefficient  -  See flow coefficient

• Void ratio  -  The ratio of the volume of voids to the volume of solids.

• Volumetric efficiency  -  The more air inside the combustion chamber, the more fuel that can be burned and the higher the output engine torque and power.

W

• Water content  -  The ratio of weight of soil to the weight of water or the quantity of water contained in a material.

• Weber number  -  Used in fluid mechanics is often useful in analysing fluid flows where there is an interface between two different fluids, especially for multiphase flows with strongly curved surfaces.

• Womersley number  -  The pulsatile flow frequency in relation to viscous effects.

X

Y

Z

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