Fluid Dynamics

fluid mechanics banner 4Fluid Dynamics, a branch of physics that deals with the effects of force and energy on fluid properties of liquids and gases.

Fluid Dynamics deals with the ability of fluid movement and the interactions when coming in contact with two or more fluids.

Fluid Statics deals fluid at rest.


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Nomenclature & Symbols



Nomenclature, Symbols, and Units


Fluid Dynamics Glossary


  • ACFM  -  Actual cubic feet per minute is a unit of volumetric flow. It is primarily used by manufacturers of blowers and compressors to measure how much air is being moved across the equipment.
  • Acoustic Flowmeter  -  This type of flowmeter (ultrasonic) can give a continuous measurement of the flow rates of both open channel or pipe.
  • Aeration  -  Aeration is the process where water is subjected to intimate air contacting for the purpose of removing contaminating gasses or to saturate it with dissolved oxygen.
  • Air–fuel Ratio  -  The mass ratio of air to fuel mixture present in an internal combustion engines. Because it is a ratio, it is unitless.
  • Amagat's Law  -  States that the volume of a mixture is equal to the sum of the partial volume of its components.
  • Archimedes Principle  -  The upward bouyancy force that is extended on a body that is completely or partially immersed in a fluid and is equal to the weight of the fluid that the body displaces.
  • Average Force  -  used when the instantaneous velocity is not measured precisely between two points.
  • Average Velocity  -  The average of any given velocities where the acceleration is constant.  The calculation for average velocity with zero accleration is shown below.


  • Barlow's Formula  -  Relates to burst pressure, the point just before or at when a pipe or tubing fails under pressure.

  • Bernoulli's Equation  -  A way of describing the conservation of energy principle in an incompressible fluid.
  • Boyle's Law  -  States that the volume occupied by a given mass of gas varies inversely with the absolute pressure if the temperature remains constant.
  • Brackish  -  A relative term used for water having a total dissolved solids concentration less than brine but more than in potable water.

  • Brake Horsepower  -  The engine's horsepower required to overcome the loss in power caused by the pump.

  • Bulk Modulus  -  The elastic relationship between an applied pressure that acts to change the volume of the substance and the ability of a substance to withstand changes in volume when under compression.

  • Bulk Modulus of Elasticity  -  See Bulk Modulus
  • Buoyancy  -  When a liquid applies upward force on an object that is completely submerged or floating.

  • Buoyancy Mass  -  The amount of matter an object has relative to the density of the liquid.


  • Chezy Coefficient  -  Dependent on surface roughness and the hydraulic mean depth.
  • Chezy Formula  -  Applies to the steady velocity with turbulent flow in a open channel.
  • Compression Ratio  -  The ratio of the maximum volume to the minimum volume in a cylinder.
  • Continuity Equation for Mass  -  This formula states that the mass entering a system is equal to the mass leaves the system both at the same rate.
  • Crirical Flow  -  The variation of specific energy with depth at a constant discharge showing the minimum in the specific energy at a depth called critical depth at which the Froude number has a value of one.
  • Cubic Inch Displacement  -  The size of the engine.  Determine by calculating the engine number of cylinders multiplied cylinder bore area multiplied by the stroke of the crankshaft.


  • Dalton’s Law  -  States that the total pressure of a gas mixture is equal to the sum of the partial pressures of the individual gas components of the mixture.
  • Darcy Friction Factor - Brkić  -  The Brkić approximation is one way of estimating the Darcy friction factor for use in pressure drop calculations.
  • Darcy Velocity  -  The volumetric flow per unit area of a porous mediun.
  • Darcy's Law  -  The law states that this rate is directly proportional to the drop in vertical elevation between two places in the medium and indirectly proportional to the distance between them.
  • Density  -  The ratio of the amount of matter in an object compared to its volume
  • Dew Point  -  The temperature at which air must be cooled to become saturated with water vapor.
  • Dry Gas  -  A natural gas with very little liquid hydrocarbons or water vapor. It consists of almost 100% of methane \((CH4)\).
  • Dynamic Shear Viscosity  -  The resistance to shearing flow of a fluid, where adjacent layers move parallel to each other in different ways.


  • Engine Horsepower  -  The estimate of engine horsepower after a quarter mile run using the trap speed.
  • Evaporation  -  The condition that occurs when heat is absorbed by the surface of a liquid and it changes to vapor.


  • Flow  -  See flow types
  • Flow Rate  -  The amount of fluid that flows in a given time past a specific point.
  • Fluid  -  A substance that deforms and changes position when put under stress.
  • Fluid Oxidation  -  A fluid degradation phenomenon that occurs when it is exposed to oxygen.  This leads to an increase in viscosity and TAN (total acid number) as well as the eventual formation of insoluble solids.
  • Pluid Power  -  Describes hydraulic and pneumatic transmit of power from one location to another.
  • Fluid Pressure  -  Fluid at rest, exerts a force perpendicular to any surface in comes in contact with.
  • Force  -  The push or pull of an object resulting in a change from rest or motion.
  • Friction  -  The mechanical resistance to the relative movement of two surfaces.



  • Hard Water  -  Water with a total hardness of 1 gpg or more as calcium carbonate equivalent.
  • 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.
  • Head Gasket Volume  -  The head gasket volume based on the cylinder bore and the gasket's compressed thickness.
  • Head Loss  -  A pressure loss due to the resistance of the fluid and obstructions along the way in a pipe.

  • Head Pressure  -  The pressure at a specific point to the vertical distance at another specific point.
  • Head Velocity  -  The liquid is allowed to flow from the tank while additional liquid is being added.  The movement of the liquid through the pipe is converted to kinetic energy that is called head velocity.
  • Horsepower  -  A measure of power or the rate of doing work.  It is typically used to describe the work of an engine or machine and is the equivalent of moving 33,000 pounds through the distance of one foot in one minute.
  • Hydraulic Diameter  -  Normally used when the flow is in non-circular pipe or tubes and channels.
  • Hydraulic Efficiency  -  The ratio of hydrodynamic energy in the form of fluid to the amount of mechanical energy delivered to the rotor.
  • Hydraulic Energy  -  Is presurized fliud, static or in motion, used to achieve machine motion.
  • Hydrostatic Weighting  -  You can determine the average density of any object without nead for volume or measuring the mass per unit volume.





  • Laminar Flow  -  Generally happens when dealing with low Reynolds numbers in pipes.  This could be due to low velocities, large diameters or high viscosities.
  • Lift Force  -  An body moving through a fluid (gas or liquid) or air, is the force exerted perpendicular to the direction of travel.


  • Mean Depth  -  The average water depth in a stream channel or conduit computed by dividing the cross-section area by the surface width.
  • Mechanical Efficiency  -  The ratio of theoretical power the pump needs to operate to the actual power delivered to the pump itself.
  • Mole Fraction  -  The number of molecules (moles) of a solution in a mixture divided by the total number of moles present in the solution.
  • Momentum  -  An object is the amount of mass in motion.


  • Natural Gas  -  Has very little liquid hydrocarbons or water vapor. It consists of almost 100% of methane \((CH4)\).
  • Newton's Law of Viscosity  -  States that shear stress between adjacent fluid layers is porportional to the velocity gradients between the two layers.


  • Operating Pressure  -  The operating pressure of equipment occurs when the plant or process is operating at steady state conditions.  


  • Pascal's Law  -  States that the increase in pressure is uniformly applied in all directions in a confined fluid.
  • Piston Deck Volume  -  Calculation with a known bore, piston deck distance, volume of piston bumps and volume of piston depressions.
  • Pressure  -  The force exerted perpendicular to the surface of an object and is expressed as force per unit area.



  • Raw Water  -  Untreated water or any water before it reaches a specific water treatment device or process.
  • Rayleigh–Taylor Instability  -  The instability of an interface between two fluids of different densities which occures when the lighter fluids is pushing the heavier fluid.
  • Reduced Specific Volume  -  A fluid is ratio of the specific volume of a substance's critical pressure and temperature.


  • Saturated Liquid  -  Absorbed as much heat energy as it can without vaporizing.
  • Saturated Vapor  -  A vapor at a temperature of the boiling point on the verge of condensing.
  • Seepage Velocity  -  Represents the actual velocity of a fluid flowing through the void spaces in the soil.
  • Slug Flow  -  Occurs when the speed of the vapor phase pushes the waves from the wavy flow regime onto each other.
  • Soft Water  -  Any water which contains less than 1.0 gpg (17.1 mg/l) of hardness minerals, expressed as calcium carbonate equivalent.

  • Softened Water  -  Any water that is treated to reduce hardness minerals, expressed as calcium carbonate equivalent.
  • Souders-Brown Equation  -  Used to find the maximum allowable vapor velocity in vapor-liquid seperation vessels.

  • Spray Flow  -  Also known as mist or dispersed flow occurs when two-phase flow where the liquid phase is the dispersed phase and exists in the form of many droplets, while the gas phase is the continuous phase.
  • Static Head  -  The vertical distance from a reference point of the liquid to a lower reference point of the liquid.
  • Stokes' Law  -  The force that is put on a small sphere, slowing down the movement through a viscous fluid.
  • Subcooled Liquid  -  If a liquid is not saturated, meaning the liquid is not at its boiling point, it is called subcooled.
  • Surge  -  A temporary and fairly large increase in the voltage or current in an electric circuit or cable.
  • Suspended Solids  -  This figure is the sum of all of the suspended material found in a water sample or process stream.
  • System Head  -  The difference between the total discharge head minus the total suction head.


  • Torricelli's Law  -  The fluid efflux velocity flowing through an orifice under gravity at the bottom of a tank filled to a depth.  This is the same velocity from the orifice as a falling body over the same height due to gravity.
  • Total Acid Number (TAN)  -  Measures the fluid acidity and indication of fluid oxidation.  It is determined by the amount of potassium hydroxide in milligrams that it needs to neutralize the acids in one gram of fluid.
  • Total Discharge Head  -  The total of the friction, pressure, and static discharge.

  • Total Displacement  -  In engine design, the total displacement is the amount of volume displaced by engine cylinders.
  • Total Head  -  The sum of the discharge flange and the sum of the suction flange.
  • Turbidity  -  The cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye.
  • Turbulence  -  Any deviation from parallel flow in a pipe due to rough inner walls obstructions or direction changes.


  • Underwater Pressure  -  The added pressure exerted underwater directly above a spot at the water level.


  • Valve Flow Coefficient Cv/Kv  - The number of US gallons of water at 60°F that will flow throuhg a valve in one minute when the pressure differential across the valve is one pound per square inch (psi).
  • Vapor  -  The gas state of a liquid or solid.  Both vapor and gas are basically the same thing.
  • Vaporization  -  A physical change of a substance from a liquid phase to a gas phase.
  • Vena Contracta  -  The narrowest point in a fluid stream where the diameter of the stream and fluid velocity is at its maximum, resulting in lower pressure at that point.
  • Viscosity  -  The measure of the internal friction/resistance to the flow of a liquid.
  • Viscosity Index  -  A measure of a fluid's sensitivity to change in viscosity with change in temperature.
  • Volumetric Efficiency  -  The calculation for an internal combustion engine.  This is the calculation for the volumetric efficiency for an internal combustion engine.


  • Water Absorption  -  The increase in weight expressed as a percentage of its dry weight after immersion in water for a specified time.
  • Water Hardness  -  The hardness of water is a measure of the constituents in solution which tend to precipitate from the water when heated.
  • Water Quality  -  A term used to describe chemical and/or physical characteristics of a water stream or source as they apply to its ultimate use.
  • Water Resistance  -  Capable of withstanding limited exposure to water.
  • Water Softening  -  Water Softening is the process that reduces the ions (cations) that cause water hardness.
  • Water Vapor Diffusion  -  The process by which water vapor spreads or moves through permeable materials caused by a difference in water vapor pressure.
  • 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. 





Display #
Absolute Pressure
Acoustic Flowmeter
Actual Vapor Pressure