Fluid Dynamics

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Fluid Dynamics is the study of the motion and behavior of fluids, both liquids and gases.  It is a branch of physics that focuses on understanding the fundamental principles that govern the movement of fluids and how they interact with their surroundings.

Fluid Dynamics Index

Fluid dynamics has many practical applications in engineering, including the design of aircraft, ships, and automobiles, the design of fluid delivery systems, and the development of energy efficient technologies.  It is also important in a wide range of scientific fields, including meteorology, oceanography, and geology, as well as in the study of the behavior of fluids in the human body.


classification of fluids 1Fluid Classification

  • Real Fluid  -  A fluid that possesses viscosity.  All fluids are considered to be real fluids.
  • Ideal Fluid  -  It is incompressible and has no viscosity.  Ideal fluid is only an imaginary fluid, as all the fluids known have some viscosity.
  • Ideal Plastic Fluid  -  A fluid in which the shear stress is more than the yield value and shear stress is proportional to the rate of shear strain or velocity gradient.
  • Newtonian Fluid  -  A real fluid in which the shear stress is directly proportional to the rate of shear strain or velocity gradient.
  • Non Newtonian Fluid  -  A real fluid in which the shear stress is not directly proportional to the rate of shear strain or velocity gradient.
  • Compressible Fluid  -  A fluid in which the density of fluid changes with the change in external force or pressure.  All gases are considered to be compressible.
  • Incompressible Fluid  -  A fluid in which the density of a fluid does not change with the change in external force or pressure.  All liquids are considered to be incompressible.


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Fluid Dynamics

 Fluid Dynamics Glossary


  • Absolute Pressure  -  A pressure at absolute zero can only exist in a total vacuum and any pressure above this is called absolute pressure.
  • 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.
  • Actual Vapor Pressure  -  The measurement to the amount of water vapor that is in a volume of air.
  • 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 Flow Rate  -
  • Air Flow Rate through Piping  -
  • Air Pipe Sizing  -
  • Air Pressure Loss through Piping  -
  • Air Velocity through Piping  -
  • 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.
  • Avogadro's Law  -  When temperature and pressure are held constant, the volume of a gas is proportional to the number of moles of gas present.


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  • 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.

  • Borda-Carnot Equation  -  A empirical description of the mechanical loss energy losses of the fluid due to a sudden flow expansion.

  • 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 and Compressibility Relationship  -
  • Bulk Modulus of Elasticity  -  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.
  • 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.
  • Coriolis Force  -  Experienced by an object traversing a path in a rotating frame of reference.
  • 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 rate 6 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.
  • Fluid 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.
  • velocity headHead 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.






  • roughness coefficient 2Manning's Roughness Coefficient  -  Measures the frictional resistance exerted by a channel, culvert or pipe on the flow.
  • 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. 





  • 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.
  • strainer 1Strainer  -  A closed vessel that collect solid particles to be separated while passing a fluid through a removable screen.
  • 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.


  • torricellis law 1Torricelli'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.
  • Torricelli's Principle  -  See Torricelli's Law
  • Torricelli's Theorem  -  See Torricelli's Law
  • 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 Dissolved Solids  -  The matter that remains in water which remains in solution.
  • Total Head  -  The sum of the discharge flange and the sum of the suction flange.
  • Total Pump Efficency  -  The conversion of one form of energy into another form of energy.
  • Total Suction Head  -  The total of the friction, pressure, and static discharge.
  • 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.







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Display #
Brake Horsepower
Bulk Modulus
Bulk Modulus and Compressibility Relationship
Buoyancy Mass

Tags: Physics