Written by Jerry Ratzlaff on . Posted in Fluid Dynamics

Fluidfluid banner

A fluid is a substance that deforms and changes position when put under stress.  Fluids can be both liquids and gases.  All liquids share certain characteristics, these can be used to distinguish between a liquid or a gas.

  • Compressibility - Liquids are only slightly compressable, but for most purposes they are non compressable.  Gasses are highly compressable.
  • Molecular spacing -Liquids molequles are held quite close togeather and have a strong attraction force.  The travel distance of liquid molecules is quite short between there collisions and they also have low kinetic energy.     
  • Pressure - Pressure on a liquid is the same in all directions from any point in the liquid.  Any fluid pressure on a solid surface is normal to that surface.
  • Resistance to motion - Since liquids are affected by viscosity, there is resistance to the change in velocity.  Gasses have a very low viscosity.
  • Shape and volume - Since liquids and gasses do not support shear, they take on the shape of the container or surface of the object.  The volume of a liquid is not significantly affected by pressure or temperature and has a fixed volume no matter the size of the container.  Gases also take on the shape of the container or surface of the object but the volume of a gas can change by pressure or temperature.
  • Shear resistance - Liquids and gasses deform continuously with little force which does not support shear.

Types of Fluids

  • Fluids
    • Ideal Fluids - This assumes to have no viscosity, meaning no resistance or shear, and not compressable.  In an ideal fluids there is uniform velocity distribution when flowing.
    • Real fluids - This fluid does not have uniform velocity distribution, it is compressable, shows finite viscosities and experience friction and turbulance in flow
      • Newtonian fluids - This fluid's viscosity is constant no matter how much shear is applied for a constant temperature.
      • non-Newtonian fluids - This is the opposite of Newtonian fluids and does not following Newton's law of viscosity.  These fluids having variable viscosity when shear is applied.
        • Dilatant (shear thickening) fluids - Viscosity of the fluid increases with increased stress.
        • Pseudoplastic (shear thinning) fluids - The opposite of dilatant fluids. Viscosity of the fluid decreases with increased stress.
        • Rheopectic (time dependent) fluids - Viscosity of the fluid increases with stress over time.
        • Thixotropic (time dependent) fluids - Viscosity of the fluid decreases with stress over time.
        • Bingham (plastics) fluids - The fluid appears solid up to a particular stress and with further increase in stress there will be shear deformation.

Fluid Viscosity

The viscosity ( \(\mu\) (Greek symbol mu) ) of a fluid is the measure of that's fluids resistance to flow when acted upon by an external force such as pressure differential or gravity.  When the temperature is raised most fluids will flow easier.  Fluid viscosity can be refered to as absolute viscosity or dynamic viscosity 

Fluid Velocity

Fluid velocity is how fast the process is traveling in a pipe.  To calculate the velocity of the fluid, use one of the equations below.

To get flow rate in Feet per Second

  • Step 1 - Convert the flow rate to cubic feet per second.  This is done by multiplying the flow rate by one of the values shown on the Cubic Feet per Second page.
  • Step 2 - divide by the flow area.  The information for flow area can be found on the Carbon Steel Pipe Properties page.  Make sure that the units match and convert the area into square feet.

Fluid Velocity FORMULA

\(v = \frac { Q } { A  }  \)


\(v\) = velocity

\(Q\) = volumetric flow rate

\(A\) = area