A distinction is made between the flow velocities of the individual particles and the mean flow velocity over a line, area or volume element or time interval.
The flow speed of water is the average speed at which the water moves downstream and is on the order of 1 meter per second. In contrast, that of groundwater is millimeters per second or centimeters per day, ie a few orders of magnitude lower.
- : Flow
- the point is the time derivative in physical notation
The flow velocity vectors carry one time line over into the next.
Flow velocity in the potential field
In the potential field , the flow velocity follows the equation of motion
The specific energy equation applies :
Flow velocity in the Newtonian fluid
Examples of formulas in more specific application areas are:
- The Bernoulli equation for smooth liquids and gases:
- Bernoulli's general law for frictionless, eddy-free , stationary flows
- The Bernoulli law at constant density
- Bernoulli equation with acceleration term for non-stationary flow:
- Bernoulli equation of relative motion
- Bernoulli equation at higher speeds (about> 150 m / s):
Typical formulas for measurement methods
- Dynamic pressure in the open air flow:
- Lateral discharge from an open vessel:
- mean flow velocity with a non-constant cross-section
- Velocity at a point on the cross-section as a function of the location , with the direction of flow , so that .
The determination of the flow rate can be done with different techniques. Flow velocity in free air flow:
Flow velocity of currents in pipelines :
General measurement methods:
- Fluid Mechanics section . In: Karl-Heinrich Grote, Jörg Feldhusen (Hrsg.): Dubbel - pocket book for mechanical engineering . div.