Flow velocity
The flow velocity , also flow velocity or flow velocity , is the velocity in a flow , a directed movement of particles or continuous bodies ( fluids ).
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.
definition
The flow velocity is the change in location of the individual point (location) along its path .

 : Flow
 the point is the time derivative in physical notation
The flow velocity vectors carry one time line over into the next.
Average flow velocities can be determined using a streamline , the flow crosssection or the flow rate ( volume flow element , mass flow ).
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
The flow velocity in a field of a flow in a Newtonian fluid is calculated from the NavierStokes equations , in their general formulation:
 : Density of the fluid
 : Time
 : Pressure
 : 1. Lamé constant
 : 2. Lamé constant, dynamic viscosity
 : Partial derivative with respect to time
 : Nabla operator
For this basic equation of fluid mechanics , a system of nonlinear partial differential equations of the 2nd order, there are numerous simplifications, special cases and numerical approaches.
NonNewtonian fluids such as blood, glycerine or dough, which show a nonproportional, erratic flow behavior (see rheology ), behave differently .
Application formulas
Examples of formulas in more specific application areas are:
 The Bernoulli equation for smooth liquids and gases:
 Bernoulli's general law for frictionless, eddyfree , stationary flows
 The Bernoulli law at constant density
 Bernoulli equation with acceleration term for nonstationary flow:

Bernoulli equation of relative motion

 : Circumferential speed  application for blade rings

 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:
Medium speeds
 mean flow velocity with a nonconstant crosssection

 Velocity at a point on the crosssection as a function of the location , with the direction of flow , so that .

The flow velocity has an influence on the velocitydependent parameters Reynolds number and Froude number .
Measurement methods
The determination of the flow rate can be done with different techniques. Flow velocity in free air flow:
 Pitot tube
 Prantl's pitot tube
 Pressure probe (measurement of static pressure)
Flow velocity of currents in pipelines :
Flow velocity of bodies of water and currents in open channels :
General measurement methods:
literature
 Fluid Mechanics section . In: KarlHeinrich Grote, Jörg Feldhusen (Hrsg.): Dubbel  pocket book for mechanical engineering . div.