Secondary flow

Secondary flow is a term used in fluid dynamics so the movement, for example, air or water.

It refers to an additional flow comparatively low speed in the plane transverse to the main flow direction.

Together with the easily imaginable mainstream lead to secondary flows helical or complex railway lines and make fluidic predictions and calculations to a difficult matter.

Are triggered secondary flows often by inequalities as barriers, friction, or the change of a cross section. Because they cause additional speeds, additional energy is required for secondary flows and often there may be additional forces. Therefore, secondary flows in the art are usually undesirable; in special cases, but they can also be induced consciously to bring about desirable effects (eg, mixing and equalization or stabilization) - even if it is a " banana edge" ( see below).

Simple example: teacup

The best-known example of a secondary flow is the flow inside a teacup: When stirring, the main flow direction is circular in the horizontal plane. In the bottom boundary layer, the water particles move in spiral paths but inward. There is therefore a secondary flow toward the center of the teacup in front of. This cup of middle -flowing water has to go somewhere; it rises to the liquid level, from there to the cup rim and the wall of the cup down to the ground. A side effect is that at the moment of ascension up the heavier leaves the movement can no longer follow and be deposited in the middle of the cup bottom.

Distinguishing features: first and second type

• This secondary flow due to curved streamlines is also called secondary flow of the first kind.
• Occur, for example in tubes of non- circular cross-section to turbulence; in this cause is called secondary flow of the second kind

Complex example: Bananenflanke

Secondary currents are involved in the Bananenflanke. Here the main flow direction is the actual direction of flight of the ball. The lying on the surface of the ball air particles that are "carried along " by the rotation of the ball perpendicular to the main flow direction, form a secondary flow of the first kind and are responsible for the Magnus effect. Other effects are added, because the speed ratios are different in the different layers of the ball ( in sketches is usually only the " equatorial plane " shown; way up to the "poles " of the ball, however, is a decreasing transverse velocity before ). They bring a complex and turbulent flow anyway as secondary flow of the second kind the whole is also "chaotic" and unsteady.