Secondary flow

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In fluid mechanics , i.e. the movement of fluids , a secondary flow or cross flow is an additional flow in the plane transverse to the main flow direction at a comparatively low speed .

Together with the main flow , secondary flows lead to spiral or even more complex trajectories and make flow predictions and calculations difficult.

Secondary flows are often triggered by irregularities such as obstacles, friction or changes in a cross-section . Since they cause additional velocities, additional energy is often required for secondary flows and additional forces can arise. Therefore secondary flows are mostly undesirable in technology; In special cases, however, they can also be brought about consciously in order to bring about desirable effects (for example mixing and equalization or stabilization).

Differentiation: first and second kind

  • The secondary flow described in the following simple example as a result of curved streamlines is also called secondary flow of the first type .
  • In the case of turbulence , for example in pipes with a non-circular cross-section, one speaks of secondary flow of the second type .

Simple example: cup

The best known example of a secondary flow is the flow inside a cup: When stirring, the main flow direction is circular within the horizontal plane. In the soil boundary layer, on the other hand, the water particles move inward in spiral paths. So there is a secondary flow in the direction of the center of the cup. This water flowing towards the middle of the cup has to go somewhere; it rises up to the liquid level, from there to the edge of the cup and down the wall of the cup to the bottom. A side effect is that at the moment of ascent, the heavier tea leaves can no longer follow the movement and are deposited in the middle of the cup base.

Complex example: banana flank

Secondary currents are involved in the banana flank . Here the main flow direction is the actual flight direction of the ball. The air particles lying on the surface of the ball, which are "carried away" by the rotation of the ball transversely to the main flow direction, form a secondary flow of the first type and cause the Magnus effect .

There are additional effects because the speed ratios in the different layers of the ball are different (in sketches mostly only the " equatorial plane " is shown; however, the lateral speed decreases up to the " poles " of the ball). They cause a complex, turbulent flow and thus a secondary flow of the second kind. The whole thing is also "chaotic" and unsteady .

literature

  • Heinz Schade, Ewald Kunz: Fluid Mechanics . De Gruyter, Berlin et al. 1980, ISBN 3-11-007921-6 .