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Posted by on Oct 30, 2017 in Tell Me Why |

When Does a Slip Current Occur?

When Does a Slip Current Occur?

A slip current occurs when a steady stream of air tries to resume its even flow after being diverted by an object at right angles to it. The less streamlined the object, the greater the disturbance, or turbulence, of the stream of air. A good example is provided by a bus moving along a road.

The air pushed aside by the bus is turned back to fill the empty space when the vehicle has passed. Thus air currents are set flowing in the same direction as that being taken by the bus. A sensation of being pulled forward may be felt if you ride a bicycle too close to the rear of a large moving object.

sensation of being pulled forward

Turbulence or turbulent flow is a flow regime in fluid dynamics characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow regime, which occurs when a fluid flows in parallel layers, with no disruption between those layers.

Turbulence is commonly observed in everyday phenomena such as surf, fast flowing rivers, billowing storm clouds, or smoke from a chimney, and most fluid flows occurring in nature and created in engineering applications are turbulent. Turbulence is caused by excessive kinetic energy in parts of a fluid flow, which overcomes the damping effect of the fluid’s viscosity. For this reason turbulence is easier to create in low viscosity fluids, but more difficult in highly viscous fluids.

turbulent flow

In general terms, in turbulent flow, unsteady vortices appear of many sizes which interact with each other, consequently drag due to friction effects increases. This would increase the energy needed to pump fluid through a pipe, for instance. However this effect can also be exploited by such as aerodynamic spoilers on aircraft, which deliberately “spoil” the laminar flow to increase drag and reduce lift.

The onset of turbulence can be predicted by a dimensionless constant called the Reynolds number, which calculates the balance between kinetic energy and viscous damping in a fluid flow. However, turbulence has long resisted detailed physical analysis, and the interaction within turbulence creates a very complex situation. Richard Feynman has described turbulence as the most important unsolved problem of classical physics.

Content for this question contributed by Vincent Majerowicz, resident of Healdsburg, Sonoma County, California, USA