Characteristic length
A characteristic length is intended to characterize the extent of a generally three-dimensional geometry and has the dimension of a length. Characteristic lengths play a role in the formulation of various dimensionless key figures , especially in fluid mechanics , for example with the Reynolds or Nusselt number .
In the similarity theory , geometrically similar geometries, that is to say scaled up or downsized, are assumed. In the case of geometrical similarities, the specification of a defined dimension is sufficient to describe the dimensions of the system under consideration (mostly a body ). The characteristic length is defined (or to be defined) for the respective geometry and the problem under consideration.
A meaningfully defined characteristic length can be:
- for a pipe flow the inside diameter
- for the flow around a wire or round rod the (outer) diameter
- for hydrofoils the length (depth) of the profile .
For the model of the ideally stirred container in heat transfer theory , the ratio of volume to surface is used as the characteristic length (= reciprocal of the A / V ratio ):
