Where is L measured from in the formula for the Reynolds Number. I know it is a linear dimension of the system measured in metres but unsure of how to measure it.
L is described as a characteristic dimension, it's function is to indicate the overall scale of the system. For example in circular pipes it would be the pipe diameter and for a wing you would generally use the chord.
Having said that the main function of Reynolds number is comparative, for example comparing a scale model in a wind tunnel to the real thing so it doesn't necessarily matter exactly what you pick as long as it is consistent. But there will be conventions for particular circumstances.
$\begingroup$ Perfect thanks very much for the description. $\endgroup$– ConorApr 29, 2017 at 14:25
Reynolds number is a dimensionless constant used to describe the flow characteristics of a flow system. It is a tool for ccomparative analysis. So the absolute value of Reynolds number does not hold any physical meaning unless the flow conditions are specified. The L is a parameter used to quantify the extent or size of flow.It can be choosen to be any length which gives an idea of the extent of the flow.But once we compute Reynolds number of a flow by choosing a particular length(say, the chord for a wing), it should be made sure that we have to use the same length in computing the Reynolds number of another flow if both are intended to be compared. Hope you got it.