Quote:
Originally Posted by Nigel_S
The flow in the pipe should not be turbulent, maybe "laminar" is not the best description for a tube but there is certainly a boundary layer against the edge and the main flow goes down the centre, unless the pipe is curved in which case it will be off centre and you need to worry about keeping the flow attached to the inside of the curve - it is laminar flow.
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Turbulent flow will still have a boundary layer, and will still have the potential flow separation problem, you're right, but it's still turbulent flow - the velocity profile is quite different between laminar flow and turbulent:
Obviously laminar is preferable as the overall rate is higher, so you don't want to muck it up if you have it - but going past the edge of the valve, the valve guide, and taking a ~90 degree turn out of the head ensures that the flow is turbulent to start with.
Even if you were to start with a smooth laminar flow, it will transition to turbulent after a given length of pipe (dependent on the Reynolds Number) anyway (quite soon, for the flow rate and cross section of an exhaust pipe), and once you go turbulent you don't go back. Now, the *amount* of turbulence can still change, but if you start out with turbulent flow, you don't have to worry about a (slightly) rough surface finish or gentle curves, because you're no longer concerned with avoiding a premature laminar-turbulent transition.