Question

What is external flow? What is internal flow? What is the Reynolds number? What is its physical interpretation? What are turbulent and laminar flows? Plot the variation of the local heat transfer coefficient for flow over a flat plate

Answer 1

External and Internal flow

The flow of the fluid under consideration is called external flow if the flow is an unbounded one such as over a surface. Whereas the flow will be called internal flow if it is a bounded by a surface. For example flow over a flat plate will be considered as an unbounded flow because there is it not bounded there by any surface and is free or open to the surrounding. Whereas the flow in a pipe will be a bounded flow because there it is bounded by a surface, the wall of the pipe.

Reynolds number and its physical interpretation

Reynolds number is a dimensionless quantity given by the ratio of Inertia force to Viscous force. Physically it is used to distinguish between the flows as Laminar flow and Turbulent flow. If this ratio is less than 2000 then it will be a laminar flow and if it is more than 2000 it is called as Turbulent flow. The formula for Reynolds number is given by

Turbulent and Laminar flow

Fluid flow is classified in two different flows

(1) Laminar flow : A flow were the fluid slides or flows in parallel to its own flow without having any swirl or eddy with respect to itself is called a Laminar flow. It can also be called a streamline flow because no two streamlines are cutting each other here. Reynolds number for this type of flow is generally less than 2000.

(2) Turbulent flow : A flow were the fluid particle layers cross each other to form eddies and swirl and follow a irregular pattern is called a turbulent flow. Here Reynolds number is greater than 2000.