CLOSED CONDUIT FLOW: REYNOLDS EXPERIMENT
It consists of a constant head tank filled with water, a small tank containing dye, a horizontal glass tube provided with a bell-mouthed entrance a and regulating valve. The water was made to flow from the tank through the glass tube in to the atmosphere and the velocity if flow was varied by adjusting the regulating valve. The liquid dye having the same specific weight as that of water was introduced in to the flow at the bell – mouth through a small tube
From the experiments it was disclosed that when the velocity of flow was low, the dye remained in the form of a straight line and stable filament passing through the glass tube so steady that it scarcely seemed to be in motion with increase in the velocity of flow a critical state was reached at which the filament of dye showed irregularities and began of waver.
Further increase in the velocity of flow the fluctuations in the filament of dye became more intense and ultimately the dye diffused over the entire cross-section of the tube, due to intermingling of the particles of the flowing fluid.
Reynolds‟s deduced from his experiments that at low velocities the intermingling of the fluid particles was absent and the fluid particles moved in parallel layers or lamina, sliding past the adjacent lamina but not mixing with them. This is the laminar flow.
At higher velocities the dye filament diffused through the tube it was apparent that the intermingling of fluid particles was occurring in other words the flow was turbulent.
The velocity at which the flow changes from the laminar to turbulent for the case of a given fluid at a given temperature and in a given pipe is known as Critical Velocity.
The state of flow in between these types of flow is known as transitional state or flow in transition.
Reynolds discovered that the occurrence of laminar and turbulent flow was governed by the relative magnitudes of the inertia and the viscous forces.
At low velocities the viscous forces become predominant and flow is viscous.
At higher velocities of flow the inertial forces predominance over viscous forces.
Reynolds related the inertia to viscous forces and arrived at a dimension less parameter.
Where μ/ρ = kinematic viscosity of the flowing liquid ν
The Reynolds number is a very useful parameter in predicting whether the flow is laminar or turbulent.
Re < 2000 viscous / laminar flow
Re - 2000 to 4000 transient flow
Re > 4000 Turbulent flow
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