Laminar and turbulent f low
- 1. Title: LAMINAR AND TURBULENT FLOW BACKGROUND: A flow can be Laminar, Turbulent or Transitional in nature. This becomes a very important classification of flows and is brought out vividly by the experiment conducted by Osborne Reynolds (1842 - 1912). Into a flow through a glass tube he injected a dye to observe the nature of flow. When the speeds were small the flow seemed to follow a straight line path (with a slight blurring due to dye diffusion). As the flow speed was increased the dye fluctuates and one observes intermittent bursts. As the flow speed is further increased the dye is blurred and seems to fill the entire pipe. These are what we call Laminar, Transitional and Turbulent Flows. Laminar flow: Smooth, steady and orderly flow of fluid in a tube. All the fluid molecules move in straight line. Therefore they move parallel layers or laminae with no disruption between the layers. Velocity of flow is greatest in the axial stream (center of the tube). It becomes progressively slower as the layers move to the periphery. Axial stream velocity is twice the mean flow velocity. Velocity of the layer in contact with the wall is virtually zero. Turbulent flow: Fluid does not move in orderly manner. The fluid molecules become more disorganized. They form swirls and eddies as they move down the pressure gradient in haphazard manner. There is increased resistance to flow as the eddy currents interfere with each other. Therefore greater energy is required for a given flow rate compared to when the flow is laminar.
- 2. Reynolds Number, Re: Reynolds number is a dimensionless parameter which determines whether the flow in a closed conduit or a pipeline is laminar or turbulent. Where: D = inside diameter of the pipe, m = liquid velocity, m/s = density of the fluid, kg/m3 = dynamic viscosity, Pa-s or Poise Note: 1 poise = 1 = 0.1 Pa-s If the dynamic viscosity is divided by the density, kinematic viscosity, v can be obtained: V = . Therefore: Where: V = kinematic viscosity, m2/s Note: 1 stroke = 1 cm2/sec Values of kinematic viscosity of various fluids can be determined from page 116 of Mechanical Engineering Tables and Charts SI units 5th Edition MRII. If Re ≥ 3000 the flow is turbulent Re ≤ 2000 the flow is said to be laminar Viscosity Viscosity is the property of a fluid that causes it to resist flow. Viscosity of a liquid decreases with increased temperature, while viscosity of a gas increases with increased temperature. EQUIPMENTS: 1. Hydraulic Bench 2. Laminar and Turbulent Apparatus