University of Texas at Arlington MAE 3183 Measurements II Laboratory Viscous FlowUniversity of Texas at Arlington MAE 3183 Measurements II Laboratory Viscous Flow 2/12 Motivation Purpose of this experiment is to clarify the idea of viscous flow in pipes. The concepts of pressure head, fluid friction, laminar and turbulent flow, pressure loss mechanism in them, Calculation of pressure drop and friction in pipes would be revised here. Before performing this experiment you must go through 1. Friction factor 2. Head loss in pipes 3. Absolute and relative friction in pipes. 4. Moody diagram and its utility 5. Reynolds number. Boundary layer. 6. Laminar and turbulent flow in a pipe. Pressure drop in a laminar and turbulent flows Introduction The term “fluids” is understood by scientists and engineers to mean both gases and liquids. Gases and liquids have a strong family resemblance is demonstrated by the fact that gas can be liquefied by reducing temperatures and increasing pressures as liquids can be classified by the converse. Certain fluids (liquids) are considered to be incompressible, although this is not strictly so, since liquids can be compressed very slightly; however, it is substantially true for all practical purposes. There is a considerable range of environmental operating conditions in which common gases, such as air, are considered to be incompressible. For example, the compressibility of air is changed mainly by velocity and temperature. Velocity alone begins to change the performance of aerodynamic bodies at about three quarters the velocity of sound (Mach number = 0.75). Up until recent years the maximum velocity of aircraft was subsonic, well below the velocity of sound, and aerodynamic theory relating to performance, and calculations relating to structural strength did not recognize any influence due to air compression. More recently, in engineering problems associated with aircraft capable of flying near the speed of sound, and other supersonic velocities where a compression wave literally piles up in front of the aircraft, an appropriate mathematical treatment must be employed. The physical laws which control the behavior of fluids, in both compressible and incompressible type flow, are generally the same with certain deviations. In the experimental work in which this manual is used, only incompressible flow needs to be considered. These experiments serve as an introduction to hydraulic engineering, a vast subject occupied with the transportation and control of fluids, the means for increasing the energy content of fluids, and the ways in which energy either already existing or added may be extracted as power. These processes occur in plants where power is generated by heating water to produce steam and then using the thermal energy to drive engines or turbines to deliver useful mechanical power. Huge hydraulic power plants make use of potential energy of water stored high in lakes behind dams. The water is transported down to large turbines which extract energy converted from potential to kinetic by gravity and finally the turbines drive electric generators to power factories and supply domestic heat, light, and power for cities. Canals and locks have served as highways for shipping from ancient to modern times. The Romans were advanced hydraulic engineers in their time, and today the great works of the St. Lawrence Seaway have no equal in engineering achievement in all world history. Bulk fluids are transported today in ships plying canals, rivers and the oceans. No less important is the network of pipe lines crisscrossing this and other continents through which gas, oil, and powered coal are pumped on their way to consumers, night and day. Pumping machinery, turbines (all kinds), compressors, flotation and buoyancy equipment, the hydrodynamics of ships hulls, the aerodynamics of aircraft, offshore well drilling, the science of cryogenics - all of these are the concern of the fluid dynamics engineer. Theory The physical properties of fluids are normally understood to be defined by pressure, temperature, density, and viscosity. Pressure, P, when associated with experimentation of the type of apparatus herein described is usually expressed in inches of water column (which can subsequently be converted to psi,University of Texas at Arlington MAE 3183 Measurements II Laboratory Viscous Flow 3/12 Pa, etc.). Temperature, T, is normally expressed in degrees Fahrenheit. Density, , is expressed in slugs per cubic foot, however, you should be aware that in practice most engineers will use the units of pounds per cubic foot and refer to this as the density when in actuality it is the specific weight. Viscosity of a fluid is the property which causes resistance to flow due to shearing forces within the fluid itself. A characteristic called dynamic (absolute) viscosity, , is the ratio of shearing stress to the rate of shear. In general, changes of temperature have a great effect on the viscosity; however, changes in pressure have only a slight effect. The force required to overcome the shearing resistance between adjacent layers of a fluid moving at different velocities is directly proportional to the absolute viscosity, the area in shear, the relative velocity between adjacent moving fluid layers, and is inversely proportional to the thickness of the moving films. Consider Figure 1 below showing stationary plate (1), and a moving plate (2), in between which is a fluid film (3). Figure 1, Fluid Shear Force Assume there is no relative movement between each plate and the molecules of fluid immediately in contact with the plates. There is a shearing displacement of the molecules in the fluid itself across the profile of the fluid. Therefore, the shearing force, F, required to move the upper plate at a velocity, v, is: FAvds or FAvds As the physical characteristics of a fluid vary, so does the energy content of the fluid. For example if pressure or temperature or both are increased in a fluid, its energy will be increased. When temperature and pressure are reduced, as in the case of the fluid performing work, then the energy is reduced and converted to work. Energy is considered to be one of these three types; 1) Potential Energy is the energy possessed by a fluid by reason of it’s height with reference to a datum plane; for example, one pound of a fluid at an elevation of 100 feet above a zero plane would posses