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Average (Bulk) Velocity

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Fluid Flow LAMINAR AND TURBULENT FLOW Average (Bulk) Velocity In many fluid flow problems, instead of determining exact velocities at different locations in the same  flow  cross-section,  it  is  sufficient  to  allow  a  single  average  velocity  to  represent  the velocity of all fluid at that point in the pipe.   This is fairly simple for turbulent flow since the velocity profile is flat over the majority of the pipe cross-section.  It is reasonable to assume that the average velocity is the same as the velocity at the center of the pipe. If the flow regime is laminar (the velocity profile is parabolic), the problem still exists of trying to represent the "average" velocity at any given cross-section since an average value is used in the fluid flow equations.  Technically, this is done by means of integral calculus. Practically, the student should use an average value that is half of the center line value. Viscosity Viscosity is a fluid property that measures the resistance of the fluid to deforming due to a shear force.    Viscosity  is  the  internal  friction  of  a  fluid  which  makes  it  resist  flowing  past  a  solid surface  or  other  layers  of  the  fluid.   Viscosity  can  also  be  considered  to  be  a  measure  of  the resistance of a fluid to flowing.  A thick oil has a high viscosity; water has a low viscosity.  The unit of measurement for absolute viscosity is: µ = absolute viscosity of fluid (lbf-sec/ft2). The  viscosity  of  a  fluid  is  usually  significantly  dependent  on  the  temperature  of  the  fluid  and relatively independent of the pressure.  For most fluids, as the temperature of the fluid increases, the  viscosity  of  the  fluid  decreases.   An  example  of  this  can  be  seen  in  the  lubricating  oil  of engines.  When the engine and its lubricating oil are cold, the oil is very viscous, or thick.  After the  engine  is  started  and  the  lubricating  oil  increases  in  temperature,  the  viscosity  of  the  oil decreases significantly and the oil seems much thinner. Ideal Fluid An  ideal fluid  is one  that is incompressible and has no viscosity.   Ideal fluids do not  actually exist, but sometimes it is useful to consider what would happen to an ideal fluid in a particular fluid flow problem in order to simplify the problem. Reynolds Number The flow regime (either laminar or turbulent) is determined by evaluating the Reynolds number of the flow (refer to figure 5).   The Reynolds number, based on studies of Osborn Reynolds, is a dimensionless number comprised of the physical characteristics of the flow.   Equation 3-7 is used to calculate the Reynolds number (NR) for fluid flow. NR = v D / µ gc (3-7) Rev. 0 Page 19 HT-03



   


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