Reynolds Number: origin and calculation
Hello all.There is oftentimes confusion over laminar flow,turbulent flow,boundary layers,separation,etc.,and all have to do with Reynolds Number effects,so I thought it would serve us to look at the Reynolds Number and have it available to all members and lurkers to use as a computational tool. The number was a necessary invention,as air has mass and viscosity, and a tool was necessary for prediction of the air's effects when designing everything from a vacuum cleaner to an airplane. Reynolds Number = force (inertia)/ force (viscous)  The development of the formula begins with: N = [ ( velocity)x ( length)x (specific weight)]/ [ ( viscosity )x(acceleration due to gravity) ]. specific weight/ gravitational constant = rho,so formula becomes  [(velocity)x(length)x(rho)]/ viscosity. rho/viscosity is the inverse of kinematic viscosity(v),so formula can be simplified to N= [(velocity)x(length)x(inverse of kinematic viscosity)]. For" standard air"and U.S.Standard units,v= 0.0001567 seconds/ftsq,and it's inverse is 6380. So here at the end,N= velocity x length x 6380. An example for use would be my T100 at 70mph.Multiplying 70mph by 5280ft/mile gets me into feet,then dividing by 3,600 seconds per hour gets me into feet per second(my unit of length for calculations. At 16.5 feet Length(without the boattail), and 102.667 ft/sec Velocity,multiplying by 6380 gives me a Reynolds Number of 10,807,720. When comparing various shapes and their Cds as a function of Reynolds Number,one can calculate crucial data,especially if you intend to model,so that appropriate "scaling factors" can be accounted for. That's it,hope it helps dispel some of the mystery of aero noodling.
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