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 12-13-2008, 04:17 PM #36 (permalink) aerohead Master EcoModder     Join Date: Jan 2008 Location: Sanger,Texas,U.S.A. Posts: 12,631 Thanks: 20,296 Thanked 6,282 Times in 3,888 Posts 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/ft-sq,and it's inverse is 6380.--------------------------------- So here at the end,N= velocity x length x 6380.---------------------------------------------------------------- An example for use would be my T-100 at 70-mph.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. __________________ Photobucket album: http://s1271.photobucket.com/albums/jj622/aerohead2/