headwax wrote:
Characteristic length
5 in
Viscosity
.862 cP
Density
1000 kg/m³
Velocity
20 ft/s
RE=898134.570765662 (ie turbulent flow)
Characteristic length
24 in
Viscosity
.862 cP
Density
1000 kg/m³
Velocity
20 ft/s
RE=0.431104593968 (ie laminar flow)
In other other words: surfboards in the main have laminar flow, but in the important parts they have soemthing else.
Whoa...something is wrong here as Reynolds number increases with the characteristic length scale, so the Reynolds number based on a dimension of 24 inches should be greater than for 5 inches, i.e. :
Re = (flow speed x characteristic dimension)/(kinematic viscosity)
For a fin with a chord of 5 inches:
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Avg. max speed of 48 measurements of a collection of shortboards, kneeboards, bodyboards, (1) longboard, ridden by various riders on waves ranging from chest high to 2x ovrhead surf was observed to be 19.5 mph (median = 19.6 mph)
Hence avg. max speed = 19.5 x (5280/3600) = 28.6 ft/sec = 343 in/sec = 872 cm/sec
Chord length = 4 in = 12.7 cm
Typical kinematic viscosity = 0.01 cm^2/sec
Re = (872 x 12.7)/(.01) = 1,100,000
For a length of 24 inches (e.g. average wetted length of the wetted area of a shortboard/kneeboard):
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Re = 1,100,000 x (24/5) = 5,300,000
Note also that the Reynolds number determining whether the flow is laminar or turbulent depends on the shape of the object. The values of 2000-4000 are for flow in a pipe, but are not representative for flow about a foil or along a flat plate.
For a fin/foil, a Reynolds number of 1,000,000 is generally considered to be marginally into fully turbulent flow (significant reductions in the maximum "lift" force occur as the Reynolds number falls below this value).
For a flat plate (e.g. approximating the bottom of the board when taking the curvature of the wave into account), the transition depends strongly on the smoothness of the plate and the presence of any natural disturbances in the flow. According to Schlicting (Boundary Layer Theory, McGraw-Hill Science Series), the lower transition limit should be considered to be about 320,000; for exceptionally disturbance-free flows, laminar flow has been observed to Reynolds numbers of 1,000,000 (and higher).
Thus both the flow on the board and across the foil are turbulent (but the foil marginally so, and even less so for a foil with an average chord more on the order of 3 inches--e.g. a side fin).
The characteristic dimension in the Reynolds number should also not be used to estimate the spacing that determines whether two objects (significantly) influence the flow about each other. That typically depends on the alterations in the bulk flow, rather than the smaller scale turbulence within that flow.
Experience gained is in proportion to equipment ruined.
