Since the air (and water) viscosity is small, in many ﬂows encountered in everyday life the Reynolds number Re is very large. The two most common fluids are air and water. The Reynolds number Re = U L /ν depends on the fluid kinematic viscosity ν, which is the property of the fluid, and the characteristic velocity U and the characteristic length L.
The following table gives several examples of the values of Re for particular values of the speed and length for air and water.
|Size L||Speed U||Re in air (at 20 C)||Re in water (at 20 C)|
|1 cm||1 cm/sec||7||102|
|1 m||1 m/sec||66,200||1.02×106|
|10 m||10 m/sec||6.62×106||1.02×108|
Note that Re in water is roughly 15 times higher than Re in air for the same characteristic length and speed.
The second table shows the typical values of the Reynolds number for various applications.
|Bacterium||Re ∼ 0.000,01|
|Blood flows||0.002 < Re < 2,000|
|Housefly||Re ∼ 100|
|Big butterfly||Re ∼ 4,000|
|Large fish or large bird||Re ∼ 50,000|
|Person running||Re ∼ 500,000|
|Cars||5×106 < Re < 6×107|
|Aircraft||3×107 < Re < 8×108|
|Ship||5×107 < Re < 5×109|
|Atmospheric flows||Re > 109|
Hence, flows at large Reynolds numbers deserve a special attention.