As objects move through substances, such as liquids and gases, the molecules rearrange and distribute themselves to move around the object. The interface between the molecules and the object creates turbulent forces, which act on the object. The magnitudes of these forces are relatively related to the objects mass, shape, and the nature of the turbulent forces. Turbulent forces, most commonly associated with swimming, such as viscosity plays a critical role in understanding the physics of a body moving through water. Once we acknowledge that turbulent forces and inertia are related we can examine how swimmers and the body of water around them interact and produce a state of flow. Reynolds number examines the state of flow.
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V
is the mean fluid velocity (SI
units: m/s)
L is a
characteristic linear dimension, (traveled length of
fluid, or hydraulic radius when dealing with river
systems) (m)
μ
is the dynamic
viscosity of the fluid
(Pa·s or N·s/m² or kg/m·s)
ν
is the kinematic
viscosity (ν = μ / ρ) (m²/s)
p
is the density
of the fluid (kg/m³)
Q
is the volumetric flow
rate (m³/s)
A is the
pipe cross-sectional area (m²)