Download OH 5: Fluid Dynamics

Fluid Dynamics
 All athletic events take place in a fluid environment
 water (swimming), air (cycling), both (water polo)

 Drag and lift forces are perpendicular to each other,
produce different effects, and are affected by different
factors
 Flow velocity –
Fluid Drag Forces
 Two types:
 Both skin friction and profile drag are proportional to
relative flow velocity, cross-sectional area, shape of
object, smoothness of surface, and density of liquid
Skin Friction Drag
 Caused by the fluid tending to rub (shear) along the
surface of the body

 The layer of fluid next to the skin sticks to the body;
however, the next layer is towed along and therefore
slides relative to the innermost layer



Laminar flow
 The smooth, layered, flow pattern of a fluid around an
object with no disturbance

Profile Drag
 The main form of drag in skiing, cycling, running , all
projectiles events, and swimming

 Characterized by turbulent flow in which the pressure on
the leading surface of a body is greater than the pressure
on the trailing surface
Turbulent flow

 This causes a large, turbulent, low-pressure zone to form
behind the object
 The region of low pressure increases the amount of work
done on the object

Fluid Lift Forces
 Always directed perpendicular to the flow velocity

 More air flows over the upper curved surface than the
lower flat surface, such that the difference in velocity
across the surfaces results in a pressure difference
between the two sides
 The external force resulting from the pressure difference
is perpendicular to the direction of flow velocity, and can
change the motion of the object
 Bernoulli’s principle –
Angle of Attack
 Refers to the tilt of an object relativeto the flow velocity

 Function of the shape of an object and the flow velocity
 If the angle of attack increases too much, it approaches a
critical maximum angle (stall angle), beyond which the lift
force decreases as the drag force becomes dominant
The Magnus Effect

 The changes in flight path are always perpendicular to the
flow velocity of the projectile
Why a Curveball Curves (topspin, under spin, sidespin)
Body Balance and Stability Control

 Generally depends upon the location of the centre of mass
and how stable that centre of mass is
 The stick figure model is useful
Equilibrium
 The state of a system that is not experiencing any change
in its direction or speed
 Static Equilibrium:
 Dynamic Equilibrium:
Balance
 Process whereby the body’s state of equilibrium is
controlled for a given purpose

 If the line of gravity passes through some part of the
body’s base of support, the body will be balanced
Stability
 A measure of the difficulty with which equilibrium can be
disturbed

 There is a trade-off between maximizing stability and
acquiring speed off a mark
Increasing Stability in Static Equilibrium

 Increase the inertia of the body

 Increase the distance between the point where a vertical
line from the centre of mass intersects the base of
support
Increasing Stability in Dynamic Equilibrium
 Enlarge the body’s base of support in the direction of the
external horizontal forces
 Adopt a starting position in which the centre of pressure
is close to the edge of the base of support whenever a
quick acceleration is important
 When slowing down or reversing directions in running, lean
backwards but keep normal frictional forces high to
prevent slipping
