Drag Flashcards
fluid mechanics
The natural science of fluids (liquids or gases) in motion
In some cases fluid forces have little effect on an objects motion
drag
impacts horizontially
lift
impacts vertically
boundary layer
A thin layer of air surrounding or “attached” to the ball
can be laminar or turbulent
laminar flow
A type of fluid flow in which the fluid move smoothly in individual streams or layers
Flow that occurs in parallel to each other
turbulent flow
Flow in which the velocity at any point varies erratically
When the nice neat pattern breaks down the flow becomes turbulent
Fluid often changes from laminar to turbulent when they go over an abrupt feature like wind going over a ridgeline of a mountain
boundary layer seperation
Where the boundary layer breaks away from the ball
The earlier the separation the greater the pressure gradient between the front and back of the ball which leads to more drag
factors affecting boundary layer separation point
velocity
surface roughness
shape
velocity
○ Low velocity
§ the boundary layer clings to the surface so separation occurs well towards the rear
□ This results in minor drag
○ High velocity
§ Separation occurs further forward
This results in increased drag
surface roughness
○ Rough surfaces create turbulent boundary layers, which reduces the affect of drag
§ Dimpled golf ball vs smooth
Swimsuits are designed to have rough surfaces
shape
§ the boundary layer clings to the surface so separation occurs well towards the rear
□ This results in minor drag
○ Round ball
§ Separation occurs further forward
This results in increased drag
lift/drag - fluid resistance relationship
lift = y
drag = x
fluid resistance: y=x
fluid resistance
- As an object moves through a fluid, it disturbs it
The greater the disturbance to the fluid, the greater the transfer of energy from object to fluid
factors that affect fluid resistance
○ Density (air)
§ The denser the fluid, the more disturbed it becomes and hence the greater the resistance
○ Viscosity (liquid)
§ The more viscous the fluid the more disturbed it becomes and hence the greater the resistance
Swimmer vs runner
types of drag
surface drag
form drag
wave drag
surface drag
- The friction produced between a fluid and the surface of a moving object
- Factors affecting surface drag
○ Relative velocity to moving object
○ Relative roughness of surface object
○ Viscosity of the fluid
○ Surface area of the object
- Factors affecting surface drag
form drag
- The resistance created by the pressure differential between the front and the back of an object travelling through a fluid
- Factors affecting form drag
○ The cross sectional area of the object presented to the fluid
○ Velocity of the object
○ Surface roughness
○ Shape of the object
- Factors affecting form drag
wave drag
- The creation of waves forms resistance at the point where air and water interact
- Seen as the major form of drag affecting swimmers
- Factors that affect wave drag
○ Relative velocity of the wave
○ Technique
swim lower = less
○ Open water vs closed conditions (ocean vs pool)
Lane ropes are used to reduce wave drag by helping dissipate waves through the surface of the water
factors affecting drag
Drag coefficient
○ Measure used to quantify the drag or resistance of an object in a fluid environment
○ Directly related to cross sectional area
Mass
○ The greater mass of the ball the less effect of drag
Velocity
○ The faster the ball moves through the air, the earlier the boundary layer separates from the ball, creating a large pressure differential between the front and rear of the ball hence more drag
Surface roughness
○ Rougher surfaces allows the air stream to cling to the ball for longer periods of time, resulting in a smaller pressure differential and therefore less drag
Cross sectional area
○ A linear relationship exists between CSA exposed to fluid and drag
Shape
○ Round ball vs oval ball
○ NFL ball cuts through air more cleanly than a soccer ball
Oval shape encourages air to cling to surface all the way to the end of the tail meaning smaller pressure differential and less drag
