Fluid mechanics Flashcards
What are the two types of forces exerted by fluids?
Buoyant force and dynamic force
What is fluid mechanics?
The study of forces that fluids exert on objects moving through them
What is buoyancy force?
- Due to immersion of the object
- pressure from water weight above, beside, below
what is dynamic force?
- Due to relative movement of the fluid
- Drag force, lift force
what happens when you submerge an object deeper and deeper?
increase in above and below pressure
equations for buoyancy force?
R lower - R upper = Buoyant force
which is always higher, R lower of R upper?
R lower
what is archimede’s Principle?
the buoyancy force will always equal the water weight displaced… weight always equals object in the end
how does buoyant force affect an object?
buoyant for will always act up on the object, related to weight of fluid displacement.
how can you determine if an object will sink or swim?
difference between the buoyant force and weight of the object
what happens when the buoyant force is equalled to the weight ?
static equilibrium
what happens if BF is bigger than mg?
the object will rise
what happens if the BF is smaller than mg?
The object will sink
what is density?
the amount of matter in a given volume
equation for density?
P = m/v
what is muscles density in respect to water?
1000kg/m^3, more dense than water… will sink
what is fats density in respect to water?
less dense than water… will rise… olive oil on water
where does the buoyancy act on a body?
centre of volume
where does force of gravity act upon a body?
centre of mass
what is dynamic fluid force?
force due to relative motion
what is dynamic fluid force affected by?
- density of fluid
- SA of object
- (relative velocity)^2
what is the most important in determining fluid forces?
relative velocity
what is relative velocity?
difference between objects velocity and fluids velocity
what is drag force?
component of fluid force that opposes (parallel/opposite direction) the relative motion of object
what is surface drag?
sum of fiction forces acting between fluid molecules on surface and object
what is form drag?
how fluid molecules travel around an object due to the objects shape
what is skin friction/viscosity drag (surface drag)?
- molecules slide past object’s surface, friction creates layer called boundary layer
what can a swimmer do to minimize friction of boundary layer?
minimize surface drag by shaving and using swim caps
what is surface drag affected by?
- coefficient of drag (roughness of fluid and viscosity of fluid)
- fluid density
- objects cross section
- relative velocity ^2
what is the fluid travelling around a smooth object called?
Laminar flow
what does the divergence of fluid depend on?
shape of object
what does converge of fluid depend on?
natural flow of rebalance pressure
what is form drag produced by?
shape and size of object
what is fluid pressure like when object passes it?
high pressure in the front, low pressure in the back
front pressure slows down object and back pressure pulls object (both slowing down object)
what is the ideal shape than minimizes form drag?
shape that cuts through fluid and slowly allows fluid to recreate original trajectory
what is form drag affected by?
- shape of object
- drag coefficient
- density of fluid
- cross section of object
- relative vel ^2
why are there dimples on a golf ball?
dimples diminish form drag by changing laminar flow, holding molecules closer to the mall
what is lift force?
dynamic force component that acts perpendicular to motion of object
what is lift force caused by?
lateral deflection of fluid molecules
how does deflection work?
object exerts lateral force on molecules, molecules do the same but opposite direction, causing lift
what is lift force proportional to?
lateral acceleration of fluid molecules and mass of molecules
what can alter lift force?
shape of object
what is the Magnus effect?
air flow on an object causing spin
what is bernoulli’s principle?
- fast fluid exert less force than slow fluid
ball spinning in direction of fluid is less pressure. vice versa.
what happens to spinning a ball?
changes relative velocity of fluid on that object (magnus effect)
what is topspin?
molecules on the top slow down due to friction as bottom molecules don’t change speed. therefore more pressure on the top, net downward. Vice versa for backspin.
