Lecture 26 - Fluid Forces Flashcards
what is a fluid?
- liquid or gas
- anything that will fill the container it is in
what is drag force?
- when we move through a fluid, the fluid applies a drag force
- drag force slows our motion down
- we must split the fluid and close it behind us which takes energy (drag force)
- drag opposes the motion (like friction)
what reduces drag force in sports?
- when someone in front of you is breaking the fluid, you can stay in the space behind and use less energy
- used by cyclists and nascar
what is the equation for calculating drag force?
- Fdrag = 1/2 pv^2CdragA
- Fdrag = is drag force
- p (roe) is density of air
- Cdrag = drag of coefficient of the projectile
- A = cross-sectional area
how is drag coefficient affected by streamlining?
- drag coefficient is smaller with streamlining
what is the cross-sectional area?
- the area of the structure ‘presented to the flow’
- can make this smaller by bringing their arms in
what is relative velocity?
- the velocity between the fluid and the object
compare the drag and velocity of head wind vs tail wind
- head wind = anterior wind, against the direction of movement, larger relative velocity with greater drag (ex: 20+5 = 25 m/sec)
- tail wind = posterior wind, assisting the direction of movement, smaller relative velocity with less drag (ex: 20-5 = 15 m/sec)
what is sidewind and relative velocity?
- fluid is moving to the side (alongside head/tail wind)
- lateral component to the fluid velocity that the object fells
- drag force is angled relative to the objects pathway
- ex: plane landing in a storm
what is drag force direction?
- the drag force direction changes during the flight since the direction of the velocity of the object changes with its parabolic path
- ex: as the ball goes up, drag force pulls down, as the ball falls down, drag force pushes up
where does drag force come from? (what are the two sources)
- skin drag and profile drag
what is skin drag?
- the friction between the surface of the object and the fluid as it passes over
- the fluid immediately next to the surface is in fact stuck to the surface and not moving
what is profile drag?
- the drag caused by the separation of the boundary layer from a surface and the wake created by that separation
- when an object “splits” the fluid, and the fluid has to reconnect
- the lower pressure behind the object that “sucks” the object backwards
what is the boundary layer?
- the space between the non-moving surface layer and the undisturbed fluid flow
- this is opposed by the viscosity of the fluid
- gets thin and viscous in water/honey
what is laminar flow?
- when viscosity is really high or moving really slow
how does viscosity affect skin drag?
- greater viscosity has greater skin drag (aka water > honey)
why does a golf ball have dimples?
- make the boundary level more turbulent (narrower)
- reduces skin drag and allows the ball to travel further since its drag is less (makes a thinner wake)
how does region size affect profile drag?
- smaller region = less profile drag
- larger region = higher profile drag
what is the angle of attack?
- the angle at which the object approaches the fluid flow
- larger angle of attack = larger profile drag
what is streamlining?
- get rid of the wake (fill the back with material)
- reduces the pressure pull in the back and helps reconnect the fluid flow
how is streamlining beneficial to profile drag?
- must be oriented properly with respect to the fluid flow
- otherwise can actually increase profile drag
what is lift force?
- a force that acts at a right angle to drag
- arises due to Bernoulli’s principle (faster fluid flow creates lower pressure)
- lower pressure ‘sucks’ on the object
- perpendicular to relative flow direction and drag force
what is the equation for lift force?
F lift = 1/2 pv^2C lift A
- C lift = life coefficient
how does angle of attack influence lift force?
- greater angle of attack = greater lift
- angle of attack is the angle between direction of flow and discus
what is stalling?
- too large of an angle of attack causes the drag to be too large and therefore makes the object stall
what is the lift to drag ratio?
- high enough angle to get lift, but low enough angle to get limited drag
