Midterm 2: PQ/Quiz Flashcards
What is a venturi meter?
device used to measure the velocity of a fluid by passing it from a tube with a large cross-sectional area into a constriction (tube with smaller cross-sectional area)
manometer filled with liquid is placed with one arm connected to the large tube, and the other the small tube
when the fluid flows from the large to the small tube, its velocity increases and its pressure drops, causing a pressure differential across the manometer which draws fluid towards the lower pressure in the small tube
How is a prairie dog burrow like a Venturi meter?
has two openings – one at the top of a raised mound of earth, and one flush with the ground
as air flowing over the ground hits the mound, it is forced to flow up and over the top, which causes the flow to be constricted, forcing it to accelerate and drop in pressure
the burrow is like a manometer, while the raised mound of earth acts like the constriction of the smaller tube in the Venturi meter
BUT unlike the manometer, the burrow isn’t filled with liquid, so the low pressure at the opening of the raised burrow draws air through the burrow, down a pressure gradient, frmo the higher pressure at the other burrow entrance flush with the ground
Fluids flow from a region of high pressure to a region of low pressure. But in the following diagram, the fluid flowing from point B to point C is flowing against a pressure gradient. How is this possible?
height of the water in the vertical tubes shows the static pressure of the water in the three sections of pipe, but does not show the dynamic pressure
as fluid flows from A to B, the velocity of the fluid increases, thus part of the static pressure of the
fluid in (A) is converted into dynamic pressure in (B)
while the static pressure of the water in section (B) is lower than in section (C), the total energy of the fluid (which is equal to the sum of the static, dynamic and potential pressure) is still higher in (B) than in (C
therefore, the water will continue to flow down an energy gradient from B to C
What does a Pitot tube measure? How does it do this?
pitot tube measures the dynamic pressure of a fluid, which is proportional to the velocity of the fluid – it can therefore be used to measure fluid velocity (or airspeed)
it consists of a tube with an opening that faces directly into the oncoming fluid flow
- moving fluid hits the opening of the tube and stagnates (stops), converting its dynamic pressure into static pressure
- stagnation pressure at this point is therefore the sum of the static and dynamic pressures of the fluid flow
second opening in the tube parallel to the fluid’s flow is called the static port, and is used to measure the
static pressure of the fluid
- pressure gauge (manometer) placed between these two ports will measure the dynamic pressure only
What is Reynolds number?
ratio of inertial forces in a fluid (related to fluid density, velocity, and characteristic length) and viscous forces in a fluid (dynamic viscosity)
number used to predict whether flow is laminar (low Re) or turbulent (high Re)
What is the difference between dynamic and kinematic viscosity?
dynamic viscosity: measure of the relationship between the shear stress applied to a fluid and the resulting shear strain rate (units: Pa s)
kinematic viscosity: ratio of dynamic viscosity of a fluid to its density (units: Stokes)
What is the difference between the
dynamic viscosity of air and water compared to the difference in kinematic viscosity?
dynamic viscosity of water is 0.001 Pa s, which is 55 times larger than air (0.0000181 Pa s)
kinematic viscosity of the two fluids differs by only 15x
Swimming at low Reynolds number is a challenging prospect.
Explain what type of motion is required to move in this environment. Give some examples.
swimming at low Re requires a motion in which the power stroke of a locomotory structure to push the animal forward and the recovery stroke to re-set the position of the structure for the next power stroke are not the exact opposite of one-another (ie. are not time reversible)
ie. beating flagellum moving like a cork-screw or cilia, which bend during the recovery stroke, can both provide a non time-reversible motion
What is the coefficient of pressure (Cp) for the stagnation point?
1
What is lift?
force generated perpendicular (at right angles) to the direction of fluid flow
What is the coefficient of lift (CL)?
dimensionless number that relates the lift (FL) generated by a structure (aerofoil, wing, animal, or aircraft) to the reference area (S) of the lift generating
surface (generally the planform area of the wings) and dynamic pressure experienced by the structure due to the velocity and density of the fluid flow past it
What is drag?
force generated parallel and in the same direction of fluid flow
What is the coefficient of drag (Cd)?
dimensionless number that relates the drag (Fd) generated by a structure (aerofoil, wing, animal, or aircraft) to the reference area (S) of the structure (generally the planform area of the wings, but could be projected frontal area/wetted area etc. when
considering a whole animal) and dynamic pressure experienced by the structure due to the velocity and density of the fluid flow past it
What is the Magnus effect? How can this effect be used to generate lift?
effect by which a rotating object (cylinder/sphere, etc.) can generate lift
rotating object placed in a fluid flow accelerates the fluid on the side of the object where the fluid’s direction of travel and the object’s direction of rotation
coincide
- fluid flow that encounters the opposite side of the rotating object is slowed down
Bernoulli states that high velocity air has lower pressure, while low velocity air has higher pressure – thus the object’s rotation combined with a fluid flow creates a pressure difference on opposite sides of the object, producing a force at right angles to the direction of fluid flow (lift)
What is the critical angle of attack?
angle between the chord of the aerofoil and the relative wind that produces the maximum lift (proportional to coefficient of lift)
What happens to the magnitude of lift and drag once
the critical angle of attack is exceeded? Why? What is this phenomenon called?
once the critical angle of attack is exceeded, the boundary layer of air flowing across the top surface of the wing begins to separate
- this is caused by the air in the boundary layer over the top of the wing being unable to flow to the trailing edge due to an increasingly unfavourable pressure
gradient behind the wing generated by the increasing angle of attack
once the boundary layer separates from the surface of the wing, a turbulent wake is formed behind the wing
- this dramatically reduces the lift and increases the pressure drag produced by the wing
this condition is called a STALL
How much weight could a wing potentially carry in level flight?
to fly straight and level, lift force must balance weight
How much thrust would need to be produced to maintain a constant airspeed?
to maintain a constant forward speed while flying requires a forward thrust that is equal to the drag
Can a symmetrical aerofoil at 0° angle of attack generate lift? Why/why not?
NO
air flowing past the aerofoil will produce the same pressure distribution on both the top and bottom surface – pressure will be high at the leading edge, then low on either side of the aerofoil where air must accelerate around the curved surface, slowly rising towards the trailing edge
with no net pressure difference between the top and bottom of the aerofoil, there is no net force generated perpendicular to the direction of flow, thus no lift
What is induced drag? Why does it occur?
induced drag occurs as a by-product of lift production, which produces low pressure above the wing and high pressure below
at the wingtip, the high pressure air flows up and around the wingtip to the low pressure region above the wing, producing wingtip vortices
- these vortices produce an area of downward moving air directly behind the trailing edge of the wing
this downwash causes the relative wind to be pushed down at the trailing edge of the wing
- this creates an effective relative wind that is at an angle halfway between the angle of wind in the downwash and the oncoming relative wind
since lift is generated perpendicular to the effective relative wind, it is tilted rearwards and directs more force rearwards
- this backwards directed force is the induced drag
What is parasitic drag?
combined drag caused by the skin friction, pressure, and interference drag produced by the non-lift producing surfaces of a flying animal or aircraft
Why are some large birds required to run a long distance before take-off is achieved? (ie. swan running across the water, or albatross running down a hill)
because of their high wing loading
wing loading is a measure of the amount of lift that needs to be produced per unit area of the wing in order to balance weight
- weight of the flying/gliding animal (mass × g)
divided by the total wing area gives wing loading
high wing loading means more lift must be produced per unit wing area, and this can be achieved by flying faster, thereby increasing the airspeed over the wing, and increasing lift
- the higher the wing loading, the harder it will be for the animal to take off and fly at low speeds
swans have a high wing loading, which means they suffer from this problem
- by running across the lake, they generate high airspeed over their wings, which is necessary to generate the large amount of lift they require to get airborne
albatross also have a high wing loading and must run down hills into oncoming wind in order to generate a fast enough air speed to fly
What forms of energy are used in powered flight?
- kinetic energy
- potential energy
- metabolic/chemical energy
What forms of energy are used in
gliding?
- kinetic energy
- potential energy
What is the difference in the descent angle in a gliding organism and a parachuting organism? Why does this difference in descent angle exist?
descent angle of gliding organism: generally < 45º
descent angle of parachuting organism: usually > 45º
this difference is due to the fact that gliders are able to generate lift as they descend through the air, gliding forwards
parachutes produce maximum drag vertically, in the opposite direction to their descent path, to drift slowly to the ground
What factor does weight affect in gliding?
gliders speed (but NOT its glide angle)
At low (Re «_space;1) Reynolds number, what happens to the coefficient of drag (Cd) for an object?
becomes much greater than 1 because skin friction drag dominates, and Cd is the ratio of the measured total drag divided by the calculated pressure drag
What happens when a wing stalls?
drag (Fd) increases
Total drag acting on a flying animal…
decreases with airspeed to some intermediate low point before increasing again
For an animal that is in an un-accelerating, equilibrium glide…
aerodynamic resultant vector must be equal and opposite the weight vector
What is induced drag caused by?
lift producing surfaces of a flying animal
What can you determine from an animal’s L/D ratio?
- their angle of descent
- distance travelled to height lost
- ratio of their coefficients of lift and drag
What direction is drag always in?
in the direction of incoming flow
What do adverse pressure gradients require?
require a pressure drop to occur where the fluid accelerates, which will only
occur if the object can divert the flow either with curved surface or with a non-zero angle of attack.
Do adverse pressure gradients occurs in the boundary layer?
yes – gradients also occur in the boundary layer where viscous effects, and therefore energy loss, are
important in resulting in the velocity gradient profile
How do rough surfaces affect adverse pressure gradients?
rough surfaces can move the separation point of the boundary layer towards the trailing edge, but they don’t eliminate them
What is pressure drag dependent on?
cross-sectional area perpendicular to the flow direction
What is skin friction drag a function of?
viscosity, velocity, length, and surface area
As Reynolds number increases, why does CD suddenly decrease at the point signifying a drag crisis?
pressure drag decreases due to the separation point moving downstream on the object
during the drag crisis, C𝑑 drops because the boundary layer remains attached for longer/further
downstream – this reduces the size of the low pressure wake at the back and decreases pressure drag
What direction is lift always in?
perpendicular to the direction of flow
What is the equal transit theory?
flow above the airfoil “meeting up” with flow below airfoil
PROVEN TO NOT BE TRUE
Why won’t a symmetrical airfoil with an AoA of 0 not produce lift?
will have equal pressure distribution above and below the airfoil, generating zero lift overall
How can you increase thrust?
increase titling the resultant aerodynamic force vector forward by increasing lift vector
- increase velocity
- increase lift coefficient – by increasing AoA or camber
reduce drag vector by decreasing drag coefficient
- streamline airfoil
- increase Re (up to a certain point)
What is stall angle?
angle of attack when the lift begins to drop
How can lift coefficient be changed to increase lift force?
lift coefficient is a value that is normalized for density, velocity and reference area
therefore, the only thing that can increase lift coefficient is angle of attack
If you reduce the aspect ratio while maintaining the same wing area, what happens to induced drag?
increases
if you reduce the aspect ratio of the wing, a larger portion of the wing is subject to the downwash from
the wing-tip vortices
as a result, a larger portion of the wing will be affected by a reduction in it’s angle of attack, and therefore, induced drag overall will increase
