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fluid mechanics and energy transfer > fluid mechanics 2 > Flashcards

fluid mechanics 2 Flashcards

1
Q

What is the distinction between drift and lift?

A

DRAG force is parallel to the direction of flow LIFT force is perpendicular to the direction of flow

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2
Q

What causes drag forces?

A

Inertia and viscosity

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3
Q

What is Cp?

A

The pressure drag coefficient, depending on the shape of the object and the reynolds number.

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4
Q

What is the general equation for drag?

A

dF=0.5*V2*ρ*cf*dA

c is the drag coefficient

dA is an area

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5
Q

What is an area used for pressure drag?

A

The frontal area e.g. that at right angles to the direction of flow

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6
Q

What is the area used to calculate friction drag?

A

BL

B is the transverse width (span) of the body

L is the length of surface parallel to flow

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7
Q

What is the no slip condition?

A

The fluid has zero velocity at the surface of a stationary solid body immersed in the flow

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8
Q

How does friction and pressure drag come to be?

A

Without viscosity there would be no drag at all. Friction drag exists because of viscosity and pressure drag is non-zero only because there is separation of boundary layer, so that the flow field outside of the boundary layer is disturbed from its ideal shape and the summation (integration) of pressures around the body does produce non-zero result

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9
Q

How do you find the friction force of a plate on fluid?

A

B is the width of the plate, going into or out of the page

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10
Q

What is α?

A

α=∫01 f(η)(1-f(η)) dn

f(η) is a function defining the shape of the boundary layer.

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11
Q

How can you find f(η)?

A

u/U = f(y/δ) =f(η)

δ is the thickness of the boundary layer, y is the distance to the plate surface.

If laminar u/U=2η-η2

If turbulant u/U=f(η)=n1/7

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12
Q

What is β?

A
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13
Q

How do you find the shear stress acting on the wall due to a fluid?

A

τw=μUβ/δ

τw is shear on the wall

U is the initial velocity

δ is the thickness of the boundary layer

μ is the viscosity

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14
Q

How do you find the boudnary layer thickness δ, when flow is laminar?

A
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15
Q

What is Xc?

A

The critcal distance. Before this flow is laminar. After it is turbulant.

It is often found from the critical reynolds number which is often 500000

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16
Q

How do you find the boudnary layer thickness δ, when flow is turbulant ?

A
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17
Q

What is streamlining?

A

Find a compromise between reduced pressure drag (by keeping the point of separation as far back as possible, which is achieved in a turbulent boundary layer) and the increased friction drag (which would be smaller in a laminar boundary layer). If separation cannot be avoided, early “tripping” of the laminar layer into turbulent reduces drag. This can be achieved by either inducing turbulence in flow or roughening the surface upstream. This is why golf balls are dimpled.

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18
Q

Why does a golf ball have dimples?

A

A moving object has a high-pressure area on its front side. Air flows smoothly over the contours of the front side and eventually separates from the object toward the back side. A moving object also leaves behind a turbulent wake region where the air flow is fluctuating or agitated, resulting in lower pressure behind it. The size of the wake affects the amount of drag on the object. Dimples on a golf ball create a thin turbulent boundary layer of air that clings to the ball’s surface. This allows the smoothly flowing air to follow the ball’s surface a little farther around the back side of the ball, thereby decreasing the size of the wake. A dimpled ball thus has about half the drag of a smooth ball.

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19
Q

What is stokes law?

A

Very low Reynolds numbers Re < 1 (Stokes’ Law)

𝐶𝐷 = 24/𝑅𝑒 (sphere)

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20
Q

How do you find the density of air ?

A

ρ(kg.m-3)=P(K.N.m-2)/(RairT)

Rair=287 J kg-1 K-1

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21
Q

What is the formula for the surface area of a sphere?

A

A=4πr2

22
Q

Equation for lift in terms of circulation?

A

FL=UΓρ

Γ = w ∙ h, where w is the average downwash velocity component parallel to y

23
Q

What is the magnus effect?

A
24
Q

What is the lift force on a rotating cylinder?

A

Γ=2πωR2

thus, FL=ρU2πωR2

25
Q

What is the equation for lift in terms of a lift coefficient?

A

Fl=clAρ*0.5*V2

A is the projected area of the airfoil (B∙C), which is perpendicular to the lift vector FL B is span & C the mean chord length of the airfoil.

26
Q

What is the spin ratio?

A

ωD/2V

27
Q

What are stagnation points?

A

Places where the velocity is 0, and therefore they have the highest static pressure.

If they are on exaclty opposite sides of a wing, there will be no lift.

28
Q

What is the aspect ratio?

A

Aspect ratio = B/C = B2/A

B=span

A=projected area of air foil which is perpendicular to the lift vector

C=mean chord length

29
Q

What is total drag equal to?

A

= Profile Drag (skin friction + pressure drag) + Induced Drag

30
Q

What is CDo?

A

drag coefficient for zero lift

31
Q

How can you find the induced drag coefficient?

A

CDi=CL2 / π*B2*A-1

32
Q

Practically what does compressible mean?

A

ρ isn’t constant

33
Q

What is enthalpy?

A

Enthalpy is by definition the sum of internal energy and a product of pressure and volume. Specific enthalpy (per unit mass of fluid) is: h = u + pυ

34
Q

How do you find the speed of sound in a gas?

A

C=(γRT)0.5

R is the gas constant specifc to that gas, γ is the ratio of specfic heats. T in kelvin

35
Q

What is the mach number?

A

M=V/c

V=local flow velocity

C=local speed of sound

36
Q

What is the critcal velocity?

A

local speed of sound

37
Q

What is hs?

A

Stagnation enthalpy. Sum of local enthalpy and local kinematic energy in flow.

At stagation points.

38
Q

When do we assume self similarity?

When is it a valide assumption?

A

When we use equations to find the local shear stress on the wall.

The self-similarity of velocity profiles in the boundary layer is supported by experimental evidence, provided there is no pressure gradient along the surface and there is no transition from laminar to turbulent layer within the region considered – i.e. the layer is either laminar all the way from the leading edge, or turbulent all the way

39
Q

When do critical conditions occour?

A

When the local mach number Ml is 1

40
Q

What is the critical temperture ratio?

A

Tlocal is the general temperture

Ts is the stagnation temperture (velcity = 0)

41
Q

What is the critical pressure ratio?

A
42
Q

What is the critical density ratio?

A
43
Q

What is couette flow?

A

The flow between two parallel plates, one of which is moving, is called Couette Flow

44
Q

What is Simple Couette Flow ?

A

There is no pressure difference to make the fluid flow, it flows because one of the plates is moving.

45
Q

When fluid is used as a lubricant between moving parts, is it laminar or tubulent?

A

Although flow velocities may be high, the thickness of oil film is so small that Reynolds numbers are well below critical values and the flow is laminar

46
Q

Rayleigh step bearing

A

T/b is 25% more (for the same length and minimum clearance) than the corresponding value for an inclined slipper bearing.

 Unfortunately Rayleigh Step Bearings are more prone to sideways leakage which reduces the load carrying capacity

47
Q

What is the significance of the centre of pressure on a slipper bearing?

A

Position of centre of pressure. Centrally pivoted slippers have been found to work satisfactorily , which makes possible the use of inclined slippers bearings for movement in either direction.

48
Q

What does the inclinded slipper bearing looklike?

A
49
Q

What is reynolds number?

A

interial forces / viscous forces

= ρ*u*d/ dynamic viscosity

= u*d/ kinematic viscousity

50
Q

What are the units of kinematic viscosity?

A

m2 s-1

51
Q

What are the units of dynamic viscosity?

A

Ns m-2

52
Q

Mass flow rate for a chocked nozzle?

A

Mass low rate for a choked nozzle does not depend on pressure downstream once it falls below the critical pressure. This flow rate can be increased only by increasing the stagnation pressure ps upstream

fluid mechanics and energy transfer (4 decks)

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