2 - Viscous Flow Flashcards

1
Q

What is the definition of viscosity?

A

The measure of resistance to the relative movement between two neighbouring particles of a fluid

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2
Q

What is the effect of increased temperature on the viscosity of a liquid? Why does this occur?

A

Viscosity decreases

The bonds between liquid molecules becomes more relaxed and less restricted and therefore able to move freely

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3
Q

What is the effect of increased pressure on the viscosity of a gas?

A

If temperature remains the same viscosity increases as the number of collisions between gas molecules increases

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4
Q

What is the effect of increased temperature on the viscosity of a gas? Why does this occur?

A

Viscosity increases

The internal energy of the gas increases, molecules of the gas move faster and collide more frequently, resulting in more resistance to the change of relative velocity between them

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5
Q

Does a viscous fluid flowing through a pipe satisfy Bernoullis theorem? What will happen?

A

No, energy will be consumed to overcome friction between fluid particles resulting in a loss in pressure downstream

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6
Q

What does dynamic viscosity change with?

A

Temperature and pressure, how it changes depends on the fluid

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7
Q

What is fluidity

A

1/μ

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8
Q

What is Hf? (Bernoullis/Viscous flow)

A

Loss of pressure head (due to viscous friction)

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9
Q

What is Reynolds number?

A

A dimensionless number used to characterise the regime of a flow

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10
Q

What ratio does Reynolds number show?

A

The ratio of inertia (dynamic) force of the fluid to the friction force due to the fluids’ viscosity

Dynamic: ρvl
Viscosity: μ

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11
Q

What is the boundary layer?

A

The layer of fluid formed near the surface of an object, in which the speed of the fluid particles is smaller than in the mainstream, and gradually increase from 0 to the speed of mainstream

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12
Q

What is the speed of particles at the surface of the boundary layer? Why?

A

0, the effect of viscosity causes them to be “stuck”

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13
Q

What are the general characteristics of the flow in the boundary layer over a plate? (Speed, thickness, pressure)

A
  • speed of the fluid is 0 on the surface and increases with distance perpendicular to the surface
  • thickness, δ, of the BL is the distance between the surface and the location where the speed is 0.99 of the full local speed of the main flow
  • the pressure of the fluid is constant in the direction perpendicular to the surface, the pressure in the direction of the flow changes with the pressure outside of the BL
  • close to LE BL is laminar, it then becomes transitional, and then turbulent further aft
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14
Q

What is the structure and speed profile in the laminar BL? What Re number corresponds?

A

Thin, speed of the fluid increases approximately linearly in the direction perpendicular to the surface

Re < 5x10^5

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15
Q

What is the structure and speed profile in the turbulent BL? What Re number corresponds?

A

Thicker, speed of the fluid increases steeply close to the surface and then becomes uniform near the edge of the BL. Has viscous laminar sublayer underneath, buffer zone between the sublayer and the turbulent flow.

Re > 5x10^6

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16
Q

What is the Re when the flow is transitional?

A

5x10^5 < Re < 5x10^6

17
Q

What are the two regimes of flow?

A

Laminar - orderly layered
Turbulent - disorderly (random collisions)

18
Q

How does ice affect the aerofoil?

A

Changes the shape of the surface and the pressure distribution around the aerofoil, encourages turbulent flow and reducing lift production.

Increases weight of the aerofoil, for the same amount of lift to be produced the aerofoil must be flown at a higher aoa. Increasing aoa leads to early BL separation and therefore the BL will be thicker, more turbulent, and produce greater form and skin drag. Can also lead to a stall especially while climbing.

19
Q

What is form drag? What causes it?

A

Drag caused by the fore-aft pressure difference caused by airflow separation.

  • Fore-aft pressure difference
  • Oscillation (from vortices detaching) consumes energy

Caused by;
- Adverse pressure gradient
- Non streamlined shapes
- Sudden change of surface shape

20
Q

What is Karman Vortex street a function of?

A

Re

21
Q
A