Viscous Flow

Question 1

What is viscosity?

Answer 1

Is the measure of resistance to relative movement between 2 neighbouring particles in a fluid.

Question 2

How does fluidity change with temperature and pressure?

Answer 2

Gases: inc temp inc viscosity as more Eint so more collisions and therefore resistance.
Fluids: inc temp viscosity decreases as molecular bonds relax reducing resistance.
Both: inc pressure inc viscosity as results in compression and therefore more resistance

Question 3

Difference between laminar and turbulent regimes? What determines this?

Answer 3

Laminar flow is orderly layered whilst turbulent is disordered and random collisions.
Anything between is in transition.
The fluid, fluid velocity, characteristic scale of the flow, flow regime and the quality of the surface

Question 4

What is Reynolds number?

Answer 4

A dimensionless number which is used to characterise the regime of a fluid flow. Ie tell you if turbulent, transition or turbulent.
Is a ratio of the inertia force of the fluid to friction due to viscosity

Question 5

Where is the boundary layer found? What does it contain?

Answer 5

Is found near the surface of an object and contains a fluid flow that is slower than the mainstream due to viscosity.

Question 6

What is the speed of the viscous flow in the boundary layer on the surface?

Answer 6

0m/s relative to the surface

Question 7

What is the thickness of the boundary layer?

Answer 7

From the surface to where v= 0.99 free stream

Question 8

How does pressure change in the boundary layer?

Answer 8

Pressure is constant perpendicular to the surface. P/Y=0

Pressure changes parallel to the surface depending on if it is fore/aft of thickest part of the wing

Question 9

How is the increase in speed in laminar vs turbulent?

What flow is the viscous sub layer zone? Where is it found?

Answer 9

L: linear
T: non linear
Found in the turbulent zone as a laminar flow

Question 10

Draw the boundary layer diagram

Answer 10

Look at notes

Question 11

What numbers of RE tell us the regime of a flow?

Answer 11

Re<5x10*5 laminar

Re>5x10*6 turbulent

Question 12

Does laminar or turbulent produce more friction drag? Why?

Answer 12

U/Y=shear stress, and using graph it can be seen this value is higher for turbulent than laminar flow.
Average speed is greater in turbulent flow as it has a greater Ek, which can be drawn in from the mainstream.

Question 13

What direction does “y” act?

Answer 13

Perpendicular to the surface

Question 14

What are the two conditions for boundary layer separation to occur?
Why does boundary layer separation occur?

Answer 14

• Particles reach stagnation points u/y=0
• Adverse pressure gradient p/x>0 at the stagnation point

There is skin friction drag along the surface of an aerofoil due to viscosity of air, and particles consume Ek to overcome this friction. V on the surface is 0 but will increase in y direction. The longer particles spend near the surface in a BL they will loose Ek due to SFD, and u/y will decrease. At some point there is not enough Ek to overcome the shear stress and u/y=0 and a stagnation point has been reached.
In the region of the stagnation point there will be an accumulation of lazy particles, which increase the pressure in the area and this causes them to be pushed away from the surface, creating turbulent eddies. If these particles meet an adverse pressure gradient (p/x>0), which occurs after the thickest part of the wing, there will be a backward flow of particles (u/y<0).
The fact that the particles leave the surface is called boundary layer separation.

Question 15

Can the wing produce lift where the flow is turbulent? What about separated?

Answer 15

Yes turbulent, no separated.

Question 16

What effect does boundary layer separation have on flight?

Ie)with AoA

Answer 16

The part of the wing where the BL separates is unable to produce lift. So as AoA increases, the CoP moves forward, Cl will increase up until the critical angle. If BL separation occurs too early, a stall can occur and will produce excessively high drag.

Question 17

What causes the buffet?

Answer 17

Wake oscillation hitting the tailplane

Question 18

What is aileron reversal?

Answer 18

Where BL separation occurs over the aileron. If a/c is wanting to bank (right) and as a result left aileron is deflected to create a higher AoA, BL separated may occur over this aileron if deflection is too large, resulting in a loss of lift on this wing and the left wing will drop, the opposite as intended.

Question 19

Describe how pressure changes with x/y and speed changes with x/y in the boundary layer.

Answer 19

Refer to drawing in notes

Question 20

Why does the boundary layer thicken?

Answer 20

Is due to fluids consuming Ek to overcome the shear stresses, and the longer they spend near the surface the more Ek they consume so require more distance to reach 99% free stream speed

Question 21

What 3 types of stalls are there?

How can we avoid them

Answer 21

• Aileron reversal
• Low speed/high AoA (occurs in the turbulent layer on upper surface)
• Accelerated stall (occurs near LE when rapid increase in AoA

Avoid rapid movements and want a smooth surface.

Question 22

Are the transition point and separation point the same thing?

Answer 22

No, transition is when flow goes from laminar to turbulent.

Separation is where flow goes from turbulent to wake

Question 23

What is skin friction drag? How does it differ in the laminar, transition and turbulent flows with a changing Re?

Answer 23

Skin friction drag is caused by viscosity of fluids and is a result of the shear stresses between the particles of a fluid within the boundary layer.
Refer to drawing in notes but…

With increasing Re, SFD decreases in laminar/turbulent flows, higher in TF, which is explained due to the higher shear stresses.
With increasing Re, SFD increases in transition flows.
Could tie in how with increase in speed, Re decreases and how this would affect them…

Question 24

How can we reduce skin friction drag?

Answer 24

Need to encourage laminar flow so:

• smooth surface (contamination)
• max thickness further rearwards
• decrease size/surface area
• reduce AoA
• decrease speed

Question 25

Draw a diagram showing du/dy

Answer 25

Look in notes

Question 26

What happened to Cf when Re increases in all 3 regimes?

Answer 26

Laminar & turbulent: dec

Transition: inc

Question 27

What is form drag?

Answer 27

Is the drag produced as airflow leaves the streamline of a boundary layer and the associated wake, causing for/aft px differences.

Question 28

What can help cause form drag? How do we reduce it?

Answer 28

Non streamline surfaces, sudden change in shape, adverse px gradients.
Reduce by streamlining.

Question 29

How does Re affect the wake?

Answer 29

Inc Re inc the wake

Question 30

Why does turbulent flow have a higher Ek?

Answer 30

As it travels faster, it also has higher shear forces so draws energy from the mainstream flow to overcomes the forces… which increase the Ek

Question 31

What is hf in Bernoulli’s Therom?

Answer 31

Is the loss of pressure head along a streamline between sections due to viscosity.

Question 32

What are the units for kinematic and dynamic viscosity?

What is the relationship between the two?

Answer 32

K: m*2/s (v)
D: Pa x s (u)

Dynamic/ro=kinematic so Re=VL/v

Question 33

The change in static head two levels in a Venturi represents what?

Answer 33

Difference in static pressure which is equal to the change in dynamic pressure.

Question 34

Nu vs mu

Answer 34

Nu is dynamic viscosity (u) in PaS

Mu is kinematic viscosity (v) in m^2/S