01 boundary layer and reynolds number

8/2/2019 01 Boundary Layer and Reynolds Number

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The Boundary Layer and Reynolds Number

Viscous Flow = flow with friction

Friction/Viscosity effectsand boundary layers

turbulent

laminar

Reynolds Numbers

Airflow Separation

Scale Effect


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Friction Effects

Fig 1.24 top


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Boundary Layer Theory

As airflow slows it tends to become less

stable and mixes


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Boundary Layer Theory

Air at the surface stops (transfer ofmomentum)

The further from the surface the flow speed

increases (not affected as much by viscosity)

When the flow reachesfree stream velocity

boundary layer terminates


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Velocity Gradients

Flow velocities are faster close to the surfacefor turbulent boundary layers


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Friction Effects

Boundary Layer Development


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Friction Effects


Laminar boundary layer

relatively thin layer occurring near leading edge

smooth streamlines little vertical exchange of air particles

stable airflow

Transition Region

smooth flow starts to break down

waviness starts


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Friction Effects


Turbulent Boundary Layer

thicker layer some distance aft of leading edge

random streamlines significant vertical exchange of air particles

unstable airflow

laminar sub-layer may occur

heat exchange greater than laminar flow


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Laminar Boundary Layers

Pros

The slower velocities near the surface causeless friction drag > laminar flow airfoils tend

to be low drag airfoils Cons

Since the flow is slower near the surface itwill come to a stop sooner resulting in a stall

at lower AOA

Laminar flow airfoils do not do well at highangles of attack (AOA) > stall sooner


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Turbulent Boundary Layers

Pros

The faster velocities (possess higher kinetic

energy) near the surface are harder to slowdown > this fact enables a wing to achieve ahigher angle of attack and create more liftbefore stalling.

Cons The faster velocities near the surface create

more skin-friction drag.


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Pressure Distribution for Conventional

Airfoil


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Pressure Distribution for Laminar Flow

Airfoil


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Friction Effects


Low skin friction makes laminar flow

desirable for streamlined objects.

Low kinetic energy makes laminar flow

undesirable at high angles of attack

which increases the probability of flow

separation and the accompanying largeincrease in drag.


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Reynolds Number

Laminar vs. Turbulent

velocity

viscosity, distance from leading edge

density,

Reynolds Number

dimensionless parameter

indicator of B.L. condition

laminar

turbulent

RNx = Vx/Where:

RNx = Reynolds Number at

distance x along the chord,ft.

V=free stream velocity, fps

= viscosityNote: decreases with

altitude but /increases with altitude


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Reynolds Number

RN Lower

short chord

low speed high altitude

RN Higher

long chord

high speed low altitude

For a given flow the RN is proportional to the ratio of dynamic forces to friction

forces. A flow with a higher Reynolds number

is less viscous than one with a lower Reynolds number. We use RN to compare

flow characteristics.


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When Does the Boundary Layer Change

from Laminar to Turbulent?

TheReynolds Numberis used topredict the

type of boundary layer that will occur.

RNx = Vx/


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Reynolds Number

Flat Plate Laminar to turbulent transition starts at RN 530,000

Transition complete at RNs of 20 to 50 million

RNs of 1 to 5 million - partly laminar partly turbulent

RN effect on friction drag


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Reynolds Number


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Airflow Separation

Character of boundary layer influenced by

pressure gradient

favorable gradient(proverse/dropping)

assists laminar flow

unfavorable gradient(adverse/increasing)

impedeslaminar flow

Increasing velocity = decreasing pressure

Decreasing velocity = increasing pressure


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Friction and Airflow Separation

Friction in the flow (viscous flow) causes

a tugging force (skin friction drag)

slowing of the flow (loss of KE) and a pressure rise (adverse pressure gradient) and if

the KE is not great enough

airflow separation, which causes

drag (pressure drag due to airflowseparation)

loss of lift


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Airflow Separation and Pressure Drag


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Distribution of Pressure


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Airflow Separation or Stall

Friction and adverse pressure gradient causes the

boundary layer to slow, reverse direction, and

eventually to separate from the surface

The oncoming free stream sees this region as a barrier

and flows over it/around it (airflow separation)

This results in a loss of lift and increased drag


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Stall can be delayed by encouraging high

speed air to get closer to the surface

This is called turbulating the boundary layer

Vortex generators accomplish this


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A boundary layer can also be turbulated

by surface roughness (i.e. dimpled golf ball)


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Drag on a Golf Ball

The turbulated boundary layer will stay attachedto the ball/wing longer (higher kinetic energy) >Reducing the size of the wake (or flow disturbance)behind the ball.

The smaller the wake the lower the drag due topressure differences

The net result of dimpling the ball(increasingsurface roughness) is a reduction in total drag

the pressure drag decreases more than the frictionaldrag from the turbulent flow increases


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Friction Effects

Laminar flow = low skin friction drag

Turbulent flow = higher skin friction

drag

Separated flow = high pressure drag

Attached flow = low pressure drag

Golf/Tennis/Baseballs (ping pong balls?)

Vortex generators


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Streamlining and Drag


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Airflow Separation

Skin friction dragreduces boundary layerkinetic energy.

Premature stagnation of

boundary layer occurs when lower levels lack

sufficient kinetic energy

in the presence of adversepressure gradient

Reverse flow on surface

Subsequent airflowoverruns stagnationpoint


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Airflow Separation

Airflow separation occurs from: High angle-of-attack

upper pressure gradient too adverse

boundary layer cannot adhere to surface

Shock waves at transonic speeds

static pressure increases sharply through shock wave

boundary layer loses energy through shock

separated flow behind shock

compressibility buffet

Extreme surface roughness on aircraft (heavy frostor skin damage) will increase skin friction drag andearlier airflow separation will cause reduction ofClmax and increased stall speed.


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Airflow Separation


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Airflow Separation

Prevention of boundary layer separation Boundary Layer Control (BLC)

Energize boundary layer

Laminar versus turbulent boundary layer

Vortex generators

Slots/slats

Blowing

Remove de-energized (lower) portion of boundarylayer

Suction


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Scale Effects


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Scale Effect

Scale Effect

variation of aerodynamic characteristics

with RN = scale effect

extremely important in correlating wind

tunnel data of scale models with actual flight

characteristics of full size aircraft

produce variations in stall angle-of-attack / max lift coefficient /drag

negligible affect on pitching moments


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Scale Effect

So lift coefficient is

actually a function of RN

(i.e., in addition to being

a function of AoA andshape)

Effect of increasing RN

on a given section

Clmax increases

stall AoA increases

Cd decreases


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Scale Effect


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Scale Effect

Fact: For a given shape, lift coefficient

and drag coefficient are a function of

AOA, RN, and Mach Number(MN) so a scale model will have the same lift and

drag characteristics as the full scale item as

long as the RN and MN are the same

(thus RN and MN are referred to as

similarity parameters)


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Scale Effect

B-747 wing root RN

68.3 million

Mach 0.8 & FL 350

8.5 million

150 kts & S.L.

450,000 1/20 scale model

150 kts & S.L.

01 boundary layer and reynolds number

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