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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
Boundary Layer Development
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
Boundary Layer Development
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
Boundary Layer Development
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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Airflow Separation or Stall
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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Airflow Separation or Stall
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.