Transcript Drag - Delta State University

Drag
Lecture 6
Chapter 3
What is Drag?
What are different types of Drag?
Drag
• Drag is the term used to denote resistance
to airflow.
– Example: Hold your arm out of the window of
a moving car. What happens? You are
experiencing drag.
Pressure Drag
• Most of the drag experienced by holding
your hand perpendicular to the ground
outside a vehicle window to the airstream
is pressure drag.
• Pressure drag results from the difference in
pressure between the fore and aft sides of
the hand.
Skin Friction Drag
• A frictional force over a surface is called
skin friction drag or viscous drag.
– A flat plate parallel to the airflow experiences
skin friction drag.
• Figure 3-2 page 57.
Parasite Drag
• An object will have friction along the
surface(skin friction drag) and some
pressure drag because a wake is being
formed behind it.
• Pressure drag and skin friction drag are not
entirely independent because the size of
the wake is dependent on the point of
separation of flow.
Parasite Drag
• The sum of pressure drag and skin friction
drag.
– Figure 3-2 page 57 shows an airfoil with both
pressure drag and skin friction drag.
Boundary Layer
• Boundary layer is the free airstream above
the surface and the point where the
velocity builds up above the surface.
– Figure 3-3 page 58
• Within the boundary layer the velocity takes on a
profile an gradually reduces from the free stream to
zero at the surface.
– The reaction to the retardation of the flow within the
boundary layer is the skin friction drag.
Back to Skin Friction Drag
• The boundary layer is the mechanism by
which skin friction drag is created.
• The extent of skin friction drag depends on
the shape and thickness of the boundary
layer.
Laminar & Turbulent
Boundary Layers
• Laminar Flow is a smooth, layered fashion,
in which the streamlines all remain in the
same relative position with respect to each
other.
• Turbulent Flow is one in which the
streamlines break up & become all
intermingled, moving in random, irregular
patterns.
Transition from Laminar
to Turbulent
• Transition region is the area where a
boundary layer changes from laminar to
turbulent.
– Examples: Cigarette Smoke
• Figure 3-4, 3-5 page 59
Reynolds Numbers
• Scientist Osborne Reynolds discovered
many of the principles of fluid viscosity
and boundary layers.
• Remember viscosity of a fluid is the
stickiness of a fluid.(all fluids have some
amount of viscosity)
• A reynolds number is used to measure to
viscous qualities of fluid.
Reynolds Number
•
•
•
•
Re= V x d/ v
Re = Reynolds number
V= Fluid velocity
d= Distance downstream from leading
edge
• v= Kinematic viscosity of a fluid
– A low reynolds number is laminar
– A high reynolds number is turbulent
Reynolds Numbers
• The higher velocities or longer distances
downstream tend to produce higher
Reynolds numbers (greater potential for
turbulent flow)
• Reynolds numbers are different for
different operating airspeeds.(length is
average wing chord)
Wakes & Pressure Drag
• The imbalance of pressure between the
forward face of the plate and that on the aft
face(in the wake) results in drag force.
– Larger wake produces more drag
• Figure 3-7 page 62.
Adverse Pressure Gradient
• A flow moves along a surface it is creating
friction from the boundary layer.
• As the boundary layer becomes thicker more
friction drag is created.
• As the cross-sectional area of the body is getting
smaller in the downstream direction, the velocity
is less, pressure increases causing an adverse
pressure gradient. (the pressure is working against the
flow rather than with it)
Skin Friction & Adverse
Pressure Gradient
• The combination of skin friction/adverse
pressure gradient gang up on air flow an
prevent it from traveling farther along the
surface.
• It is ideal to delay the separation.
– The longer it remains attached, the smaller the
resulting wake and pressure drag.
Bluff Bodies
• Bluff bodies are bodies with diameters that
are fairly large in relation to their length,
pressure drag is the greatest offender.
• Fuselages,nacelles,landing gear wheels are
bluff bodies.
– Figure 3-9 page 64.
Sphere/Cylinder
• At a low Reynolds number,(low
velocity/small diameter)the flow will
remain entirely laminar and separate early
forming a large wake.
• At a high Reynolds number the flow will
transition to a turbulent boundary layer
before reaching the separation point.
Laminar/Turbulent Boundary
Layers
• Turbulent boundary layers have more than
laminar boundary layers, the flow is
turbulent, the separation is delayed and a
smaller wake is formed.
• Why do golf balls have dimples?
Why do golf balls have dimples?
• Roughing the surface will promote early
transition and have less drag.
Drag Coefficient
• Drag(like lift) is proportional to the
dynamic pressure of the air and the area on
which it acts.
• CD = drag coefficient CD= Drag/q x A
•
•
•
•
* = density
V = velocity
A = area
q = dynamic pressure
Drag Coefficient
• The drag coefficient can also be thought of
as the ratio of drag force to dynamic
pressure force.
• The drag is actually being generated by a
three-dimensional body, yet the drag is
proportional to only two dimensions of the
body.(when using a drag coefficient)
Projected Area viewed from all
sides
• Projected areas (fuselage, nacelle, landing
gear) have different shapes when view
from different directions.
– Frontal, planform, side
• The drag components are also different for
each side.
– Porsche vs. Van
Induced Drag
• Induce drag is the direct result from the
production of lift.
– Figure 3-13 p. 69
• The lift vector, being perpendicular to the actual
airflow is tilted backward, resulting in a component
of lift in the streamwise direction. (drag)
• Production of induced drag by downwash pushing
down the airstream vector resulting in tilted vector.
Reducing Downwash
• Longer wingspan places the wingtips
farther apart.
• Higher aspect ratio.
• Reduce lift coefficient( because induced
drag is proportional lift coefficient)
Factors that increase
Induce Drag
•
•
•
•
•
High Weight
Less efficient wing (than elliptical)
High Altitude
Low velocity
Low wingspan
Quiz on Lecture 6
Chapter 3
Please take out a sheet of paper
Include today’s date and your name
Quiz on lecture 6
Chapter 3
• Compare and contrast parasite and induced
drag. Include examples of each.
• What is a reynolds number?
• Compare and contrast the two types
boundary layers discussed today.