Improving Vehicle Efficiency with Aerodynamic Modifications by Phil

Improving Vehicle Efficiency with
Aerodynamic Modifications
by Phil Knox
Improving aerodynamics is just one of many fuel-saving techniques discussed on the ecomodder.com forum
Today’s presentation is a departure from discussions of how to power and improve efficiency for our
homes, in that we are turning our attention to the other main part of our energy use and carbon
footprint: our vehicles.
So we are familiar with cars modified to go faster or draw more attention with their appearance, but
there are actually folks out there that modify their vehicles and driving habits for the purpose of
achieving better fuel economy. Among this online group, Phil Knox has emerged as one of the
leading authorities on making aerodynamic improvements. Whereas I was active in this field of
automotive aerodynamics for a couple of years, Phil has been experimenting with aerodynamic
modifications for decades.
We actually collaborated on this presentation. I put together this powerpoint slideshow from Phil’s
material, and I am actually going to run through the dry underlying technical concepts as an
introduction. After this Phil will present real world applications of these principles by showing his
various projects.
Factors contributing to the force of aerodynamic drag:
• Size of the object
• Shape and orientation of object
• How hard the wind is blowing
Let’s start with an example from common experience: Think about handling plywood sheets
outside on a windy day. It does not take long to notice what affects that drag force you feel. So
the size of the sheet matters; a two foot square sheet is easier to handle than a 4 by 8 sheet.
The fact that it is a very blunt shape and how it faces the wind makes a big difference. Finally,
the force depends on how hard the wind is blowing.
1/2ρV2
CD
Area
Drag Force=(Wind Pressure)(Shape and orientation factor)(size factor)
So all these factors are taken into account in the equation for drag force. Don’t worry, this is the only
equation in the whole presentation. The expression for how hard the wind is blowing is ½ rho times
velocity squared. Rho is the density of the fluid, so if you were driving under water, this equation would
still be valid. Accordingly, it is vastly easier to walk against a 5 mph breeze as opposed to swimming
against a 5 mph current. So V is the relative speed between the object and the upstream fluid. Note that
this is velocity squared! Accordingly the pressure of a 40 mph wind is not just twice that of a 20 mph wind
but 4 times more. The next factor, the drag coefficient, takes into account the shape of the object and its
orientation to the wind. So in a wind tunnel the forces are measured, the dynamic pressure and area are
determined and this equation is rearranged to solve for the drag coefficient. For cars the area we use is
the largest transverse cross-section.
What things may be done to reduce the drag force? If we drove at a higher altitude that would reduce
rho some- not really a helpful technique. Driving more slowly has a big impact, so driving behavior is
important. These last two terms, Cd and area involve changing the geometry of the vehicle.
Here is a listing of drag coefficients for the various shapes we learned in kindergarten. Fortunately
most of us drive cars that are shaped better than these. Down here at the bottom are the really
good shapes.
This page compares these bodies having the same cross-sectional area. The barn door is the
reference for a high drag shape. Going to a circle is a big improvement over the barn door,
but it is still far from optimal. A stubby tear drop shape is a further step in the right direction.
Finally we get to the airfoil shape and realize that fish have been onto something fairly
optimal for a very long time. Note that the size of the trailing wake goes down with the
reduction in resistance. These wakes are regions of low pressure. The barn door is really bad
because it has high pressure all over its leading side and a really broad area of low pressure
on its trailing side. The maximum pressure that you can get on the front side is the entirety
of the one half rho v-squared of dynamic pressure. The low pressure side can actually go
further in the opposite directions. You might get something like a negative 2 of the dynamic
pressure value.
Sharp changes in contour are highly upsetting to fluid flow
This next picture was made with bubbles in the flow with the camera shutter open long enough to show
them as streaks. The fluid has mass so there are inertial forces acting on it that make it not interested in
making sharp turns. If there were a dumpster behind a building, this is about how you would steer to get
around it. These swirly patches are flow separations. In fact they are often referred to as separation
bubbles. This speed bump is sending pressure disturbances (otherwise known as sound waves) in all
directions so the whole flow field knows that the obstacle is there. The fluid particles over here have
already heard the traffic report so they are not going to just drive into a wall. This picture also shows why
you should not put a wind turbine close to a building and the need to put it high up if it is behind a
building.
Flow Separation
Fluid flow can also be reluctant to slow down and spread out. Try pouring water backwards through a
funnel and see what happens. This is a fairly gentle looking curve but the condition of the flow is such that
it does not stay attached.
Flow that stays in neat layers is called laminar flow. Flow that is too jumpy to stay in layers is turbulent
flow. Turbulent flow occurs when the inertial forces on the fluid particles become too large for the viscous
forces to dampen out. So when freestream flow encounters a surface, the flow is laminar at first and then it
transitions to turbulent flow. Typically for a car, there may be an inch or two of laminar flow at the leading
edge, but the rest is turbulent. The good news is that turbulent boundary layers do not separate as readily
as laminar boundary layers because more energy gets imported from the outer flow to the flow near the
surface. So for this image, the flow is laminar to right here and then it transitions to turbulence. If the flow
had been turbulent before the dip, it may have stayed attached.
This drawing just shows the velocity distributions for a laminar boundary layer separation.
The flow field around an
automobile is
characterized by flow
separations.
The point that I have been leading up to is that the flowfield around a car is quite messy rather than
neat and tidy. There are often flow separations in addition to the big wake at the back end. So this
example shows a separation bubble right on the leading edge of the hood which will show up if the
front is not rounded sufficiently and there is also one at the base of the windshield. One thing I have
wondered about the Prius is if it has a bubble at the base of the windshield because there is not much
of a contour change at all from the hood to the windshield. This example also shows a pair of vortices
starting at the A pillars. The flow has to go faster over the top of a car than it does for the sides or the
bottom, so by the Bernoulli principle the pressure at the top is lower so that why the air wants to get
on top. Airplane wings make these kind of vortices from the tips.
So when I went online to look for smoke visualization photos of cars, I noticed that most of them
actually looked neat and pretty. There would be these nice lines of smoke that kind of disappeared at
the back of the vehicle because of the mixing in the wake. I found this picture that is far more
revealing. It looks like they are injecting this red smoke from ports on roof of the car. That makes it
where you can see all the tumbling and oscillation in the flow in the wake. Next time you drive behind
a truck when it is raining look at how the drops swirl around. There will be a vortex shed from one side
and then the next one is shed from the other side so the flow has an oscillation pattern to it. If you are
really observant you can feel how these alternating vortices shake your vehicle.
1970-80’s
Since the ‘80’s car drag
coefficients are closer
to 0.3
So how low of a drag coefficient can be obtained for road vehicle?
For a completely streamlined body of revolution with nothing else around it the drag coefficient may be
lower than .04. Now what I think they mean for this next one down is a body of a more representative
length and width ratio for a car rather than a really long and slender body.
When you bring a low drag body close to the ground and put wheels on it the drag coefficient jumps up.
So the wheels are part of the increase, but when you put a shape close to the ground the flow can’t just
go anywhere it wants because it is constrained by this ground plane. So the drag that results from putting
a body close to something else is referred to as interference drag. In this case it makes the body seem
stubbier to the airflow than it would if it were away from the ground plane. So if you want to account for
this interference and get the right boundary condition for a perfectly straight streamline under the vehicle
– this is how to do it (next slide). So the representative drag coefficient for a body close to a ground plane
is equivalent to a shape and its reflection out in a uniform flow field.
(Back to previous slide)
This figure is from a book published in the late ‘80’s so the information is out-of-date. About this time
(the ‘80’s) car companies began to get much more serious about aerodynamics, so today typical drag
coefficients are much better. So half the distance to the ideal has been covered over the last few
decades.
The last figure I am going to talk about is the historic trend in car drag coefficients. Again this figure is not up
to date because drag coefficients are now down here. So we started out with boxes on wheels and then
streamlining got introduced into styling. Fuel was cheap and plentiful for a while, and then things changed….
Beginning sometime in here, all production cars would get a thorough work-over in a wind tunnel. The
manufactures would eventually start spending as much wind tunnel time reducing noise as reducing drag. I
also know that car companies check-out their competitor’s cars with wind tunnel testing.
My involvement in the field of automotive aerodynamics wound down in 1991, but our speaker today actually
started before me and has kept it up ever since. So thanks for coming today Phil, and I am handing the show
over to you.
Automakers know
how to make
highly
aerodynamic cars
but they generally
do not choose to
market them.
Late ‘80’s simulation set-up by CMR Executed as a batch job on a mainframe
computer – got the results the next day
Questions?