Well, first the short answer to the question:
No, waxing a typical car will have no measurable effect
on it's drag or gas mileage. There are simply too many other "bad"
aerodynamic features on an automobile that completely swamp any possible
effect that a smoother surface would have.
However, that answer really doesn't quite satisfy me as an engineer,
because I wanted to know what the effect would be, from a theoretical standpoint,
even if it is not measurable from a practical standpoint. However, not
being very well acquainted with fluids and aerodynamics, I was not really
sure how to approach the question to give a definitive answer and at least
a guess as to the actual magnitude of the effect, even if it is very, very
small. So, I asked some 'experts': I posted the question on two Usenet
News groups, "sci.aeronautics" and "rec.autos.tech".
First, some basics: what we are dealing with here is a question of fluid
flow and aerodynamics (air is considered a "fluid"). There are
broadly speaking two types of fluid flow across a surface, laminar and
turbulent:
- Laminar flow is a smooth flow over a surface, with the velocity of
the fluid being zero at the surafce, and fluid moving in theoretical layers
(laminates) in ever increasing speeds away from the surface. This type
of flow occurs over smooth surfaces, like over the wings of an airplane,
or maybe over the hood of a car. Because the fluid flow at the surface
is zero, the friction-drag coefficient is pratically independent of surface
roughness.
- Turbulent flow occurs around non-smooth protrusions of the object into
the fluid stream, and causes irregular fluid motions around it. For example,
the flow is probably very turbulent around the front grill of a car, or
underneath around all the pipes. Frictional-drag around these areas can
be affected by surface roughness.
There are also two types of drag to be concerned with:
- frictional drag, or skin friction. This is the drag with which the
question is concerned.
- Eddy-making drag, also called form drag, or pressure drag. This drag
occurs due to the "wake" in the fluid caused by the shape of
the object. A flat plate moving edgewise through a fluid has very little
form drag, but still can have skin friction drag; a flat plate moving normal
to fluid flow has LOTS of form drag, and skin friction drag (alothough
it may exist) is insignificant.
From the Usenet groups, I got several responses; here are a couple of
the more useful ones:
- from Clarence Chang, Ph.D., NASA Lewis Research Center,
Cleveland, Ohio:
Below 35 mph (there about) the skin drag is dominant. Above that speed,
the pressure drag dominates. Skin drag is linear with speed. Pressure drag
is proportional to speed square. Polishing the painted surface won't do
anything for the pressure drag, only the skin drag.
However, the turbulent drag from the various protrusions are much more
dominant than the skin drag. For example, if you enclose the bottom of
the car in a smooth flat skin (that is, no exhaust pipe and other stuff
exposed), you can easily get the drag coefficient to drop another 20%.
The drag coefficient on a typical car is about 0.35. 20% of that is alot.
If you open the windows, for example, you can easily increase the drag
5 to 10% or so.
Making the surface smooth only works for bodies with very low Cd, say in
the neighborhood of below 0.1. There are several easy ways to reduce drag.
You can enclose the bottom of cars with a smooth skin, but of course the
shops don't like it since you'd have to take the panels off to work on
the car. If however, you lower the body so that the distance between the
floor pan and the ground is closer, drag goes down too. You can also put
an air dam in the front of the car to route the air around instead of letting
it go under the car. That works too.
- From Prof. Greg Locock in Geelong, Australia:
I am heavily involved in solar cars, so I can't answer your question directly,
but here are a few observations: Our solar car is designed to be laminar
for about 60% of its length, by the use of a laminar flow section and a
good (surfboard quality) finish. We get a measurable increase in CdA going
from a mould-perfect glassfibre shell, to one that has solar cells set
in, and encapsulated, and then hand finished to a mirror like finish. On
our previous car this took us from a (frontal area reference) Cd of 0.10
to about 0.13.
A normal car (and most other solar cars) is laminar for up to 30%, and
more typically 20% of its length. The (skin) drag coefficient of the turbulent
part of the flow is so great in comparison with the laminar contribution
that we don't bother working the laminar part out. The proportion of the
drag in a modern car at highway speeds that is due to skin friction is
a very small part of the total. The defect size associated with a scruffy
paint finish is pretty small compared with the boundary layer thickness.
Glider pilots claim that bugs squashing on their laminar flow wings degrade
the performance to worse than normal airfoils. So I guess I'd conclude
that I would expect a tiny difference due to waxing the car, and a larger,
but still unmeasurably small, improvement due to cleaning the grot off
the paint.
My question back to him was: what is the difference in Cd before and
after hand-finishing the encapsulation material?
Unfortunately we did not measure the difference, but I think it would
be unmeasurable on the road. By the way, there is a good deal of experimental
error in these measurements. We are taking the car into a wind tunnel soon,
if we have time we could run an experiment. Any suggestions for a test?
I'd rather it did not involve messing the solar array up! I'd suggest sticking
PVC electrical insualtion tape across the car, say one strip every 100
mm.
I told Professor Locock that it was very kind of him to offer to do
an experiment in the wind tunnel for you to answer this question, but not
to devote their limited time there to it. But it just goes to show that
this question is rather difficult to answer in a theoretcal sense, and
that even these guys were unsure of the effects of surface smoothness on
drag for their vehicle. (I actually suggested that if they did do
the experiement, if it might tell them that effort devoted to making the
surface to a mirror-fine finish is really a waste of time! But it certainly
makes it look nice.)
So, in the end, I guess I still can't quite give you a good estimate
of the effects of waxing a car. It is certainly unmeasurable for a typical
car, or probably even a sleek sportscar. It is possible that for a very
aerodynamic vehicle, like an experimental solar car in which they are trying
to get every conceivable bit of efficiency, that it may make some very
small difference. But even for those vehicles, much greater fuel efficiencies
can be gained by improving the shape of the shell to minimize the form
drag.
I still have some inquiries out to various solar car teams at universities
around the world - if I hear from them, or get other useful info from
the Usenet particiapants, I will let you know by email directly. Sorry
it took so long to get an answer, and that it is only a partial answer
at that.
By the way, in my search I came across some interesting Web sites that
you might like to look at:
Regards,
Tod
------------------------------------------------------------
Tod A. Flak
Lawrence Berkeley National Lab
1 Cyclotron Road, MS 70A-3363
Berkeley, CA 94720
phone: (510) 486-5721
fax: (510) 486-5857
email: taflak@lbl.gov
Web page: http://todflak.lbl.gov/
------------------------------------------------------------
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