springman

These dicksmokes make Chitty Chitty Bang Bang look like a well researched documentary.

Nicole

Why didn't they just rent a wind tunnel and use the same car forwards and backwards to measure wind resistance and lift?

It would have so much easier and more precise.

Comparing two different models, one with spoilers not available when the myth was born, both with different engines, etc - it makes no sense to me!

Landseer

Yet for Popular Science and this Mythbusters franchise, the win is pretty big.
Americans love the story. Hype and outrageousness and glamor.
Not precision and logic.

Sense doesn't sell, at least not much.

Now that the 928 is exposed 10's of thousands more rednecks know what is under the skin, and that can't be good.

rjr0928

Rednecks???? To Late
http://www.youtube.com/user/Davidsfa.../0/z-9o-5qudrI

rjr0928

http://www.youtube.com/user/Davidsfa.../0/z-9o-5qudrI

G Man

I have heard F-1 cars generate so much downforce they could be driven upside down. When are they gonna put that one to the test?

mark kibort

that was a pretty lame episode I sure they could have go 3 other cars that coasted further and shorter with that kind of test. so many variables, I lost count. but, what was most telling was the windtunnnel test. another test would to be to find two vehicles with a huge disparity in drag coefficients. I guarantee, that there is not 2 seconds in the quarter mile worth of difference. that can be plugged in to any simulator to be proved right there.
However, VERY cool to see our car on TV!!! that red thing looked very nice. how could they destroy such a nice car!!????

slate blue

I agree it was poorly done, lots of improvements could have been implemented, like using a G-Tech for the timing of the runs.

One thing I found interesting was the water tank test, the model could have been better but the dead zone behind the car could well be correct. if so I am looking forward to the implementation of the base bleed and other aero features that will lead to better pressure recovery and smaller wakes.

Greg

F451

x 928.

What a waste of a perfectly good 928.

Asshats...

mark kibort

also, ANY differenc in aerodynamic characteristics of the car facing forward vs backward, was COMPLETELY negated by the FACT that the car had a totally different horizontal orientation. (car was NOT level after the mod) The entire show was good and bad. GREAT that they spent time playing around with the 928, and it got a lot of air time. (very cool) and stupid that this test was one of the worst ones that they have EVER done as far as controlling variables. I was thinking of the things that could have effected the MPG tests, or even the 0-60/1/4 mile tests. when they removed the body, they certainly could have effected the air flow to the engine itself, which probably is the biggest factor, aside from the temperature sensors that also change mixture dynamically. weight distribution changing rolling friction is another. so many variables, so little control on this one. Sorry boys, i give this test, a thumbs down.

Doug&Julie

The number one rule of television is....it's entertainment first, everything else second. Wind tunnels are cool (for geeks, like me), but they're not good television. Accurate data equipment? Meh...just floor it and run fast, that will be good TV!

I love the show. I'm fine with the premise that it's science (and, fundamentally, it is). But I also recognize it's TV and they're going to blow a lot of sh*t up. It's annoying to me, but I'll live with it. It's all good fun.

..and +1 for being totally in love with Kari Byron. Hubba hubba.

Z

Exactly. The vast majority of people out there in TV land have never heard of a wind tunnel, and have no idea what any kind of results from it might mean, no matter how superior that way of doing it might be. An awful lot of the general public consists of people like this:

Warren928

Yea, they suck at doing earnest research. Their f----ing HACKS!

mark kibort

they did a great job on the airplaine conveyor belt

Glenn Evans

The flow in the water tank was not representative of a car at highway speeds, and not representative of the airflow over a 928. I posted the following to the Mythbusters forum on discovery.com(http://tinyurl.com/6x3vt2v). I am an aeronautical engineer, BTW. I am amazed that NASA permitted its name to be used without insisting that the water flow visualisation be explained in context.


The model tested in the water tank and wind tunnel was a 928S4, minus its rear wing. The model was stated to be 1/24th scale. In the wind tunnel, it was mounted clear of the floor, so that it would be clear of the slower-moving boundary layer close to the surface of the floor and fully exposed to the 100mph airflow. The measured drag was 0.34 pounds for the forward-facing model and 0.37 pounds when it was reversed. This corresponds to Cds of about 0.36 and 0.39 respectively.

The water tank visualisations, however, were not representative of the aerodynamics of the full-sized car. This was not because they were performed in water, but because they were representative of velocities in air in the order of two to three feet per second (two miles per hour or less). The behaviour of the boundary layer (the slower-moving air close to the surface of the car) at such low velocities is different to that over most of a car's surface at normal road speeds. Surely the NASA engineer would have told Adam and Jamie this.

The red dye over the roof shows that the water flow separates from the surface of the model's roof as soon as the roofline starts to angle downward. This is characteristic of a laminar boundary layer, which would not extend anywhere near that far back on a car at highway speeds.

The early separation of the flow from the model's surface, and the huge, expanding, separated wake behind it, is not consistent with the Cd of 0.36 measured in the wind tunnel. It certainly is not representative of the full-sized 928S4's Cd of 0.34

The red dye behind the model reveals that there is no water flowing under it. This means that the velocity up to 5mm (1/24 of the real car's ground clearance) above the floor of the tank is imperceptible, and that the boundary layer at that point of the tank floor is considerably thicker than that. This is another indication that the velocity is very low, as the thickness of a boundary layer decreases as velocity increases.

So, in asserting that the Mythbusters' water tank visualisations were not representative, I've used the terms “boundary layer”, “laminar”, “turbulent” and “separated” in relation to the water flow but have stated that water (a liquid, therefore incompressible) actually can be used to visualise flows in air (a gas, therefore compressible).

In fact, a flow of gas will behave as if it were incompressible at low Mach numbers (Mach 1 is the speed of sound). So, when air flows around an object, it will not compress to make room but increase its velocity, just like a river flowing through a narrow gorge, and its static pressure will decrease. (Static pressure is the local pressure which you would measure with a barometer, and dynamic, or ram, pressure is that which you feel due to the movement of a fluid. The total pressure remains constant ie the dynamic pressure plus the static pressure in a fluid flow equals the barometric, or static pressure if the velocity of the fluid were zero.)

Now, think of a fluid as one molecule-thick sheets laid on top of each other which flows edge first. Away from any disturbance eg an object in the flow, these sheets move as a single mass and do not slide in relation to each other. This is termed “steady flow” or the “free stream”in fluid dynamics.

If there is an object in the flow, or the fluid flows over a surface, the sheet of molecules adjacent to the surface does not move over that surface ie its velocity is zero. Each sheet above that will slide smoothly over the one below it – ie its velocity will be greater – until a sheet is moving at the same velocity as the free stream. This layer of fluid moving at a lower velocity than the free stream is termed the “boundary layer”. Where the analogy of molecular sheets holds true, the boundary layer flow is said to be “laminar”. The force required to make the sheets slide over each other is a measure of the fluid's viscosity ie its resistance to flow.

A laminar boundary layer has low drag ie the sheets of molecules slide over each other relatively easily and the free stream velocity is reached a relatively short distance above the surface. (A steady flow outside the boundary layer often is incorrectly called “laminar” This is incorrect because the sheets do not slide in relation to each other.)

The flow in the boundary layer may become turbulent ie the layers break into tiny vortices and mix with each other, while the boundary layer itself remains attached to the surface and the flow outside the boundary layer remains steady. There is much greater exchange of energy between the layers in a turbulent boundary layer, so the velocity increases much more slowly above the surface of an object in the flow, and the boundary layer is much thicker than for a laminar boundary layer. The drag (resistance force) on an object will be greater where the boundary layer is turbulent.

When fluid flows over a smooth object, the boundary layer typically starts off being laminar but transitions to being turbulent. This will occur even if the fluid is flowing over a very smooth, flat surface. If the surface is rough, the boundary layer will become turbulent much sooner.

When a fluid flows over a smoothly curved surface, its velocity will increase and its static pressure will decrease while the surface curves upward, and the reverse occurs as the surfaces curves back downward. A laminar boundary layer will remain laminar for a much greater distance on an upward-curved surface ie where the static pressure is decreasing. When a laminar boundary layer meets increasing static pressure, however, it does not have enough energy to maintain its momentum and slows very quickly, and cannot stay attached to the surface. The flow behind that point of separation will be turbulent, and be so for much further above the surface than the thickness of the boundary layer. Such a separated flow has far greater drag than an attached turbulent boundary layer.

A turbulent boundary layer, however, has much more energy than a laminar one and will remain attached to the surface for a much greater distance against increasing pressure. In practice, the boundary layer will become turbulent after a very short distance in most situations. This has the benefit of allowing a flow to remain attached for much longer when flowing over a smooth, downward-curved surface (like an aircraft wing or the roof of a car), keeping the drag much lower than if the flow separated from the surface.

An object's drag depends on the velocity, density and viscosity of the fluid flowing over it, and the size and shape of the object. Its shape determines its Cd, and (for cars) frontal area is used for size. I have said, however, that the position of the points where the boundary layer transitions from laminar to turbulent, and where the flow separates from the body, are affected by velocity. This means that Cd is not constant at all velocities. The position of these points also are affected by the distance the fluid has flowed over the surface.

This, however, does not account for different test fluids, or the fact that the flow does not have to travel as far over the surface of a scale model as it does for the full-sized object; so the transition of the boundary layer from laminar to turbulent, and separation of the flow, will occur further back on a model's body than they would on the body of a full-sized car, if the velocity, density and viscosity of the fluid is the same.

These variables of size, velocity and fluid properties are combined in the Reynolds number (R), which is equal to (fluid density) x (velocity of the free stream) x length / (fluid viscosity). Flows around geometrically similar bodies of different sizes, and in different fluids, will behave identically when the Reynolds number and Mach number are the same. For practical purposes, this holds true at low Mach numbers, at which compressibility effects are negligible compared to viscous effects. In other words, the flow around a scale model will behave the same as that around a full-sized object at the same Reynolds number, provided that the Mach number is kept low. The Reynolds number doesn't have dimensions, and is the same whether metric or Imperial units are used for the physical properties from which it is derived, provided that they are consistent with each other.

The flows in the Mythbusters visualisations can be analysed utilising the known dimensions of the 928S4 and the scale of the model, and by timing the shift of disturbances in the dye to estimate the velocity of the water flows. The velocities in the visualisations vary from around 0.1 to 0.2 feet per second, and the Reynolds numbers at the separation points indicated by the dye vary from around 5000 to 8000. At 50mph, the Reynolds number at the corresponding point on the roof of a 928 is about 5.1 million.

I stated previously that a boundary layer will transition from laminar to turbulent even when flowing over a smooth, flat surface. This will occur at a Reynolds number in the order of 500,000. At 50mph on the full-sized 928, the Reynolds number is 500,000 a little over one foot from the front of the car. The relationship between Reynolds number and the transition of a boundary layer from laminar to turbulent is described in an authoritative reference work on aerodynamics, “Theory of Wing Sections” by Ira H. Abbott and Albert E. von Doenhoff, on page 105 (Section 5.12) of the Dover Edition published in 1958. (Abbott was the Director of Aeronautical and Space Research at NASA, and von Doenhoff was a NASA research engineer.) It states:

“For viscous flows at very low Reynolds numbers, as in oil, all disturbances are damped out by the viscosity, and the flow is laminar regardless of the magnitude of any disturbance. As the Reynolds number is increased, a condition is reached at which some particular types of disturbances are amplified and eventually cause transition. This value of the Reynolds number is called the “lower critical”. Further increase of the Reynolds number causes amplification to occur for a greater variety of disturbances and increases the rate of amplification. Under these circumstances the Reynolds number at which transition occurs depends on the magnitude of the disturbances. Transition can be delayed to high values of the Reynolds number only by reducing all disturbances such as stream turbulence, unsteadiness, and surface roughness to a minimum. … The upper value appears to depend only on the care taken in conducting the experiment.”

Bear in mind, however, that a laminar boundary layer will quickly separate from a surface when it meets increasing pressure, such as occurs behind (ie downstream) of the point of maximum thickness of a wing section or the highest point on the roof of a car.

So, what the Mythbusters have shown in the water tank is a flow clearly below the lower critical Reynolds number. The boundary layer has remained laminar even after flowing over where the windscreen meets the roof (where it surely must detach but must be able to reattach and remain laminar while the roofline is still rising).

Hopefully, I have convinced you that the visualisations of flow in the water tank on Mythbusters were not representative of the aerodynamics of a full-size car at highway speeds.

The fact is that the original, spoilerless 928's Cd of 0.41 was considerably lower than the vast majority of its contemporaries in 1977, and for years afterwards. Porsche knew how to style low-drag cars: the original 356 (1948) had a Cd of 0.375; the original 911 (1963) 0.381 and the 924 (1976) 0.36, which then was close to, if not the lowest Cd of a production car. Low drag, however, is not as important as stability and minimising lift. The original 928's Cd of 0.41 was a compromise of all these, based on the knowledge of the time, to enable the car to reach over 140mph and do so safely.

Twenty years after the introduction of the 928, Audi got the aerodynamics of the TT badly wrong, and Mercedes did so with its CLR at Le Mans in 1999. You can see two of the three spectacular flips of CLRs at Le Mans on youtube.