Models that don't work on real cars

[JulianEdgar]

Aerohead has a belief, that has unfortunately been apparently adopted by many people here, which is quite wrong.

That belief is that The Template (and roughly similar shapes) have zero lift. Of course, these shapes have in fact very high lift.

Aerohead often quotes references to support this incorrect belief, but the clearest and most accessible he has quoted is Figure 2.4, page 51 of Hucho (second edition).

So let's look at that diagram. (Note that I not going to (mis)quote or paraphrase, as Aerohead does so often. Instead, I will reproduce the diagrams and text.)

The diagram Aerohead references:



Now, what does Aerohead say about it? He said:

Originally Posted by aerohead

If you'll revisit Figure 2.4, page 51 of Hucho, you can see how,over the last 14.5% of the body, local pressure rises all the way back to local barometric pressure. Depending on rear overhang, and low pressure under the body, due to a diffuser, rear lift can be zero, like the VSPORT's.

Note how there are no caveats or qualifications in what Aerohead says - he's accepting the diagram as being applicable to real cars. But look at the caption. The diagram is for inviscid fluid - that is, an imaginary fluid without viscosity!

And does that matter? Let's look at what is written on the next page:



Note from the passage:

On the rear part of the vehicle's upper surface a steep pressure rise occurs, and it is in this region where considerable differences exist between the real flow of a viscous fluid and the inviscid flow shown here.

and

If all X-components of the pressure distribution on the vehicle surface are integrated, the result for the drag will be D=0.

and

In the real, viscous flow there exists a drag force, but it cannot be explained by considering an ideal, inviscid fluid.

None of this is much of a surprise to me - if you measure real stuff on real cars on real roads, you quickly find where people have misunderstood (or mis-applied) theory.

The idea that Aerohead constantly pushes that low drag = low lift is simply rubbish. It's one of many misconceptions he has, most of which are very easily shown by getting away from the keyboard and doing some real-world measurements on the road.

It's not hard - and it might prevent silly mistakes like referencing a diagram for inviscid fluid as if it applies to real cars.

[oil pan 4]

When I drove a "new beetle" which is about as close as you can get to the template on a street driven car, that thing generated lift and had horrible cD.
It had to use a spoiler so the back wheels would stay planted to the road surface.
But quoting 2 generations out of date stuff is kind of his thing.

[mpg_numbers_guy]

Beetles slop down way too fast to conform to the template anyway, not exactly an apples to apples comparison.

[M_a_t_t]

The simplest way to look at it:

Rounded surface creates lower pressure. Look at an air foil, the top surface creates lift by lowering the pressure. Overall shape of the car is rounded, thus creates lower pressure on that surface (varying levels). Low pressure on the top half of the car is lift.

Flip a car upside down and you get down force right?

[freebeard]

It's been done.

When I drove a "new beetle" which is about as close as you can get to the template on a street driven car, that thing generated lift and had horrible cD.

That's because it isn't. At least the first generation. The styling trope can be traced back to the Audi Avus or Quattro, the stylistically arched daylight opening. It made production as the Audi TT. Spoilers save TT owner's lives.

To follow The Template, max camber needs to be at 30% of the length.

Thanks OP, I hadn't seen this one before:



What do you make of the internal dotted lines? They seem to imply that pressure differentials transmit through a solid body. ???

I'm patiently waiting for the inexorable: duckduckgo.com/?q=CFD Open+VDB and Blender. The best link appears to be:

Fluid Simulation with Blender - FetchCFD
Search domain http://www.fetchcfd.com/view-project...n-with-Blender
Blender's physics fluid simulator uses the Lattice Boltzmann methods to simulate the fluids and allows easy adjustment of resolution. Blender source file also attached. THIS WORK CAN BE REUSED UNDER THE CC BY LICENSE.

https://www.fetchcfd.com/view-project/192

This will bring it down from supercomputer level to the hobbyist. You don't even need to have a garage full of 2nd hand game consoles Beowolfed into a render farm.

[JulianEdgar]

Originally Posted by freebeard

Thanks OP, I hadn't seen this one before:



What do you make of the internal dotted lines? They seem to imply that pressure differentials transmit through a solid body. ???

The lines in the upper part of the diagram show flow directions. The dotted lines show flow around the car.

The key point is that the pressures shown on this diagram do not apply to real cars.

[freebeard]

Got it. According to Simulation Theory it's all varying degrees of verisimilitude.

[kach22i]

Originally Posted by M_a_t_t

The simplest way to look at it:

Rounded surface creates lower pressure. Look at an air foil, the top surface creates lift by lowering the pressure. Overall shape of the car is rounded, thus creates lower pressure on that surface (varying levels). Low pressure on the top half of the car is lift.

Flip a car upside down and you get down force right?

I found the first few sections of the link below informative.


https://www.sciencedirect.com/topics...rodynamic-lift

Vehicle body aerodynamics
Heinz Heisler MSc., BSc., F.I.M.I., M.S.O.E., M.I.R.T.E., M.C.I.T., M.I.L.T., in Advanced Vehicle Technology (Second Edition), 2002

The aerodynamic lift coefficient CL is a measure of the difference in pressure created above and below a vehicle's body as it moves through the surrounding viscous air..........

14.3.2 Vehicle lift (Fig. 14.20)
When a car travels along the road the airstream moving over the upper surface of the body from front to rear has to move further than the underside airstream which almost moves in a straight line (see Fig. 14.20). Thus the direct slower moving underside and the indirect faster moving top side airstream produces a higher pressure underneath the car than over it, consequently the resultant vertical pressures generated between the upper and under surfaces produce a net upthrust or lift.

[Vman455]

Originally Posted by JulianEdgar

The dotted lines show flow around the car.

I don't think this is the case; Hucho is quite clear that this is a two-dimensional section. I believe the dotted lines are meant to represent possible duct paths between intakes and outlets for cooling and cabin ventilation air. The paragraph after the one you posted explains:

From the pressure distribution shown in Fig. 2.4, suitable positions for the air intake and outlet for the cooling and ventilation system can be selected. The intake should be placed in regions of high pressure, e.g. in the nose region or in the scuttle, whereas the outlet may be placed in a region of suction.

[JulianEdgar]

Originally Posted by Vman455

I don't think this is the case; Hucho is quite clear that this is a two-dimensional section. I believe the dotted lines are meant to represent possible duct paths between intakes and outlets for cooling and cabin ventilation air. The paragraph after the one you posted explains:

Yes I think you are right.