Negative pressures on forward-facing surfaces

[Vman455]

I read something interesting the other day about the non-intuitive nature of aerodynamics: It is possible, the author claimed, to shape the front of a car such that most of the surface is exposed to negative differential pressure and thus contributes thrust. Most people, he went on, don't see how this could be since they think, intuitively, that air behaves the same as a moving solid would, "hitting" the front surfaces exposed to flow and thus exerting positive differential pressure on them.

Well, could that be true?, I wondered. In order to find out, I took some centerline pressure readings on the front of my car, which is just the sort of continuous curve the author recommended. The results:



Yes! In fact, the pressure is below atmospheric from just above the emblem, all the way up the hood, and then across the upper part of the windshield.

I used a digital manometer and pitot tube ($50 on Amazon), 40 feet of clear vinyl tubing, one of my new Scanivalve pressure patches (less than $10), and some painter's tape to gather this data.

[kach22i]

Originally Posted by Vman455

..........
Yes! In fact, the pressure is below atmospheric from just above the emblem, all the way up the hood, and then across the upper part of the windshield.

I used a digital manometer and pitot tube ($50 on Amazon), 40 feet of clear vinyl tubing, one of my new Scanivalve pressure patches (less than $10), and some painter's tape to gather this data.

I cannot comment on your testing methods, but you have put a lot of work into this. Bravo!

A post of yours from 2018......

https://ecomodder.com/forum/showthre...e-11745-5.html

Originally Posted by Vman455

........... Take a look at a CFD pressure distribution map of the 2012 Prius:


That line of green at the scallop edge is lower pressure than the yellow across the rest of the hood, and the pressure at the cowl behind it isn't any higher than across the hood. It looks stupid, but it's apparently effective enough to use on multiple models.

The 2018 post has no link or citation credit from where it was sourced. However I'm going to assume it is of standard color coding.

If somebody can post a typical CFD color key I'd appreciate it.

I just want to compare the two data sets.

[Piotrsko]

Recall the Naca cowling put on radial engine powered aircraft. IIRC, one such cowl put on a Beechcraft biplane produced so much thrust it destroyed either its mounts or the front of the airplane. I believe the GB2 series aircraft used them also.

[Vman455]

Originally Posted by kach22i

I cannot comment on your testing methods, but you have put a lot of work into this. Bravo!

A post of yours from 2018......

https://ecomodder.com/forum/showthre...e-11745-5.html


The 2018 post has no link or citation credit from where it was sourced. However I'm going to assume it is of standard color coding.

If somebody can post a typical CFD color key I'd appreciate it.

I just want to compare the two data sets.

If I remember correctly, I grabbed that screenshot from an Ansys video; they did the CFD for the Prius Plus package. It's a shame they left the windows clear.

I assume the coloring is--

red = high pressure/low velocity
blue = low pressure/high velocity

[aerohead]

The centerline forebody pressure profile, as far as any potential 'thrust' was concerned, would have to be considered within the context of the vehicle's entire pressure distribution, and not just centerline, but the entire body surface area.
An easy example would be the HUMMER H-1, which would demonstrate a remarkably low pressure spike over the grille-to- bonnet transition ( more dramatic than the Prius ), however that spike would be overwhelmed by the lack of any significant roof camber, and then premature separation off the back of the greenhouse, where the local pressure at the separation line is virtually the same as the lowest pressure on the HUMMER, at the windshield-to-roof transition. This super-low pressure would telegraph all the way across the rear, to the base, where the negative low base pressure would completely overshadow any potential ' thrust' at the forebody.
The reason for its Cd 0.70.
Laser Doppler Anemometry is one technique available, which can scan an entire vehicle surface, to ascertain all spatial local velocities and static pressures, from which the whole-car pressure distribution can be derived.
It's not an easy thing for us, especially if vorticity is present, virtually impossible to visualize outside of a laboratory.

[kach22i]

Originally Posted by aerohead

.............
Laser Doppler Anemometry is one technique available, which can scan an entire vehicle surface, to ascertain all spatial local velocities and static pressures, from which the whole-car pressure distribution can be derived.
It's not an easy thing for us, especially if vorticity is present, virtually impossible to visualize outside of a laboratory.

I think that may be the green flashing light in sequential serial like lines over form as seen in several still photos and videos posted over the years.

If so, I have no memory of it capturing vortex presence.

Frustrating that smoke in wind tunnel testing does little to illuminate vortex presence.

[aerohead]

On You-Tube, there is footage of the Bramos Racing Team's 'winged,' Porsche 911 in the A2 Wind Tunnel, undergoing smoke-flow visualization.
You can clearly discern the longitudinal vorticity, off the C-pillars, attacking the rear wing, and surviving beyond the wing, in spite of the wing's capping plates, which are conventionally understood to 'kill' tip vortices.
Hucho has stated that pseudo-Jaray 'fastback' vortices are the strongest, and longest lived, among all body types.

[freebeard]

They have those big indoor canals with a gantry crane that they test ship hulls in. They are huge. A full size (even oversize?) car model upside down with hydrogen bubbles (pace Graysgarage), and multi-camera motion capture might be illuminating.

[aerohead]

Originally Posted by freebeard

They have those big indoor canals with a gantry crane that they test ship hulls in. They are huge. A full size (even oversize?) car model upside down with hydrogen bubbles (pace Graysgarage), and multi-camera motion capture might be illuminating.

Yes, the US NAVY, David Taylor Model Basin, West Bethesda, Maryland is quite the candy store!
Texas Tech University built a small version of it to originally test the Ford Taurus underhood flow, later parachute designs for the Pentagon.
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Hydrogen bubbles would be great, as they're neutral buoyancy, have the same mass as air, and behave just like 'air.'

[Charlie Cheap]

To lower under hood temp I cut an eyebrow in the hood of my 65 Mustang 6-cylinder to allow heat to escape. Using wool tufts taped at the opening I found air did exit up to around 55 mph...then was sucked in. By that speed air-in or air-out did not matter. It did run cooler in traffic and had no effect on the highway. Looking close at the attached picture you can see the small eyebrow cut. The LH-6 on the hood showed I was PROUD it was not a V8. A cool-air intake, 2-bbl carb, hot ignition, bored .060", free-flowing exhaust, 2.83 rear axle, and 15" tires, made a good combo. With 4-speaker stereo and cold air it was comfy too. Maybe that is why I owned it 27 years.