Tonneau/half tonneau.....spoiler/half template

[freebeard]

The Ridgeline, and the Cadillac EXT/Chevy Avalanche remind me of the 1969 El Camino. Just not as sweet.

I don't pretend to understand this, but it suggests to me that sail panels are beneficial, as kach22i says.


https://ecomodder.com/forum/showthre...tml#post581181

Consider the humble superellipse:



It transitions smoothly from a perfect square to a perfect circle; neither of which you want. Still...

[hat_man]

I hate to resurrect an old thread but I have a new idea/question about the pressure differential concept of the half tonneau that Aerohead mentioned. Now that I'm mobile again from the broken foot it's time to start some fab work. I'm still going with the simple idea I had from the start and for now no wing. But reading the Aerodynamics of Road Vehicles book while I was off got my mind wandering again and led to new questions.

So, here goes........

I like the look of the Honda Ridgeline with the sail panels and sloped bed rails in the attached picture. I would never do it, but what if you took that section and magically attached it to the top of the bed rails on a Ranger. Something like in the other picture. Does this create a larger volume of air being acted on? And if it does, how does this affect the pressure difference?

I guess a better way to think of it might be a half tonneau on a Honda Ridgeline like in the last picture. If you put the half tonneau at the level of the red line it would create a specific volume of air being acted on and if you put it on the yellow line it creates a larger volume. (I'm probably using the wrong terms here, but I hope you understand what I'm getting at.)

Also, if you put it at the level of the red line does this create an area above the tonneau that helps direct air flow rearward and keeps it from spilling out randomly over the sides? If it does is that beneficial or just a moot point? I can see how putting it at the level of the yellow line might create a downward slope but way below the "template" line.

Just trying to wrap my head around how changing the volume of air trapped below the tonneau could affect the pressure differential at the tailgate. Always trying to learn something new.

Thanks

[freebeard]

Half tonneaus don't work that way. They're at the yellow line, but only cover the back half of the bed.

What you're cribbing from the Ridgeline is more of an half aerocap. Try 'half tonneau' in the Search box. I looked at a few but they were dead ends. This one is on-point:

https://ecomodder.com/forum/showthre...ate-36859.html

edit: Never mind, that thread is this one. There was a thread where someone took a canopy and cut it down to half length. A true aerocap will have taper above the bed rails.

[kach22i]

The upper yellow line, and lower red line in your last image is a bit of a distraction in my opinion.

Just compare top of bed to bottom of bed and keep things simple.

Splitting hairs or doing something half-way serves no purpose.

Originally Posted by hat_man

Also, if you put it at the level of the red line does this create an area above the tonneau that helps direct air flow rearward and keeps it from spilling out randomly over the sides?

I will comment on the mid-position (red line) in passing, I suspect the little bit of the tailgate sticking up would act as a Gurney flap / Wickerbill. This could produce a tube-like vortex of swirling air that may pull airflow over it and lower drag while providing a little down-force........... as the rear spoiler on my pickup truck does (link in signature below). Mind you that your bed has to be long enough, and the top edge high enough to fill out the Aero-Template or close enough to start talking about air-flow.

I have not tested this next theory, but would like to one day.

I suspect the cab sail panels do more than a roof wing. The Texas U diagram did not test cab side sail panels as I recall.

The reason for this theory is after having watched many wind-tunnel videos, it looks to me that if the smoke wand in front is even a little bit left or right of dead center the smoke air flow veers off to the sides by rolling diagonally across the windshield then travels along the sides of the vehicle.

My conclusion; more air is flowing off to the sides than up and over the top.

What if anything does this say about "air pressure"?

There is higher velocity air traveling a longer path over the centerline top of the vehicle as in an airplane wing - causing lift.

Higher velocity air has lower pressure than slower moving air.

There is lower pressure at the sides therefore the airflow meanders in that direction in an attempt to equalize.

The air does not want to be accelerated at all, it takes energy to do this, so it will always attempt to be restored to a resting state.

I know what you are probably thinking, that none of this air is actually moving, it is the vehicle that is moving. I know, I know, I just cannot get my brain to work at that advanced level - I've tried, I'm just too stupid and untrained.

[hat_man]

I guess what I was seeing my warped and twisted mind was sort of a hybrid. A half tonneau like discussed earlier in this thread. The rearward section of a truck bed covered at the height of the stock Ranger bed rails, with the addition of the Ridgeline side sail panels and Ridgeline bed rail section above the stock Ranger bed rails.

Aerohead was explaining (an I hope I have this right) that the low pressure area created behind the cab is transferred (or extended?) to the area under the half tonneau creating a small pressure differential across the tailgate. Lower pressure inside the bed and higher pressure outside the bed. I am assuming that it works best in conjunction with a wing attached to the cab. My thoughts earlier in this thread were that there might be some small gain with having the half tonneau and no wing. The half tonneau is an easy project where the wing is possibly more involved. I was going to do the half tonneau and as I get back to work I will have a nice A-B test platform (with the 60 mile drive each direction) to test it on. That would give me some time to try and design a wing arrangement and then test that too.

Those plans led my to thinking about how adding side panels (in addition to the half tonneau and the wing) and extending them (sloped) to the tailgate might affect the results. If I have the thinking right the air moving across the roof creates the low pressure area behind the cab. I wasn't sure how the air moving across the doors/windows interacted with this low pressure. Does it help create it (A+B=AB) or does it disturb the creation of the low pressure and lessen it (A+b=Ab), but still create a low pressure area where AB>Ab>0. (Maybe that should be < as we are talking about lower pressure and not higher pressure, but I think you can see what I'm getting at.) If you sip through a straw with a crack in it you can still maintain enough pressure to move liquid through the straw. But if you seal the crack there wouldn't be a pressure loss and would create a lower pressure in the straw. So my thought was IF the "side air" was like a "crack" in the low pressure area behind the cab then would the addition of the side/sail panels be like sealing that "crack" and create a slightly lower pressure area behind the cab?

In theory, the red and yellow lines are half covers. The trapped area with the red "cover" would be smaller in volume than the trapped area with the yellow"cover". I also couldn't quite figure out how this would affect the pressure differential created across the tailgate. 35psi in a tire is one thing, but the same 35psi in a 36" diameter pipe is quite another . Air escaping from the tire is fairly harmless even at the moment of escape, but in the pipe would probably sever an arm off. I didn't know if the math worked the same in reverse. Given a specific air flow across the roof, is the pressure differential across the tail gate directly/indirectly proportional to the volume of trapped air being acted upon? And does the addition of side/sail panels increase/decrease or have no effect on the volume of trapped air?

Again, I have no plans at this point other than the half tonneau and no wing, and recording those results as soon as I get back on the road to work. The cab wing is definitely on the table, but at a much later date after recording how the half tonneau alone works. I'm just trying to learn how all these other variables interact with each other to expand my knowledge base.

We know that a full aero cap is the way to go, and there are some excellent threads showing other members work and their FE improvements. I don't think anyone has tried the half tonneau and wing arrangement yet. I think I saw someone had done just the cab wing and saw some improvements. I thought I would be the unofficial test bed for the half tonneau idea and eventually add the wing to track the gains/losses of both.

[kach22i]

Originally Posted by hat_man

I guess what I was seeing my warped and twisted mind was sort of a hybrid.

In my opinion, you need to cartoon diagram or draw these ideas out for a clearer indication - it's just taking too many words.

The process of drawing is similar to the process of writing in that it flushes out your thoughts and forces you to organize them for communication.

You end up communicating with yourself and solving problems, that is what designers do with concept sketches.

The nice thing about sketches is that you can mark them up and edit them, erase or white-out them as needed, overlays and revisions - they are only a tool for thought.

If rough sketches are not literal enough for you to process then try looking at the images someone did for me on my project.

Pickup Truck Aerodynamics - CFD Study Chevy S10
https://ecomodder.com/forum/showthre...s10-35043.html

[COcyclist]

Originally Posted by freebeard

There was a thread where someone took a canopy and cut it down to half length. A true aerocap will have taper above the bed rails.

Originally Posted by aardvarcus

This one? He did notice an improvement but later added a full aerocap IIRC.

[hat_man]

Here are some really bad diagrams of what I am getting at and the red/yellow options.

[hat_man]

I would really love to source one of these and combine it with a half tonneau, but after searching the net repeatedly I believe they are only available in the UK and NZ. With the new Rangers just now being offered over here (2019) if they are optional equipment over here, I'm sure they will be very expensive. Seems to be available in Ebay UK for between $45 and $90 American. The Wildtrak has been available over there since at least 2012.

[freebeard]

Does it help create it (A+B=AB) or does it disturb the creation of the low pressure and lessen it (A+b=Ab), but still create a low pressure area where AB>Ab>0. (Maybe that should be < as we are talking about lower pressure and not higher pressure, but I think you can see what I'm getting at.)

I skipped ahead to kach221's response.

Each 'particle' of air is seeking the path of least resistance to repose at each point as the vehicle passes under it (or the air moves over it in the case of snerodynamics). This means lateral flow over the surface. The only shape that avoids this is Thee Holy Template.

Do you know those cylindrical hay bales? Imagine one the width of the bed and 80% the height of the cab, ahead of the half tonneau, and spinning. I think the half tonneau creates a vortex that acts like a solid object in that space.

What your sail panel thumbnails lack is the pronounced bevel the Ridgeline has for a little Coanda effect.