Vehicle section to trailer sextion transitions

[Tesla]

Starting with a typical vehicle cross section, rectangle/hexagon/squircle type of shape in most cases, how do we transition to a simpler semicircular aeroform to work with?

Are we guided by the points that lag most on vehicle according to template, these would be the corners where roof and sides meet or can we average the transition and assume the air where transition is slower will move across to fill area where transition is faster.

I'm looking at a vehicle to trailer canopy situation.
I've done a bit of a chart to help visualise it.

The outer black shape represents the vehicle shape at rear
The smaller black shape represents where the shape would be according to template at 4' from vehicle.
The red ellipse is the start of trailer canopy and the other ellipses represent sections along length of trailer canopy.

The areas shaded orange are where the ellipse shape has transitioned faster then template, whilst the rest of ellipse is slower then template.

Can we safely assume that the air from these areas would travel across as indicated by arrows to fill void where transition is faster, thus avoiding the vorticies that can form off the corners?

Is it as simple as comparing and balancing the area differences under the respective curves?

Edit: Freudian slip in title, it may well end up being a camper trailer, so "bedding" may be involved.

[freebeard]

Threshold Strategy - Halving the Drag Coefficient Seems Possible - ATZ online

kach22i posted this link today. Take a look at figures 5 and 6 on page 4. Also the area you denote in orange will likely have vortexes that apparently begin at the base of the A-pillar.

[Tesla]

I'm looking at the rear of the vehicle, I imagine the A-Pillar vorticies have already done their damage and are decomposing, it's more the C-Pillar vorticies that I was referring to which rotate in the opposite direction to the A-Pillar ones.

So the question really is if the rate of change is reduced in centre of roof line and lower sides, can this compensate for the more rapid change coming down and in from the corners and hence reduce risk of C-Pillar vorticies.

[euromodder]

Why not start from the inner black line, i.e. the shape of the towing vehicle + 4' of template, then start from there and work it into a template-shaped boattail ?
Rather than starting from the bigger red shape as the front of the trailer.

Within the 4' of towbar , some blunt nose could be used .

[freebeard]

I'm looking at the rear of the vehicle, I imagine the A-Pillar vorticies have already done their damage and are decomposing, it's more the C-Pillar vorticies that I was referring to which rotate in the opposite direction to the A-Pillar ones.

I went back and looked, you're right.

So the question really is if the rate of change is reduced in centre of roof line and lower sides, can this compensate for the more rapid change coming down and in from the corners and hence reduce risk of C-Pillar vorticies.

I'm not picturing it. Are you talking about the tow vehicle or the trailer?

How about a half-rounded trailer with blisters right behind the orange area. Something larger than a running light but smaller than an aircraft canopy. To offer something for the air to reattach to.

[Tesla]

Originally Posted by euromodder

Why not start from the inner black line, i.e. the shape of the towing vehicle + 4' of template, then start from there and work it into a template-shaped boattail ?
Rather than starting from the bigger red shape as the front of the trailer.

Within the 4' of towbar , some blunt nose could be used .

I have looked at a variety of ideas, but think the eliptical profile, if possible would be the best way to go for the entire trailer canopy.
Through it's inherent shape it would be the strongest/lightest structure and would also be more generic to a range of vehicles, I would also have a rounded nose on it to help close the gap to vehicle.

Originally Posted by freebeard

I'm not picturing it. Are you talking about the tow vehicle or the trailer?

How about a half-rounded trailer with blisters right behind the orange area. Something larger than a running light but smaller than an aircraft canopy. To offer something for the air to reattach to.

Sorry rambling is a bit confusing, in actuality I am looking at both, to make trailer canopy as generic as possible using eliptical profile for reasons mentioned above and then also look at what needs to be done to trailer/vehicle/gap to smooth the transition from one shape to the other.

I like your idea for the corner transition on trailer, as it will have a double life as utility and camper, that would be an ideal location to mount some skylights, if one could find domed teardrop units they would be perfect for the position.

Another thing I was looking at is using deflectors or aero fences on vehicle to guide airflow into the corner void in such a way to increase pressure there and minimise resk of vorticies.

[aerohead]

Originally Posted by Tesla

Starting with a typical vehicle cross section, rectangle/hexagon/squircle type of shape in most cases, how do we transition to a simpler semicircular aeroform to work with?

Are we guided by the points that lag most on vehicle according to template, these would be the corners where roof and sides meet or can we average the transition and assume the air where transition is slower will move across to fill area where transition is faster.

I'm looking at a vehicle to trailer canopy situation.
I've done a bit of a chart to help visualise it.

The outer black shape represents the vehicle shape at rear
The smaller black shape represents where the shape would be according to template at 4' from vehicle.
The red ellipse is the start of trailer canopy and the other ellipses represent sections along length of trailer canopy.

The areas shaded orange are where the ellipse shape has transitioned faster then template, whilst the rest of ellipse is slower then template.

Can we safely assume that the air from these areas would travel across as indicated by arrows to fill void where transition is faster, thus avoiding the vorticies that can form off the corners?

Is it as simple as comparing and balancing the area differences under the respective curves?

Edit: Freudian slip in title, it may well end up being a camper trailer, so "bedding" may be involved.

I believe that it is about area.Or 'sectional density' which is associated with it,which NASA's Richard Whitcomb worked with.
You'd take the variance in cross-sectional area at any position along a template,then alter your station bulkhead at that location as it morphs away from the original section shape in a way to maintain an identical sectional area.
The volume of air displaced at that position remains constant,although where the displacement occurs is varying.

[Tesla]

Thank you, that's the confirmation I was looking for.
It's where all my scratchings and calculations were pointing me, it's more about 3D air pressure and velocity at a particular point in time, rather than a 2D angle on a surface in a singular plane, although one still needs to be wary of a too agressive change at any point, there is some lee way if this is compensated in the adjacent areas.

One of the things I've been looking at with all these different equations is the rate of change of the angle and I think the ideal form has an ever decreasing rate of change, so although the angle is increasing to a max of 22-23°, the actual rate of change between sections, is decreasing, this fits with reducing air pressure as the sectional area gets smaller, so the void is filled slower and hence to avoid seperation each subsequent void must be smaller than the previous one.
If the sectional area decreases too quickly, the void cannot be filled, low pressure areas and drag are the end result.

[euromodder]

Originally Posted by Tesla

I have looked at a variety of ideas, but think the eliptical profile, if possible would be the best way to go for the entire trailer canopy.

If you use the red line as a start , you're going outside the (rear of the towing vehicle + 4' of template) , which means you'd be adding frontal area.
Then, you'd also need to get rid of that again in the tail ...
Seems more logical to me to not add the extra area.

Through it's inherent shape it would be the strongest/lightest structure and would also be more generic to a range of vehicles,

It'd also be the least useful or practical shape - useful internal volume is important if the trailer has to do double duty as a camper / utility trailer.

I would also have a rounded nose on it to help close the gap to vehicle.

Which would be situated before "the red line", I presume ?

When thinking about "my" ideal aero-trailer, I end up with something shaped more or less like the Airstream Basecamp.

Plan :

Side :


But with a blunter nose to improve internal volume, without all the external disturbances, without the rounded rear (avoid that !), sized to match the intended towing vehicle, and continuing the teardrop a good distance further aft / down.

Streamline plates can then cover the gap between towing vehicle and trailer, hinged on the towing vehicle, and free-riding on the round(ish) nose and roof of the trailer.

[Tesla]

As both of those inner lines are theoretical points with the outline of the rear of vehicle, neither would add any additional frontal area, they would just be different tapering plans for the transition. As confirmed by aerohead above it is the area under the curve that needs to be considered, if both shapes have the same area, then the air has the same size void to fill and the result should be the same.

Yes as for rounded front, it would extend forward of the canopy towards the vehicle, ultimately I would like to close gap completly, by using appropriate panels & fixtures on both vehicle & trailer for a completely smooth transition.

For this concept of a camper, the canopy would primarily form the cover of a sleeping area, so the curved shape is not an issue, cooking, washing and storage facilities would be pull out drawer sections underneath in the trailer body.

As for the utility aspect, most of the time trailer is underloaded and would carry mostly air, so it would be just like a lightweight, empty boattail. For those times when something particularly large needs to be transported, then take canopy off and just suffer aero drag, easy removal is another reason I am focussing on lightweight construction.

The idea so far is to have a tilt up rear with fold out walls, so when not in transit it is more like a tunnel with uniform roof height, looking a bit like the wagons of the "wild west" you see in old movies with the domed roof.