Flow Through Ventilation??

[Bob the Builder]

Just curious again. I'm trying to keep my roof profile as low as possible. My Sunrader shell has the typical 14x14 (about 4 inches tall), roof vent at the rear of the roof. I would rather not have anything that tall above the roof line back there so I will probably remove and seal the shell there. The over the cab front profile is considerably lower than the roof at the rear. I'm thinking of installing a streamlined vent/fan arrangement at the front to pull air in over the front bunk and put either one or two small exhaust fans in the rear wall to create circulation when not wanting to run the ac. Pros, cons, would it work or not worthwhile?

[linda s]

The slope in the front pushes wind up and over your rig creating almost negative pressure on the main roof. Your existing vent is not creating any wind resistance where it is. The wind is just flowing above it. Look at this wind tunnel diagram. See how far the lines move away from the vehicle as the wind flows over it. 

Any alterations to that front slope will decrease your aerodynamics, not help it. 

Maybe put that exhaust fan on the back and just open a window for air.

Linda S

[Bob the Builder]

Not really worried about the aerodynamics. Was trying to keep the overall height low enough to be able to put this through my 8ft garage door when I have my final height set. Would be much easier to continue work on the interior this winter if I can get it inside and to maintain it after that. Not quite close enough to see where it will end up as far as height yet but getting there. Thanks

[mpanzar]

With all due respect, the diagram of the car is not applicable to any toyota RV.

[linda s]

With all due respect the movement of air over a slope will all have varying degrees of a still similar outcome. Be it sloped hood, windshield or the slope of a Sunrader overhead. Air doesn't care how big you are. This is hard to see but the first big rig has a wind deflector and the second doesn't. See the black space on the front of the roof of the first pic. That black space is where the wind isn't hitting. Negative air space because of the slope of the deflector. 

Linda S

[mpanzar]

@linda s I agree with both posts regarding two examples shown. and i agree with the principles you mentioned.  Regarding the air deflector on the tractor trailers, that was designed and likely optimized over several iterations. As for our lovely sunraders, i am assuming they were not designed for aerodynamic performance, and if they were i doubt there was much testing and iteration. (if there is aerodynamic testing data with images, i think we need t-shirts made ). On our sunraders, the deflector like geometry just after the cabover section as you move towards the aft, may trip the flow (similar to the first truck image) due to the relatively sharp nature of the geometry as compared to the examples of the car and truck deflector.  I'm not an expert in aerodynamics/gas flow or CFD but i have some design experience related to the subject and have encountered similar phenomenon due sharp geometry. Im not positive on the engineering, but i believe the sunraders deflector would cause boundary layer separation and promote turbulent flow. I guess there is only one way to find out!

 

Edited May 25, 2023 by mpanzar

[WME]

Vent fan, top mount OR side mount...https://www.etrailer.com/RV-Vents-and-Fans/Ventline/VP-543SP.html

[Bob the Builder]

Now that looks very promising. Lots of options there. Thanks.

[IdahoDoug]

I am a former Product Planner for General Motors, as well as Lexus and would caution against adding an air opening on the rear of the Sunrader.  We had to be very deliberate in preventing exhaust fumes from getting into the interior on automobiles, so you'll see if you remove a bumper, for example, that all air openings on the rear to dry the body cavities will have simple one way flapper valves that shut when air tries to come in.  

 

A larger, more boxy shape increases the danger of this reverse air coming in the rear of a vehicle underway, vs a slippery car.  Linda is correct on the roof vents not adding much/any to the aero drag, but it would add drag if placed on the slope of the cab over.  

 

To the OP's question. When sitting, having an air exhaust fan somewhere will obviously pull air into the cabover bunk, so that will work.  Perhaps put it on the side, and if you don't have a preference - behind the stove, where it would double as a massive exhaust fan to pull cooking smells and cooking heat immediately outside?  

 

Interesting thought, and I have been pondering using a pair of strong computer fans on our VW Vanagon to pull night air through the van while sleeping.  They're super quiet, which is why I was looking at/considering them.  Plus programmable, highly variable, etc. Let us know what you come up with!