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Vents

Printed From: BHPC Forum
Category: Public: Open to anyone
Forum Name: Building
Forum Description: Anything to do with building HPVs
URL: http://forum.bhpc.org.uk/forum_posts.asp?TID=6948
Printed Date: 09 December 2019 at 5:23am
Software Version: Web Wiz Forums 11.02 - http://www.webwizforums.com


Topic: Vents
Posted By: simon
Subject: Vents
Date Posted: 30 August 2019 at 7:22pm
We all know about NACA inlet ducts Does anyone know about outlet ducts ,do NACA ducts work to draw air out of the fairing if fitted backwards

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SIMON



Replies:
Posted By: Yanto
Date Posted: 01 September 2019 at 8:07am
AFAIA any hole as long as it's in a low pressure area will let air out, all a properly profiled NACA duct will do is reduce turbulence (drag) when reintroducing the exiting air into a laminar flow, I don't believe they actually pull air out, but I've been wrong many times so don't take my word for it, there is lots of reading on the net, some of which may be correct Big smile .

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Ian, racing again No 6.


Posted By: Balor
Date Posted: 01 September 2019 at 10:17pm
I think he saw that video:
http://www.youtube.com/watch?v=rtEsdUzlIEk" rel="nofollow - https://www.youtube.com/watch?v=rtEsdUzlIEk

So yea, they kind of do - if that is indeed that case, that is.
But what do you need it for? Boundary layer suction?


Posted By: simon
Date Posted: 07 September 2019 at 3:24pm
After the worlds this year I was wanting to increase air flow to the rider and we noticed no out vents on faired bikes

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SIMON


Posted By: RoyMacdonald
Date Posted: 07 September 2019 at 9:16pm
Probably only an issue if it was possible to build a sealed bike, but as long as it has wheels in contact with the road......

Roy 


Posted By: Balor
Date Posted: 28 September 2019 at 8:21pm
That's a good point... however, both leading AND trailing edges of a streamlined body are high pressure zones, and dumping ventilation airflow at the sides would likely lead to bad aerodynamics.
I think using battery powered fans is a better idea, but it is not a strictly HPV solution (so are battery-powered lights though).


Posted By: Balor
Date Posted: 28 September 2019 at 9:08pm
For better cooling insulating (foam-core or even foam-sheet) shells are preferable, painting it white and it might be possible to incorporate some evaporative cooling as "low-tech" temperature control. We already have some built in, but it is possible to lose water to sweating faster that it is possible to absorb it, so having a spray bottle handy might be helpful.
More advanced, semi-automatic methods might be employed for indirect evaporation cooling in case of high humidity, like making some shell panels of absorbing (think melamine sponge) material and wetting them out using capillary action or low-powered pump.


Posted By: GeoffBird
Date Posted: 28 September 2019 at 9:37pm
With a streamlined bike, the air probably exits out of the hole for the front wheel, as long as it's not sealed, as this is a low pressure region.

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Right Time - Right Place - Wrong Speed


Posted By: Brucey
Date Posted: 09 October 2019 at 9:54pm
there are two problems with internal airflow

1) internal drag; the airflow inside the machine isn't for free. 
2) external drag; subtracting air from the flow and then adding it again may increase the drag significantly. 

If you want to estimate the 'cost' of the internal  airflow, just imagine what it would take in the way of fans (when the machine is stationary) to shove the air in the directions it goes in when the machine is on the move. 

Estimating the effects on the external drag is more difficult.  It is not too difficult to arrange a reasonably efficient air inlet (eg at a stagnation point on the front of the machine) and even 'scraping' some of the flow off the outside of the shell (e.g. via a slot) probably isn't that bad either. The biggest problems come with the exhaust; you stand every chance of generating turbulent flow, or even  inducing flow separation on the lee side of the machine. 

I have always thought that perhaps,  if the exhaust were configured in the right way, it could possibly be made to help the flow remain attached rather than hinder it.  Sure, there would likely be some kind of an (increased internal drag, probably ) penalty for altering the exhaust, but maybe this could be made to 'pay for itself' by allowing a different fairing shape, eg allowing a more adverse pressure gradient, and a smaller wetted area or something?

FWIW NACA ducts, even as inlets, may not be the best solution for HPVs. They were designed as a low(er) drag inlet vs a raised scoop, for use at high speeds.  However they have consequences; one of which is that the boundary layer downstream of the duct is pretty messed up and  this is unlikely to be desirable on an HPV. With a NACA duct you lose much of any ram effect; is it, I wonder, possible that a smaller duct which retains more ram effect could be a better duct at low speeds because it flows better?

cheers


Posted By: simon
Date Posted: 16 October 2019 at 9:07pm
You need airflow so I was wondering how to exhaust better than just around the back wheel which can’t be that good ,or if the wheels were covered so it couldn’t exhaust ,would a side duct of any type be better than a hole at the rear end where the two sides join

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SIMON


Posted By: RoyMacdonald
Date Posted: 17 October 2019 at 4:16pm
The rear wheel usually runs in a low pressure area so air usually goes in rather than out at the rear wheel. Completely Overzealous is designed to use that effect for ventilation.

Roy 


Posted By: Balor
Date Posted: 29 October 2019 at 9:38am
Something does not add up - shouldn't air move from high pressure to low pressure?
I think you meant that rear wheel is "high" pressure area, which makes sense given airfoil pressure distribution.


Posted By: GeoffBird
Date Posted: 29 October 2019 at 9:17pm
Yes, the rear of an airfoil is a high pressure zone and the widest point, a low pressure zone, so you'd expect air to enter the rear wheel aperture and exit the front wheel aperture.

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Right Time - Right Place - Wrong Speed



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