Screen/Aerodynamics question

[sinbad]

This seems odd to me, I wonder if anyone can shed some light on it for me.

Whilst riding at speed, I am subjected to more wind noise when I tuck in behind the screen, and it gets quieter than normal if I stick my head up as high as I can.

So normal riding position = medium
tucked down in the bubble head low = a lot noisier
head up high = much quieter

I find this bizarre, I'm thinking about trying no screen at all to see what happens, but what I'm wondering is will a double bubble type screen actually make me more comfortable, or will it just be more like when I'm tucked in?

Pointy S half faired btw.

[muffles]

I don't know the answer but I've experienced the noisier bit when tucking in. Well, to be precise it gets more "echoey" which I think is because I am out of the main airflow and in a bit with no airflow.

[gettin2dizzy]

All of the airflow that has to flow around the bike will be channeled over the lip of the fairing/screen.

[Jools'SV Now]

simple really; the sound you hear is mainly air hitting/going around your lid.

so bolt upright it's just ambient air (for want of a better term)
normal - you get ambient air and some coming over the fairing, possibly onto the lower part of your lid only.
tucked in - loads of air hitting top of your lid - so more noise

maybe:confused:

[Aiki]

It's turbulent air coming off the screen which is noisy, laminar flow in clear air is quieter.

[OldBoy]

I'm looking at this problem at the moment with a home built fairing, going as far as sticking short lengths of wool on the screen which shows the direction and turbulence of the air flow.

I've found that the noise comes from where the air flowing over the screen meets the still air behind the screen producing turbulence as Aiki mentioned in his post. On my bike this is just about visor height, I can even feel it if I lift my visor. I'm having a go at making something like this to cure it
http://www.laminarlip.com/index.php

[jimmy__riddle]

could it be the angle of your head is changed slightly and so the air is flowing differently over any airvents in your lid?

[Baph]

On the subject of riding a half faired pointy with no screen...

I normally ride with a Fabbri DB, but had the front end off to work on it, and couldn't be bothered putting the screen back on.

I didn't notice the noise levels, but when sat upright at NSL speeds, I felt like the front was about to lift off the floor just due to wind drag on me!!

The screen quickly got fitted again!!

[arenalife]

It's not just flow, it's pressure. Behind the screen is a low pressure area relative to the surrounding air. That's going to pull your eardrums back and fore like a mofo giving that unpleasant pumping feeling. The only screen that ever felt truly comfortable for me was on my Deauville which pushed the air so wide there wasn't such a large differential flow over my bonce.

[Warthog]

I was going through this exact question as I have a naked and have just fitted a flyscreen to it. With no screen, you are just hit full on with windblast. With the flyscreen, there is no pressure on my chest anymore, but there is turbulence just where the flyscreen ends. With my head very high up there is normal windblast sound. Then as I sit back normally, my helmet lands in the turbulence and it is much louder. As I duck down the raindrops off my visor run very rapidly downwards showing that there is a circular airflow. The noise gets worse as it hits my helmet dead around my ears, then if I lie my head on the tank everything goes whisper quiet. My guess is that you are either better off well out of the turbulence in the wind blast or you want a big enough screen that you can properly hide under.

[OldBoy]

This shows the airflow round different bikes http://people.bath.ac.uk/ensmjc/Research/Motorcycles/aero.html
Unless you are laying on the tank you get the full blast in yer face on a sports bike.

Pehaps more important is this http://www.isvr.co.uk/at_work/m_cycle.htm

[OldBoy]

I was going through this exact question as I have a naked and have just fitted a flyscreen to it. With no screen, you are just hit full on with windblast. With the flyscreen, there is no pressure on my chest anymore, but there is turbulence just where the flyscreen ends. With my head very high up there is normal windblast sound. Then as I sit back normally, my helmet lands in the turbulence and it is much louder. As I duck down the raindrops off my visor run very rapidly downwards showing that there is a circular airflow. The noise gets worse as it hits my helmet dead around my ears, then if I lie my head on the tank everything goes whisper quiet. My guess is that you are either better off well out of the turbulence in the wind blast or you want a big enough screen that you can properly hide under.
Agree with you on screen size I made mine some three inches too short and I have the same with the rain drops which is useful its just the noise.
And yes using the tufts of wool shows a cicular air motion between screen, tank and rider.

[Dangerous Dave]

Whilst riding at speed, I am subjected to more wind noise when I tuck in behind the screen
The air flow as it flows over the screen and crosses another airflow stream creates turbulance/dirty air. If you ever see a wind tunnel session you will see the smoke start to spiral, this is the dirty air.

Computer Image below of a wind tunnel, see the dirty air behind the wheels?
http://www.compusys.co.uk/NR/rdonlyres/B52574BC-71F8-49FD-93DE-1E5036F71F12/0/ACFDFormula1Windtunnel.jpg

[sinbad]

It certainly is intriguing to me. I'm just surprised that the fairing does not provide a "pocket" of quietness :)
I understand the principles of aerodynamics, I know about pressure and turbulence, this doesn't explain why things get noisier though. If someone said to you, you can stand in a 90mph blast of wind, or a 45mph blast of wind, which will be louder? You'd say 90mph.

Maybe the turbulence sends air up inside my helmet that wouldn't normal get there. There definitely is an echoiness to it, though (as muffles said), it feels like your head is in a big plastic container drum.

I've tried moving my helmet to different angles also, to very little effect. Next time I'm out I'm going to reach as far forwards as possible to see if being right behind the screen quietens things down a bit.

I'm going to get a double bubble, but when I get it I think I will certainly experiment with cutting the oem screen down a bit to see if lowering the deflected air which tumbles from the lip will give me the more silent volumes that I currently only get if I stick my head up as high as I can.

[Aiki]

The fairing will provide a pocket of quietness but it is quite small and just by the top yoke. What happens is that an area of low pressure forms right behind the screen and when the high speed air comes off the screen it is sucked down behind the screen which causes vortices to form which are large(ish) swirls of air which is the buffeting you feel. The way to solve this is either a lower screen to reduce the pressure differential, or a higher screen so you can tuck behind it and then the vortices will go over the top of you and anoy your pillion.

Another way to reduce the effect might be too introduce an air bleed where the screen joins the fairing, this would bleed air into the area behind the screen thereby increasing the pressure and lessening the pressure differential and therefore reducing the intensity of the vortices.

[Warthog]

I had to carry a 10 kg bag of dry ice today (don't ask) and so I had my tank bag expanded to full size. It had a huge effect on the turbulence, I was really shocked! The air can't circulate in that space and so noise went down by about half! It was really good, I am seriously thinking about riding with a huge tankbag all the time now.