The Wind that Blows: the science of screens

[Guest MadMax]

I've been reading various threads about windscreens and trying to come up with an explanation that puts some science behind the screen dilemma. I'm going to lay out my thinking so that some of the physicists or sailors lurking out there can comment or add detail.

You see, as a sailor, it seems to me that a windscreen is very similar to a sail. The air flowing over the external curved surface is of a higher pressure than the wind flowing under it. This is because the outside air has to force its way around the curvature which is an obstacle to its progress. Inside the curve there is no obstacle and the pressure required to pass is lower. This is what generates lift in a sail or an airplane wing and I believe it has to be the same for a motorcycle screen.

For this reason, I imagine that filling in the gap behind the screen would be a risk. The normal difference in pressure between the inside and outside airstream would have the effect of lifting the screen away from your face or chest. If you filled in the gap, you would just have the outside pressure and this may bend the screen to extremes in some circumstances.

Shortening the screen to the degree that some riders have considered would also seem to me to be self defeating. The shorter the screen, the less pronounced the aerofoil effect. After a while, you may as well be on a naked bike.

A screen will work best at an angle that preserves the airflow (the lamina flow). As the screen angle reaches its stalling point, the adjustable lips that some designs use may overcome the tendency of the airflow to become turbulent by adding extra curvature at the point where the airflow leaves the screen and heads for your face or chest.

I'm guessing that the physics involved makes it hard to tailor a screen to individuals of different sizes. Because we can't play very easily with the angle at which the main part of our screens meets the air, we can only fiddle with the height factor, try a different shape or vary the angle of the edge by using an adjustable lip.

What do you think, especially the scientists? Does this make sense?

[VFRBulldawg]

It's early, but I'll take a SWAG at this. As an aerospace engineer I just can't help myself.

First, your description of laminar flow over and airfoil is a little backwards. When talking about a typical wing shaped airfoil the the curvature makes the upper surface longer than the lower surface. This requires the airflow over the upper section to have to increase velocity relative the airflow over the lower section to move the same linear distance from leading edge to trailing edge. Fluid mechanics tells us that when air flow increases velocity while holding temperature constant that the air pressure drops. I know this can sound counter intuitive to some people, but higher airflow velocity means lower air pressure, but don't confuse air pressure with drag forces caused by airflow moving around an object. Drag force is primarily a function of the airflow velocity squared, not air pressure.

I don't think your laminar airfoil flow model really applies well, here. There is a very small opening at the front of the windscreen letting in a small amount of airflow under the windscreen relative to the amount of airflows over it. I can't say for 100% sure, but I feel pretty confident that the area under the windscreen is a low pressure pocket, and not a high pressure pocket, relative to the air pressure moving over the windscreen. That small amount of airflow from the inlet spreads out to cover a greater volume as it moves back.

Well, I have to run at the moment. I'll try to come back later.

[Guest MadMax]

Thanks for the initial response. This is intended to be a conversation starter. I'm not a physicist and I'll soon get out of my depth..

However, when you come back, do you agree that a windscreen is more like a sail than a wing? If so, does that make any difference from the fluid mechanics perspective?

Anyway, given that my own DIY physics is pretty suspect, I'd be interested in other models that help explain why some windscreens work better for some people rather than others.

[Monk]

While we wait for some more expert opinions....I'd say it's more like a sail than a wing, but I don't think it applies anyway since the sail is catching the air and slipping close to the ripple for the most effect... air pressure being on the inside of the curve...Where the wind screen is basicly pushing though the air(angle will push bike down Screen only)....Some have holes at the bottom some don't(the intended purpose was to get rid of the eddy effect.......... I have a laminar airfoil on mine and I find the effect is defusing the blast over the top of stock, which is putting a softer blast on my helmet......... But due to riders size differences and also how far or closer the screen is to a rider is going to be different for each rider........

I do like my laminar airfoil, by the way.

[gll429]

it acts like neither a wing or a sail... both help things MOVE!!

the screen acts a bug collector, a scratch magnet, and a the weak spot that cracks LAST when you drop your bike. :biggrin:

[mello dude]

I've been reading various threads about windscreens and trying to come up with an explanation that puts some science behind the screen dilemma. I'm going to lay out my thinking so that some of the physicists or sailors lurking out there can comment or add detail.

You see, as a sailor, it seems to me that a windscreen is very similar to a sail. The air flowing over the external curved surface is of a higher pressure than the wind flowing under it. This is because the outside air has to force its way around the curvature which is an obstacle to its progress. Inside the curve there is no obstacle and the pressure required to pass is lower. This is what generates lift in a sail or an airplane wing and I believe it has to be the same for a motorcycle screen.

For this reason, I imagine that filling in the gap behind the screen would be a risk. The normal difference in pressure between the inside and outside airstream would have the effect of lifting the screen away from your face or chest. If you filled in the gap, you would just have the outside pressure and this may bend the screen to extremes in some circumstances.

Shortening the screen to the degree that some riders have considered would also seem to me to be self defeating. The shorter the screen, the less pronounced the aerofoil effect. After a while, you may as well be on a naked bike.

A screen will work best at an angle that preserves the airflow (the lamina flow). As the screen angle reaches its stalling point, the adjustable lips that some designs use may overcome the tendency of the airflow to become turbulent by adding extra curvature at the point where the airflow leaves the screen and heads for your face or chest.

I'm guessing that the physics involved makes it hard to tailor a screen to individuals of different sizes. Because we can't play very easily with the angle at which the main part of our screens meets the air, we can only fiddle with the height factor, try a different shape or vary the angle of the edge by using an adjustable lip.

What do you think, especially the scientists? Does this make sense?

I once envisioned a screen service based on the rider and the bike. Using the finite element method CFD or Computational Fluid Dynamics analysis, after a few computer model runs, the wonderkind vendor would kick out the perfect screen for the customer. -- Dream on......

[YoshiHNS]

it acts like neither a wing or a sail... both help things MOVE!!

the screen acts a bug collector, a scratch magnet, and a the weak spot that cracks LAST when you drop your bike. :biggrin:

:laughing6-hehe:

But yeah. It doesn't act like a wing or a sail. If anything, it's more like a spoiler. And with the speed you'd be riding at, I'm not even sure the air flowing over it would still be laminar by the time it reaches the end of the windscreen.

[Burns]

I've always been under the impression that the vent on the bottom of the screens was to induce a little high pressure in the cockpit area... help spoil the low-pressure turbulence that would normally be seen from the air rolling off the top of the wind-screen? the laminar lips would just be trying to do the same thing... just 'extending' the turbulance further back... I dunno.. I'm just moderately versed in this stuff.

[Guest CousinJack]

I've always been under the impression that the vent on the bottom of the screens was to induce a little high pressure in the cockpit area... help spoil the low-pressure turbulence that would normally be seen from the air rolling off the top of the wind-screen? the laminar lips would just be trying to do the same thing... just 'extending' the turbulance further back... I dunno.. I'm just moderately versed in this stuff.

I'm fairly certain the vent is pretty useful in preserving some sort of laminar flow, and preventing major turbulence from the top edge of the screen. My last bike was a naked Bonneville, and the lack of a screen was a PITA above 70mph, so I fitted an aftermarket screen. You know the sort, half-round recess in the bottom edge to fit around the headlight. Well it looked good, worked a treat at 40mph, but at 90 I literally could not focus my eyes, my head was being battered so much by turbulence. By trial and error found that tilting it back and leaving a gap for airflow at the bottom of the screen solved the problems. I strongly suspect the VFR vent works the same way.

[The mailman]

do you agree that a windscreen is more like a sail ?

African or European?

Sorry, can't help myself at times...

[Burns]

do you agree that a windscreen is more like a sail ?

African or European?

Sorry, can't help myself at times...

and just that fast, the thread has devolved into a discussion about coconuts. :laughing6-hehe:

[VFRBulldawg]

I've always been under the impression that the vent on the bottom of the screens was to induce a little high pressure in the cockpit area... help spoil the low-pressure turbulence that would normally be seen from the air rolling off the top of the wind-screen? the laminar lips would just be trying to do the same thing... just 'extending' the turbulance further back... I dunno.. I'm just moderately versed in this stuff.

I agree, it is there to help with flow under the windscreen and to help reduce turbulence behind the screen. In all likelihood, without it you would get a low pressure vortex up under there. I had a long day at work and my engineering brain is shot at the moment. I'll be having some beers with some old college buddies who did aerospace engineering with me later tonight. Perhaps we can hash this out and come up with only a mildly beer influenced psuedo scientific answer.

[TexOz]

I've always been under the impression that the vent on the bottom of the screens was to induce a little high pressure in the cockpit area... help spoil the low-pressure turbulence that would normally be seen from the air rolling off the top of the wind-screen? the laminar lips would just be trying to do the same thing... just 'extending' the turbulance further back... I dunno.. I'm just moderately versed in this stuff.

I agree, it is there to help with flow under the windscreen and to help reduce turbulence behind the screen. In all likelihood, without it you would get a low pressure vortex up under there. I had a long day at work and my engineering brain is shot at the moment. I'll be having some beers with some old college buddies who did aerospace engineering with me later tonight. Perhaps we can hash this out and come up with only a mildly beer influenced psuedo scientific answer.

You crazy engineer guys. Way to catch up with your buddies VFRBulldawg - I can see the scene at the pub tonight:

VFRBulldawg: "Okay guys, really great to catch up, it's my shout so I'll get the beers. While I'm gone John I want you to start deriving the formula for lift from first principles, paying particular attention to verifying the coefficient of lift."

John: "Can do Bulldawg, can't wait to get my teeth into it - and some hot chicken wings "

VFRBulldawg: "Fred I want you to consider the profile drag for a 5th Gen ZG windscreen, let's review at all speed gates in 10mph increments from zero to supersonic, that should give us enough data to reach some decent conclusions"

Fred: "Check - no problems buddy"

VFRBulldawg: "Bobby, you weren't an honors student so you're the scribe, write everything down on the back of these 50 beer coasters I've collected."

Bobby: "But Bulldawg, can't I do some calculations?"

VFRBulldawg: "Bobby, we've been through this before, you should've studied harder at college, now enough of these complaints, remember there's no I in team"

Fred: "Dawg, are we going to think about any 7th Gen Aerodynamics tonight?"

VFRBulldawg: "Let's not get ahead of ourselves boyz - one VFR generation at a time"

Fred: "Copied all brother"

VFRBulldawg: "Righto, so has everybody got their calculators, a thirst for good aerospace engineering, and a thirst for beer?"

All: "Sure have, you bet, can't wait" <hi-5s all round>

And so the evening progresses in a fog of beer, applied algebra, and good times with some old mates ... :beer:

And you guessed it right - I'm the "Bobby" in this story - although I didn't study engineering! :happy:

[Polarbear]

:laughing6-hehe: :laughing6-hehe: :laughing6-hehe:

[Guest MadMax]

"African or European" "Coconuts" ?? Something has been seriously lost in translation here for me.... :computer-noworky:

[vfrcapn]

I've always been under the impression that the vent on the bottom of the screens was to induce a little high pressure in the cockpit area... help spoil the low-pressure turbulence that would normally be seen from the air rolling off the top of the wind-screen? the laminar lips would just be trying to do the same thing... just 'extending' the turbulance further back... I dunno.. I'm just moderately versed in this stuff.

I agree, it is there to help with flow under the windscreen and to help reduce turbulence behind the screen. In all likelihood, without it you would get a low pressure vortex up under there. I had a long day at work and my engineering brain is shot at the moment. I'll be having some beers with some old college buddies who did aerospace engineering with me later tonight. Perhaps we can hash this out and come up with only a mildly beer influenced psuedo scientific answer.

You crazy engineer guys. Way to catch up with your buddies VFRBulldawg - I can see the scene at the pub tonight:

VFRBulldawg: "Okay guys, really great to catch up, it's my shout so I'll get the beers. While I'm gone John I want you to start deriving the formula for lift from first principles, paying particular attention to verifying the coefficient of lift."

John: "Can do Bulldawg, can't wait to get my teeth into it - and some hot chicken wings "

VFRBulldawg: "Fred I want you to consider the profile drag for a 5th Gen ZG windscreen, let's review at all speed gates in 10mph increments from zero to supersonic, that should give us enough data to reach some decent conclusions"

Fred: "Check - no problems buddy"

VFRBulldawg: "Bobby, you weren't an honors student so you're the scribe, write everything down on the back of these 50 beer coasters I've collected."

Bobby: "But Bulldawg, can't I do some calculations?"

VFRBulldawg: "Bobby, we've been through this before, you should've studied harder at college, now enough of these complaints, remember there's no I in team"

Fred: "Dawg, are we going to think about any 7th Gen Aerodynamics tonight?"

VFRBulldawg: "Let's not get ahead of ourselves boyz - one VFR generation at a time"

Fred: "Copied all brother"

VFRBulldawg: "Righto, so has everybody got their calculators, a thirst for good aerospace engineering, and a thirst for beer?"

All: "Sure have, you bet, can't wait" <hi-5s all round>

And so the evening progresses in a fog of beer, applied algebra, and good times with some old mates ... :beer:

And you guessed it right - I'm the "Bobby" in this story - although I didn't study engineering! :happy:

lol, I'm actually the Bobby in this story and I did study fluid dynamics in my ME program. Anyone have an extra beer coaster...? :laughing6-hehe:

[Rush2112]

"African or European" "Coconuts" ?? Something has been seriously lost in translation here for me.... :computer-noworky:

I believe it may be a Monty Python reference... "Holy Grail" if I'm not mistaken.

[spud786]

Greetings my Good Man,

I'd like to Elaborate further with some of my findings

""Shortening the screen to the degree that some riders have considered would also seem to me to be self defeating. The shorter the screen, the less pronounced the aerofoil effect. After a while, you may as well be on a naked bike."""

THe vfr you can sit straight up on the interstate at 100 mph in comfort, cant do that on a non windscreen bike. I see riders on 600 bikes all scrunched down at highways speed , obviously the airflow is causing issue. THey have lower windshileds,

You do have to be very careful, giving up one issue and creating another.

Thats why when I test screens, I require more than 500 mile in a series of varying conditions. Ive had pefect clean air screen turn into a buffeting bitch in traffic.

One reason I m still running the stock screen , overall, it about as good as it gets

Edited October 16, 2010 by spud786