Question about Aerodynamics Boundary Layer ! Help !

[firasf1dream]

hello everyone,
i would like please to understand something about the boundary layer:
i understand that boundary layer should be a thickness in which there is absence of flow ?
or there is never absence of flow even in separation state ? i understood that a boundary layer is absence between the vehicle surface and the free streamlines and when the boundary is very thick, a separation will happen and the flow will become turbulent ?

[Vyssion]

Hi FirasF1dream,

The boundary layer isn't a region where there is an "absence of flow" as you describe it so to speak... Air is a fluid with low viscosity and plays an important role in keeping flow attached to the moving body. It is, however, not the same thing as density and in no way is indicative of a high density if the fluid also has a high viscosity. Because air has this viscosity property, at the boundary layer, air tends to adhere to a body as it moves through it; and where there is relative motion between any layers of a substance, there are shear force interactions. This adhesion to the surface of the body is often referred to as the air being attached to the surface. Further away from the surface, however, the interaction with this boundary layer no longer has any effect and the flow velocity of the air stream is unaffected.
Shear forces interact between the boundary layer and the free stream air, there is a loss of energy. This causes a thickening of the boundary layer around the aerofoil which may begin as only a few millimeters but widen to a couple of centimeters across the length of an average car. This widening of the boundary layer actually causes the relative thickness of the body to increase. As this happens and the boundary layer gradually increases more and more, small eddies begin to form within this layer which travel in the opposite direction to the fluid flow. The reversal of flow at the surface causes the boundary layer to be forced off the surface to a point at which the sum of pressure vectors is zero. Flow separation then greatly increases the thickness of an aerofoils profile.

You kind of have the right idea, just the actual physics reasoning behind "why" the separation occurs isn't right

[firasf1dream]

Hi FirasF1dream,

The boundary layer isn't a region where there is an "absence of flow" as you describe it so to speak... Air is a fluid with low viscosity and plays an important role in keeping flow attached to the moving body. It is, however, not the same thing as density and in no way is indicative of a high density if the fluid also has a high viscosity. Because air has this viscosity property, at the boundary layer, air tends to adhere to a body as it moves through it; and where there is relative motion between any layers of a substance, there are shear force interactions. This adhesion to the surface of the body is often referred to as the air being attached to the surface. Further away from the surface, however, the interaction with this boundary layer no longer has any effect and the flow velocity of the air stream is unaffected.
Shear forces interact between the boundary layer and the free stream air, there is a loss of energy. This causes a thickening of the boundary layer around the aerofoil which may begin as only a few millimeters but widen to a couple of centimeters across the length of an average car. This widening of the boundary layer actually causes the relative thickness of the body to increase. As this happens and the boundary layer gradually increases more and more, small eddies begin to form within this layer which travel in the opposite direction to the fluid flow. The reversal of flow at the surface causes the boundary layer to be forced off the surface to a point at which the sum of pressure vectors is zero. Flow separation then greatly increases the thickness of an aerofoils profile.

You kind of have the right idea, just the actual physics reasoning behind "why" the separation occurs isn't right

hello Vyssion,
thank you so much for your reply,
so i understand that there is never absence of flow which means around the body there is always flow even when there is vortices or eddies, and when the pressure is zero as you said is that the state where you lose downforce and gain drag ?
so about absence of flow the only time that this happens is when an aircraft break the speed of sound and makes that boom sound, when the air is separated so low pressure in this area and big pressure around it, and as the flow goes from high to low so the flow will get in this area fast enough to make that sound : Boom ? or not !

[Vyssion]

so i understand that there is never absence of flow which means around the body there is always flow even when there is vortices or eddies, and when the pressure is zero as you said is that the state where you lose downforce and gain drag ?

Kind of... to the first part of your question, there is always flow in the sense that there isn't a mini vacuum in the middle of the boundary layer vortices. There are molecules/atoms in the middle of the vortices, they just aren't moving all that much when compared to the outer vortex perimeter.
To the second part, when I mentioned "sum of the pressure vectors" I was referring to the specific vectors which would add up to objects downforce/drag. Essentially, when you have an aerofoil (for example), there is something called a pressure gradient which exists. Oversimplifying things here, a "favourable" pressure gradient (FPG) would be one where the aerofoil curves up into the flow (e.g. the top surface of a downforce producing wing). An "adverse" pressure gradient (APG) is one where the aerofoils surface curves away from the flow (e.g. the bottom surface of a downforce producing wing. As the air flows within this APG, as the flow tries to follow the surface curving away from itself, some of the pressure vectors' magnitude is taken up by that process. The boundary layer begins laminar over the aerofoil surface, but as the surface curves further away, the boundary layer is less able to withstand the increasing APG, and so when all of the pressure magnitude of those vectors is taken up by trying to follow the aerofoil surface, the flow separates and forms little eddies/vortices. Now, this process "adds" energy to the boundary layer, which makes the air transition into a turbulent boundary layer, which is much more able to withstand a larger APG (this point is usually referred to as a separation bubble). So the flow reattaches and continues over the surface. Full separation only occurs when that turbulent boundary layers pressure vectors sum up to zero similar to the laminar case. Drag is always increasing as this happens, but downforce doesn't begin to drop off until the turbulent layer separates.

so about absence of flow the only time that this happens is when an aircraft break the speed of sound and makes that boom sound, when the air is separated so low pressure in this area and big pressure around it, and as the flow goes from high to low so the flow will get in this area fast enough to make that sound : Boom ? or not !

I haven't done too much hypersonic aerodynamics, but the basic concept detailed above is the same for it. The air which hits the nose of the aeroplane literally can't move out of the way fast enough. So the air compresses at the nose and is basically forced out faster than it can move itself out in a cone around it. You don't want your wings to intersect that cone or it'll cause massive stress on them. The sonic boom itself is caused by the shock waves of the compressed air at the nose of the aeroplane bunching up closer and closer until they merge into one giant shockwave traveling at the speed of sound through the air. The air does separate from the aeroplane for the same reasons as an aerofoil, but because air is compressible, you need to factor that into it as well. Generally, we accept that below 0.3Mach, air can be considered incompressible.

[firasf1dream]

so i understand that there is never absence of flow which means around the body there is always flow even when there is vortices or eddies, and when the pressure is zero as you said is that the state where you lose downforce and gain drag ?

Kind of... to the first part of your question, there is always flow in the sense that there isn't a mini vacuum in the middle of the boundary layer vortices. There are molecules/atoms in the middle of the vortices, they just aren't moving all that much when compared to the outer vortex perimeter.
To the second part, when I mentioned "sum of the pressure vectors" I was referring to the specific vectors which would add up to objects downforce/drag. Essentially, when you have an aerofoil (for example), there is something called a pressure gradient which exists. Oversimplifying things here, a "favourable" pressure gradient (FPG) would be one where the aerofoil curves up into the flow (e.g. the top surface of a downforce producing wing). An "adverse" pressure gradient (APG) is one where the aerofoils surface curves away from the flow (e.g. the bottom surface of a downforce producing wing. As the air flows within this APG, as the flow tries to follow the surface curving away from itself, some of the pressure vectors' magnitude is taken up by that process. The boundary layer begins laminar over the aerofoil surface, but as the surface curves further away, the boundary layer is less able to withstand the increasing APG, and so when all of the pressure magnitude of those vectors is taken up by trying to follow the aerofoil surface, the flow separates and forms little eddies/vortices. Now, this process "adds" energy to the boundary layer, which makes the air transition into a turbulent boundary layer, which is much more able to withstand a larger APG (this point is usually referred to as a separation bubble). So the flow reattaches and continues over the surface. Full separation only occurs when that turbulent boundary layers pressure vectors sum up to zero similar to the laminar case. Drag is always increasing as this happens, but downforce doesn't begin to drop off until the turbulent layer separates.

aha ! ok i think i got it, the small vortices that formes in the beginning of surface change yes yes ! small bubbles and then they disappear because the flow will catch up again
about the energy, you said it's added to the boundry layer, you mean stress is added on the wing right ? there is the case of tip vortices when they form on the tips of a wings they create lots of stress (if we are talking FEA) so the end plate plus the aerodynamical values, it's job is to break the stress from the tips of the wing plus less vibrations will be induced i think (correct me please if i am wrong)

[Vyssion]

about the energy, you said it's added to the boundry layer, you mean stress is added on the wing right ? there is the case of tip vortices when they form on the tips of a wings they create lots of stress (if we are talking FEA) so the end plate plus the aerodynamics values, it's job is to break the stress from the tips of the wing so less vibrations (correct me please if i am wrong)

Mmmm you're beginning to enter into the more detailed realm of aerodynamics with the Navier Stokes Equations' turbulent kinetic energy production terms and stuff with the first bit of what you said, but regarding the second part, tip vortices form just due to the natural flow of high pressure to low pressure and there not being a wall present to prevent that flow.
Any sort of vibrations are more due to the wing's general flexibility than the tip vortices.

Regarding endplates, they play an important part in increasing the wing elements downforce and can even have positive effects on drag reduction. Above a racing wing, there is a high pressure zone, and below the element, there is a low pressure. This differential in pressure is proportional to the downforce generated. Ideally, it would be beneficial to have a wing with an infinite span in order to ensure that there was no interaction between these two pressure differences until the wing was past the point where the air could have any effect on it. However, in the real world, this is not possible, and as such, endplates are employed in order to try and prevent pressure interactions between the two zones and thereby keep downforce high.
In free stream, a wing’s effective aspect ratio is less than the theoretical value calculated. This is primarily due to “spillage” over the edges of the element which have a negative impact on lift generation and drag. By utilizing bigger endplates, the effective aspect ratio of such elements can be increased.
There are various reasons for designing an endplate with most of its planar area below the element than above; and not purely due to rules and regulations. The main reason as stated above to do with the pressure differentials. The magnitude of the increase of pressure on the upper surface of the element, is less than the magnitude of the decrease in pressure below the element when compared to normal atmospheric pressures. This also implies that the higher downforce the wing assembly produces, the larger the optimal endplates would need to be due to the greater difference in pressures exhibited by higher downforce profiles.

[firasf1dream]

about the energy, you said it's added to the boundry layer, you mean stress is added on the wing right ? there is the case of tip vortices when they form on the tips of a wings they create lots of stress (if we are talking FEA) so the end plate plus the aerodynamics values, it's job is to break the stress from the tips of the wing so less vibrations (correct me please if i am wrong)

Mmmm you're beginning to enter into the more detailed realm of aerodynamics with the Navier Stokes Equations' turbulent kinetic energy production terms and stuff with the first bit of what you said, but regarding the second part, tip vortices form just due to the natural flow of high pressure to low pressure and there not being a wall present to prevent that flow.
Any sort of vibrations are more due to the wing's general flexibility than the tip vortices.

Regarding endplates, they play an important part in increasing the wing elements downforce and can even have positive effects on drag reduction. Above a racing wing, there is a high pressure zone, and below the element, there is a low pressure. This differential in pressure is proportional to the downforce generated. Ideally, it would be beneficial to have a wing with an infinite span in order to ensure that there was no interaction between these two pressure differences until the wing was past the point where the air could have any effect on it. However, in the real world, this is not possible, and as such, endplates are employed in order to try and prevent pressure interactions between the two zones and thereby keep downforce high.
In free stream, a wing’s effective aspect ratio is less than the theoretical value calculated. This is primarily due to “spillage” over the edges of the element which have a negative impact on lift generation and drag. By utilizing bigger endplates, the effective aspect ratio of such elements can be increased.
There are various reasons for designing an endplate with most of its planar area below the element than above; and not purely due to rules and regulations. The main reason as stated above to do with the pressure differentials. The magnitude of the increase of pressure on the upper surface of the element, is less than the magnitude of the decrease in pressure below the element when compared to normal atmospheric pressures. This also implies that the higher downforce the wing assembly produces, the larger the optimal endplates would need to be due to the greater difference in pressures exhibited by higher downforce profiles.

well i studied navier stokes in my second year in the fluid mechanics course but i forgot it now,
yes so all about the pressure, the difference of pressure will let the flow go from high the low therefore the wing tip when open will let flow pass from high to low and so create the vortices, you said end plate should be below more than above, i think the reason is make barriers with the ground, which lead to ground effect theory ?

about vortices, there is something that i did not understand is how vortices can be used to generate downforce ?!!! well i understand that they induce more drag and no downforce but when used properly, they generate downforce ! can you please explain that ?

[rjsa]

I guess I picture is worth a few tousand words here:

[Vyssion]

yes so all about the pressure, the difference of pressure will let the flow go from high the low therefore the wing tip when open will let flow pass from high to low and so create the vortices, you said end plate should be below more than above, i think the reason is make barriers with the ground, which lead to ground effect theory ?

about vortices, there is something that i did not understand is how vortices can be used to generate downforce ?!!! well i understand that they induce more drag and no downforce but when used properly, they generate downforce ! can you please explain that ?

The reason for the endplates extending below the aerofoil more so than above is because the magnitude of the difference between atmospheric pressure under the wing is much greater than above the wing... So endplates well below the wing confine that low pressure region to a bigger volume which allows for a larger magnitude drop.

Vortices aren't used to produce downforce exactly... The way they help is via a similar aim to endplates; basically, creating a vortex under an aerofoil (like the front wing of an F1 car within the half circle tunnel like feature) increases that low pressure drop magnitude. A vortex has a low pressure core and that helps increase the difference between under the wing and above - which means more downforce

[bhall II]

...as such, endplates are employed [on race car wings] in order to try and prevent pressure interactions between the two zones and thereby keep downforce high.

I don't think this holds true for ground effect. Edge vortices created by the interaction you've described are highly beneficial to an inverted airfoil in ground effect. They keep air flow attached to the underside of the wing at angles of attack that would otherwise be impossible, especially if it's a wing that has rotating wheels immediately downstream, and they create downforce in and of themselves - like vortex lift on a delta wing.

That's why we saw the development of front wing end plates like these...



...until the current style was essentially made mandatory by the regulations. Those designs deliberately encouraged high-pressure air flow to spill over the sides.

[firasf1dream]

I guess I picture is worth a few tousand words here:

http://www.consultkeithyoung.com/media/ ... nation.png

hello rjsa, thank you for your reply, well yes this was my question since the beginning, the place where there is the arrow saying boundry layer thickness there is air particles

[Vyssion]

...as such, endplates are employed [on race car wings] in order to try and prevent pressure interactions between the two zones and thereby keep downforce high.

I don't think this holds true for ground effect. Edge vortices created by the interaction you've described are highly beneficial to an inverted airfoil in ground effect. They keep air flow attached to the underside of the wing at angles of attack that would otherwise be impossible, especially if it's a wing that has rotating wheels immediately downstream, and they create downforce in and of themselves - like vortex lift on a delta wing.

That's why we saw the development of front wing end plates like these...

http://s27.postimg.org/6tkqvf16b/MG_7935.jpg

...until the current style was essentially made mandatory by the regulations. Those designs deliberately encouraged high-pressure air flow to spill over the sides.

FW GE definitely wants those vortices - totally agree there with ya.... Was keeping it simple

[firasf1dream]

...as such, endplates are employed [on race car wings] in order to try and prevent pressure interactions between the two zones and thereby keep downforce high.

I don't think this holds true for ground effect. Edge vortices created by the interaction you've described are highly beneficial to an inverted airfoil in ground effect. They keep air flow attached to the underside of the wing at angles of attack that would otherwise be impossible, especially if it's a wing that has rotating wheels immediately downstream, and they create downforce in and of themselves - like vortex lift on a delta wing.

That's why we saw the development of front wing end plates like these...

http://s27.postimg.org/6tkqvf16b/MG_7935.jpg

...until the current style was essentially made mandatory by the regulations. Those designs deliberately encouraged high-pressure air flow to spill over the sides.

thank you bhall for your reply, well let's go to the beginning of the new era of F1 to the 2009 season, we can see most of the car has very simple front wing design at the beginning of the season we do not see all the barriers on the endplates example this picture http://i42.tinypic.com/euk9at.jpg

[firasf1dream]

The reason for the endplates extending below the aerofoil more so than above is because the magnitude of the difference between atmospheric pressure under the wing is much greater than above the wing... So endplates well below the wing confine that low pressure region to a bigger volume which allows for a larger magnitude drop.

here i do not understand what you mean magnitude of the difference, is it that the very low pressure under the wing has more influence so it's a strong vaccum to the particles of the above region ?
let's go back to the perfect condiction which is the wing without specific length, in this case we do not have any problem of endplate, when we calculate the lift it's to the max, but when we define a specific length, we found that there is drop in downforce created by the free end which means vortices are formed (particles tend to go from high pressure region to low pressure region) so they added vertical simple endplate and found that this will make downforce rise again, and if the endplate was hight from above they found that the downforce will even more ?

Vortices aren't used to produce downforce exactly... The way they help is via a similar aim to endplates; basically, creating a vortex under an aerofoil (like the front wing of an F1 car within the half circle tunnel like feature) increases that low pressure drop magnitude. A vortex has a low pressure core and that helps increase the difference between under the wing and above - which means more downforce

you mean they are used to help keep the downforce from dropping ?

[bhall II]

...the 2009 season, we can see most of the car has very simple front wing design at the beginning of the season we do not see all the barriers on the endplates...

Current front wings are essentially diffusers, and Honda/Brawn was the first team to figure that out and really explore the possibilities of the concept. The advantage gained through that work, especially early in the season before other teams had a chance to follow suit, carried them to a World Championship.



Even though it wasn't as sexy as double-diffusers at the time, and it certainly didn't get as much attention, development of the so-called outwash front wing was probably more important.