Drag Bucket Paradox, sometimes less aero = less drag

[ConnClark]

Originally Posted by Tesla

This is all related to the golfball dimple effect, boundary layers etc.

A worse front end is acting like a dimpled golf ball, creating more turbulance and a thicker boundary layer although this has an increase in skin/body drag, the thicker layer allows the air to bend tighter around the rear before seperation resulting in a smaller, higher pressure wake and less drag overall.

Smooth out the front, the boundary layer gets thinner, less skin/body drag, bends less, it seperates sooner, the wake is lower pressure and bigger resulting in higher drag overall.

Ummmmm.... this isn't how it works.

Golfball dimples mix are used to mix the faster moving air with the slower moving air of the boundary layer thus thinning it and maintaining dynamic pressure which allows it to follow the corners and bends in the body. A thicker boundary layer tends to form turbulence and thus is more prone to separation and hence more drag.

[Tesla]

Not quite sure what you meant with the pressures, but,

As I understand it,
Theoretically, without any outer boundary's, a flat plate moving through the air (or any fluid),has a boundary layer that increases proportionally to the length of the plate, I haven't studied the equations to determine if it is linear, exponential or whatever, but it is a proportional relationship.

So if we assume a linear relationship,
a small car with 3' of roof has say, 3/8" of boundary layer at the end of the roof,
A van with 12' of roof would have 1 1/2" of boundary layer and,
A Semi Trailer with 40' of flat roof would have about 5" of boundary layer.

So this then begs the question,
If a longer vehicle has a naturally thicker boundary layer, and a thicker boundary layer is more stable around a curve than a thinner one, can the taper at the rear of a longer vehicle be sharper than that on a shorter vehicle and still have the airflow attached?

We see the Semi Trailer boattail flaps at quite sharp angles of around 10-15°, but the template suggests anything greater than 3-5° at the start of taper and you will get immediate seperation.

So then I have to ask is the template application dependant on vehicle length and shape, how long are the flat sections before tapering, does this have a bearing on how aggressive the taper can be?

We must remember the template taper from the point of greatest camber to the tail, is based on the frontal profile of the template which is a bit of an elongated hemisphere, this profile will roughly fit most road vehicles, like sedans and hatches, but when you start to apply it to wagons, vans and trucks there is an extended section of flat roof and sides which I am starting to think has a significant bearing on template application.

For these long vehicles, it may not be necesary to trip the boundary layer to thicken it, as it is already thicker, it may be that we just need to compress the template a little and actually use a more agressive taper.

[Tesla]

Originally Posted by ConnClark

Ummmmm.... this isn't how it works.

Golfball dimples mix are used to mix the faster moving air with the slower moving air of the boundary layer thus thinning it and maintaining dynamic pressure which allows it to follow the corners and bends in the body. A thicker boundary layer tends to form turbulence and thus is more prone to separation and hence more drag.

I am still getting my head around this and the fact that laminar flow and boundary layers are just mathematical constructs, assume you are talking in a relative sense, as it is the golf ball that moves through stationary air, but either way one assumes a measure for the limit of the boundary layer, 95 - 99.99% of free stream velocity depending on source, so if you mix the stationary air at the balls surface with the moving air further away, the mixed air is has a lower velocity than the free stream and is the boundary layer.

You can't mix stationary air with fast air and expect it move at free stream velocity, any air below that velocity is part of the boundary layer.

With the dimpled golf ball compared to a smooth ball, the smooth ball has less skin friction on the face, and has a more laminar flow, but seperation occurs immediately after the equator and has a much larger wake, where as the dimpled ball has greater skin friction on the face because of the increased turbulence, but this increased turbulence (boundary layer) results in higher pressures on it's surface which result in delayed seperation and a smaller wake and lower drag which allows it to maintain a greater velocity for longer.

[aerohead]

Originally Posted by Tesla

I'm kind of in the same boat, I have done a number of mod's, car goes better, wind noise is down, handling much better, but FE static, so there must be something sucking up those improvements and it is quite possible that it is the wake gaining more intensity as the flow elswhere improves.
I will continue smoothing things out and may try look at some trip mechanism then to see if it changes things, for better or worse.

The Fiat Aero Lab published an SAE paper back around 1986 in which some changes which reduced upper body drag increased under-body drag,or visa- versa, for a net zero effect to overall drag.
As I recall,grille-blocking could back-fire,if it shunted air below a car with a torture-chamber underside.
If your vehicle is very tall compared to width,you might treat it as a symmetrical wing section/strut,getting as much air around it rather than over it.And use the L/W= 4 2-D profile,rather than the 2.5:1 3-D form.

[Tesla]

Originally Posted by aerohead

The Fiat Aero Lab published an SAE paper back around 1986 in which some changes which reduced upper body drag increased under-body drag,or visa- versa, for a net zero effect to overall drag.
As I recall,grille-blocking could back-fire,if it shunted air below a car with a torture-chamber underside.
If your vehicle is very tall compared to width,you might treat it as a symmetrical wing section/strut,getting as much air around it rather than over it.And use the L/W= 4 2-D profile,rather than the 2.5:1 3-D form.

That's kind of the direction I am heading in.

In my case 6' high from ground level, 6' wide, a clear 1+' of ground clearance and 8' of flat roof.

I believe this requires a different approach for a vehicle that is 4' high 6' wide, only 6" ground clearance and 4' of flat roof.

The dimensional differences result in it not fitting within the template profile at all, where as the std road vehicles are generally a rough fit.

I think the underside needs to be treated more like the solar racers, leaving the elevation, getting good smoothing and allowing more free flow, the question then begs, where to position the stagnation point at the front, as we basically want to achieve a natural distribution between top, sides & under.

My feeling is a split somewhere around 35% each side and 15% over and 15% under, with a stagnation point & radiator inlet in the lower half of the front under bonnet face, which is where it basicall is after the upper grille block, which I did with an upward tapered panel to get smooth transition to the bonnet.

When you say L/W of 4 in 2d profile does that basically translate to 2:1 in the symetrical form, so shorter than the template proportionally?

[Tesla]

I just watched this Video on form, Lift and Drag,
It's a bit older, but explained a lot for me as I am trying to get up to speed.

http://www.youtube.com/watch?v=4q5ffroIMMc

[ConnClark]

Originally Posted by Tesla

So if we assume a linear relationship,
a small car with 3' of roof has say, 3/8" of boundary layer at the end of the roof,
A van with 12' of roof would have 1 1/2" of boundary layer and,
A Semi Trailer with 40' of flat roof would have about 5" of boundary layer.

So this then begs the question,
If a longer vehicle has a naturally thicker boundary layer, and a thicker boundary layer is more stable around a curve than a thinner one, can the taper at the rear of a longer vehicle be sharper than that on a shorter vehicle and still have the airflow attached?

A thicker boundary layer is NOT more stable around a curve.

We see the Semi Trailer boattail flaps at quite sharp angles of around 10-15°, but the template suggests anything greater than 3-5° at the start of taper and you will get immediate seperation.

So then I have to ask is the template application dependant on vehicle length and shape, how long are the flat sections before tapering, does this have a bearing on how aggressive the taper can be?

We must remember the template taper from the point of greatest camber to the tail, is based on the frontal profile of the template which is a bit of an elongated hemisphere, this profile will roughly fit most road vehicles, like sedans and hatches, but when you start to apply it to wagons, vans and trucks there is an extended section of flat roof and sides which I am starting to think has a significant bearing on template application.

For these long vehicles, it may not be necesary to trip the boundary layer to thicken it, as it is already thicker, it may be that we just need to compress the template a little and actually use a more agressive taper.

You don't trip the boundary layer to thicken it. You trip it to re-energize it. Also trucks and vans have long flat surfaces along the roof in sides for only one reason, to increase cargo volume. Trucks and vans also have a reason for a sharp taper on the end, to decrease length. Do not confuse design features to curtail cost and fit in size regulations as rules for aerodynamics.

[Tesla]

Originally Posted by ConnClark

A thicker boundary layer is NOT more stable around a curve.


You don't trip the boundary layer to thicken it. You trip it to re-energize it. Also trucks and vans have long flat surfaces along the roof in sides for only one reason, to increase cargo volume. Trucks and vans also have a reason for a sharp taper on the end, to decrease length. Do not confuse design features to curtail cost and fit in size regulations as rules for aerodynamics.

As said, new to all of this and just trying to suck it all in.

As this is all relative, with regard to a moving vehicle or wing section there are two boundaries to the layer, that on the surface which is stationary and that adjacent to the freestream, which is at 99% of freestream velocity.

In the Laminar condition, it is thin and there is a consistant gradient of change in velocity, in the turbulent boundary layer, it is thicker and there is pressure and velocity variations throughout the layer, but still generally follows the velocity gradient.

Laminar is low pressure high velocity whereas turbulent is higher pressure lower velocity, when the slope changes after the point of maximum camber, the laminar flow seperates and tumbles into major turbulance, it does not have the volume of air or the pressure to fill the gap created, so the freestream tumbles in to fill the void. In the same circumstance the turbulent boundary layer has a higher pressure and greater volume of air and hence can maintain contact with the surface for a longer distance after the point of max camber. This effect I imagine would allow it to maintain attachment for a longer distance around a curve.

I'm still trying to understand the distinction that people are trying to place on the energizing/re energizing term, when you mix freestream with the slower moving surface air, it creater turbulence, it is no longer freestream and the boundary layer becomes thicker.
For a short distance you will have some higher velocities closer to the surface, but the layer will reorganize itself quickly to it's normal state.

What is the difference between a 2" boundary layer at the end of a long vehicle and a 2" boundary layer at the end of a short vehicle that has been tripped artificially?

I wasn't suggesting that semi trucks were built long for the purpose of a thicker boundary layer, just that because they are longer the boundary layer is thicker, hence aerodynamic principles need to be applied in a different way.

[aerohead]

Originally Posted by Tesla

That's kind of the direction I am heading in.

In my case 6' high from ground level, 6' wide, a clear 1+' of ground clearance and 8' of flat roof.

I believe this requires a different approach for a vehicle that is 4' high 6' wide, only 6" ground clearance and 4' of flat roof.

The dimensional differences result in it not fitting within the template profile at all, where as the std road vehicles are generally a rough fit.

I think the underside needs to be treated more like the solar racers, leaving the elevation, getting good smoothing and allowing more free flow, the question then begs, where to position the stagnation point at the front, as we basically want to achieve a natural distribution between top, sides & under.

My feeling is a split somewhere around 35% each side and 15% over and 15% under, with a stagnation point & radiator inlet in the lower half of the front under bonnet face, which is where it basicall is after the upper grille block, which I did with an upward tapered panel to get smooth transition to the bonnet.

When you say L/W of 4 in 2d profile does that basically translate to 2:1 in the symetrical form, so shorter than the template proportionally?

Looking down from above (plan view) the section would be 4X as long as it is wide.If you look at the aft portion of this section,you will see that this tail is actually longer than that of the 'Template.'
Fachsenfeld used this in his patent application illustrations,whereas most of the streamlining was done along the sides.The body began to narrow at the A-pillar,a bit more radical than EV1,which began its taper midway down the door.
A number of VERY low drag cars have used this approach.
No matter what you do,the roofline or body sides should never exceed 22-23 degrees.A turbulent boundary layer cannot support attached flow with a steeper curvature unless you provide blown or suctioned slots.
If you have a notchback car go ahead and use VGs as on the Lancer.
For large trucks and vans there is little evidence (none that I personally know of)that anything which might help an airplane would help a road vehicle,except slots.Hucho had no evidence that VGs or turning vanes could help lower drag of a truck.
In free flight,the aft-body of an aircraft fuselage,or bullet enjoys a jet-pumping action which doesn't occur in ground proximity.On the ground,the flow is asymmetric and causes a doubling of drag.
The reason for the 'Template' was that it was developed from a separation-free form.Since aerodynamic streamlining is predicated on reduction or elimination of flow separation,it is a 'natural.'
And the curvature is already as steep as it can be without triggering separation.If you make the tail steeper you will increase pressure drag (the big no-no!) If you make the tail longer you're adding additional skin friction ( the lesser of two evils).
And don't be looking at 'laminar' sections either.They would only help you alone on a closed course,with zero wind,at around 20 mph.If the wind came up,or you got into traffic,or went any faster,the section would transition right into a turbulent boundary layer and skin friction increase.

[Tesla]

Originally Posted by aerohead

Looking down from above (plan view) the section would be 4X as long as it is wide.If you look at the aft portion of this section,you will see that this tail is actually longer than that of the 'Template.'
Fachsenfeld used this in his patent application illustrations,whereas most of the streamlining was done along the sides.The body began to narrow at the A-pillar,a bit more radical than EV1,which began its taper midway down the door.
A number of VERY low drag cars have used this approach.
No matter what you do,the roofline or body sides should never exceed 22-23 degrees.A turbulent boundary layer cannot support attached flow with a steeper curvature unless you provide blown or suctioned slots.
If you have a notchback car go ahead and use VGs as on the Lancer.
For large trucks and vans there is little evidence (none that I personally know of)that anything which might help an airplane would help a road vehicle,except slots.Hucho had no evidence that VGs or turning vanes could help lower drag of a truck.
In free flight,the aft-body of an aircraft fuselage,or bullet enjoys a jet-pumping action which doesn't occur in ground proximity.On the ground,the flow is asymmetric and causes a doubling of drag.
The reason for the 'Template' was that it was developed from a separation-free form.Since aerodynamic streamlining is predicated on reduction or elimination of flow separation,it is a 'natural.'
And the curvature is already as steep as it can be without triggering separation.If you make the tail steeper you will increase pressure drag (the big no-no!) If you make the tail longer you're adding additional skin friction ( the lesser of two evils).
And don't be looking at 'laminar' sections either.They would only help you alone on a closed course,with zero wind,at around 20 mph.If the wind came up,or you got into traffic,or went any faster,the section would transition right into a turbulent boundary layer and skin friction increase.

Ok,
As I understand it you may be suggesting a taller vehicle may need an even longer proportional taper profile.

Would this be the same whether that vehicle had a longer body or shorter one with the same frontal area?

This then also contradicts the use the narrower section to govern the taper as has been mentioned on numerous posts in regard to boattail design.

Got the 22-23° rule, never break it under any circumstances.

Got the Laminar thing, not relevant to road going vehicles.

Got the VG thing, many questions still remain unanswered and no conclusive evidence of benefits shown except in rare specialised cases.

But things still don't add up:
The Dryden van boattail, now they reported significant benefits and yes they did get seperation further down the on the original tail before they truncated it, but they most likely went way past the 22° point then as well.

As I understand the template theory, they should have had seperation on that tail in the first 6", yet flow remained attached for a good 4'.



from this thread:
http://ecomodder.com/forum/showthrea...n-21952-7.html

And then there is all the designs, patents and fitted boattails to Semi Trailers, none of these are anywhere near the template, and most are in the same range as the Dryden Van 10-15°, I assume they have had some proven testing over the last 10years, otherwise we would not be seeing an increase in their application.

I don't dispute the effectiveness of the template profile in boattail design, but am just trying to answer some of the contradictions.

I do know now that in fluid motion the rules change with size, materials and conditions, infact it seems we really don't know what the rules are.
We have a set of rules that can effectively describe an outcome under a fixed set of conditions, then as conditions change we need to change the rules.
Doesn't seem to be far from quantum mechanics, everything is just a model for an observed effect, but actual governing laws are ever elusive.