Effect on engine power from Coanda exhaust

[olefud]

As the desired effect of the Coanda exhausts is for the exhaust plume to follow the contour of the body work to be directed to the right area of the diffuser and then create a vortex to seal it from the surrounding air streams, theoretically a less turbulent air stream should maintain its flow stability and direction better than more turbulent air and result in it arriving at the diffuser with more energy.

Rather than reducing the orifice size at the exhaust, could using a flow straightener (which would effectively reduce the area of the orifice) such as those used in wind tunnels, fire hoses and decorative, large scale water displays, like those in the foyer of the Burj Al-Arab in Dubai and the forecourt of the Bellagio in Las Vegas, to reduce flow turbulence and increase exhaust plume integrity and result in a more energetic air stream and therefore more efficient vortex creation and better sealing of the diffuser area?? Of would the use of a flow straightener cause other unforeseen problems??

I pretty much agree with your comments –but there MAY be multiple factors. Velocity and temperature are interrelated but differing parameters. Your comments deal with velocity rather well, though I see this as being a means as well as an end. The exhaust nozzles, to my mind, are means of getting to plume to the desired location –though also a factor in generating vortices. I think so because once subjected to ambient pressure the plume expands, cools a bit and tends to slow fairly rapidly. But of course velocity also influences the formation of vortices.

Temperature alone also has a significant influence. Air (referencing the case of brakes) increases its viscosity with temperature. It is much “tackier” at 1200 or 1800 degrees F than at ambient. At such temps it becomes a thicker, more adherent boundary layer and displays enhanced Coanda effect. Thus, if the heated plume is properly ducted, it can attach to and direct larger quantities of ambient air –which IMO is one of the objects of utilizing the heated exhaust plume.

Your comments are appreciated. I can almost see the wheels turning as you developed the subject.

[aussiegman]

I pretty much agree with your comments –but there MAY be multiple factors. Velocity and temperature are interrelated but differing parameters. Your comments deal with velocity rather well, though I see this as being a means as well as an end. The exhaust nozzles, to my mind, are means of getting to plume to the desired location –though also a factor in generating vortices. I think so because once subjected to ambient pressure the plume expands, cools a bit and tends to slow fairly rapidly. But of course velocity also influences the formation of vortices.

Thanks for the positive feedback.

Yep, I am struggling to remember everything we all talked about and I had totally forgotten the part about temperature and its effect on gas viscosity. IIRC, it was centred on dynamic and kinematic viscosity's and that the kinematic viscosity would increase with higher temperatures. A very good point which I wasn't at all clear with first off!! Must be getting old

When using Hot Blowing, there were only small temperature differentials in the exhaust plume (at the exhaust opening) between on and off throttle applications and so there was very little aero sensitivity when in operation for flow and temperature changes in the exhaust plume were limited. This was one of its biggest advantages of Hot Blowing, stability of effect.

In contrast, Cold Blowing (which is really an alternation between Hot and Cold Blowing) sees velocity, volume and temperature differentials which are in the vicinity of 400-500C and approximately 25% for flow and velocity. However, the eventual temperature at the diffuser interface between Hot and Cold Blowing showed a much smaller differential.

As you rightly pointed out, the exhaust plume starts to cool rapidly as soon as exits the exhaust opening and by the time it
reached the vortex generator at the diffuser interface the temperature differential was not as high as at the exhaust tip between the two methodologies. This was again explained to me that the hotter exhaust plume expands more rapidly and therefore dissipates more heat over a similar time period through increased convection, advection and expansion effects.

Where Cold Blowing showed a performance difference was that while temperature generally varied less over all and only slightly lower than Hot Blowing , the velocity and volume did change during by around 25%. This did alter the effectiveness of the sealing from the vortices created at the diffuser interface. This made the car more aero sensitive as it had less downforce under Cold Blowing conditions.

The largest differences between Hot and Cold Blowing is found in the stability of the effect, with Hot Blowing more consistent and in the ability of the exhaust plume to follow the series of acute angles which is due to velocity but also as you said gas viscosity. Hot Blowing thanks to the temperature of the exhaust plume at the tip allows the bodywork to contain more acute angles as the hotter gas will remain attached. However, as it travels further and looses its heat and therefore decreases in viscosity, the bodywork radii and angles must increase to prevent detachment of the exhaust plume and loss of the directional stability of the flow.

Or something like that, I think..

[aussiegman]

Temperature alone also has a significant influence. Air (referencing the case of brakes) increases its viscosity with temperature. It is much “tackier” at 1200 or 1800 degrees F than at ambient. At such temps it becomes a thicker, more adherent boundary layer and displays enhanced Coanda effect. Thus, if the heated plume is properly ducted, it can attach to and direct larger quantities of ambient air –which IMO is one of the objects of utilizing the heated exhaust plume.

Your comments are appreciated. I can almost see the wheels turning as you developed the subject.

Need to think about this one a lot more so this is me thinking out load.....

Current F1 “Coanda exhausts” are trying to replicate the sealing of the diffuser area by using high energy vortices from the directed exhaust plume to create an "air wall". The energy they use to create these vortices and maintain the "air wall" integrity is the kinetic energy or velocity of the exhaust plume. The higher the velocity the greater the energy of the vortex created and higher the integrity of the air wall.

So, while I absolutely agree temperature helps with directional flow attachment and as such is an advantage for increasing the effectiveness of the Coanda exhausts in directing the exhaust plume to the diffuser interface to overcome the higher exhaust tip placement, I'm not positive it substantially aids in the actual replication of the diffuser sealing.

So, the basic question is, does a higher temperature gas stream assist with the actual sealing of the diffuser through either aiding vortex creation or increasing air wall integrity or does it only assist with maintaining flow attachment, stream integrity and general velocity in getting it to the diffuser interface?

IMO, the gas temperature is now only such an issue as it aids gas viscosity and the exhaust plume needs to maintain flow attachment to reach the diffuser interface using the Coanda effect.

Once there, the thermal energy of the gas has little bearing on the ability to create the required vortices or increase their integrity as this relies on the use of the high kinetic energy in the gas stream to create and maintain vortex integrity against the outside pressures influences such as tyre squirt and the higher pressure air outside the diffuser bleeding into the lower pressure created. Also, much of the gas temperature has been lost by the time it reaches the diffuser interface to the effects of convection, advection and expansion.

So the heated air from the brakes may have high thermal energy, however it has low kinetic energy, as this is generally limited to that of the forward movement of the car (due to drag considerations and the assumption that the designers would not want to substantially increase effective drag ), which ranges to a maximum of approximately 315kph (which equates to approx. 195mph or 87.5m/s or 287ft/s) but will generally be much slower with average speeds around 225kph (140mph or 62.5m/s or 205f/s).

Additionally, substantially elevated temperatures bring with them other associated issues with the burning, charring or melting of surrounding suspension and bodywork components as well as possibly affecting their structural integrity. To negate the temperature effects you'd have to either increase weigh through insulation or larger mass components or change their materials altogether to survive the elevated temperatures. Standard carbon fibre constructions do not typically like 1100C plus temps and may require titanium ducting and components or basalt based fabrics and specialist resins. Both are generally heavier and if use in suspension will increase the inertia of the suspension systems.

Exhaust gases exit the exhaust tip at approximately 540kph or 150m/s and so has much more kinetic energy. As such, once it reaches the diffuser interface it has a greater ability to effectively seal the diffuser across the various vehicle speeds than the variable and lower energy plume from the possible brake system. Attempts to increase the air speed of the brake plume would result in a trade off for increased drag.

The diffuser can become effective at much lower speed than the average so having the higher kinetic energy exhaust becomes an even greater advantage if you can use it to “drive” the diffuser as well at lower speed.

So to my thinking, ducting the very hot but low kinetic energy brake plume to help drive the Coanda exhaust may help with flow attachment to some degree it would likely not increase the effectiveness of the sealing of the diffuser. It may help slightly to drive the diffuser through the expansion and cooling of the brake plume if it could be directed into the right area, however without a high kinetic energy it will only ever match or be lower than the vehicle speed due to other considerations which brings my thinking to the next set of variables:

1 ) induced drag from ducting and flow restriction to channel or energise brake plume
2 ) induced drag from compromised aerodynamic flow to the rear wing area as the ducting extends from the outboard brake
3 ) structure to the inboard exhaust and diffuser sections
4 ) increased complexity of the system
5 ) increase weight from ducting etc
6 ) inadequate plume velocity
7 ) possible vehicle handling sensitivities due to event timing of braking applications affecting aero balance
8 ) issues over-cooling the brakes due to trying to meet flow requirements

As a summary, there may be some general, small benefits, however I am not sure the weight, complexity, compromise and possible negatives could be effectively overcome or ignored to see the system provide a benefit over other solutions.

Then again I might be totally wrong..

[olefud]

As a summary, there may be some general, small benefits, however I am not sure the weight, complexity, compromise and possible negatives could be effectively overcome or ignored to see the system provide a benefit over other solutions.

Then again I might be totally wrong..

Again, yea, right on. I wasn’t thinking about the diffuser specifically, though the front brakes could feed the diffuser inlet. This is more a blank sheet of paper exercise. Given –I can back this up but won’t do so here to stay focused- a plume of 1000 degree F at the brakes, are there any aero devices that could benefit? Keep in mind this is a new concept so the existing schemes are not limiting.

With reference to your specific variables;

1 ) induced drag from ducting and flow restriction to channel or energize brake plume.
In fact the means that forms the heated plume avoids the need for convection ducting though some forced convection may be necessary to cool the pads. Should reduce drag relative to the present rather extensive brake ducting.

2 ) induced drag from compromised aerodynamic flow to the rear wing area as the ducting extends from the outboard brake.

More localized aero assistance if possible would avoid this.

3 ) structure to the inboard exhaust and diffuser sections.

Well taken if this is the only utility.

4 ) increased complexity of the system. and
5 ) increase weight from ducting etc.

At the brake the proposed system is quite elegant and substantially simplifies things. I’m not being quite fair on this as I’m trying to stay focused on the aero (being in a
Coanda aero thread), but if there’s interest I can expand in a new thread.

6 ) inadequate plume velocity.

As generated it would have a velocity roughly that of the slipstream though not functionally caused thereby. Velocity direction is a more open question.

7 ) possible vehicle handling sensitivities due to event timing of braking applications affecting aero balance.

Good point. But should aid the critical braking period as hot blowing aids acceleration.

8 ) issues over-cooling the brakes due to trying to meet flow requirements.

I think this could be controlled. Actually, the problem with cast iron rotor testing is that the rotor runs too cool relative to the pad. I’ve don’t have detailed information on carbon or ceramic rotors, but these tend to like higher temperatures.

One non-aero final point. I have been thinking a bit about F-1 since the tremendous amount of radiant energy rejected by the brakes is largely absorbed by the tires in other than in the tread area and wheels which hardly seems like a good thing. If this energy could instead be rejected in a hopefully useful controlled plume tire management would be simplified.

[skgoa]

The answer is "none."

Actually, Lotus have said, both by means of Raikkonen and the team's technical director James Allison that it takes away some engine power. It's maybe not the case for all configurations, but it has to do with the length of the exhaust, and maybe also the exhaust channel wherein it exits, instead of a more free stream of air.

But that's more of a problem stemming from using the wrong engine / exhaust pipe combination.

[olefud]

The answer is "none."

Actually, Lotus have said, both by means of Raikkonen and the team's technical director James Allison that it takes away some engine power. It's maybe not the case for all configurations, but it has to do with the length of the exhaust, and maybe also the exhaust channel wherein it exits, instead of a more free stream of air.

But that's more of a problem stemming from using the wrong engine / exhaust pipe combination.

But to get the exhaust exit and orientation in the proper position for Coanda and downwash to influence the exhaust plume to the desired diffuser/wing position, the “wrong” exhaust length and/or nozzle is needed.

[PlatinumZealot]

1) Opening the exhaust into a low pressure zone is helpful. Do the current Coanda exhaust configurations promote low pressure at the exhaust upening?

2) I can not say that I have noticed that the exhaust chokes down for the Coanda systems. What is the benefit of 'increasing the speed of exhaust flow at the exit'? Does it help with flow redirection?

Brian

Diameter is the same. No reason to make it smaller for Coanda in 2012.

2011 was the opposite, some teams had a narrow exhaust like Mercedez, some teams had a wide exhaust like Mclaren.

[Crucial_Xtreme]

1) Opening the exhaust into a low pressure zone is helpful. Do the current Coanda exhaust configurations promote low pressure at the exhaust upening?

2) I can not say that I have noticed that the exhaust chokes down for the Coanda systems. What is the benefit of 'increasing the speed of exhaust flow at the exit'? Does it help with flow redirection?

Brian

Diameter is the same. No reason to make it smaller for Coanda in 2012.

Actually the diameter of the exhaust pipes was decreased this year because it(smaller pipes) while decreasing horsepower roughly 15bhp increases exhaust velocity by up to 80%.

Here's Pat Symonds take on it via RaceTech Mag



Exhaust close up
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