Saturday, January 12, 2013

Analysis: Coanda Exhausts

When the 2012 Technical regulations were published last year the biggest change was around the exhaust position. Since 2010 when Red Bull reintroduced the exhaust blown diffuser (EBD), teams had progressively lowered their exhaust outlets and pointed them at the diffuser to gain additional aerodynamic effect. For 2012 the exhaust position was mandated into a higher position with the exhaust tailpipe pointed upwards. On paper the rules appeared to prevent any exhaust effect on the diffuser. However two aerodynamic effects have been adopted to direct the exhaust plume downwards and recreate some of the blown diffuser effect; Coanda and Downwash. Although initially different teams adopted different solutions and differing amounts of these two effects. McLaren found a way of shaping the sidepod and exhaust fairing to use these effects, which were then able to consistently direct the exhaust plume at the gap between the diffuser and rear tyre. McLaren’s exhaust\sidepod solution has proven to be the most widely adopted and has been termed a ‘Coanda’ exhaust by the media. Even though it’s arguably an inaccurate term, as the downwash effect is probably greater than the Coanda effect in comparison to other sidepod shapes, such as Sauber’s or Red Bull’s.

Why blow the diffuser?
Like any aerodynamic device, a diffuser requires a moving airflow to create its effect (downforce). With a racing car the air speed varies as the car manoeuvres around the track, the conundrum is that cars tend to need downforce in slower corners, just when the air flow is at its slowest. If you could artificially make the airflow faster over the car, then you would create more downforce for faster cornering. By using the exhaust, the teams are literally using the engine as a fan to speed up airflow over the aerodynamic surfaces. The fast moving exhaust plume is blown over the diffuser for two effects; firstly it drives more airflow through the diffuser. The more airflow that passes through the diffuser the more downforce it creates. The second factor is sealing the diffusers edge and this is increasingly the primary benefit for EBDs. Unable to blow directly into the diffuser since the 2011 rule changes, instead the exhaust now blows along the side of the diffuser, this serves as fluid skirt to seal the side of the diffuser from leakage. As the diffuser has low pressure inside, this will draw in high pressure airflow from outside the diffuser reducing its downforce. Additionally, the tyres wake sends an unwanted jet of airflow into the diffuser, known as ‘tyre squirt’. Having the exhaust blow along the diffusers edge keeps these pressure regions separate and prevent the tyre squirt upsetting the diffuser. Preventing these effects means the diffuser is more efficient, an additional benefit is it means the team can run a higher rear ride height, which effectively makes the diffuser larger and further increases it’s potential to make downforce. All these effects multiply to create efficient downforce at the back of the car.

How to redirect the exhaust plume?

Unaffected by other Aero effects, the exhaust plume would blow upwards

The 2012 rules place the exhausts tailpipe a minimum of 25cm above the reference plane and well ahead of the diffuser; they also prohibit any bodywork over the exhaust plume, which could be used to deflect the plume downwards. Since the 2009 the sidepod bodywork is already restricted by curvature and minimum radius rules, so teams are heavily restricted with what they can do to manipulate the plume. Several teams thought laterally about a solution and the Coanda exhaust solution was conceived by McLaren.
With this set up the exhaust plume is curved downwards by both the shape of the bodywork aft the tailpipe (coanda) and by the airflow passing over the sidepod (downwash).

The curved surface of the Exhaust duct encourages the exhaust plume to bend via the coanda effect

Coanda – The tendency of a gas or liquid coming out of a jet to travel close to the wall contour, even if the wall’s direction of curvature is away from the jet’s axis.

McLaren’s sidepod sees the exhaust tailpipe exit inside deep trough-like duct in the sidepod, the bottom of this duct starts under the exhaust and then curves downwards pointed towards the edge of the diffuser. The exhaust plume exits at 10 degrees upwards, but due to the Coanda effect the plume tries to follow the curve of the duct; this helps the plume start to point downwards. The duct ends with a sharp edge and the exhaust plume continues on a downwards trajectory until it hits the floor.

Using the Coanda effect the exhaust blow the floor just to the side of the diffuser

Downwash – A downward deflection of airflow

The shape of the sidepod ahead of the tailpipe slopes downwards, this creates a downwards airflow called downwash. McLaren have also fitted vanes at the front of the sidepod to increase the downwash effect. When this downwash hits the exhaust plume, it bends it downwards, increasing the repointing of the plume towards the floor. This effect is well documented and often termed “jet in cross flow”.
With these two effects, even at low speed the plume is diverted towards the floor, making use of the EBD effect in slow turns, just when downforce it is required most.

The Downwash over the sidepod, bends the exhaust plume aiming it towards the diffuser

One of the problems in designing the exhaust flow in the coke bottle (the narrow tail of the sidepod) area is that there are two primary airflows. Firstly the coke bottle flow, where the airflow around the flanks of the sidepods passes in-between the rear wheels to drive flow over the diffuser. The second is the exhaust flow, which trying to get to the diffusers edge. At some point these two flows want to cross over and will disrupt each other. For pure ramped solutions like Sauber’s or Red Bull’s early season sidepods, little was done to mitigate this clash. Red Bull subsequently created tunnels in its sidepod to allow the coke bottle flow to reach the back of the car and not cross the exhaust plume. McLaren’s solution partially resolves this problem. The exhaust exiting in the bulged fairing out of the flank of the sidepod, allows a lot of coke bottle flow to pass under the fairing. Some of the coke flow will inevitably hit the exhaust plume redirecting the plume inwards slightly. As a result the exhaust is actually pointed further outwards than appears necessary, when at speed the flow around the flank of the sidepods pushes the plume back into correct alignment with the diffusers edge.

EBDs existed as long ago as 1983; they eventually fell out of favour due to their sensitivity to throttle position. So when the driver is off the throttle there is very little extra downforce being produced, when the throttle is wide open and the revs are high there’s a lot of extra downforce being produced, the variation can make the cars response hard to predict. In the past two years teams have created engine mappings that keep the throttles open (cold blowing) and fuel being burnt in the exhausts (hot blowing) to keep exhaust velocities up when the driver is off the throttle. These mappings have been restricted (not specifically banned) this year, so some softening of the blown effect between on and off throttle is still possible. But the new exhaust position being further from the diffuser and less effective than the low exhausts used last year, means the 2012 EBD is less sensitive to throttle position than in 2011. Thus the demand for complex mapping is less critical in getting the EBD working predictably.

Trying to understand where the exhaust plume hits the floor is harder to confirm as the floor on the McLaren is not coated with heat shielding or heat reflective coatings. Although the exhaust plume is around 750c at the tail pipe, the exhaust rapidly cools as it leaves the tail pipe and passes downward towards the floor. Temperatures of 150c are typical at floor level with a Coanda exhaust and are far easier to deal with than the 2010\2011 style EBD, where the tailpipe blew directly on the carbon fibre diffuser bodywork. Most of the time we only see a sacrificial film applied to the floor, to keep it from being dulled by the exhausts heat.

Adoption by other teams
With the McLaren style exhaust\sidepod being increasingly adopted and few teams have yet to follow this design. Ferrari were closest in concept to the McLaren with their original Acer duct exhaust design. It was Force India, Toro Rosso, Sauber and Caterham who soon followed McLaren and Ferrari subsequently altered their Acer duct to follow the McLaren style more closely. Mercedes, Williams and latterly Lotus now sport a coanda exhaust. This leaves just Red Bull following the fully ramped sidepod design, with Marussia having a design very close to the original Sauber\Red Bull set up. Only HRT remain with a simple periscope exhaust set up.

Next year there are no changes to the regulations regarding exhaust position, so we can expect teams develop their new cars with coanda set up in mind from their very inception. This will mean the entire sidepod and its internals being repositioned to create the ideal downwash and the tail end of the car rearranged to allow for the bulged exhaust fairing and slim coke bottle shape. It’s possible some teams may stick with the Sauber\Red Bull format, especially as Sauber have returned to a sidepod more closely related to their early ramped design than the bulged side exit as they adopted for the Spanish GP.


Luiz Bueno Ian Burgess

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