Tuesday, March 03, 2015

Driver core-skin temperature gradients and blackouts

Whilst it is highly beneficial to reduce the surface-to-bulk temperature gradient of a racing-tyre, the same cannot be said for the cognitive organisms controlling the slip-angles and slip-ratios of those tyres.

A 2014 paper in the Journal of Thermal Biology, Physiological strain of stock car drivers during competitive racing, revealed that not only does the core body temperature increase during a motor-race, (if we do indeed count a stock-car race as such), but the skin temperature can also rise to such a degree that the core-to-skin temperature delta decreases from ~2 degrees to ~1.3 degrees.


The authors suggest that a reduced core-to-skin temperature gradient increases the cardiovascular stress "by reducing central blood volume." Citing a 1972 study of military pilots, they also suggest that when such conditions are combined with G-forces, the grayout (sic) threshold is reduced.

Intriguingly, in the wake of the Fernando Alonso's alien abduction incident at Barcelona last week, they also assert that "A consequence of this combination may possibly result in a lower blackout tolerance."

Monday, March 02, 2015

McLaren front-wing vortices, circa 2003

Academic dissertations conducted in association with Formula 1 teams tend to be subject to multi-year embargoes. Hence, Jonathan Pegrum's 2006 work, Experimental Study of the Vortex System Generated by a Formula 1 Front Wing, is somewhat outdated, but might still be of some interest to budding aerodynamicists.

Currently an Aerodynamics Team Leader at McLaren, Pegrum's study concentrated on a front-wing configuration not dissimilar from that on an MP4-18/19 (2003-2004).

A constellation of four co-rotating vortices were created: (i) a main bottom edge vortex, generated by the pressure difference across the endplate due to the low pressure under the wing; (ii) a top edge vortex, generated by the pressure difference across the endplate due to the high pressure above the wing; (iii) a canard vortex, a leading edge vortex generated by the semi-delta wing ('canard') attached to the outer surface of the endplate; and (iv) a footplate vortex, generated by the pressure-difference across the footplate operating in ground-effect. 


Pegrum shows (in the absence of a wheel, below), that the strongest vortices are the bottom-edge and top-edge vortices, but all four mutually interact in the manner of unequal, co-rotating vortices, undergoing the early stages of a merger.

Now, whilst co-rotating vortices have a tendency to merge, counter-rotating vortices have a tendency to repel. Pegrum highlights the 1971 work of Harvey and Perry, Flowfield Produced by Trailing Vortices in the Vicinity of the Ground, which demonstrated that when a vortex spinning around an axis in the direction of the freestream passes close to a solid surface, it tends to pull a counter-rotating vortex off the boundary layer of the solid surface, (as illustrated below by Puel and de Saint Victor, Interaction of Wake Vortices with the Ground, 2000). 


The interaction between these counter-rotating vortices is such that the primary vortex is repelled away from the solid surface. This phenomenon, of course, is still very much of interest when it comes to the Y250 vortex and its cousins.

Thursday, February 19, 2015

Proof that Formula 1 was better in the past

If you're a long-time Formula 1 fan, then the chances are that you believe the sport was better in the past. However, the chances are that you will have also read arguments from younger journalists and fans, to the effect that Formula 1 in the modern era is better than it was in the past.

Fortunately, there is an objective means to resolve this dispute: churn.

In sport, churn provides a straightforward measure of the uncertainty of outcome. Churn is simply the average difference between the relative rankings of the competitors at two different measurement points. One can measure the churn at an individual race by comparing finishing positions to grid positions; one can measure the churn from one race to another within a season by comparing the finishing positions in each race; and one can measure the inter-seasonal churn by comparing the championship positions from one year to another.

The latter measure provides an objective means of tracking the level of seasonal uncertainty in Formula 1, and F1 Data Junkie Tony Hirst has recently compiled precisely these statistics, for both the drivers' championship and the constructors' championship, (see figures below). In each case, Hirst compiled the churn and the 'adjusted churn'. The latter is the better measure because it normalises the statistics using the maximum possible value of the churn in each year. The maximum can change as the number of competitors changes.

The results for the drivers' championship indicates that churn peaked in 1980. Given that the interest of many, if not most spectators, is dominated by the outcome of the drivers championship, this suggests that Formula 1 peaked circa 1980.


The results for the manufacturers' championship are slightly different, suggesting that uncertainty peaked in the late 1960s, (although the best-fit line peaks in the middle 1970s).

  
One could, of course, make the alternative proposal that the churn within individual races is more important to spectators' interest, but at the very least we now have an objective statistical measure which provides good reason for believing that Formula 1 was better in the 1970s and early 1980s.

Monday, February 16, 2015

Lovelock and emergentism

In James Lovelock's 2006 work, The Revenge of Gaia, he concludes the chapter entitled What is Gaia? with a description of the regulator in James Watt's steam engine, and the following argument:

"Simple working regulators, the physiological systems in our bodies that regulate our temperature, blood pressure and chemical composition...are all outside the sharply-defined boundary of Cartesian cause-and-effect thinking. Whenever an engineer like Watt 'closes the loop' linking the parts of his regulator and sets the engine running, there is no linear way to explain its working. The logic becomes circular; more importantly, the whole thing has become more than the sum of its parts. From the collection of elements now in operation, a new property, self-regulation, emerges - a property shared by all living things, mechanisms like thermostats, automatic pilots, and the Earth itself.

"The philosopher Mary Midgley in her pellucid writing reminds us that the twentieth century was the time when Cartesian science triumphed...Life, the universe, consciousness, and even simpler things like riding a bicycle, are inexplicable in words. We are only just beginning to tackle these emergent phenomena, and in Gaia they are as difficult as the near magic of the quantum physics of entanglement."

Now Lovelock is an elegant and fascinating author, but here his thought is lazy, sloganistic and poorly-informed. There are multiple confusions here, and such confusions are endemic amongst a number of writers and journalists who take an interest in science, so let's try and clear them up.

Firstly, we encounter the slogan that a system can be 'more than the sum of its parts'. Unfortunately, the authors who make this statement never seem to conjoin the assertion with a definition of what they mean by the phrase 'sum of its parts'. Most scientists would say that the sum of the parts of a system comprises the parts of the system, their properties, and all the relationships and interactions between the parts. If you think that there is more to a whole system than its parts, their properties and the relationships between the parts, then that amounts to a modern form of vitalism and/or dualism, the notion that living things and/or conscious things depend upon non-physical elements. Calling it 'emergentism' is simply a way of trying to dress up a disreputable idea in different language, rather in the manner than creationism was re-marketed as 'intelligent design'.

Assertions that a system can be more than the sum of its parts are frequently combined with attacks on so-called 'reductionistic' science. Anti-reductionistic authors can often be found pointing out that whole systems possess properties which are not possessed by any of the parts of which that system is composed. However, if such authors think this is somehow anti-reductionistic, then they have profoundly mis-understood what reductionistic science does. Scientists understand that whole systems possess properties which are not possessed by any of the parts; that's precisely because the parts engage in various relationships and interactions. A primary objective of reductionistic science is to try and understand the properties of a whole system in terms of its parts, and the relationships between the parts: diamond and graphite, for example, are both composed of the same parts, (carbon atoms), but what gives diamond and graphite their different properties are the different arrangements of the carbon atoms. Explaining the different properties of carbon and diamond in terms of the different relationships between the parts of which they are composed is a triumph of so-called 'reductionistic' science.

The next confusion we find in Lovelock's argument is the notion that twentieth-century science was somehow linear, or Cartesian, and non-linear systems with feedback somehow lie outside the domain of this world-view. Given the huge body of twentieth-century science devoted to non-linear systems, this will come as something of surprise to many scientists. For example, in General Relativity, (that exemplar of twentieth-century science), the field equations are non-linear. Lovelock might even have heard the phrase 'matter tells space how to curve, and space tells matter how to move'; a feedback cycle, in other words! Yet General Relativity is also a prime exemplar of determinism: the state of the universe at one moment in time uniquely determines its state at all other moments in time. There is clearly no reason to accept the implication that cause-and-effect must be confined to linear chains; non-linear systems with feedback are causal systems just as much as linear systems.

It is amusing the note that Lovelock concludes his attack on so-called 'Cartesian' science with an allusion to quantum entanglement. Clearly, quantum entanglement is a product of quantum physics, that other exemplar of twentieth century physics. So, in one and same breath, twentieth century science is accused of being incapable of dealing with emergentism, yet also somehow yields the primary example of emergentism!

Authors such as Lovelock, Midgley, and their journalistic brethren, are culpable here of insufficient curiosity and insufficient understanding. The arguments they raise against twentieth-century science merely indicate that they have failed to fully understand twentieth-century science and physics.

Tuesday, December 23, 2014

Formula 1 turbines and enthalpy


A couple of interesting developments occurred around the exhaust systems on both the Ferrari and Mercedes-engined Formula 1 cars in 2014: the Ferrari-engined vehicles acquired insulation around the exhaust-pipes, and the Mercedes-equipped cars appeared with a so-called log-type exhaust.

The purpose of the insulation was to increase the temperature of the exhaust gases entering the turbine. Similarly, increasing the exhaust gas temperature was a purported beneficial side-effect of the log-type exhaust on the Mercedes.

A couple of general points about the physics of turbines might provide some useful context here. First, the work done by the exhaust gases on the turbine comes from the total enthalpy (aka stagnation enthalpy) of the exhaust gas flow.


This is perhaps a subtle concept. The total energy E in the fluid-flow through any type of turbine consists of:

E = kinetic energy + potential (gravitational) energy + internal energy

However, to understand the change of fluid-energy between the inlet and outlet of a turbine, it is necessary to introduce the enthalpy h, the sum of the internal energy e and the so-called flow-work pv:

h = e + pv ,

where p is the pressure, and v is the specific volume, (the volume occupied by a unit mass of fluid).

One way of looking at the flow-work is that it is part of the energy expended by the fluid maintaining the flow; the fluid performs work upon itself, (in addition to the external work it performs exerting a torque on the turbine), and this work can be divided into that performed by the pressure gradient and the work done in compression/expansion.

Another way of looking at it is that the energy released into the fluid from a combustion process may have been released at a constant pressure as the fluid performed work expanding against its environment. The internal energy e doesn't take that into account, but the enthalpy h = e + pv does. As the diagram above from Daniel Schroeder's Thermal Physics suggests, the enthalpy counts not only the current internal energy of a system, but the internal energy which would be expended creating the volume which the system occupies.

For a system which is flowing, it possesses energy of motion (kinetic energy) in addition to enthalpy. The so-called total enthalpy hT is simply the sum of the enthalpy and kinetic energy:

 hT= e + pv + 1/2 ρ v2 ,

where ρ is the mass density and v is the fluid-flow velocity.

This quantity is also called the stagnation enthalpy because if you brought a fluid parcel to a stagnation point, at zero velocity, without allowing any heat transfer to take place to adjacent fluid or solid walls, the kinetic energy component of the total energy in that parcel would be transformed into enthalpy.

In the case of a Formula 1 turbine, there is no difference in the potential energy of the exhaust gas at the inlet and outlet, so this term can be omitted from the expression for the change in energy. What remains entails that the rate at which a turbine develops power is determined by subtracting the enthalpy-flow rate at the outlet from the enthalpy-flow-rate at the inlet. The greater the decrease in total enthalpy, the greater the power generated by the turbine.

As the exhaust gases pass through the turbine, they lose both kinetic energy and static pressure, but gain some internal energy due to friction. As a consequence, the entropy of the exhaust gas increases, and the enthalpy reduction is not quite as large as it would otherwise be (see diagram above from Fluid Mechanics, J.F.Douglas, J.M.Gasiorek and J.A.Swaffield).

However, (and here is the crux of the matter), for a given pressure difference between the turbine inlet and outlet, the reduction in total enthalpy increases with increasing temperature at the inlet. In other words, this is another expression of the fact that the thermal efficiency of a turbine is greater at higher temperatures (a fact which also dominates the design of nuclear reactors).

So, all other things being equal, increasing exhaust gas temperature with insulation or a log-type exhaust geometry will increase the loss of total enthalpy between the inlet and outlet of the turbine, increasing the power generated by the turbine.

However, there is another side to this coin: the required pressure drop between the turbine inlet and outlet for a desired enthalpy-reduction, decreases as the inlet temperature increases. Hence, if there is a required turbine power-level, it can be achieved with a lower pressure drop if the exhaust gases are hotter. This could be important, because the lower the pressure at the inlet side of the turbine, the lower the back-pressure which otherwise potentially inhibits the power generated by the internal combustion engine upstream. So increasing exhaust gas temperatures might be about getting the same turbine power with less detrimental back-pressure on the engine.

Saturday, September 20, 2014

Coral reefs and vortices

It seems that counter-rotating vortices are everywhere. The September 2014 edition of the Proceedings of the (US) National Academy of Sciences has published a fascinating study which reveals that coral reefs actively create quasi-steady arrays of counter-rotating vortices.

Corals exist in a symbiotic relationship with algae, which live within the tissue of the coral, and photosynthesise the organic carbon used by the corals to build their calcium-carbonate skeletons. In return, the corals have to provide nutrients for the algae, and remove the excess oxygen produced by photosynthesis.

Until now, it's been assumed that corals were dependent upon molecular diffusion alone to achieve the necessary mass transport. A concentration boundary layer exists at the surface of the coral: the concentration of a molecular species produced by the coral (such as molecular oxygen, O2) is highest at the surface of the coral, and a concentration gradient exists in the direction normal to the surface of the coral until the edge of the boundary layer is reached, where the concentration matches the ambient level. This concentration gradient drives outward molecular diffusion.

In the presence of an ambient flow, the boundary layer becomes thinner, increasing the steepness of the concentration gradient, and thereby enhancing the mass transfer rate. However, many parts of many coral reefs often experience periods of very low ambient flow, and there was evidence to believe that mass transfer rates were actually higher than could be explained by the ambient flow conditions. (Here there is a similarity with heat transfer within a bundle of nuclear fuel rods, where the rate of thermal mixing was higher than could be explained by turbulent diffusion and thermal conduction alone).

The research just published has revealed that the cilia (tiny hairlike entities) on the surface of the coral polyps are able to create a pattern of counter-rotating vortices which enhance mass transfer rates even in conditions of stagnant ambient flow (see image below). The counter-rotating vortices seem to be produced by the coordinated sweeping motion of the cilia, with one group of cilia sweeping in direction, and another group sweeping in the opposite direction.


The research revealed that the vortices are able to transport dissolved molecules by ~1mm in ~1sec, under conditions which would otherwise require ~1000secs to traverse the same distance by molecular diffusion alone.

It was also found that the location and shape of one such vortex was stable over the 90min period under which the concentration levels of oxygen were measured. The latter produced the image below, showing that one side of the vortex, flowing towards the surface of the coral, had ambient levels of oxygen, whilst the other side of the same vortex transports the oxygenated water away.

Sunday, August 31, 2014

CFD lessons from nuclear reactors


The fissile fuel in a commercial nuclear reactor is typically packaged into rods, which are collected together in arrays and placed within vertical cylindrical channels (as seen below for the case of the UK's Advanced Gas-Cooled reactor design). The coolant flows through the vertical channels, and the heat generated by fission is transferred from the surface of the fuel rods to the coolant. The efficiency and safety of the reactor therefore depends upon the efficiency with which the heat is transferred from the surface of the solid elements to the fluid flow. It is well-known that turbulent mixing enhances the efficiency of the heat transfer, and this is duly utilised within reactor design.



One of the requirements of reactor design is to homogenise the cross-channel temperature distribution, from one fuel rod to another, and it was noted in the 1960s that there was a greater degree of cross-channel heat transfer within a bundle of fuel rods than could be accounted for by turbulent diffusion alone.

The geometry created by the bundle of rods is rather differerent from a simple channel-flow problem. Taking a cross-section through a vertical channel, one has a collection of solid discs, each of which is separated from its nearest neighbour by a specified gap. The packing of adjacent cylindrical fuel elements creates a network of sub-channels, joined together by the gaps (see diagram below from A Keshmiri, Three-dimensional simulation of a simplified Advanced Gas-Cooled reactor fuel element, 2011). The coolant naturally flows in an axial direction through both the gaps and the sub-channels.


Experimental work noted that there was cross-channel heat transfer taking place through the gaps between sub-channels. For more than 20 years, it was thought that this heat transfer could be explained by 'secondary flow'. In a turbulent channel flow, the anisotropy of the turbulent stresses induce a component to the mean velocity flow-field which lies in a plane normal to the primary streamwise flow. Unfortunately, the magnitude of this secondary flow was way too small to explain the magnitude of the observed cross-channel mixing.

Only in recent decades has it been realised that the cross-channel mixing is due to a train of periodic vortices created in the sub-channels. The continual passage of these vortices creates a quasi-periodic cross-channel flow pulsation at particular stations along the bundle of fuel-rods. Steady-state CFD studies revealed nothing more than a turbulent channel flow pattern, and completely failed to represent the mixing of the coolant between adjacent sub-channels.

The cross-channel mixing was caused by an unsteady flow pattern which was smeared away in steady-state CFD, yet the coherent vortical structures make a contribution to the thermal mixing which has the same order of magnitude as that from the turbulent diffusion.

The exact mechanism responsible for the creation of this vortex train is not yet fully understood. The basic idea, however, is that the fluid flow is slower in the gaps between the fuel rods than it is in the larger sub-channels, and this creates a shear layer. The shear layer is intrinsically unstable, and breaks up into a train of vortices, in a manner possibly similar to Kelvin-Helmholtz instability. Adjacent sub-channels inherit counter-rotating vortices, so the patterns are not dissimilar to those of a von Karman vortex street shed behind a bluff body (see diagram below from Turbulent vortex trains in narrow square arrayed rod bundles of a dual-cooled nuclear reactor, Taehwan et al).


Note, however, that the vortex train in the bundle of fuel rods is not created by separation, as such. Rather, it is the result of the instability of the shear layers within the interior of the fluid. It is ultimately the geometrical configuration of the fuel rods which creates the unsteady flow pattern, and indeed the cross-channel pulsations are seen to vary as the gap between the fuel elements, and the diameter of the fuel elements, are varied.

The message is clear: even in the absence of separation, be very wary of steady-state CFD...

Thursday, August 07, 2014

Adrian Newey and unsteady CFD

The September 2014 issue of Motorsport Magazine contains an interesting article in which Adrian Newey discusses his favourite F1 cars. For disciples of modern F1 aero design, however, two statements catch the attention.

With respect to the 2009 Red Bull RB5, Adrian remarks that "we had a really great design group. We did some good research, understood the flow physics and the packaging." Then, recalling the research conducted for the exhaust-blown area around the spat on the 2011 RB7, Newey states that "it was very clear that the area around the rear tyres was critical...Then the whole research started developing...from steady-state CFD to tyre-dependent CFD and we worked with Renault to understand how the pulsing and acoustics of the exhaust worked."

This suggests that the recent aerodynamic success of the Red Bull has been based upon using unsteady CFD to understand the flow physics in that complex area around the spat. When the car pitches and rolls, not only does the rear ride-height change, but the rear tyre sidewall deforms, and given the sensitivity of the flow in the spat area, this sidewall deflection can crucially affect the performance of the diffuser.

The phrase 'tyre-dependent CFD' could, in isolation, merely imply that a set of steady CFD simulations were conducted, each representing a different degree of roll. However, by placing this phrase in opposition to 'steady-state CFD', it implies that Red Bull conducted unsteady CFD simulations which represented the roll of the car, including the time-evolution of the tyre sidewall profile.

Having said that, even if the solid geometry remains fixed, there is ample reason to believe that unsteady CFD simulations are indispensable for understanding the flow physics of a Formula 1 car.  

Steady-state CFD generates time-averaged images of the flow, and these can be misleading, both because they smear away time-dependent fluctuations in the flow, but also because the time-averaging procedure sometimes generates fictional flow structures which don't actually exist in the any of the instantaneous flow fields.

The image on the left, taken from Jacques Heyder-Bruckner's PhD research on wing-wheel interaction, vividly illustrates how the time-averaged image (top) smears away much of the structure associated with the breakdown of a front-wing endplate vortex (bottom).

The fictional potential of steady-state CFD is exemplified by the common wisdom used to explain the function of a Gurney flap. This claims that there is a stable, counter-rotating vortex pair formed behind the Gurney. As a case in point, the All-American Racers website proffers the following explanation:

"At the trailing edge, the airflow immediately beneath the wing rolls into a small anti-clockwise vortex behind the Gurney. Immediately above this, a second small vortex, rotating in the opposite direction, is formed by the airflow traveling above the wing as it passes over the gurney's lip. together these two vortices form a small separation bubble - a rotating mass of air removed from the main flow - which is somewhat taller overall than the gurney itself.

In clearing this separation bubble, the airflow's vertical deflection is increased and hence downforce increases. Additionally, separation of airflow from the wing's lower surface is postponed, allowing a higher angle of attack to be used before stall, which further enhances the wing's effectiveness."

In reality, there is no such stable vortex pair. Research conducted by David Jeffrey and David Hurst at the turn of the century established that the flow behind a Gurney is intrinsically unsteady, consisting of the continual alternate shedding of discrete vortices, which convect downstream (see the PIV images below, obtained by Jonathan Zerihan, which depict the vorticity contours associated with a Gurney flap in ground-effect at four different ride-heights). The process is not dissimilar to that associated with the von Karman vortex street behind a bluff body:

"The first stage in this shedding cycle begins as the separating shear layer on one side of the body rolls up to form a vortex. As it does so, it draws the separating shear layer over from the other side of the body. This second shear layer contains vorticity of opposing sign, and as it crosses the wake centerline it cuts off the supply of vorticity to the shear layer that is rolling up. At this point, the vortex is shed and moves downstream, while the shear layer on the opposite side starts to roll up, repeating the process.
 

With the Gurney flap the offsurface edge provides a fixed separation point for the pressure-surface shear layer, and this interacts with that separating from the suction surface to form a vortex street, in a manner similar to other bluff bodies."

To understand the flow physics in such circumstances, it necessary to compile a sequence of instantaneous flow images, (a storyboard, if you will). Studying the frozen and often fictional images generated by steady-state CFD simply doesn't cut the mustard.







Wednesday, July 02, 2014

Formula 3 airflow restrictors and black holes

The July issue of RaceTech magazine contains a decent article by Marco de Luca and Angelo Camerini on the principles behind airflow restrictors, such as those used to limit engine power in endurance racing and Formula 3.

The authors explain that as the RPM of the engine increases, the suction of the engine lowers the pressure in the downstream portion of the air intake duct. With an airflow restrictor, this duct consists of an inlet, a converging section, a throat, and (in some cases, but not in F3) a diverging section. As the downstream pressure lowers, the airflow velocity through the throat increases until, eventually, supersonic speeds are reached. In this condition, the flow in the duct is 'choked'.

Under normal conditions, when the RPM of the engine increases the pressure drop is communicated by means of pressure waves travelling upstream to the external atmosphere at the speed of sound. When the flow velocity becomes supersonic in the throat of the intake duct, these pressure waves can no longer breach the throat, and the engine's demand for greater mass-flow is unsatiated.


What the RaceTech authors sadly omit to mention, however, are the similarities between such airflow regimes and the event horizons of black holes. 

Physicists have been aware for some decades that fluid flow can influence the propagation of sound in the same way that black holes can influence the propagation of light. The classic example of such an analogy is perhaps the Laval nozzle. This contains a converging section, which accelerates a subsonic upstream flow so that it reaches the speed of sound in the throat of the nozzle. Then, (unlike the case of the air restrictor), the airflow is maintained in a supersonic condition downstream. 

The subsonic region of the 'acoustic geometry' corresponds to the exterior of the blackhole spacetime geometry; the throat corresponds to the event horizon; and the supersonic region corresponds to the interior of the black hole. 

Sound waves propagating upstream in the subsonic region are doppler shifted to longer wavelengths, just like the light escaping from the clutches of a black hole. Moreover, all sound waves in the supersonic region are swept further downstream, just as light is unable to escape the interior of a black hole. (See this Scientific American article by Theodore A. Jacobson and Renaud Parentani for an accessible introduction to the field of acoustic black holes.)


I look forward to a subsequent article by Marco, which explains the analogies between exhaust systems and white holes.

Saturday, February 15, 2014

Nigel Bennett, Gordon Murray, and vortex generators

Erstwhile Formula 1 and Penske designer Nigel Bennett has published a superb autobiography, Inspired to Design, which provides a reminder that several important aerodynamic concepts, prevalent in Formula 1 to this day, were actually invented in Indycar.

One of the recollections in the book even suggests that the use of vortex generators to enhance underbody downforce, was co-conceived by Bennett and Tony Purnell:

"Tony Purnell and I discussed some research he was doing at Cambridge University regarding laser viewing of vortex sheets, an element of which was trying to measure the low pressure generated at the centre of a vortex. Tony explained that if the vortex was trained to run between two plain surfaces, the low pressure would act on those surfaces.

"So, in our wind-tunnel tests, we set out to see if we could use this phenomenon to create more downforce from the car, and sure enough, it worked in that by creating a vortex at the front of the underbody such that it directed air at the underwing and chassis intersection, we were able to gain some 30-40lb [13-18kg] of downforce (full-size at 150mph) without an increase in drag. We developed a series of triangular sharks' teeth, fitted at an angle to the normal air stream just in front of the lower edge of the radiator intake duct, and the air would spill off these and form the swirling vortex. Later work using flow visualisation techniques showed where this vortex ran, and indeed, other vortices from the outer shelf edge did much the same thing in the outer rear corners," (p97).

It seems, then, that Bennett and Purnell were the first to systematically investigate and apply vortex generators. This work appears to have been undertaken as part of the design for the 1988 Penske PC 17. However, it should be recalled that Gordon Murray (featured in this month's Motorsport magazine podcast) introduced inch-deep vortex generators on the underside of the 1975 Brabham BT44, also with the intention of creating downforce. (Murray explains this in diagrammatic form when interviewed by Steve Rider for Sky Sports' F1 Legends Series).

For those seeking a rigorous insight into vortex generators, Lara Schembri Puglisevich has recently submitted a PhD thesis at the University of Loughborough, reporting the results of Large-Eddy Simulations of vortical flows in ground effect. This work includes a comparison (pictured below) of a vortex generator above: (i) a smooth, stationary ground plane; (ii) a smooth, moving ground plane; and (iii) a rough, stationary ground plane. The images show vorticity isosurfaces, colour-contoured by streamwise velocity. The flow is from left-to-right, with the vortex generator suspended from the floor above.

This is the first attempt to understand the potential interaction between a vortex and the roughness of the ground plane. Unfortunately, it wasn't possible to make the rough ground plane a moving plane, hence the stationary ground plane builds up its own boundary layer, which interacts with the vorticity shed by the vortex generator.

Nevertheless, these LES images vividly demonstrate just how 'messy' real vortices are.