Friday, August 11, 2017

Curved flow and the Arrows A3

After something of a sustained gestation period, the publication of F1 Retro 1980 is imminent, so it's a good opportunity to take a look at one of the more interesting aerodynamic experiments seen that season: the underbody venturi extensions on the Arrows A3 at Brands Hatch. 

This was the latest in a series of attempts to improve upon the original F1 ground-effect concept. In 1979, the Lotus 80 and the Arrows A2 had both attempted to extend the area of the underbody, but both had failed to reap the expected benefits.

The Lotus 80, in its initial configuration, featured skirts under the nose, and separate skirts extending all the way from the leading edge of the sidepods, inside the rear wheels, to the back of the car. The failure of the Lotus 80 is commonly attributed both to an ineffective skirt system, and an insufficiently rigid chassis.  

The Arrows A2 featured an engine and gearbox inclined at 2.5 degrees in an attempt to exploit the full width of the rear underbody. In its original configuration the A2 also dispensed with a conventional rear-wing, replacing it with a flap mounted across the rear-deck. The sidepod skirts were complemented by a parallel pair of skirts running inside the width of the rear wheels to the back of the car. Unfortunately, the higher CoG at the back entailed the car had to be run with a stiff rear anti-roll bar, detracting from the handling, (Tony Southgate - From Drawing Board to Chequered Flag, MRP 2010, p108).

The 1980 Arrows A3 was a more conventional car, with the engine and gearbox returned to a horizontal inclination. However, at Brands Match in 1980, Arrows experimented, like the initial Lotus 80, with skirts under the nose. Developed in the Imperial College wind-tunnel, the Arrows version of the idea had skirts suspended from sponsons attached to the lower edges of the monocoque, running back beneath the lower front wishbones to the leading edge of the sidepods. At the same event, the team also tried extending the rear underbody all the way to the trailing edge of the rear suspension, with bulbous fairings either side of the gearbox fairing. This was done with the avowed intention of sealing the underbody from the detrimental effects of rear wheel turbulence.

Sadly, although the nose-skirts were intended to cure understeer, it was reported that they actually exacerbated the understeer.

Now, many aerodynamic difficulties encountered in this era of Formula One were actually just a manifestation of inadequate stiffness in the chassis or suspension. However, for the sake of argument, let's pursue an aerodynamic hypothesis to explain why the nose-skirts on the A3 worsened its understeer characteristic.

The nose skirts on the Lotus 80 and Arrows A3 would have suffered from the fact that a Formula 1 car has to generate its downforce in a state of yaw. Thus, in a cornering condition, a car is subjected to a curved flow-field. This is difficult to replicate in a wind-tunnel, hence a venturi tunnel design which worked well in a straight-ahead wind-tunnel condition could have failed dramatically under curved flow conditions. To understand this better, a short digression on curved flow and yaw angles is in order.

The first point to note is that a car follows a curved trajectory through a corner, hence if we switch to a reference frame in which the car is fixed but the air is moving, then the air has to follow a curved trajectory. If we freeze the relative motion mid-corner, with the car pointing at a tangent to the curve, then the air at the front of the car will be coming from approximately the direction of the inside front-wheel, while the air at the back of the car will be coming from an outer direction.

That's the simplest way of thinking about it, but there's a further subtlety. The negotiate a corner, a car generates: (i) a lateral force towards the centre of the corner's radius of curvature; and (ii) a yaw moment about its vertical axis.

Imagine the two extremes of motion where only one of these eventualities occur. In the first case, the car would continue pointing straight ahead, but would follow a curved path around the corner, exiting at right-angles to its direction of travel. In the second case, it would spin around its vertical axis while its centre-of-mass continued to travel in a straight line.

In the first case, the lateral component of the car's velocity vector corresponds to a lateral component in the airflow over the car. The angle which the airflow vector subtends to the longitudinal axis of the car, is the same along the length of the vehicle.

In the second case, the spinning motion also induces an additional component to the airflow over the car. It's a solid body spinning about its centre of mass with a fixed angular velocity, and the tangential velocity of that spin induces an additional component to the airflow velocity along the length of the car. However, the further away a point is from the axis of rotation, the greater the tangential velocity; such points have to sweep out circles of greater circumference than points closer to the centre of mass, hence their tangential velocity is greater.

Curved-flow, side-slip and yaw-angle. (From 'Development methodologies for Formula One aerodynamics', Ogawa et al, Honda R&D Technical Review 2009).
Now imagine the two types of motion combined. The result is depicted above, in the left-part of the diagram. The white arrows depict the component of the airflow due to 'side-slip': the car's instantaneous velocity vector subtends a small angle to the direction in which its longitudinal axis is pointing. In the reference frame in which the car is fixed, this corresponds to a lateral component in the direction of the airflow which is constant along on the length of the car.

When the yaw moment of the car is included (indicated by the curved blue arrow about the centre-of-mass), it induces an additional airflow component, indicated by the green arrows. Two things should be noted: (i) the green arrows at the front of the car point in the opposite direction from the green arrows at the rear; and (ii) the magnitude of the green arrows increases with distance from the centre of mass. The front of the car is rotating towards the inside of the corner, while the rear of the car is rotating away, hence the difference in the direction of the green arrows. And, as we explained above, the tangential velocity increases with distance from the axis of rotation, hence the increase in the magnitude of the green arrows.

The net result, indicated by the red arrows, is that the yaw-angle of the airflow has a different sign at the front and rear of the car, and the magnitude of the yaw angle increases with distance from the centre-of-mass. (The red arrows in the diagram are pointing in the direction in which the car is travelling; the airflow direction is obtained by reversing these arrows).

So, to return to 1980, the Arrows A3 design trialed at Brands Hatch moved the mouth of the venturi tunnel forward to the nose of the car. The further forward the mouth, the greater the angle of the curved onset flow to the longitudinal axis of the car, and the further away it is from the straight-ahead condition. Hence, the curved flow might well have separated from the leading edge of the skirt on the side of the car facing the inside of the corner, injecting a turbulent wake directly down the centre of the underbody. In this respect, the conventional location of the venturi inlets on a 1980 F1 car, (i.e., behind the front wheel centreline), would have reduced yaw sensitivity.

Front-wings and rear-wings certainly have to operate in state of yaw, and do so with a relatively high level of success. However, such devices have a larger aspect-ratio than an underbody venturi, which has to keep its boundary layer attached for a much longer distance.

It should also be noted that the flow through the underbody tunnels, like that through any type of duct, suffers from ‘losses’ which induce drag. The energy budget of a flow-field can be partitioned into kinetic energy, pressure-energy, and ‘internal’ heat energy. Viscous friction in the boundary layers, and any turbulence which follows from separation in the duct, creates heat energy, and irreversibly reduces the sum of the mean-flow kinetic energy and the pressure energy.

These energy losses are proportional to the length of the duct, the average flow velocity through the duct, and inversely proportional to the effective cross-sectional diameter of the duct. Due to such losses, it is not possible for full pressure recovery to be attained in the diffuser and its wake, and this will contribute to the total drag of the car. Hence, whilst underbody downforce comes with less of a drag penalty than that associated with inverted wings in freestream flow, it is nevertheless true that the longer the venturi tunnels, and the greater the average velocity of the underbody flow, the greater the drag of the car. 
Moreover, the longer the mouth and throat of a venturi tunnel, the thicker the boundary layer at the start of the pressure recovery region, and the more prone it will be to separation in that adverse pressure gradient. All of which mitigates against a quick and easy gain from extending the area of the underbody.

Monday, August 07, 2017

Driverless cars and cities

Driverless cars are somewhat in the news this year, with Ford investing $1bn to meet their objective of launching a fleet of autonomous cars in 2021. Coincidentally, the July 2017 issue of 'Scientific American' features an article extolling the virtues of a driverless future in modern cities. The article is written by Carlo Ratti and Assaf Biderman, who work for something called the 'Senseable Lab' at the Massachusetts Institute of Technology. 

 A number of the claims made in the article are worth reviewing. Let's start with the following:

"On average, cars sit idle 96 percent of the time. That makes them ideal candidates for the sharing economy...The potential to reduce congestion is enormous...'Your' car could give you a lift to work in the morning and then, rather than sitting in a parking lot, give a lift to someone else in your family - or to anyone else in your neighbourhood or social media community...a city might get by with just 20 percent the number of cars now in use...fewer cars might also mean shorter travel times, less congestion and a smaller environmental impact."

A number of thoughts occur in response to these claims:

1) Ride-sharing would reduce the number of cars, not the number of journeys. Every journey which currently takes place would still take place, but in addition would be all the journeys made when a car needs to travel from the point where one passenger disembarks to the point where the next embarks. At present, each journey contains a passenger; with the proposed ride-sharing of driverless cars, there would be additional journeys in which the cars contain no passengers at all. All other things being equal, that would increase congestion and pollution, not reduce it. 

2) The modern technological world, including the GPUs and artificial neural networks which have created the possibility of driverless vehicles, has been built upon the wealth of a capitalist economy. Such an economy is driven by, amongst other things, the incentivization of private ownership. In particular, people like owning their own cars. It's not clear why a technological development alone, such as that of the driverless car, will prompt society to adopt a new model of shared ownership.

3) Not everyone lives in cities. Universities tend to be located in cities, hence many academics fall into the habit of thinking that everyone lives and works in cities. Many people live outside cities, and drive into them to their places of work. They drive into the cities from different places at the same time each morning. For such people, there needs to be a one-to-one correspondence between cars and passengers.

4) People like the convenience and efficacy of having a car parked adjacent to their home or place of work. If you're a parent, and your child falls ill at home, or there's an accident at school, you want to drive there immediately, not wait for a shared resource to arrive.

5) If cars are constantly in use, their components will degrade in a shorter period of time, so maintenance costs will greater, and the environmental impact of manufacturing new tyres, batteries etc. will be greater.

So that's just for starters. What else do our MIT friends have to say? Well, they next claim that "vacant parking lots could be converted to offer shared public amenities such as playgrounds, cafes, fitness trails and bike lanes."

Unfortunately, most car parks are privately owned, either by retail outlets or employers. If they become redundant, then those private companies will either extend their existing office space or floor space, or sell to the highest bidder. Car-parks are unlikely to become playgrounds.

The authors then claim that current traffic-light controlled intersections could be managed in the style of air traffic control systems: 

"On approaching an intersection, a vehicle would automatically contact a traffic-management system to request access. It would then be assigned an individualized time, or 'slot', to pass through the intersection.

"Slot-based intersections could significantly reduce queues and delays...Analyses show that systems assigning slots in real time could allow twice as many vehicles to cross an intersection in the same amount of time as traffic lights usually do...Travel and waiting times would drop; fuel-consumption would go down; and less stop-and-go traffic would mean less air pollution."

Sadly, this is a concept which seems to imagine that cities consist of grids of multi-lane highways. Most cities in the world don't. And in every city, the following 'min-max' principle of road-capacity applies:

For a sequence of interconnected roads, given the capacity (i.e., the maximum flow-rate) in each component, the capacity of the entire sequence is the minimum of those individual capacities. 

Hence, even if the capacity of every multi-lane intersection in a city is doubled, the capacity of a linked sequence is determined by the component with the lowest capacity. In many cities, multi-lane highways taper into single-lane roads, and it is the single-lane roads which limit the overall capacity. Doubling the capacity of intersections would merely change the spatial distribution of the queues.

So, all in all, not a positive advert for driverless cars.

Tuesday, July 04, 2017

Cosmology and entropy

After a hiatus of approximately seven years, I've just written a new paper, 'Cosmology and entropy: in search of further clarity'. Here's an extract:

It is a widespread belief amongst modern physicists that black holes, or their horizons, possess temperature and entropy. The putative black-hole temperature is inversely proportional to the surface area of the horizon, while the entropy is proportional to the surface area. In natural units, the entropy of a non-rotating black hole is (Penrose 2016, p271):
S_{bh} = \frac{1}{4} A = 4 \pi M^2 \,,
$$ where $A$ is the area and $M$ is the mass.

The concept that a black hole could be the bearer of entropy is often justified by claiming that the black-hole entropy compensates for the 'loss of information', or the 'lost degrees of freedom', associated with matter and radiation falling into the black hole, never to be seen again. Bekenstein's original argument went as follows:

"Suppose that a body carrying entropy $S$ goes down a black hole...The $S$ is the uncertainty in one's knowledge of the internal configuration of the body...once the body has fallen in...the information about the internal configuration of the body becomes truly inaccessible. We thus expect the black hole entropy, as the measure of the inaccessible information, to increase by an
amount $S$
," (Bekenstein 1973).

Presumably, the idea is that one loses both the actual entropy and the maximum possible entropy associated with these extinguished dimensions of phase-space. However, as Dougherty and Callender (2016) point out, Bekenstein-type arguments express an epistemic and operationalistic interpretation of entropy. They rightly complain that "The system itself doesn't vanish; indeed, it had better not because its mass is needed to drive area increase...there is no reason to believe that a body slipping past an event horizon would lose its compensation is necessary...we could observe the entropy of steam engines and the like that fall behind event horizons. Just jump in with them!"

We can make the objection more precise in general relativistic terms. For example, take the Oppenheimer-Snyder spacetime for a star collapsing to a black hole, or the Schwarzschild spacetime for a black hole itself. In each case, the spacetime is globally hyperbolic, hence it can be foliated by a one-parameter family of spacelike Cauchy hypersurfaces $\Sigma_t$, and the entire spacetime is diffeomorphic to $\mathbb{R} \times \Sigma$.

Each Cauchy surface is a complete and boundaryless 3-dimensional Riemannian manifold. There is no sense in which any Cauchy surface intersects the singularity. Each Cauchy surface which contains a region inside the event horizon also contains a region outside the horizon. Moreover, every inextendible causal curve in a globally hyperbolic spacetime $\mathbb{R} \times \Sigma$ intersects each Cauchy surface $\Sigma_t$ once and only once. Particles follow causal curves, hence because each particle will intersect each Cauchy surface exactly once, assuming that none of those particles form bound systems, it follows that no degrees of freedom are lost. The future may well be finite inside the event horizon, but that doesn't entail that any degrees of freedom are lost from the universe.

The entropy of one part of the universe can decrease, just as the entropy of a volume of water decreases when it transfers heat to some ice cubes immersed within it. Similarly, if a material system possessing entropy falls into a black hole, whilst the region of the universe exterior to the black hole loses entropy, the total entropy does not decrease from one spacelike Cauchy hypersurface to the next. To echo Dougherty and Callender, there is no reason for the event horizon of a black hole to possess entropy; there is simply no loss to compensate for.

Penrose, however, argues that "the enormous entropy that black holes possess is to be expected from...the remarkable fact that the structure of a stationary black hole needs only a very few parameters [mass, charge and angular momentum] to characterize its state. Since there must be a vast volume of phase space corresponding to any particular set of values of these parameters, Boltzmann's formula suggests a very large entropy," (2010, p179).

This appeal to the 'no-hair' theorem of black holes is based upon a sleight of hand: it is the space-time geometry of the stationary, asymptotically flat, vacuum solutions which are classified by just three parameters. Such vacuum solutions are useful idealisations for studying the behaviour of test particles in a black hole spacetime, but they do not represent the history of actual black-holes.

The spacetime of an actual black-hole contains the mass-energy which collapses to form the black hole, and any mass-energy which falls into the black-hole thereafter, including swirling accretion disks of matter and so forth. Hence, actual black holes are represented by variations of the Oppenheimer-Snyder spacetime, not the Schwarzschild space-time. As Dafermos and Rodnianski comment, "It is traditional in general relativity to 'think' Oppenheimer-Snyder but `write' maximally-extended Schwarzschild," (2013, p18).

Whilst the exterior region of a collapse solution is isometric to an exterior region of the vacuum solution, the difference in the interior solution makes all the difference in the world. Spacetimes which represent collapse to a black-hole are not classified by just three parameters; on the contrary, they are classified by a large number of parameters, characterising the specifics of the collapsing matter, including its entropy. The entropy of such black-hole spacetimes is possessed, not by the geometry of the black-hole horizon, but by the infalling mass-energy, just as it should be, (see Figure 1).

Conformal diagram of a black hole, including a pair of Cauchy
surfaces, $\Sigma_1$ and $\Sigma_2$. The shaded region represents the infalling
matter; the thin diagonal line represents the event horizon; and the jagged
line represents the singularity. Cauchy surface $\Sigma_2$ possesses a region
inside the event horizon, and a region outside the event horizon. The shaded
region possesses entropy; the horizon doesn't. (From Maudlin 2017)

Dougherty and Callender also draw attention to a number of conceptual contradictions associated with the notion that black-hole horizons possess entropy and temperature. For example:
  1. The increase in the area of a black-hole horizon, and therefore its purported entropy, is proportional to the mass-energy of the material which falls into the black-hole. Hence, if a massive object with a small entropy falls into the hole, it produces a large increase in black-hole entropy, whilst if a small object with a large entropy falls in, it produces a small increase in black-hole entropy.
  2. Entropy is an 'extensive' thermodynamic property, meaning that it is proportional to the volume of a system. In contrast, black-hole entropy is proportional to the area of the black-hole.
  3. Temperature is an 'intensive' thermodynamic property, meaning it is independent to the size of an object, yet if the size of a black-hole is increased, its temperature decreases.
  4. There is no 'equilibrium with' relationship in black-hole thermodynamics. Individual black-holes can be in equilibrium in the sense that the spacetime is stationary, but one black-hole cannot be in equilibrium with another.
  5. If two black-holes of the same area, and therefore with the same purported temperature, coalesce, then the area of the merged black-hole is greater than each of its progenitors, hence the purported entropy increases. In contrast, thermodynamics dictates that the coalescence of two entities at the same temperature is an isentropic process.
Even if it is accepted that black holes, or their horizons, possess entropy, a belief in black hole entropy is typically twinned with a belief in the eventual evaporation of black holes. For example, Penrose (2010,  p191) asserts that black holes will evaporate by Hawking radiation after the cosmic background radiation cools to a lower temperatures than the temperature of the holes. In this scenario, all the entropy in the universe eventually becomes radiation entropy. Hence, once again, it seems that the clumping of matter is nothing more than an intermediate state. If black holes can evaporate, then black holes are clearly not the ultimate means by which entropy is maximised.

An alternative scenario suggests that large black holes will not evaporate because there is a fundamental lower limit to the temperature of the cosmological radiation field, and this temperature is greater than the possible temperature of large black holes. The belief in such a lower limit is based upon the fact that a universe with a positive cosmological constant $\Lambda > 0$, such as ours currently appears to be, possesses a spacelike future conformal boundary, and the past light cone of each point on this future boundary defines an event horizon. It is then suggested that this event horizon possesses a temperature and an entropy, just as much as the event horizon of a black hole.

However, the reasons for believing that a cosmological event horizon possesses temperature and entropy are much weaker than those for believing a black hole possesses thermodynamic properties. The cosmological event horizon is entirely observer dependent, unlike the case of a black hole event horizon. Moreover, the region rendered unobservable by an event horizon is the region to the future of the event horizon, and in the case of the cosmological event horizon this is the region to the exterior of the past light cone. (In contrast, the
region to the future of the event horizon of a black-hole is the interior of the black hole). 

Penrose (2016, p278-279) points out that the region to the exterior will be of infinite volume if the universe is spatially non-compact, hence its entropy will also be infinite. It therefore makes no sense to interpret the (finite) entropy of a cosmological event horizon as the entropy/information of all the matter and radiation `lost' beyond that horizon.

Sunday, March 19, 2017

The fundamental fallacy of modern feminism

There is, within contemporary film and television, a prevailing fashion for portraying women, in various combinations and degrees, as physically strong, aggressive, competitive, risk-takers. The writers, actors, producers, and directors responsible, and their sympathetic media critics, believe that there is some form of entrenched, gender-based discrimination in society, which film and television can help to overturn. They regard themselves as agents of social-change, engaged on a type of quest.

It is a puzzling phenomenon because, far from being testimony to an industry driven by egalitarian values, it actually reveals a deep-seated dislike and contempt of femininity. These films and TV programmes portray female characters as good, or worthy of praise, in direct proportion to the extent to which their behaviour imitates that of men. It follows that masculinity, and the behaviour of men, is being assigned the highest value; masculinity is setting the standard by which female characters are to be judged.

So where does this fashion spring from? Part of the reason may be a strain of thought in feminist academia, which holds that the differences in male and female behaviour are purely contingent, and not rooted in biological differences between the sexes. It's no coincidence that this notion is largely promulgated by philosophers, psychologists, and sociologists;  i.e., those who lack a rigorous scientific education.

As something of an antidote, recall the principal scientific fact in this context: The human species has evolved by natural selection with sexual reproduction. As a consequence, sexual selection has operated, amplifying differences in appearance and behaviour between the sexes. Gendered humans experience reproductive success in proportion to the extent that they exhibit the appearance and modes of behaviour associated with their own sex. In this respect, humanity is just like many other animal species.

So what could make people think that the differences between the human sexes have anything other than a natural, biological explanation? The answer, it seems, is the concept of 'social conditioning'. In particular, this notion is presented as an independent explanatory alternative to biological explanations:

Are women's “feminine” traits the product of nature/biology or are they instead the outcome of social conditioning? (Feminist Ethics, Stanford Encyclopedia of Philosophy). conditioning creates femininity and societies...physiological features thought to be sex-specific traits not affected by social and cultural factors are, after all, to some extent products of social conditioning. Social conditioning, then, shapes our conditioning makes the existence of physical bodies intelligible to us by discursively constructing sexed bodies through certain constitutive acts. (Feminist perspectives on sex and gender, Stanford Encylopedia of Philosophy)

Not only are the differences in behaviour between the sexes attributed to social conditioning, but so also are the differences in appearance:

Uniformity in muscular shape, size and strength within sex categories is not caused entirely by biological factors, but depends heavily on exercise opportunities: if males and females were allowed the same exercise opportunities and equal encouragement to exercise, it is thought that bodily dimorphism would diminish (ibid.)

Now clearly, social conditioning exists. It is, for example, responsible for the differences in behaviour between "white working-class women, black middle-class women, poor Jewish women, wealthy aristocratic European women," (ibid). Moreover, women across all human societies are subject to different expectations than men. If women across all human societies have a set of shared characteristics (in a statistical sense), then those characteristics will correspond to a set of shared biological characteristics, and a shared stream of social conditioning.

The fallacy of modern feminism, however, is the implicit assumption that social conditioning is somehow independent of a biological explanation. It's clear from reading this type of material that the authors consider an explanation in terms of 'social causes' or 'social forces' to be an endpoint, rather than something in need of further explanation. The identification and discovery of a case of social conditioning is presented in triumph, as the culmination of the research.

Human society has emerged as a net consequence of the interactions between billions of biologically gendered individuals over thousands of generations. Society is not free-floating, it is tethered to the natural and biological world. All social phenomenon are ultimately explicable in terms of the biological processes from which they emerge. If men and women are subject to different social conditioning, then it is because men and women are biologically distinct. The differences in social conditioning are a response to the biological differences, and part of the sexual selection feedback loop which amplifies and controls those differences.

By presenting a false dichotomy between social explanations and biological explanations, modern feminists seem to have convinced a generation of film-makers and media types, not to mention a large fraction of the political classes, that the differences between men and women are social rather than biological. It's an important difference, because if you think the differences are merely social and contingent, then it follows that equality of outcome between the sexes, rather than mere equality of opportunity, is possible with the appropriate form of social re-engineering. In other words, it encourages a type of gender neo-Marxism.