sethoflagos

Aha! I wish you'd yelled 'Prandtl!' at me after post #2 ;-) This must be the effect the OP has seen. But the question is where? Years of weekly air flow measurements inside paper machine drying hoods; working with gas ducting up to 27 feet square section; and not to mention countless ciggies smoked in draughty passageways; and the effect has always been swamped by bulk flow.

This clip seems to get across the key ideas quite clearly. What catches most people out is the deep contrast between steady state flow and unsteady (or dynamic) flow. This is not synonymous with laminar vs turbulent. Laminar flow can be dynamic without being turbulent. And despite laminar often being described as streamline flow, streamlines exist in turbulent flow too. Other than maybe some extreme situations such as a shock wave front, there is always an instantaneous velocity vector field with no discontinuities between the vectors (thanks to the continuity equation), so streamlines can be drawn.

studiot's usage of the 'streamline' terminology is absolutely correct. Any other related term (pathline, streakline, timeline) would not be consistent with his explanation of it's relationship to the instantaneous flow velocity vector field. Perhaps you should be asking questions rather than trying to rubbish other people's answers........?

Nothing against aeronautics engineers btw. It's simply that flow of air around various solid body sections within very restricted ranges of pressure and temperature corresponds to an extremely narrow band of interest within the vast, rich universe of Navier-Stokes. Is it more efficient to stir coffee with a teaspoon or a fork? Strangely, I've found Wikipedia unusually silent on this commonplace, everyday dilemma.

Strangely enough.....He's presenting a simplistic model for students in a discipline that rarely need worry itself with the full picture. I guess by 'particles' he means what we usually describe as 'fluid parcels' in the literature, and, yes, in pure laminar flow, fluid parcels are constrained to simple streamlines, and eddy diffusivity is zero. But you cannot extend this reasoning down to the molecular scale because individual fluid molecules move in anything but a straight line. So at the boundaries of fluid parcels and streamlines there is constant material (and momentum, and thermal) flux across those boundaries. If there is a higher concentration of dye in one streamline, there may be no convective mixing, but there is definitely nett movement of dye in the direction of negative concentration gradient into adjacent streamlines of lower dye concentration through the mechanism of molecular diffusion. To paraphrase an ancient and not particularly good engineering joke, one may in certain instances quite reasonably commence an analysis with the phrase 'assume spherical chicken'. Even if this analysis proceeded to provide an excellent fit with experimental data, it would take a particular kind of fool to conclude from this that all chickens were indeed spherical. Beware simplifying assumptions. They are only useful approximations; they are not fact.

Please don't quote Wikipedia at me. Most of the fluid flow entries seem to have been written by aeronautical engineers, and they tend to have very limited bandwidth in the field. My comments stand,

I recall neither the OP nor myself mentioning laminar flow. 'Churn flow' has a very specific understanding in fluid mechanics, which is not relevant here, If you intended 'formation of vortices' then you are incorrect: vortices can and must occur in general regimes of laminar flow to account for conservation of angular momentum. 'Mixing' also occurs in laminar flow whether by means of convection or molecular diffusion.

It has no specific name that I can think of, perhaps because it isn't the case. Under normal 'no slip' conditions the fluid is stationary at the walls, and particularly so in the very corners. Moving away from the walls (and corners), the bulk fluid 'sees' less and less of any wall effect and ceases to have much interest at all in the shape of duct section. There is nothing to disturb the streamlines any more than any other standard section. Change direction in a smooth curve and still no problems. Turn a sharp right angle (is this what you were thinking of?) and now you're asking for infinite angular acceleration on the inside turn and downstream eddies will form. But this is true of all duct sections including circular

As a first pass, you could assume that the rate of dissolution etc is slow enough, and the sphere is small enough, for it to be falling at it's terminal velocity per Stokes' law (which is valid for low Reynolds numbers, and well dispersed particles). The equations for drag and terminal velocity can be found here https://en.wikipedia.org/wiki/Stokes'_law. As DrKrettin states, the rate of dissolution is proportional to the surface area of the sphere, and therefore varies with R^2. Specifically, you can look at the Noyes-Whitney equation at https://en.wikipedia.org/wiki/Arthur_Amos_Noyes. You can rearrange this to express for rate of change of radius with time. But you need values for the diffusion coefficient and diffusion layer thickness.The ratio of these is called the mass transfer coefficient, and is ideally determined by experiment. Alternatively, a rough and ready correlation between mass transfer coefficient and friction factor can be found from the Chilton-Colburn analogy (see https://en.wikipedia.org/wiki/Chilton_and_Colburn_J-factor_analogy) The 'f' correlates with the drag force found previously (I suspect that f may be equal to drag coefficient in this case, but life's too short to work that one out for you just now). Anyway, you get a relationship between mass transfer coefficient and velocity. Since you already have a relationship between terminal velocity and R, this, in turn, gives you a relationship between mass transfer coefficient and sphere radius. Et voila, this should end up in a simple ODE for R vs time.

He's claiming that the evolution of modern Homo sapiens was effected by conflict with the neanderthals. As we understand the moderns to have evolved in Africa, at a time when the neanderthals were in Europe (plus a few points east), it seems unlikely. As in evidence, not one shred.

No neanderthals in Africa. Small point, but I think an important one as regards evolution of the moderns.

Since thermal energy is generally liberated during dissolution, the problem is one of 'simultaneous heat and mass transfer'. The calculations become rather involved due to multiple (diffusive and convective) mechanisms being involved, and are not really amenable to this back of an envelope approach. Rogers and Mayhew "Thermodynamic and Transport Properties of Fluids" was our undergrad reference text on this area, and is more readable than many. But it is still a substantial tome and the maths is quite challenging. Dissolution generally kicks off with https://en.wikipedia.org/wiki/Arthur_Amos_Noyes https://en.wikipedia.org/wiki/Mass_transfer_coefficient https://en.wikipedia.org/wiki/Chilton_and_Colburn_J-factor_analogy The Stokesian regime problem setup you're attempting is actually more complex than assumption of a turbulent flow regime where complete mixing of the continuous phase may reasonably be assumed.

Nature is certainly very fond of spheres. .....and the continuity of conserved quantities.

Btw significant J-T cooling of a gas requires the high pressure state to be getting somewhere towards the vicinity of its critical point. 50 bar air at 200 K will give significant J-T cooling on depressurisation. Warm high pressure air, if anything tends to go a little the other way (Z>1). In the system under consideration, J-T is a herring of a deep red hue.

Your problem here is that you are faced with a process where four variables (P, V, T, and to a certain extent n) are changing simultaneously and we only have the tools to cope with two variables at a time. The general approach to these problems is to consider changes between two different states not by the direct route, but by a combination of simpler steps where only two variables are in play at the same time. So long as the initial and final states are the same, it doesn't matter which path we choose - the overall thermodynamic behaviour remains the same (google 'state variables' for background). When your piston is top dead centre, some air remains at the top of the cylinder. It will be around the piston discharge pressure (lets call it 5 bar absolute) and its elevated compression temperature (let's guess 400 K). Lets also assume the volume is 5% of the piston swept volume. We will also be assuming that the overall process is adiabatic (although you will see some interesting wrinkles). The poppet will not open until the cylinder pressure has fallen a little below atmospheric - lets say 0.95 bar absolute. Now the first wrinkle, We will invent an imaginary constant volume cooling process that will reduce the air pressure to 0.95 bar. Only pressure and temperature are in play so the Pressure Law applies - pressure is directly proportional to temperature. So temperature reduces to 400 *0.95/5 = 76 K (not in reality!!) involving a heat removal of 5 somethings times the heat capacity at constant volume (Cv) * (400-76) Joules. As the overall process is assumed adiabatic, we must return this heat energy to the air (second wrinkle), but this time at constant pressure causing the air to expand in accordance with Charles's Law - volume is directly proportional to absolute temperature. This time we use the heat capacity at constant pressure (Cp) and solve for 5*Cv*(400-76) =5*Cp*(T-76), The 5s drop out and Cp/Cv is the ratio of specific heats (1.4 or thereabouts) which gives a temperature of 307 K (which for me in Lagos is ambient). So by Charles's Law our 5% of stroke has become 5*307/76 = 20% (-ish) of stroke by the time our poppet valve opens so 15% of piston travel is doing nothing. But at least the poppet valve is open, and air will enter the cylinder as the piston drops to BDC at essentially constant pressure and temperature. There is no reason to treat the air flow through the poppet valve as anything but an isothermal process. There are no extreme conditions here. Hope this helps.

Evolution designed our immune systems based on - Being hungry most of the time. Consuming a wide variety of natural foodstuffs (especially mother's milk in infancy) Daily exposure to potentially harmful pathogens A day out in the sunlight (vitamin D is very important) A good night's sleep The more we depart from the lifestyle our immune systems were designed for, the less reliable they seem to become. In particular, if we don't exercise them with a regular diet of authentic 'dirt', they seem to have this nasty habit of picking a fight with something else. Like nuts. Or even parts of ourselves.

On the less extreme scales, maybe we can see the density of a standard crustal mineral like alpha-quartz (2.648 g/cm3) being fixed by a balance between inward compressive forces like electrostatic attraction and pressure, and outward expansive forces like thermal vibration and electron degeneracy pressure. If we increase the forces of expansion by heating, we eventually find new equilibrium crystal structures with lower density: beta-quartz (2.533 g/cm3) ; alpha-tridymite (2.265 g/cm3). But increase the pressure substantially and (with no nett change in chemical composition) we get stuff like coesite (2.911 g/cm3) and above 40 GPa, seifertite (4.294 g/cm3). Each subtle change of crystal geometry asking more questions of the constrained electrons to maintain their unique quantum states. 40 GPa is comfortably achieved in the earth's interior, so there is, here and there, geological evidence for their natural occurrence. Presumably the process continues at astronomical pressures until all electron quantum states are fully occupied and it's next stop, neutronium. Btw, this was a question. I don't actually know any of this stuff

An old one, but worth repeating. During a train trip through the countryside, an engineer, a physicist, and a mathematician observe a flock of sheep. The engineer remarks, "I see that the sheep in this region are white." The physicist offers a correction, "Some sheep in this region are white." And the mathematician responds, "In this region there exist sheep that are white on at least one side." Mathematics is founded on absolute axioms (eg Euclid), and is therefore amenable to deductive reasoning. Reality is not. Partly because we lack a 'theory of everything' in which to frame such axioms, and partly since it is unfeasible to subject every particle to in the universe to test to prove universality of an observation. We do have axioms of a sort (the postulates of thermodynamics were mentioned recently in a similar context), but they remain essentially statistical rather than absolute. Various thermodynamic relationships may be deduced from the postulates, but the foundation remains empirical, and potentially open to reinterpretation.

Perhaps the idea to focus on is why they could find no trace of the bucket. Seems to have the signs of 50 years of rusting condensed into a few milliseconds. The overall reaction is elemental metals become metal oxides. As John Cuthber effectively says: ice in; steam out. It actually subtracts a little from the overall energy release, Maybe steam is simply a catalyst that generates a fast kinetic pathway for the various key reactions.

There's a lot going on there, isn't there? My guess (some of this I'm pretty confident of) is that there are at least three phases to this blast. 1) Standard steam explosion. The liquid thermite mixture falling through the ice block tower produces steam faster than it can escape subsonically. A type of BLEVE. 2) As the presenter suggests, the steam explosion could (would?) turn the thermite mixture into an aerosol, so the steam + aerosol aluminium reaction is definitely in play, though the kinetics ask a couple of questions. 3) Choice of a galvanised bucket is interesting. The outside of the bucket is clearly getting very reactive even before the blast. Since zinc boils at 907 deg C, a steam + zinc vapour reaction has less reaction kinetics issues.

'Non-flammable' is a pretty loose word. It's meaning can vary a bit with context. Many materials that are described as non-flammable can become decidedly flammable in different atmospheres and/or at different temperatures and pressures. If they're hot enough, exothermic chemical reactions in the gas phase can emit some of the energy produced in the visible spectrum. We see this as a 'flame'. Due to the make up of our atmosphere, one of the reactants is usually oxygen, but it doesn't need to be. The reaction between hydrogen and chlorine (among many other possibilities) gives a pretty neat flame. It's just that we don't see these other reactions so often.

The book answer is to seek and follow the manufacturer's advice in the matter. Since the pressure drop across an air filter is typically less than normal fluctuations in barometric pressure (oto a few inches water gauge) it's common practice to measure the differential pressure across the filter rather than rely on either upstream or downstream absolute pressure measurement. Rigging up a manometer from plastic tube with some dyed water is a simple enough DIY job for an HVAC air filter, and quite accurate enough. A commonly used rule of thumb guide is to clean or change out the filter when the dP at full air flow is double the dP when new. The only caveat I can think of is that for low pressure fans (fan dP is not much higher than filter dP) fan air flow can vary quite appreciably with filter condition. To compensate for this, an ammeter in the fan circuit gives a fairly linear measure of changes in air flow. In this case, you can change out based on a doubling of dP/I.

It's all a bit far-fetched I think. The deep splits in the true parrots (Psittacoidea) even with the most conservative assumptions seem to be late Eocene latest. And this includes the divergence of most African parrots from the neo-tropicals. Our lovebirds (Agapornis) seem to have closer ties with their Australasian roots, splitting off even earlier. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2727385/ Any talk of basal Psittaciformes is almost certainly pre-KT. Parsimony strongly suggests that all major splits can be explained by the breakup of Gondwana, with the occasional foray into more northerly latitudes during later warmer periods.

The exact location of the mean definitely isn't universal. Particularly in this day and age we need to be able to adapt to the customs and mores of whatever community we find ourselves in, and those may be very different to the ones we were born into. As a general rule, if you listen to the guidance of those around you as to where the acceptable limits of personal conduct lie, and find somewhere within those boundaries where you feel comfortable with your own personal values, that's quite enough. Personally, as someone you has lived and worked in quite a wide variety of cultures, I find the flexibility of Aristotelian ethics suits my purpose. It allows me to remain functional so long as I avoid the more intolerant locations such as Saudi Arabia. People with fixed personal codes of conduct that are out of step with those around them tend to run into problems very quickly (I could give you a list of nationalities that typically relocate well, and those that tend to relocate badly).

Aristotle's concept of the Golden Mean (https://en.wikipedia.org/wiki/Golden_mean_(philosophy)) has always seemed a more satisfactory idea for those of us with a 'fifty shades of grey' world view.