Marxism-Leninism thread

Discussion in 'General Chat' started by HippoCrushEverything, Mar 8, 2017.

  1. For all communist purposes
  2. The communist party will get around 16 seats in parliament next election and it makes me a sad panda.
  3. This leads to the question of how is lift produced?

    All explanations I have encountered have been based on Bernoulli's principle. I still don't understand how vortices are related to the generation of lift.

    Also what the **** is vorticity?
  4. The capital of Vortex
    HippoCrushEverything likes this.
  5. One point of confusion, perhaps, is that there are dozens of equivalent explanations for the origin of lift, and every professor of fluid mechanics seems to have one that they like the most and champion for over the others. Another confounding issue is tradition. Many famous theories of lift were derived a hundred years ago, before we had sophisticated analytical or numerical methods for computing flow-fields. The mathematics used at that time leaned heavily on clever techniques which the modern student might not find valuable, but which is considered the standard on account of history. For instance, as there was no way to truly account for viscosity at the wall in 1902 (boundary layer theory was first published in 1908), when Kutta first derived the lift of an airfoil the effects of viscosity were applied as a boundary condition (the Kutta Condition) instead of solved as a continuum property.

    Bernoulli's Equation is very useful for many solutions to lift. However, one thing all aerodynamicists hate is the so-called Bernoulli Myth, or the idea that, when looking at a parcel of fluid that is split at the leading-edge of the airfoil, that those two parcels must re-join at the trailing edge. That much at least is universally agreed to be nonsense; the flow above a wing moves much faster than that, and such parcels will never re-join. Unfortunately, those trying to correct this myth have done so a little overzealously such that some people now believe that Bernoulli's Equation cannot explain lift at all; this is just as much a myth itself. The error is in applying a valid equation incorrectly, not in the equation itself.

    Fluids, like everything else in the universe, obey the conservation of momentum. For lift to produce a vertical force on a wing, the wing has to produce an opposite force on the fluid, and the fluid must therefore accelerate downwards proportional to this force. Forces in fluid mechanics can be either pressure or shear, but shear forces are always parallel and opposed to the direction of motion, while lift is (by definition) perpendicular to the flow direction, and so lift must manifest through the pressure field. So if you hear someone say "it's not pressure, it's inertia", or "it's not inertia, it's pressure" what they are really saying is "it's not the left-hand-side of the equation, but the right". Well, unsurprisingly, the left-hand-side of an equation has to equal the right, and so both statements are true. My colleagues in the UK would say that lift is all about streamline curvature. This is yet another perfectly acceptable and perfectly equivalent description.

    So where do vortices come from? It's as much a matter of physics as it is mathematical convenience. Vortices follow specific laws, and when you approximate the flow over a wing as a vortex, these vortex laws correctly predict the appearance and strength of tip vortices and the starting vortex, and collectively force the system to conserve momentum and angular momentum. There are also very simple formulas for the lift produced by a vortex, and a point vortex in a flow will produce the same streamline curvature as a wing that produces the same lift. In the early 20th century, physicists managed to produce closed-form equations for the strength of the 'bound vortex' produced by a wing based only on its angle-of-attack and camber. Vortex-based methods were the standard aeronautical tool for predicting aerodynamic forces until the advent of panel methods following the proliferation of digital computing. There are models for finite wings of varying chord and angle-of-attack and camber, swept wings, delta wings, and even unsteady pitching-and-plunging wings and aeroelasticity all based on replacing the wing with an equivalent vortex system.
    HippoCrushEverything likes this.
  6. If you consider the average angular velocity of a body of fluid, as you shrink your region of investigation down smaller and smaller, you will converge upon a single number as you approach an infinitesimally small volume. Vorticity is exactly twice this rotational velocity. We use twice the average velocity because many physical problems require this doubling, so it saves writing a coefficient, and because the curl of the velocity field (a common calculus operator) results in exactly twice this angular velocity, saving us writing a second coefficient. This can save as many as four pen strokes every time you write the number down!

    (The strength of a vortex is also defined as the integral of the vorticity field over an area, so I imagine this is how you stumbled upon the term)
    HippoCrushEverything likes this.
  7. You guys got me confused, is this about communism or fluids
  8. Hippo made a joke in the other thread and now it's leaking out.
    Veyronman and ETB4U like this.
  9. Well put a plug in it then.
    ETB4U likes this.
  10. oh gosh jokes on fluids now
    ETB4U likes this.
  11. While slowly disappearing, the kibbutz movement is/was probably one of Comunism's finest iterations. Many of the remaining kibbutzim are quite rich.

  12. Look at all this communist rhetoric, don't you understand that this is a failed system
    SEABEE likes this.
  13. Swastika-shaped plugs aren't usually found in hardware stores
  14. We have to seize the means of propulsion.
    TOXA, SEABEE and HippoCrushEverything like this.
  15. Thx, really appreciate the links. I'll have to chew on this for a while but I'm already 200% more educated than yesterday.
  16. yes comrade
    republicans really fans of russia as of november
  17. I've been a fan since Command and Conquer
  18. look at this Traitor to the american people and their comrades
    CitroenSM likes this.
  19. Pshh obvious liberal propaganda. He's obviously helping to build that wall to keep illegals out.
  20. #22 HippoCrushEverything, Mar 10, 2017
    Last edited: Mar 11, 2017
    Manufacturing modern turbine sections with active clearance control and monocrystalline blades isn't something one can do with a 3d printer, no. Sadly, the same holds true for compressors. Building VIGV and VSV systems from scratch would be horrendously difficult. Also, air temperatures between the high-pressure compressor and diffuser vary by the engine type and pressure ratio, but are somewhere between 250°C and 700°C ish, which is way too hot for carbon fibre reinforced polymers. Fan blades can be and have been made out of composite materials.

    Centrifugal compressors are used in tiny jet engines because they are cheap, simple and are more efficient across a range of inlet air speeds without air flow control systems. The efficiency of axial compressors in general drops dramatically after you go below a certain size.

    The hot side is cooled by compressor bleed air. Individual turbine blades have various air film cooling/radial duct/whatever methods. The same bleed air is used for ACC. Air-to-fuel mass flow ratio trough the engine section (not bypass/fan air), can vary a lot, usually between 45:1 and 130:1. Air-to-fuel ratio trough the primary combustion zone is kept at a fairly constant 15-ish regardless of engine type. Some older (and Russian) engines can be a bit smoky at higher power settings. This has more to do with poor airflow patterns across the combustion section than overall "richness" of the fuel/air mixture.

    Because your design is essentially a compressor directly mounted to an afterburner component, you could probably derive some efficiency estimates from existing afterburners. You'll notice that when paired to a Laval nozzle, their thermal efficiency is poor af, and propulsive efficiency sucks horse dick below mach 1.
  21. Afterburners have poor efficiency because the flow has reduced oxygen content, and because a lot of pressure has been lost through the nozzle and turbine. In his example, however, the turbine isn't needed for shaft work, and you probably haven't passed through your main nozzle yet, so you'd still be at your high working pressure. As such, your combustion efficiency may be similar to standard combustors and all of your energy can go through your main nozzle. The efficiency is thus a lot higher than you're estimating here. Afterburners are essentially a re-heating stage after partial expansion, which was absent in his description (one heat addition, one expansion).
  22. Yes.

    What sort of propulsive efficiency would you assume for this contraption at subsonic cruise, say again mach 0,85 but sea level, compared to a turbojet of equivalent size? Also pls explain pressure thrust.
  23. #25 Vanilla Ice, Mar 11, 2017
    Last edited: Mar 11, 2017
    A turbojet has three stages:

    1. Compression, or work-in (idealized at a constant entropy)
    2. Heat addition (idealized at a constant pressure)
    3. Expansion, or work-out (idealized at a constant entropy)

    Due to the heat-addition step, your expansion has a greater change of volume than your compression. Thus you have a net difference in velocity between your inlet and outlet, and this gives you a thrust. In a standard turbojet, part of this expansion step goes towards the so-called back-work ratio, to power the compressor via a turbine. Every joule of energy that goes into that turbine is energy that doesn't go into thrust. Thrust is all about your gas energy right when you hit the nozzle. So for an identical compression and heat addition, the electric-hybrid turbojet will generate more thrust, because its back-work ratio is zero.

    This isn't a totally fair comparison though, since the standard jet's only energy input is that heat addition step, whereas for the hybrid jet you get both heat addition and shaft work on the compressor. But at equal thrust I'd imagine that the total energy input for the hybrid jet is likely similar (possibly better). The thermodynamics aren't the issue here.

    Electric motors are big and heavy. Electronics don't like operating at the flame temperature of JP-8. Adding complexity is never a good idea. It's easy to pair a low speed turbine with a low speed compressor, and a high-speed turbine to a high-speed compressor with concentric shafts, but doing that with electric motors means gears or multiple motors. Seriously, a geared turbofan just means running your main fan and a different speed than your turbine shaft, a dead simple idea, but the engineering challenges on even just that were so great that it's something we've only just got to work this decade. The idea that a hybrid jet would just plug in and work right is insane, and the benefits are basically a rounding error as far as I can surmise (even discounting the weight of batteries).

    So yeah. From a thermodynamic, efficiency point of view it makes sense. It's everything else that needs work.
    HippoCrushEverything likes this.

Share This Page