Alain <kua### [at] videotronca> wrote:

> > On 12-3-2015 4:12, Alain wrote:

> >>> Thomas de Groot <tho### [at] degrootorg> wrote:
> >>>> Aliens are reported on a daily basis, in particular on YouTube.
> >>>> However,
> >>>> and invariably, they are shaky, blurred, indistinct things. Only the
> >>>> POV-Ray telescope is able to show alien ships in all their pristine
> >>>> splendour coming to us from deep space.
> >>>> --
> >>>> Thomas
> >>>
> >>> Hi Thomas,
> >>>
> >>> that is a very good start indeed, but I see some options for
> >>> improvements.
> >>> First, the structure looks great, but too many areas are lost in the
> >>> shadows.
> >>> This seems to be a general problem with space scenes, since one has
> >>> only one
> >>> light source (star) close by. May be a binary star can solve this. The
> >>> orbiter
> >>> can still have a different color in this case. The second issue is the
> >>> engine
> >>> glow. IMO it should have more turbulence at the edges. Otherwise
> >>> really great.
> >>>
> >>> Best regards,
> >>> Michael
> >>>
> >>>
> >>
> >> I can't think of any source of turbulence in the vacuum of space.
> >> Once the gas/plasma leave the engines *realy, realy* fast, each of it's
> >> particles goes in a straight line, mostly indepently from the others.
> >>
> >
> > Interesting. No interaction between the particles at all? Hmmm... I
> > suppose that would be minimal due to the eject velocity indeed, like you
> > say.
> >
> > So turbulence is another of those things - like wings - which we should
> > put in the domain of imagination :-)
>
> I did not say "no interactions at all" but mostly independently, meaning
> very little interactions.
> If you looks at the jet from a rocket in the athmosphere, you'll notice
> a central, often cone like and very smooth, part. It often display some
> repeating pattern, like a string of beads or diamonds. It's surrounded
> by a broader, more diffuse jet. On the outside of that outer layer,
> where it interact with the mostly static ambient air, is where all the
> turbulence is generated. As you progress farther from the engine, those
> turbulences progress inward.
> In space, you get the inner part forming a long and sharp cone, and the
> outside part is mostly V shaped, also smooth and a lot dimer, that stay
> that way as there is almost no exterior medium to interact with.
>
> In your later image, the exaust looks like the flame of a cheap, low
> presure, blow torch. In several cases, in films and TV shows, it's
> exactly what they used for the "rocket engines"... I've seen some cases
> whith the flame curving up for a ship going in a straight line.
> If a rocket engine had an exaust like that, it would shake itself loose
> and apart very shortly, or need to be so rugged that it's weight would
> be excessive.
>
>
> Alain

I would prefer a turbulence between both images.

Alain is right if the gas is emitted without obstacles from a source emitting in
some directions with a uniform distribution. But if you have obstacles, may be a
pipe of some kind (exhaust pipe), you will have interactions and so turbulence,
even in space.

Best regards,
Michael

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