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Search: Posts Made By: truffaldino
Forum: Main CFD Forum August 29, 2015, 23:20
Replies: 3
Views: 57
Posted By truffaldino
Since shallow water equations are approximations,...

Since shallow water equations are approximations, they are less accurate.

However, as wavelength goes to infinity they become more and more accurate.

It also worthy to note importance of 1d...
Forum: Main CFD Forum August 29, 2015, 15:33
Replies: 3
Views: 57
Posted By truffaldino
The advantage is lower dimension (3d NS reduces...

The advantage is lower dimension (3d NS reduces to 2d shalow water and 2d NS to 1d shallow water)
Forum: Main CFD Forum August 1, 2015, 09:07
Replies: 8
Views: 242
Posted By truffaldino
I think it has no significant effect on...

I think it has no significant effect on turbulence theory.

At the Kolmogorov scale the Knudsen number is still huge and not close to zero, so there is no problem with Euler equation.
Forum: Main CFD Forum July 6, 2015, 08:50
Replies: 4
Views: 164
Posted By truffaldino
"A" must be the wing planform area, so in 2d it...

"A" must be the wing planform area, so in 2d it is the chord length.
Forum: Main CFD Forum May 31, 2015, 17:38
Replies: 3
Views: 142
Posted By truffaldino
Perhaps the reason for that is moving air: water...

Perhaps the reason for that is moving air: water moves air due to non-slip on water surface, and there should be a kind of voricial air motion inside the fontain (ring vortex).

The second...
Forum: FLUENT May 22, 2015, 09:40
Replies: 9
Views: 356
Posted By truffaldino
But for that one should to do 3d simulations...

But for that one should to do 3d simulations instead 2d. Do you think it is realistic to run on PC at Re=10^4 ?
Forum: Main CFD Forum April 29, 2015, 12:57
Replies: 10
Views: 6,270
Posted By truffaldino
And dispersion is a dependence of wave speed on...

And dispersion is a dependence of wave speed on their wavelength.

In the non-dispersive medium all the waves move with the same velocity.


Numerical diffusion is a particular case of that...
Forum: Main CFD Forum March 17, 2015, 10:18
Replies: 49
Views: 864
Posted By truffaldino
That is true. In my opinion, the more or less...

That is true. In my opinion, the more or less satisfactory advance in turbulence is the Kolmogorov's theory. The rest is handwaiwing and numerical simulations.
Forum: Main CFD Forum March 16, 2015, 16:08
Replies: 49
Views: 864
Posted By truffaldino
Yes, it is clear that it must be greater than 1,...

Yes, it is clear that it must be greater than 1, but why in the case of smooth interfaces (flat plates, airfoils etc) it must be transported by 5 orders of magnitude faster?
Forum: Main CFD Forum March 16, 2015, 15:54
Replies: 49
Views: 864
Posted By truffaldino
But the question remains, why do transition is...

But the question remains, why do transition is set by high frequencies? Looks like just a reformulating the problem in momentum space.
Forum: Main CFD Forum March 16, 2015, 15:39
Replies: 49
Views: 864
Posted By truffaldino
It is interesting! Let us think about this

It is interesting! Let us think about this
Forum: Main CFD Forum March 16, 2015, 15:37
Replies: 49
Views: 864
Posted By truffaldino
Maybe I am wrong, but as far as I remember for...

Maybe I am wrong, but as far as I remember for sphere fully turbulent flow develops at Re=10^4-10^5

http://www.grc.nasa.gov/WWW/k-12/airplane/dragsphere.html

BTW,for which kind of flow...
Forum: Main CFD Forum March 16, 2015, 15:04
Replies: 49
Views: 864
Posted By truffaldino
This is exactly the main question of this thread,...

This is exactly the main question of this thread, why does it ocuur ar Re>>1. If there a simple explanation? Why does it more natural to think that transition happens when inertial term overhelms (by...
Forum: Main CFD Forum March 16, 2015, 14:17
Replies: 49
Views: 864
Posted By truffaldino
If we apply such a direct approach, assuming that...

If we apply such a direct approach, assuming that transition happens when contribution from inertial term becomes of the same oredr as from dissipative one, we will get result that Re transition...
Forum: Main CFD Forum March 16, 2015, 14:14
Replies: 49
Views: 864
Posted By truffaldino
Yes it is more dependent on Re_theta and shape...

Yes it is more dependent on Re_theta and shape factors, but still transitional Re_theta are order of 10^2 -10^3 still much more bigger than 1.
Forum: Main CFD Forum March 16, 2015, 11:18
Replies: 49
Views: 864
Posted By truffaldino
Yes Orr-Sommerfeld is a very well-known stability...

Yes Orr-Sommerfeld is a very well-known stability analysys based on linearization. It is aimed to find value of critical Re.

But the question is not finding Re, but just to qualitatively...
Forum: Main CFD Forum March 16, 2015, 10:08
Replies: 49
Views: 864
Posted By truffaldino
Ok, this is a wrong idea, but why can't we show...

Ok, this is a wrong idea, but why can't we show that critical Re should be big (not to find it, but just demonstrate that it is big) in Kolmogorov's style.

Say, to estimate order of total...
Forum: Main CFD Forum March 15, 2015, 16:05
Replies: 49
Views: 864
Posted By truffaldino
Here is an idea: why not to estimate the total...

Here is an idea: why not to estimate the total time of decay of biggest eddies through the cascade till the Kolmogorov scale and then multiply this time by freestream velocity: This will give the...
Forum: Main CFD Forum March 15, 2015, 15:21
Replies: 49
Views: 864
Posted By truffaldino
The idea to use vortex sizes and dimensional...

The idea to use vortex sizes and dimensional considerations to explain (qualitatively) big values of critical Re seems to be reasonable.
Forum: Main CFD Forum March 10, 2015, 17:52
Replies: 49
Views: 864
Posted By truffaldino
I was asking this question since friend of mine,...

I was asking this question since friend of mine, who is not a specialist in fluid mechanics, sent me a note written by his former supervisor who is a specialist in quantum field theory (qft). Using...
Forum: Main CFD Forum March 10, 2015, 10:18
Replies: 49
Views: 864
Posted By truffaldino
Yes, all this is evident and well known: The...

Yes, all this is evident and well known:

The question is what about well-posedness and ill-posedness of Euler and why does dissipative term becomes more releveant at such a small perturbation.
Forum: Main CFD Forum March 10, 2015, 09:44
Replies: 49
Views: 864
Posted By truffaldino
NS without perturbation is Euler equation. So,...

NS without perturbation is Euler equation. So, according your explanation, Euler should be ill-posed in 3D. I remember I heard something about that. Is it well known fact?

And another question:...
Forum: Main CFD Forum March 10, 2015, 08:39
Replies: 49
Views: 864
Posted By truffaldino
To me this question looks quite reasonable. ...

To me this question looks quite reasonable.

Imagine a person who takes a look at NS equation for the first time: This is an equation where all coefficients are 1 and it has one parameter nu which...
Forum: Main CFD Forum March 9, 2015, 18:46
Replies: 49
Views: 864
Posted By truffaldino
Why is transitional Re is >> 1

Recently one person asked me why transitional (laminar to turbulent) Reynolds numbers are big (10^3 to 10^5) while all coefficients in Navier-Stokes equation are of order of 1?

The person is not...
Forum: Main CFD Forum February 26, 2015, 09:02
Replies: 0
Views: 200
Posted By truffaldino
Another Free Mesher

Hello Guys,

Another free mesher in fortran is available on my blog (2D unstructured tri + boundary layer quad).

Truffaldino
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