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Search: Posts Made By: truffaldino
Forum: Main CFD Forum January 13, 2016, 00:38
Replies: 7
Views: 206
Posted By truffaldino
There is nothing unusual in this situation. This...

There is nothing unusual in this situation. This happens even for flat plate in parallel flow (when flow is laminar Cd decreases, then Cd incresases as transition point moves forward and then again...
Forum: Main CFD Forum December 9, 2015, 11:21
Replies: 27
Views: 438
Posted By truffaldino
Improvement exists, but its accuracy exceeds...

Improvement exists, but its accuracy exceeds accuracy of other terms in NS.

Taking the square of average already gives the second order accuracy (I was wrong and the error after integrating over...
Forum: Main CFD Forum December 8, 2015, 15:27
Replies: 27
Views: 438
Posted By truffaldino
If you consider integral over all faces of the...

If you consider integral over all faces of the cell (not only one face) you will get O(h^3)
Forum: Main CFD Forum December 6, 2015, 22:49
Replies: 27
Views: 438
Posted By truffaldino
I think domination of linear over non-linear...

I think domination of linear over non-linear terms is a general situation in vicinity of zeros of velocity and is applicable not only to the boundary.

Even if the gradient is big, the viscosity...
Forum: Main CFD Forum December 6, 2015, 13:32
Replies: 27
Views: 438
Posted By truffaldino
I am not a specialist in discretization schemes,...

I am not a specialist in discretization schemes, but I would give the following answer to your question.

If your mesh is fine enough, and the velocity change across the face is small then the...
Forum: Lounge October 16, 2015, 22:47
Replies: 1
Views: 328
Posted By truffaldino
The physics of floating screwdrivers

The physics of floating screwdrivers: how a jet of air can float a common screwdriver

https://www.youtube.com/watch?v=jAYP6pWrdkc
Forum: Main CFD Forum August 29, 2015, 23:20
Replies: 3
Views: 207
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: 207
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: 329
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: 274
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: 209
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: 540
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: 7,548
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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: 1,168
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.
Showing results 1 to 25 of 243

 
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