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Search took 0.01 seconds. Search: Posts Made By: truffaldino
Forum: Main CFD Forum July 12, 2021, 11:46
Replies: 0
Views: 333
Posted By truffaldino
Anybody tried transition sst for Re 10^4 ?

I would like to run cfd simulations for wing of a model aircraft for reynols numbers between 30k and 70k using transition sst model.


If anybody tried to run it for similar reynolds number? What...
Forum: Main CFD Forum December 23, 2016, 20:11
Replies: 19
Views: 48,781
Posted By truffaldino
Formally, your approach is correct (one should...

Formally, your approach is correct (one should use free stream turbulence level), but there is a big problem.

The turbulence level at which linear (e^N) theory ceases to be valid is about 1%.
...
Forum: Main CFD Forum October 4, 2016, 18:04
Replies: 2
Views: 1,451
Posted By truffaldino
You will be wasting your tme with k-omega and any...

You will be wasting your tme with k-omega and any other turbulence modelling at such a low reynolds number. Use x-foil or xflr5: if there is no massive separation/bubbles, then your results will be...
Forum: Main CFD Forum August 11, 2016, 16:36
Replies: 6
Views: 1,519
Posted By truffaldino
The knovledge of the total drag coefiicient is...

The knovledge of the total drag coefiicient is not enough. You should know distribution of pressure (or the velocity gradients) on the surface of the object.
Forum: Main CFD Forum June 19, 2016, 23:20
Replies: 1
Views: 856
Posted By truffaldino
Because you are working with very slow...

Because you are working with very slow incompressible flow.

The pressure-based approach was developed for low-speed incompressible flows, while the density-based approach was mainly used for...
Forum: Main CFD Forum January 12, 2016, 23:38
Replies: 7
Views: 1,113
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, 10:21
Replies: 27
Views: 1,828
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, 14:27
Replies: 27
Views: 1,828
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, 21:49
Replies: 27
Views: 1,828
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, 12:32
Replies: 27
Views: 1,828
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: 2
Views: 2,582
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: 1,332
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: 1,332
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: 1,263
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: 5,270
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: 1,316
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: 5,562
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: 20,388
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, 09:18
Replies: 49
Views: 3,983
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, 15:08
Replies: 49
Views: 3,983
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, 14:54
Replies: 49
Views: 3,983
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, 14:39
Replies: 49
Views: 3,983
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, 14:37
Replies: 49
Views: 3,983
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, 14:04
Replies: 49
Views: 3,983
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, 13:17
Replies: 49
Views: 3,983
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...
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