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Dynamic mesh smoothing: what is really Spring-based smoothing??

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Old   March 20, 2009, 19:37
Default Dynamic mesh smoothing: what is really Spring-based smoothing??
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Hi all,

My question is regarding to the Spring-based smoothing of the Dynamic mesh options: can I think the spring-based smoothing as the linear deformation of the body where this motion is defined?

I mean, if I can consider, by enhabling this option to a certain fluid boundary zone, the linear deformation of the body? Or even enabling this option I will need an UDF to define grid's deformation law and motion? I want to make a FSI (aeroelasticity) simulation, but without any other external software (coupling with FEM -NASTRAN, ABAQUS, whatever- with MpCCI or similar)

Many thanks,

Freeman
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Old   March 21, 2009, 14:29
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Hi there,

i don't know if i got the question right, but I'll try to answer:

well the springbased smoothing just describes how your mesh is deformed, here by deforming the cells like springs. You have to care for the motion yourself by using UDFs. What kind of FSI are u plannig to do? I'm doing some aeroelasticitiy with FLUENT without additional software as well as with MpCCI :-)

regards
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Old   March 21, 2009, 14:43
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Quote:
Originally Posted by O.D.Y. View Post
Hi there,

i don't know if i got the question right, but I'll try to answer:

well the springbased smoothing just describes how your mesh is deformed, here by deforming the cells like springs. You have to care for the motion yourself by using UDFs. What kind of FSI are u plannig to do? I'm doing some aeroelasticitiy with FLUENT without additional software as well as with MpCCI :-)

regards
Thanks O.D.Y., you understood me right: so I understand that spring based is only an algorithm, "a rule" which the grid will be remeshed, but not actually any real physical deformation: mesh node's motion is the one I have to define by means of an UDF, despite having spring-based smoothing enabled... I got the point now. I also tried an experiment by myself enabling this spring-based smoothing, but without prescribing any motion, and obviously nothing happened (no deformation of the grid at all )

By the way: you're doing aeroelasticity with Fluent and without coupling software?? do you know you're my hero now =)? Can you briefly describe me how? I understand you have your own UDF to define noe's motion, don't you? This is my first time with aeroelasticity: my goal now is to make a Flutter analysis to a little airplane's wing (aprox. root chord=3.5in, tip chord=1.5in, span=2.7in)...

I would really appreciate your help in this, as I am a little lost... Thanks a lot in advance

Freeman
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Old   March 23, 2009, 00:15
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Could you both enlighten me how come you do FSI without coupling software?

The only idea I imagine, is to KNOW the actual body deformations, so they are enforced in the model by providing UDF which at every time-step moves the body nodes and remeshes your model, is it right?

Then you MUST assume your body motions/deformation are correct, so they are not really real deformations, since you cannot get those from actual real time-step water flow actions.

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Old   March 23, 2009, 00:18
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BTW. I do not have good experience with spring-based smoothing.

I would say if it works well it only works in uni-axial movements. Even with this, one of my tests included such a case and still it used to break.

From all my tests, layering method appeared to be the most reliable, unless you've got quite complicated mesh motion with complicated mesh, then I would go for remeshing technique.

Best
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Old   March 23, 2009, 17:22
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Well ok the FSI you can do is like you imagine it. It's actually only a "one-way coupling" where you know the deformations beforehand ( in my case from modal analysis of compressor rotor blades). You can then suppose that the structure will deform that way and there's such thing like a mass parameter which tells you how big the influences of the flow on this modeshape are. So if they are negligible you can enforce the deformations with constant amplitude and you can then calculate the work done by the blade and determine whether it would be damped or not.

I think you can even go one step further by calculating the modal force with an UDF and then calculating the modal displacement with simple modal equation.

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Old   March 23, 2009, 17:32
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BTW your ideas about the remeshing are quite interesting as I just employed uni-axial blade movement up to now where the smoothing worked quite good. I'm curious about how it will work in 3D....
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Old   March 23, 2009, 17:43
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Quote:
Originally Posted by O.D.Y. View Post
Well ok the FSI you can do is like you imagine it. It's actually only a "one-way coupling" where you know the deformations beforehand ( in my case from modal analysis of compressor rotor blades). You can then suppose that the structure will deform that way and there's such thing like a mass parameter which tells you how big the influences of the flow on this modeshape are. So if they are negligible you can enforce the deformations with constant amplitude and you can then calculate the work done by the blade and determine whether it would be damped or not.

I think you can even go one step further by calculating the modal force with an UDF and then calculating the modal displacement with simple modal equation.

cheers
Hi O.D.Y.

But if you know beforehand the displacements (which are supposed to cause the displacements)... why you need to solve any CFD? You have just solved the problem, isn't it?

By the way, regarding to Flutter analysis, if we take the Flutter equation of motion:



And I have my aero-structure... does anyone know how the discretization of the aero-structure is done? Could anyone give me some paper where an example is completely done, where the structure is discretized (like it is done in NX NASTRAN aeroelasticity module) and the flutter equation is solved numerically? And where does CFD fit in this process??

Thanks a lot for your help. Regards,

Freeman
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Old   March 24, 2009, 05:03
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Well, of course you do not know the exact solution. I'll try to explain with my problem:

As far as I know from literature and industy applications its done like that:
The rotor blade will be excited due to multiples of the shaft speed in the aeroengine in certain frequencies. Those multiples can only excite the structure if they coincide with a structural eigenfrequency. And if the mass parameter (blade mass divided by mass surrounding the blade which is influenced by the vibration) is sufficiently high the eigenfrequencies and eigenmodes can be regarded as those from a pure structural calculation. The question is then, whether the flow really excites those modes. Therefore you put the mode from the structural analysis into a FSI calculation and determine the energy-input from the flow in the mode.

As for a classical flutterproblem like in your case you would determine the structural eigenfrequencies of the wing and put it into a FSI calculation with the specific flow situation you want to determine the occurance of flutter for....

I mean the flutter equation you've posted is obviously a modal equation too where you have to supply values for modal mass, modal stiffness a.so.. To calculate these values you need a structural modeshape and die influence coefficients can be determinde the way I described it above... ...at least I'm doing it that way, and it correlates well with two-way coupled calculations.

cheers
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Old   March 24, 2009, 18:11
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Hey man, you're making me to see the light: I almost see it with your last example! There is only one thing that I'm not really sure to having undersood you with your example. Let's see, you said:

Quote:
The question is then, whether the flow really excites those modes. Therefore you put the mode from the structural analysis into a FSI calculation and determine the energy-input from the flow in the mode.
From a pure point of view of frequency response (let me abbreviate it with FR) in vibrations theory, I understand that what we are measuring is the dynamical response of our system (my wing or your blades) depending on the frequency of the input. But our input is -ideally- a steady laminar flow (I mean with no vortex shedding, or other kind of flow instabilities), so we have no "sinusoidal input" to study FR. I think that what you are trying to explain me is that we are assuming our system modal equations as transfer functions in which we want to measure the response that the system has with a "step input function", as the steady flow is, isn't it?

So, when you say "you put the mode from the structural analysis into a FSI calculation and determine the energy-input from the flow in the mode", I understand that you perform an unsteady CFD simulation in Fluent and I make the structure to deform each time step in accordance to a mode shape with an UDF, and then I will see if the deformation of the mode shape decays while the time marches (also controlled with the UDF), because the flow energy input is not enough to restore the energy dissipated with the system damping. If Flutter is present, then the deformation will not decay in time, or even it will diverge! Is all this dissertation right? Are you doing your job like this? So you have dedicated UDF's to do so?

The only headache will be now to do an appropiate UDF...

PD: by the way, bist Du auf Deuscthland vielleicht? Du manchmal schreibst wie ein Deutscher... Grüsse aus Spanien
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Old   March 30, 2009, 07:47
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Si, soy aleman, y tu?

And yes, this is the way I'm doing it. And as you say the UDF is the problem, or let's better say: the most challenging task. But I will go 3D very soon and see how much effort it really is.
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