[Nek5000-users] Boundary nodes cordinates and random generator

Tue Oct 16 14:07:43 CDT 2018

That great. It is good to have a look on your work. But I was using a different method as I said using Fourier modes to generate the velocity field so the time correlation is also described in a different way. Even though I will have a deep look on your work.

Thank you

Sijo

----- Mail original -----
De: "nek5000-users" <nek5000-users at lists.mcs.anl.gov>
À: "nek5000-users" <nek5000-users at lists.mcs.anl.gov>
Envoyé: Mardi 16 Octobre 2018 20:53:54
Objet: Re: [Nek5000-users] Boundary nodes cordinates and random generator

If I understand well, then you would destroy the temporal coherency by 
creating new phases at every time step. The way that we have used is 
described (to some detail) in my very old report 
(https://www.kth.se/social/files/5bc6330556be5b1206650f12/schlatter_2001a.pdf), 
in particular eq. (2.71). The time dependence goes in via an omega. But 
of course, you might be using another way, in that disregard my comments.

Philipp

On 2018-10-16 20:43, nek5000-users at lists.mcs.anl.gov wrote:
> Ohh really? But if you don't re-create new random numbers at each time step then it will give you the same velocity filed at all time step right? Or in that case, the time correlation helps to avoid this problem?
> 
> I feel we have to generate different random numbers at each time step to maintain the turbulence nature of the flow.
> 
> 
> Sijo
> 
> ----- Mail original -----
> De: "nek5000-users" <nek5000-users at lists.mcs.anl.gov>
> À: "nek5000-users" <nek5000-users at lists.mcs.anl.gov>
> Envoyé: Mardi 16 Octobre 2018 20:34:55
> Objet: Re: [Nek5000-users] Boundary nodes cordinates and random generator
> 
> Hi,
> I typically use ran1 in fact. It is simple, and portable. Of course, not
> too many different numbers are created, but should be enough for the
> purposes of inflow conditions. But, perhaps I am not sure I understand
> fully, you would need to create these random numbers just once when you
> start the simulation, and then just evaluate the complex exponentials?
> At least in my implementation I do not re-create random numbers.
> 
> For the coordinates, I guess you get them in userbc, where you can
> check, depending on your geometry, on what boundary you are.
> 
> Philipp
> 
> On 2018-10-16 20:31, nek5000-users at lists.mcs.anl.gov wrote:
>>
>> Yes I need some temporal correlation as well. In fact for me evaluating the complex exponential part is okay, the problem was computational expense for generating random number for 150 modes for theta,Phi,Psi and Alfa. I used the built in function of fortran and I experience Segmentation fault in my computer and when I use the code from Numerical recipes (subroutine ran1) it works well. But I just want to know why this happens and what are the other possible solutions.
>>
>> By using the ran1 code from Numerical recipe, 150X4 = 600 random numbers are generated at each time step for the random Fourier mode velocities.
>>
>> Or is there anyway solutions to cut down the number of random numbers by slightly any change in the Fourier mode?
>>
>> And finally regarding the co ordinates of the inlet boundary is there any quick way to get it?
>>
>>
>> Regards
>>
>> Sijo GEORGE
>>
>>
>> Your replay for my last question:
>>
>> But I guess you want some temporal correlation as well, i.e. your modes
>> will evolve as a harmonic signal also in time? Typically, we chose a few
>> modes (say 150) with random wavenumbers and phases, but then we just
>> evolve them in space and time. So the problem is then rather to evaluate
>> the complex exponential (i.e. cosine and sine signals) than random numbers.
>>
>> So in that case, one needs to have a RNG that works well in parallel and
>> you can restart a simulation without "kinks".
>>
>> But if you do not need these properties, I would say that computing 150
>> random numbers is not very expensive. I typically use a quite "simple"
>> generator from Numerical recipes, just because it is portable and gives
>> the same numbers on all machines.
>>
>> Philipp
>>
>> _______________________________________________
>> Nek5000-users mailing list
>> Nek5000-users at lists.mcs.anl.gov
>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
> 
_______________________________________________
Nek5000-users mailing list
Nek5000-users at lists.mcs.anl.gov
https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users