[wilder_pinguin]

Hey there, this is my first post in this forum hopefully someone can help me with my problem. I`m sorry if I didn`t select the right subforum

Momentarily I'm simulating a Kármán vortex street using primarily Lattice Boltzmann Method. The basic intention was to start off with some validable example to develop an idea of the handling of this method. For the analysis of my data, I used the scalar pressure field and logged the data at a monitor point approximately 4*(cylinder_diameter) behind my cylinder. Then I performed a FFT with the corresponding pressure data. Unfortunately, the resulting dominant frequency is always about the double amount of what I would expect. This leads to a Strouhal number which is way too high. No matter how I change my discretisation settings, my fluid properties or simulation settings. For double checking I performed the same case setup in Star CCM with classic CFD approaches today and came to the same result. Concluding I must admit that I somehow using a not appropriate evaluation routine. My research on the internet did not lead to a better understanding of better performing such an evaluation till now. But I found some voices saying that the shedding frequency could be twice as high in the direction of the drag force, which should by my understanding be the direction of the wake propagation. Unfortunately, I cannot find any sources who really deal with this issue nor give an idea of how to do it better. Could you give me a hint or a paper or an idea or something to resolve my issue?
Thank you for your time and help

If any data about my Case setup is needed, I will provide it as fast as I can

I found this threat from 2003 with the same issue, but it really didn’t help a lot: karman vortex street

[LuckyTran]

1) Are you sure you're even doing the fourier transform correctly? Can you verify with your eyeballs that the envelope of the karman street matches the frequency that you get from the FFT? Have you tested it against a simple sine with a known frequency?

2) Make sure the definition of frequency you are reading is the inverse of the temporal period and not based on # of vortices passing per time. The first is the reciprocal period and does not matter if vortices are shed on only one side or alternating on both sides. In the latter definition, you could end up double counting. This mistake is unlikely since Strouhal number almost always is referring to the period.

3) Your monitor is directly behind the cylinder axis and you are only monitoring pressure. It means for the vortex from generated from either side (say one is clockwise and the other anti-clockwise), both will generate the same scalar reduction in pressure (the low pressure in the vortex core). So you end up with twice as many as you think. This happens also if you analyze the longitudinal velocity fluctuations versus the normal component.

[wilder_pinguin]

Hi LuckTran thanks for your quick reply,


1) Yes I indeed test the routine with different sin curves and it worked out quite well and I could map the exakt frequency of the sin curve with my implementation of the FFT


2) I'm not sure if I totally understood what you mean, as my output is the change of pressure over time. The problem here could underlie consequently in the way I imported and modiefied this data, so I will provide you my routine:


#################################################


clear all;

#Insertion of relevant Parameters

full_data = load("-ascii","pressure_over_time.txt");

#Velocity of Fluid in direction of inflow:

v_fluid = input("Please insert the fluid velocity in [m/s] \n");

#Diameter of the cylinder

d_cylinder = input("Please insert the diameter of the cylinder in [m] \n");

#physical Simulation time:

time_sim = input("Please insert the physical Simulation time in [s] \n");

#FFT Analysis

N = length(full_data); #total number of Samples
fs = N/time_sim; #Samples per Second
fax_bins = [0 : N-1]; #Shift of Minimum bin from 1 to 0

freq_magn_spec = abs(fft(full_data(:,2))); #FFT of pressure over bins

fax_Hz = (fax_bins*fs)/N; #Transformation from bin domain to frequency domain

#Plotting

stem(fax_Hz,freq_magn_spec);
title("FFT Analyse fuer Re = 9933 induzierte Turbulenz Vergleich")
axis([0 130])
xlabel("Frequenz / [Hz]")
ylabel("Amplitude")
grid on

#Determination of dominant frequency

fax_Hz_tr = transpose(fax_Hz);
data_1 = [fax_Hz_tr(10:130,1) freq_magn_spec(10:130,1)];
max_val = max(data_1);
a = find (data_1(:,2) == max_val(1,2));
freq = data_1(a);

#Calculation of Strouhal_Number

sr = (d_cylinder*freq)/v_fluid;

printf("Strouhal number is: "), disp(sr), printf("\n");



#################################################



3) Yes this is actually the case with my evaluation. So this means that a shedding period consists of the shedding of the upper AND the lower vortex rather than just one of them. This would explain why my results always are twice as high as expected, because I measured every single upcoming vortex downstream. Is there a better way for the evaluation of the shedding frequency. Maybe with alterring the monitor point or could I investigate another magnitude like the lift coefficent? Following I tried to sketch how I positioned my monitor point at the moment.



######################################
#----------------------------------------------------#
# ------#--------------------------------------------#
#---- #--# <--4*D--> #-----------------------------#
# ------#--------------------------------------------#
#----------------------------------------------------#
######################################



Thank you for your time and help

[Continuum]

I had a laminar flow cylinder model Re ~ 1200. I am looking at the data. If I probe the u velocity along the centerline just downstream of the cylinder, I see two modes coming out in the data. The pressure also shows two modes

I cannot post an image, but here is the time history of u velocity (s m/s). If I de-trend the data so it varies about zero and take a DFT, I see two main peaks. The primary is 15 Hz. The secondary is ~ 7.7 Hz. I need to see how this compares.

Perhaps a time history transducer capture of vorticity downstream is the better solution. Also, if I move up away from the cylinder, I see only one mode.




Regards


DFT data (frequency (Hz), power)

 2.51921676491  3.90982238694E-5 
 5.03843352983  3.08057135426E-3 
 7.55765029474  6.32156173068E-3 
 10.0768670596  3.26227611559E-3 
 12.5960838246  8.13348888352E-3 
 15.1153005895  1.71046425969E-2 
 17.6345173544  9.13153942811E-3 
 20.1537341193  1.41463314041E-3 
 22.6729508842  3.07577105965E-3 
 25.1921676491  1.68343972858E-3 
 27.711384414  1.54236683017E-3 
 30.230601179  3.44906756928E-3 
 32.7498179439  1.94480211742E-3 
 35.2690347088  3.71207358123E-4 
 37.7882514737  8.41750291866E-4 
 40.3074682386  4.86672627763E-4 
 42.8266850035  1.79845794373E-4 
 45.3459017684  4.28188987822E-4 
 47.8651185334  2.53790351048E-4 
 50.3843352983  3.57585486466E-5 
 52.9035520632  6.4627589185E-5 
 55.4227688281  3.72562113036E-5 
 57.941985593  1.45224788985E-5 
 60.4612023579  2.9632018429E-5 
 62.9804191228  2.11823574629E-5 
 65.4996358878  2.91947758187E-5 
 68.0188526527  7.74996914427E-5 
 70.5380694176  5.18746507556E-5

velocity data

 0  .3630442619 
 .002071793424  .3412983119 
 .004141993821  .3197909892 
 .00621164171  .299176544 
 .008281471208  .2808058262 
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 .04391671345  .5557858944 
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 .04819659144  .5674224496 
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[Continuum]

OK, so I went back and computed Strouhal number and predicted frequency.



I get:


Re.D.H ~ 1267
St ~ 0.1927
nu ~ 9.75 Hz


So these results are a bit counter to what I am extracting above. I need to think about this a bit.



I went back and extracted the vorticity and did the DFT on that. It is zero biased so it does not need to be detrended. The primary frequency was 7.8 Hz. The predicted St.CFD ~ 0.15. So this is considerably lower that the expected Strouhal of about 0.19.




Regards

[LuckyTran]

Originally Posted by wilder_pinguin

3) Yes this is actually the case with my evaluation. So this means that a shedding period consists of the shedding of the upper AND the lower vortex rather than just one of them. This would explain why my results always are twice as high as expected, because I measured every single upcoming vortex downstream. Is there a better way for the evaluation of the shedding frequency. Maybe with alterring the monitor point or could I investigate another magnitude like the lift coefficent? Following I tried to sketch how I positioned my monitor point at the moment.

Moving the monitor is a bandaid, it doesn't address the fundamental issue. The problem is that pressure is a stupid scalar that doesn't know its left from right. Lift coefficient would work. You could also monitor the normal velocity component (the up-down velocity).

[wilder_pinguin]

Originally Posted by LuckyTran

Moving the monitor is a bandaid, it doesn't address the fundamental issue. The problem is that pressure is a stupid scalar that doesn't know its left from right. Lift coefficient would work. You could also monitor the normal velocity component (the up-down velocity).

Perfect thanks a lot for your help! I will monitor the Lift coefficient from now on...