fluctuation velocity

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rezakamyab rezakamyab
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fluctuation velocity

Dear Dr. William,

I am writing to you about a question for fluctuation velocity. In pivlab results how we could calculate fluctuation velocity?

I look forward to receive your help

your sincerely
Reza Kamyab
roul roul
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Re: fluctuation velocity

hi, i am not William but i am also using pivlab.

i guess, by fluctuation, u mean instantaneous velocity value - mean velocity. if this is so, then

i just save the result in matlab workspace and write a few 3-4 lines of code to get the fluctuation part alone.
basically u_smoothed - u_mean.

hope this helps, until william replies.
rezakamyab rezakamyab
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Re: fluctuation velocity

hi,

Thanks a lot for your reply. I have tried to write this code but unfortunately my results are wrong. I need to calculate turbulence intensity. Does anyone have this code and send to me?

I look forward to receive yours help
 

From: roul [via PIVlab - Time-Resolved Digital Particle Image Velocimetry Tool for MATLAB] <[hidden email]>
To: rezakamyab <[hidden email]>
Sent: Friday, 4 September 2015, 8:56:25
Subject: Re: fluctuation velocity

hi, i am not William but i am also using pivlab.

i guess, by fluctuation, u mean instantaneous velocity value - mean velocity. if this is so, then

i just save the result in matlab workspace and write a few 3-4 lines of code to get the fluctuation part alone.
basically u_smoothed - u_mean.

hope this helps, until william replies.


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roul roul
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Re: fluctuation velocity

here is a bit of code snippet, maybe u can understand this and modify as per ur requirement:

xvel_inst = zeros(rows,cols);                              
% inst = instantaneous, pre-allocation of new matrix (in for loop) for speed

xvel_fluc = zeros(rows,cols);  
% fluc. = fluctuation = xvel_inst - xvel_mean, pre-allocation of new matrix (in for loop) for speed

for k = 1:cols
    xvel_inst(:,k) = u_smoothed{k,1}(:,col_no);            
end
% here, i am saving the xvel_inst values along a vertical line only for all the image pairs, but not for the entire image

clear u_smoothed;

xvel_mean = mean(xvel_inst,2);                            
% ,2 to calc mean row-wise not col-wise

for m = 1:cols
    xvel_fluc(:,m) = xvel_inst(:,m) - xvel_mean;          
end
% _dash = the fluc. part = _inst - _mean

xvel_rms = rms(xvel_dash,2);        
% i guess this is what you want, if i am correct
rezakamyab rezakamyab
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Re: fluctuation velocity

Thanks a lot.

your code seems to be useful but is not complete. An error is shown as :

Undefined function or variable 'rows'.

Error in Untitled (line 4)
xvel_inst = zeros(rows,cols);

Thanks for your help.



From: roul [via PIVlab - Time-Resolved Digital Particle Image Velocimetry Tool for MATLAB] <[hidden email]>
To: rezakamyab <[hidden email]>
Sent: Friday, 4 September 2015, 13:25:44
Subject: Re: fluctuation velocity

here is a bit of code snippet, maybe u can understand this and modify as per ur requirement:

xvel_inst = zeros(rows,cols);                              
% inst = instantaneous, pre-allocation of new matrix (in for loop) for speed

xvel_fluc = zeros(rows,cols);  
% fluc. = fluctuation = xvel_inst - xvel_mean, pre-allocation of new matrix (in for loop) for speed

for k = 1:cols
    xvel_inst(:,k) = u_smoothed{k,1}(:,col_no);            
end
% here, i am saving the xvel_inst values along a vertical line only for all the image pairs, but not for the entire image

clear u_smoothed;

xvel_mean = mean(xvel_inst,2);                            
% ,2 to calc mean row-wise not col-wise

for m = 1:cols
    xvel_fluc(:,m) = xvel_inst(:,m) - xvel_mean;          
end
% _dash = the fluc. part = _inst - _mean

xvel_rms = rms(xvel_dash,2);        
% i guess this is what you want, if i am correct




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roul roul
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Re: fluctuation velocity

of course, its not complete. i just posted the main code lines. i thought u will do the rest.

just define rows as the number of points in the y direction,
and cols. as the number of image pairs u want to use  to calculate the mean

hope now u can get the code running.

make any other changes if necessary.

regards,
roul
rezakamyab rezakamyab
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Re: fluctuation velocity

Many thanks roul.

unfortunately i am not good at writing code at all!

Thanks for your help. 

Best Regards,
Kamyab


From: roul [via PIVlab - Time-Resolved Digital Particle Image Velocimetry Tool for MATLAB] <[hidden email]>
To: rezakamyab <[hidden email]>
Sent: Friday, 4 September 2015, 16:53:24
Subject: Re: fluctuation velocity

of course, its not complete. i just posted the main code lines. i thought u will do the rest.

just define rows as the number of points in the y direction,
and cols. as the number of image pairs u want to use  to calculate the mean

hope now u can get the code running.

make any other changes if necessary.

regards,
roul




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roul roul
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Re: fluctuation velocity

hi, no probs..in case u haven't figured out the code, just use the following code lines before the ones i sent earlier:

col_no = 40;                        
% this column no. corresponds to data at x = <some value> m from lhs of image,
change this one to the column number you want your profiles at

cols = size(u_smoothed,1) -1;      
% no. of cols in new matrix = no. of frames or image-pairs; -1 bec. last cell is for mean frame

rows = size(u_smoothed{1,1},1);    
% no. of rows in new matrix = no. of datapoints along a vertical line

hope this helps..

or maybe u can trying subtracting the mean velocity from all the frames in pivlab itself..there is an option to subtract velocity..

rezakamyab rezakamyab
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Re: fluctuation velocity

hi,

many thanks for your help.

Best regards,


From: roul [via PIVlab - Time-Resolved Digital Particle Image Velocimetry Tool for MATLAB] <[hidden email]>
To: rezakamyab <[hidden email]>
Sent: Monday, 7 September 2015, 12:03:21
Subject: Re: fluctuation velocity

hi, no probs..in case u haven't figured out the code, just use the following code lines before the ones i sent earlier:

col_no = 40;                        
% this column no. corresponds to data at x = <some value> m from lhs of image,
change this one to the column number you want your profiles at

cols = size(u_smoothed,1) -1;      
% no. of cols in new matrix = no. of frames or image-pairs; -1 bec. last cell is for mean frame

rows = size(u_smoothed{1,1},1);    
% no. of rows in new matrix = no. of datapoints along a vertical line

hope this helps..

or maybe u can trying subtracting the mean velocity from all the frames in pivlab itself..there is an option to subtract velocity..






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