Remove unwanted vectors from a vector plot

I am using this program to sort through vector files and plot the average data using the quiver function. In one of my experiments, there is a wall that the flow runs into, this presents a problem in displaying that data. I want to remove the unwanted data that is caused by the wall, could someone help me modify the code to do so?

function [x,y,u_avg,v_avg,CHC_tot] = piv_averages(prefix,suffix,Nstart,Nfinish,interp)

% This program reads in a series of instantaneous PIV vector fields from
% Insight and averages them.  The user has the option of excluding
% interpolated vectors, which have CHC > 1.  (interp = 0 means do not 
% interpolate, while interp = 1 means interpolate).

% Create file name for each image
c_exit=1.22 %speed of sound
x0=1073 %origin of the jet in pixels  
y0=53.8 %origin of the jetin pixels
D=923.71 %diameter of jet exit in pixels 
v_shift=0
for i = Nstart:Nfinish
    Nstring = int2str(i);       % Convert iteration number to a character string
    if i < 10
        filename_inst = strcat(prefix,'0000',Nstring,suffix);
    elseif i < 100
        filename_inst = strcat(prefix,'000',Nstring,suffix);
    elseif i < 1000
        filename_inst = strcat(prefix,'00',Nstring,suffix);
    elseif i < 10000
        filename_inst = strcat(prefix,'0',Nstring,suffix);
    else
        filename_inst = strcat(prefix,Nstring,suffix);
    end

    % Read file name
    A_inst = csvread(filename_inst,1,0);

    x = A_inst(:,1);             % x-position (mm)
    y = A_inst(:,2);             % y-position (mm)
    u = A_inst(:,3);             % x-velocity (m/s)
    v = A_inst(:,4);             % y-velocity (m/s)
    chc = A_inst(:,5);           % number of good vectors at this location

    N = size(x,1);               % Length of entire vector array
    
    % Initialize output variables if this is the first file
    if i == Nstart
        u_tot = zeros(N,1);
        v_tot = zeros(N,1);
        CHC_tot = zeros(N,1);
    end
     
    for j = 1:N
        if interp == 0
            if chc(j,1) == 1
                u_tot(j,1) = u_tot(j,1) + u(j,1);
                v_tot(j,1) = v_tot(j,1) + v(j,1);
                CHC_tot(j,1) = CHC_tot(j,1) + 1;
            end
        elseif interp == 1
            if chc(j,1) > 0
                u_tot(j,1) = u_tot(j,1) + u(j,1);
                v_tot(j,1) = v_tot(j,1) + v(j,1);
                CHC_tot(j,1) = CHC_tot(j,1) + 1;
            end
        end
    end
end

for j = 1:N
    u_avg(j,1) = u_tot(j,1)/CHC_tot(j,1);
    v_avg(j,1) = v_tot(j,1)/CHC_tot(j,1);
end

% Set origin to jet exit centerline
x_c = x - x0;
y_c = y - y0;

% Shift by convective velocity
v = v - v_shift;

% Nondimensionalize variables
x_non = x_c/D;          % Nondimensionalize using jet diameter
y_non = y_c/D;          % Nondimensionalize using jet diameter
u_non = u/c_exit;       % Nondimensionalize using sonic speed
v_non = v/c_exit;       % Nondimensionalize using sonic speed

% Plot nondimensional vector field
figure(2)
quiver(x_non,y_non,u_non,v_non,5)
xlabel('x/D')
ylabel('H/d')
title('Nondimensional velocity field')

% Plot averaged vector field
figure(1)
quiver(x,y,u_avg,v_avg,5)
xlabel('z (mm)')
ylabel('x (mm)')
title('Dimensional velocity field')

% Output averaged data
output_avg = [x y u_avg v_avg CHC_tot];
dlmwrite('average_vels.dat','xyUVC');
dlmwrite('average_vels.dat',output_avg,'-append');

Here is an example of one of my velocity fields. The vectors along the bottom of the image need to be removed. Thank you in advance!

[Adam Danz]

It sounds to me that the problem isn't removing the quiver arrows from the plot. The problem is identifying those data points and not plotting them in the first place.

After you read in the data, determine which data points are affected by the wall and replace their values with NaNs.

As to how you'll detect those data points, that depends on how the data are generated and what each variable represents etc. For example, do all data points affected by the wall have a starting coordinate at y=50 (or whatever the bottom edges is of the image)?

[Monique Embury]

I understand that. I just am not sure how to impliment that into this code. I have over 15,000 rows in once excel spreadhseet that has x,y,U,V as coulmns. x and y are in pixels. I them have several of these files for the averages. I have attached what the excel file.

[Adam Danz]

(x_non, y_non) are the starting coordinates of each vector. If the vectors that you want to remove all start at the bottom of the image, you just need to 1) find the minimum value in y_non and 2) detect which indices of y_non are within some small window around that minimum value. Once you have a list of indices you just need to 3) replace those values with NaN prior to plotting.

[Monique Embury]

Hi Adam,

I understand this is what I need to do, thank you. I am having issues implimenting it. Can you help with the code?

Thanks!

[Monique Embury]

Adam,

Do you think you could help? I think I know how to hard code this information, but could you help me create a loop like you suggested?

[Adam Danz]

I didn't suggest a loop. It just requires a few steps (see my answer below).

Answer 1

[Adam Danz]

To eliminate any quiver arrow that start below a given threshold along the y-axis,

% 1) Choose a threshold
yThreshold = 100; %accept all vectors that start at or above y = yThreshold; 

% 2) identify all quiver arrows that meet the criteria 
acceptIdx = data.y >= yThreshold; 

% 3) plot the quiver arrows, but only the ones accepted
figure()
quiver(x(acceptIdx), y(acceptIdx), U(acceptIdx), V(acceptIdx), 5)
% add reference line at threshold if you'd like
yline(yThreshold); 

Alternatively, you can identify the yThreshold as some distance from the smallest y-value. In this example below, the yThreshold is set to the minimum y-value plus 0.1% of the range of y-data.

yThreshold = min(data.y) + range(data.y)*.001