# Streamlines do not plot

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Petr Michalek il 4 Mar 2021
Commentato: Cris LaPierre il 5 Mar 2021
Hello,
I have a problem with plotting the 2D streamlines in a flow around a cylinder. The quiver plot works fine, but the streamlines do not plot at all. I want the streamlines to start at left vertical edge at coordinates x=0 and y=(-40 to +40) mm. Please help, thanks.
I get an error:Error using matlab.internal.math.interp1
Sample points must be unique and sorted in ascending order.
I attach my code and one data file DR1v10vecstat.dat:
close all; clear;
input_folder='D:\ValeckyUT\Valec_tenky_horizontal\';
cd(input_folder);
input_file=uigetfile('*.dat');
mydata=importdata(input_file);
I=159; J=99; IJ=[159 99];
x=mydata.data(:,5); x=reshape(x,[I,J]);
y=mydata.data(:,6); y=reshape(y,[I,J]);
U=mydata.data(:,10); Ums = reshape(U,[I,J]);
V=mydata.data(:,11); Vms = reshape(V,[I,J]);
figure(22)
scale=3; quiver(x,y,Ums,Vms,scale); title(['Vector map']); axis equal;
xlabel('x[mm]'); ylabel('y[mm]');
% xs=x(:,1); ys=y(1,:);
[M,N]=size(Ums); [X,Y]=meshgrid(1:N,1:M);
starty=-40:1:40;
startx=zeros(size(starty)); hold on;
% figure(22)
streamline(X,Y,Ums,Vms,startx,starty);
##### 2 CommentiMostra 1 commento meno recenteNascondi 1 commento meno recente
Petr Michalek il 4 Mar 2021
I tried to change startx and starty, but I still get odd images like the one below.

Accedi per commentare.

### Risposta accettata

Cris LaPierre il 4 Mar 2021
I think this is causing your streamlines to not show up
startx=zeros(size(starty))
Based on the example in the documentation, I suggest using ones
startx=ones(size(starty));
##### 5 CommentiMostra 4 commenti meno recentiNascondi 4 commenti meno recenti
Cris LaPierre il 5 Mar 2021
Nice. I'll just add that the 2nd set of streamlines does not have to cover the full range of y. You can play around with it to be just what you want it to be.
scale=3;
quiver(x,y,Ums,Vms,scale);
starty=-40:2:40;
startx=ones(size(starty))*min(x,[],"all");
hold on;
h=streamline(x,y,Ums,Vms,startx,starty);
% add 2nd set of streamlines
streamline(x,y,Ums,Vms,7.5*ones(1,33),-10:0.5:6);
hold off
set(h,'Color','g'); Accedi per commentare.

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