Streamlines do not plot

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);
addpath(genpath(input_folder));
input_file=uigetfile('*.dat');
delimiterIn=' '; headerlinesIn=3; 
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); 

[darova]

Did you try to change starting points?

[Petr Michalek]

I tried to change startx and starty, but I still get odd images like the one below.

Answer 1

[Cris LaPierre]

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));

[Petr Michalek]

This helped a bit, but I got this.

[Cris LaPierre]

When I was looking at the linked example, it uses the same x, y, u and v for quiver and streamline, but your code was returning an error. I finally realized you need to transpose your x, y, U and V matrices. This causes the values in each column to correspond to the same x value, and the rows to correspond to the same y value. This is what streamline is expecting.

This code would work with zeros, but below I've modified that line to use the left-most x value instead. Here's my version of your code. I'm using a mat file to load the mydata structure.

load inputFile
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
scale=3;
quiver(x,y,Ums,Vms,scale);
title(['Vector map']);
axis equal;
xlabel('x[mm]'); ylabel('y[mm]');


starty=-40:2:40;
% modified startx so that it starts at the left edge
startx=ones(size(starty))*min(x,[],"all");
hold on;
streamline(x,y,Ums,Vms,startx,starty);
hold off

[Petr Michalek]

That is great, thanks, but I would like to draw also the two vortices behind the cylinder.

[Petr Michalek]

I managed to draw the two vortices, but I had to draw two sets of streamlines:

starty=-40:1:40; startx1=20*ones(size(starty));

streamline(x,y,Ums,Vms,startx1,starty); hold on;

startx2=ones(size(starty))*min(x,[],"all");

streamline(x,y,Ums,Vms,startx2,starty); hold on;

It is not perfect, but looks quite nice.

[Cris LaPierre]

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.

load dr1v10
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');