Horizontal Freestream Flow Issues With Symbolic Math Toolbox
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I am attempting to plot stictly horizontal freestream flow using fcontour and the Symbolic Math toolbox. A few days ago I was able to successfully achieve this with the code below. However, starting last night whenever I run the script the "symbolic method" produces vertical contour lines that indicate stricty vertical freestram flow. I have gone back to the original code to ensure nothing I changed in the script had an impact, but the issue still remains. I have included an image of the incorrect plot (from the symbolic method) vs. the correct plot (from the numerical method) to better illustrate the problem. I would love some guidance on what to try and fix this strange issue.
clear
clc
% use the symbolic method to create the stream function for freestream flow
% and plot it
syms xs ys
Uval = 1;
Vval = 0;
psi_fs = Uval*ys - Vval*xs;
figure(4)
fcontour(psi_fs,[-3 3 -5 5],'k'); %'k' makes all the lines black
% use the numerical method to create the stream function for freestream
% flow and plot it
xlocs = [-3:.1:3]; % you can use linspace for this
ylocs = [-3:.1:3];
% make the grid of points x horizontal, y vertical
[xmat,ymat] = meshgrid(xlocs,ylocs);
psi_vals_fs = Uval*ymat - Vval*xmat;
figure(5)
contour(xmat, ymat, psi_vals_fs,'k') % 'k' makes all the lines black
Accepted Answer
The ‘ZData’ matrix from the symbolic code appear to be transposed with respect to ‘psi_vals_fs’. This is likely due to the assumptions that fcontour makes with the data. See Algorithms to understand what it is doing. (It may be interpreting ‘y’ as ‘x’ since the arguments are not specifically stated.) Providing the function with appropriate argumentts seems to solve it.
clear
clc
% use the symbolic method to create the stream function for freestream flow
% and plot it
syms xs ys
Uval = 1;
Vval = 0;
psi_fs(xs,ys) = Uval*ys - Vval*xs;
figure(4)
hf = fcontour(psi_fs,[-3 3 -3 3],'k'); %'k' makes all the lines black <— LIMITS CHANGED TO MATCH THE NUMERIC CODE
get(hf)
Zm = hf.ZData
% use the numerical method to create the stream function for freestream
% flow and plot it
xlocs = [-3:.1:3]; % you can use linspace for this
ylocs = [-3:.1:3];
% make the grid of points x horizontal, y vertical
[xmat,ymat] = meshgrid(xlocs,ylocs);
psi_vals_fs = Uval*ymat - Vval*xmat
figure(5)
contour(xmat, ymat, psi_vals_fs,'k') % 'k' makes all the lines black
.
2 Comments
Trevor John
on 31 Jan 2025
Star Strider
on 31 Jan 2025
As always, my pleasure!
There are many features of MATLAB and the Toolboxes that I’m still discovering, too. New functions and capabilities continue to be added, expanded, or changed.
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