柱面流线图

Question

我制作了半径为1的圆柱体的流线图。有没有办法去除圆柱体内的什么，甚至可以使用不同颜色的圆柱体来提高圆柱体？

clear 
    % make axes 
    xymax = 2; 
    x = linspace(-xymax,xymax,100); 
    y = linspace(-xymax,xymax,100); 
    % note that x and y don't include 0 
    [X,Y] = meshgrid(x,y); 
    R = sqrt(X.^2 + Y.^2); 
    sin_th = Y./R; 
    cos_th = X./R; 
    U = 1; 
    a = 1; 
    psi = U*(R - a*a./R).*sin_th; 
    figure 
    contour(X,Y,psi,[-3:.25:3],'-b'); 

Answer 1

可以屏蔽你不想nan画什么：

psi((Y>0 & psi<0) | (Y<0 & psi>0)) = nan; 

它

，比draw a circle：

rectangle('Position', [-1 -1 2 2],'Curvature',[1 1],'EdgeColor','r') 

下面是代码和结果：

% make axes 
xymax = 2; 
x = linspace(-xymax,xymax,100); 
y = linspace(-xymax,xymax,100); 
% note that x and y don't include 0 
% [X,Y] = meshgrid(x(x<-1 | x>1),y(y<-1 | y>1)); 
[X,Y] = meshgrid(x,y); 
R = sqrt(X.^2 + Y.^2); 
sin_th = Y./R; 
cos_th = X./R; 
U = 1; 
a = 1; 
psi = U*(R - a*a./R).*sin_th; 
% mask the inner part with nans: 
psi((Y>0 & psi<0) | (Y<0 & psi>0)) = nan; 
contour(X,Y,psi,[-3:0.25:3],'-b'); 
% draw a circle: 
rectangle('Position', [-1 -1 2 2],'Curvature',[1 1],'EdgeColor','r') 
axis equal 

---

你可以尝试也直接改变X和Y（而不是Y和psi）：

psi(Y>-1 & X>-1 & Y<1 & X<1) = nan; 

bu结果有点不同。

Answer 2

这是违反直觉的，但rectangle功能可以用来画一个圆！

hold on 
rectangle('Position',[-R,-R,2*R,2*R],'Curvature',[1,1],'FaceColor',[1 1 0]) 

随意玩弄线条属性太（“EdgeColor”和“线宽”）

https://www.mathworks.com/help/matlab/ref/rectangle.html