MATLAB: What should I do in order to remove this weird HEAT GRADIENT (flow line)

heat flowMATLAB

I am trying to plot the heat flow through a plate, but for some odd reason I am getting a weird flow line on the cold side (lower right corner). Could somebody check the code for me? I have added a screenshot of the plot I am getting.
Also, How could I add a "hole" in this plot? An area which cannot be affected by the heat flow.
   %Predefined Variables
dx = 1;
dy = 1;
   %Plate dimensions
plate_length = 64;
plate_height = 96;
   %Heated & Cooled points
heated_length = 32;
heated_height = 32;
cooled_length = 16;
cooled_height = 16;
heated_temp = 100;
cooled_temp = -100;
   %Grid
x = 0:dx:plate_length;
y = 0:dy:plate_height;
nx = length(x);
ny = length(y);
[x_arr, y_arr] = meshgrid(x,y);
temp=zeros(ny, nx);
   %Heating & Cooling Points
heated_top = find((y_arr == plate_height) & (x_arr <= heated_length));
heated_left = find((y_arr >= plate_height-heated_height) & (x_arr == 0));
heated_points = union(heated_top, heated_left);
cooled_bottom = find((y_arr == 0) & (x_arr >= plate_length-cooled_length));
cooled_right = find((y_arr <= cooled_height) & (x_arr == plate_length));
cooled_points = union(cooled_bottom, cooled_right);
temp(heated_points) = heated_temp;
temp(cooled_points) = cooled_temp;
   %Iteration
for k=1:1:30
inner_x = 2:nx-1;
inner_y = 2:ny-1;
x_diff = temp(inner_y, inner_x+1) + temp(inner_y, inner_x-1);
y_diff = temp(inner_y+1, inner_x) + temp(inner_y-1, inner_x);
old_inner = temp(inner_y, inner_x);
temp(inner_y, inner_x) = (x_diff +y_diff)/4;
   %Plotting
contourf(x_arr,y_arr,temp);
%surf(x_arr,y_arr,temp);
%contour(x_arr,y_arr,temp);
%hold on;
drawnow;
end

Best Answer

you can specify the levels at which to draw the contours.
From the documentation for contourf:
  • contourf(Z,v) draws a filled contour plot of matrix Z with contour lines at the data values specified in the monotonically increasing vector v. To display a single contour line at a particular value, define v as a two-element vector with both elements equal to the desired contour level. For example, to draw a single contour of level k, use contourf(Z,[k k]). Specifying the vector v sets the LevelListMode property to manual.
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