The x and y don't quite make a grid (a point is missing in the upper-left corner near (0,2)):
xyz = readmatrix('velocity.txt');
plot(xyz(:,1),xyz(:,2),'.')
so it's most convenient to use a scatteredInterpolant:
I = scatteredInterpolant(xyz(:,1),xyz(:,2),xyz(:,3));
[x,y] = meshgrid(unique(xyz(:,1)),unique(xyz(:,2)));
contourf(x,y,I(x,y))
colorbar