function f = funcao_espiral(x,y)
R0 = 0.05;
R1 = 0.45;
F = 15;
r = sqrt(R0^2 + x^2 + y^2);
s = atan2(y,x);
t = s + F*R0/r;
u = (x^2+y^2)/R1^2;
if u < 1 
    a = r*(1 - u)^2; 
else 
    a = 0;
end
f = a*sin(t);


