include <C:\libs\OpenSCad\plot-function\plot_function.scad>   // https://github.com/rcolyer/plot-function

module PolarFuncExample(){
 
$fa=1; $fs=0.4;
function PolarFunc1(r, a) = 30 + 1.3*cos(8*a);

difference() {
  PlotPolarFunction(1, 15);
  translate([0, 0, -1]) cylinder(r=13, h=40);
}
}

PolarFuncExample();