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

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

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

PolarFuncExample();