Well, sorry, I was assuming a little bit of creativity ... How about this:
f[r_, \[Phi]_] := Sin[10 r] Cos[\[Phi]]
ContourPlot[f[Sqrt[x^2 + y^2], ArcTan[x, y]], {x, y} \[Element] Disk[]]
By "... in a circle" I guess you mean inside a circle, i.e. on a Disk[]
; this I had mentioned above.