The problem is the way the 1/3 is being evaluated in the assignment statement. Did you try substituting 0.33 and see if you get the correct answer. If you break apart the components that make up v individually you will see why the answer is zero.
Remember order of operations, the computer is seeing 1/(3piH*(R1 +r1*r2+R2).
Like the above answer, if it’s just an approximate you need do 0.33, or isolate the 1/3 : (1/3) * Pi *H…