In my lecture notes on Fluid Dynamics there is a derivation on the velocity potential $\Phi$ of waves propagating in a rigid cylinder of radius $R$. The problem is solved by seperation of variables $\Phi(r,\phi,z,t)=Q(r)H(\phi,z,t)$ and after some calculations they arrive at the following equation: $$\frac{d^2Q}{dr^2}+\frac{1}{r}\frac{dQ}{dr}+\left(K^2-\frac{m^2}{r^2}\right)Q=0$$
after which they state: "This is a Bessel equation. There are only bounded solutions that satisfy $v_r=\frac{d\Phi}{dr}=0$ at $r=R$ if $K^2>0$."
I don't see why the this statement holds or why it is obvious. Can it be seen easily by looking at the equation or are there some lines of calculations required to proof the result? Any help would by welcome
Thanks in advance.