Acoustical and optical non-diffracting beams are potentially useful for manipulating particles and larger objects. An extended optical theorem for a non-diffracting beam was given recently in the context of acoustics. The theorem relates the extinction by an object to the scattering at the forward direction...

We present a method to determine, using only velocity field data, the time-averaged energy flux and total radiated power P for two-dimensional internal gravity waves.   Both and P are determined from expressions involving only a scalar function, the stream function .  We test the method using data...