[EE] Ultrasonic reflector design and simulation?

Hi,

I'm involved in the design of a system that uses ultrasonic transducers to
measure the velocity of fluids. For various mechanical reasons, we have to
use one or more acoustic reflectors between the transmitter and receiver.

Has anyone here ever used any design software for these sorts of acoustics?
Can anyone personally recommend any literature?

We have prototypes that work, but they probably could be a lot better.

-- 
Jason White