The Flow Noise Simulator (FNS) of the 1st Research Center of TRDI/JDA (Japan Defense Agency) is a large, variable pressure, recirculating water tunnel with very low background noise level. (Figure 1) The hydrodynamic goals of the FNS include an excellent flow uniformity of 1% deviation, low turbulence intensity level of 0.5% and it requires careful design of circuit components, for example, contraction, diffusers, bends and turning vanes. In the conceptual design of the FNS, a series of detailed CFD analysis and component model experiment has been conducted to determine the geometry of the circuit component. Further, the entire circuit performances have been evaluated by following scale model studies. This paper discusses some hydrodynamic design considerations for the FNS.Copyright © 2003 by ASME