Viscous flow computations for the diffuser section of a large water tunnel

A new research project, involving the construction of a modern, large, closed-circuit depressurised high-speed water tunnel to support the detailed hydro-acoustic, hydrodynamic, cavitation and flow visualization based experimental campaigns, is to be completed this year in Istanbul Technical University. An extensive computational study was conducted to design mainly the most critical sections of the tunnel. The results of the simulations concerning the hydrodynamic properties of the contraction and test sections of the tunnel were previously presented in NAV2015 conference. The present paper covers the fundamental viscous flow computations focusing the design of the diffuser section of the tunnel. In order to discharge the flow from the test section with minimum energy loses, diffuser takes a critical place at the downstream side of the test section for water tunnels. Therefore, achievable minimum pressure loss is directly related with length scale of the flow separation region along the diffuser. Furthermore, this flow phenomenon directly affects the acoustical performance of the tunnel with decreasing overall back noise level as well as the flow uniformity in test section. The paper does not only cover the hydrodynamic results of a constant-expansion-angle diffuser section but involves the design of a diffuser with three-step expansion structure aiming no or minimum flow separation region. Incompressible Reynold-Averaged-Navier-Stokes computations were performed for the simulations. The effect of several design parameters, which includes the expansion ratio and length of the diffuser geometry, was investigated. The influence of the chamfered corners was also considered.