A new research project, involving the construction of a modern, large, closed-circuit depressurized high-speed water tunnel to support the detailed hydro-acoustic, hydrodynamic, cavitation and flow visualization based experimental campaigns, has recently been started in Istanbul Technical University. The present paper covers the fundamental viscous flow computations concerning the design of the critical parts of the tunnel, which includes the contraction, test section and diffuser. Incompressible Reynolds-Averaged-Navier-Stokes computations were performed for the analyses. Experimental and computational results of a cavitation tunnel available in the associated literature were used for the benchmark study.  The simulations were carried out considering the pre-defined dimensions of the test section. The sixth-order polynomial-based geometry of the asymmetric contraction was determined aiming a high quality flow structure in the test section. The effect of several design parameters, such as the length and ratio of the contraction as well as the wall curvature properties, were analyzed in order to determine the best contraction geometry. Various opening angle and length selections were also considered for the diffuser to obtain a separation-free steady fluid flow without excessively increasing the headloss in the circuit. The influence of the introduction of slight angles to the test section’s vertical walls are also discussed.