ONRT Free-Running CFD and System Based Simulations for Transient Maneuvers

A comprehensive evaluation of free-running Computational Fluid Dynamics (CFD) and System-Based (SB) approaches is presented for simulating transient maneuvers of the Office of Naval Research Tumblehome (ONRT) surface combatant, with a current focus on operations in calm water. The transient maneuvers considered include crashback, crash-ahead, turning from rest [Acceleration Turning (AT)], turning at rest, and full in-behind four-quadrant propulsion operation. Simulations are conducted across multiple vessel speeds using two CFD solvers (CFDShip-Iowa V4.5 and ReFRESCO), and one SB method (aNySim-XMF). The research is performed in support of NATO AVT-399, which aims to assess the predictive capabilities of current tools relative to the STANAG maneuvering performance criteria. According to the CFD and SB predictions for cruising speed, the ONRT will satisfy STANAG criteria regarding stopping and turning from rest abilities. Notable differences between CFD and SB predictions arise in turning at rest scenarios due to complex propeller inflow effects and the omission of propeller side-force modeling in the SB approach. AT simulations exhibit significant differences in transient maneuvering metrics such as Time to reach 90° (\(\:{T}_{90}^{*}\)) and Advance (AD) compared to previous CFD and experimental results for conventional turning circle maneuvers, attributable to reduced initial forward speed. Ongoing and future efforts include the development of scale-effect correlation curves, additional crashback and transient Propeller Open-Water (POW), and targeted experiments in the Iowa Institute of Hydraulic Research (IIHR) wave basin. These efforts aim enhance the predictive fidelity of both CFD and SB approaches in support of mission-specific naval performance requirements.