Modelling of the Plume of a Submerged Exhaust System

The Royal Netherlands Navy (RNN) currently operates four diesel-electric Walrus class submarines. The submarines sail submerged on electric engines and periodically recharge the batteries with diesel engines at periscope depth. While recharging, air is taken in with a snorkel mast and exhaust gases are dispelled at the back of the sail, below water level. Figure 1 shows a situation sketch and photograph of this. During the sea trials of the current class of submarines in the nineties, a disturbance on the surface behind the sail was observed. The rising exhaust caused a local surface elevation of 1 to 2 m, resulting in a limited backwards visibility, water flooding the air intake and an increased change of detection. The elevation was deemed unacceptable and model tests were performed to modify the design and reduce the surface elevation. However, the Walrus class is scheduled for replacement around 2025, and a similar power configuration could be used then. To enable evaluating several exhaust configurations the Defense Material Organisation (DMO), responsible for the design and maintenance of the fleet of the RNN, has expressed desire in a numerical model to predict the surface elevation
The numerical modeling is divided in two sections. First a turbulent jet is modeled to determine the required numerical settings, secondly the submarine simulations are performed. This article describes the influence of different turbulence models on the turbulent jet, the verification of the turbulent jet, and the validation of the submarine simulations with experimental data obtained by MARIN. The numerical solver used for all simulations is ReFRESCO, DMO has a preference for this code due to previous experiences and a longstanding collaboration with the Maritime Research Institute Netherlands (MARIN) for the design of naval vessels, including submarines. ReFRESCO (MARIN (2017)) is a multiphase (unsteady) incompressible viscous flow solver using the RANS equations, complemented with turbulence models, cavitation models and volume-fraction transport equations for different phases. For turbulence modeling, both RANS/ URANS and Scale-Resolving Simulations (SRS) models such as SAS, DDES/IDDES, XLES, PANS and LES approaches can be used.