Exhaust Dispersion on Naval Vessels
An LES Informed Evaluation of RANS Scalar Transport Closure Models to Improve Predictive Accuracy
T. de Kemp (TU Delft - Aerospace Engineering)
A.H. van Zuijlen – Mentor (TU Delft - Aerospace Engineering)
M.I. Gerritsma – Graduation committee member (TU Delft - Aerospace Engineering)
C. Garcia Sanchez – Graduation committee member (TU Delft - Architecture and the Built Environment)
E.W.M. Roosenboom – Mentor (Damen Naval)
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Abstract
Modern naval vessels increasingly integrate exhaust outlets within the ship superstructure, placing the gases directly in the highly turbulent wake generated by its superstructure. Reliable prediction of exhaust gas dispersion is essential during ship design to ensure crew safety, prevent contamination of air intakes, radars and sensors, and helicopter operating areas. The validity of practical Reynolds Averaged Navier-Stokes (RANS) scalar transport models however remains insufficiently understood. This study therefore aims to improve the predictive reliability of practical Computational Fluid Dynamics (CFD)
tools for naval exhaust dispersion.
This research started with an evaluation of a fundamental study into a canonical wall mounted cube with an exhaust located in its wake as a simplified representation of ship superstructure flow. An Large Eddy Simulation (LES) was performed to resolve the dominant turbulent structures responsible for scalar mixing and was compared to experimental data. The resulting turbulence statistics were used to perform both a priori and a posteriori analyses of commonly used scalar transport closures, the Gradient Diffusion Hypothesis (GDH) and the Generalised Gradient Diffusion Hypothesis (GGDH). Finally the findings were applied in the context of a realistic geometry of a Commissioning Service Operating Vessel (CSOV).
The results show that LES predicts more realistic concentration distributions compared to the conventional RANS simulations, although both LES and RANS under predict the magnitude of the concentration compared to experimental observations. Deficiencies were found in the modelling of the turbulent scalar fluxes for both gradient based models. The inclusion of anisotropy through the Reynolds stress tensor in GGDH improves the alignment of the predicted scalar fluxes with the LES data and therefore also reduced the global prediction errors compared to the conventional isotropic GDH formulation. Constant model coefficients were determined from the mode of the LES inferred values of the coefficient. The improvement this made to the dispersion estimate was far greater than increasing the model complexity, the calibrated GDH model outperformed the uncalibrated GGDH simulation. Spatially varying model coefficients provided only limited additional benefit over optimised constant values, from a global prediction error perspective.
Applying the calibrated coefficients to a realistic ship geometry revealed that scalar concentration predictions exhibit strong sensitivity to local flow structures introduced by the different meshes investigated, even when global flow quantities such as aerodynamic force coefficients appear converged. The findings demonstrate that improving the reliability of practical exhaust dispersion simulations requires not only improved scalar closure models and coefficient selection, but also rigorous verification
of the underlying flow solution.