Atmospheric Boundary Layer Effects on Ship Airwakes Using Synthetic Eddy Method in Lattice-Boltzmann Simulations
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Kurban, Erk
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Abstract
Shipdeck landing is amongst the most challenging operations a modern navy deals with. To ensure such operations are safely carried out, extensive training of prospective pilots is required. One of the first steps of such training is exercises with flight simulators. To realistically simulate the conditions in the wake of a ship, Computational Fluid Dynamics (CFD) simulations should be conducted. There are different CFD methodologies to obtain an accurate ship airwake simulation, one of them is the Lattice-Boltzmann Method (LBM). It has been proven that the LBM is a computationally efficient mid-fidelity method to solve various flow cases. Previous studies showed that the accuracy of the LBM ship airwake predictions was acceptable, hence results were deemed suitable to be used in flight simulators.
Many aspects need to be considered in a ship airwake simulation to represent real-world conditions as closely as possible. Ship geometry modeled in simulations, boundary conditions used to model ship, and inflow modeling are among those important aspects. Ship geometry fundamentally affects the flow field over the landing deck, hence, choice of an appropriate ship model is crucial. NATO Generic Destroyer (NATO-GD) is a more accurate representation of modern naval combatant designs in comparison to widely-adopted Simple Frigate Shape 2 (SFS2). Therefore, it is important to understand the difference between the airwakes generated by different ship geometries. To be able to properly model curved surfaces of NATO-GD, simple boundary conditions like bounce-back fall short, as it transforms geometries into voxelized Cartesian approximation. Rather, boundary conditions that can model non-Cartesian aligned surfaces in the LBM simulations would be preferred such as Mei-Luo-Shyy (MLS) and Grad immersed boundary conditions.
The Atmospheric Boundary Layer (ABL), being a natural phenomenon, needs to be modeled in such simulations. It is relatively straightforward to model the steady part of the ABL as it can be modeled by power law or logarithmic law profiles. However, inherent turbulence of the ABL is not represented in such mean velocity profiles. There are numerous methods to simulate the inherent turbulence in numerical simulations. While some of those methods have been proven to be ineffective, some others have been proven to be computationally expensive. With the Synthetic Eddy Method (SEM) it is possible to retain the computational efficiency of the LBM, while accurately simulating the inherent turbulence.
While it was not the primary objective of this study, a new method was proposed to evaluate pilot workload for ship deck landing. Using solely the vertical flow fluctuation statistics obtained from the LBM simulations, this new method eliminates the subjectivity inherent in pilot workload evaluation. The comparisons with pilot workload ratings available for SFS2 in the literature demonstrated good correlation, indicating the potential of this new method.
This thesis aims to inspect the effects of the ship geometry and the ABL on the ship airwake by simulating a realistic ABL by using the Synthetic Eddy Method. It also evaluates the solid wall boundary conditions to model ship geometry accurately in the LBM simulations. Additionally, the effects of floating-point precision and Lattice-Boltzmann velocity sets on ship airwake simulation accuracy were evaluated, allowing for the determination of minimum viable computational resources. The accuracy of simulation results was evaluated by comparisons with multiple experimental measurement data sets and high-fidelity CFD results.
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2025-12
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Dissertation (PhD)