Stealthier submarines

21-06-2023 | Posted by Joaquín Martí

Submarino sigiloso

Stealth and detectability are critical aspects of the design of a submarine. The present post Is based on one published by Dassault Systèmes on that topic.

A successful submarine design must withstand the pressures of deep water, the stresses of waves, and the shocks of combat. It must travel silently with minimal drag while minimizing its radar cross-section (RCS).

Its hydraulic, electrical, thermal, ventilation and mechanical systems are safety-critical and need to operate reliably for months. Large design teams, with many different specialist groups, must work together.

A unified approach to modelling and simulation (MODSIM) enables this, shortening the development process for producing a submarine that achieves stealth requirements without compromising other objectives.

The detectability of a submarine depends on several factors. The noise signature of a submarine affects how readily the vessel is detected by passive sonar. The sound is not just a result of vibrations and noise from machinery inside the vessel; the design of the hull itself affects noise as water flows turbulently, particularly around control surfaces.

Another important factor is its RCS. When radar waves strike the vessel, they are reflected and scattered; reflections towards the receiver are what allows detection and must be minimized. Also, for submerged vessels near the surface, the hydrodynamics result in a complex free surface deformation pattern. Both effects must be considered, thus naval architects, radar engineers and computational fluid dynamics (CFD) experts all need to work together.

The project begins by building a CAD model with CATIA, with parameters allowing optimization later, and making it available for simulation on the 3DEXPERIENCE platform. The submarine’s dynamic behaviour is modelled using the Dymola Behavior Modeling (DBM) application, which generates multibody models from CAD data and provides 3D visualization and animation capabilities. This generates a multibody model, including control surfaces, that allows studying the hydrodynamic behaviour under different conditions. The results are compared with the requirements and, if not met, the model parameters and topology can be modified until the target behaviour is achieved.

Once the geometry and control surfaces are optimized, the hydrodynamic wake can be studied with SIMULIA Fluid Dynamics Engineer, which uses a state-of-the-art Reynolds-Averaged Navier-Stokes (RANS) solver to model the flow, including turbulence. The interface between water and air, where the surface wake forms, is solved using the multiphase Volume of Fluid (VOF) formulation. The free surface VOF iso-contour plots are the data needed for the next stage of the simulation.

A shooting-and-bounding ray (SBR) model with the Asymptotic Solver in CST Studio Suite is used to study the RCS, a method demonstrated to be accurate for periodic and random rough surfaces. The CST Studio Suite SBR method includes shadowing and multi-path effects for greater accuracy, making it an effective tool for RCS evaluation in maritime environments.

The free-surface VOF iso-contour plots are directly translated into a tessellated mesh into CST Studio Suite through the 3DEXPERIENCE platform.

This provides the single source of truth, collecting all the different modelling and simulation data, accessible to all team members, reducing the risk of mistakes and preserving traceability.

Also, periscopes and optronics masts are a major source of hydrodynamic disturbance and turbulence. They result in an unsteady flow and a high steep bow wave at some speeds, a wave identifiable by radar or visual observation. To address this issue, strakes can be added to the periscope to disrupt the development of the bow wave, parameterized so the thickness and pitch of the helix can be optimized.

MODSIM enables engineers to develop designs faster and more efficiently. For an undersea defence vessel, it allows a full RCS analysis for stealth purposes, covering multiple physical disciplines. The unification of CAD and CAE means that the submarine design data can be easily converted into a simulation-ready model, and the integration of different simulation tools on the 3DEXPERIENCE platform allows systems, electromagnetic and fluid simulations to be combined to capture the complex interaction of radar scattering and hydrodynamic wakes.

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