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The plesiosaur and the lily pad: real-time fluid dynamics for biologically inspired engineering

Potential uses for Computational Fluid Dynamics

There are many potential uses for Computational Fluid Dynamics in the maritime industry

On Tuesday 12 June, Dr Gabriel Weymouth, Associate Professor for the Marine and Maritime Institute at the University of Southampton, gave a seminar on advancements in fluid simulation software and the biologically-inspired engineering designs this has enabled.

This research is motivated by recent interest in both marine and maritime research; the first of these is related to the field of engineering, and the second to biology. Over the past decade, there have been significant changes in the construction of maritime vessels, such as stealth/underwater/autonomous vehicles, after several decades of stillness. On the other side, we have seen heavy interest in the study of marine animals - such as the plesiosaur, which will be discussed later on - and in the improvement of human swimming speeds. Hand in hand with this, scientific software and hardware has become increasingly more powerful and user-friendly.

You might expect these to compliment each other, leading to a continuous scientific advancement in the field. However, you would be making the same mistake as hundreds of undergraduates each year, in assuming that Computational Fluid Dynamics (CFD) software is somewhat straightforward to use. In fact, due to a combination of the incredibly slow computation and the overwhelming complexity of the grids, CFD software often becomes an impediment to a project rather than a solution.

Dr Weymouth presents an alternative in the form of Lily Pad, a CFD solver whose goal is to lower the barrier to CFD by using simple high-speed methods and giving immediate visual feedback to the user.

Lily Pad

Recognising the issues with mainstream CFD software, Dr Weymouth has been developing Lily Pad with the aim to help students learn fluid dynamics, and to introduce inexperienced people to CFD. His objective is to use simple and fast methods with modern programming features and with a high degree of interactivity with the user. He describes the grid complexity as the primary issue, as it can lead to models with millions of free parameters, with the physics being very sensitive to them.

Lily Pad solves these issues by:

  • Switching a boundary fitted grid to a Cartesian grid and applying the Volume of Fluid (VOF) method to slice the grid
  • Using the Boundary Data Immersion Method (BDIM) to impose boundary conditions for solid edges, leading to the same simple equation in each domain

Through the use of a few visually striking demonstrations, Dr Weymouth shows that, despite the simplifications, Lily Pad is capable of matching experimental data and achieving the same results as the more common boundary fitted methods in a much shorter time. This software has helped researchers to solve long-term problems and it has been already applied to many different fields thanks to the general formulation of the numerical method.

Applications

The main application which is discussed is the use of Lily Pad by a group of undergraduate students to analyse the swimming style of the plesiosaur, a large marine reptile from the Jurassic period with a unique 4-flipper swimming style which enables a high degree of manoeuvrability. The analysis helped them to construct a cost-effective underwater vehicle which can be used to explore/inspect features near the seafloor. In terms of performance, the prototype shows higher maneuverability, increased thrust and efficiency, improved force variation and enhanced payload. This entire undergraduate project was made possible by the simplicity and accuracy of Lily Pad.

The vehicle built by undergraduate students based on analysis using Lily Pad

The vehicle built by undergraduate students based on analysis using Lily Pad

Lily Pad's big brother, Lotus, currently in development, is able to tackle 3D problems, and has been used in PhD projects where 2D models are not feasible. However, 3D simulations increase the computational cost significantly and so it loses somewhat the main advantage of Lily Pad.

In his conclusion, Dr Weymouth explains that Lotus is his current project, but also introduces a few other plans for the future. These include:

  • Speeding up the 3D solver by using 2D turbulence modelling
  • Porting Lily Pad to Python/Julia
  • Developing a Lotus package for Python

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