• Open
  • Closing date: 25 August 2021

MISSION TO METRICS

The mission of The Ocean Cleanup is to develop advanced technologies to rid the world’s oceans of plastic. To do so, we must design our systems to effectively capture and retain plastic by creating a difference of velocity between them and the plastics, the so-called delta-V. Therefore, it is important to know the Lagrangian trajectories and velocities of the plastics across different scales (particle to global). This internship will allow you to address how plastic particles are transported on a scale of a few hundred meters up to a kilometer. 

“The best part of working at The Ocean Cleanup is that I get to use my computational modeling skills to tackle real-life problems.” - Andriarimina Daniel Rakotonirina, Computational Modeler

THE ASSIGNMENT

The Stokes drift velocity is the average velocity for a pure wave motion when following a specific fluid parcel as it travels with the fluid flow. A particle floating on the free surface of water waves experiences a net Stokes drift velocity in the direction of wave propagation. The Stokes drift is important for the transport of plastics by a wave field. A background in nonlinear and regular waves on the Stokes drift can be seen in [1, 2] but incorporating irregular nonlinear waves [3] in such studies is still a challenge. Since the Stokes drift velocity decays exponentially with depth, it is important to include the latter. In general, understanding the Stokes drift of plastic particles in random wave fields and their directional spreading on a scale of a few hundred meters would help us scale up our efforts to rid the world's oceans of plastic.

Several methods exist to derive the Stokes drift of Lagrangian positively buoyant particles via perturbation theory or numerical modeling, assuming the linear wave theory. However, these theories are not suited to capture the intrinsic features of the waves in terms of the velocity field, especially higher-order terms and breaking waves. In this internship, a CFD approach is considered, which solves the two-phase Navier-Stokes equations and the motion of Lagrangian particles. This approach is implemented in the code Basilisk [4], a versatile Free Software program for solving partial differential equations on adaptive Cartesian meshes. With Basilisk, a high-resolution simulation of 3D waves will offer an insight on the Stokes drift using different types of boundary conditions, especially those that can replicate irregular nonlinear waves and directional spreading. One of the interests of this thesis is that we will simulate experiments conducted at MARIN (Dutch Maritime Institute) earlier in 2020.


Your responsibilities include:

  • Hands-on use of Basilisk
  • Setting scales at which we will operate: (10^1 m) for comparison with experimental results carried out at MARIN and (10^2 m) for mesoscale studies
  • Studying boundary conditions, especially for irregular ones, using CFDwavemaker
  • Developing simulations on O(10^1 m) with regular and irregular waves + comparison with MARIN
  • Developing simulations on O(10^2 m) with regular and irregular waves
  • Developing the same simulations with Lagrangian particles

PROFESSIONAL QUALIFICATIONS

  • Preparing for a master’s degree in mechanical- or marine engineering, or equivalent with strong numerical fluid mechanics skills
  • Strong C/C++ and Linux/Unix-based platforms skills
  • Knowledge of Parallel Computing and Python is a plus

PERSONAL QUALIFICATIONS

  • Able to perform well in a fast-paced and highly challenging environment
  • Meticulous, detail-oriented, structured
  • Team player, diplomatic 
  • Has an affinity for numbers
  • Excellent communication skills
  • Ability to work with tight deadlines
  • Intrinsic motivation to work on our ambitious and meaningful mission

REFERENCES

[1] M.S. Longuet-Higgins (1953). "Mass transport in water waves". Philosophical Transactions of the Royal Society A. 245 (903): 535–581.


[2] J.M. Williams (1981). "Limiting gravity waves in water of finite depth". Philosophical Transactions of the Royal Society A. 302 (1466): 139–188.


[3] Thomas Berge Johannessen. The effect of directionality on the nonlinear behaviour of extreme transient ocean waves. Ph.D. thesis, Imperial College London (University of London), 1997.


[4] S. Popinet, Basilisk flow solver and PDE library, http://basilisk.fr/

Curious to hear how it is to work at The Ocean Cleanup? Listen to our team members explain their work in our podcasts.

Starting from:
ASAP
Work permit needed:
European Union