The laminar to turbulent transition occuring on marine propeller blades is known to be critical for the body perfomance and its structural integrity. Previous experimental laboratory researches have shown that under relatively high Reynolds numbers (Re=300,000 to 800,000), highly transitional flows are observed on laminar propeller section, which induces important structural vibrations with low damping, that can in some case get close to the resonance. However, these experiments are mainly based on wall pressure and vibration measurements, and hence the interaction process has not been clearly identified and understood, and requires numerical and/or experimental observation of the boundary layer flow. The objective of this paper is to numerically investigate the behaviour of Laminar Seperation Bubble(LSB) induced vibration on a NACA66 hydrofoil section. For this, a massively parallelized open source DNS code NEK5000 is used to solve the boundary layer flow. It uses higher order spectral element method to solve the incompressible Navier Stokes equation. The DNS domain is reduced to the near wall region, and velocity profiles are taken from the URANS calculation, implemented at the boundaries to reproduce the adverse pressure gradient inducing laminar seperation. To study the transition induced vibration, a two degree of freedom system (2DOF) is considered at the elastic axis of the hydrofoil in order to reproduce the motion of a section, induced by the two first vibration modes i.e. bending (inducing heave motion) and pitch (inducing pitch motion). As a consequence, an equation of motion which consists of mass, stiffness and damper and the hydrodynamic loads computed by DNS is implemented inside Nek5000 . It has to be noted that even though the flow is solved in 3D, periodic boundary conditions are set in the spanwise boundary conditions, with reduced domain so that the equation of motion slove a 2D motion. Hence, this numerical setup will allow to investigate the full interaction between highly transitional flow and the first modes of a hydrofoil. This is not applicable on a full 3D geometry, because of the limitation of the CPU cost required for DNS, and the limitation of RANS methods to capture transitional flows.
Since fluid structure interaction in transitional flows has not been tested in Nek5000, a validation case is first performed on transition induced pitching of NACA0012 airfoil which is already investigated experimentally and numerically. A lower Reynolds number of Re=64,000 is taken with 0 degree initial angle of attack. It is a 1 degree of freedom system where the foil only pitch about it's elastic axis. The lift and drag from the fluid solver is given as input to the equation of motion and the output velocity is feeded as the mesh velocity into the fluid solver to make the motion of the body. This model is validated by comparing the amplitude and frequency of the pitch oscillations and it has shown the ability of the current method to simulate the interaction mechanism. The transition, pitching and natural frequency of the airfoil is fully captured and compares well with the literature.

In the final version of the paper, the effect of fluid structure interaction will be analyzed for different degrees of flexibility on the NACA66 hydrofoil at Re=450,000.