Free surface effects on the hydrodynamics of an underwater vehicle

Underwater vehicles conducting missions are often subjected to different boundary conditions, be it the seabed, when deeply submerged or near the free surface. When an underwater vehicle travels near the free surface, it behaves similarly to a conventional surface vessel, with its drag exhibiting a hump and hollow behaviour, generating a Kelvin wake. Most of the work on near free surface vehicles has been experimental due to the high computational costs required to numerically simulate an underwater vehicle and its interaction with the free surface. However, recent advancements in computational fluid dynamics (CFD), be it a better understanding or more efficient solvers, as well as more readily available computing power, have allowed for investigations into near free surface conditions. This work first involves verifying the numerical model and its grid dependence. After this, it is validated against experimental data over a combination of test conditions. Validation was conducted on an underwater vehicle operating near the free surface at a range of submergence depths and speeds, undergoing a rotating arm manoeuvre, and operating at a steady drift angle. The numerical model is then used to investigate the effects of several variables on the hydrodynamic behaviour of an underwater vehicle which include Froude number, submergence depth, length-to-diameter ratio, pitch angle, drift angle, and a combination of several variables. This combination of variables serves to investigate and represent the conditions typically experienced by an underwater vehicle, when the vehicle is at angles of incidence, and also the change in hydrodynamic coefficients with a change in hull form.
The results show that the hydrodynamic coefficients of an underwater vehicle operating near the free surface vary with speed and are a direct consequence of the wavelength of the wave field generated by the shallowly submerged body. The magnitude of the wave field follows an exponentially diminishing behaviour with respect to increasing submergence depth. The effects of length-to-diameter ratio were investigated using a parallel mid-body and manifested as a phase shift in the hydrodynamic coefficients with respect to Froude number, as well as a change in the proportions of the hydrodynamic coefficient peaks.
The effects of pitch angle on an underwater vehicle operating near the free surface were also investigated as a steady-state alternative to surfacing manoeuvres. Pitch angle causes asymmetry between the pressure distribution of the upper and lower faces of the underwater vehicle. Pitch angle is also inherently destabilising, where a small initial pitch angle can cause large effects that increase with time. Large pitch angles when shallowly submerged also causes air entrainment over the bow and stern, which significantly changes the behaviour of the underwater vehicle.
The drift angle strongly affected the proportions of the surge coefficient peaks. In addition to this relative change in magnitude between the two peaks, the depth of the trough also changes with an increase in drift angle. The Froude numbers at which the peaks and troughs occur also shift towards a lower Froude number with an increase in drift angle. As for sway and yaw coefficient, proximity to the free surface also causes humps to form with respect to Froude number. Heave and pitch coefficients show the most non-linear behaviour with respect to Froude number, especially at larger drift angles.
Additionally, the interaction between the seabed and the underwater vehicle is another area still not yet approached hydrodynamically but rather structurally. This thesis takes a different approach, analysing the interaction between the seabed and the underwater vehicle purely on its hydrodynamic performance and serves to supplement the findings from free surface investigations. This allows for the comparisons in hydrodynamic behaviour between a rigid boundary (seabed) and one that is free to deform (free surface).
This thesis has developed CFD workflow for ANSYS Fluent on investigating the effects of near free surface operations on underwater vehicle hydrodynamics. The simulations conducted extend upon previous work, with a much broader combination of variables, allowing for the accurate representation of underwater vehicle operating conditions, and serves as a guide to determine the safe operational envelope or operational limits of an underwater vehicle. Having accurately quantified the effects of near free surface operation and the hydrodynamic interaction between the free surface and the hull, this thesis serves to also inform on future work to model these effects in manoeuvring models. Finally, this developed numerical model can also be extended for future work involving appended underwater vehicles with more complex hydrodynamic interactions, with a similar end-goal of incorporating the findings into the manoeuvring models.