Modelling a Moving Propeller System in a Stratified Fluid Using OpenFOAM

Abstract

Moving propeller systems can introduce significant disturbances in stratified environments by mixing the surrounding fluid. Restorative buoyancy forces subsequently act on this region/patch of mixed fluid, causing it to eventually collapse vertically and spread laterally in order to recover the original stratification. This work describes the use of an OpenFOAM solver, modified using existing functionality, to simulate a moving propeller system in a stratified environment. Its application considers a rotating KCD-32 propeller in a laboratory-scale wave tank which mimics published experiments on mixed patch collapse. The numerically-predicted collapse behaviour is compared with empirical data and scaling laws. The results agree closely, both qualitatively and quantitatively, thereby representing a successful step towards the validation of the numerical model.

Hydrographical implications of ship-induced turbulence in stratified waters, studied through field observations and CFD modelling

“…The observed internal waves were expected, as ships in stratified water are known to induce internal waves (Lin and Pao, 1979;Watson et al, 1992;Jacobs, 2020), and the estimated internal wave speed of 1.1-1.7 m s -1 is realistic for these strongly stratified conditions. A rough estimate of the long internal wave speed is (g'h) 1/2 where h is the surface layer depth and g' = g (r 2 -r 1 )/r 2 , with g the gravitational acceleration, and r 1 and r 2 the surface and bottom layer densities.…”

“…This could potentially also contribute to the deeper wakes and more intermittent and irregular wake shape, compared to the Oresund wakes, but the difference in stratification is likely the main cause. It is well known that strong stratification impede the vertical and expand the horizontal extent of the wake (Merritt, 1972;Lin and Pao, 1979;Brucker and Sarkar, 2010;Jacobs, 2020), hence our dataset could provide the first observations of this process in the field for full-scale ships.…”

“…From numerical modelling and model scale laboratory experiments it is well known that stratification impacts the turbulent wake development (Merritt, 1972;Lin and Pao, 1979;Brucker and Sarkar, 2010;Jacobs, 2020). In addition, turbulent ship wakes have been reported to penetrate in situ stratification (Jürgensen, 1991;Lindholm et al, 2001;Loehr et al, 2001;Nylund et al, 2021), thereby entraining deeper water from below the stratification.…”

“…The intermediate water will spread and stabilize at a depth where the surrounding water has the same density (Merritt, 1972;Arneborg, 2002). Moreover, strong currents and shear have been observed to shift the wake horizontally (Benilov et al, 2001;Loehr et al, 2001) and a strong stratification can reduce the vertical extent of the turbulent wake (Lin and Pao, 1979;Brucker and Sarkar, 2010;Voropayev et al, 2012;Jacobs, 2020). Stratification is important for the turbulent wake development, still, few previous studies have been conducted during conditions where the wake depth reached the stratification (Jürgensen, 1991;Lindholm et al, 2001;Loehr et al, 2001;Weber et al, 2005;Voropayev et al, 2012;Nylund et al, 2021).…”