Niels Gjøl Jacobsen

Numerical investigation of flow and scour around a vertical circular cylinder

Flow and scour around a vertical cylinder exposed to current are investigated by using a three-dimensional numerical model based on incompressible Reynolds-averaged Navier–Stokes equations. The model incorporates (i) k-ω turbulence closure, (ii) vortex-shedding processes, (iii) sediment transport (both bed and suspended load), as well as (iv) bed morphology. The influence of vortex shedding and suspended load on the scour are specifically investigated. For the selected geometry and flow conditions, it is found that the equilibrium scour depth is decreased by 50% when the suspended sediment transport is not accounted for. Alternatively, the effects of vortex shedding are found to be limited to the very early stage of the scour process. Flow features such as the horseshoe vortex, as well as lee-wake vortices, including their vertical frequency variation, are discussed. Large-scale counter-rotating streamwise phase-averaged vortices in the lee wake are likewise demonstrated via numerical flow visualization. These features are linked to scour around a vertical pile in a steady current.

Sand Dunes in Steady Flow at Low Froude Numbers: Dune Height Evolution and Flow Resistance

The development of sand dunes in an erodible bed exposed to flowing water is considered using a numerical flow model. The bed is initially given a small perturbation, which is followed in the time domain. Examples are given where a sinusoidal perturbation reaches the fully developed steady equilibrium shape. The flow modeling is based on a k-ω turbulence closure. The sediment transport is assumed to be bed-load only, with an avalanche-like movement on the steep dune front. The model is also found capable of predicting the growth in wavelength if the initially prescribed wavelength is sufficiently short. Results of the dune development are presented for different initial wavelengths. It is demonstrated that an equilibrium shape is developed for a range of wavelength-depth ratios as long as this ratio is sufficiently small.

Breaking Wave Impacts on Offshore Wind Turbine Foundations: Focused Wave Groups and CFD

Extreme wave loads from breaking waves on a monopile foundation are computed within a 3D CFD model. The wave impacts are obtained by application of focused wave groups. For a fixed position of the monopile, the focus location of the wave group is varied to produce impacts with front shapes that varies from early stages of breaking to broken waves. The CFD results for in-line force are compared to load estimates obtained from the Morison equation. The peak loads determined with this simple method are smaller than those of the CFD solution. The computational results appear to suggest that for the impacts of spilling breakers the peak force gets smaller the more developed the breaking is. This is in qualitative agreement with a finding from shallow water impacts on vertical walls: the strongest wave loads are associated with breakers that hit the structure with slightly overturning front. Extensions of the study are discussed.Copyright

Physically-consistent wall boundary conditions for the k-ω turbulence model

A model solving Reynolds-averaged Navier–Stokes equations, coupled with k-ω turbulence closure, is used to simulate steady channel flow on both hydraulically smooth and rough beds. Novel experimental data are used as model validation, with k measured directly from all three components of the fluctuating velocity signal. Both conventional k = 0 and dk/dy = 0 wall boundary conditions are considered. Results indicate that either condition can provide accurate solutions, for the bulk of the flow, over both smooth and rough beds. It is argued that the zero-gradient condition is more consistent with the near wall physics, however, as it allows direct integration through a viscous sublayer near smooth walls, while avoiding a viscous sublayer near rough walls. This is in contrast to the conventional k = 0 wall boundary condition, which forces resolution of a viscous sublayer in all circumstances. Subsequent testing demonstrates that the zero-gradient condition allows the near-bed grid spacing near rough walls to be based on the roughness length, rather than the conventional viscous length scale, hence offering significant computational advantages.

Cross-Shore Redistribution of Nourished Sand near a Breaker Bar

AbstractThis paper focuses on the optimal location for dumping nourished sand on a barred coastline. This is done by investigating the short-term behavior of the cross-shore redistribution of nourished sediment on a breaker-bar profile in a two-dimensional vertical plane. This is achieved by the use of a complete numerical description of the surf-zone processes with respect to both hydrodynamics and sediment transport. The numerical model is based on the finite-volume approach with a free surface-tracking method, also known as the volume of fluid (VOF), and the sediment transport is calculated applying the Engelund and Fredsoe deterministic concept. The methodology is as follows: a Dean/Bruun equilibrium profile is exposed to regular waves (chosen as H=1.3 m and T=4.8 s). These waves will form bars on the Dean/Bruun equilibrium profile, and the wave impact is continued until a quasi-steady behavior of the bars has been obtained, i.e., bars that have stopped growing in height and only migrate slowly offsho...

Vertical Wave Impacts on Offshore Wind Turbine Inspection Platforms

Breaking wave impacts on a monopile at 20 m depth are computed with a VOF (Volume Of Fluid) method. The impacting waves are generated by the second-order focused wave group technique, to obtain waves that break at the position of the monopile. The subsequent impact from the vertical run-up flow on a horizontal inspection platform is computed for five different platform levels. The computational results show details of monopile impact such as slamming pressures from the overturning wave front and the formation of run-up flow. The results show that vertical platform impacts can occur at 20 m water depth. The dependence of the vertical platform load to the platform level is discussed. Attention is given to the significant downward force that occur after the upward force associated with the vertical impact. The effect of the numerical resolution on the results is assessed. The position of wave overturning is found to be influenced by the grid resolution. For the lowest platform levels, the vertical impact is found to contribute to the peak values of in-line force and overturning moment.Copyright

A Full Hydrodynamic Modelling of 2D Breaker Bar Development

The free surface simulation of breaking waves is studied using a combination of VOF and RANS closures. Further, a numerical model for the detailed study of sediment transport and morphological development is presented. In the present study it is applied to the case of sediment transport in the surf zone. The temporal change of a cross shore beach profile under both regular and bichromatic waves is considered. The dependency on the morphological time scale on the regularity of the incident waves is discussed. The feedback onto the hydrodynamics due to a changing bed level is discussed in the case of regular waves.