Numerical and experimental investigation of flow and scour around a circular pile

Abstract

The flow around a vertical circular pile exposed to a steady current is studied numerically and experimentally. The numerical model is a three-dimensional model. The model validation was achieved against new experimental data (which include two-component laser-Doppler anemometry (LDA) flow measurements and the hot-film bed shear stress measurements, and reported in the present paper) and the data of others, and a $k$–$\omega $ turbulence model was used for closure. The model does not have a free-surface facility and therefore is applicable only to cases where the Froude number is small ($\hbox{\it Fr}\,{<}\,O(0.2)$). The flow model was used to study the horseshoe vortex and lee-wake vortex flow processes around the pile. The influence on the horseshoe vortex of three parameters, namely the boundary-layer thickness, the Reynolds number and the bed roughness, was investigated. In the latter investigation, the steady solution of the model was chosen. A study of the influence of the unsteady solution on the previously mentioned flow processes was also carried out. The ranges of the parameters covered in the numerical simulations are: The boundary-layer-thickness-to-pile-diameter ratio is varied from 2$\,{\times}\, 10^{-2}$ to 10$^{2}$, the pile Reynolds number from 10$^{2}$ to $2\,{\times}\, 10^{6},$ and the pile diameter-to-roughness ratio from 2 to about 10$^{3}.$ The amplification of the bed shear stress around the pile (including the areas under the horseshoe vortex and the lee-wake region) was obtained for various values of the previously mentioned parameters. The steady-state flow model was coupled with a morphologic model to calculate scour around a vertical circular pile exposed to a steady current in the case of non-cohesive sediment. The morphologic model includes (i) a two-dimensional bed load sediment-transport description, and (ii) a description of surface-layer sand slides for bed slopes exceeding the angle of repose. The results show that the present numerical simulation captures all the main features of the scour process. The equilibrium scour depth obtained from the simulation agrees well with the experiments for the upstream scour hole. Some discrepancy (up to 30%) was observed, however, for the downstream scour hole. The calculations show that the amplification of the bed shear stress around the pile in the equilibrium state of the scour process is reduced considerably with respect to that experienced at the initial stage where the bed is plane.

Investigations on Scour Development at Tripod Foundations for Offshore Wind Turbines: Modeling and Application

“…The transport formulation for each computational face on the bottom boundary grid has been implemented in vectorial form, generally based on the idea given in Kovacs & Parker (1994), including slope-dependent modifications of the resulting shear stresses. The overall approach is thereby similar to that used in Roulund et al (2005) and Jacobsen (2011). To account for the steep-slope mechanisms, a sediment sliding procedure has further been incorporated.…”

Numerical Modeling of Backfilling Process Around Monopiles

“…Based on the authors' experience, the present model can be expected to behave similarly to other k-ω variants, e.g. the k-ω SST models used in our previous studies of Roulund et al (2005) and Dixen et al (2013), while requiring fewer closure coefficients. Wilcox (2006) further discusses the new version of the k-ω, the version used in the present study, and earlier versions created by Wilcox et al, see Wilcox (2006, p. 127 and p. 152).…”

“…In the calculations, θc is calculated, taking into account the bed-slope effect (as in Roulund et al, 2005), with a base-line value (i.e., with no bed-slope effect) being 0.05. Throughout the present work, the reference level is taken as b=α1d=3.5d, which is somewhat larger than the more traditionally used b ≈ 2d (e.g.…”

Experimental and Numerical Study of the Local Scour Caused by Sediment Releasing Through a Dam Gate

“…The   k SST model 8) was used for the closure turbulence model, since it is often applied to the boundary layer flow under the inverse pressure gradient. The local scour phenomenon is often caused by the inverse pressure gradient at hydraulic structures, and the   k SST model has often been applied to simulate the local scour 9) . In each time step, we computed the excess tractive stress 10) , which considers the effect of local bed slope with a normal direction (…”