Showing papers by "Jørgen Fredsøe published in 2006"

The sequence of sediment behaviour during wave-induced liquefaction

Abstract: This paper presents the results of an experimental investigation of the complete sequence of sediment behaviour beneath progressive waves. The sediment was silty with d50 = 0.060 mm. Two kinds of measurements were carried out: pore-water pressure measurements (across the sediment depth), and water-surface elevation measurements. The process of liquefaction/compaction was videotaped from the side simultaneously with the pressure and water-surface elevation measurements. The video records were then analysed to measure: (i) the time development of the mudline, (ii) the time development of liquefaction and compaction fronts in the sediment and (iii) the characteristics of the orbital motion of the liquefied sediment including the motion of the interface between the water body and the sediment. The ranges of the various quantities in the tests were: wave height, H = 9–17 cm, wave period, T = 1.6 sec, water depth = 42 cm, and the Shields parameter = 0.34–0.59. The experiments reveal that, with the introduction of waves, excess pore pressure builds up, which is followed by liquefaction during which internal waves are experienced at the interface of the water body and the liquefied sediment, the sequence of processes known from a previous investigation. This sequence of processes is followed by dissipation of the accumulated excess pore pressure and compaction of the sediment which is followed by the formation of bed ripples. The present results regarding the dissipation and compaction appear to be in agreement with recent centrifuge wave-tank experiments. As for the final stage of the sequence of processes (formation of ripples), the ripple steepness (normalized with the angle of repose) for sediment with liquefaction history is found to be the same as that in sediment with no liquefaction history.

Control of Scour at Vertical Circular Piles under Waves and Current

Abstract: An experimental study on the control of scour at vertical circular piles under monochromatic waves and a steady current is presented. The experiments on wave and steady currents were carried out under live-bed and clear-water regimes, respectively. In waves, splitter plate attached to the pile along the vertical plane of symmetry and threaded pile (helical wires or cables wrapped spirally on the pile to form threads) were found to be effective to reduce the scour depth. For the Keulegan-Carpenter numbers 6-100, the vortex shedding is the main mechanism of scour under waves. The splitter plate and threaded pile disrupt the vortex shedding. The average reduction of the scour depth by the splitter plate was 61.6%. For threaded piles, different combinations of cable and pile sizes were tested, and the best combination was found for a cable-pile diameter ratio equaling 0.75, in which average scour depth reduction was 51.1%. The average reductions of scour depths for other cable-pile diameter ratios of 0.33 and 0.5 were 43.2 and 48.1%, respectively. On the other hand, in a steady current, the threaded pile proved to be effective to control scour depth to a great extent. Cables wrapped spirally forming threads on the pile help to weaken the downflow and horseshoe vortex, which are the principal agents of scour under a steady current. The experimental results showed that the scour depth consistently decreases with an increase in cable diameter and the number of threads, and with a decrease in thread angle. The maximum reduction of scour depth observed was 46.3% by using a triple threaded pile having a thread angle of 15° and a cable-pile diameter ratio of 0.1. The proposed methods of controlling scour are easy to install and are economical.

Liquefaction around Pipelines under Waves

Abstract: This paper presents the results of an experimental study on liquefaction around a pipeline buried in a soil exposed to a progressive wave. The soil used in the experiments was silt with d50=0.045 mm. The pore-water pressure was measured in the far field and on the pipe simultaneously. The tests indicate that the buildup of pore pressure and the resulting liquefaction in the soil are influenced by the presence of the pipe. The pore pressure builds up much more rapidly at the bottom of the pipe than in the far field at the same level as the pipe bottom. By contrast, the buildup of pore pressure at the top of the pipe is not influenced radically by the presence of the pipe. The tests further indicate that as the liquefaction initially occurs in the very top layer and develops downwards, this picture changes in the vicinity of the pipe; in the latter case, the liquefaction initially occurs at the bottom of the pipe, and develops along the perimeter of the pipe upwards. The influence of the "no-slip" condition at the pipe surface on the end results has been investigated and found very significant. The influence of the wave height, the influence of the pipe diameter, the influence of the no-liquefaction-regime conditions, and the influence of a sinking pipe have also been investigated.

Critical Flotation Density of Pipelines in Soils Liquefied by Waves and Density of Liquefied Soils

Abstract: This paper summarizes the results of an experimental and theoretical investigation of: (1) pipeline flotation in a soil (liquefied under waves); and (2) density of the liquefied soil. In the experiments, the soil was silt with d50 =0.078 mm . Pipeline models of 2 cm diameter were used. They were buried in the soil at different depths in the range 3–15.5 cm . The total depth of the silt layer was 17.5 cm . Waves (with 17 cm wave height and 1.6 s wave period, the water depth being 42 cm ) were used to liquefy the soil. The pipes with specific gravity smaller than 1.85–2.0 floated when the soil was liquefied, the critical specific gravity for pipe flotation. The lower bound of the above range corresponds to the initial pipe position near the surface of the bed, and the upper bound to that near the impermeable base. Furthermore, the pipe floated (or sank) to a depth where the pipe specific gravity was equal to the previously mentioned critical specific gravity for flotation. The density of liquefied soil was ...

Large Eddy Simulation of the ventilated wave boundary layer

Abstract: [1] A Large Eddy Simulation (LES) of (1) a fully developed turbulent wave boundary layer and (2) case 1 subject to ventilation (i.e., suction and injection varying alternately in phase) has been performed, using the Smagorinsky subgrid-scale model to express the subgrid viscosity. The model was found to reproduce experimental results well. However, in case 1, the near-bed ensemble averaged velocity is underestimated during the acceleration stage, probably due to the Smagorinsky subgrid-scale model not being able to capture the physics well in that region. Also, there is a general overestimation of the streamwise turbulence intensity, while an underestimation of the intensities in the two other directions. This may be an effect from the stretched computational mesh in the streamwise direction, since the Smagorinsky subgrid viscosity assumes proportionality to one scalar expressing the overall (local) grid size. The results indicate that the large eddies develop in the resolved scale, corresponding to fluid with an effective viscosity decided by the sum of the kinematic and subgrid viscosity. Regarding case 2, the results are qualitatively in accordance with experimental findings. Injection generally slows down the flow in the full vertical extent of the boundary layer, destabilizes the flow and decreases the mean bed shear stress significantly; whereas suction generally speeds up the flow in the full vertical extent of the boundary layer, stabilizes the flow and increases the mean bed shear stress significantly. Ventilation therefore results in a net current, even in symmetric waves.

Experimental Investigation of Coherent Structures in Wave Boundary Layers

Abstract: (29/03/2019) Experimental Investigation of Coherent Structures in Wave Boundary Layers Synchronized flow visualization experiments, wall shear stress and LDA measurement of the combined oscillatory flow and current boundary layer were made in the same oscillating water tunnel as the one used in the work by Jensen et al. (1989) and Lodahl et al. (1998). The range of wave Reynolds-number Rew = aUm/ν, was from practically 0 to 5.0×10 5; in which ν is the kinematic viscosity, a is the amplitude of the free-stream particle motion and Um is the amplitude of the freestream velocity. For the current Reynolds-number Rec = 4RhV/ν, the range was from 0 (oscillatory flow alone) to 2.7×10 4; in which Rh is the hydraulic radius and V is the mean velocity of the current.