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Experiments by Invited Researchers

 

Experimental study of forces on a submarine outfall: influence of incident wave direction, of stabilizing concrete weights and pipe distance from bottom.

Project acronym: HyIV-DHI-04
Name of Group Leader: Dr. Maria da Graça Reis e Silva de Oliveira Neves
User-Project Title: Experimental study of forces on a submarine outfall: influence of incident wave direction, of stabilizing concrete weights and pipe distance from bottom.
Facility: Shallow water basin
Proceedings TA Project: EXPERIMENTAL STUDY OF FORCES ON A SUBMARINE OUTFALL: INFLUENCE OF INCIDENT WAVE DIRECTION OF STABILIZING CONCRETE WEIGHTS AND PIPE DISTANCE FROM THE BOTTOM
Data Management Report: Report

User-Project Objectives

The main objective of the proposed 3D tests was to provide new insight into the flow and forces around a submarine outfall by generating a unique consistent dataset of forces. Therefore, several series of regular and irregular wave tests were devised with different outfall geometries (with and without concrete weights), different wave obliquities (90º, 30º and 10º) and different pipe distances from the bottom (0 and 0.02 m - approximately 0.3 D) and 0.033 m – approximately 0.5D, where D is the pipe diameter, equal to 0.07 m).

Short description of the work carried out

The most relevant achievement was the fact that the planned test programme was almost completed. The most challenging part of the tests was the change of pipe direction and pipe geometry (with or without weights). The requirement that part of the pressure sensors had to be always inside water made the changing operations more difficult, together with the multiple instrument cables and the length of the pipe (more than 10 m) to be moved. The exact position of the three wave gauges located over the pipe was difficult to guaranty after wave gauges calibration due to the 0.70 m water depth. Some of the instruments had problems. For the geometry without weights, pipe movements were not realistic, which influenced force measurements and made the measurements less reliable. For 90º wave obliquity, only one group of 8 pressure sensors was measuring and in this group one pressure sensor was not measuring at all. The pipe itself broke apart in one section but was fixed. The load cells had problems and were changed during the tests. The wave generator stopped sometimes due to some paddles that were not properly aligned. The absorbing beach suffered longitudinal heterogeneous erosion during the tests, varying the reflection coefficient from around 14% to 23% and contributing to create a transversal component of the flow relatively to the wave propagation direction. Transversal waves were created in the tank that made it impossible to do the tests using the active absorption system in wave generation.

Highlights of important research results

An important volume of data was collected to be analyzed carefully, especially in what concerns the loads. The influence in the forces of the incident wave angle and of the presence of weights is the most important results that can be obtained from the collected data. It was observed that the presence of the weights alters the flow around the pipe, leading to a strong reduction on measured forces for small incident wave angles (10º and 30º), while the pipe distance from the bottom had very small influence on the measured forces.

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