Duration: 2012-2015

Principal Investigator: Athanassios A. Dimas

Funding: GSRT- Program ARISTEIA - 1718

Research Areas: Coastal Engineering, Turbulent Flow

Services: Computations – Simulation, Laboratory Measurements

Abstract
Coastal erosion/accretion is a serious problem in Europe/Greece, and is expected to intensify in the future due to the increase of human activities in the coastal zone and the influence of climatic change. The analysis of coastal processes in the surf zone, which are the main cause of coastal erosion/accretion, is currently performed with numerical simulation or physical modeling at Reynolds numbers substantially smaller than in the natural environment. The objective of the present research project was to use High Performance Computing methods and resources in order to achieve top performance on the emerging petascale computing platforms for the three-dimensional numerical simulation of coastal processes at Reynolds number about two orders of magnitude higher than the state of the art. The methodology was based on the development of a scalable solver of the Navier-Stokes equations using the immersed boundary method with adaptive mesh refinement. Free-surface boundary conditions were introduced by a two-phase flow approach using the level set method. Turbulence modeling was incororated as in large-eddy and large-wave simulations. Laboratory measurements, using particle image velocimetry, of wave propagation and breaking was also carried out for code adjustment and validation. The main research team was based at the Department of Civil Engineering, University of Patras, and was aided by Prof. Balaras of the George Washington University, USA. The strength of this cooperation was the added value brought to the execution of the project by the group in Greece, with experience on coastal processes and numerical simulations, with support by the group in the USA, with experience on high performance computing and turbulence. The project is organized into four workpackages.

WP1
In the present study, the numerical simulation of wave propagation over a constant slope beach and over a submerged breakwater is attempted, as well as the simulation of oscillatory flows over a porous breakwater or the vicinity of pipelines. The simulation of such flows becomes quite painful with the conventional methods of spatial discretization, both because of the free surface movement in cases whereas the flow is considered as a two phase one, and because of the difficulty to deal with complex geometries. Through the Immersed Boundary Method, it becomes very easy to deal with such flows, which are very important for various reasons. More specifically, wave propagation and breaking is a natural and important process in the surf zone. The surf zone is a region of great complexity as it is characterized by patterns such as vortices, low frequency waves, and currents (Battjes, 1988). In addition, submerged breakwaters in the coastal zone are widely used for shoreline or harbor protection or to protect beach from erosion. These structures have been increasingly popular due to their ability to reduce wave energy and nourish the shoreline with sediment and have a mild aesthetic impact on the natural environment. At last, marine pipelines are used in a wide array of engineering applications (oil and gas transportation, sea outfalls, etc.). Their design depends on the oscillatory forces due to the wave-induced oscillatory flow and its interaction with the seabed.

WP2
Surface waves in the coastal zone induce oscillatory flow motions in the vicinity of the seabed. These motions interact with the bed sediment and modify the bed shape by generating coherent structures, which are generally known as ripples. The presence of ripples in oscillatory flows is important due to the dramatic effect they have on the seabed roughness. The bed roughness directly affects the near-bed boundary layer hydrodynamics, which in turn controls sediment transport in coastal areas. Consequently, accurate prediction of sediment transport rates is an important element in morphological studies in coastal marine environments. In the present study, coupled simulations of oscillatory flow and sediment transport (bed and suspended) over a rippled bottom, are performed. Another aim of this study was the parallelization of the numerical models. The computational cost of such simulations is particularly important due to the extremely large number of computing nodes. When parallelizing a serial numerical model, simultaneous execution of code sections from multiple processors is achieved. This results in the significant reduction of the computational cost of the simulations.

WP3
In the present study wave propagation and breaking was simulated for three different bottom configurations and two different incoming wave directions. Specifically the case of a constant beach slope of 1/35, a constant beach slope of 1/15 and a beach with varying beach slope between 1/15 at deep water and 1/12 at the shoreline with the Larson profile (1988) were examined. In addition, the cases of vertical incoming waves on the shoreline and propagation of incoming waves with 30^ο between the wave direction and the perpendicular to the shoreline were examined. The urgency of addressing with such flows comes from the fact that the most important processes taking place in the surf zone are the wave breaking and the generation of wave currents. The two main types of currents are the longshore current, which occurs when the propagating direction of the braking wave is slanted relative to the shoreline, and the current that is perpendicular to the shoreline, which is known as the undertow current.

WP4
The flow which is developed at the inner coastal zone due to wave propagation and wave breaking mechanisms, has been and still is the objective of several research studies, as it affects significantly the stability of harbor and coastal structures, as well as the alongshore and cross shore sediment transport. In the present study a series of experimental measurements of that flow was performed, over bottoms of different slope and roughness, and particularly over a rough steep bottom with slope 1/3 and a smooth mild bottom with slope 1/15. The results include detailed measurements of the velocity field, as well as surface elevation timeseries. The experimental data were further utilized for extracting period- and phase-averaged mean and r.m.s. statistics of the flow and to draw conclusions about the turbulence, respectively.

Researchers

• Athanassios A. Dimas, Professor
• Gerasimos (Makis) Kolokythas, Postdoctoral Researcher
• Konstantina (Nantia) Galani, PhD Candidate
• Theofano (Fani) Koutrouveli, PhD Candidate
• Giorgos Leftheriotis, PhD Candidate
• Efstratios (Stratos) Fonias, PhD Candidate

Publications

1. Galani, K.A., Dimou, G.D., Karageorgopoulos, E.G. and Dimas, A.A., 2013. PIV Measurements of Turbulent Flow Induced by Waves Above a Rock-Armored Slope. Proc. Coastal Dynamics 2013, Arcachon, France.
2. Γαλάνη, Κ.Α., Δήμου, Ι.Δ., Καραγεωργόπουλος, Ε.Γ. και Δήμας, Α.Α., 2013. Μετρήσεις Τυρβώδους Ροής Υπεράνω Πρανούς Ογκολίθων Εργαστηριακού Ομοιώματος Κυματοθραύστη με τη Μέθοδο Ταχυμετρίας μέσω Απεικόνισης Σωματιδίων (PIV). Πρακτικά Έκτο Πανελλήνιο Συνέδριο Λιμενικών Έργων, 23-32, Αθήνα.
3. Φονιάς, Ε.Ν. και Δήμας Α.Α., 2013. Αριθμητική Προσομοίωση Παλλόμενης Ροής Γύρω από Υποβρύχιο Αγωγό Κοντά σε Πυθμένα με τη Μέθοδο του Εμβαπτισμένου Ορίου. Πρακτικά Έκτο Πανελλήνιο Συνέδριο Λιμενικών Έργων, 43-52, Αθήνα.
4. Κουτρουβέλη, Θ.Ι. και Δήμας Α.Α., 2013. Αριθμητική Προσομοίωση Ροών με Ελεύθερη Επιφάνεια κατά τη Διάδοση Κυμάτων μέσω της Μεθόδου Εμβαπτισμένου Ορίου. Πρακτικά Έκτο Πανελλήνιο Συνέδριο Λιμενικών Έργων, 33-42, Αθήνα.
5. Fonias, E.N., and Dimas, A.A., 2014. IMMERSED BOUNDARY METHOD FOR SIMULATION OF OSCILLATORY FLOW PAST A SUBMERGED CYLINDER CLOSE TO A HORIZONTAL BED. Proc. 2014 International Conference on Ocean, Offshore and Arctic Engineering (OMAE), OMAE2014-23986, June 8-13, San Francisco, California.
6. Koutrouveli, Th.I., and Dimas, A.A., 2014. NUMERICAL SIMULATION OF WAVE PROPAGATION OVER SUBMERGED COMPOSITE BREAKWATERS USING THE IMMERSED BOUNDARY METHOD. Proc. 2014 International Conference on Ocean, Offshore and Arctic Engineering (OMAE), OMAE2014-24055, June 8-13, San Francisco, California.
7. Galani, K.A., Dimou, G.D., and Dimas, A.A., 2014. EXPERIMENTAL STUDY OF TURBULENT FLOW INDUCED BY REGULAR AND IRREGULAR WAVES ABOVE A ROCK-ARMORED SLOPE. Proc. 2014 International Conference on Ocean, Offshore and Arctic Engineering (OMAE), OMAE2014-23993, June 8-13, San Francisco, California.
8. Leftheriotis, G.A., and Dimas, A.A., 2014. COUPLED SIMULATION OF OSCILLATORY FLOW, SEDIMENT TRANSPORT AND MORPHOLOGY EVOLUTION OF RIPPLES BASED ON THE IMMERSED BOUNDARY METHOD. Proc. 2014 International Conference on Ocean, Offshore and Arctic Engineering (OMAE), OMAE2014-24006, June 8-13, San Francisco, California.
9. Leftheriotis, G.A. and Dimas, A.A., 2014. Coupled Numerical Simulation of Flow, Sediment Transport and Morphology Evolution of Dunes based on the Immersed Boundary Method. Proc. River Flow 2014, Lausanne, Switzerland.
10. Kolokythas, G.A., Leftheriotis, G.A. and Dimas A.A., 2014. Numerical Simulation of Coastal Flow and Sediment Transport Over Rippled Beds. ERCOFTAC Bulletin 100, 1-9.