Analysis of the Kelvin-Helmholtz instability in seagrass-oscillatory flow interaction

The aquatic vegetation is ubiquitous worldwide, from low temperature areas to tropical shallow coastal zones (Van Der Heide et al., 2007). These ecosystems provide a productive life cyle being the habitat for many marine animal species (Marion et al., 2014). At the coastal region, the aquatic vegetation (seagrass) attenuates the currents, dissipates the wave height and stabilises the coastlines (Maxwell et al., 2017; Pinsky et al., 2013) being considered as a soft system to reduce the risk of ooding and erosion under sea level rise and extreme wave events (Ondiviela et al., 2014). Based on the above described services, estimations of economical annual values provided by the aquatic vegetations is over $10 trillion (Nepf, 2013). Control _eld studies such as those by Schanz et al. (2002); Schanz and Asmus (2003) demonstrate the interdependence between the hydrodynamic and biological processes, since there is a cascading e_ect in which for a speci_c ow conditions, di_erent species can live and proliferate within the seagrass meadows or be washed away, depending on the energetic wave conditions and seagrass density. Regarding the interaction between the submerged seagrass and the surrounding ow, it is well known that the presence of the seagrass canopies attenuate the momentum by the work done on the ow by the stems (Finnigan, 2000). The e_ects in the velocity _eld can be di_erentiated and thus studied in terms of scales; 1) processes with spatial scales of the order of the stem diameter or spacing between stems; and 2) processes with scales of the order of the drag length scale. The turbulent structures at the scale of the stems are called wake scales and are produced by the shadow zone downstream the stems (Nepf, 2012; Zhang et al., 2018). Turbulent processes at the drag length scale are governed by the density of the canopy and the ow dynamics (Nepf, 2012). These processes modulate the water renewal between the water inside and above the canopy and the amount of suspended sediments along the water column (Luhar and Nepf, 2013). The turbulent processes at the drag length scale can be analysed as a plane mixing layer by two co-owing streams that present a shear layer at the top of the canopy (Raupach et al., 1996). This shear-layer-ow is characterized by an inexion point in the velocity pro_le (two water bodies moving at di_erent velocities), responsable for the vertical mass exchange at the top of the canopy (Ghisalberti and Nepf, 2009). The shear layer facilitates the generation of Kelvin-Helmholtz instability type vortex (Ghisalberti and Nepf, 2002). The Kelvin-Helmholtz type vortex has been widely studied in steady ows (Raupach et al., 1996; Finnigan, 2000; Ghisalberti and Nepf, 2002; Nepf, 2012; Mandel et al., 2017), characterizing its e_ect on the seagrass movement, the Reynolds stresses, the sediment distribution, the vertical mixing and the free surface. However, the formation and e_ect of Kelvin-Helmholtz type vortices in the wave oscillatory ow is still far to be completly understood. Indeed, it is still not clear which are the dominant terms in the Navier-Stokes equations for the oscillatory-seagrass-ow interaction. Ghisalberti and Schlosser (2013) reported some \`necessary" conditions in the ow in order to produce Kelvin-Helmholtz instabilities; Abdolahpour et al. (2017) analysed a steady current released by the presence of the shear layer and its relation with the shear layer magnitude and Abdolahpour et al. (2018) used the seagrass-steady-ow interaction formulation of Ghisalberti and Nepf (2002) to estimate the Kelvin-Helmholtz frequency range in oscillatory ows. The evolution of vortices downstream submerged structures in oscillatory dominant ows is assumed to be dissipated by the viscosity and the e_ects on the wave breaking process have not been yet analysed. Indeed, a theoretical model to solve the Kelvin- Helmholtz instability modes as a function of the free surface and a general characterization of the turbulent spectra is an open question that will provide new insights in order to improve models and simulations of relevant hydrodynamic processes at coastal scale. The aim of this Thesis is to understand the relation between the free surface frequency and the Kelvin-Helmtholtz instability modes in seagrass-oscillatory-ow interaction. For this, I will _rst analyze the e_ects of a vortex by an isolated submerged stem interacting with a surface wave. Then, I develop an analytical model to determine the dominant terms in the momentum equation in seagrass-oscillatory-ow interaction. Finally I close the scienti_c question by solving the Kelvin-Helmholtz instability modes in seagrass-oscillatory ow interaction as a function of the free surface wave applying the Piecewise method to a simpli_ed velocity pro_le. This thesis is structured as follows. Chapter 2 analyses the e_ects of backwards wave breaking process induced by a strong transport of mass in a vortex produced by an isolated submerged stem. In chapter 3, a simpli_ed seagrass-oscillatory-ow model is developed by dimensional analysis of the Navier-Stokes equation. Here, some reference variables are de_ned according to the free surface wave parameters . The vali dation of the simpli_ed model is performed against experimental data from a ap type wavemaker system and a random seagrass distribution. Finally, chapter 4 presents a theoretical model for the Kelvin-Helmholtz instability modes as a function of the incoming free surface wave. The model applies the piecewise linear method to the Rayleigh's equation in an ideliazed vertical velocity pro_le. It is important to remark that this thesis is composed by three papers: Chapter 2 has been published in the Ocean Engineering Journal, Chapter 3 is in _nal revisión for the Experiments in Fluids Journal and Chapter 4 is under review for the Journal of Fluid Mechanics.Doctorad

Modelamiento de oleaje a escala de laboratorio enfoque numérico y físico

En el campo de la ingeniería, enfocada a la hidrodinámica marina, existen tres líneas base para el entendimiento de los procesos físicos que ocurren en el océano. Estas líneas son las mediciones en campo y posterior análisis y conclusiones de los datos medidos; experimentación en laboratorio, lo que permite recrear ciertos escenarios simplificados de la realidad a escala reducida; y la modelación numérica, que usa las ecuaciones de gobierno de los fluidos o las teorías de onda para simular eventos presentes en la naturaleza (Hughes, 1993). Los estudios o mediciones de campo arrojan los mejores resultados, pero es más costoso obtenerlos y existen muchas variables de la naturaleza presentes en las mediciones, lo que hace que sea más complejo el entendimiento de los datos recolectados y la interacción entre los procesos físicos ocurrentes. A diferencia de los estudios hechos en campo, la modelación física (laboratorio), es un poco menos compleja, ya que los experimentos a estudiar pueden ser controlados y el escenario de estudio simplificado, para lograr un mayor entendimiento de los procesos que ocurren en el fenómeno en cuestión, sin dejar de lado la física base del problema (Hughes, 1993). La modelación matemática sirve como una herramienta para replicar los fenómenos que ocurren en la naturaleza, tanto a escala real como reducida; es una herramienta económica, debido a que la creación de los escenarios de estudio son diseñados en computador y no se necesita de infraestructura física para realizarlos; además ni en campo ni en el laboratorio es posible medir todas las variables deseadas, debido a que no se cuenta con los instrumentos requeridos o su capacidad no es la suficiente para obtener datos de una variable en particular a una escala espacio temporal dada. Es importante resaltar que en la modelación matemática se pueden obtener aspectos globales de los fenómenos de interés en toda la región de estudio, mientras que en campo se mide u observa en puntos definidos. En laboratorio se podrían obtener las mediciones requeridas en todo el dominio, pero se necesita una gran cantidad de instrumentos o repetir los ensayos sin hacer cambios a la configuración de los mismos hasta tener los datos deseados, lo cual es poco eficiente. Si bien la herramienta numérica es muy potente y útil, su desmesurado uso puede no traer buenos resultados, ya que muchos de los fenómenos en la naturaleza no son bien entendidos aún y se necesita acoplar los resultados numéricos con datos de campo o con experimentación física para refinar y mejorar lo obtenido por el método matemático. Como los estudios de campo son muy costosos, es allí donde la modelación física juega un papel importante en el entendimiento de un fenómeno en particular y sirve como proyección del comportamiento del prototipo permitiendo obtener observaciones que no son fáciles de realizar en la naturaleza (Hughes, 1993). A este tipo de modelaciones se les llama híbridos, ya que usa la componente experimental enlazada con la matemática, tomando los resultados obtenidos en laboratorio en casos complejos como datos de entrada o condiciones de contorno para un exhaustivo modelado numérico. Alternativamente, los resultados de la modelación numérica pueden ser útiles, si se está seguro de que los resultados son cercanos a la realidad, para alimentar las condiciones iniciales de un modelo físico o sus condiciones de contorno (Hughes, 1993).Maestrí

On the Observed Wind-Driven Circulation Response in Small Semienclosed Bays

International audienceAbstract This study analyzes horizontal and vertical wind-driven circulation responses in small semienclosed bays, the associated offshore dynamic conditions, and the relative importance of each term in the momentum balance equations using a multiplatform observational system. The observational platform consists of three ADCPs and a land-based radar monitoring the velocity field within the bay and in the contiguous offshore area. The wind-driven patterns in the bay can switch from a barotropic cyclonic or anticyclonic circulation to a two-layer baroclinic mode response as a function of the wind regime (its direction and magnitude). For the baroclinic mode, the vertical location of the inflection point in the velocity profile can vary according to the proximity of the boundary current to the entrance of the bay. The influence of offshore combined meteorological and marine conditions on the inner-bay dynamics is evidenced under moderate to strong wind conditions and is almost nonexistent under negligible wind. The momentum balance analysis as well as the nondimensional numbers evidence the impact of wind stress, coastline shape, stratification, and the nonlinear advective terms. Advection can be at the same order of magnitude as pressure gradient, Coriolis, or wind stress terms and can be greater than the bottom stress terms. The nonlinear terms in the momentum equations are frequently neglected when analyzing wind-driven circulation by means of in situ data or analytical models

Car-EWaves: Extreme waves in the Caribbean Sea from 1958 to 2017

Maximum monthly aggregate value of significant wave height for the Caribbean Sea from 1958 to 2017. The mean wave period, mean wave direction and the wind velocity (U10 and V10) related to the date of the maximum aggregation significant wave height value, are included in this dataset.Peer reviewe

Metodología para el cálculo de variables hidrodinámicas en canales de oleaje usando técnicas de video.

El uso de sistemas de video para el cálculo de diferentes fenómenos físicos, es una herramienta que ha mostrado ser efectiva y exacta, tanto en estudios de laboratorio como de campo de zonas costeras. Este trabajo se enfocó en la detección de la superficie libre de un fluido de forma no invasiva (fuera del movimiento del fluido a estudiar) en laboratorio, buscando encontrar resultados comparables con sensores de tipo intrusivos; usando modelos matemáticos de segmentación de imágenes (SOBEL), para la búsqueda de los cambios fuertes en la intensidad de los píxeles (gradientes), y al tratar las imágenes como superficies en forma de funciones discretas. Al seleccionar dicha derivada como la superficie libre, se encuentran coordenadas UV asociadas a un punto de la imagen. Además usando un modelo de rectificación, se puede asociar dichos puntos a las coordenadas reales, Osorio, A. F., et al. (2008), referenciadas a un cero inicialmente fijado. Los resultados encontrados muestran muy buena similitud con las mediciones de los sistemas de tipo intrusivo

Extreme waves in the Caribbean Sea: spatial regionalization and long-term analysis

The extreme wave height distribution in the Caribbean Sea is studied using a new method based on the maximum basin-wide aggregate of significant wave height, Hs, values per month. Besides, by means of the Self-Organizing Maps (SOM) technique, we identify coherent geographical regions with similar extreme wave height variability in the Caribbean Sea. Our findings revealed three primary regions: the eastern side with comparatively lower values, the central region with intermediate values, and the western side with the highest extreme wave heights. The study also examines the wind forcing conditions driving the spatial and temporal variability of the extreme waves, highlighting the influence of the low-pressure belt dynamics as well as the role played by the Caribbean Low-Level Jet (CLLJ) index, and the impact of cold fronts and hurricanes on extreme wave heights. Additionally, we explore the relationship between the extreme wave height distribution and climatic indices, such as the Atlantic Multidecadal Oscillation (AMO), the Atlantic Meridional Mode (AMM), the North Atlantic Oscillation (NAO) and the Oceanic Niño (ONI). The results reveal that the spatial distribution of extreme wave heights in the Caribbean Sea is mostly ruled by the influence of the CLLJ, with correlations close to 80%. In addition, significant correlations were observed between the extreme wave heights and the ENSO in the central Caribbean, as well as positive correlations between the extreme wave heights and NAO in the eastern part of the basin, and significant values of correlation with the negative phases of AMO and AMM in the whole basin. We show that, unlike conventional (or broadly used) methods deployed to identify extreme wave height, such as percentile 99th, Hs99, our methodology allows a further assessment of the wind and climate forcing conditions associated with the extreme wave events. Although, we acknowledge that the method here presented has limitations to capture extreme wave height outliers, it has the advantage of being used concomitant with the wind forcing to develop multivariate wave climate analysis at basin scale, and could be extended to a more local scale when studying coastal processes

Breakdown of Near-Surface Sea Current from High-Frequency Radar Data

International audienceAbstract To assess the contribution of wind drag and Stokes drift on the near-surface circulation, a methodology to isolate the geostrophic surface current from high-frequency radar data is developed. The methodology performs a joint analysis utilizing wind field and in situ surface currents along with an unsupervised neuronal network. The isolation method seems robust in the light of comparisons with satellite altimeter data, presenting a similar time variability and providing more spatial detail of the currents in the coastal region. Results show that the wind-induced current is around 2.1% the wind speed and deflected from the wind direction in the range [18°, 23°], whereas classical literature suggests higher values. The wave-induced currents can represent more than 13% of the ageostrophic current component as function of the wind speed, suggesting that the Stokes drift needs to be analyzed as an independent term when studying surface sea currents in the coastal zones. The methodology and results presented here could be extended worldwide, as complementary information to improve satellite-derived surface currents in the coastal regions by including the local physical processes recorded by high-frequency radar systems. The assessment of the wave and wind-induced currents have important applications on Lagrangian transport studies

Backwards wave breaking by flow separation vortices under solitary waves

In this work, the backward wave breaking process by the presence of flow separation vortices under a solitary wave is studied. Based on a set of non-dimensional variables defined from the Buckingham Π theorem, a set of numerical experiments are performed in order to analyze the effect of varying the submerged obstacle geometry on the surrounding flow and free surface by using a RANS-VOF model. Model simulations are tested against available experiments showing a good performance of numerical results. The structure-submergence ratio and the structure-based Reynolds number modulate the type of breaking (collapsing-plunging). The structure aspect ratio defines the location and number of backward breaking points for rectangular structures and the breaking wave direction for triangular structures (either forward or backward). Moreover, when the flow separation vortex diameter is comparable to the local water depth a backward wave breaking process is originated. The vortex behaves as a rigid body that accelerates the flow at the upper side of the vortex, accumulating mass at the downstream side where a very large slope on the free surface makes the water fall backward due to gravity.A. Cáceres-Euse wants to thank COLCIENCIAS for the scholarship #647. A. Orfila thanks financial support from LAMARCA Project (PID2021-123352OB-C31) funded by MCIN/AEI/ and by “ERDF A way of making Europe”. I. Hernández-Carrasco is supported by the TRITOP Project (UIB2021-PD06) funded by the Universidad de las Islas Baleares – FEDER (UE) . M. Wyssmann thanks support by the Chancellor’s Fellowship awarded by the University of Tennessee at Knoxvill

Coastal HF radars in the Mediterranean: status of operations and a framework for future development

Abstract. Due to the semi-enclosed nature of the Mediterranean Sea, natural disasters and anthropogenic activities impose stronger pressures on its coastal ecosystems than in any other sea of the world. With the aim of responding adequately to science priorities and societal challenges, littoral waters must be effectively monitored with High-Frequency radar (HFR) systems. This land-based remote sensing technology can provide, in near real-time, fine-resolution maps of the surface circulation over broad coastal areas, along with reliable directional wave and wind information. The main goal of this work is to showcase the current status of the Mediterranean HFR network and the future roadmap for orchestrated actions. Ongoing collaborative efforts and recent progress of this regional alliance are not only described but also connected with other European initiatives and global frameworks, highlighting the advantages of this cost-effective instrument for the multi-parameter monitoring of the sea state. Coordinated endeavours between HFR operators from different multi-disciplinary institutions are mandatory to reach a mature stage at both national and regional levels, striving to: i) harmonize deployment and maintenance practices; ii) standardize data, metadata and quality control procedures; iii) centralize data management, visualization and access platforms; iv) develop practical applications of societal benefit, that can be used for strategic planning and informed decision-making in the Mediterranean marine environment. Such fit-for-purpose applications can serve for search and rescue operations, safe vessel navigation, tracking of marine pollutants, the monitoring of extreme events or the investigation of transport processes and the connectivity between offshore waters and coastal ecosystems. Finally, future prospects within the Mediterranean framework are discussed along with a wealth of socio-economic, technical and scientific challenges to be faced during the implementation of this integrated HFR regional network