Vortex induced vibration (VIV) is an important problem for offshore structures, where flow-induced vibrations can lead to fatigue damage. Prediction of the coupled forces and motions from VIV are critical in estimating fatigue for design purposes and also for estimating potential motions in offshore operations. Recent work has demonstrated the importance of considering combined in-line and cross-flow motions in prediction, however semi-empirical prediction methods that rely on force coefficient databases are difficult to implement with combined in-line and cross-flow motions due to the sheer number of experiments required to cover the relevant motion parameter space. This paper builds on previous forced motion experimental work to construct a force coefficient database based on combined in-line and cross-flow motions of a cylinder in a free stream. Hydrodynamic forces are measured over a wide range of normalized in-line and cross-flow amplitude, reduced velocity, and phasing between the in-line and cross-flow motion, while Reynolds number is held constant at a value of 7620. In-line and cross-flow motions are prescribed to be sinusoidal in order to limit the number of variable experimental parameters. Decomposed forces such as added mass and lift in phase with velocity are mapped over the range of parameters to investigate how these quantities change as a function of the combined motion. One interesting finding that was not previously recognized in earlier experiments of this type is the presence of large mean lift forces over some values of motion amplitude and phasing between in-line and cross-flow motion. In some cases, the mean lift is very large and comparable to oscillating lift forces at other motion parameter combinations. The presence of a mean lift is significant as it implies an asymmetry to the wake that may occur due to forced motions. On long, slender structures, if this type of forced motion were to occur at locations along the length of the structure, this could lead to kiting or mean deflections of the structure perpendicular to the direction of the flow.
Gedikli, Ersegun Deniz (Norwegian University of Science and Technology (NTNU), University of Rhode Island) | Chelidze, David (University of Rhode Island) | Dahl, Jason M. (University of Rhode Island)
The dynamics of offshore structures resemble to the dynamics of long flexible cylinders. However, the complexity of flexible body interactions have pushed industry to rely on the dynamics of rigid cylinders ignoring the effect of spatial response of such structures completely. In this work, we aim to clarify the dynamics of a flexible cylinder that undergoes vortex-induced vibrations in a systematically designed experiment to see the effect of structural mode shape on the excitations. In the experiments, we test three bending dominated flexible cylinders and compare the results with a tension dominated flexible cylinder under uniform flow conditions. We study the nonlinear modal interactions by means of analyzing the spatial response using a multivariate analysis technique called generalized smooth orthogonal decomposition. Using this technique, we show that a flexible cylinder is unable to oscillate with an even mode excitation in the in-line direction, although it has second mode frequency characteristics. It is also shown that a mode switch in the in-line direction is highly dependent on the cross-flow motion where for a possible mode switch in the in-line direction, a mode switch in the cross-flow direction is necessary. In other words, a cylinder cannot oscillate with higher modes in the in-line direction keeping the cross-flow shape constant. This shows the inherent coupled response of in-line and cross-flow motions.
Technology, law, and world's appetite for more energy pushed oil and gas exploration farther from the shores. This need—moving into deeper waters—brought extra challenges with it, for example, offshore structures inherently became more prone to environmental loads that may easily lead to nonlinear responses. Therefore, understanding the nonlinear interactions of such structures became more important than ever.
Vortex-induced vibration (VIV) is an inherent problem seen in offshore structures where coupled fluid-structure interaction may lead large structural motions that can have a significant effect on structural fatigue and offshore operations. Majority of previous research initially focused on understanding the response of a flexibly mounted rigid cylinder that is allowed to move only in cross-flow (CF) direction (also known as one-degree-of-freedom (1-DOF) response), and later concentrated on understanding combined in-line (IL) and CF responses (also known as two-degrees-of-freedom (2-DOF) response). Papers by Bearman (1984), Sarpkaya (2004), Williamson and Govardhan (2004) give insights into 1-DOF cylinder dynamics, and papers by Jauvtis and Williamson (2004), Dahl et al. (2006), etc. illustrate how a rigid cylinder behaves in 2-DOF VIV system.
Guérin, Charles-Antoine (Université de Toulon) | Grilli, Stéphan T. (University of Rhode Island) | Moran, Patrick (University of Rhode Island) | Grilli, Annette (University of Rhode Island) | Insua, Tania (Ocean Networks Canada (ONC))
A High-Frequency (HF) radar was installed by Ocean Networks Canada in Tofino, BC, to detect tsunamis from far- and near-field sources on the Pacific Ocean side of Vancouver Island; in particular, from seismic sources in the Cascadia Subduction Zone. Based on a classical analysis of the Doppler spectrum, this HF radar can measure ocean surface currents up to a 85-110 km range depending on sea state. However, an inherent limitation of detection of small and short-lived tsunami currents is the conflicting requirement for short integration time and sufficient accuracy (resolution) of the Doppler spectra. This limits a direct tsunami detection typically to shallow water areas over the continental shelf where tsunami currents have become sufficiently strong due to wave shoaling.
To overcome this limitation, the authors have recently proposed a new detection method, referred to as “Time-Correlation Algorithm (TCA)”, that does not require inverting Doppler spectra for the tsunami currents and can thus potentially detect an approaching tsunami in deeper water, beyond the continental shelf. This algorithm is based on computing space-time correlation of the raw radar signal in different radar cells aligned along precomputed tsunami wave rays, and time-shifted by the precomputed tsunami propagation time between cells. A change in pattern of such correlations indicates the presence of a tsunami. They validated the TCA with numerical simulations for both idealized (Grilli et al., 2016a) and realistic (Grilli et al., 2016b, 2017) tsunami wave trains and seafloor bathymetry, using data simulated with a radar simulator.
Here, the TCA is for the first time applied to actual radar data measured with the ONC WERA HF radar and numerically modified by a synthetic tsunami current. Using a state-of-the-art long wave model we perform tsunami simulations with realistic source and bathymetry, and combine the resulting currents with the background currents and radar backscattered signal measured by the HF radar system. This combination makes it possible to evaluate the performance of the proposed TCA detection algorithm, based on an experimental rather than numerically simulated, data set of radar signal. Our findings confirm that an actual detection can be achieved beyond the continental shelf, where tsunami currents are small (as low as 5 cm/s), in deeper water than when using an algorithm based on a direct inversion of currents from the measured radar Doppler spectra.
O'Reilly, C. M. (University of Rhode Island, Navatek Ltd.) | Grilli, S. T. (University of Rhode Island) | Harris, J. C. (Université Paris-Est) | Mivehchi, A. (University of Rhode Island) | Janssen, C. F. (Hamburg University of Technology (TUHH)) | Dahl, J. M. (University of Rhode Island)
We report on recent progress and validation of a 3D hybrid model for naval hydrodynamics problems based on a perturbation method, in which both velocity and pressure are expressed as the sum of an inviscid flow with a viscous perturbation. The far- to near-field inviscid flows can be solved with a Boundary Element Method (BEM), based on fully nonlinear potential flow theory, and the near-field perturbation flow is solved with a NS model based on a Lattice Boltzmann Method (LBM) with a Large Eddy Simulation (LES) of the turbulence. We summarize the hybrid model formulation and latest developments regarding the LES, and particularly a new wall model for the viscous/turbulent sub-layer near solid boundaries, that is generalized for an arbitrary geometry. The latter are validated by simulating turbulent flows over a flat plate for Re ∈ [3.7 × 104; 1.2 × 106], for which the friction coefficient computed on the plate agrees well with experiments. We then simulate the flow past a NACA0012 foil using the hybrid LBM-LES with the wall model, for Re = 1 × 106, and show a good agreement of lift and drag forces with experiments. Results obtained with the hybrid LBM model are either nearly identical or improved relative to those of the standard LBM, but for a smaller computational domain, demonstrating the benefits of the hybrid approach.
The simulation of large ship motions and resistance in steep waves is typically performed using linear or (more rarely) nonlinear potential flow solvers, usually based on a higher-order Boundary Element Method (BEM), with semi-empirical corrections introduced to account for viscous/turbulent effects. However in some cases, viscous/turbulent flows near the ship's hull, and breaking waves and wakes must be accurately modeled to capture the salient physics. Navier-Stokes (NS) solvers can and have been used to model such flows, but they are computationally expensive and often too numerically dissipative to model wave propagation over long distances.
Mivehchi, Amin (University of Rhode Island) | Harris, Jeffrey C. (University of Paris-Est) | Grilli, Stephan T. (University of Rhode Island) | Dahl, Jason M. (University of Rhode Island) | O'Reilly, Chris M. (University of Rhode Island, Navatek Ltd.) | Kuznetsov, Konstantin (University of Paris-Est) | Janssen, Christian F. (Hamburg University of Technology (TUHH))
We report on recent developments of a 3D hybrid model for naval hydrodynamics based on a perturbation method, in which velocity and pressure are decomposed as the sum of an inviscid flow and a viscous perturbation. The far- to near-field inviscid flows are solved with a Boundary Element Method (BEM), based on fully nonlinear potential flow theory, accelerated with a fast multipole method (FMM), and the near-field perturbation flow is solved with a Navier-Stokes (NS) model based on a Lattice Boltzmann Method (LBM) with a LES modeling of turbulent properties. The BEM model is efficiently parallelized on CPU clusters and the LBM model on massively parallel GPGPU co-processors.
The hybrid model formulation and its latest developments and implementation, in particular, regarding the improvement and validation of the model for naval hydrodynamics applications, are presented in a companion paper by O'Reilly et. al (2017), in this conference. In this paper, we concentrate on the BEM model aspects and show that the BEM-FMM can accurately solve a variety of problems while providing a nearly linear scaling with the number of unknowns (up to millions of nodes) and a speed-up with the number of processors of 35-50%, for small (e.g., 24 cores) to large (e.g., hundreds of cores) CPU clusters.
The simulation of the dynamic response of maritime structures in waves and wave-induced forces is typically based on linear wave models, such as AEGIR (Kring et al.,1999), or in case of large motions and/or steep waves, on using nonlinear wave models based on potential flow theory (PFT), usually solved with a higher-order Boundary element method (BEM). For structures with a forward speed, semi-empirical corrections are often made to account for viscous/turbulent effects in the total resistance. While standard Computational Fluid Dynamics (CFD) models based on the full Navier-Stokes (NS) equations can also be used to simulate such problems, their computational cost is typically too prohibitive and their accuracy for long-term wave modeling usually less than that of PFT-BEM models. However, in some cases, the viscous/turbulent flow around the structure's hull and possible breaking waves and wakes require to be more accurately modeled to capture the salient physics of the problem.
Torres, Marissa J. (University of Rhode Island) | Hashemi, M. Reza (University of Rhode Island) | Hayward, Scott (University of Rhode Island) | Ginis, Isaac (University of Rhode Island) | Spaulding, Malcolm (University of Rhode Island)
A coupled ADCIRC+SWAN model was forced with parametric and global wind products from NHC and ECMWF databases, respectively, to simulate and examine the storm surge responses of Hurricanes Bob (1991) and Irene (2011) in Rhode Island coastal waters. Comparisons with NOAA, USGS, and USACE observation/hindcast data showed that the location of the hurricane track in relation to the study area is paramount when determining which wind product should be used in storm surge simulations. Parametric winds better predict storm surge at locations within the RMW, where global winds lack horizontal resolution, and vice versa for locations outside the RMW.
New England is not a frequent location for land-falling hurricanes, though it has weathered its share of storms over the past several decades. Tropical cyclones developing in the south-western Atlantic typically travel northward along the East Coast and swirl out into the Atlantic without reaching New England. However, the storms that have made landfall in New England since the 1900s have caused moderate to severe damage. The most notable of which was the Great Hurricane of 1938 that came without warning and produced in excess of 4 meters of storm surge in some areas, and recent Hurricane Sandy in 2012 which led to major economic loss in this region.
Climate scientists have been studying the frequency and intensity of hurricanes over time and space, and have developed global and regional climate models to better predict the characteristics of future hurricanes. In parallel, similar efforts have been made, by ocean scientists/engineers, to predict storm surge and waves generated by these storms. The primary model used by the National Weather Service for predicting storm surge due to hurricanes is the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model. A popular tool for numerical simulation of storm surge is the ADvanced CIRCulation (ADCIRC) model, which provides the solution over a flexible, unstructured computational domain. In terms of waves which are generated during hurricanes, Simulating WAves Nearshore (SWAN) is a popular spectral wave model that solves the spectral action balance equation, and is coupled with ADCIRC for use in this study. With regard to wind forcing of tropical storms, ADCIRC has a range of options, including the Holland parametric wind model (Holland, 1980) to compute wind velocities at each node, and using wind field data (wind velocity and surface pressure) over a regular grid, where ADCIRC will interpolate this forcing over the domain.
Harris, Jeffrey C. (Université Paris-Est) | Kuznetsov, Konstantin (Université Paris-Est) | Peyrard, Christophe (Université Paris-Est, EDF R&D) | Saviot, Sylvain (EDF R&D) | Mivehchi, Amin (University of Rhode Island) | Grilli, Stephan T. (University of Rhode Island) | Benoit, Michel (Aix-Marseille Univ.)
We report on recent developments of a three-dimensional (3D) model for wave propagation and wave-structure interaction. The velocity field is solved with a boundary element method (BEM), based on fully non-linear potential flow. This approach is efficiently parallelized on CPU clusters. Recent progress is presented for extending the model for the use of higher-order elements (i.e., cubic B-splines), and outline the future steps necessary to a high-order approach on completely arbitrary meshes necessary for complex industrial applications. Particular care is taken with regards to the corner compatibility condition along the intersection between the body and free-surface, which is necessary for high-accuracy modeling with the BEM approach. Applications are shown for academic tests as well as for the computation of wave-induced forces and moments on gravity-based foundations, where we compare numerical results against laboratory experiments. Such applications are of interest to the continued development of foundations for offshore wind farms, and extensions to this model are being implemented for simulating floating structures and coupling to other models including viscous effects, which can be important in some cases.
A large variety of ocean wave models have been applied to investigate wave-structure interaction; ever since the work of Longuet-Higgins and Cokelet (1976), the boundary integral approach based on potential flow theory has shown some interesting advantages, particularly as the calculations are only performed on the surfaces and not the interior of the domain. In the models, different ways to handle the free-surface have been proposed, both in frequency and time-domain, but in some cases where fully nonlinear effects are important, the standard approach has been to solve Laplace's equation for the velocity potential (mass conservation) at each time step (optionally multiple times, or for the time-derivative of the velocity potential), then updating the BEM mesh nodes and free surface boundary conditions with a mixed Eulerian-Lagrangian (MEL) approach. Tanizawa (2000) made a review to date of this technique.
Grilli, Stéphan T. (University of Rhode Island) | Shelby, Michael (University of Rhode Island) | Grilli, Annette (University of Rhode Island) | Gúerin, Charles-Antoine (Université de Toulon, CNRS, Aix Marseille Université) | Grosdidier, Samuel (Diginext Ltd.) | Insua, Tania (Ocean Networks Canada (ONC))
A shore-based High-Frequency (HF) WERA radar was recently installed by Ocean Networks Canada (ONC) near Tofino, British Columbia (Canada), to mitigate the elevated tsunami hazard along the shores of Vancouver Island, from both far- and near-field seismic sources and, in particular, from the Cascadia Subduction Zone (CSZ). With this HF radar, ocean currents can be measured up to a 70-85 km range, depending on atmospheric conditions, based on the Doppler shift they cause in ocean waves at the radar Bragg frequency. In earlier work, the authors (and others) have shown that tsunami currents need to be at least 0.15-0.20 m/s to be reliably detectable by HF radar, when considering environmental noise and background currents (from tide and mesoscale circulation). This would limit the direct detection of tsunami-induced currents to shallow water areas where they are sufficiently strong due to wave shoaling and, hence, to the continental shelf. It follows that, in locations with a narrow shelf, warning times based on such a tsunami detection method may be small.
To detect tsunamis in deeper water, beyond the shelf, the authors have proposed a new algorithm that does not require “inverting” currents, but instead is based on spatial correlations of the raw radar signal at two distant locations/cells located along the same wave ray, time shifted by the tsunami propagation time along the ray. A pattern change in these correlations indicates the presence of a tsunami. They validated this algorithm for idealized tsunami wave trains propagating over a simple seafloor geometry in a direction normally incident to shore. Here, this algorithm is further developed, extended, and validated for realistic case studies conducted for seismic tsunami sources and using the bathymetry, offshore of Vancouver Island, BC. Tsunami currents, computed with a state-of the- art long wave model, are spatially averaged over cells aligned along individual wave rays, within the radar sweep area, obtained by solving the wave geometric optic equation. A model simulating ONC radar’s backscattered signal in space and time, as a function of the simulated tsunami currents, is applied on the Pacific Ocean side of Vancouver Island. Numerical experiments are performed, showing that the proposed algorithm works for detecting a realistic tsunami. Correlation thresholds relevant for tsunami detection can be inferred from the results.
Since the devastating earthquake of 2010 in Haiti, significant efforts were devoted to estimating future seismic and tsunami hazard in Hispaniola. In 2013, the UNESCO commissioned initial modeling studies to assess tsunami hazard along the North shore of Hispaniola (NSOH), which is shared by the Republic of Haiti (RH) and the Dominican Republic (DR). This included detailed tsunami inundation for two selected sites, Cap Haitien in RH and Puerto Plata in DR. This work is reported here.
In similar work done for critical areas of the US east coast (under the auspice of the US National Tsunami Hazard Mitigation Program), the authors have modeled the most extreme far-field tsunami sources in the Atlantic Ocean basin. These included: (i) an hypothetical Mw 9 seismic event in the Puerto Rico Trench; (ii) a repeat of the historical 1755 Mw 9 earthquake in the Azores convergence zone; and (iii) a hypothetical 450 km3 flank collapse of the Cumbre Vieja Volcano (CVV) in the Canary Archipelago. Here, we perform tsunami hazard assessment along the NSOH for these 3 far-field sources, plus 2 additional near-field coseismic tsunami sources: (i) a Mw 8 earthquake in the western segments of the nearshore Septentrional fault, as a repeat of the 1842 event; and (ii) a Mw 8.7 earthquake occurring in selected segments of the North Hispaniola Thrust Fault (NHTF).
Based on each source parameters, the initial tsunami elevation is modeled and then propagated with FUNWAVE-TVD (a nonlinear and dispersive long wave Boussinesq model), in a series of increasingly fine resolution nested grids (from 1 arc-min to 200 m) based on a one-way coupling methodology. For the two selected sites, coastal inundation is computed with TELEMAC (a Nonlinear Shallow Water long wave model), in finer resolution (down to 30 m) unstructured nested grids. Although a number of earlier papers have dealt with each of the potential far-field tsunami sources, the modeling of their impact on the NSOH as well as that of the near-field sources, presented here as part of a comprehensive tsunami hazard assessment study, are novel.
Where coastal tsunami hazard is governed by near-field sources, such as Submarine Mass Failures (SMFs) or meteo-tsunamis, tsunami propagation times may be too small for a detection based on deep or shallow water buoys. To offer sufficient warning time, it has been proposed to implement early warning systems relying on High Frequency (HF) radar remote sensing, that can provide a dense spatial coverage as far offshore as 200-300 km (e.g., for Diginext’s Stradivarius radar). Shorebased HF radars have been used to measure nearshore currents (e.g., CODAR SeaSonde® system (http://www.codar.com/), by inverting the Doppler spectral shifts, these cause on ocean waves at the Bragg frequency. Both modeling work and an analysis of radar data following the Tohoku 2011 tsunami, have shown that such radars could be used to detect tsunami-induced currents and issue tsunami warning. However, long wave physics is such that tsunami currents will only raise above noise and background currents (i.e., be at least 10-15 cm/s), and become detectable, in fairly shallow water, which would limit direct HF radar detection to nearshore areas, unless there is a very wide shelf.
Here, we use numerical simulations of both tsunami propagation (in the Mediterranean basin) and HF radar remote sensing to develop and validate a new type of tsunami detection algorithm that does not have these limitations. This algorithm computes correlations of HF radar signals at two distant locations, shifted in time by the tsunami propagation time computed between these locations (easily obtained based on bathymetry). We show that this method allows detection of tsunami currents as low as 5 cm/s, i.e., in deeper water, beyond the shelf and further away from the coast, thus providing an earlier warning of tsunami arrival.