Mitigation of Current-Driven, Vortex-Induced Vibrations of a Spar Platform via “SMART” Thrusters

2004 ◽  
Vol 126 (1) ◽  
pp. 96-104 ◽  
Author(s):  
F. Joseph Fischer ◽  
Stergios I. Liapis ◽  
Yannis Kallinderis
Keyword(s):  
Active Control ◽  
Control Strategies ◽  
Low Frequency ◽  
Three Dimensions ◽  
Navier Stokes ◽  
Frequency Wave ◽  
Fluid Forces ◽  
Vortex Induced Vibrations ◽  
Helical Strakes ◽  
The Cost

The possibility of using active-control technology to mitigate long-period (100–300 s) motions of spar platforms was investigated. In particular, the technical feasibility and cost-effectiveness of using a thruster-based active-control system was examined. Only vortex-induced vibrations (VIV) of a moored spar were considered for this study, although it is believed that similar active-control systems can contend equally well with low-frequency wave-drift and wind-induced motions. VIV has been selected for this demonstration due to the considerable attention it has received for spars, because of the high cost for passive VIV-mitigation means. The investigation employed numerical simulations of the hydrodynamics around the spar in two and three dimensions. In particular, for this study, Navier-Stokes methods were used to compute the strongly nonlinear interactions between the current-flow field and the spar hull. The fluid forces (hull-integrated pressures) were then used to compute the unsteady motions of the moored spar. The effect of the motion-controlling thrusters was included as an additional external force—that also varies with time. Of the several different control strategies that were examined, it was found that the most effective one is “D-Control” wherein thruster forces are proportional to spar velocities. Using such control, spar motions can be kept below 35% of the spar diameter, whereas uncontrolled responses were computed to be as high as 80% of the spar diameter. Similar uncontrolled motions have actually been observed in current-tank model tests of spars. As part of this study, discussions were held with engineers from three different thruster companies to ascertain the number and type of thrusters that would be required to achieve desirable VIV mitigation—given the computed thruster-forces, and to provide prudent reliability. Associated costs were also very much of interest. For the spar system examined here, which is representative of those introduced into or being considered for the Gulf of Mexico, it was found that the cost of the thrusters and their electric motors is around $8 million—a cost that is significantly less than that estimated for passive VIV-control devices such as helical strakes, or discontinuous hull geometries. Furthermore, the proposed thruster system could also be employed to contend with (to mitigate) other undesirable, large-amplitude, near-resonant spar motions, e.g., low-frequency drift, for which there are no alternative remedies at present.

Dynamic Positioning Systems: Comparison Between Wave Filtering Algorithms and Their Influence on Performance

2003 ◽  
Author(s):  
Eduardo A. Tannuri ◽  
Tiago T. Bravin ◽  
Celso P. Pesce
Keyword(s):  
Analog Circuits ◽  
High Frequency ◽  
Control Strategies ◽  
Low Frequency ◽  
Design Parameters ◽  
Dynamic Positioning ◽  
Frequency Wave ◽  
Filtering Algorithms ◽  
Positioning Systems ◽  
Dynamic Positioning System

Wave filtering is an essential function of a Dynamic Positioning System, being responsible for the separation between high-frequency wave induced motions and low-frequency ones, which must be controlled. Low attenuation of the first components may cause oscillatory control action, high fuel consumption and can damage propeller systems. On the other hand, depending on filtering design, high levels of attenuation may be associated with non-admissible delay-times, which may cause instability in the closed-loop system. Traditionally, low-pass or notch-type filters have been used since the first DP systems, due to the simplicity, acceptable performance and possibility of being implemented using analog circuits. Alternatively, observers based on Kalman Filtering Theory have also been used, based on simplified ship models, separating low and high frequency motions. Two wave filters, representing each of those categories, were implemented in a complete DP simulator. All DP components and algorithms are considered and modeled in the simulator, including propellers, thrust allocation, wind-feedforward and different control strategies. Environmental loads are evaluated using fully validated models, including wind, current and wave actions. A pipe-laying barge under typical Campos Basin environmental conditions has been considered as example. Several aspects of the filtering algorithms were analyzed and compared, involving the influence of each filter in the overall DP performance, relationship of design parameters with physical system, ease of commissioning and tuning. The trade off between low frequency tracking (which is associated with the delay time) and wave response suppression is analyzed and discussed for both categories of filters.

Structural and Acoustic Noise Sources Due to Turbulent Flow Through an Elbow: Formulation of Analysis Methods

2003 ◽  
Author(s):  
Stephen A. Hambric ◽  
L. Joel Peltier ◽  
John B. Fahnline ◽  
David A. Boger ◽  
John E. Poremba
Keyword(s):  
Power Transmission ◽  
Acoustic Noise ◽  
Low Frequency ◽  
Navier Stokes ◽  
Fe Model ◽  
Noise Sources ◽  
Turbulent Fluid Flow ◽  
Fluid Forces ◽  
Piping Systems ◽  
Flow Through

The low-frequency structure- and fluid-borne noise from elbows excited by fluctuating forces within turbulent fluid flow is investigated. Computational Fluid Dynamics (CFD) Reynolds Averaged Navier Stokes (RANS) analyses of the flow through a piping elbow with a radius to diameter ratio of 2.8 compare favorable to measurements made by previous investigators. The CFD RANS solutions are post-processed to estimate the spectra of the fluctuating wall pressures beneath the turbulent boundary layer (TBL) flow. The CFD RANS solutions are also used to identify regions within the core flow that might excite acoustic modes within the piping fluid. A finite element (FE) model of the piping walls is coupled with a boundary element (BE) model of the interior acoustic fluid and is excited by the fluctuating wall and fluid forces estimated from the CFD RANS solutions. The power transmission through the inlet and discharge ports of the elbow is computed and separated into its structure-borne and fluid-borne components. The influence of both structural and acoustic resonances on the power transmission is evident for both excitation mechanisms. The power transmission curves at the elbow ports may be used as source inputs to transfer matrix models of piping systems that contain elbows.

Low-Frequency Thermoacoustic Instability Mitigation Using Adaptive-Passive Acoustic Radiators

2011 ◽  
Author(s):  
Reza Kashani ◽  
Jeff Monfort
Keyword(s):  
Active Control ◽  
Control Strategies ◽  
Low Frequency ◽  
Acoustic Energy ◽  
Acoustic Damping ◽  
Thermoacoustic Instability ◽  
Acoustic Radiator ◽  
Passive Acoustic ◽  
Instability Frequency ◽  
Combustion Environment

A commonly used technique for mitigating thermoacoustic instability in an enclosed combustion environment is removing more acoustic energy from the combustor, at the frequency corresponding to the acoustic mode(s) of the combustor which are sympathetic to such instability. This approach is based on adding tuned acoustic damping to the combustion environment. By incorporating in-situ adjustability into acoustic damping devices, they can change their mechanical attributes, e.g., mass and/or stiffness, and adapt themselves in a semi-active manner to the varying instability frequency. Adaptive-passive thermoacoustic mitigation solutions have less weight penalty than the alternative active solutions mainly because the adaptation is done in a semi-active way, at slow pace, with a small and less power-hungry actuation mechanisms. Moreover, the flexibility they offer make them highly desirable for land and marine instability mitigation applications. In this work, semi-active adjustment of a novel tuned acoustic damper, namely an acoustic radiator, is explored. The paper describes the inner working of a semi-active (adaptive-passive) acoustic radiator and the relevant control schemes to adapt them to the instability frequency on hand. The damping effectiveness of the proposed damper, is demonstrated experimentally. It should be mentioned that the semi-active control strategies developed for acoustic radiators can also be used, with minor modifications, for semi-active control of other acoustic damping mechanisms such as Helmholtz resonators and quarter-wave tubes.

Singularities

2018 ◽  
Author(s):  
S. G. Rajeev
Keyword(s):  
Stokes Equations ◽  
Negative Answer ◽  
Three Dimensions ◽  
Picard Iteration ◽  
Navier Stokes ◽  
Scale Invariant ◽  
Navier Stokes Equations ◽  
L2 Norm ◽  
Two Dimensional Flow ◽  
Physical Consequences

The initial value problem of the incompressible Navier–Stokes equations is explained. Leray’s classic study of it (using Picard iteration) is simplified and described in the language of physics. The ideas of Lebesgue and Sobolev norms are explained. The L2 norm being the energy, cannot increase. This gives sufficient control to establish existence, regularity and uniqueness in two-dimensional flow. The L3 norm is not guaranteed to decrease, so this strategy fails in three dimensions. Leray’s proof of regularity for a finite time is outlined. His attempts to construct a scale-invariant singular solution, and modern work showing this is impossible, are then explained. The physical consequences of a negative answer to the regularity of Navier–Stokes solutions are explained. This chapter is meant as an introduction, for physicists, to a difficult field of analysis.

scholarly journals Model Predictive Control Strategies to Activate the Energy Flexibility for Zones with Hydronic Radiant Systems

Energies ◽  
2021 ◽  
Vol 14 (4) ◽  
pp. 1195
Author(s):  
Ali Saberi Derakhtenjani ◽  
Andreas K. Athienitis
Keyword(s):  
Control Strategies ◽  
High Price ◽  
Electricity Prices ◽  
Peak Demand ◽  
Heating Systems ◽  
Electrical Grid ◽  
Demand Reduction ◽  
The Cost ◽  
Floor System ◽  
Radiant Floor Heating

This paper presents control strategies to activate energy flexibility for zones with radiant heating systems in response to changes in electricity prices. The focus is on zones with radiant floor heating systems for which the hydronic pipes are located deep in the concrete and, therefore, there is a significant thermal lag. A perimeter zone test-room equipped with a hydronic radiant floor system in an environmental chamber is used as a case study. A low order thermal network model for the perimeter zone, validated with experimental measurements, is utilized to study various control strategies in response to changes in the electrical grid price signal, including short term (nearly reactive) changes of the order of 10–15 min notice. An index is utilized to quantify the building energy flexibility with the focus on peak demand reduction for specific periods of time when the electricity prices are higher than usual. It is shown that the developed control strategies can aid greatly in enhancing the zone energy flexibility and minimizing the cost of electricity and up to 100% reduction in peak power demand and energy consumption is attained during the high-price and peak-demand periods, while maintaining acceptable comfort conditions.

scholarly journals Active Control of Greenhouse Climate Enhances Papaya Growth and Yield at an Affordable Cost

Agronomy ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 378
Author(s):  
Irene Salinas ◽  
Juan José Hueso ◽  
Julián Cuevas
Keyword(s):  
Active Control ◽  
Skin Color ◽  
Natural Ventilation ◽  
Control Strategies ◽  
Soluble Solids ◽  
Growth And Yield ◽  
Climate Control ◽  
Tropical Fruit ◽  
Greenhouse Climate ◽  
Solids Content

Papaya is a tropical fruit crop that in subtropical regions depends on protected cultivation to fulfill its climate requirements and remain productive. The aim of this work was to compare the profitability of different climate control strategies in greenhouses located in subtropical areas of southeast Spain. To do so, we compared papayas growing in a greenhouse equipped with active climate control (ACC), achieved by cooling and heating systems, versus plants growing in another greenhouse equipped with passive climate control (PCC), consisting of only natural ventilation through zenithal and lateral windows. The results showed that ACC favored papaya plant growth; flowering; fruit set; and, consequently, yields, producing more and heavier fruits at an affordable cost. Climate control strategies did not significantly improve fruit quality, specifically fruit skin color, acidity, and total soluble solids content. In conclusion, in the current context of prices, an active control of temperature and humidity inside the greenhouse could be a more profitable strategy in subtropical regions where open-air cultivation is not feasible.

scholarly journals Evolution of a narrow-band spectrum of random surface gravity waves

2003 ◽  
Vol 478 ◽  
pp. 1-10 ◽  
Author(s):  
KRISTIAN B. DYSTHE ◽  
KARSTEN TRULSEN ◽  
HARALD E. KROGSTAD ◽  
HERVÉ SOCQUET-JUGLARD
Keyword(s):  
Three Dimensional ◽  
Low Frequency ◽  
Three Dimensions ◽  
Band Spectrum ◽  
Nls Equation ◽  
Random Surface ◽  
Narrow Bandwidth ◽  
Quasi Stationary State ◽  
The Stability ◽  
Three Dimensional Simulations

Numerical simulations of the evolution of gravity wave spectra of fairly narrow bandwidth have been performed both for two and three dimensions. Simulations using the nonlinear Schrödinger (NLS) equation approximately verify the stability criteria of Alber (1978) in the two-dimensional but not in the three-dimensional case. Using a modified NLS equation (Trulsen et al. 2000) the spectra ‘relax’ towards a quasi-stationary state on a timescale (ε2ω0)−1. In this state the low-frequency face is steepened and the spectral peak is downshifted. The three-dimensional simulations show a power-law behaviour ω−4 on the high-frequency side of the (angularly integrated) spectrum.

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