Random Wind Forces Research Articles

Study on the behavior of weakly nonlinear water waves in the presence of random wind forcing

Specific solutions of the nonlinear Schrodinger equation, such as the Peregrine breather, are considered to be prototypes of extreme or freak waves in the oceans. An important question is whether these solutions also exist in the presence of gusty wind. Using the method of multiple scales, a nonlinear Schrodinger equation is obtained for the case of wind-forced weakly nonlinear deep water waves. Thereby, the wind forcing is modeled as a stochastic process. This leads to a stochastic nonlinear Schrodinger equation, which is calculated for different wind regimes. For the case of wind forcing which is either random in time or random in space, it is shown that breather-type solutions such as the Peregrine breather occur even in strong gusty wind conditions.

The Effect of Random Wind Forcing in the Nonlinear Schrödinger Equation

The influence of a strong and gusty wind field on ocean waves is investigated. How the random wind affects solitary waves is analyzed in order to obtain insights about wave generation by randomly time varying wind forcing. Using the Euler equations of fluid dynamics and the method of multiple scales, a random nonlinear Schrödinger equation and a random modified nonlinear Schrödinger equation are obtained for randomly wind forced nonlinear deep water waves. Miles theory is used for modeling the pressure variation at the wave surface resulting from the wind velocity field. The nonlinear Schrödinger equation and the modified nonlinear Schrödinger equation are computed using a relaxation pseudo spectral scheme. The results show that the influence of gusty wind on solitary waves leads to a randomly increasing ocean wave envelope. However, in a laboratory setup with much smaller wave amplitudes and higher wave frequencies, the influence of water viscosity is much higher. This leads to fluctuating solutions, which are sensitive to wind forcing.

Reliability-Based Design Optimization of Frame-Supported Tensile Membrane Structures

AbstractDue to the inherent flexibility of tensile membrane structures (TMS), they need to remain in a stable equilibrium condition in the presence of gusty winds as well as in their absence. This paper is aimed at the reliability-based optimization of frame-supported tensile membrane structures subjected to uncertain wind loads. The transient membrane displacement is minimized under this random loading constrained to a stable TMS form and a maximum failure probability against membrane tearing. A particle swarm optimization algorithm is used, combined with Latin hypercube sampling and response surface approach, for obtaining the optimum initial prestress required. These algorithms balance the computationally demanding dynamic relaxation method required for the membrane structural analysis. The proposed methodology is demonstrated through the example of a frame-supported conic membrane structure. The results show that the proposed method can effectively optimize the TMS performance under random wind forces...

Dynamic and Buffeting Analysis of Suspension Pipeline Bridge

Suspension aerial crossing structures are broadly applied for supporting petroleum pipelines across special terrain for their optimal structural style and constructional benefit. Due to the general flexibility of the structures, pipeline suspension bridges easily vibrate under the action of random wind forces. One of the typical vibration responses, known as buffeting, is considered to be an important factor for the serviceable safety of suspension bridges. In this paper, the dynamical model of a suspension pipeline bridge is presented and buffeting analysis under the action of wind loads is carried out through the Finite Element Method. It is shown that the frequency spectrum of the suspension pipeline bridge is composed of densely distributed modal frequencies. Low frequencies are mainly focused on horizontal and vertical bending motions of the bridge. Based on the standard harmonic response analysis, the Pseudo-Excitation Method (PEM) is introduced to obtain the buffeting vibration in response to the wind excitation. The correlative formulas of quasi-static buffeting force model are derived, and the buffeting analysis of the bridge using PEM is achieved on the solution platform Ansys.

Asymmetry of Free Circulations in Closed Ocean Gyres

Abstract Steady flows in regions of closed geostrophic contours, such as in ocean basins or over seamounts, are examined by calculating solutions to the nonlinear quasigeostrophic and shallow-water equations with topography. For oceanically realistic choices of parameters, the solutions are asymmetric in that those with cyclonic circulation resemble the topography while those with anticyclonic circulation exhibit small-scale structure and cross-isobath flow. These small-scale structures reflect topographic wave modes, which are stationary with the anticyclonic circulation. The implication of the asymmetry is that random wind forcing is much more likely to excite persistent cyclonic than anticyclonic flow. This may explain why the circulation in the Norwegian and Greenland Gyres is most often cyclonic.

Dynamic modeling and active control of a cable-suspended parallel robot

Cable-suspended parallel robot (CPR), in which cables are utilized to replace links to manipulate objects, is developed from parallel and serial cable-driven robot. Compared with the parallel robot, this kind of robot has more advantages. The cooperative variation of lengths of six cables pulls the feed cabin to track radio source with six degrees of freedom (DOFs). Similar to a parallel robot, the cable-cabin mechanism for 500-m aperture spherical radio telescope (FAST) can be viewed as a CPR. According to its 5-m scaled model, based on the inverse kinematics analysis the inverse dynamic formulation of CPR with non-negligible cable mass is established using Lagrangian dynamic formulation. Then, considering random wind forces acted on the cabin are simulated based on the characters of the mechanism, a fuzzy plus proportional–integral control (FPPIC) method, which can enhance the control performance for steady-state errors, is utilized to control the wind-induced vibration of the trajectory tracking of the feed cabin. Finally, we provide the examples of simulation and experiment to justify the dynamic modeling for control and to test the proposed method.

The 3D Quasigeostrophic Fluid Dynamics Under Random Forcing On Boundary

The three-dimensional baroclinic quasigeostrophic flow model has been widely used to study basic mechanisms in oceanic flows and climate dynamics. In this paper, we consider this flow model under random wind forcing and time-periodic fluctuations on fluid boundary (the air-sea surface). The time-periodic fluctuations are due to periodic rotatios of the earth and thus periodic exposure of the earth to the solar radiation. After overcoming the difficulty due to the low regualrity of an associated Ornstein-Uhlenbeck process, we establish the well-posedness of the baroclinic quasigeostrophic flow model in the state space. Then we demonstrate the existence of the random attractors, again in the state space. We also discuss the relevance of out result to climate modeling.

Enstrophy dynamics of stochastically forced large-scale geophysical flows

Enstrophy is an averaged measure of fluid vorticity. This quantity is particularly important in rotating geophysical flows. We investigate the dynamical evolution of enstrophy for large-scale quasi-geostrophic flows under random wind forcing. We obtain upper bounds on the enstrophy, as well as results establishing its Hölder continuity and describing the small-time asymptotics.

Ergodicity of stochastically forced large scale geophysical flows

We investigate the ergodicity of 2D large scale quasigeostrophic flows under random wind forcing. We show that the quasigeostrophic flows are ergodic under suitable conditions on the random forcing and on the fluid domain, and under no restrictions on viscosity, Ekman constant or Coriolis parameter. When these conditions are satisfied, then for any observable of the quasigeostrophic flows, its time average approximates the statistical ensemble average, as long as the time interval is sufficiently long.

An on-line trained adaptive neural controller

This article presents a neural network approach for online industrial tracking control applications. In comparison to several existing neural control schemes, the proposed direct neural controller is characterized by the simplicity of its structure and its practical applicability for real-time implementation. In order to enhance the adaptive ability of the neural controller, a set of fuzzy rules is set up for selecting interim training targets. With minor qualitative knowledge about the plant, the scheme is designed for controlling the nonlinear behavior of the plant under conditions of disturbances and noise. Simulations of a ship course-keeping control under random wind forces and measurement noise have been investigated and comparison of performance has been made with a conventional PID controller. Results presented clearly demonstrate the feasibility and adaptive property of the proposed scheme. >

Response of articulated tower platforms to random wind and wave forces

The displacement response of an articulated tower is mainly governed by its rigid body mode of vibration which has a very low frequency. Since the wind velocity spectrum of the fluctuating component has a low frequency energy content, the wind-induced vibration of an articulated tower may be significant. The combined wind-, wave- and current-induced vibration of the tower is an important phenomenon to study because of the complex hydrodynamic interactions. In this paper, the response of a model articulated tower to random wind force alone and to the combined effect of wind, wave and current forces is investigated. Only the first-order forces due to the waves have been considered. The response of the tower is determined by a frequency domain iterative method and is evaluated for a parametric study. Both wind velocity and random sea are characterized by Power Spectral Density Functions. A Monte-Carlo simulation procedure is used to obtain sample records of time histories of both the wind velocity and water particle kinematics. The response analysis considers the nonlinearities due to writing the equilibrium equation based on the instantaneous position of the tower and relative-velocity squared drag force. Using this method of analysis a parametric study is conducted to investigate the behaviour of the tower under different important factors which include the pressure drag effect, the effect of nonlinear drag on both wind and wave forces, the mean wind velocity at the reference height, the current velocity and different combinations of wind, wave and current forces.

Adaptive Prediction of the Motion of Marine Vehicles

A predictor of the future motion of a vessel subject to random wave and wind forces, would have a variety of applications in ocean engineering. Most previous work has assumed that the wave spectrum is known and that the vessel is modeled accurately; both factors affect the predictor performance strongly. In practice, the relevant data is difficult to measure on a manoeuvering vessel and can change significantly with operating conditions. Here were describe the application of an adaptive algorithm which predicts the future of a signal from its history. The predictor adapts to the signal and varies its parameters to optimise the prediction. Operating on a signal with a stationary spectrum, the predictor tends to a steady performance; if the spectrum changes, the predictor quickly adjusts to the new situation. We illustrate the performance of the system with examples.

Environmental Load Effect Analysis of Guyed Towers

A general method for dynamic load effect analysis of slender offshore structures subjected to short crested random waves, current and wind, is given. The structure is represented by a three-dimensional space frame model utilizing dash-pots and linear or nonlinear spring elements to represent guy lines and coupling between structure and foundation. The component mode synthesis formulation is adopted for reduction of the number of degrees of freedom. The hydrodynamic forces are computed by Morison’s equation, accounting for finite wave elevation, directionality, and relative fluid-structure motion. Various kinematic models for the fluid field in the splashing zone are compared. To get a reasonable representation of nonlinearities in the loading and the structural model, a Monte Carlo approach is adopted. Starting with simulated samples of the random fluid field and wind forces, time series of structural responses are found by numerical time integration utilizing the Newmark β-family of time integration operators. Numerical results for a guyed tower at 450-m water depth are presented. The statistical uncertainties associated with the stochastic time domain simulations are discussed. A significant discrepancy is found between linearized frequency domain solutions and the present nonlinear time domain formulation. The importance of an adequate representation of superharmonic responses is particularly discussed. The differences in results due to various solution methods are found to vary significantly with sea-state conditions.

Optimal design of active TMD for buildings control

Active TMD were previously designed assuming the structural parameters of the optimal passive TMD. However, the active control technique introduces to both the building and the TMD an active stiffness and active damping which change the structural parameters of the controlled structure. This paper presents a process for designing optimal active TMD for tall building control. It shows that the relationship between the first mode natural frequency of the building and the natural frequency of the tuned mass damper depends completely on the required degree of control in the building. A numerical example for a tall building subjected to stationary random wind forces is given.

The action of flexible bridges under wind, II: Buffeting theory

By using the analytical and conceptual format set forth in the first of these two companion papers, the problem of bridge buffeting under natural wind is considered. Again, bridges having vibration modes that are not necessarily simple are dealth with. The buffeting action of random wind forces in the presence of self-excited, or bridge motion-induced, forces is discussed, as are energy considerations associated with bridge stability. A possible explanation is offered for the well-known fact that turbulence in the incident wind often delays the onset of flutter.