In this paper, we present a data-driven approach to construct a reduced-order model (ROM) for the unsteady flow field and fluid-structure interaction. This proposed approach relies on (i) a projection of the high-dimensional data from the Navier-Stokes equations to a low-dimensional subspace using the proper orthogonal decomposition (POD) and (ii) integration of the low-dimensional model with the recurrent neural networks. For the hybrid ROM formulation, we consider long short term memory networks with encoder-decoder architecture, which is a special variant of recurrent neural networks. The mathematical structure of recurrent neural networks embodies a non-linear state space form of the underlying dynamical behavior. This particular attribute of an RNN makes it suitable for non-linear unsteady flow problems. In the proposed hybrid RNN method, the spatial and temporal features of the unsteady flow system are captured separately. Time-invariant modes obtained by low-order projection embodies the spatial features of the flow field, while the temporal behavior of the corresponding modal coefficients is learned via recurrent neural networks. The effectiveness of the proposed method is first demonstrated on a canonical problem of flow past a cylinder at low Reynolds number. With regard to a practical marine/offshore engineering demonstration, we have applied and examined the reliability of the proposed data-driven framework for the predictions of vortex-induced vibrations of a flexible offshore riser at high Reynolds number.

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Interior and exterior noise generated by multi-engine, turboprop aircraft remains a serious problem. In this paper, two phase locked loop schemes are used for engine synchro-phasing to effect noise cancellation in a twin engine OV-ZOA aircraft. For interior noise, a sampled microphone technique serves as the phase detector. For exterior noise, a digital timer/wunter board monitoring engine pulse tachometers is used Both schemes use high bandwidth, inner frequency lock loops to improve capture range. Principle of Noise CancellationThe noise generated by turboprop aircraft engmes poses problems to crew and passengers in the cabin during flight, and to surrounding communities in the far field during take-off and landing. Multi-engine turboprop aircraft noise can be minimized if the proper speed and phase relationship are maintained between propellers or engine shafts [ 1-51. This approach is illustrated in Figure 1 for a twin-engine aircraft. Unlike conventional Power lever (Cabin) ave engine l/rev si pnal Figure 1. Nobe cancella~on co~fig~ratioR for OV-10A. active noise control metho$s which require adding canceling sources, the phase lock oach &criM in this paper is simple and relatively heTraditional engine synchro phase angle between the propeller shafts thus avoiding the acoustic beats arising from varying speed and phase differences between them. A conventional phase locked loop consists of a phase detector, a loop compensator and a voltage controlled oscillator (the engine and propeller, in our case)[ 6 , q . The reference frequency is the master engine speed while the output frequency is the slave engine speed. In the approach presented in this paper, an inner frequency locked loop is used to extend capture range, while an the phase difference trated in Figure 2. The frequency frequency while the phase locked loop uses feedback of phase. The frequency difference detector and phase detector can be realized through analog or digital means [6,7].yields a fixed angle that may not m m t for varying ~~~ In this paper, the approach for in that " h s the cabin noise at a desired ~~a t i Q n by cons monitoring noise at that location using a ~~~h o~.Once per revolution pulses from m a s t e r and slave engine shafts are used for the phase locked li troller is used with erence angle is de interior and exterior c~e~~t i Q n 1104

A general formulation applicable to multi-segment mooring systems is derived from first principles using the concept of lumped mass model and presented in this paper. The problem of seabed contact is addressed using an elastic-dissipative model of seabed. The contribution of seabed force along with the tension force is considered in creating global tangent stiffness matrix of the system. An implicit generalized-α method is used for time integration and it is modified for the current problem by developing an incremental iterative version of the method with corresponding predictor and corrector terms. An in house code named LM3D is developed based on the derived formulation. The code is verified with orcaflex results and also validated with experimental results. The main purpose of this paper is to showcase the reduction in tension observed in a simple branched mooring system when compared to a single line. Three configurations of branched system were considered and compared against a single line arrangement with same material properties. The dynamic analysis of this branched system was carried out with the developed in-house code.

In this paper, a general data-driven approach to construct a reduced-order model (ROM) for the coupled fluid-structure interaction (FSI) problem of a transversely vibrating bluff body in an incompressible flow is presented. The proposed data-driven approach relies on the Eigensystem Realization Algorithm (ERA) to design ROM models in a state-space format. The stability boundaries of the coupled FSI system are obtained by examining the eigenvalue trajectories of the ERA-based ROM. These stability boundaries provide us valuable quantitative insights into the lock-in phenomenon of the bluff-body vibration. We demonstrate the present ERA-based ROM technique for various configurations of bluff bodies such as an isolated single cylinder, the side-by-side and the tandem cylinder arrangements. A comparative study on the effect of different appendages to suppress the VIV of a cylinder is also presented using the ERA-based stability analysis. The validity of the proposed method for the FSI stability analysis on such variety of configurations has not been presented before and is the novel contribution of this paper. Overall, the proposed data-driven framework is found to be much more effective in terms of computational costs and the predicted lock-in regions are comparable to high-fidelity full-order simulations. This work has a potential for a profound impact on the design optimization and control of bluff body structures used in offshore industry.