Influence of Cavity Shape on Hydrodynamic Noise by A Hybrid LES-FW-H Method

The flow past various mechanical cavity, which is a common structure on the surface of the underwater vehicle, and generating hydrodynamic noise has attracted considerable attention in recent years. In this paper, a hybrid method is presented to investigate the hydrodynamic noise induced by mechanical cavities with various shapes. With this method, the noise sources in the near wall turbulences or in the wake are computed by the large eddy simulation （LES） and the generation and propagation of the acoustic waves are solved by the Ffowcs Williams-Hawkings （FW-H） acoustic analogy method with acoustic source terms extracted from the time-dependent solutions of the unsteady flow. The feasibility and reliability of the current method was verified by comparing with experimental data （Wang, 2009）. The 2D cavity models with different cross-section shapes and 3D cavity models with different cavity mouth shapes （rectangular and circular） are developed to study the influence of cavity shape on the hydrodynamic noise. By comparing the flow mechanisms, wall pressure fluctuations, near-field and far-field sound propagation distributions, it is found that the quadrangular cavity with equal depths of leading-edge and trailing-edge is preferred for its inducing lower hydrodynamic noise than the cylindrical cavity does.

An overview of flow field computational methods for hydrodynamic noise prediction

Recently,the hydrodynamic noise is becoming a research hotspot because it not only affects the concealment and comfort of ships,but also affects the living condition of underwater mammals.Accurate prediction of hydrodynamic noise requires that the detailed flow field has been simulated temporally and spatially with high fidelity method.In this paper,we introduce the current issues and challenges for the prediction of hydrodynamic noise,and provide an overview to several detailed flow field simulation methods which aim to resolve these issues.The overview could point the future directions for hydrodynamic noise prediction.

Research on the flow stability and noise reduction characteristics of quasi-periodic elastic support skin

To enhance flow stability and reduce hydrodynamic noise caused by fluctuating pressure,a quasiperiodic elastic support skin composed of flexible walls and elastic support elements is proposed for fluid noise reduction.The arrangement of the elastic support element is determined by the equivalent periodic distance and quasi-periodic coefficient.In this paper,a dynamic model of skin in a fluid environment is established.The influence of equivalent periodic distance and quasi-periodic coefficient on flow stability is investigated.The results suggest that arranging the elastic support elements in accordance with the quasi-periodic law can effectively enhance flow stability.Meanwhile,the hydrodynamic noise calculation results demonstrate that the skin exhibits excellent noise reduction performance,with reductions of 10 dB in the streamwise direction,11 dB in the spanwise direction,and 10 dB in the normal direction.The results also demonstrate that the stability analysis method can serve as a diagnostic tool for flow fields and guide the design of noise reduction structures.

Experimental Research on the Double-peak Characteristic of Underwater Radiated Noise in the Near Field on Top of a Submarine

The double-peak characteristic of underwater radiated noise in the near field on top of the target submarine was analyzed in depth on the basis of submarine test data on the sea. The contribution of three major noise sources to the radiated noise of a submarine were compared and analyzed, and emphasis was put on the original source, production mechanism, and their correlative characteristics. On the basis of analysis on underwater tracking and pass through characteristics of the target submarine, the double-peak phenomenon was reasonably interpreted. Furthermore, the correctness of the theoretical interpretation was verified adequately in real submarine tests. The double-peak phenomenon indicates that the space distributing character on submarine radiated noise are both asymmetrical with time and space, whereas that is provided with directivity. Studying the double-peak phenomenon in depth has important reference value and meaning in engineering practice for understanding the underwater radiated noise field of submarines.

Suppression of self-sustained oscillations of incompressible flow over aperture-cavities and its mechanisms

Experiments and large-eddy simulations(LESs)are conducted to study the effectiveness and the underlying physical mechanisms of a passive control technique for suppressing the self-sustained oscillations of incompressible flow over aperture-cavities.The control technique is implemented by installing a wedge block above the chamfered leading-edge.The experiments are carried out in a low-speed water tunnel with the freestream velocity ranging from 0.4 m/s to 4.4 m/s,while the large-eddy simulations are carried out corresponding to the experiment at a velocity of 4.0 m/s.The wall pressure fluctuations measured along the cavity floor show that a significant suppression of the self-sustained oscillations of the shear layers can be achieved by the control device.Furthermore,the suppression performance is improved as the freestream velocity increases,not limited to the design point of the control device.The analysis of numerical simulation results focuses on three aspects,the vorticity fields,the velocity fields and the pressure fields,and the physical effects of the control device on the incompressible aperture-cavity flow are visualized.Three mechanisms of suppressing the cavity oscillations are identified from the numerical results,which are the destruction of the large vortex structures by the high frequency vortical excitations,the inhabitation of the intracavity recirculation feedback by introducing the lower shunt flow,and the attenuation of the trailing-edge impingement by thickening the shear layer.