Hydro-Acoustic Signature Prediction for Submarines-A New Hybrid Calculation Approach.pdf

1、This PhD dissertation focuses on the use and improvement and enhancement of numerical method for predicting hydroacoustic signature of a submarine. The hydroacoustic signature is one of the most important signature param

2、eters in ASW(Anti Submarine Warfare) and a comprehensive view is a vital qualification in the acoustic design of a modern vessel. Hydrodynamics and Acoustics are closely related in the study of ‘flow noise source mechani

3、sms’ involved in submarines. In order to increase submarine’s operational efficiency, submarines in transit move at a maximum speed which guarantees the submarine covertness. Generally this speed does not increase 15 kno

4、ts(7. 5 m/s). At this speed the flow noise, so-called‘hydrodynamic noise’; predominates. Underwater radiated noise can be used for detection, localization and classification of the vessel. The flow noise strongly depends

5、 on the submarine’s speed, and generally the Sound Level, SL is proportional to the speed raised the sixth power. This means that doubling the speed will increase SL by 18 dB. A submarine in transit is 15-20 dB noisier t

6、han one in ‘quite’ mode. The acoustic pressure fluctuations(perceived as noise) tend to be orders of magnitude smaller than the dynamic fluctuations and propagate over large distances far from their source.
　　 Variou

7、s approaches have been proposed to simulate these phenomena. The direct method solves the Navier-Stokes equations and simultaneously resolves the flow and acoustic contributions. However, for practical applications this

8、method is prohibitively expensive. Hybrid methods decouple the flow and acoustic parts, solving the hydrodynamic part first to determine acoustic sources, and solving an acoustic system to obtain the associated field rad

9、iation sources. Examples of such methods are: acoustic analogies(e.g. Lighthill, Ffowcs-Williams Hawkings), linearised Euler methods, Kirchhoff methods, etc. Hydroacoustic predictions are carried out by commercially deve

10、loped computer programs with incorporating new values of turbulence model coefficients through regression analysis and testing the results on different configurations with verified experimental results. For low frequency

11、 assessments FEM(Finite Element Method) together with BEM(Boundary Element Method) are a powerful combination. The problem arises to interface FEM results with BEM results. CFD General Notation System, CGNS has been used

12、 to interface the CFD results into acoustic analysis. The present results have some relevance for the design of new submarine design concepts. The main innovations of the present thesis are as follows,
　　 1. This stu

13、dy constitutes the harbinger to model designing that aims to analyze Lighthill's stress tensor calculated from Computational Fluid Dynamic techniques; and acoustical results from Boundary Element Method. The results are

14、interfaced with the help of CFD General Notation System CGNS using application program interface(API) in ‘C Complier’. The system consists of two parts:(1) a standard format for recording the data; and(2) software that r

15、eads, writes, and modifies data in that format. The format is a conceptual entity established in binary format. It eventually develops boundary condition strategies that can be tested initially on simple configurations a

16、nd eventually applied in practical applications.
　　 2. A methodology is developed that compares different turbulent models like large eddy simulation(LES), Shear Stress Transport(SST) and k-ε model etc. to calculate

17、Lighthill's stress tensor and a variational formulation of Lighthill's acoustic analogy to solve the acoustic problem using Boundary Element Method. Numerical experimentations were conducted in the mechanical engineering

18、 department, University College London, UK. Tremendous CFD calculations were conducted and various models were compared and optimized to calculate Lighthill's stress tensor. A new form of Schmidt number is proposed, whic

19、h gives better estimation of acoustic sources for a variety of configurations.
　　 3. Lighthill stress tensors for different models, speeds and angles have been calculated and each tensor component is compared at incr

20、easing Froude number. The Lighthill stress tensor strength tensor is plotted against sound level dB to analyze separately. Different aspects of submarine configurations 1ike, mainsail with or without hydroplanes and diff

21、erent submarine nose shapes are also modeled and analyzed acoustically from the practical and theoretical point of view. The initial predictions of the hot spots for sound sources have been monitored using CFD results co

22、nverted into CGNS file format in LMS Virtual Lab to find embedded physical meaning in calculated data. ‘Sound source hot spots’ were analyzed for different speeds. Different turbulence models are compared using the entir

23、e procedure. With this procedure, sound levels and hot spots in a submarine model have been successfully predicted and area of interest has been successfully identified.
　　 4. Numerical calculation of structure borne

24、 radiated noise from a submarine double cylinder has been successfully predicted with multi-excitation forces. The results were compiled with linear superposition principle. The acoustic and vibration characteristics hav

25、e been fully analyzed including a comprehensive directivity analysis for each frequency and excitation level.
　　 5. Fluid borne structural noise for a submarine was also calculated using CFD-FEM-BEM approach. Model r

26、eduction by comparative constraints method was successfully implemented and proposed for submarine hull with enclosed pressure hull. Complete Flow - Vibro - Acoustic characteristic results of submarine with pressure hull

27、 are thoroughly analyzed. Furthennore the directivity of submarine with pressure hull is also given for lower and higher frequency ranges. The acoustic radiation characteristic of submarine produced by the vibration of s

28、ubmarine structure induced by the flow when water moves from the surface of submarine is successfully predicted by this method.
　　 Keywords: Hydroacoustics; Fluid Induced Structure Vibration; Numerical Experimentatio