Protection of sensors and reduction of hydrodynamical vibrations/drag of the sensors is usually ensured by mounting the passive sonar sensor hydrophones inside stiff sonar windows. Nonetheless, research has shown that once the vehicle is in motion, the structure becomes excited by the turbulent flow. Consequently, the generated turbulence causes additional noise. Worse off, the acoustic waves as a result of these contributions propagate to the position of the hydrophone and perturb the incoming signal. At present, much has already been published with regard to sound radiated by boundary layer driven structures. Unfortunately, no one has reported on an accurate analytical solution for the coupled problem of boundary layer driven structures that automatically takes into account the entirely fluid loading.
To this note, Dr. Christian Henke from the ATLAS ELEKTRONIK GmbH in Germany carried out an in-depth assessment of flow noise generation and transport mechanisms with respect to different materials and geometries. To be more specific, he showed for the first time and in contrast to earlier studies that fluid loading could be characterized once and for all without the distinction between ‘‘light’’ and ‘‘heavy’’ fluid loading. His work is currently published in Journal of Fluids and Structures.
The research method he employed commenced with indulging in a detailed description of the modelling approach and the underlying assumptions of the system to be used. Next, he considered the coupling effects by a set of partial differential equations and boundary conditions. He then carried out the analytical solution of the aforementioned problem where he successfully invert the system matrix by a biorthogonal expansion method that he later expanded with a damping mechanism. Lastly, he demonstrated the sensitivity of the main solution parameters with examples.
The author observed that by changing the parameters and the source term of the analytical solution, the results obtained could be generalized to different rectangular geometries, material parameters, flow speeds and damping ratios. In addition, he noted that if it were possible to shade the reflected wave through the hydrophone holder, then the results could be potentially be applied for non-rectangular geometries. Lastly, a good agreement in the ω-4 range of the frequency spectrum and the pattern in the wave number–frequency plots was achieved and showed that the accuracy of the modelling approach was within the scope of sea trial measurement accuracies.
In conclusion, the study by Christian Henke presented a vivid demonstration on how to resolve the problem of flow noise generation and propagation in hull-mounted sonar systems using a numerical/analytical solution approach. The knowledge presented here on the underlying mechanism of the subject matter has potential to maximize the signal to noise ratio. Altogether, this work has developed a novel analytical solution that is based on a biorthogonal system which can be solved in the spectral domain without the usual simplifying assumptions.
Christian Henke. Flow noise in planar sonar applications. Journal of Fluids and Structures, volume 78 (2018) page 263–276.Go To Journal of Fluids and Structures