Towed Array Tensioning for Low Frequency Flow Noise Reduction
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Solution Overview
Problem
Towed acoustic sensor arrays face significant challenges in reducing flow noise, primarily due to turbulent boundary layers and vortex shedding, which current designs struggle to effectively mitigate, especially at low frequencies.
Innovation Solution
The method involves determining an optimal tension for the towed array by establishing a relationship between tension and wall pressure fluctuation spectral levels, and either adding additional hose length or deploying a drogue to achieve this tension, thereby reducing flow noise across all stream-wise wavenumbers at lower frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If standard towed array design is used with hydrophone grouping and stand-off distance, then higher wavenumber noise is filtered, but low wavenumber flow noise remains unmitigated
Solution Approach 1:
The patent changes the physical parameter of hose tension to mitigate flow noise. By increasing the tension in the towed array hose, the natural frequency of the hose is raised, which reduces the coupling between low wavenumber flow disturbances and the hydrophones. This parameter change directly addresses low wavenumber noise without requiring complex reconfiguration of the array geometry.
Solution Approach 2:
The patent applies preliminary action by pre-tensioning the hose before deployment or during operation. The tensioning mechanism is activated in advance to establish the optimal tension level that minimizes flow noise, rather than attempting to correct noise issues after the array is already deployed in a relaxed state.
2Object-affected harmful factors
If additional hose or drogue is added to increase tension, then flow noise is reduced, but array handling and deployment complexity increases
Solution Approach 1:
The patent introduces a drogue as an intermediary device to achieve the desired hose tension. The drogue, deployed behind the towed array, uses drag force to tension the hose without requiring direct mechanical intervention on the array itself. This intermediary approach reduces flow noise while minimizing the complexity of modifying the array structure.
Solution Approach 2:
The patent segments the tensioning function from the array structure by using separate components (additional hose sections or drogues) that can be independently deployed and adjusted. This allows the tensioning mechanism to be optimized for noise reduction without complicating the core array design and handling procedures.
3Measurement precision
If hose tension is increased to reduce flow noise, then signal to noise ratio improves, but risk of hose damage or hydrophone detachment increases
Solution Approach 1:
The patent applies partial action by increasing tension only to the optimal level needed for noise reduction, rather than maximizing tension. The optimal tension is calculated based on the balance between noise reduction benefits and the mechanical limits of the hose and hydrophone mounting structures, ensuring sufficient margin of safety.
Solution Approach 2:
The patent employs feedback by monitoring the tension level in the hose and adjusting it to maintain the optimal value. Sensors or calculation models provide feedback on the current tension state, allowing the system to adjust the tensioning mechanism (additional hose deployment or drogue position) to maintain noise reduction while preventing excessive tension that could cause damage.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces flow noise by 15 dB at frequencies below 20 Hz, as demonstrated by experimental data, improving the signal-to-noise ratio and handling ease of the towed acoustic sensor array.
Implementation Method 1
a turbulent boundary layer 34 of thickness δ forms around the exterior of hose wall 24
Implementation Method 2
vortex shedding induced during turns, are primary sources of flow noise
Implementation Method 3
An optimal towed array tension is calculated using the determined relationship. Tension is increased to the optimal towed array tension
Data Source
AI summary
A method for reducing noise in a towed acoustic array includes providing a towed array having a length, diameter, frequency range and tow speed. The relationship between tension in the towed array and wall pressure fluctuation spectral levels is determined. Wall pressure fluctuation spectral levels are related to the noise in the array. An optimal towed array tension is calculated using the determined relationship. Tension is increased to the optimal towed array tension by either provided additional towed array hose or a drogue at the end of the towed array.