Measurement and Analysis of Sub-Convective Pressure Fluctuations in Turbulent Boundary Layers: A Novel Methodology

Abstract

Surface flow noise results from fluid-surface interactions, manifesting as surface vibrations or far-field noise. Decomposing the surface pressure field reveals distinct components, with the sub-convective component being particularly critical due to its coupling with structural modes, inducing vibrations. This component, characterized by wavenumbers lower than convective wavenumbers, is significantly weaker than its convective counterpart, making it difficult to measure and model accurately. Existing studies rely on limited measurements, constrained by instrumentation and facility capabilities, leading to empirical wall pressure models with restricted accuracy and applicability.

This study presents the first high-resolution measurements of sub-convective pressure fluctuations, enabling validation of wall pressure spectrum models. A novel measurement approach inspired by acoustic metamaterials was developed, employing sub-resonant cavity sensors that integrate seamlessly into existing geometries. These sensors, leveraging off-the-shelf pressure transducers, operate effectively in grazing flow environments without disturbing the flow. Their dynamic response, determined by geometry, can be optimized for specific flow conditions, offering versatility across applications.

To minimize aliasing effects at low wavenumbers, an optimized sensor array with spanwise-elongated geometries was deployed linearly along the flow direction. Wind tunnel experiments across varying Reynolds numbers and pressure gradients provided crucial insights. Long statistical averages ($\mathcal{O}(10^6\delta/U_e)$) revealed the statistical characteristics of large-scale turbulent motions. Results showed an asymmetric convective ridge about the convective line, a sharp transition into the sub-convective domain, and sub-convective levels 30–35 dB below convective levels.

Comparisons with existing models revealed discrepancies, with all models overpredicting measured levels. While the Chase model aligned over certain ranges, deviations highlight the need for improved wall pressure models. This study lays the groundwork for enhanced vibroacoustic analysis and model refinement through innovative measurement techniques. Overall, these measurements provide a refined insight into the nature of sub-convective pressure fluctuations and will aid in the development of more accurate wall pressure models, crucial for fluid-structure interaction analysis.