Experimental Characterization of Turbulent Motions Using Wall-Pressure Measurements in Low Reynolds Number Turbulent Boundary Layers

Abstract

The wall-pressure fluctuations induced by low Reynolds number turbulent boundary layers are experimentally studied using flush-mounted microphones. The spatial coherence of the energy is characterized using traditional time-averaged statistical descriptors. A novel analysis is developed, based on the wavelet transform, to study the organization of coherent turbulent events, and their corresponding wall-pressure signatures. This analysis identified that induced irrotational motions/entrained fluid, between neighbouring packets, have wall-pressure signatures below 100 Hz, and packets of hairpin vortices contribute to the wall-pressure energy between 100 Hz and 250 Hz. The packets contain a hierarchy of organized, well-defined events, which contribute to the wall-pressure fluctuations at frequencies above 250 Hz. It is estimated that wallpressure signatures from packets can be retained for up to seven boundary layer thicknesses in the streamwise direction. The composition of events within hairpin packets depends on Reynolds number, showing a shift towards higher-frequency events, with increasing Reynolds number. iv To my parents, thank you for always supporting me. v Table of Contents viii List of Tables xi List of Figures xiii Nomenclature xxi List of References 134

The Design, Fabrication, and Commissioning of a High-Speed Aeroacoustic Wind Tunnel for Studies of Surface Pressure Fluctuations Beneath Turbulent Boundary Layers

“…It is universally accepted that viscous effects dominate in this region and thus the spectrum best collapses when scaled based on kinematic viscosity, ν, and other inner layer variables (wall shear stress, τ w ; and friction velocity, u τ ) [7]. Early measurements conducted by Schewe [31] within this frequency region indicated a dependence of the spectrum on ω −7/3 , however more recent measurements with smaller transducer sizes have shown an attenuation slope ranging from ω −5 to ω −7 for very high frequencies [8,17,22,23,26,28,30]. The transition from the ω −7/3 minor decay region to the larger decay region seems to relate to the transition from the buffer layer to the viscous sub-layer within the TBL [17].…”

“…Early measurements conducted by Schewe [31] within this frequency region indicated a dependence of the spectrum on ω −7/3 , however more recent measurements with smaller transducer sizes have shown an attenuation slope ranging from ω −5 to ω −7 for very high frequencies [8,17,22,23,26,28,30]. The transition from the ω −7/3 minor decay region to the larger decay region seems to relate to the transition from the buffer layer to the viscous sub-layer within the TBL [17]. There is not much consensus in literature with regards to the behavior of the spectrum in the high frequency region, as opposed to other regions, due to the fact that sensors with finite transducer sizes act as low-pass filters in terms of their frequency content measurement capabilities.…”

“…There is therefore a need to not only assess these inconsistencies with experimental methods that cover a wide scope of flow conditions and speeds, but also to clarify the fundamental physical relationship between the turbulent structures within the boundary layer and their contributions to the pressure fluctuation signature. Carleton University has dedicated resources to this area of research through statistical studies of TBL pressure fluctuation phenomena through experimental measurements in low-speed environments using the Medium Speed Aeroacoustic Wind Tunnel Facility (MSAWT) [16,17]. The next phase of this research initiative, and the ultimate focus of this thesis, involves expanding Carelton's capabilities through the design and fabrication of an experimental platform capable of studying the nature of surface pressure fluctuations at a wide range of subsonic flow speeds; including those that are more representative of typical cruise flight conditions of general business and commercial aircraft.…”

“…The panel and support structure can also be removed to facilitate other aeroacoustic test section configurations. A support structure, which is fixed to the top of the nozzle channel assembly, houses the removable traverse system and flow probe instrumentation holder that was designed for the MSAWT set-up [17]. This instrumentation probe traverse system can house a variety of flow measurement devices such as a hot-wire or pitot-static probe which can be used to characterize mean flow and boundary layer properties.…”

A Theoretical and Experimental Investigation on Ejector Acoustics and Ejector Silencer Design

“…There is also an uncertainty associated with the microphones. This was discussed by Van Blitterswyk [91]. The manufacturer's calibration uncertainty is ± 0.3 dB (1.04 Pa), rel 1 Pa and this includes the uncertainties relating to the pre-amplifier gain, non-linearity, repeatability and rounding errors.…”