The goal of this research was to investigate the structure and the dynamics of unsteady/transient separated turbulent boundary layers for 4×103 < ReΘ< 1.4×104. Central to the study is the characterization of the streamwise and spanwise fluctuating skin friction and wall pressure fluctuations in and around the separation zone.

During the initial stage of the project, a photonic skin friction and wall pressure sensor will be implemented to measure directly and at the same spatial location the unsteady skin friction and wall pressure. The photonic sensor will be an extension of previous sensor development efforts, which were limited to low to moderate-frequency wall shear stress measurements.

The proposed work will be carried out in the specially designed test section of a low-speed wind tunnel. There is a need of reliable wall shear stress sensors to (a) corroborate existing theories and (b) provide precise two-dimensional, skin friction data well-resolved in time and space for the development and validation of models that can predict the onset and extent of stall in transient separated turbulent boundary layers.

The main goal is to generate high fidelity data, taking into account the evolution of the boundary layer from its origin, and to identify key issues and challenges regarding current understating of unsteady separated turbulent boundary layer flows. Detailed velocity mapping will be carried out in and around the separation region using hotwire anemometry, laser doppler velocimetry (LDV) and particle image velocimetry (PIV). These detailed velocity field measurements, together with the unsteady skin friction and wall pressure, will fully characterize the unsteady separated flow region. In addition, the extensive data collected (velocity, turbulence quantities, wall shear stress, wall pressure, and higher order moments) will be analyzed in a systematic way to allow for the prediction of the onset and extent of stall in transient separated turbulent boundary layers.

This research also responds to previous works on unsteady/steady turbulent boundary layers that have highlighted a number of related issues: (a) the urgent need of high-resolution direct measurements of the mean and fluctuating components of the skin friction, since it seems that the behavior of the near wall as well as the outer wall region depends on the friction velocity and in all these years of turbulent boundary layer research (especially zero pressure gradient) the friction velocity has been inferred, for the most part, from the Clauser chart. Some attempts have been made to measure directly the wall shear stress, yet some of these measurements have not been taken into consideration since they do not agree with the law of the wall. In addition, the most reliable way to identify and characterize the bursting process as well as large organized structures of the turbulent boundary layer is through the measurement of the frequency fluctuations of the wall shear stress; (b) the need for carefully controlled boundary layer experiments over a wide range of Reynolds numbers; (c) information regarding the development of the boundary layer (tripping devices, transition) from its initial stage, since it is not clear what is the necessary development length for a turbulent boundary layer to be independent from its initial condition; (d) the need for clearly defined initial conditions; (e) the need for new data on unsteady separated turbulent boundary layers since each available data reports the dependence of the boundary layer on a single parameter.

Central to these studies is the characterization (using direct measurement) of the two-dimensional unsteady fluctuating skin friction and wall pressure fluctuations. Photonic skin friction sensors and wall pressure sensors will be developed and implemented to measure simultaneously, at the same spatial location, the unsteady fluctuating streamwise and spanwise skin friction and wall pressure. The sensing approach is based on the whispering gallery mode (WGM) of dielectric micro-cavities. In optics, the whispering gallery mode phenomenon (WGM) arise from total internal reflection of light at the internal surface of a high index of refraction dielectric resonator embedded in a surrounding medium of lower refractive index.

This work was done by Tindaro Ioppolo of Southern Methodist University for the Army Research Office. For more information, download the Technical Support Package (free white paper) below. ARL-0235


This Brief includes a Technical Support Package (TSP).
Characterization of Turbulent Unsteady Separation Using Photonic Micro-Skin Friction and Wall Pressure Sensors

(reference ARL-0235) is currently available for download from the TSP library.

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This article first appeared in the April, 2021 issue of Aerospace & Defense Technology Magazine.

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