Inner-Scaled Helmholtz-Resonators for Turbulent Boundary Layer Flow Control

An experimental study is presented towards inner-scaled Helmholtz-Resonators (HRs) as a passive turbulent boundary layer (TBL) flow control method. Using acoustic pressure-velocity coupling at the orifice of the HR, it is aimed to attenuate the kinetic energy of the grazing turbulence to reduce the mean skin friction. The HRs are tuned towards the spatial and temporal scales of the near-wall cycle turbulence events that play a major role in the production of turbulence. The adopted HR design strategy has a spatial tuning towards the most efficient attenuation of sweep events, based on recent studies on micro-cavity arrays. The temporal tuning is based on the streamwise wavelength of the most energetic wall-normal velocity fluctuations. These interact with the wall to create pressure fluctuations and form the primary excitation source of the HRs. A parameter study on HRs under a fully developed grazing TBL at Reτ ≈ 2200 was performed. A strong pressure-velocity coupling was found between the HR and the grazing TBL when the HR design frequency either matched the frequency of the most energetic wall-normal velocity fluctuations or when the HR design frequency was below this frequency. The pressure-velocity coupling extends to y+ ≈ 25, in which phase-interlocking of the grazing TBL occurs. Clear inflow and outflow regions were identified with a streamwise width related to the HR resonance frequency. The inflow and outflow regions cause an increase and decrease in streamwise velocity, respectively. These velocity fluctuations appear as an increase in spectral energy around the wavelengths of the HR resonance and are accompanied by a reduction in spectral energy at higher wavelengths. While quadrant analysis indicates an increase in turbulence production from an increased relative strength of Q2 and Q4 events, variable-interval time averaging indicates no significant changes to the intensity and duration of the near-wall cycle turbulence events as a result of the HR resonance. To date, no significant changes to the mean boundary layer statistics were found that could directly be attributed to the achieved pressure-velocity coupling. Note that only a single HR has been the focus of the current study, with valuable information about HR scaling, resonance and domain-of influence on the TBL flow. Future arrays of HRs may be able to show a more pronounced global effect on the mean flow.Aerospace Engineerin