Small-scale Helmholtz resonators with grazing turbulent boundary layer flow

Helmholtz resonators flush-mounted in a wall beneath turbulent boundary layer flow are studied by focusing on their flow-induced excitation and effect on the grazing turbulent flow. A particular focus lies on single resonators tuned to the most intense spatio-temporal fluctuations in the near-wall vertical velocity and wall-pressure, residing at a spatial scale of $ \lambda _x<^>+ \approx 250 $ lambda x+approximate to 250 (or temporal scale of $ T<^>+ \approx 25 $ T+approximate to 25). Resonators are examined in a boundary layer flow at $ Re_\tau \approx 2280 $ Re tau approximate to 2280. Two neck-orifice diameters of $ d<^>+ \approx 68 $ d+approximate to 68 and 102 are considered, and for each value of $ d<^>+ $ d+ three different resonance frequencies are studied (corresponding to a period of $ T<^>+ \approx 25 $ T+approximate to 25, as well as one lower, and one higher, period). The response of the TBL flow is analysed by employing velocity data from hot-wire anemometry and particle image velocimetry measurements. Passive resonance only affects streamwise velocity fluctuations in the region $ y<^>+ \lesssim 25 $ y+less than or similar to 25, while vertical velocity fluctuations due to resonance reach up to $ y<^>+ \approx 100 $ y+approximate to 100. A narrow-band increase in streamwise turbulence kinetic energy at the resonance scale co-exists with a more than 20% attenuation of lower-frequency energy. Current findings on single resonator cases will aid in the development of passive surfaces with distributed resonators for boundary-layer flow control.