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 $ λ x + ≈ 250 (or temporal scale of $ T^+ \approx 25 $ T + ≈ 25). Resonators are examined in a boundary layer flow at $ Re_\tau \approx 2280 $ R e τ ≈ 2280. Two neck-orifice diameters of $ d^+ \approx 68 $ d + ≈ 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 + ≈ 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 + ≲ 25 , while vertical velocity fluctuations due to resonance reach up to $ y^+ \approx 100 $ y + ≈ 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. [ABSTRACT FROM AUTHOR]