  Response details are presented of small-scale Helmholtz resonators excited by
grazing turbulent boundary layer flow. A particular focus lies on scaling of
the resonance, in relation to the spatio-temporal characteristics of the
near-wall velocity and wall-pressure fluctuations. Resonators are tuned to
different portions of the inner-spectral peak of the boundary-layer
wall-pressure spectrum, at a spatial scale of $\lambda_x^+ \approx 250$ (or
temporal scale of $T^+ \approx 25$). Following this approach, small-scale
resonators can be designed with neck-orifice diameters of minimum intrusiveness
to the grazing flow. Here we inspect the TBL response by analysing velocity
data obtained with hot-wire anemometry and particle image velocimetry
measurements. This strategy follows the earlier work by Panton and Miller (J.
Acoust. Soc. Am. 526, 800, 1975) in which only the change in resonance
frequency, due to the grazing flow turbulence, was examined. Single resonators
are examined in a boundary layer flow at $Re_\tau \approx 2\,280$. Two
neck-orifice diameters of $d^+ \approx 68$ and 102 are considered, and for each
value of $d^+$ three different resonance frequencies are studied (targeting the
spatial scale of $\lambda_x^+ \approx 250$, as well as sub- and
super-wavelengths). Passive resonance only affects the streamwise velocity
fluctuations in the region $y^+ \lesssim 25$, while the vertical velocity
fluctuations are seen in a layer up to $y^+ \approx 100$. A narrow-band
increase in streamwise turbulence kinetic energy at the resonance scale
co-exists with a more than 20% attenuation of lower-frequency (larger scale)
energy. Current findings inspire further developments of passive surfaces that
utilize the concept of changing the local wall-impedance for boundary-layer
flow control, using miniature resonators as a meta-unit.
