Small-scale Helmholtz resonators with grazing turbulent boundary layer flow

Abstract

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 (Formula presented.) (or temporal scale of (Formula presented.)). Resonators are examined in a boundary layer flow at (Formula presented.). Two neck-orifice diameters of (Formula presented.) and 102 are considered, and for each value of (Formula presented.) three different resonance frequencies are studied (corresponding to a period of (Formula presented.), 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 (Formula presented.), while vertical velocity fluctuations due to resonance reach up to (Formula presented.). 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.