Turbulent skin friction drag accounts for approximately 50% of the total drag in large aircraft. Helmholtz resonators (HRs) tuned to interact with near-wall turbulent structures have shown promise as a passive flow control technique for reducing this drag. Previous studies have shown that HRs attenuate velocity fluctuations at sub-resonance frequencies while amplifying them at resonance and super-resonance.
This study experimentally investigates whether inclining the HR orifice against the flow can induce a phase shift between the orifice pressure and velocity. If achieved, this could lead to attenuation at the resonance frequency as well. In addition, since ambient noise is significant in practical applications and its effect on resonator-turbulence interaction remains unexplored, the influence of noise on a single HR in grazing flow is also investigated. The impact of noise and orifice orientation on sweep, ejection, and turbulence production is also studied. Experiments were conducted in the Delft University Boundary Layer Facility at Reτ ≈ 2500. Time-resolved particle image velocimetry, hot-wire anemometry, and microphone measurements were employed. Three HR orientations (Vertical, Flow-Opposed, and Flow Aligned) were tested under no-noise conditions, tonal excitation at the resonance frequency of the HR, and white noise at two levels. The hot-wire results show that the Flow-Opposed HR attenuates more at sub-resonance and amplifies less at resonance than the other configurations. This suggests that a phase shift is induced, but it is not sufficient to achieve attenuation at resonance. The effect of tonal noise is strongly orientation-dependent: Vertical HR generally shows attenuation of Reynolds stresses, while Flow-Opposed HR shows amplification. Quadrant analysis reveals that sweep and ejection events follow the same trend across all configurations and frequencies.