Wall-pressure fluctuations are a practically robust input for real-time control systems aimed at modifying wall-bounded turbulence. The scaling behaviour of the wall-pressure-velocity coupling requires investigation to properly design a controller with such input data so that it can actuate upon the desired turbulent structures. A comprehensive database from direct numerical simulations (DNS) of turbulent channel flow is used for this purpose, spanning a Reynolds-number range. Spectral analysis reveals that the streamwise velocity is most strongly coupled to the linear term of the wall pressure, at a Reynolds-number invariant distance-from-the-wall scaling of (and for the wall-normal velocity). When extending the analysis to both homogeneous directions in and, the peak coherence is centred at and for and, and and, respectively. A stronger coherence is retrieved when the quadratic term of the wall pressure is concerned, but there is only little evidence for a wall-attached-eddy type of scaling. An experimental dataset comprising simultaneous measurements of wall pressure and velocity complements the DNS-based findings at one value of k, with ample evidence that the DNS-inferred correlations can be replicated with experimental pressure data subject to significant levels of (acoustic) facility noise. It is furthermore shown that velocity-state estimations can be achieved with good accuracy by including both the linear and quadratic terms of the wall pressure. An accuracy of up to 72 % in the binary state of the streamwise velocity fluctuations in the logarithmic region is achieved; this corresponds to a correlation coefficient of 0.6. This thus demonstrates that wall-pressure sensing for velocity-state estimation - e.g. for use in real-time control of wall-bounded turbulence - has merit in terms of its realization at a range of Reynolds numbers.

This work explores the dynamic response of a turbulent boundary layer to large-scale reactive opposition control, at a friction Reynolds number of Reτ≈2240. A surface-mounted hot-film is employed as the input sensor, capturing large-scale fluctuations in the wall-shear stress, and actuation is performed with a single on/off wall-normal blowing jet positioned 2.4δ downstream of the input sensor, operating with an exit velocity of vj=0.4U∞. Our study builds upon the work of Abbassi et al. [Int. J. Heat Fluid Flow 67, 30 (2017)0142727X10.1016/j.ijheatfluidflow.2017.05.003] and includes a control-calibration experiment and a performance assessment using PIV- and PTV-based flow field analyses. With the control-off calibration-experiment conducted a priori, a transfer kernel is identified so that the velocity fluctuations that are to-be-controlled can be estimated. The controller targets large-scale high-speed zones in an "opposing"mode and low-speed zones in a "reinforcing"mode. A desynchronized mode was tested for reference and consisted of a statistically similar control mode, but without synchronization to the incoming velocity fluctuations. An energy-attenuation of about 40 % is observed for the opposing control mode in the frequency band corresponding to the passage of large-scale motions. This proves the effectiveness of the control in targeting large-scale motions: an energy-intensification of roughly 45% occurs for the reinforcing control mode instead, while no change in energy, within the wall-normal range targeted, appears with the desynchronized control mode. Moreover, direct measures of the skin-friction drag are inferred from PTV data. Results indicate that the opposing control logic yields the lowest wall-shear stress (3% lower than the desynchronized control, and 10% lower than the uncontrolled flow). Finally, a FIK-decomposition of the skin-friction coefficient revealed that the off-the-wall turbulence follows a consistent trend with the PTV-based wall-shear stress measurements, although biased by an increased shear in the wake of the boundary layer given the formation of a plume due to the jet-in-crossflow actuation.

Wall-pressure spectra and coherence between wall-pressure and streamwise velocity in a turbulent pipe flow are presented. An experimental investigation was conducted in the CICLoPE long-pipe facility at friction Reynolds numbers in the range of 4700≲Re_τ≲46000. Wall-pressure energy spectra reveal an alignment of the inner-spectral peak location in terms of λ_x⁺≈250, as well as an increase in overall energy content with increasing Re_τ. Linear coherence spectra between wall-pressure and streamwise velocity in the logarithmic region follow a Reynolds-number-independent wall-scaling. Identification of such a universal scaling contributes to compelling evidence that wall-pressure sensing, as an input for real-time flow control, is a feasible approach for implementation in practical engineering systems.