Fig. 4: Temporal fluctuations in wall stress signals and their spectra illustrate the effects of wall actuation.
From: An energy-efficient pathway to turbulent drag reduction
a Sample of time-series measurements of the wall stress τw at \({{{{{{{{\mathrm{Re}}}}}}}}}_{\tau }=\) 6,000 without actuation and with small-eddy actuation (\({A}^{+}=12,\ {T}_{{{{{{{{\rm{osc}}}}}}}}}^{+}=140,\ {\kappa }_{x}^{+}=0.0014\)), shown in solid black and blue lines, respectively. Mean values of each signal are denoted by dashed lines, whose difference indicates that the wall oscillation results in DR = 24%. b Spectra of τw at \({{{{{{{{\mathrm{Re}}}}}}}}}_{\tau }=\) 6,000 with and without small-eddy actuation with same parameters as in (a). c Spectra of τw at \({{{{{{{{\mathrm{Re}}}}}}}}}_{\tau }=\) 9,700 with and without large-eddy actuation (\({A}^{+}=4.6,\ {T}_{{{{{{{{\mathrm{osc}}}}}}}}}^{+}=604,\ {\kappa }_{x}^{+}=0.0008\)), shown in red and black lines, respectively. Spectra in (b, c) are obtained using hot-wires with 7.5 ≤ l+ ≤ 12 for \(6000\le {{{{{{{{\mathrm{Re}}}}}}}}}_{\tau }\le 9700\), where l is the length of the hot-wire sensor.
