Turbulent drag reduction by spanwise traveling ribbed surface waves

The combination of passive and active drag reduction techniques is investigated by particle-image velocimetry (PIV) and micro-particle tracking velocimetry (mu-PTV) measurements. To be more precise, the impact of a riblet-structured surface, which represents the passive control technique, undergoing spanwise transversal wave motion, which defines the active flow control method, on the wall-shear stress distribution of a zero-pressure gradient turbulent boundary layer is analyzed. The experimental setup consists of a flat plate equipped with an insert to generate a spanwise traveling wave on a riblet-structured aluminum sheet. The PIV and mu-PTV measurements are conducted downstream of the actuated riblet surface at two momentum thickness based Reynolds numbers Re-theta = 1200 and 2080. The surface deformation is generated by an electromagnetic actuator system at three amplitudes A = 0.25, 0.30, and 0.375 mm with a wavelength of lambda = 160 mm and a frequency of f = 81 Hz. The nondimensional quantities in inner wall units defined by the friction velocity of the non-actuated flat plate flow at Re-theta = 1200 are the amplitudes A(+) = 6, 7, and 9, the wavelength lambda(+) = 3862, and the frequency T+ = 110 and the corresponding quantities for Re-theta = 2800 are A(+) = 11, 14 and 17, lambda(+) = 7170, and T+ = 380. The results are twofold. On the one hand, the combination of the riblet surface with the transversal wave motion increases the local drag reduction (DR). That is, compared to a boundary layer flow over a non-actuated smooth surface a local drag reduction of up to DR = 9.4 % is determined at the chosen measurement position whereas the drag reduction due to the riblet surface is DR = 4.7%. On the other hand, the dependence of the local drag reducing efficiency of the riblets on their geometric dimensions with respect to the flow field could be decreased. That is, the transversal wave motion extends the positive drag reducing efficiency range of the riblets. (C) 2015 Elsevier Masson SAS. All rights reserved.