Experimental assessment of square wave spatial spanwise forcing of a turbulent boundary layer

• Published in: arXiv.org
• Main Authors: Knoop, Max W, Hartog, Friso H, Schrijer, Ferdinand F J, Olaf W G van Campenhout, Michiel van Nesselrooij, van Oudheusden, Bas W
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 16.03.2024
• Subjects: Actuation; Boundary conditions; Boundary layer flow; Drag reduction; Flow control; Kinetic energy; Particle image velocimetry; Physics - Fluid Dynamics; Turbulence; Turbulent boundary layer; Turbulent flow; Velocity distribution; Waveforms
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2308.04122

We present an experimental realisation of spatial spanwise forcing in a turbulent boundary layer flow, aimed at reducing the frictional drag. The forcing is achieved by a series of spanwise running belts, running in alternating spanwise direction, thereby generating a steady spatial square-wave forcing. SPIV in the streamwise-wall-normal plane is used to investigate the impact of actuation on the flow in terms of turbulence statistics, drag performance characteristics, and spanwise velocity profiles, for a non-dimensional wavelength of \(\lambda_x^+ = 397\). We confirm that a significant flow control effect can be realised with this type of forcing. The scalar fields of the higher-order turbulence statistics show a strong attenuation of stresses and production of turbulence kinetic energy over the first belt already, followed by a more gradual decrease to a steady-state energy response over the second belt. The streamwise velocity in the near-wall region is reduced, indicative of a drag-reduced flow state. The profiles of the higher-order turbulence statistics are attenuated up to a wall-normal height of \(y^+ \approx 100\), with a maximum streamwise stress reduction of 45% and a reduction of integral turbulence kinetic energy production of 39%, for a non-dimensional actuation amplitude of \(A^+ = 12.7\). An extension of the classical laminar Stokes layer theory is introduced, to describe the non-sinusoidal boundary condition that corresponds to the current case. The spanwise velocity profiles show good agreement with this extended theoretical model. The drag reduction was estimated from a linear fit in the viscous sublayer in the range \(2 \leq y^+\leq 5\). The results are found to be in good qualitative agreement with the numerical implementations of Viotti et al. (2009), matching the drag reduction trend with \(A^+\), and reaching a maximum of 20%.