Modification of burst events in the near-wall region of turbulent boundary layers by synthetic jets

Abstract

Burst events in the near-wall region of turbulent boundary layers are the main contributors to skin friction drag in wall flows. This paper shows how synthetic jets modify the near-wall burst events in turbulent boundary layers, leading to local drag reduction. Different characteristics of synthetic jets including various non-dimensional jet frequencies, ranging from 0.025 to 0.063, at two non-dimensional jet amplitudes of 0.1 and 0.2 were examined by conducting hot-wire measurements at a Reynolds number of $R{e}_{\theta }\approx$ 1050. Statistical analysis utilizing a variable-interval time-averaging (VITA) technique illustrated a reduction of up to 15% in burst intensity and a decrease of approximately 10% in burst durations within the near-wall region, $y^{+}\le 12$, indicating that synthetic jets lifted turbulent kinetic energy and weakened burst events in this region. Consequently, there was a reduction of up to approximately 14% in turbulence intensity near the wall, contributing to diminished shear stresses and local skin-friction drag. Furthermore, the synthetic jets generated a displacement of the inner peak of turbulent energy away from the wall, indicating that the synthetic jets shifted turbulent kinetic energy away from the wall. As the jet frequency or amplitude increased, the modification of the boundary layer became more pronounced.