Relaminarized and recovered turbulence under nonuniform body forces

Turbulence in a wall-bounded flow of supercritical fluid can be significantly modulated by nonuniform body forces. This study presents direct numerical simulations performed with a nonuniform streamwise body force varying in the wall-normal direction in a fully developed channel flow. A quasilaminar state and reorganized turbulence were obtained by changing the amplitude of the nonuniform body force which distorts the parabolic mean velocity profile and thus alters the turbulence production due to mean shear. Weak production of the flattened mean velocity profile leads to a relaminarization. In the quasilaminar state, all components of the Reynolds stress tensor are fairly weak except the streamwise fluctuations distant from the wall, which indicates the collapse of the near-wall turbulence self-sustaining cycle. The remaining streamwise fluctuations away from the wall exhibit elongated streaks, which is shown by premultiplied spectra. In the recovered turbulence regime, the Reynolds shear stress has a negative range in the bulk connected with a positive range near the wall. This corresponds to the nonmonotonic shear stress resulting from an M-shaped velocity profile. In a subsequent quadrant analysis of the Reynolds shear stress, the sweep and ejection events are found to dominate near the wall only while inward Q(1)and outward Q(3) motions are significant in the bulk. Both kinds of high-speed events can be attributed to the velocity maxima of the M-shaped mean velocity profile and are found to penetrate towards the wall and the channel center. Interestingly, the flow topology shows the typical teardrop shape once turbulence recovered. Our study contributes to the understanding of flow relaminarization in mixed convection and assists in developing further flow control techniques.