Targeted turbulent structure control in wall-bounded flows via localized heating

A targeted turbulent flow control strategy, based on selective heating of streamwise-aligned heat strips, is assessed for drag reduction using direct numerical simulations of variable viscosity and compressible turbulent channel flows. As increasing the temperature of a gas increases its viscosity, heating is generally an unfavorable drag mitigation approach. However, through a selective spatial arrangement of the heating array, the slight increase in viscosity and decrease in density can serve to modify the organization of the streamwise-aligned structures and the likelihood of the ejection and sweep events near the wall. This can, under specific conditions, lead to a very modest drag reduction. The optimal spatial arrangement is identified using a bidimensional empirical mode decomposition and targets the near-wall, large-scale turbulent motion. The drag coefficient, at constant mass flow rate, remains unchanged with heating despite up to an 11% increase in the local viscosity above the heating strips. When accounting for the viscosity variation in the drag reduction calculation, an effective drag reduction of 6% is observed.