Intermittent Flow and Thermal Structures of Accelerating Transitional Boundary Layers: Part 2—Fluctuation Quantities

Abstract

The conditionally sampled fluctuation quantities of nonaccelerating and accelerating heated transitional boundary layers were analyzed. The results indicated that the values of u′, υ′, uv , and ut , in the turbulent part of the transitional flow were higher than those values in the fully developed turbulent flow. These higher values were believed to be manifestations of the vigorous activities involved in the transition process. The contributions to the unconditioned u′ by “mean-step” change due to the alternating behavior between turbulent and nonturbulent flows are about 20 percent in the near-wall region, but are negligible for Y+ > 30. The turbulent part uv values are higher than the fully turbulent and unconditioned values in the inner boundary layer but lower in the outer boundary layer. The mean-step change has negligible effect on unconditioned uv values. As acceleration increases, both u′ and t′ in the turbulent part are suppressed; however, turbulent part u′ is still higher than the unconditioned u′ Acceleration promotes streamwise Reynolds heat flux (ut ) transport in both turbulent and nonturbulent parts. A second peak of the turbulent part ut occurs at around Y+ = 120 as acceleration increases. The turbulent part eddy viscosity values are much lower than those in the fully turbulent flow.