Jinyul Hwang

Contribution of velocity-vorticity correlations to the frictional drag in wall-bounded turbulent flows

The relationship between the frictional drag and the velocity-vorticity correlations in wall-bounded turbulent flows is derived from the mean vorticity equation. A formula for the skin friction coefficient is proposed and evaluated with regards to three canonical wall-bounded flows: turbulent boundary layer, turbulent channel flow, and turbulent pipe flow. The frictional drag encompasses four terms: advective vorticity transport, vortex stretching, viscous, and inhomogeneous terms. Drag-reduced channel flow with the slip condition is used to test the reliability of the formula. The advective vorticity transport and vortex stretching terms are found to dominate the contributions to the frictional drag.

Influence of a large-eddy breakup device on the frictional drag in a turbulent boundary layer

A direct numerical simulation of a spatially developing turbulent boundary layer with a large-eddy breakup (LEBU) device was performed to investigate the influence of the LEBU device on the near-wall turbulence and frictional drag. The LEBU device, which is thin and rectangular in shape, was located at 80% of the boundary layer thickness (δ). The LEBU device reduced the skin-friction coefficient (Cf) up to 17%. The breakdown of the outer structures passing through the LEBU device reduced the energy of the long wavelength motions (λz+>200) along the wall-normal direction. The reduction of Cf mainly arose from the contribution of the Reynolds shear stress by the difference in the spatial coherence of the outer high- and low-speed structures. We investigated the relationship between the large-scale motions and the velocity–vorticity correlations (vωz and −wωy), which directly contribute to Cf. The contributions of vωz and −wωy accounted for 80% of the total Cf reduction. The amount of the Cf reduction induce...

Wall-attached structures of velocity fluctuations in a turbulent boundary layer

Wall turbulence is a ubiquitous phenomenon in nature and engineering applications, yet predicting such turbulence is difficult due to its complexity. High-Reynolds-number turbulence arises in most practical flows, and is particularly complicated because of its wide range of scales. Although the attached-eddy hypothesis postulated by Townsend can be used to predict turbulence intensities and serves as a unified theory for the asymptotic behaviours of turbulence, the presence of coherent structures that contribute to the logarithmic behaviours has not been observed in instantaneous flow fields. Here, we demonstrate the logarithmic region of the turbulence intensity by identifying wall-attached structures of the velocity fluctuations (

Turbulent boundary layer over a divergent convergent superhydrophobic surface

u_{i}

Inner–outer interactions of large-scale structures in turbulent channel flow

) through the direct numerical simulation of a moderate-Reynolds-number boundary layer (

Large-scale motions in a turbulent channel flow with the slip boundary condition

Re_{\unicode[STIX]{x1D70F}}\approx 1000

Influence of large-scale accelerating motions on turbulent pipe and channel flows

). The wall-attached structures are self-similar with respect to their heights (

Influence of large-scale motions on the frictional drag in a turbulent boundary layer

l_{y}

Contribution of large-scale motions to the skin friction in a moderate adverse pressure gradient turbulent boundary layer

), and in particular the population density of the streamwise component (

Role of wall-attached structures in the interface of the quiescent core region in turbulent pipe flow

u