Kyoungyoun Kim

Effects of background noise on generating coherent packets of hairpin vortices

We examine the autogeneration process by which new hairpin vortices are created from a sufficiently strong hairpin vortex, leading to the formation of a hairpin packet. Emphasis is placed on the effects of background noise on packet formation. The initial conditions are given by conditionally averaged flow fields associated with the second quadrant (Q2) event in the fully turbulent channel flow direct numerical simulation (DNS) database at Reτ=395. The nonlinear evolution of the initial vortical structure is tracked by performing a spectral simulation. Background noise is introduced by adding small amplitude perturbations to the initial field or by imposing momentum forcing. The background noise gives rise to chaotic development of a hairpin packet. The hairpins become asymmetric, leading to much more complicated packet structures than are observed in the symmetric hairpin vortex train of the flow with a clean background. However, the chaotic packets show the same properties as the clean packet in terms o...

Effects of Periodic Blowing from Spanwise Slot on a Turbulent Boundary Layer

(A + =0:25;0:50, and 1:00) at a e xed blowing frequency of f + =0:017. The effect of steady blowing (SB) was also examined, and the SB results were compared with those for periodic blowing (PB). PB reduced the skin friction near the slot, although to a slightly lesser extent than SB. PB was found to generate a spanwise vortical structure in the downstream of the slot. This vortex generates a reverse e ow near the wall, thereby reducing the wall shear stress. The wall-normal and spanwise turbulence intensities under PB are increased as compared to those under SB, whereas the streamwise turbulent intensity under PB is weaker than that under SB. PB enhances more energy redistribution than SB. Theperiodicresponseof thestreamwiseturbulence intensity to PBispropagated to a lesser extent than that of the other components of the turbulence intensities and the Reynolds shear stress.

Assessment of Local Blowing and Suction in a Turbulent Boundary Layer

Effects of local blowing or suction from a spanwise slot on a turbulent boundary layer flow are investigated using the direct numerical simulation technique. Three different blowing or suction velocities are imposed on the slot keeping blowing or suction flow rate constant. The recoveries of mean wall pressure for the different blowing velocities collapse well on the same recovery line. However, each wall pressure recovers just after the slot for suction. In the blowing case, the relaxation of rms wall pressure fluctuations and pressure gradient after the slot is seen and the relaxation distance from the slot center is nearly constant for three different blowing velocities. In the suction case, the flow recovers from the immediate rear of the slot. These features are also observed in three-dimensional views of the near-wall vortices.

Effects of unsteady blowing through a spanwise slot on a turbulent boundary layer

The effects of localized periodic blowing on a turbulent boundary layer were investigated by direct numerical simulation. Time-periodic blowing was applied through a spanwise slot by varying the wall-normal velocity in a cyclic manner from 0 to

Wall Pressure Fluctuations in a Turbulent Boundary Layer After Blowing or Suction

2A^ +

Tensorial time scale in turbulent gradient transport of Reynolds stresses

. Time-periodic blowing was applied at frequencies in the range

A decoupled monolithic projection method for natural convection problems

0\,{ \le}\, f^ +\,{ \le}\, 0.08

Analysis of velocity-components decoupled projection method for the incompressible Navier-Stokes equations

at a fixed blowing amplitude of

Reynolds Shear Stress Distribution in Turbulent Channel Flows

A^ +

Numerical Study on the Effects of GDL Porosity on the PEMFC Performance

= 0.5. Simulations of a spatially evolving turbulent boundary layer were carried out for two Reynolds numbers,