Hyung Jin Sung

Control of turbulent separated flow over a backward-facing step by local forcing

An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13000 ⩽ ReH⩽ 33000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 ⩽ A0 ⩽ 0.07) and forcing frequency (0 ⩽ StH⩽ 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment, which lead to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length (xr)min/xx0was obtained at Stθ≅ 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.

Simulation of flexible filaments in a uniform flow by the immersed boundary method

An improved version of the immersed boundary (IB) method is developed for simulating flexible filaments in a uniform flow. The proposed IB method is based on an efficient Navier-Stokes solver adopting the fractional step method and a staggered Cartesian grid system. The fluid motion defined on an Eulerian grid and the filament motion defined on a Lagrangian grid are independently solved and their interaction force is explicitly calculated using a feedback law. A direct numerical method is developed to calculate the filament motion under the constraint of inextensibility. When applied to the case of a swinging filament analogous to a rope pendulum, the proposed method gave results very similar to those of the analytical solution derived using the perturbation method. For a flexible filament flapping in a uniform flow, the mechanism by which small vortex processions are produced was investigated. The bistable property of the system was observed by altering the filament length, and the effects of the boundary condition at the fixed end (simply supported or clamped) were studied. For two side-by-side filaments in a uniform flow, both in-phase flapping and out-of-phase flapping were reproduced in the present simulations. A repulsive force was included in the formulation to handle collisions between the free ends of side-by-side filaments undergoing out-of-phase flapping.

Drag Reduction by Spanwise Wall Oscillation in Wall-Bounded Turbulent Flows

Drag reduction in turbulent channel and pipe e ows by spanwise (circumferential) wall oscillations is studied numerically. The ine uence of the wall oscillation on near-wall streamwise vortices is examined. By the use of the Stokes second problem, a wall-normal distanceparameter and an acceleration parameterare obtained toestimate the drag reduction rate. A simple equation is derived for expressing the drag reduction rate by spanwise wall oscillations. The relation between near-wall streamwise vortices and low- and high-speed e uids is scrutinized to extract the key parameters. The drag reduction mechanism is analyzed in terms of the attenuation of Reynolds shear stress.

Multiple-arrayed pressure measurement for investigation of the unsteady flow structure of a reattaching shear layer

Spatio-temporal characteristics of wall pressure fluctuations in separated and reattaching flows over a backward-facing step were investigated through an extensive pressure-velocity joint measurement with an array of microphones. The experiment was performed in a wind tunnel with a Reynolds number of 33 000 based on the step height and the free-stream velocity. Synchronized wavelet maps showed the evolutionary behaviour of pressure fluctuations and gave further insight into the modulated nature of large-scale vortical structures. To see the relationship between the flow eld and the relevant spatial mode of the pressure eld, a new kind of wavenumber ltering, termed ‘spatial box ltering’ (SBF), was introduced and examined. The vortical flow eld was reconstructed using every single-point velocity measurement by means of the conditional average based on the SBF second mode of pressure fluctuations. The flow eld showed a well-organized spanwise vortical structure convected with a speed of 0:6U0 and a characteristic ‘sawtooth’ pattern of the unsteady trace of reattachment length. In addition to the coherent vortical structures, the periodic enlargement/shrinkage process of the recirculation region owing to flapping motion was analysed. The recirculation region was found to undergo an enlargement/shrinkage cycle in accordance with the lowpass-ltered component of pressure fluctuations. In addition, such modulatory behaviour of the vortical structure as the global oscillation phase was discussed in connection with the conditionally averaged flow eld.

Numerical simulation of the flow behind a rotary oscillating circular cylinder

A numerical study was made of flow behind a circular cylinder in a uniform flow, where the cylinder was rotationally oscillated in time. The temporal behavior of vortex formation was scrutinized over broad ranges of the two externally specified parameters, i.e., the dimensionless rotary oscillating frequency (0.110⩽Sf⩽0.220) and the maximum angular amplitude of rotation (θmax=15°, 30°, and 60°). The Reynolds number (Re=U∞D/ν) was fixed at Re=110. A fractional-step method was utilized to solve the Navier–Stokes equations with a generalized coordinate system. The main emphasis was placed on the initial vortex formations by varying Sf and θmax. Instantaneous streamlines and pressure distributions were displayed to show the vortex formation patterns. The oscillatory forcing was in the vicinity of the lock-on range, which can be applied to flow feedback control afterwards. The vortex formation modes and relevant phase changes were characterized by measuring the lift coefficient (CL) and the time of negative ma...

Enhanced photoluminescence, Raman spectra and field-emission behavior of indium-doped ZnO nanostructures

Indium-doped (In-doped) ZnO nanostructures with four different morphologies, namely nanochips, nanotripods, nanorods, and nanodisks, have been successfully synthesized by a simple hydrothermal method. The effects of indium dopant and various morphologies on the structural, optical and field-emission properties of these ZnO nanostructures were investigated. The XRD patterns demonstrated that the In-doped ZnO nanostructures exhibited the hexagonal wurtzite structure with preferential orientation along the (0 0 2) crystal plane, and a slight difference in lattice parameters was detected among the samples with various morphologies. The doped nanostructures were found to be single crystals grown along the c-axis. The composition of the doped ZnO nanostructures was confirmed by X-ray diffraction (XRD), X-ray photospectroscopy (XPS), and energy-dispersive spectroscopy (EDS). The photoluminescence (PL) spectra of doped ZnO nanostructures having a blue-shift in the UV region show a prominent tuning in the optical band gap, without any significant peak relating to intrinsic defects. From Raman spectra, the 437 cm−1 mode corresponds to the E2 mode for wurtzite ZnO crystals with very sharp features which revealed the better crystallinity of samples. The lowest turn-on field of the field-emission was found to be ∼2.5 V μm−1 and the highest emission current density of 1.13 mA cm−2 was also obtained for In-doped ZnO nanochips under a field of 6.3 V μm−1. The field enhancement factor β was estimated to be 10 640 ± 3 in the case of doped nanochips, which was much higher than that of any previous report. Moreover, the doped ZnO nanostructures exhibit good long period emission current stability with a variation of less than 5% during 25 h under a field of 6.3 V μm−1. The superior field-emission properties were attributed to the better morphologies, In-doping and better crystallinity of In-doped ZnO nanostructures.

Direct numerical simulation of the turbulent boundary layer over a rod-roughened wall

The effects of surface roughness on a spatially developing turbulent boundary layer (TBL) are investigated by performing direct numerical simulations of TBLs over rough and smooth walls. The Reynolds number based on the momentum thickness was varied in the rangeReθ = 300 ∼ 1400. The roughness elements were periodically arranged two-dimensional spanwise rods, and the roughness height was k =1 .5θin ,w hereθin is the momentum thickness at the inlet, which corresponds to k/δ =0 .045 ∼ 0.125, δ being the boundary layer thickness. To avoid generating a rough-wall inflow, which is prohibitively difficult, a step change from smooth to rough was placed 80θin downstream from the inlet. The spatially developing characteristics of the rough-wall TBL were examined. Along the streamwise direction, the friction velocity approached a constant value, and self-preserving forms of the turbulent Reynolds stress tensors were obtained. Introduction of the roughness elements affected the turbulent stress not only in the roughness sublayer but also in the outer layer. Despite the roughnessinduced increase of the turbulent Reynolds stress tensors in the outer layer, the roughness had only a relatively small effect on the anisotropic Reynolds stress tensor in the outer layer. Inspection of the triple products of the velocity fluctuations revealed that introducing the roughness elements onto the smooth wall had a marked effect on vertical turbulent transport across the whole TBL. By contrast, good surface similarity in the outer layer was obtained for the third-order moments of the velocity fluctuations.

Very-large-scale motions in a turbulent boundary layer

Direct numerical simulation of a turbulent boundary layer was performed to investigate the spatially coherent structures associated with very-large-scale motions (VLSMs). The Reynolds number was varied in the range Re θ = 570–2560. The main simulation was conducted by using a computational box greater than 50δ o in the streamwise domain, where δ o is the boundary layer thickness at the inlet, and inflow data was obtained from a separate inflow simulation based on Lunds method. Inspection of the three-dimensional instantaneous fields showed that groups of hairpin vortices are coherently arranged in the streamwise direction and that these groups create significantly elongated low- and high-momentum regions with large amounts of Reynolds shear stress. Adjacent packet-type structures combine to form the VLSMs; this formation process is attributed to continuous stretching of the hairpins coupled with lifting-up and backward curling of the vortices. The growth of the spanwise scale of the hairpin packets occurs continuously, so it increases rapidly to double that of the original width of the packets. We employed the modified feature extraction algorithm developed by Ganapathisubramani, Longmire & Marusic ( J. Fluid Mech ., vol. 478, 2003, p. 35) to identify the properties of the VLSMs of hairpin vortices. In the log layer, patches with the length greater than 3δ–4δ account for more than 40% of all the patches and these VLSMs contribute approximately 45% of the total Reynolds shear stress included in all the patches. The VLSMs have a statistical streamwise coherence of the order of ~6δ; the spatial organization and coherence decrease away from the wall, but the spanwise width increases monotonically with the wall-normal distance. Finally, the application of linear stochastic estimation demonstrated the presence of packet organization in the form of a train of packets in the log layer.

Microchannel Anechoic Corner for Size-Selective Separation and Medium Exchange via Traveling Surface Acoustic Waves

We demonstrate a miniaturized acoustofluidic device composed of a pair of slanted interdigitated transducers (SIDTs) and a polydimethylsiloxane microchannel for achieving size-selective separation and exchange of medium around polystyrene particles in a continuous, label-free, and contactless fashion. The SIDTs, deposited parallel to each other, produce tunable traveling surface acoustic waves (TSAWs) at desired locations, which, in turn, yield an anechoic corner inside the microchannel that is used to selectively deflect particles of choice from their streamlines. The TSAWs with frequency fR originating from the right SIDT and propagating left toward the microchannel normal to the fluid flow direction, laterally deflect larger particles with diameter d1 from the hydrodynamically focused sample fluid that carries other particles as well with diameters d2 and d3, such that d1 > d2 > d3. The deflected particles (d1) are pushed into the top-left corner of the microchannel. Downstream, the TSAWs with frequency fL, such that fL > fR, disseminating from the left SIDT, deflect the medium-sized particles (d2) rightward, leaving behind the larger particles (d1) unaffected in the top-left anechoic corner and the smaller particles (d3) in the middle of the microchannel, thereby achieving particle separation. A particle not present in the anechoic corner could be deflected rightward to realize twice the medium exchange. In this work, the three-way separation of polystyrene particles with diameters of 3, 4.2, and 5 μm and 3, 5, and 7 μm is achieved using two separate devices. Moreover, these devices are used to demonstrate multimedium exchange around polystyrene particles ∼5 μm and 7 μm in diameter.

Liquid transfer between two separating plates for micro-gravure-offset printing

Experimental investigations of the liquid transfer between two separating plates have been carried out, with the aim of increasing the ink transfer ratio in micro-gravure-offset printing. A sessile droplet is divided into two by moving one plate vertically with respect to a fixed plate. The surface contact angles of the sessile droplet on the two plates were of particular interest in this study because the volume ratio of the two droplets varies with the contact angles. Various methods for controlling the surface contact angle, including chemical treatment, plasma surface modification and electrowetting-on-dielectric were assessed. Image analysis of the droplets was carried out to estimate the surface contact angles and to measure the volumes of the droplets. The liquid transfer process and related phenomena, such as satellite droplet generation and the variation in the contact areas between the droplets and the plates, were observed. The experimental results help us to determine the optimal surface contact angles of the plate cylinder, blanket cylinder and substrate for micro-gravure-offset printing.