• The Journal of the Korea institute of electronic communication sciences
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• v.13 no.1
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• pp.253-264
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• 2018

The GPS radiosonde is designed to conduct a full synoptic sounding to balloon burst using data generated from precision meteorological sensors and the GPS satellite network. The GPS radiosonde include proven, accurate temperature, humidity and capacitance aneroid pressure sensors. The atmospheric boundary layer was intensively observed in three islands of the west sea from 18 LST on March 9, 2016 to 06 LST on March 12, 2016. We investigated the restriction of nocturnal stable layer and rather the development of the mixed layer at night. On March 9, nocturnal mixed layer was developed by buoyancy heat flux. On the other hand, on March 10, the shear production was higher especially at 21 LST when the mixed layer height was the highest during the intensive observation period. The wind shear and the surface heat flux which produce the turbulent kinetic energy played an important role to grow the mixed layer even at night.

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.191-194
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• 2006

In this paper, we present a detailed mechanism of drag reduction by dimples and roughness on a sphere by measuring the streamwise velocity above the dimpled and roughened surfaces, respectively. Dimples cause local flow separation and trigger the shear layer instability along the separating shear layer, resulting in generation of large turbulence intensity. With this increased turbulence, the flow reattaches to the sphere surface with high momentum near the wall and overcomes strong adverse pressure gradient formed in the rear sphere surface. As a result, dimples delay main separation and reduce drag significantly. The present study suggests that generation of a separation bubble, i.e. a closed-loop streamline consisting of separation and reattachment, on a body surface is an important flow-control strategy for drag reduction on a bluff body such as the sphere and cylinder. In the case of roughened sphere, the boundary layer flow is directly triggered by roughness and changes to a turbulent flow. Due to this change, the drag significantly decreases. As the Reynolds number further increases, transition to turbulence occurs earlier on the sphere surface. Because of faster growth of turbulent boundary layer by roughness, earlier transition thickens the boundary layer, resulting in earlier separation and drag increase with increasing Reynolds number

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.9
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• pp.1290-1306
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• 1998

An experimental study has been performed on a three-dimensional boundary layer over a rotating disk with an impinging jet at the center of the disk. The objective of the present study is to investigate the turbulence statistics of the three-dimensional turbulent boundary layer, which may be regarded as one of the simplest models for the flow in turbomachinery. Six components of the Reynolds stresses and ten triple products are measured by aligning the miniature X-wire probe to the mean velocity direction. The ratio of the wall-parallel shear stress magnitude to twice the turbulent kinetic energy in the near-wall region is strongly decreased by the impinging jet. In the case of the free rotating disk flow the shear stress vector lags behind the mean velocity gradient vector in the whole boundary layer, while the lag is weakened as the impinging jet speed increases.

• Water Engineering Research
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• v.3 no.3
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• pp.163-176
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• 2002

In this study, laboratory experiments have been performed to investigate characteristics of the velocity fields and turbulence for non-buoyant plane jet in the vicinity of the jet nozzle using PIV system. The experimental results show that, in the transition region, the lateral velocity profile is in good agreement with Gaussian distribution. However, the coefficient of Gaussian distribution, $\K_{u,}$, decreases with longitudinal distance in the transition region. The existing theoretical equation for the centerline velocity tends to overestimate the measured data in the transition region. A new equation for the centerline velocity derived by incorporating varying $k_{u}$ gives better agreement with the measured data than the previous equation. The results of the turbulence characteristics show peak values are concentrated on the shear layers. The Reynolds shear stress profile shows the positive peak in the upper layer and negative peak in the lower layer. The turbulent kinetic energy also provides double peaks at the shear layers. The peak of the Reynolds shear stress and the turbulent kinetic energy increases until x/B=8, and then it decreases afterwards.s.

• Journal of computational fluids engineering
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• v.7 no.4
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• pp.8-18
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• 2002

The primary objective of the present study is evaluation of the k-ε-vv-f turbulence model for prediction of natural convection in a rectangular cavity. As a comparative study, the two-layer k-ε model is also considered. Both models, with and without algebraic heat flux model, are applied to the analysis of natural convection in a rectangular cavity. The performances of turbulence models are investigated through comparison with available experimental data. The predicted results of vertical velocity component, turbulent heat fluxes, turbulent shear stress, local Nusselt number and wall shear stress are compared with experimental data. It is shown that, among the turbulence models considered in the present study, the k-ε-vv-f model with an algebraic heat flux model predicts best the vertical mean velocity and velocity fluctuation, and the inclusion of algebraic heat flux model slightly improves the accuracy of results.

• International Journal of Aeronautical and Space Sciences
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• v.18 no.1
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• pp.48-55
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• 2017

Aeroacoustic computation of a fully-developed turbulent pipe flow at $Re_{\tau}=175$ and M = 0.1 is conducted by LES/LPCE hybrid method. The generation and propagation of acoustic waves are computed by solving the linearized perturbed compressible equations (LPCE), with acoustic source DP(x,t)/Dt attained by the incompressible large eddy simulation (LES). The computed acoustic power spectral density is closely compared with the wall shear-stress dipole source of a turbulent channel flow at $Re_{\tau}=175$. A constant decaying rate of the acoustic power spectrum, $f^{-8/5}$ is found to be related to the turbulent bursts of the correlated longitudinal structures such as hairpin vortex and their merged structures (or hairpin packets). The power spectra of the streamwise velocity fluctuations across the turbulent boundary layer indicate that the most intensive noise at ${\omega}^+$ < 0.1 is produced in the buffer layer with fluctuations of the longitudinal structures ($k_zR$ < 1.5).

• Journal of Advanced Marine Engineering and Technology
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• v.24 no.6
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• pp.120-127
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• 2000

This study is concerned with evaluating the effects of acoustic excitation on the development of two stream mixing layer generated by split plate. The ratios of two velocities U1 and U2 either side of the splitter plate were such that $U_1/U_2$=1.0 (uniform flow) or $U_1/U_2$<1.0(shear flow). The mixing layers were disturbed acoustically through the edge of split plate. Quantitative data were obtained with hot-wire anemometry. Flow visualization with smoke-wire was also employed for qualitative study. the results show that the large scale structures of mixing layers are strongly affected by excitation frequency and amplitude in both uniform and shear flows. The maximum streamwise and vertical turbulent intensities of the excited flow fields are apt to be decreased as compared with those of without excitation. The flow characteristics of uniform flow are more influenced by acoustic excitation than those of shear flow.

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.401-402
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• 2006

The turbulent flow around a sphere was investigated using two experimental techniques: smoke-wire flow visualization in wind tunnel at Re=5300, 11000 and PIV measurements in a circulating water channel. The smoke-wire visualization shows flow separation points near an azimuthal angle of $90^{\circ}$, recirculating flow, transition from laminar to turbulent shear layer, evolving vortex roll-up and fully turbulent eddies in the sphere wake. The mean velocity field measured using a PIV technique in x-y center plane demonstrates the detailed near-wake structure such as nearly symmetric recirculation region, two toroidal vortices, laminar separation, transition and turbulent eddies. The PIV measurements of turbulent wake in y-z planes show that a recirculating vortex pair dominates the near-wake region.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.463-466
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• 2002

In order to examine the possibility of using a cavity as a passive device for reduction of skin friction and heat transfer, an intensive parametric study over a broad range of the cavity depth and length at different Reynolds numbers is performed for both laminar and turbulent boundary layers in the present study. Direct and large eddy simulation techniques are used for turbulent boundary layers at low and moderate Reynolds numbers, respectively. for both laminar and turbulent boundary layers over a cavity, a flow oscillation occurs due to the shear layer instability when the cavity depth and length are sufficiently large and it plays an important role in the determination of drag and heat-transfer increase or decrease. For a cavity sufficiently small to suppress the flow oscillation, both the total drag and heat transfer are reduced. Therefore, the applicability of a cavity as a passive device for reduction of drag and heat transfer is fully confirmed in the present study. Scaling based on the wall shear rate of the incoming boundary layer is also proposed and it is found to be valid in steady flow over a cavity.

• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.379-381
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• 2011

Direct numerical simulations (DNSs) of spatially developing turbulent boundary layers (TBLs) over two-dimensional (2-D) and rod and three-dimensional (3-D) cube rough walls were performed to investigate the effects of streamwise spacing on the properties of the TBL The 2-D and 3-D roughness were periodically arranged in the downstream direction with pitches of px/k=2, 3, 4, 6, 8 and 10 and for the cube, the spanwise spacing is fixed to pz/k=2 with staggered array, where px and pz are the streamwise and spanwise spacings of the roughness and k is the roughness height. Inspection of the Reynolds stresses showed that except for px/k=2 and 3 over the 2-D rough walls, the effects of the surface roughness extend to the outer layer over the 2-D and 3-D rough walls and the magnitude of the Reynolds shear stress in the outer layer is increased with proportion to px/k. However, such results are contrary to the trends of form drag, roughness junction and roughness length against px/k, which showed the maximum values at px/k=8 and 4 over the 2-D and 3-D rough walls respectively.