• Journal of Mechanical Science and Technology
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• 제19권8호
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• pp.1682-1691
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• 2005

Direct numerical simulation database of an axial turbulent boundary layer is used to compute frequency and wave number spectra of the wall shear-stress fluctuations in a low-Reynolds number axial turbulent boundary layer. One-dimensional and two-dimensional power spectra of flow variables are calculated and compared. At low wave numbers and frequencies, the power of streamwise shear stress is larger than that of spanwise shear stress, while the powers of both stresses are almost the same at high wave numbers and frequencies. The frequency/streamwise wave number spectra of the wall flow variables show that large-scale fluctuations to the rms value is largest for the stream wise shear stress, while that of small-scale fluctuations to the rms value is largest for pressure. In the two-point auto-correlations, negative correlation occurs in streamwise separations for pressure, and in span wise correlation for both shear stresses.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2000년도 춘계학술대회 논문집
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• pp.75-80
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• 2000

We revisit the arrested Ekman boundary layer problem, using a fully non-linear numerical model with the subgrid dissipation modeled by the large eddy simulation method (LES). The main objective of this study is to find out whether the dynamic balance of the arrested Ekman boundary layer explained by MacCready and Rhines (1991) is valid for high Reynolds number. The model solution indicates that for high Reynolds number and low Richardson number flows, the density anomaly diffusion by near-wall turbulent action may become intense enough to homogenize completely the density structure within the boundary layer, in the direction perpendicular to the sloping wall. Then the buoyancy effect becomes negligible allowing a near-equilibrium Ekman boundary layer flow to persist for a long period.

• Tribology and Lubricants
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• 제35권3호
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• pp.190-198
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• 2019

Gas foil bearings (GFBs) enable small- to medium-sized turbomachinery to operate at ultra-high speeds in a compact design by using ambient air or process gas as a lubricant. When using air or process gas, which have lower viscosity than lubricant oil, the turbomachinery has the advantage of reduced power loss from bearing friction drag. However, GFBs may have high Reynolds number, which causes turbulent flows due to process gas with low viscosity and high density. This paper analyzes gas foil journal bearings (GFJBs) with high Reynolds numbers and studies the effects of turbulent flows on the static and dynamic performance of bearings. For comparison purposes, air and R-134a gas lubricants are applied to the GFJBs. For the air lubricant, turbulence is dominant only at rotor speeds higher than 200 krpm. At those speeds, the journal eccentricity decreases, but the film thickness, power loss, and direct stiffness and damping coefficients increase. On the other hand, the R-134a gas lubricant, which that has much higher density than air, causes dominant turbulence at rotor speeds greater than 10 krpm. The turbulent flow model predicts decreased journal eccentricity but increased film thickness and power loss when compared with the lamina flow model predictions. The vertical direct stiffness and damping coefficients are lower at speeds below 100 krpm, but higher beyond that speeds for the turbulent model. The present results indicate that turbulent flow effects should be considered for accurate performance predictions of GFJBs with high Reynolds number.

• 수산해양교육연구
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• 제28권5호
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• pp.1388-1394
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• 2016

This paper presents a experimental study of two-phase flow boiling of FC-72 in multi channels. Flow boiling heat transfer coefficients are obtained with mass flux ranging from 152.9 to $353.9kg/m^2s$ and heat flux from 5.6 to $46.1kW/m^2$. The experimental results show that the heat transfer is governed by nucleate boiling mechanism in the low heat flux region. However, it is found that the effects of nucleate boiling and forced convection boiling are combined as the heat flux increases. A new correlation to predict the heat transfer coefficient is developed by using the dimensionless number such as Reynolds number, Weber number, boiling number. This correlation shows good predictive accuracy against the measured data.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2011년도 제37회 추계학술대회논문집
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• pp.178-181
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• 2011

Recently, micro shock tube is being extensively used in various fields of engineering applications. The flow characteristics occurring in the micro shock tube may be significantly different from that of conventional macro shock tube due to very low Reynolds number and Knudsen number effects which are, in general, manifested in such flows of rarefied gas, solid-gas two-phase, etc. In these situations, Navier-Stokes equations cannot properly predict the micro shock tube flow. In the present study, a two-dimensional CFD method has been applied to simulate the micro shock tube, with slip velocity and temperature jump boundary conditions. The effects of wall thermal conditions on the unsteady flow in the micro shock tube were also investigated. The unsteady behaviors of shock wave and contact discontinuity were, in detail, analyzed. The results obtained show much more attenuation of shock wave, compared with macro-shock tubes.

• 대한기계학회논문집
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• 제17권8호
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• pp.2110-2121
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• 1993

A numerical investigation has been carried out for the flow characteristics after exchange of some filters from the original layer to the new low pressure loss layer with equal filtering efficiency. The solution domain includes upper plenum, filter layer, clean space, access panels, and lower plenum. The concept of the distributed pressure resistance was applied to describe the momentum loss in filter layer and access panels. The evolution of the flow field is simulated using the low Reynolds number k-.epsilon. over bar turbulent model and SIMPLE algorithm based on the finite volume method. As a result, after the exchange of filter layer the power requirement can be reduced by 8-9 percent. The results also demonstrate that the perpendicularity of the flow near access panels may become worse at new filter layer than origianl filter layer. But the situation can be recovered by adjusting the jopening ratio of access panels.

• Advances in aircraft and spacecraft science
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• 제7권4호
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• pp.371-385
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• 2020

A forward-facing aerospike attached to a payload fairing of a satellite launch vehicle significantly alters its flowfield and decreases the aerodynamic drag in transonic and low supersonic speeds. The present payload fairing is an axisymmetric configuration and consists of a blunt-nosed body along with a conical section, payload shroud, boat tail and followed by a booster. The main purpose of the present numerical simulations is to evaluate flowfield and assess the performance of aerodynamic drag coefficient with and without aerospike attached to a payload fairing of a typical satellite launch vehicle in freestream Mach number range 0.8 ≤ M_∞ ≤ 3.0 and freestream Reynolds number range 33.35 × 10⁶/m ≤ Re_∞ ≤ 46.75 × 10⁶/m whichincludes the maximum aerodynamic drag and maximum dynamic conditions during ascent flight trajectory of the satellite launch vehicle. A numerical simulation has been carried out to solve time-dependent compressible turbulent axisymmetric Reynolds-averaged Navier-Stokes equations. The closure of the system of equations is achieved using the Baldwin-Lomax turbulence model. The aerodynamic drag reduction mechanism is analysed employing numerical results such as velocity vector plots, density and Mach contours in conjunction with the experimental flow visualization pictures. The variations of wall pressure coefficient over the payload fairing with and without aerospike are exhibiting different kind of flowfield characteristics in the transonic and low supersonic speeds. The numerically computed results are compared with schlieren pictures, oil flow patterns and measured wall pressure distributions and exhibit good agreement between them.

• 한국전산유체공학회지
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• 제8권2호
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• pp.1-7
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• 2003

The V2F turbulence model, which has shown very good performance in several test cases at low speeds, has been applied to supersonic ramp flow with 20. corner angle at the free stream Mach number of 2.79. The flow is known to manifest strong shock wave/turbulent boundary layer interactions. As a comparative study, low-Reynolds k-ε models are also considered. While the V2F model predicts wall-pressure distribution well, it relatively predicts larger separation bubble and higher skin-friction after the reattachment than the experimental data. Although the ellpticity of f equation is the characteristics of incompressible flows, the converged solutions are acquired in the compressible flow with shock waves. The effect of the realizability constraints used in the model is also examined. In contrast to the result of impinging jet flows, the realizability bounds proposed by Durbin deterioate the overall solutions of the supersonic ramp flow.

• 대한기계학회논문집B
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• 제27권4호
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• pp.447-457
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• 2003

An experiment in a turbulent non-premixed flat flame was carried out in order to investigate the effect of swirl number on the flow and combustion characteristics. First. stream lines and velocity distribution in the flow field were obtained using PIV method. In contrast with the axial flow without swirl, highly swirled air induced stream lines along the burner tile. and backward flow was caused by recirculation in the center zone of the flow field. In the combustion. the flame with swirled air also became flat and stable along the burner tile with increment of the swirl number. Flame structure by measuring OH and CH radicals intensity and by calculating Damkohler number(Da) and turbulence Reynolds number(Re$_{T}$) was examined. It appeared to be comprised in the wrinkled laminar-flame regime. Backward flow by recirculation of the burned gas decreased the flame temperature and emissions concentrations as NO and CO. Consequently, the stable flat flame with low NO concentration was achieved.d.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 춘계학술대회
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• pp.1237-1242
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• 2004

In this study, single-phase heat transfer experiments were conducted with oil cooler with offset strip fin using water. An experimental water loop has been developed to measure the single-phase heat transfer coefficient in a vertical oil cooler. Downflow of hot water in one channel receives heal from the cold water upflow of water in the other channel. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the on cooler with offset strip fin remains turbulent. The present data show that the heat transfer coefficient increases with the Reynolds number. Based. On the present data, empirical correlation of the heat transfer coefficient was proposed. Also, performance prediction analysis for oil cooler were executed and compared with experiments. ${\varepsilon}-NTU$ method was used in this prediction program. Independent variables are flow rates and inlet temperature. Compared with experimental data, the accuracy of the program is within the error bounds of ${\pm}5$% in the heat transfer rate.