• Journal of Ocean Engineering and Technology
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• v.16 no.3
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• pp.8-18
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• 2002

Formulation of the far-field method for the prediction of time-mean hydrodynamic force and moment acting on a 3-D surface-piercing body in waves is reviewed. It is found that the inequality between the weight of the floating body and its buoyancy force permits the replacement of the fluid particles inside the control surface by the fluid particles outside the control surface. Under such circumstances, momentum exchanges across the control surface make the time-mean value of the time rate of the momentum of the fluid inside the control surface non-vanishing. It is a second-order quantity which is hard to calculate by the far-field method. The drift forces and moments on half-immersed ellipsoids are calculated by both the far-field method and the near-field method. The discrepancy between two numerical results is presented and discussed.

• Communications of the Korean Mathematical Society
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• v.33 no.2
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• pp.571-580
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• 2018

The object of the present paper is to prove the non-existence of pseudo projective Ricci symmetric spacetimes $(PW\;RS)_4$ with different types of energy momentum tensor. We also discuss whether a fluid $(PW\;RS)_4$ spacetime with the basic vector field as the velocity vector field of the fluid can admit heat flux. Next we consider perfect fluid and dust fluid $(PW\;RS)_4$ spacetimes respectively. Finally we construct an example of a $(PW\;RS)_4$ spacetime.

• Membrane and Water Treatment
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• v.10 no.5
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• pp.387-394
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• 2019

Membrane distillation (MD) is one of the water treatment processes which involves the momentum, heat and mass transfer through channels and membrane. In this study, CFD modeling has been used to simulate the heat and mass transfer in the direct contact membrane distillation (DCMD). Also, the effect of operating parameters on the water flux is investigated. The result shows a good agreement with the experimental result. Results indicated that, while feed temperature is increasing in the feed side, water flux improves in the permeate side. Since higher velocity leads to the higher mixing and turbulence in the feed channel, water flux rises due to this increase in the feed velocity. Moreover, results revealed that temperature polarization coefficient is rising as flow rate (velocity) increases and it is decreasing while the feed temperature increases. Lastly, the thermal efficiency of direct contact membrane distillation is defined, and results confirm that thermal efficiency improves while feed temperature increases. Also, flow rate increment results in enhancement of thermal efficiency.

• Atmosphere
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• v.26 no.3
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• pp.473-482
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• 2016

An accurate determination of turbulent fluxes over an urban area is a challenging task due to its morphological diversity and associated flow complexity. In this study, an eddy covariance (EC) method is applied over a highly heterogeneous urban area in a small city (Gongju), South Korea to investigate the quantitative influence of 'coordinate tilt' in determining the turbulent fluxes of sensible heat, latent heat, momentum, and carbon dioxide mass. Two widely-used coordinate transform methods are adopted and applied to eight directional sections centered on the site to analyze a 1-year period EC measurement obtained from the urban site: double rotation (DR) and planar fit (PF) transform. The results show that mean streamline planes determined by the PF method are distinguished from the sections, representing morphological heterogeneity of the site. The sectional pitch angles determined by the DR method also compare well with those in the PF method. Both the PF and DR methods show large variabilities in the determined streamline planes at each directional section, implying that flow patterns may form in a complicate way due to the surface heterogeneity. Resulting relative differences of the turbulent fluxes, defined by $(F_{DR}-F_{PF})/F_{DR}$, are found on average +13% in sensible heat flux, +21% in latent heat flux, +37% in momentum flux, and +26% in carbon dioxide mass flux, which are larger values than those reported previously for fairly homogeneous natural sites. The fractional differences depend significantly on wind direction, showing larger differences in northerly winds at the measurement site. It is also found that the relative fractional differences are negatively correlated with the mean wind speed at both stable/unstable atmospheric conditions. These results imply that EC turbulent fluxes determined over heterogeneous urban areas should be carefully interpreted with considering the uncertainty due to 'coordinate tilt' effect in their applications.

• Journal of ILASS-Korea
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• v.20 no.3
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• pp.168-174
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• 2015

Combustion instability is critical problem in developing liquid rocket engine. There have been many efforts to solve this problem. In this study, the method was sought through the injector as part of these efforts to suppress combustion instability. If the injector can suppress the disturbance coming from the supply line as a kind of buffer it will serve to reduce combustion instability. Especially we target at gas propellant oscillation in gas-centered swirl coaxial injector. The phenomenon is simulated with acoustic excitation of speaker. The film thickness response at injector exit was measured by using a liquid film electrode. Also the response of spray to the disturbance was observed by high-speed photography. Gas-liquid momentum flux ratio and the frequency of feeding gas oscillation were changed to investigate the effect of these experimental parameters. The trend of response by varying these parameters and the cause of weak points was studied to suggest the better design of injector for suppressing combustion instability.

• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.1
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• pp.55-63
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• 2012

The turbulent flow characteristics in a coaxial injector were investigated by the nonlinear $k-{\varepsilon}-f_{\mu}$ model of Park et al.[1] and large eddy simulation (LES). In order to analyze the geometric effects on the scalar mixing for nonreacting variable-density flows, several recessed lengths and momentum flux ratios are selected at a constant Reynolds number. The nonlinear $k-{\varepsilon}-f_{\mu}$�� model proposed the meaningful characteristics for various momentum flux ratios and recess lengths. The LES results showed the changes of small-scale structures by the recess. When the inner jet was recessed, the development of turbulent kinetic energy became faster than that of non-recessed case. Also, the mixing characteristics were mainly influenced by the variation of shear rates, but the local mixing was changed by the adoption of recess.

• 유체기계공업학회:학술대회논문집
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• 2004.12a
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• pp.172-179
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• 2004

Average Bi-directional flow tube was suggested to measure single and two phase flow rate. Its working principle is similar with Pilot tube, however, it makes it possible to eliminate the cooling system which is normally needed to prevent from flashing in the pressure impulse line of Pilot tube when it is used in the depressurization condition. The suggested instrumentation was tested in the air-water vertical test section which has 80mm inner diameter and 10m length. The flow tube was installed at 120 of L/D from inlet of test section. From the test, single air and water flow rate was measured successfully. For the emasurement of two phase flow rate, Chexal drift-flux correlation was used. In the test a new correlation of momentum exchange factor was suggested. The test result shows that the suggested instrumentation using the measured void fraction and Chexal drift-flux correlation can predict the mass flow rates within $10\%$ error of measured data.

• Journal of the Korean Society of Propulsion Engineers
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• v.7 no.2
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• pp.15-22
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• 2003

Computational study using the 2-dimensional, compressible, Navier-Stokes equations is performed to predict the discharge coefficient of air flow through a film-cooling hole. In order to investigate the effect of internal/external flows on discharge coefficient, the present computational results which are obtained for three flow cases, only external flow, only internal flow, and no flow, are compared with experimental ones. It is found that the computational results predict the discharge coefficient of the film-cooling hole in a reasonable accuracy and the external crossflow reduces the discharge coefficient, while the internal crossflow increases the discharge coefficient in a range of momentum flux ratio $I_{c-jet}$ > 1 due to the total pressure loss and boundary layer effect.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.04a
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• pp.57-60
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• 2011

The spray characteristics according to the recess length of the GCSC injector and the change of momentum flux ratio(MFR) of the gas and the liquid have been examined through high pressure cold flow test using a high pressure chamber. The liquid in this experiment was water, and the gas was nitrogen. The spray images were taken by a back-lit strobe imaging technique. Results showed that the spray was a wide hollow cone at the lower MFR(liquid velocity was fixed) and the spray became a narrow solid cone as the MFR was increased. And the injector with short recess length produced a narrow solid cone at the higher MFR.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.04a
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• pp.16-19
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• 2011

An experimental study on the spray characteristics of Gas-centered swirl coaxial injectors(GCSCI) for high-performance staged combustion rocket engines has been carried out using cold flow tests. In this study, water and gaseous nitrogen are used as working fluids and a back-lit photography technique with image processing for the measurements of spray characteristics. Our study is focused on the effect of injector geometries like as gap thickness of liquid nozzle and gas nozzle and momentum flux ratio for fundamental understanding of the injectors.