• Journal of the Society of Naval Architects of Korea
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• v.56 no.4
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• pp.327-334
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• 2019

Recently, there has been a growing interest in artificial supercavitation as a way to reduce friction drag of submerged vehicles. A cavitator plays an important role to generate the supercavity, so many studies have focused on the case of cavitator only. However, the body shape behind the cavitator affects the growth of the supercavity and this effect must be considered for evaluating the overall performance of the system. In this work, we conducted experimental investigation on artificial supercavitation generated by different combinations of the cavitator and body. We observed the supercavity pattern by using a high-speed camera and measured the pressure inside the cavity by using an absolute pressure transducer. We estimated the relation between the amount of injected air and the supercavity shape for different combinations. In summary, the disk type cavitator generates larger supercavity than that of the cone and ellipsoidal cavitators, but cavity development speed is relatively slower rather than the others. Furthermore, fore body angle plays an important role to generate the supercavity enveloping the entire body.

• Journal of the Korean earth science society
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• v.21 no.3
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• pp.315-322
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• 2000

In this study, the data used for the models were a set of 56 geologic estimates of long-term fault slip rates. The hest models were those in which mantle drag was convergent on the Transverse Ranges in the San Andreas fault system, and faults had a low friction (${\mu}$= 0.3). It is clearly important to decide whether these cases of low strength are local anomalies or whether they are representative. Furthermore, it would be helpful to determine fault strength in as many tectonic settings as possible. Analysis of data was considered by unsuspected sources of pore pressure, or even to question the relevance of the friction law. To contribute to the solution of this problem, three attempts were tried to apply finite element method that would permit computational experiments with different hypothesized fault rheologies. The computed model has an assumed rheology and plate tectonic boundary conditions, and produces predictions of present surface velocity, strain rate, and stress. The results of model will be acceptably close to reality in its predictions of mean fault slip rates, stress directions and geodetic data. This study suggests some implications of the thermoelastic characteristics to interpret the relationship with very low strength of San Andreas fault system.

• Journal of Korean Tunnelling and Underground Space Association
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• v.15 no.3
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• pp.301-310
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• 2013

Jet fans are installed in road tunnels in order to maintain critical velocity when fire occurs. Generally the number of jet fans against fire are calculated by considering critical velocity and flow resistance by wall friction, vehicle drag force, thermal buoyance force and natural wind. In domestic case, thermal buoyance force is not considered in estimating the number of jet fans. So, in this study, we investigated the pressure loss due to the thermal buoyance force induced by tunnel air temperature rise and the impact of thermal buoyance force on the number of jet fans by the numerical fire simulation for the tunnel length(500, 750, 1000, 1500, 2000, 3500m) and grade (-1.0, -1.5, -2.0%). Considering the thermal buoyance force, number of jet fans have to be increased. Especially in the case of 100MW of heat release rate, the pressure loss due to thermal buoyance force exceed the maximum pressure loss due to vehicle drag resistance, so it is analyzed that number of 2~11 jet fans are needed additionally than current design criteria. Thus, in case of estimating the number of jet fans, it must be considered of thermal buoyance force induced tunnel air temperature rise by fire.

• Journal of the Korean Society of Marine Environment & Safety
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• v.24 no.1
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• pp.119-125
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• 2018

The particle based computational fluid dynamics (CFD) method, which follow Lagrangian approach for fluid dynamics, fluid particle behavior by tracking all particle calculation physical quantities of each particle. According to basic concept of particle based CFD method, it is difficult to satisfy continuum theory and measure influences from neighboring particle. Article number density and weight function were used to solve aforementioned issue. Difficulties continuum mean simulate non-continuum particles such as solid including granular and sand. In this regard, the particle based CFD method modified solid particle problems by replacing viscous and surface tension forces friction and drag forces. In this paper, particle interaction model for solid particle friction model implemented to simulate solid particle problems. The broken dam problem, which is common to verify particle based CFD method, used fluid or solid particles. The angle of repose was observed in the simulation results the solid particle not fluid particle.

• Advances in nano research
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• v.16 no.4
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• pp.325-340
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• 2024

There are several novel uses for dispersing many nanoparticles into a conventional fluid, including dynamic sealing, damping, heat dissipation, microfluidics, and more. Therefore, melting heat and mass transfer characteristics of a 3-D MHD Hybrid Nanofluid flow over a rotating disc with presenting dufour and soret effects are assessed numerically in this study. In this instance, we investigated both ferric sulfate and molybdenum disulfide as nanoparticles suspended within base fluid water. The governing partial differential equations are transformed into linked higher-order non-linear ordinary differential equations by the local similarity transformation. The collection of these deduced equations is then resolved using a Chebyshev spectral collocation-based algorithm built into the Mathematica software. To demonstrate how different instances of hybrid/ nanofluid are impacted by changes in temperature, velocity, and the distribution of nanoparticle concentration, examples of graphical and numerical data are given. For many values of the material parameters, the computational findings are shown. Simulations conducted for different physical parameters in the model show that adding hybrid nanoparticle to the fluid mixture increases heat transfer in comparison to simple nanofluids. It has been identified that hybrid nanoparticles, as opposed to single-type nanoparticles, need to be taken into consideration to create an effective thermal system. Furthermore, porosity lowers the velocities of simple and hybrid nanofluids in both cases. Additionally, results show that the drag force from skin friction causes the nanoparticle fluid to travel more slowly than the hybrid nanoparticle fluid. The findings also demonstrate that suction factors like magnetic and porosity parameters, as well as nanoparticles, raise the skin friction coefficient. Furthermore, It indicates that the outcomes from different flow scenarios correlate and are in strong agreement with the findings from the published literature. Bar chart depictions are altered by changes in flow rates. Moreover, the results confirm doctors' views to prescribe hybrid nanoparticle and particle nanoparticle contents for achalasia patients and also those who suffer from esophageal stricture and tumors. The results of this study can also be applied to the energy generated by the melting disc surface, which has a variety of industrial uses. These include, but are not limited to, the preparation of semiconductor materials, the solidification of magma, the melting of permafrost, and the refreezing of frozen land.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.12
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• pp.805-814
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• 2011

In the present study, the detailed flow structure and heat transfer characteristics in the newly-designed heat transfer surface geometry were investigated. The surface geometry proposed in the present study is a traditional dimple structure combining with a protrusion inside the dimple, which is named a protrusion-in-dimple in this study. The basic idea underlying the present surface geometry is to enhance the flow mixing and the corresponding heat transfer in the flow re-circulating region generated by a conventional dimple cavity. The present study was performed by the direct numerical simulation at a Reynolds number of 2800 based on mean velocity and channel height and Prandtl number of 0.71. Three different protrusion heights for protrusion-in-dimples were considered as the main design parameter of the present study. The calculated pressure drop and heat transfer capacity were assessed in terms of the Fanning friction factor and Colburn j factor. The overall performances estimated in terms of the volume and area goodness factor for protrusion-in-dimple cases were higher than the conventional dimple case.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.5
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• pp.1189-1196
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• 1988

Thermal entrance lengths of turbulent tube flow for viscoelastic polymer solutions are investigated experimentally in the recirculating flow system with tubes of inside diameters 8.5mm(L/D=710) and 10.3mm(L/D=1158), respectively. In the present system, the hydrodynamic and thermal boundary layers develop simultaneously from the beginning of the test section. To provide the boundary condition of constant heat flux at the wall, the test tubes are heated directly by electricity. The polymer solution used in the current study is 1000 wppm aqueous solution of polyacrylamide(Separan AP-273). The apparent viscosity of the polymer solutions circulating in the flow system are measured by the capillary tube viscometer at regular time intervals. Thermal entrance lengths vary due to the rate of degradation. The entrance lengths of degraded polymer solutions are about 500~600 times the diameter. However, the entrance lengths of fresh polymer solutions are greater than the lengths of the test tubes used in this study suggesting that thermal entrance lengths for viscoelastic polymer solutions are greater than 1100 tube times the diameters. Friction factor is almost insensitive to the degradation, but the heat transfer $j_{H}$-factor is affected seriously by degradation. Based on the present experimental data of fresh solutions a correlation for the heat transfer $j_{H}$-factor is presented.ted.

• Advances in aircraft and spacecraft science
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• v.7 no.3
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• pp.229-249
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• 2020

This paper is the logical follow-up of four papers by the author on the subject "aerodynamics in Mars atmosphere". The aim of the papers was to evaluate the influence of two Mars atmosphere models (NASA Glenn and GRAM-2001) on aerodynamics of a capsule (Pathfinder) entering the Mars atmosphere and also to verify the feasibility of evaluating experimentally the ambient density and the ambient pressure by means of the methods by McLaughlin and Cassanto respectively, therefore to correct the values provided by the models. The study was carried out computationally by means of: i) a code integrating the equations of dynamics of an entry capsule for the computation of the trajectories, ii) two Direct Simulation Monte Carlo (DSMC) codes for the solution of the 2-D, axial-symmetric and 3-D flow fields around the capsule in the altitude interval 50-100 km. The computations verified that the entry trajectories of Pathfinder from the two models, in terms of the Mach, Reynolds and Knudsen numbers, were very different. The aim of the present paper is to continue this study, considering other aerodynamic problems and then to provide a contribution to a long series of papers on the subject "aerodynamics in Mars atmosphere". More specifically, the present paper evaluated and quantified the effects from the two models of: i) chemical reactions on aerodynamic quantities in the shock layer, ii) surface temperature, therefore of the contribution of the re-emitted molecules, on local (pressure, skin friction, etc.) and on global (drag) quantities, iii) surface recombination reactions (catalyticity) on heat flux. The results verified that the models heavily influence the flow field (as per the shock wave structure) but, apart from the surface recombination reactions, the effects of the different conditions on aerodynamics of the capsule are negligible for both models and confirmed what already found in the previous paper that, because of the higher values of density from the NASA Glenn model, the effects on aerodynamics of a entry capsule are stronger than those computed by the GRAM-2001 model.

• Journal of Aerospace System Engineering
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• v.13 no.5
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• pp.32-38
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• 2019

The flying wing configuration with high sweep angles and rounded leading edge represent a complex flow of structures by the leading edge vortex. For control of the tailless flying wing configuration with unstable directional stability, flaperon is used. In this study, we conducted numerical simulations for a non-slender flying wing configuration with a rounded leading edge and analyzed the effect of the sideslip angle and flaperon. Through aerodynamic coefficient analysis, it was found that the effect of AoS on lift and drag coefficient was minimal and the side force and moment coefficient were markedly influenced by AoS. As the sideslip angle increased, the pitch break, which is related to the pitching moment coefficient, was delayed. Through stability analysis, the directional and lateral static stability of the flying wing configuration were increased by flaperon. Also, the structure and behavior of the leading edge vortex were analyzed by observing the contour of the pressure coefficient and the skin friction line.

• Tribology and Lubricants
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• v.35 no.3
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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.