• Journal of Mechanical Science and Technology
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• v.18 no.9
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• pp.1668-1679
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• 2004

The design of an axial piston pump for electro-hydrostatic transmission systems requires accurate information where and how much the internal friction and flow losses are produced. This study is particularly focused on the friction losses of a bent-axis type hydraulic piston pump, aiming at finding out which design factors influence its torque efficiency most significantly. To this end, the friction coefficients of the pump parts such as piston heads, spherical joints, shaft bearings, and valve plate were experimentally identified by a specially constructed tribometer. Applying the experimental data to the equations of motion for pistons as well as to the theoretical friction models for the pump parts, the friction torques produced by them were computed. The accuracy of the computed results was confirmed by the comparison with the practical input torque of the pump. In this paper, it is shown that the viscous friction forces on the valve plate and input shaft bearing are the primary source of the friction losses of the bent-axis type pump, while the friction forces and moments on the piston are of little significance.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.5
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• pp.483-490
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• 2008

Minimization of nozzle induces many flow losses in micro-propulsion system. In this study, we studied about thermal transpiration based micro propulsion system to overcome these losses. Thermal transpiration device(Knudsen pump) having no moving parts can self-pump the gaseous propellant by temperature gradient only (cold to hot). We designed, fabricated the Knudsen pump and analyzed pressure gradient efficiency of membrane according to Knudsen number under vacuum condition. Experimental results showed that thick membranes are more effective than thin membranes in transition flow regime, and pressure gradient efficiency according to Knudsen number is increased to maximum 82% apart from membrane thickness in free molecular regime.

• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.5
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• pp.98-106
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• 2002

The airflow characteristics of an air-conditioning duct with multiple diffusers were investigated through one-dimensional analysis, CFD simulation and experimental measurement. One-dimensional program based on Bernoulli's equation and minor loss equations was developed in order to evaluate the air distribution rate at each diffuser. In CFD simulation, three-dimensional flow characteristics inside air-conditioning duct were computed for incompressible viscous flow, adopting the RNG k-$\xi$turbulence model. Also, in an effort to equalize the discharge flow rate at each outlet, the optimization procedure has been performed to obtain the optimum diffuser area. In this process, square of difference between maximum discharge rate and minimum discharge rate is used as an object function. Diffuser area and discharge velocity are established as constraints. After optimization process, determined design variables are applied again in CFD simulation and experiment to validate the optimized result by one-dimensional program. Comparison with the experimental data of airflow rate distribution showed that the developed program seems to be acceptable and can be useful design tool for an automotive air-conditioning duct in an initial design stage.

• Journal of Biosystems Engineering
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• v.33 no.4
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• pp.282-287
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• 2008

Love wave is one of the shear horizontal waves and it can propagate between two layers in liquid without energy loss. The SAW (surface acoustic wave) sensor using Love wave is very useful for real time measurement of the viscosity of liquid with high sensitivity. In this study, the 77 MHz and 155 MHz Love wave SAW sensors were fabricated and use to measure the viscosity of low viscous liquid. To generate the surface acoustic wave, the inter-digital transducers were fabricated on the quartz crystal wafer. In order to obtain the optimal thickness of the coating film (novolac photoresist) generating the Love wave on the surface of SAW device, theoretical calculation was performed. The performances of fabricated Love wave SAW sensors were tested. As test liquid, pure water and glycerol solutions having different concentrations were used. Since the determination coefficients of the regression equations for measuring the viscosity of liquid are greater than 0.98, the developed Love wave SAW sensors in this study will be very useful for precise measurement of viscosity of liquid.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.8
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• pp.1950-1963
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• 1995

A modification of the approximate-factorization method is made to accelerate the convergency rate and to take sufficiently large Courant number without loss of accuracy. And a stable implicit finite-difference scheme for solving the incompressible Navier-Stokes equations employed above modified method is developed. In the present implicit scheme, the volume fluxes with contravariant velocity components and the pressure formulation in curvilinear coordinates is adopted. In order to satisfy the continuity condition completely and to remove spurious errors for the pressure, the Navier-Stokes equations are solved by a modified SMAC scheme using a staggered gird. The upstream-difference scheme such as the QUICK scheme is also employed to the right hand side. The implicit scheme is unconditionally stable and satisfies a diagonally dominant condition for scalar diagonal linear systems of implicit operator on the left hand side. Numerical results for some test calculations of the two-dimensional flow in a square cavity and over a backward-facing step are obtained using both usual approximate-factorization method and the modified one, and compared with each other. It is shown that the present scheme allows a sufficiently large Courant number of O(10$^{2}$) and reduces the computing time.

• Korea-Australia Rheology Journal
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• v.19 no.1
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• pp.35-42
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• 2007

The effects of particle size distribution on the magnetorheological response of inverse ferrofluids was investigated using controlled mixtures of two monodisperse non-magnetisable powders of sizes $4.6\;{\mu}m\;and\;80{\mu}m$ at constant volume fraction of 30%, subjected to large amplitude oscillatory shear flow. In the linear viscoelastic regime (pre-yield region), it was found that the storage and loss moduli were dependent on the particle size as well as the proportion of small particles, with the highest storage modulus occurring for the monodisperse small particles. In the nonlinear regime (post yield region), Fourier analysis was used to compare the behaviour of the $1^{st}\;and\;3^{rd}$ harmonics ($I_{1}\;and\;I_{3}\;respectively$) as well as the fundamental phase angle as functions of the applied strain amplitude. The ratio of $I_{3}/I_{1}$ was found to become more pronounced with decreasing particle size as well as with increasing proportion of small particles in the bidisperse mixtures. Furthermore, the phase angle was able to clearly show the transition from solid-like to viscous behaviour. The results suggested that the nonlinear response of a bidisperse IFF is dependent on particle size as well as the proportion of small particles in the system.

• International Journal of Aeronautical and Space Sciences
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• v.12 no.4
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• pp.318-330
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• 2011

This paper investigates the convergence characteristics of the modified artificial compressibility method proposed by Turkel. In particular, a focus is mode on the convergence characteristics due to variation of the preconditioning factor (${\alpha}_u$) and the artificial compressibility (${\beta}$) in conjunction with an upwind method. For the investigations, a code using the modified artificial compressibility is developed. The code solves the axisymmetric incompressible Reynolds averaged Navier-Stokes equations. The cell-centered finite volume method is used in conjunction with Roe's approximate Riemann solver for the inviscid flux, and the central difference discretization is used for the viscous flux. Time marching is accomplished by the approximated factorization-alternate direction implicit method. In addition, Menter's k-${\omega}$ shear stress transport turbulence model is adopted for analysis of turbulent flows. Inviscid, laminar, and turbulent flows are solved to investigate the accuracy of solutions and convergence behavior in the modified artificial compressibility method. The possible reason for loss of robustness of the modified artificial compressibility method with ${\alpha}_u$ >1.0 is given.

• YAKHAK HOEJI
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• v.60 no.2
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• pp.73-77
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• 2016

Alginic acid and carboxymethyl cellulose sodium are dietary fibers from plants. They have a swelling property and delay the gastric emptying time, thereby resulting in feeling satiated after oral administration, which may eventually contribute to loss of body weight. The goal of this study was to compare swelling property of three commercial matrix tablets based on alginic acid and carboxymethyl cellulose sodium. When the swelling was determined by the Korean Ministry of Food and Drug Safety (MFDS) guideline, the tablet prepared by direct compression method with highly viscous swelling agent showed the highest swelling in acidic conditions. Water uptake of these tablets was rapid and completed within 30 min. Moreover, when the pH was changed from 2.5 to 6.8 buffer, the water uptake was not significantly changed in all tablets.

• Preventive Nutrition and Food Science
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• v.13 no.3
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• pp.245-248
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• 2008

The effects of native sweet potato starch (NSPS) and sweet potato starch modified by acetylation (MSPS) on dynamic rheological properties of surimi sols were investigated by small-deformation oscillatory measurements. Dynamic frequency sweeps of surimi sols at $10^{\circ}C$ showed that the addition of NSPS and MSPS resulted in a reduction of storage modulus (G') and loss modulus (G"). The tan $\delta$ values (ratio of G"/ G') of all samples were in the range of $0.15{\sim}0.54$ over a wide range of frequency, indicating that all surimi sols are more elastic than viscous. The characteristic G' thermograms of surimi sols during heating from 10 to $90^{\circ}C$ were influenced by the addition of starch. The tan $\delta$ values of all samples were maintained nearly constant above $45^{\circ}C$, showing that the G' is proportional to the G" irrespective of starch effects.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.29 no.3
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• pp.127-133
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• 2017

Performance analysis has been carried out on a high side scroll compressor that had a fixed scroll equipped with a circular oil groove on its thrust surface. Oil was supplied to the oil groove through an intermittent opening from a high pressure oil reservoir formed inside the orbiting scroll hub. Oil in the groove was then delivered to both suction and back pressure chambers by pressure differentials and viscous pumping action of the orbiting scroll base plate. Mathematical modeling of this oil groove system was incorporated into a main compressor performance simulation program for an optimum oil groove design. The study findings were as follows. Pressure in the oil groove can be controlled by changing its configuration and the oil passage area. With an enlarged oil passage, the pressure in the oil groove heightens due to an increased flow rate, but the pressure elevation in the back pressure chamber is small, resulting in reduced friction loss at the thrust surface between the two scrolls. On the other hand, by increasing the oil passage area, the oil content in the refrigerant flow increases. Considering all these factors, the energy efficiency ratio could be improved by about 3.6% under the ARI condition by an optimal oil groove design.