• 유체기계공업학회:학술대회논문집
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• 2001.11a
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• pp.454-459
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• 2001

The present study is concerned with the flow patterns induced by other impellers in a rectangular tank Impellers are FBT(Flat blade turbine), PBT(Pitched blade turbine), Shroud turbine, Rushton Turbine, and Helical ribbon turbine. The solution of flows in moving reference frames requires the use of 'moving' cell zone. The moving zone approaches are MRF(Multiple reference frame), which is a steady-state approximation and Sliding method, which is a unsteady-state approximation. Numerical results using two moving zone approaches are compared with experiments by Ranade & Joshi, which have done extensive LDA measurements of the flow generated by a standard six-bladed Rushton turbine in a cylindrical baffled vessel. In this paper we simulated the flow patterns with above mentioned moving zone approaches and impellers. Turbulence model is RNG k-$\epsilon$ model.

• Transactions of the Korean Society of Automotive Engineers
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• v.7 no.8
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• pp.150-159
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• 1999

Exhaust system is composed of several parts. Among, them , design of muffler system strongly influences on engine efficiency and noise reduction. So , through comprehension of flow characteristics inside muffler is necessary . In this study , three-dimensional steady and unsteady compressible flow analysis was performed to understand the flow characteristics, pressure loss and amplitude variation of pulsating pressure. The computational grid generation was carried out using commercial preprocessor ICEM CFD/CAE. And the three-dimensional fluid motion inside the muffler was analyzed by STAR-CD, the computational fluid dynamics code. RNG k-$\varepsilon$ tubulence model was applied to consider the complexity of the geometry and fluid motion. The steady and unsteady flow field inside muffler such as velocity distribution, pulsating pressure and pressure loss was examined. In case of unsteady state analysis, velocity of inlet region was converted from measured pulsating pressure. Experimental measurement of pressure and temperature was carried out to provide the boundary and initial condition for computational study under three engine operating conditions. As a result of this study, we could identify the flow characteristics inside the muffler and obtain the pressure loss, amplitude variation of pulsating exhaust gas.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.7 no.4
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• pp.622-632
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• 1995

The present numerical study is aimed to investigate time-dependent characteristics of a two-dimensional lid-driven square cavity flow of three high Reynolds numbers, $7.5{\times}10^3$, $10^4$ and $3{\times}10^4$. A conservative convection term on irregular grids was adopted by renewing the MAC type difference schemes on regular grids. Relaxation of velocity and pressure is implemented by SOLA algorithm. In case of $Re=7.5{\times}10^3$, flow behavior converges to steady state after a transient period. But for $Re=10^4$, periodic unsteady sinusoidal fluctuation of local velocity and kinetic energy is found and continuous movements of small eddies in the secondary flow regions are also discovered. Random generation of eddies and their active migrating behavior are detected for $Re=3{\times}10^4$, resulting in complete unsteady and non-linear flow characteristics. And, an organized structure similar to a Moffat vortex is also observed from the time-mean flow patterns. Furthermore, a typoon-like vortex(TLV) appears intemittently and rotates along the separation regions and boundary layers.

• Journal of Mechanical Science and Technology
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• v.19 no.3
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• pp.877-886
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• 2005

Two-dimensional incompressible Navier-Stokes equations are solved using SIMPLER method in the intrinsic curvilinear coordinates system to study the unsteady viscous flow physics over two-dimensional ellipses. Unsteady viscous flows over various thickness-to-chord ratios of 0.6, 0.8, 1.0, and 1.2 elliptic cylinders are simulated at different Reynolds numbers of 200, 400, and 1,000. This study is focused on the understanding the effects of Reynolds number and elliptic cylinder thickness on the drag and lift forces. The present numerical solutions are compared with available experimental and numerical results and show a good agreement. Through this study, it is observed that the Reynolds number and the cylinder thickness affect significantly the frequencies of the force oscillations as well as the mean values and the amplitudes of the drag and lift forces.

• Transactions of Materials Processing
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• v.10 no.6
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• pp.485-492
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• 2001

The paper is concerned with plastic flow in the port and welding chamber of rectangular hollow section extrusion through the porthole die. The extrusion process is analyzed by numerical simulation and experiments in the unsteady state. The effects of types of inlet with and without taper on the flow and extrusion load are mainly discussed and compared by FEA and experiments. Experiments are carried out by using the plasticine as a model material at room temperature. To visualize the plastic flow in the extrusion process, some split dies and punches are designed and manufactured by EDM. The theoretical predictions by FEM are reasonable agreements with experimental results on the deformed configurations and welding lines.

• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.2
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• pp.899-913
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• 2019

In order to provide a complementary perspective to the effects of the maneuvering motions on the unsteady propeller performance, the numerical simulation of the flow field of the hull-rudder- propeller system is performed by Unsteady Reynolds-averaged Naiver-Stokes (URANS) method. Firstly, the flow fields around the submarine model without the presence of propeller in straight ahead motion and the steady diving maneuvers with submergence rudder deflections of 4°, 8° and 12° are predicted numerically. The non-uniformity characteristic of the nominal wake field is exacerbated with the increase submergence rudder angle. Then the flow field around the SUBOFF-G submarine fitted with the 4381 propeller is simulated. The axial, transverse and vertical unsteady propeller forces in different maneuvering conditions are compared. In general, as the submarine maneuvers more violently, the harmonic amplitudes of the unsteady force at the 2BPF and 3BPF increased more significantly than that at BPF.

• International Journal of Fluid Machinery and Systems
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• v.8 no.3
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• pp.155-168
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• 2015

No-load speed is an important performance factor for the safe operation of hydropower systems. In turbine design, the manufacturers must conduct several model tests to calculate the accurate value of no-load speed for the complete range of operating conditions, which are expensive and time-consuming. The present study presents steady and unsteady methods for calculating no-load speed of a Francis turbine. The steady simulations are implemented using a commercial flow solver and an iterative algorithm that relies on a smooth relation between turbine torque and speed factor. The unsteady method uses unsteady RANS simulations that have been integrated with a user subroutine to compute and return the value of runner speed, time step and friction torque. The main goal of this research is to evaluate and compare the two methods by calculating turbine dynamic parameters for three test cases consisting of high and medium head Francis turbines. Overall, the numerical results agreed well with experimental data. The unsteady method provided more accurate results in the opening angle range from 20 to 26 degrees. Nevertheless, the steady results showed more consistency than unsteady results for the three different test cases at different operating conditions.

• Journal of the Korean Society of Propulsion Engineers
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• v.23 no.3
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• pp.18-27
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• 2019

The flow mixing characteristics for the heated dual jets were numerically studied by using URANS (unsteady Reynolds-averaged Navier-Stokes). The increased turbulent diffusion was obtained for the compressible flow, and the thermal diffusion of incompressible flow increased more than that of compressible flow. From the results of FFT and phase portraits, periodic and quasi-periodic states were observed as the jet spacing increased. It was observed that linear variations of merging points and combined points were different because unsteady flow determined the flow mixing characteristics for a large jet spacing.

• 한국전산유체공학회:학술대회논문집
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• 2010.05a
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• pp.496-500
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• 2010

Understanding train-wind is the best method to know how to optimize subway ventilation system. The capacity and efficiency of the subway ventilation system are known by pressure and velocity while train runs. Analysis of the internal flow in subway tunnel and around subway station are studied using numerical methods. Characteristics of internal flow and influence of subway ventilation system for the subway station with platform screen door and tunnel are analyzed by unsteady state analysis. Velocity and pressure of train wind transformation are compared at around subway ventilation system and the internal flow is investigated at the subway tunnel.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.225-228
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• 2002

Supersonic jet impingement on a flat plate has been investigated to show the flow physics for different jet heights and to demonstrate the adequacy of the characteristics-based flux-difference Wavier-Stokes code Current study also compares the steady-state solutions obtained with variable CFL number for different grid spacing with the time-accurate unsteady solutions using the inner iterations, displaying a good agreement between the two sets of numerical solutions. The unsteady nature of wall fluctuations due to bouncing of the plate shock is also uncovered for high pressure ratios. The methodology is then applied to a complex vertical launcher system where the jet plume hits the bottom wail, deflects into the plenum and eventually exits through the vertical uptake. Flow structures within vertical launcher system are captured and solutions are partially verified against the flight test data. Present jet impingement study thus shows the usefulness of CFD in designing a complex structure and predicting flow behavior within such a system.