• International Journal of Aeronautical and Space Sciences
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• 제10권1호
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• pp.104-111
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• 2009

An unstructured overset mesh method has been developed for the simulation of unsteady flow fields around isolated rotors and rotor-fuselage configurations. The flow solver was parallelized for the efficient calculation of complicated flows requiring a large number of cells. A quasi-unsteady mesh adaptation technique was adopted to enhance the spatial accuracy of the solution and to better resolve the rotor wake. The method has been applied to calculate the flow fields around rotor-alone and rotor-fuselage configurations in forward flight. Validations were made by comparing the predicted results with those of measurements. It was demonstrated that the present method is efficient and robust for the prediction of unsteady time-accurate flow fields involving multiple bodies in relative motion.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2001년도 춘계 학술대회논문집
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• pp.121-126
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• 2001

An accelerating flow field through a compressor cascade is studied numerically by unsteady computational simulation. The two-dimensional Navier-Stokes equations for compressible flow is used for the study of unsteady high incidence angle flow, with preconditioning scheme to cover the wide range of Mach number and $\kappa-\omega$ model for the turbulent viscous flow analysis. A DCA(double circular arc) compressor blade is accelerated artificially in this study to understand the unsteady effect by comparing the present results with the existing steady-state experimental and computational results. Also, the accelerating flow field during the starting phase of gas turbine is studied with actual experimental data for the understanding of flow field and performance characteristics at off-design condition.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.2127-2132
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• 2003

A computational study is performed to better understand the choke phenomenon of unsteady gas flow through a critical nozzle. The axisymmetric, unsteady, compressible, Navier-Stokes equations are solved using a finite volume method. In order to simulate the effects of back pressure fluctuations on the critical nozzle flow, a forced sinusoidal pressure wave is assumed downstream the exit of the critical nozzle. It's frequency is 20kHz and amplitude is varied below 15% of time-mean back pressure. The results obtained show that for low Reynolds numbers, the unsteady effects of the pressure fluctuations can propagate upstream of the throat of critical nozzle, and thereby giving rise to applicable fluctuations of mass flow through the critical nozzle. The effect of the amplitude of the excited pressure fluctuations on the choke phenomenon is discussed in details.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2005년도 연구개발 발표회 논문집
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• pp.223-230
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• 2005

The unsteady nature of vortex structures has been investigated by a large eddy simulation (LES) in an axial flow fan with a shroud covering only the rear region of its rotor tip. The simulation shows that the tip vortex plays a major role in the structure and unsteady behavior of the vortical flow in the fan. The movements of the vortex structures induce high-pressure fluctuations on the rotor blade and in the blade passage. Frequency characteristics of the fluctuating pressure on the rotor blade are analyzed using wavelet transform. The dominant frequency of the real-time pressure selected at the high pressure fluctuation region corresponds well to that of the fluctuating rotor torque and the experimental result of fan noise. It is mainly generated due to the unsteady behavior of the vortical flow, such as the tip vortex and the leading edge separation vortex.

• Journal of Advanced Marine Engineering and Technology
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• 제23권2호
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• pp.176-183
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• 1999

This paper is aimed to investigate behaviour of vortex in 2-D step cavity with high Reynolds numbers$(3.2{\times}10^{3},\;10^{4},\;3{\times}10^{4},\;5{\times}10^{4}\;and\;7{\times}10^{4})$. The SOLA algorithm which is MAC type was adopted to solution method computing the flow field on irregular grid. In case of $Re=7{\times}10^{4}$ flow behavior is steady bu periodic unsteady sinusoidal fluctuation of local velocity and kinetic energy is found for $Re=10^{4}$ Continuous movements of small eddies in the secondary flow regions are discov-ered for $3{\times}10^{4}$ Generation of eddies and their active migrating behavior are detected over $Re=5{\times}10^{4}$ resulting in complete unsteady and non-linear flow characteristics Furthermore a typhoon-like vortex(TLV) appears intermittently and rotates along the separation regions and boundary layers.

• International Journal of Fluid Machinery and Systems
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• 제3권1호
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• pp.58-66
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• 2010

A steam control valve is used to control the flow from the steam generator to the steam turbine in thermal and nuclear power plants. During startup and shutdown of the plant, the steam control valve is operated under a partial flow conditions. In such conditions, the valve opening is small and the pressure deference across the valve is large. As a result, the flow downstream of the valve is composed of separated unsteady transonic jets. Such flow patterns often cause undesirable large unsteady fluid force on the valve head and downstream pipe system. In the present study, various flow patterns are investigated in order to understand the characteristics of the unsteady flow around the valve. Experiments are carried out with simplified two-dimensional valve models. Two-dimensional unsteady flow simulations are conducted in order to understand the experimental results in detail. Scale effects on the flow characteristics are also examined. Results show three types of oscillating flow pattern and three types of static flow patterns.

• 한국유체기계학회 논문집
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• 제12권1호
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• pp.22-27
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• 2009

The important problems that arise in the design and performance of the axial flow turbine are the prediction and control of secondary flows. Some progresses have been made on understanding flow conditions that occur when the inlet endwall boundary layer separates at the point in the endwall and rolls up into the horseshoe vortex. And the flows though an axial turbine tend to be extremely complex due to its inherent unsteady and viscous phenomena. The passing wakes generated from the trailing edge of the stator make an interaction with the rotor. Unsteady flow should be considered rotor/stator interactions. The main purpose of this research is control of secondary flow and improvement efficiency in turbine by leading edge modification in unsteady state. When the wake from the stator ran into the modified leading edge of the rotor, the leading edge generated the weak pressure fluctuation by complex passage flows. In conclusion, leading edge modification(bulb2) results in the reduced total pressure loss in the flow field.

• 한국전산유체공학회지
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• 제18권3호
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• pp.26-33
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• 2013

In the present study, unsteady flow simulations of a variable geometry nozzle were conducted using a two-dimensional flow solver based on hybrid unstructured meshes. The variable geometry nozzle is used to achieve efficient performances of aircraft engines at various operating conditions. To describe the motion of the variable geometry nozzle, an algebraic method based on the basis decomposition of normal edge vector was used for the deformation of viscous elements. A ball-vertex spring analogy was used for inviscid elements. The aerodynamic data were obtained for a range of nozzle pressure ratios, and the validations were made by comparing the present results with available experimental data. The unsteady nozzle flows were simulated with an oscillating diverging section and a converging-diverging section. It was found that the nozzle performances are influenced by the nozzle exit flow characteristics, mass flow rate, as well as unsteady effects. These unsteady effects are shown to behave differently depending on the frequency of the nozzle motion.

• 한국전산유체공학회지
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• 제18권1호
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• pp.36-42
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• 2013

The performance of the wind turbine is determined by wind speed and unsteady flow characteristics. Unsteady wake flow causes not only the decline in performance but also structural problems of the wind turbine. In this paper, conceptual designs for the wind turbine tower are conducted to minimize unsteady wake flow. Numerical simulations are performed to inspect the shape effect of the tower. Through the installation of additional structures at the rear of the tower, the creation of Karman vortex is delayed properly and vortex interactions are reduced extremely, which enhance the stability of the wind turbine. From the comparative analysis of lift and drag coefficients for each structure, it is concluded that two streamwise tips with a splitter plate have the most improved aerodynamic characteristics in stabilizing wake flow.

• 한국가시화정보학회지
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• 제10권1호
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• pp.40-46
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• 2012

Recently micro shock tube is extensively being used in many diverse fields of engineering applications but the detailed flow physics involved in it is hardly known due to high Knudsen number and strong compressibility effects. Unlike the macro shock tube, the surface area to volume ratio for a micro shock tube is very large. This unique effect brings many complexities into the flow physics that makes the micro shock tube different compared with the macro shock tube. In micro shock tube, the inter- molecular forces of working gas can play an important role in specifying the flow characteristics of the unsteady shock wave flow which is essentially generated in all kinds of shock tubes. In the present study, a CFD method was used to predict and visualize the unsteady shock wave flows using the unsteady compressible Navier-Stokes equations, furnished with the no-slip and slip wall boundary conditions. Maxwell's slip equations were used to mathematically model the shock movement at high Knudsen number. The present CFD results show that the propagation speed of the shock wave is directly proportional to the initial pressure and diameter of micro shock tube.