• Journal of the Korea Institute of Military Science and Technology
• /
• v.10 no.2
• /
• pp.188-197
• /
• 2007

Active flow control, in the form of steady and unsteady momentum injection via jet blowing was studied. A jet was obtained by pressing a plenum inside the airfoil and ejecting flow out of a thin slot. The normal and drag forces were measured with leading edge or trailing edge blowing Jet and compared with the results obtained with no blowing. The blowing jet has been shown to improve the aerodynamic performance of the airfoil. The steady jet proved more effective than pulsating jet in these experimental conditions. Furthermore for the case of leading edge steady blowing jet, the alleviation of non-linearity in the normal force curve slope can be seen at higher angles of attack. No effective trailing edge jet was observed in this highly separated flow. This shows that the stall control is highly depends on the characteristics of the boundary layer near the jet slot.

• Proceedings of the KSME Conference
• /
• 2008.11b
• /
• pp.2530-2535
• /
• 2008

The Visualizations of the flowfield through tube banks with in-line and staggered arrangements were investigated by PIV. Strouhal numbers, velocity vectors and velocity profiles around the cylinders with in-line and staggered arrangements were observed at the pitch ratio $P_t/D=2.0$ and Reynolds number of $Re=4.0{\times}10^3$. As the results The flow patterns through tube banks were almost a straight line in case of the in-line arrangement while it was almost 八 type in case of the staggered arrangement in the direction of the wake. The average velocity in the rear region of the tube banks with the staggered arrangement was far smaller than that with the in-line arrangement. The Strouhal number in the last rank was far smaller than that in the front ranks in both of the in-line and staggered arrangements. The wake of each cylinder changed with time and with the position of the cylinder.

• Journal of the Korean Society of Marine Environment & Safety
• /
• v.14 no.4
• /
• pp.333-338
• /
• 2008

The coastal zone is a delicate and dynamic area in which the majority of a water kinetic energy is dissipated. These processes are subsequent to the transport of the beach materials. In comparison to emerged breakwaters, submerged structures permit the passage of some wave energy and in turn allow for circulation along the shoreline zone. This research aims to examine the beach erosion prevention capability of submerged structure by laboratory model. The flow characteristics behind a submerged obstacle with bottom gap were experimentally investigated at Re = $1.2{\times}10^4$ using the two-frame PIV(CACTUS 2000) system. Streamline curvature field behind the obstacle has been obtained by using the data of time-averaged mean velocity information. And the large eddy structure in the separated shear layer seems to have signification influence on the development of the separated shear layer. As bottom gap size increases, the recirculation occurring behind the obstacle moves toward downstream and its strength is weakened.

• Wind and Structures
• /
• v.26 no.4
• /
• pp.215-229
• /
• 2018

The appearance of a rivulet at the upper surface of a stay cable is responsible for rain-wind-induced vibration (RWIV) of cables of cable-stayed bridges. However, the formation mechanism of the upper rivulet and its aerodynamic effects on the stay cable has not been fully understood. Large eddy simulation (LES) method is used to investigate flow around and aerodynamics of a circular cylinder with an upper rivulet at a Reynolds number of 140,000. Results show that the mean lift coefficients of the circular cylinder experience three distinct stages, zero-lift stage, positive-lift stage and negative-lift stage as the rivulet located at various positions. Both pressure-induced and friction-induced aerodynamic forces on the upper rivulet are helpful for its appearance on the upside of the stay cable. The friction-induced aerodynamic forces, which have not been considered in the previous theoretical models, may not be neglected in modeling the RWIV. In positive-lift stage, the shear layer separated from the upper rivulet can reattach on the surface of the cylinder and form separation bubbles, which result in a high non-zero mean lift of the cylinder and potentially induces the occurrence of RWIV. The separation bubbles are intrinsically unsteady flow phenomena. A serial of small eddies first appears in the laminar shear layer separated from the upper rivulet, which then coalesces and reattaches on the side surface of the cylinder and eventually sheds into the wake.

• Journal of Advanced Marine Engineering and Technology
• /
• v.33 no.1
• /
• pp.44-51
• /
• 2009

The Characteristics of the flowfield through tube banks with in-line and staggered arrangements were investigated by PIV. Strouhal numbers, velocity vectors and velocity profiles around the cylinders with in-line and staggered arrangements were observed at the pitch ratio Pt/D=2.0 and Reynolds number of Re=$Re=4.0{\times}10^3$. As the results The flow patterns through tube banks were almost a straight line in case of the in-line arrangement while it was almost 八 type in case of the staggered arrangement in the direction of the wake. The average velocity in the rear region of the tube banks with the staggered arrangement was far smaller than that with the in-line arrangement. The Strouhal number in the last rank was far smaller than that in the front ranks in both of the in-line and staggered arrangements. The wake of each cylinder changed with time and with the position of the cylinder.

• Journal of Mechanical Science and Technology
• /
• v.18 no.2
• /
• pp.272-285
• /
• 2004

The losses at off-design points from a compressor cascade occur due to the deviation from a design incidence angle at the inlet of the cascade. The self-noise from the blade cascade at off-design points comes from a separated boundary layer and vortex sheddings. If the incidence angle to the cascade increases, stalling in blades may occur and the noise level increases significantly. This study applied Large-Eddy Simulations (LES) using deductive and deductive dynamic SGS models to low Mach-number, turbulent flow with each incidence angle to the cascade ranging from -40$^{\circ}$ to +20$^{\circ}$ and compared numerical predictions with measured data. It was observed that the oscillating separation bubbles attached to the suction surface do not modify wake flows dynamically for cases of negative incidence angles. However, an incidence angle greater than 8$^{\circ}$ caused a separated vortex near the leading edge to be shed downstream and created stalling. The computed performance parameters such as drag coefficient and total pressure loss coefficient showed good agreement with experimental results. Noise from the cascade of the compressor is summarized as sound generated by a structure interacting with unsteady, turbulent flows. The hybrid method using acoustic analogy was observed to closely predict the measured overall sound powers and directivity patterns at design and off-design points of blade cascade.

• 한국전산유체공학회:학술대회논문집
• /
• 2000.10a
• /
• pp.15-24
• /
• 2000

The transient response of an airfoil to a rapidly deploying spoiler is numerically investigated using the turbulent compressible Navier-Stokes equations in two dimensions. Algebraic Baldwin-Lomax model, Wilcox $\kappa-\omega$ model, and SST $\kappa-\omega$ turbulence model are used to calculate the unsteady separated flow due to the rapid spoiler deployment. The spoiler motion relative to a stationary airfoil is treated by an overset grid hounded by a Dynamic Domain-Dividing Line which has been devised by the authors. The adverse effects of the spoiler influenced by the spoiler location and the hinge gap are expounded. The numerical results are in reasonably good agreement with the existing experimental data.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.6 no.5
• /
• pp.111-118
• /
• 1998

Surge pressure problem at the oil return line of the hydraulic circuit of an active suspension system for passenger cars was investigated by experiments and numerical analyses. In the numerical analyses, the method of characteristics was used for simulating unsteady flow in the hydraulic system and gas discrete model was adopted for estimating gas volume variation in separated liquid column. In the experiments and analyses, effects of the physical parameters of the accumlator on smoothing surge pressure was elucidated.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2006.05a
• /
• pp.36-40
• /
• 2006

An analysis is made of flow and rocket motion during a supersonic separation stage of air-launching rocket(ALR) from the mother plane. Three-dimensional compressible Navier-Stokes equations is numerically solved to analyze the steady/unsteady flow field around the rocket which is being separated from the mother plane configuration(F-4E Phantom). The simulation results clearly demonstrate the effect of shock-expansion wave interaction between the rocket and the mother plane. To predict the behavior of the ALR according to the change of the C.G., three cases of numerical analysis are performed. As a result, a design-guideline of supersonic air-launching rocket for the safe separation is proposed.

• Wind and Structures
• /
• v.34 no.6
• /
• pp.499-510
• /
• 2022

Computational Wind Engineering has rapidly grown in the last decades and it is currently reaching a relatively mature state. The prediction of wind loading by means of numerical simulations has been proved effective in many research studies and applications to design practice are rapidly spreading. Despite such success, caution in the use of simulations for wind loading assessment is still advisable and, indeed, required. The computational burden and the know-how needed to run high-fidelity simulations is often unavailable and the possibility to use simplified models extremely attractive. In this paper, the applicability of some well-known 2D unsteady RANS models, particularly the k-ω SST, in the aerodynamic characterization of extruded bodies with bluff sections is investigated. The main focus of this paper is on the drag coefficient prediction. The topic is not new, but, in the authors' opinion, worth a careful revisitation. In fact, despite their great technical relevance, a systematic study focussing on sections which manifest a fully detached flow configuration has been overlooked. It is here shown that the considered 2D RANS exhibit a pathological behaviour, failing to reproduce the transition between reattached and fully detached flow regime.