• Journal of the Society of Naval Architects of Korea
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• v.48 no.5
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• pp.414-420
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• 2011

Decades of studies of geophysical flow have unveiled that the flow downstream of obstacles in stratified flow consists of attached wake and strong gravity waves, or separated, fluctuating wake and persistent late wakes. Among unique and interesting characteristics of the stratified flow past obstacles is the generation of coherent vortex in the late wake far downstream of the object. Unlike in homogeneous fluid, the flow field downstream self-develops coherent vortex even after diminishing of the near wake, no matter how small the stratification is. This paper present a computational approach to simulate the generation of the coherent vortex structure in the late wake of a moving sphere submerged in weakly stratified fluid. The results are in consistent with several experimental observations and the vortex stretching mechanism is employed to explain the process of coherence.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.1863-1868
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• 2003

Decades of studies of geophysical flow have unveiled that the flow downstream of obstacles in stratified flow consists of attached wake and strong internal waves, or separated, fluctuating wake and persistent late wakes. Among unique and interesting characteristics of the stratified flow past obstacles is the generation of coherent vortex the late wake far downstream of the object. Unlike in homogeneous fluid, the flow field downstream self-develops coherent vortex even after diminishing of the near wake, no matter how small the stratification is. This paper present a computational approach to simulate the generation of the coherent vortex structure in late wake of a moving sphere submerged in weakly stratified fluid. The results are in consistent with several experimental observations and the vortex stretching mechanism is employed to explain the process of coherence.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.986-991
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• 2003

A turbulent wake generated by a cylinder in nozzle contraction affects to the jet flow characteristics. In this study, a computational work to investigate the effect of the turbulent wake on two-dimensional subsonic jet was carried out with three different kinds of nozzle. Computations are applied to the two-dimensional unsteady, Navier-Stokes equations. Several kinds of turbulent models and wall functions are employed to validate the computational predictions. It was known that the wake flow enhanced the spread of the jet flow, compared with no wake flow condition. It was also found that the jet core is shortened by the wake flow developed from a control cylinder.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1625-1630
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• 2004

An experimental study was carried out to investigate influence of flow conditions on a boundary layer to the near-wake of a flat plate. The flow condition in the vicinity of trailing edge that is influenced by upstream condition history is an essential factor that determines the physical characteristics of a near-wake. Various tripping wires were used to change boundary layer flow condition of upstream at the freestream velocity of 6.0 m/sec. Measurements of the boundary layer and near-wake according to the change of upstream conditions were conducted by using both I-probe(55P14 for boundary layer) and X-probe(55P61 for wake). Normalized velocity profiles of the boundary layer were shown the flow types such as laminar boundary layer, transition, and turbulent boundary layer at 0.95C from the leading edge. The velocity and turbulence intensity profiles of the near-wake for the case of laminar boundary layer at the flat plate surface exhibited a defect and a double peak showing perfect symmetry, respectively.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.6
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• pp.771-787
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• 1998

A Navier-Stokes code with a modified low Reynolds number k-.epsilon. turbulence model was used to study the unsteady transitional boundary layer flow due to rotor-stator interaction. The modification, proposed by Launder, to improve prediction of stagnation flows was incorporated to the low Reynolds number k-.epsilon. turbulence model by Fan-Lakshminarayana-Barnett. Numerical solution is shown to capture well the calmed laminar flow as well as the wake induced transitional strip due to rotor-stator interaction and shows improvement, in terms of onset of transition and its length, over previous Euler/boundary layer solution. The turbulent kinetic energy shows local maximum along the upstream rotor wake in the wake induced transitional strip and this characteristics is observed untill the end of transition. The wake induced strip also shown apparent even in the laminar sublayer as the upstream rotor wake penetrates inside the boundary layer.

• Wind and Structures
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• v.16 no.3
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• pp.249-261
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• 2013

Flexible stay cables on cable-stayed bridges are three-dimensional. They sag and flex in the complex wind environment, which is a different situation to ideal rigid cylinders in two-dimensional wind flow. Aerodynamic interference and the response characteristics of wake galloping of full-scale parallel cables are potentially different due to three-dimensional flows around cables. This study presents a comprehensive wind tunnel investigation of wake galloping of parallel stay cables using three-dimensional aeroelastic cable models. The wind tunnel study focuses on the large spacing instability range, addressing the effects of cable separation, wind yaw angle, and wind angle of attack on wake galloping response. To investigate the effectiveness of vibration suppression measures, wind tunnel studies on the transversely connected cable systems for two types of connections (flexibility and rigidity) at two positions (mid-span and quarter-span) were also conducted. This experimental study provides useful insights for better understanding the characteristics of wake galloping that will help in establishing a guideline for the wind-resistant design of the cable system on cable-stayed bridges.

• 한국전산유체공학회:학술대회논문집
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• 1999.11a
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• pp.181-186
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• 1999

The effects of the frequency of upstream gust on the unsteady boundary characteristics on a downstream blade was simulated by using a Navier-Stokes code. The Navier-Stokes code is based on an unstructured finite volume method and uses a low Reynolds k-e turbulence model to close the momentum equations. The MIT flapping foil experiment set-up is used to simulate the interaction between the upstream wake and a blade. The frequency of the upstream wake is simulated by varying rate of pitching motion of the flapping airfoils. Three reduced frequencies. 3.62. 7.24. and 10.86. are simulated. As the frequency increases, the unsteady fluctuation on the surfaces of the downstream hydrofoil is shown to decrease while the upstream flapper wake has larger first harmonics of y-velocity component. The unsteady vortices are shown to interact with each other and. as a result. the upstream wake becomes undiscernible inside the inner layer. The turbulence kinetic energy shows a similar behavior. Limiting streamlines around the trailing edge of the flapper are shown to conform with the unsteady Kutta condition for a round trailing edge. while limiting streamlines around the trailing edge of the hydrofoil conforms with the unsteady Kutta condition for a sharp edge.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.3 s.234
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• pp.322-329
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• 2005

The flow characteristics in the blade passage and in the wake region of a low speed axial flow fan have been investigated by experimental analysis using a rotating hot-wire sensor for design and off-design operating conditions. The results show that the tip leakage vortex is moved upstream when flow rate is decreased, thus disturbing the formation of wake flow near the rotor tip. The tip leakage vortex interfaces with blade pressure surface, and results in high velocity fluctuation near the pressure surface. From axial velocity distributions downstream of the fan rotor, large axial velocity decay near the rotor tip is observed at near stall condition, which results in large blockage compared to that at the design condition. Although the wake flow downstream of the rotor blade is clearly measured at all operating conditions, the trough of the high velocity fluctuation due to Karmann vortex street in the wake flow is mainly observed at a higher flow condition than the design flow rate.

• Journal of Advanced Research in Ocean Engineering
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• v.2 no.4
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• pp.169-178
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• 2016

Recently, artificial reefs (ARs) have been frequently used primarily owing to the development in AR materials and projects for relatively complicated, large ARs. Among several engineering issues of ARs, wake region of an AR has been characterized because these regions have a high probability of recruiting seaweed spores, providing an energy saving zone, and facilitating deposition of sediments, nutrients, and bio-deposits. To characterize an efficiency index of an AR wake region and its dependency on the prevailing water flow directions, this study proposes a so-called efficiency index diagram. This characterization is done by normalizing the wake volumes with respect to the real AR volume and illustrating how efficiency indices vary with respect to the inlet flow directions. As a result, according to the diagram characteristics such as an averaged efficiency index, fundamental symmetric angle, secure angles, and principal directions, we can easily figure out how a target AR should be aligned along the main water flows to maximize the wake region around the AR. In addition, six ARs are considered and their efficiency index diagrams are illustrated to pinpoint the physical characteristics.

• Proceedings of the KIEE Conference
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• 1998.11c
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• pp.1061-1064
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• 1998

To examine the influence of unsteady wake on the flow and heat transfer characteristics, an experiment has been conducted in a four-vane linear cascade. Flow and heat transfer measurements are made for the inlet Reynolds number of 66000(based on chord length and free-stream velocity). Turbulent intensity and stress were measured using hot wire anemometer, and to measure the convective heat transfer coefficients on the blade surfaces liquid crystal/gold film Intrex technique was used. The disturbance by the unsteady wake is characterized by the unresolved unsteadiness. The unsteady wake enhances the turbulent motion of flow in the cascade passage. It also promotes the boundary layer development and transition. The results show that heat transfer coefficients on the suction surface increase with increasing unresolved unsteadiness.