• 한국가시화정보학회:학술대회논문집
• /
• 2004.11a
• /
• pp.40-43
• /
• 2004

The effects of free surface on wake behind a rotating propeller were investigated experimentally in a circulating water channel with the variation of water depth. Instantaneous velocity fields were measured using two-frame PIV technique at tow different blade phases and ensemble-averaged to investigate the phase-averaged flow structure in the wake region. For an isolated propeller, the flow behind the propeller is influenced by the propeller rotation and the free surface. The phase-averaged mean velocity fields show that the potential wake and the viscous wake are formed by the boundary layers developed on the blade surfaces. The interaction between the tip vortices and the slipstream causes the oscillating trajectory of tip vortices. Tip vortices are generated periodically and the slipstream contracts in the near-wake region. The presence of free surface affects the wake structure largely, when the water depth is less than 0.6D. The free surface modifies the vortex structure, especially the tip and trailing vortices and flow structure in slipstreams of the propeller wake behind X/D = 0.3.

• Proceedings of the KSME Conference
• /
• 2002.08a
• /
• pp.631-634
• /
• 2002

The study of the flow characteristics in two types of turbo-fans for a vacuum cleaner was performed in a previous study. In present study an analysis of a new modified model to reduce fan noise was performed by using CFD. The characteristics of three models calculated for various rotating speeds and flow rates are obtained and compared with available measured data. The results show that the modified model gives stable flow characteristics in operating range than the original model, while both models show similar performance characteristics at the range of high flow rate. Since in the modified model it takes much longer for an impeller blade to pass a diffuser blade than in the original model, and thus the peak pressure at BPF can be relieved, it is anticipated that the modified model gives much lower noise level with similar performance than the original one, which remains to be verified by unsteady computation and measurements.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.25 no.2
• /
• pp.164-170
• /
• 2001

The influence of unsteady wake on the flow and heat transfer characteristics in a four-vane linear cascade was experimentally investigated. The unsteady wake was generated with four rotating rectangular plates located upstream of the cascade. Tested inlet Reynolds number based on chord length was set to 66,000 by controlling free-stream velocity. A hot-wire anemometer system was employed to measure turbulent velocity components. For the convective heat transfer coefficients measurement on turbine blade surface, thermochromic liquid crystal and gold film Intrex were used. It was found that the unsteady wake enhances the turbulent motion in the cascade passage and accordingly promotes the development and transition of boundary layer. It was found that the heat transfer coefficients on the blade surface increase as the plate rotating speed increases. However, the increasing of heat transfer coefficients is not significant in the case that Strouhal number is higher than 0.503.

• 한국가시화정보학회:학술대회논문집
• /
• 2004.12a
• /
• pp.169-175
• /
• 2004

The characteristics of flow around a rotating propeller were investigated using PIV technique. For each of four different blade phases of $0^{\circ},\;18^{\circ},\;36^{\circ}\;and\;54^{\circ}$four hundred instantaneous velocity fields were ensemble averaged to investigate the spatial evolution of the flow around a propeller. The phase-averaged mean velocity fields show that the viscous wake formed by the boundary layers developed on the blade surfaces and the slipstream contraction in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. The boundary layer developed along the ship hull bottom surface of the ship stern provides a strong turbulent shear layer, affecting the vortex structure in the propeller near-wake. As the flow develops in the downstream direction, the trailing vortices formed behind the propeller hub move upward slightly due to the presence of the hull wake and free surface. The turbulence intensity has large values around the tip and trailing vortices. As the wake moves downstream, the strength of the vorticity diminishes and the turbulence intensity increases due to turbulent diffusion and active mixing between the tip vortices and adjacent wake flow.

• Journal of computational fluids engineering
• /
• v.13 no.2
• /
• pp.55-61
• /
• 2008

The Reynolds-Averaged Navier-Stokes(RANS) analysis of the 3-D steady flow around the NREL Phase VI horizontal axis wind turbine(HAWT) rotor was performed. The CFD analysis results were compared with experimental data at several different wind speeds. The present CFD model shows good agreements with the experiments both at low wind speed which formed well-attache flow mostly on the upper surface of the blade, and at high wind speed which blade surface flow completely separated. However, some discrepancy occurs at the relatively high wind speeds where mixed attached and separated flow formed on the suction surface of the blade. It seems that the discrepancy is related to the onset of stall phenomena and consequently separation prediction capability of the current turbulence model. It is also found that strong span-wise flow occurs in stalled area due to the centrifugal force generated by rotation of the turbine rotor and it prevents abrupt reduction of normal force for higher wind speed than the designed value.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.66 no.10
• /
• pp.1516-1522
• /
• 2017

We propose a novel monitoring system for diagnosing crack faults of the wind turbine using image information. The proposed method classifies a normal state and a abnormal state for the blade parts of the wind turbine. Specifically, the images are input to the proposed system in various states of wind turbine rotation. according to the blade condition. Then, the video of rotating blades on the wind turbine is divided into several image frames. Motion vectors are estimated using the previous and current images using the motion estimation, and the change of the motion vectors is analyzed according to the blade state. Finally, we determine the final blade state using the Support Vector Machine (SVM) classifier. In SVM, features are constructed using the area information of the blades and the motion vector values. The experimental results showed that the proposed method had high classification performance and its $F_1$ score was 0.9790.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
• /
• 2009.04a
• /
• pp.266-271
• /
• 2009

A modeling method for the modal analysis of a multi-packet blade system having a crack undergoing rotational motion is presented in this paper. Each blade is assumed as a slender cantilever beam. The stiffness coupling effects between blades due to the flexibilities of the disc and the shroud are modeled with discrete springs. Hybrid deformation variables are employed to derive the equations of motion. The flexibility due to crack, which is assumed to be open during the vibration, is calculated basing on a fracture mechanics theory. To obtain more general information, the equations of motion are transformed into dimensionless forms in which dimensionless parameters are identified. The effects of the dimensionless parameters related to the angular speed, the depth and location of a crack on the modal characteristics of the system are investigated with some numerical examples.

• Journal of the Korean Society for Precision Engineering
• /
• v.20 no.8
• /
• pp.54-61
• /
• 2003

The first stage rotating blade of industrial gas turbine is one of the components that is normally run in exposed state at the highest temperature of the combustion gas stream. For this reason superior materials and advanced cooling technology are required to allow higher heat resisting characteristics of the component. The 1st stage blade of a selected commercial gas turbine blade made of directionally solidified Ni-based superalloy has a row of cooling holes on its trailing edge. In most cases, minor cracks have been found at some of the root cooling holes after one cycle operation (24,000 hrs) or even shorter operation time because of the high temperature gradient and the frequently alternating thermal stress. In the repair process, unfortunately, it is usually very difficult to get rid of the damage due to the fact that cracks are initiated at the root cooling hole and propagated deep into the hole. In this study, the feasibility of removing the sidewall cracks in the hole by utilizing EDM drilling has been investigated. Also the criteria of surface integrity for EDM drilling were established to achieve high quality repair as well as machining accuracy.

• Journal of the Korean Society of Marine Environment & Safety
• /
• v.25 no.3
• /
• pp.373-380
• /
• 2019

In this paper, a pico hydro turbine employing low head circulation water at fish farms is designed and evaluated. Due to the advantages of simple structures, small head requirements, and low-cost investment, the constant thickness propeller turbine is considered as a feasible solution. The design process based on the free vortex method is presented in full detail, and a 4-blade runner is built using BladeGen. The turbine performance is analyzed both numerically and via experimental methods. Despite slight differences, the results show similar trends between CFD simulations and experiments carried out on factory test-rigs in a wide range of working conditions. At the design flow rate, the turbine achieves the best efficiency of 70 %, generating 3.5 kW power when rotating at 420 rpm. The internal flow field, as well as the turbine's behavior, are investigated through the distribution of blade streamlines, pressure, and velocity around the runner. Moreover, the pressure coefficient on the blade surface at 3 span positions is plotted while the head loss for each simulation domain is calculated and displayed by charts.

• Advances in aircraft and spacecraft science
• /
• v.6 no.1
• /
• pp.31-49
• /
• 2019

Flutter is a dangerous phenomenon encountered in flexible structures subjected to aerodynamic forces. This includes aircraft, helicopter blades, engine rotors, buildings and bridges. Flutter occurs as a result of interactions between aerodynamic, stiffness and inertia forces on a structure. The conventional method for designing a rotor blade to be free from flutter instability throughout the helicopter's flight regime is to design the blade so that the aerodynamic center (AC), elastic axis (EA) and center of gravity (CG) are coincident and located at the quarter-chord. While this assures freedom from flutter, it adds constraints on rotor blade design which are not usually followed in fixed wing design. Periodic Structures have been in the focus of research for their useful characteristics and ability to attenuate vibration in frequency bands called "stop-bands". A periodic structure consists of cells which differ in material or geometry. As vibration waves travel along the structure and face the cell boundaries, some waves pass and some are reflected back, which may cause destructive interference with the succeeding waves. In this work, we analyze the flutter characteristics of a helicopter blades with a periodic change in their sandwich material using a finite element structural model. Results shows great improvements in the flutter forward speed of the rotating blade obtained by using periodic design and increasing the number of periodic cells.