• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.1027-1033
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• 2000

This paper describes the characteristics of vibration of a hydraulic turbine mainly due to cavitation occurrence. The analysis of vibration spectra of the turbine shaft shows that hub vortex cavitation occurs in the downstream of the turbine runner, which is verified from coherence analysis between shaft vibration displacement and dynamic pressure at the draft tube. Even though acceleration level measured at the guide vane lever, which is usually used for evaluation of cavitations performance, is decreased during forced aeration, it is found from the analysis of dynamic pressure spectra that cavitation around runner blades still remains unchanged. It is also found that lateral vibration of the turbine shaft is mainly due to the hub vortex cavitation of the turbine runner.

• Ocean Systems Engineering
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• v.9 no.3
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• pp.241-259
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• 2019

This paper presents a fully three-dimensional numerical approach for analyzing deepwater drilling riser-conductor system vortex-induced vibrations (VIV) including nonlinear soil-structure interactions (SSI). The drilling riser-conductor system is modeled as a tensioned beam with linearly distributed tension and is solved by a fully implicit discretization scheme. The fluid field around the riser-conductor system is obtained by Finite-Analytic Navier-Stokes (FANS) code, which numerically solves the unsteady Navier-Stokes equations. The SSI is considered by modeling the lateral soil resistance force according to nonlinear p-y curves. Overset grid method is adopted to mesh the fluid domain. A partitioned fluid-structure interaction (FSI) method is achieved by communication between the fluid solver and riser motion solver. A riser-conductor system VIV simulation without SSI is firstly presented and served as a benchmark case for the subsequent simulations. Two SSI models based on a nonlinear p-y curve are then applied to the VIV simulations. Also, the effects of two key soil properties on the VIV simulations of riser-conductor systems are studied.

• Wind and Structures
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• v.34 no.1
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• pp.115-125
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• 2022

The high-Reynolds number flow around a rectangular cylinder, having streamwise to crossflow length ratio equal to 5 is analyzed in the present paper. The flow is characterized by shear-layer separation from the upstream edges. Vortical structures of different size form from the roll-up of these shear layers, move downstream and interact with the classical vortex shedding further downstream in the wake. The corresponding mean flow is characterized by a recirculation region along the lateral surface of the cylinder, ending by mean flow reattachment close to the trailing edge. The mean flow features on the cylinder side have been shown to be highly sensitive to set-up parameters both in numerical simulations and in experiments. The results of 21 Large Eddy Simulations (LES) are analyzed herein to highlight the impact of the lateral mean recirculation characteristics on the near-wake flow features and on some bulk quantities. The considered simulations have been carried out at Reynolds number Re=DU_∞/ν=40 000, being D the crossflow dimension, U_∞ the freestream velocity and ν the kinematic viscosity of air; the flow is set to have zero angle of attack. Some simulations are carried out with sharp edges (Mariotti et al. 2017), others with different values of the rounding of the upstream edges (Rocchio et al. 2020) and an additional LES is carried out to match the value of the roundness of the upstream edges in the experiments in Pasqualetto et al. (2022). The dimensions of the mean recirculation zone vary considerably in these simulations, allowing us to single out meaningful trends. The streamwise length of the lateral mean recirculation and the streamwise distance from the upstream edge of its center are the parameters controlling the considered quantities. The wake width increases linearly with these parameters, while the vortex-shedding non-dimensional frequency shows a linear decrease. The drag coefficient also linearly decreases with increasing the recirculation length and this is due to a reduction of the suctions on the base. However, the overall variation of C_D is small. Finally, a significant, and once again linear, increase of the fluctuations of the lift coefficient is found for increasing the mean recirculation streamwise length.

• Journal of The Korean Astronomical Society
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• v.54 no.2
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• pp.61-70
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• 2021

We investigate 20 post-coronal mass ejection (CME) blobs formed in the post-CME current sheet (CS) that were observed by K-Cor on 2017 September 10. By visual inspection of the trajectories and projected speed variations of each blob, we find that all blobs except one show irregular "zigzag" trajectories resembling transverse oscillatory motions along the CS, and have at least one oscillatory pattern in their instantaneous radial speeds. Their oscillation periods are ranging from 30 to 91 s and their speed amplitudes from 128 to 902 km s-1. Among 19 blobs, 10 blobs have experienced at least two cycles of radial speed oscillations with different speed amplitudes and periods, while 9 blobs undergo one oscillation cycle. To examine whether or not the apparent speed oscillations can be explained by vortex shedding, we estimate the quantitative parameter of vortex shedding, the Strouhal number, by using the observed lateral widths, linear speeds, and oscillation periods of the blobs. We then compare our estimates with theoretical and experimental results from MHD simulations and fluid dynamic experiments. We find that the observed Strouhal numbers range from 0.2 to 2.1, consistent with those (0.15-3.0) from fluid dynamic experiments of bluff spheres, while they are higher than those (0.15-0.25) from MHD simulations of cylindrical shapes. We thus find that blobs formed in a post-CME CS undergo kinematic oscillations caused by fluid dynamic vortex shedding. The vortex shedding is driven by the interaction of the outward-moving blob having a bluff spherical shape with the background plasma in the post-CME CS.

• Advances in aircraft and spacecraft science
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• v.6 no.5
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• pp.371-389
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• 2019

The implementation of a robust $H_{\infty}$ Control, which is numerically efficient for uncertain nonlinear dynamics, on longitudinal and lateral autopilots is realised for a quarter scale Piper J3-Cub model accepted as an unmanned aerial vehicle (UAV) under the condition of sensor noise and disturbance effects. The stability and control coefficients of the UAV are evaluated through XFLR5 software, which utilises a vortex lattice method at a predefined flight condition. After that, the longitudinal trim point is computed, and the linearization process is performed at this trim point. The "${\mu}$-Synthesis"-based robust $H_{\infty}$ control algorithm for roll, pitch and yaw displacement autopilots are developed for both longitudinal and lateral linearised nonlinear dynamics. Controller performances, closed-loop frequency responses, nominal and perturbed system responses are obtained under the conditions of disturbance and sensor noise. The simulation results indicate that the proposed control scheme achieves robust performance and guarantees stability under exogenous disturbance and measurement noise effects and model uncertainty.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.1
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• pp.18-26
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• 2007

The steady-state aerodynamic characteristics of wings on the formation flight were analyzed using the Vortex Lattice Method. When two wings were at formation flight, the sectional lift coefficient of a rear wing was increased due to a front wing. The result showed that the lift drag ratio increased as the rear wing were placed downward and decreased as the lateral spacing between wings increased. The difference of lift drag ratio between forward wing and rear wing increase as the aspect ratio of wings increased. When a rear wings and a forward wings placed at the same height, wings on the formation flight had the maximum lift drag ratio. The results showed that the benefit of the formation flight increased as the number of wings on the formation flight increased.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.46 no.12
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• pp.986-993
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• 2018

Lateral thrust jet has better maneuverability performance than the control surface like the conventional fin for attitude control or orbital transition of guided weapons. However, in the supersonic region, a jet interaction flow occurs due to the lateral thrust jet during flight, and a complicated flow structure is exhibited by the interaction of the shock wave, boundary layer flow, and the vortex flow. Especially, hit-to-kill interceptors require precise control and maneuvering, so it is necessary to analyze the effect of jet interaction flow. Conventional jet interaction analyses were performed under low altitude conditions, but there are not many cases in the case of medium altitude condition, which has different flow characteristics. In this study, jet interaction flow analysis is performed on the lateral jet controlled interceptor operating at medium altitude. Based on the results, the structural characteristics of the flow field and the changes of aerodynamic coefficient are analyzed.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2873-2878
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• 2007

The mixing vanes attached to the spacer grid of rod bundles are used to improve the heat transfer in heat exchanger devices by controlling the characteristics of the flow structures and turbulence. In this study, velocity patterns induced by two types of mixing vane(split and swirl vane) are measured by the PIV technique to better understand how to effect on the cross and secondary vortex flow patterns in $5{\times}$ rod bundle simulating the fuel assembly of the nuclear reactor. A successful measurement of the lateral velocity patterns was conducted using a specially designed beam sheet generator and experimental loop at KAERI. As the result, we found that for the cross flow between subchannels, the split vane is more effective than the swirl vane, while for the secondary vortex flow in each subchannel, the swirl vane's one is larger and longer than split vane's one.

• Wind and Structures
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• v.30 no.5
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• pp.475-483
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• 2020

This work investigates Reynolds number Re (= 50 - 200) effects on the flows around a single cylinder and the two tandem (center-to-center spacing L^⁎= L/D = 4) cylinders, each of a diameter D. Vorticity structures, Strouhal numbers, and time-mean and fluctuating forces are presented and discussed. For the single cylinder, with increasing Re in the range examined, the vorticity magnitude, Strouhal number and fluctuating lift all monotonically rise but time-mean drag, vortex formation length, and lateral distance between the two rows of vortices all shrink. For the two tandem cylinders, the increase in Re leads to the formation of three distinct flows, namely reattachment flow (50 ≤ Re ≤ 75), transition flow (75 < Re < 100), and coshedding flow (100 ≤ Re ≤ 200). The reattachment flow at Re = 50 is steady. When Re is increased from 75 to 200, the Strouhal number of the two cylinders, jumping from 0.113 to 0.15 in the transition flow regime, swells to 0.188. The two-cylinder flow is more sensitive to Re than the single cylinder flow. Fluctuating lift is greater for the downstream cylinder than the upstream cylinder while time-mean drag is higher for the upstream cylinder than for the other. The time-mean drags of the upstream cylinder and single cylinder behaves similar to each other, both declining with increasing Re.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.8
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• pp.790-797
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• 2010

Mode transition of the axi-symmetric screech tone in the low supersonic Mach number range from 1.0 to 1.20 is numerically analyzed. The axi-symmetric Navier-Stokes equations and the k-e turbulence model are solved in the cylindrical coordinate system. The dispersion-relation-preserving(DRP) scheme is applied for space discretization and the optimized four levels marching method are used for time integration. At low supersonic Mach numbers with an axi-symmetric A1 mode in the simulation, it is shown that acoustic propagation due to the nonlinear effects is seen in the lateral direction and the screech tone frequency is the same as the vortex passing frequency due to the generation of intense large-scale vortical motions.