• International Journal of Fluid Machinery and Systems
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• v.3 no.3
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• pp.235-244
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• 2010

Recent studies on the moment whirl due to leakage flow in the back shroud clearance of hydro-turbine runners or centrifugal pump impellers are summarized. First, destabilizing effect of leakage flow is discussed for lateral vibrations using simplified models. Then it is extended to the case of whirling motion of an overhung rotor and the criterion for the instability is obtained. The fluid moment caused by a leakage clearance flow between a rotating disk and a stationary casing was obtained by model tests under whirling and precession motion of the disk. It is shown that the whirl moment always destabilizes the whirl motion of the overhung rotor while the precession moment destabilizes the precession only when the precession speed is less than half the rotor speed. Then vibration analyses considering both whirl and precession are made by using the hydrodynamic moments determined by the model tests. For larger overhung rotors, the whirl moment is more important and cause whirl instability at all rotor speed. On the other hand, for smaller overhung rotors, the precession moment is more important and cancels the destabilizing effect of the whirl moment.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2008.04a
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• pp.190-196
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• 2008

The dynamical rotor system is investigated through the derivation and formulations of the dynamic equation of the rotating system in terms of both inertial and fixed frame of the system as well as quaternion. The investigation is aimed at analyzing the dynamical rotating system precession speed. The resulting equations of motion consist of the consistent mass matrix and gyroscopic matrix. The formulation shows its features and difference between its linearity and nonlinearity.

• International Journal of Fluid Machinery and Systems
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• v.4 no.1
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• pp.179-190
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• 2011

The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The influence of outlet bubble cavitation and air injection is also investigated for low cavitation number. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.29 no.7
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• pp.540-549
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• 2018

Classifying the front body of the missile and debris of a high-speed missile in intercepting a high-speed missile is an important issue. The motion of the front body of the missile is characterized by precession, but the motion of the debris in the boosting part separation phase is characterized by tumbling. There are periodic patterns caused by the precession or tumbling motion on the dynamic radar cross section (RCS). In addition, there are statistical properties caused by the change pattern of the dynamic RCS. A method is proposed to classify the front body of the missile and debris using periodic and statistical properties of the dynamic RCS. Three kinds of feature vector are extracted from the periodic and statistical properties of the dynamic RCS. The front body of the missiles and debris was classified using a support vector machine.

• Journal of Magnetics
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• v.11 no.2
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• pp.61-65
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• 2006

The physical origin of complex dynamic domain configuration in nonequilibrium magnetic systems with mesoscopic length scales has been studied. An increasing complexity in the spatial feature of the evolution is found to accompany the increasing reversal speed, when a ferromagnetic element is driven by progressively faster switching fields applied antiparallel to the initial magnetization direction. As reversal rates approach the characteristic precession frequencies of spin fluctuations, the thermal energy can boost the magnetization into local configurations which are completely different from those experienced during quasistatic reversal. The sensitive dependence of the spatial pattern on switching speed can be understood in terms of a dynamic exchange interaction of thermally excited spins; the coherent modulation of the spins is strongly dependent on the rise time of switching pulses.

• Proceedings of the KIEE Conference
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• 1999.07b
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• pp.553-555
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• 1999

In this paper, we developed the new-type digital gyrocompass for ship. This is made of DTG(Dynamically Tuned Gyro) and two accelerometers. The simple structure of two-axis free gimbals and the digital precession by CPU unit are main differences from Sperry or Anschutz type gyrocompass. As experimental results, the developed gyrocompass seeks the true north within 1 hour and support maximum speed 100[knot], maximum latitude $75[^{\circ}]NS$. By the comparison with other company's Products, we show the uniqueness and the excellence of the proposed gyrocompass.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.9
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• pp.73-80
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• 2005

This paper addresses a newly proposed start-up method of the HAUSAT-2 momentum wheel. The HAUSAT-2 is a 25kg class nanosatellite which is stabilized to earth pointing by 3-axis active control method. A momentum wheel performs two functions. It provides a pitch-axis momentum bias while measuring satellite pitch and roll attitude. Pitch control is accomplished in the conventional way by driving a momentum wheel in response to pitch attitude errors. Precession control and nutation damping are provided by driving the pitch axis magnetic torquer. A momentum wheel is nominally spinning at a particular rate and changes speed. This simulation study investigates the feasibility and performance of a proposed strategy for starting-up the wheel. A proposed strategy to start-up the wheel shows that a pitch momentum wheel can be successfully started-up to its nominal speed from rest and be stabilized to nadir pointing.