• Title/Summary/Keyword: Friction Drag

Search Result 181, Processing Time 0.03 seconds

Turbulent Drag Reduction Using the Sliding-Belt Device (미끄러지는 벨트 장치를 이용한 난류 항력 감소)

  • Choi, Byunggui;Choi, Haecheon
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.23 no.11
    • /
    • pp.1481-1489
    • /
    • 1999
  • The sliding-belt concept introduced by Bechert et al. (AIAA J., Vol. 34, pp. 1072~1074) is numerically applied to a turbulent boundary layer flow for the skin-friction reduction. The sliding belt is moved by the shear force exerted on the exposed surface of the belt without other dynamic energy input. The boundary condition at the sliding belt is developed from the force balance. Direct numerical simulations are performed for a few cases of belt configuration. In the ideal case where the mechanical losses associated with the belt can be ignored, the belt velocity increases until the integration of the shear stress over the belt surface becomes zero, resulting in zero skin friction on the belt. From practical consideration of losses occurred In the belt device, a few different belt velocities are given to the sliding belt. It is found that the amount of drag reduction is proportional to the belt velocity.

An Adaptive and Robust Controller for the Undersea Robot Manipulator

  • Young-Sik kim;Park, Hyeung-Sik
    • International Journal of Precision Engineering and Manufacturing
    • /
    • v.4 no.2
    • /
    • pp.13-22
    • /
    • 2003
  • To coordinate the robot manipulator along the desired trajectory, the exact model of the dynamics is required. The added mass and added moment of inertia, buoyancy, drag force, and friction mainly affect the dynamics of the undersea robot manipulator, and they are quite complex and unknown. In this reason. the exact model of the undersea robot manipulator is difficult to obtain. In this paper, instead of having efforts to get the exact model of the robot dynamics, a control-based approach was performed. We modeled the dynamics of the undersea robot manipulator whose parameters are unknown, and then applied a proposed direct adaptive and robust control, which is different from previous studies. The unknown added mass, and added moment of inertia, drag force and friction are estimated by the direct adaptive control scheme, and the drag force which is dominant disturbance is compensated by the robust control. Also, stability of the proposed control scheme is analyzed.

Wind Tunnel Test of 2D Model for Plasma Flow Control using DBD Plasma Actuator (DBD 플라즈마 구동기를 이용한 2차원 모델의 플라즈마 유동제어 풍동시험)

  • Yun, Su-Hwan;Kim, Tae-Gyu
    • Proceedings of the Korean Society of Propulsion Engineers Conference
    • /
    • 2012.05a
    • /
    • pp.527-528
    • /
    • 2012
  • DBD (Dielectric Barrier Discharge) plasma actuator was designed for aerodynamic drag reduction using plasma flow control, and the drag reduction was measured by wind-tunnel tests using 2D test model. At the zero wind velocity, the plasma flow control had no effect on the drag reduction because the flow separation and surface friction drag were not occurred. At the wind velocity of 2m/s, 9.7% of drag was reduced by the flow separation control. The drag reduction decreased as the wind velocity increased.

  • PDF

Experimental Investigation on the Drag Reduction Mechanism of Outer-layer Vertical Blades Array using Stereoscopic Time-Resolved PIV (스테레오 시간분해 입자영상유속계를 이용한 외부경계층 수직날 배열에 의한 마찰저항 저감 기구에 관한 실험적 조사)

  • Lee, Inwon;Park, Seong-Hyeon;Chun, Ho-Hwan;Hwang, Arom;An, Nam-Hyun
    • Journal of Power System Engineering
    • /
    • v.17 no.6
    • /
    • pp.95-101
    • /
    • 2013
  • A stereo PIV measurements in a circulating water channel has been performed to investigate the skin friction reduction mechanism of the outer-layer vertical blades first devised by Hutchins. In a recent PIV measurement study, considerable skin friction reduction was achieved as much as 2.73%~7.95% by outer-layer vertical blades array. In the present study, the influence of vertical blades array upon the characteristics of the turbulent coherent structures was analyzed by proper orthogonal decomposition method. It is observed that the vortical structures are cut and deformed by blades array and also the turbulent intensity and the Reynolds stress were weakened by the blades. These phenomena strongly associate the skin-friction drag reduction mechanism in the turbulent boundary layer flow.

GAS-DYNAMICAL FRICTION OF A PERTURBER MOVING ON A CIRCULAR ORBIT

  • Kim, Hyo-Sun;Kim, Woong-Tae
    • Journal of The Korean Astronomical Society
    • /
    • v.40 no.4
    • /
    • pp.179-182
    • /
    • 2007
  • Dynamical friction plays an important role in reducing angular momenta of objects in orbital motions. While astronomical objects usually follow curvilinear orbits, most previous studies focused on the linear-trajectory cases. Here, we present the gravitational wake due to, and dynamical friction on, a perturber moving on a circular orbit in a uniform gaseous medium using a semi-analytic method. The circular orbit causes the density wakes to bend along the orbit into asymmetric configurations, resulting in the drag forces in both opposite (azimuthal) and lateral (radial) directions to the perturber motion, although the latter does not contribute to the orbital decay much. For a subsonic perturber, the bending of a wake is only modest and the resulting drag force in the opposite direction is remarkably similar to the linear-trajectory counterpart. On the other hand, a supersonic perturber is able to overtake its own wake, possibly multiple times, creating a high-density trailing tail. Despite the dramatic changes in the wake morphologies, the azimuthal drag force is in surprisingly good agreement with the formulae of Ostriker for the linear-trajectory cases, provided $V_pt=2R_p,\;where\;V_p\;and\;R_p$ are the velocity and orbital radius of the perturber, respectively.

Characteristics of friction and stiction in head/disk interface (헤드/디스크 시스템의 마찰력 측정 및 stiction 특성)

  • 이성창;정구현;김대은
    • Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
    • /
    • 1997.10a
    • /
    • pp.45-50
    • /
    • 1997
  • In recent years the recording density of hard disk has been increasing largely due to the decreasing flying height of head. However, as the flying height is decreased the reliability issue becomes ever more critical. This is because the reliability and durability of hard disk is related to the head/disk interaction as the two components come into partial or full contact. In this work characteristics of friction and stiction in head/disk interface was investigated using constant speed drag test and CSS(contact-start-stop) test. The purpose of this research is to identify the frictional properties of head/disk interface

  • PDF

Analysis of the Dimensionless Torque in Cone Drum False Twisting Mechanism

  • Lee, Choon-Gil;Kang, Tae-Jin
    • Fibers and Polymers
    • /
    • v.4 no.4
    • /
    • pp.161-168
    • /
    • 2003
  • An investigation of the dimensionless torque in the newly developed cone drum twister texturing mechanism is reported. The cone drum twister is one of the outer surface contacting friction-twisting devices in false-twist texturing. In this cone drum twister, a filament yam passes over the surface of the cone drum that rotates by the passing yarn without a special driving device. This research is composed of the theoretical analysis of the false twisting mechanism and the experimental analysis at room temperature. The equations have been derived which shows interrelationship of the conical angle of cone drum, the wrapping angle, the drag angle, and the yam helix angle. Theoretical values of dimensionless torque were calculated and were compared with the experimental results. It is shown that, as the conical angle and the projected wrapping angle increased, the dimensionless torque also increased. But the conical angle was reached to ${30.75}^{\circ}C$, the dimensionless torque decreased.

A Study on Running Resistance of Rolling Stock (철도차량의 주행저항에 관한 연구)

  • Kim, Eung-Cheon;Lee, Jae-An;Lee, Ha-Hee
    • Proceedings of the KSR Conference
    • /
    • 2008.11b
    • /
    • pp.1782-1793
    • /
    • 2008
  • Republic of korea has begun operating high speed train service according as KTX service operation starts in 2004. Also, EMU whose maximum speed is over 150 kph will be starting to service with electrification and improvement of existing railroad. Moreover, metropolitan electric railways have begun an express service to increase scheduled speed. Therefore, running resistance of rolling stock becomes more important factor effects on the performance. Running resistance of rolling stock is the factor which is necessary for the performance or operation plan of rolling stock, and it's related to rolling friction, slip friction, drag force, gradient, acceleration, curvature, tunnel condition and so on. It is possible to be calculated by CFD (Computational Fluid Dynamics). However it is predicted by experimental equation from running resistance test because of the complex calculation and manifold variables. In this paper, studies about running resistance of rolling stock is introduced, and each term of experimental equation is studied through theoretical approximation. Also, running resistance of rolling stock is estimated by the result of running resistance test, and effects being related to friction, drag force, gradient is examined.

  • PDF

Prediction of Turbulent Flow Over L-Shaped Riblet Surfaces with $k-\varepsilon$ Turbulence Models ($k-\varepsilon$ 난류모델에 의한 L-형 리브렛 주위 난류유동 예측)

  • Myeong, Hyeon-Guk
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.22 no.1
    • /
    • pp.93-103
    • /
    • 1998
  • The paper reports the outcome of a numerical study of flow over idealized L-shaped ribleted surfaces with two-equation turbulence models. In the present study, the Launder and Sharma's k-.epsilon. turbulence model (LS model) is basically N employed, but with a little modification of the additional .epsilon.-source term without affecting its level under 2-dimensional straining in which the term has been calibrated. Compared to the original LS model, the present model has predicted greatly improved drag reduction behavior for this geometry. As a drag reduction mechanism, it is found that the skin-friction in the riblet valleys might be sufficient to overcome the skin-friction increase near the riblet tip. The present predicted results are in good agreement with the recent DN S ones by Choi et al. (1993): differences in the mean velocity prof ile and turbulence quantities are found to be limited to the riblet cavity region. It is also found that turbulent kinetic energy and Reynolds shear stress above the riblets are also reduced in drag-reducing configurations.

Effects of Cooling Flow Rate on Gas Foil Thrust Bearing Performance (냉각 유량이 가스 포일 스러스트 베어링의 성능에 미치는 영향)

  • Sung Ho Hwnag;Dae Yeon Kim;Tae Ho Kim
    • Tribology and Lubricants
    • /
    • v.39 no.2
    • /
    • pp.76-80
    • /
    • 2023
  • This paper describes an experimental investigation of the effect of cooling flow rate on gas foil thrust bearing (GFTB) performance. In a newly developed GFTB test rig, a non-contact type pneumatic cylinder provides static loads to the test GFTB and a high-speed motor rotates a thrust runner up to the maximum speed of 80 krpm. Force sensor, torque arm connected to another force sensor, and thermocouples measures the applied static load, drag torque, and bearing temperature, respectively, for cooling flow rates of 0, 25, and 50 LPM at static loads of 50, 100, and 150 N. The test GFTB with the outer radius of 31.5 mm has six top foils supported on bump foil structures. During the series of tests, the transient responses of the bearing drag torque and bearing temperature are recorded until the bearing temperature converges with time for each cooling flow rate and static load. The test data show that the converged temperature decreases with increasing cooling flow rate and increases with increasing static load. The drag torque and friction coefficient decrease with increasing cooling flow rate, which may be attributed to the decrease in viscosity and lubricant (air) temperature. These test results suggest that an increase in cooling flow rate improves GFTB performance.