• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1383-1388
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• 2004

The flow characteristics in the blade passage 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 the relative velocity distributions near the rotor tip, large axial velocity decay is observed at near stall condition, which results in large blockage compared to that at the design condition. Througout the flow measurements using a quasi-orthogonal measuring points to the tip leakage vortex, it is noted that the radial position of the tip leakage vortex is distributed between 94 and 96 percent span for all flow conditions. High spectrum density due to the large fluctuation of the tip leakage vortex is observed near the blade suction surface below the frequency of 1000 Hz at near stall condition.

• International Journal of Fluid Machinery and Systems
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• v.4 no.3
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• pp.307-316
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• 2011

In order to clarify the features of tip leakage vortex near blade tip region in a half-ducted axial fan with large bellmouth, the experimental investigation was carried out using a 2-dimensional LDV system. Three sizes of tip clearance (TC) were tested: those sizes were 1mm (0.55% of blade chord length at blade tip), 2mm (1.11% of blade chord length at blade tip) and 4mm (2.22% of blade chord length at blade tip), and those were shown as TC=1mm, TC=2mm and TC=4mm, respectively. Fan characteristic tests and the velocity field measurements were done for each TC. Pressure - flow-rate characteristics and two-dimensional velocity vector maps were shown. The vortex trace and the vortex intensity distribution were also illustrated. As a result, a large difference on the pressure - flow-rate characteristics did not exist for three tip clearance sizes. In case of TC=4mm, the tip leakage vortex was outflow to downstream of rotor was not confirmed at the small and reference flow-rate conditions. Only at the large flow-rate condition, its outflow to downstream of rotor existed. In case of TC=2mm, overall vortex behaviors were almost the same ones in case of TC=4mm. However, the vortex trace inclined toward more tangential direction. In case of TC=1mm, the clear vortex was not observed for all flow-rate conditions.

• 유체기계공업학회:학술대회논문집
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• 2003.12a
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• pp.496-501
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• 2003

Reynolds averaged Wavier-Stokes simulations based on the Reynolds stress model was performed to investigated the effect of inlet flow angle on the distributions of the Reynolds stress tensor inside tip leakage vortex of a linear compressor cascade. Two different inlet flow angles ${\beta}=29.3^{\circ}$(design condition) and $36.5^{\circ}$(off-design condition) were considered. Stress tensor analysis, which transforms the Reynolds stress into the principal direction, was applied to show an anisotropy of the normal stresses. Whereas the anisotropy was highest in the region where the tip leakage vortex collides the suction side of the blade and tip leakage flow enters between blade tip of the pressure side and the endwall, it had the lowest value at the center of tip leakage vortex. It was also found that the magnitude of maximum shear stress at design condition was greater than that of off-design condition.

• International Journal of Aeronautical and Space Sciences
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• v.6 no.2
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• pp.84-96
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• 2005

Helical tip vortex is known as stable vortex structure, however the specific frequency component of far wake perturbation induces the vortex pairing in hover and axial flight. It is expected that the tip vortex pairing phenomena may happen in transition flight and very low advance ratio flight so that inflow may be most nonuniform in the low advance ratio flight. The objectives of this paper are that a tip-vortex instability during the transition from hover into very low advance ratio forward flight is numerically predicted to understand a physics by using a time-marching free-wake method. To achieve the objectives, numerical method is firstly validated in typical axial and forward flights cases. Present scheme with trim routine can predict airloads and inflow distribution of forward flight with good accuracy. Then, the transition flight condition is calculated. The rotor used in this wake calculation is a small-scale AH-1G model. By using a tip-vortex trajectory tracking method, the tip-vortex pairing process are clearly observed in transient flight($\mu$=0.03) and disappears at a slightly higher advance ratio($\mu$=0.05). According to the steady flight simulation at $\mu$=0.03, it is confirmed the tip-vortex pairing process is continued in the rear part of rotor disk and not occurs in the front part. Time averaged inflow in this case is predicted as smooth distribution.

• Journal of the Society of Naval Architects of Korea
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• v.47 no.6
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• pp.770-775
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• 2010

As considerable interests in noise emission from the ships have been increased, control of the propeller cavitation generating vibration and radiating noise is looming large. In general, the tip vortex cavitation is first produced in case of full scale propellers, and noise levels rise dramatically from that moment. In order to reduce induced noise from the tip vortex cavitation and hence increase the cavity inception speed, we propose the mass injection method. Water injected from the propeller tip decreases rotating speed of the tip flow, and it restrains growing the tip vortex cavity. Experimental investigations of the model tests carried out in a large cavitation tunnel show that the tip vortex cavitation is effectively controled by water injection from the propeller tip.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.845-850
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• 2003

Unsteady nature of a tip leakage vortex in an axial flow fan operating at a design and off-design operating conditions has been investigated by measuring the velocity fluctuation in a blade passage with a rotating hotwire probe sensor. Two hot-wire probe sensors rotating with the fan rotor were also introduced to obtain the cross-correlation coefficient between the two sensors located in the vortical flow as well as the fluctuating velocity. The results show that the vortical flow structure near the rotor tip can be clearly observed at the quasi-orthogonal planes to a tip leakage vortex. The leakage vortex is enlarged as the flow rate is decreased, thus resulting in the high blockage to main flow. The spectral peaks due to the fluctuating velocity near the rotor tip are mainly observed in the reverse flow region at higher flow rates than the peak pressure operating condition. However, no peak frequency presents near the rotor tip for near stall condition.

• 유체기계공업학회:학술대회논문집
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• 2003.12a
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• pp.404-411
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• 2003

Three-dimensional vortical flow and separated flow topology near the casing wall in an axial flow fan having two different tip clearances have been investigated by a Reynolds-averaged Wavier-Stokes (RANS) flow simulation. The simulation shows that the tip leakage vortex formed close to the leading edge of the blade tip on suction side grows in the streamwise direction. On the casing wall, a separation line is formed upstream of the leakage vortex center due to the interference between the leakage vortex and main flow. The reverse flow is observed between the separation line and the attachment line generated downstream of the trailing edge, and increased with enlarging tip clearance. The patterns of a leakage velocity vector including a leakage flow rate are also analyzed according to two tip clearances. It is noted that the understanding of the distribution of a limiting streamline on the casing wall is very important to grasp the characteristics of the vortical flow in the axial flow fan.

• Journal of Advanced Marine Engineering and Technology
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• v.28 no.5
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• pp.735-745
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• 2004

A linear cascade of NACA 65-1810 profiles are investigated for tip leakage flow characteristics. and calculation results are compared with experimental result. STAR-CD commercial code was used to solve the three dimensional incompressible Navier-Stokes equation that was adopted for steady flow and high Reynolds $\kappa$- $\varepsilon$turbulent model. Numerical calculation of a linear cascade is carried out to investigate effect of tip clearance on pitchwise variations of velocity Profiles. and static pressure distributions on the blade surface at spanwise positions. In case of evolution of tip vortex core location. tip vortex geometry and static pressure at the center of the tip vortex core compared with experimental results. Calculation results are agreed well with the experimental data, and validated. The static pressure losses by tip leakage flow at 2% tip clearance were more than those at 1% tip clearance.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.3
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• pp.336-345
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• 1998

The substantial loss behind axial flow rotor was generated by wake, various vortices in the hub region and the leakage vortex in the tip region. Particularly, the leakage vortex formed near blade tip was one of the main causes of the reduction of performance, the generation of noise and the aerodynamic vibration in rotor downstream. In this study, the three-dimensional flowfields in an axial flow rotor for various tip clearances were calculated, and the numerical results were compared with the experimental ones. The numerical technique was based on SIMPLE algorithm using standard k-.epsilon. model (WFM). Through calculations, the effects of the tip clearance on the overall performance of rotor and the loss distributions, and the increase in the displacement, momentum, and blade-force-deficit thickness of the casing wall boundary layer were investigated. The mass-averaged flow variables behind rotor agreed well with the experimental results. The presence of the tip leakage vortex behind rotor was described well. Although the loci of leakage vortex by calculation showed some differences compared with the experimental results, its behavior for various tip clearances was clarified by examining the loci of vortex center.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.8 s.227
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• pp.902-909
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• 2004

A steady-state Reynolds averaged Navier-Stokes simulation was conducted to investigate the distribution of the Reynolds stress tensor inside tip leakage vortex of a linear compressor cascade. Two different inlet flow angles ${\beta}=29.3^{\circ}$(design condition) and $36.5^{\circ}$(off-design condition) at a constant tip clearance size of $1\%$ blade span were considered. Classical methods of solid mechanics, applied to view the Reynolds stress tensor in the principal direction system, clearly showed that the high anisotropic feature of turbulent flow field was dominant at the outer part of tip leakage vortex near the suction side of the blade and endwall flow separation region, whereas a nearly isotropic turbulence was found at the center of tip leakage vortex. There was no significant difference in the anisotropy of the Reynolds normal stresses inside tip leakage vortex between the design and off-design condition.