• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.5
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• pp.685-694
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• 2002

Effects of wake-passing orientation and frequency on the wake-induced boundary layer transition on a NACA0012 airfoil are investigated. The wakes are generated by rotating cylinders clockwise (CW) and counterclockwise (CCW) around the airfoil. Time- and phase-averaged streamwise mean velocities and turbulent fluctuations are measured with a single hot-wire probe. Wall skin frictions are estimated by the Computational Preston Tube Method (CPM). The pressure distribution on the airfoil is different according to the wake-passing orientation and frequency. Turbulent patches are generated in the laminar boundary layer due to the passing wake and the boundary layer becomes temporarily transitional. The transition process is significantly affected by the pressure gradient and the turbulent patches. For the receding wake, the turbulent patches propagate more rapidly than those for the approaching wake because adverse pressure gradient becomes larger. As the frequency increases, onset location of transition moles upstream and the boundary layer near the trailing edge becomes more transitional.

• Journal of Mechanical Science and Technology
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• v.15 no.12
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• pp.1860-1868
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• 2001

An experimental investigation was carried out to clarify unsteady flow fields with rotating stall cell, especially behavior of stall cell, in a high specific-speed diagonal flow fan. As its specific-speed is vary high for a diagonal flow fan, its pressure-flow rate curve tends to indicate unstable characteristics caused by rotating stall similar to axial flow fan. Although for an axial flow fan many researchers have investigated such the flow field, for a diagonal flow fan tittle study has been done. In this study, velocity fields at rotor Inlet in a high specific-speed diagonal flow fan were measured by use of a single slant hot-wire probe. These data were processed by using the "Double Phase-Locked Averaging"(DPLA) technique, i. e. phases of both the rotor blade and the stall cell were taken into account. The behaviors of stall cell at rotor inlet were visualized for the meridional, tangential and radial velocity.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.9
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• pp.1335-1348
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• 1998

Flow characteristics in transitional boundary layers on a flat plate were experimentally investigated under three different freestream conditions i. e. uniform flow with 0.1 % and 3.7% freestream turbulent intensity and cylinder-wake with 3.7% maximum turbulent intensity. Instantaneous streamwise velocities in laminar, transitional and turbulent boundary layers were measured by I-type hot-wire probe. For estimation of wall shear stresses on the flat plate, measured mean velocities near the wall were applied to the principle of Computational Preston Tube Method (CPM). Distributions of skin friction coefficients were reasonably predicted in all developed boundary layers. Intermittency profiles, which were estimated using Conditional Sampling Technique in transitional boundary layers, were also consistent with previously published data. It was predicted that the incoming turbulent intensity had more influence on transition onset point and transition process than freestream turbulent intensity existed just over the transition region. It was also confirmed that non-turbulent and turbulent profiles in transitional boundary layers could not be simply treated as Blasius and fully turbulent profiles.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.03a
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• pp.358-363
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• 2008

The spray cross-section characteristics of two-phase spray that using external-mixing nozzle injected into a subsonic cross flow were experimentally studied with various ALR ratio that is $0{\sim}59.4%$. Suction type wind tunnel was used and experiments were conducted to ambient environment. Several plain orifice nozzles with L/d of 30 and orifice diameter of 0.5 mm and orifice length 1.5 mm were tested. Free stream velocity profiles at the injection location were measured using hot wire. Spray images were captured to study collision point and column trajectory. Phase Doppler particle analyzer(PDPA) was utilized to quantitatively measuring droplet SMD, volume flux. Measuring probe of PDPA positions was moved 3-way transverse machine. SMD distributions were layered structure and peaked at the top of the spray plume and low value at bottom of the spray. Volume flux of spray was distributed to the two side region and volume flux quantity decreased when ALR ratio increased. It was found that the perpendicularly injected two-phase spray jet of external mixing into a cross flow showing that mistlike spray moved away from the test section bottom region.

• Journal of the Korean Geotechnical Society
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• v.27 no.7
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• pp.17-33
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• 2011

Recently, the utilization of recycled aggregates for backfilling a power transmission pipeline trench has been considered due to the issues of eco-friendly construction and a lack of natural aggregate resource. It is important to identify the physical and thermal properties of domestic recycled aggregates that can be used as a backfill material. This paper evaluated thermal properties of concrete-based recycled aggregates with various particle size distributions. The thermal properties of the recycled aggregates and river sand provided by local vendors were measured using the transient hot wire method and the transient needle probe method after performing the standard compaction test. The needle probe method considerably overestimated the thermal resistivity of recycled aggregates especially at the dry of optimum water content because of experiencing disturbance while the needle probe is being inserted into the specimen. Similar to silica sand, the thermal resistivity of recycled aggregates decreased when the water content increased at a given dry density. Also, this paper evaluated some of the existing prediction models for the thermal resistivity of recycled aggregates with the experimental data, and developed a new prediction model for recycled aggregates. This study shows that recycled aggregates can be a promising backfill material substituting for natural aggregates when backfilling the power transmission pipeline trench.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.11
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• pp.1055-1063
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• 2012

Fluctuating wall pressures were measured using an array of 16 piezoelectric transducers beneath a turbulent boundary layer. The coating used in this experiment was an open-cell, urethane-type foam with a porosity of approximately 50 ppi. The ultimate objective of the coating is to provide a mechanical filter to reduce the wall pressure fluctuations. The boundary layer on the flat plate was measured by using a hot wire probe, and the CPM method was used to determine the skin friction coefficient. The wall pressure autospectra and streamwise wavenumber-frequency spectra were compared to assess the attenuation of the wall pressure field by the coating. The coating is shown to attenuate the convective wall pressure energy. However, the relatively rough surface of the coating in this investigation resulted in a higher mean wall shear stress, thicker boundary layer, and higher low-frequency wall pressure spectral levels compared to a smooth wall.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.687-690
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• 2002

Multimode boundary-layer transition on a NACA0012 airfoil is experimentally investigated under periodically passing wakes and the moderate level of free-stream turbulence. The periodic wakes are generated by rotating circular cylinders clockwise or counterclockwise around the airfoil. The free-stream turbulence is produced by a grid upstream of the rotating cylinder, and its intensity(Tu) at the leading edge of the airfoil is $0.5\;or\;3.5\;{\%}$. The Reynolds number ($Re_c$) based on chord length (C) of the alrfoil is $2.0{\times}10^5$, and Strouhal number ($St_c$) of the passing wake is about 0.7. Time- and phase-averaged streamwise mean velocities and turbulence fluctuations are measured with a single hot-wire probe, and especially, the corresponding wall skin friction is evaluated using a computational Preston tube method. The wake-passing orientation changes pressure distribution on the airfoil in a different manner irrespective of the free-stream turbulence. Regardless of free-stream turbulence level, turbulent patches for the receding wakes propagate more rapidly than those for the approaching wake because adverse pressure gradient becomes larger. The patch under the high free-stream turbulence ($Tu=3.5{\%}$) grows more greatly in laminar-like regions compared with that under the low background turbulence ($Tu=0.5{\%}$) in laminar regions. The former, however, does not greatly change the original turbulence level in the very near-wall region while the latter does it. At further downstream, the former interacts vigorously with high environmental turbulence inside the pre-existing transitional boundary layer and gradually lose his identification, whereas the latter keep growing in the laminar boundary layer. The calmed region is more clearly observed under the lower free-stream turbulence level and for the receding wakes. The calmed region delays the breakdown further downstream and stabilizes more the boundary layer.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.9 s.252
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• pp.921-928
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• 2006

Wake-induced boundary-layer transition on a NACA0012 airfoil with zero angle of attack is experimentally investigated in periodically passing wakes under the moderate level of free-stream turbulence. The periodic wakes are generated by rotating circular cylinders clockwise or counterclockwise around the airfoil. The free-stream turbulence is produced by a grid upstream of the rotating cylinder, and its intensities $(Tu_{\infty})$ at the leading edge of the airfoil are 0.5 and 3.5%, respectively. The Reynolds number (Rec) based on chord length (C) of the airfoil is $2.0{\times}10^5$, and Strouhal number (Stc) of the passing wake is about 1.4. Time- and phase-averaged streamwise mean velocities and turbulence fluctuations are measured with a single hot-wire probe, and especially, the corresponding wall skin friction is evaluated using a computational Preston tube method. The patch under the high free-stream turbulence $(Tu_{\infty}=3.5%)$ grows more greatly in laminar-like regions compared with that under the low turbulence $(Tu_{\infty}=0.5%)$ in laminar regions. The former, however, does not greatly change the turbulence level in very near-wall region while the latter does it. At further downstream, the former interacts vigorously with high environmental turbulence inside the pre-existing transitional boundary layer and gradually loses its identification, whereas the latter keeps growing in the laminar boundary layer. The calmed region is more clearly observed under the lower free-stream turbulence level and with the receding wakes.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.11
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• pp.1080-1088
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• 2009

An actuator using piezoceramic material was designed in order to perform a flow control for flat plate flow. Boundary layer measurements were carried out to explore the flow disturbances by the designed actuator that was activated at low excitation frequency(15Hz). The mean velocity and fluctuation in the boundary layers were measured at $x/{\delta}^*=31.9$ downstream from the actuator tip by a one-dimensional hot-wire probe(55P14). Results reveal that low- and high-velocity regions were observed in the vicinity of the actuator center and in the outer area of the actuator respectively, and the formation of counter-rotating streamwise vortices was predicted. The fluctuations were persistently found in the outer part of the actuator and an inflection point in the spanwise gradient of the streamwise velocity was observed. Boundary layer instability was amplified at both the actuator excitation frequency and the T-S wave frequency when the actuator was excited at low frequency.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.6
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• pp.1931-1940
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• 1996

The effect of periodic passing wake on the film-coolant flow issuing normally from a flat plate was investigated experimentally. The passing wake was generated by rotating thin circular bars. Depending on the rotational direction the test plate could be simulated as a pressure surface or a suction surface of a gas turbine blade. The phase-averaged velocity components were measured using an X-type hot-wire probe. The Reynolds number based on the free-stream velocity and injection hole diameter was 23, 500 and the velocity ratio which is the ratio of film coolant velocity to free-stream velocity was 0.5. The velocity-triangle induced by the wake was similar to that induced by the one generated at the blade trailing edge. The vertical velocity component induced by the passing wake, which approaches to the suction surface and moves away from the pressure surface, played a dominant role in the variation of the flow field. The variation in the phase-averaged velocity on the pressure surface was greater than on the suction surface, but the turbulence kinetic energy variation on the suction surface appeared larger than on the pressure surface.