• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.6
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• pp.786-798
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• 2001

This paper describes the phenomena of wake-induced transition of the boundary layers on a NACA0012 airfoil using measured phase-averaged data. Especially, the phase-averaged wall shear stresses are reasonably evaluated using the principle of Computational Preston Tube Method. Due to the passing wake, the turbulent patch is generated in the laminar boundary layer on the airfoil and the boundary layer becomes temporarily transitional. The patches propagate downstream with less speed than free-stream velocity and merge with each other at further down stream station, and the boundary layer becomes more transitional. The generation of turbulent patch at the leading edge of the airfoil mainly depends on velocity defects and turbulent intensity profiles of passing wakes. However, the growth and merging of turbulent patches depend on local streamwise pressure gradients as well as characteristics of turbulent patches. In this transition process, the present experimental data show very similar features to the previous numerical and experimental studies. It is confirmed that the two phase-averaged mean velocity dips appear in the outer region of transitional boundary layer for each passing cycle. Relatively high values of the phase-averaged turbulent fluctuations in the outer region indicate the possibility that breakdown occurs in the outer layer not near the wall.

• 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.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.62.1-62.1
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• 2018

Galaxies in the cluster environment interact with the intracluster medium (ICM), losing the interstellar medium (ISM) and alternating their evolution. Observational evidences of the extraplanar ISM stripped by the ICM's ram pressure are prevalent in HI imaging studies of cluster galaxies. However, current theoretical understanding of the ram pressure stripping (or ICM-ISM interaction in general) is still limited mainly due to the lack of numerical resolution at ISM scales in large-scale simulations. Especially, self-consistent modeling of the turbulent, multiphase ISM is critical to understand star formation in galaxies interacting with the ICM. To achieve this goal, we utilize the TIGRESS simulation suite, simulating a local patch of galactic disks with high resolution to resolve key physical processes in the ISM, including cooling/heating, self-gravity, MHD, star formation, and supernova feedback. We then expose the ISM disk to ICM flows and investigate the evolution of star formation rate and the properties of the ISM. By exploring ICM parameter space, we discuss an implication of the simple ram pressure stripping condition (so called the Gunn-Gott condition) to the realistic ISM.

• Journal of computational fluids engineering
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• v.17 no.3
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• pp.87-92
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• 2012

The presence of a layer of vegetation which is relevant in river engineering or coastal engineering can modify the overall flow resistance, turbulent characteristics of flow. The patch of vegetation can be modelled and studied in a simple porous cylinder by previous researchers. Fully three dimensional Large Eddy Simulation is conducted in flow past a porous cylinder with a solid volume fraction (SVF) 0f 20%. The porous cylinder of diameter D contains 89 smaller cylinders which diameter is 0.048D in a regular staggered way. Reynolds number based on porous cylinder diameter D and the bulk velocity is 10,000. The large scale shedding is qualitatively similar to the one observed in the non-porous case (SVF=100%). The difference in the dynamics of the separated shear layer and the streamwise flow penetrating through the porous cylinder are compared with those in the non-porous cylinder. In particular, the wake billows form a larger distance from the back of the porous cylinder.

• 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.