• 대한기계학회논문집
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• 제19권1호
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• pp.240-250
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• 1995

A CPM (computational preston tube method) was developed to measure wall shear stress in a transitional boundary layer on a flat plate using Preston tubes. Correlation for the displacement factor of Preston tubes was improved for a CPM to be used in the transitional boundary layer. The distribution of skin-friction coefficient was reasonably predicted in the uniform free stream of 3.1% turbulence intensity. Reasonable and accurate estimation of displacement factor of Preston tubes was found to be of crucial importance for the CPM, especially in the laminar boundary layer. The mean velocity profiles of the boundary layer on the plate were also measured and presented.

• 대한기계학회논문집
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• 제16권12호
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• pp.2349-2357
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• 1992

본 연구에서는 일정한 조도의 거친 평판이 갑자기 매끈한 평판으로 변할 때 (Fig.1 참고) 평판 위에서 발달하는 난류경계층에 관하여 연구하였다. 이때 하류에 서 난류 경계층은 위에서 설명했듯이 완전히 발달한 난류 경계층과는 상당히 다르다. 특히 대부분의 난류 모델은 완전히 발달된 평형상태의 경계층에서의 실험자료를 이용 하여 개발되었기 때문에 이러한 과도 구역에서 합리적으로 적용되기가 어렵다. 과도 구역에서 평균속도와 난류 특성치를 체계적으로 계측하기 위해서는 전단응력을 직접 계측하는 것이 중요하다. 그러나 아직 전단응력을 직접 계측한 연구는 없는 실정이 다. 본 논문에서는 최근에 제안된 CPM(computational preston tube method)를 이용 하여 과도지역에서 전단응력을 직접 측정하여 난류 경계층의 구조를 연구하였다.

• Journal of Mechanical Science and Technology
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• 제14권11호
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• pp.1305-1318
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• 2000

The local wall shear stress in transitional boundary layer was estimated from the near-wall mean velocity data using the principle of Computational Preston Tube Method(CPM). The previous DNS and experimental databases of transitional boundary layers were used to demonstrate the accuracy of the method and to provide the applicable range of wall unit y(sup)+. The skin friction coefficients predicted by the CPM agreed well with those from previous studies. To reexamine the applicability of CPM, near-wall hot-wire measurement were conducted in developing transitional boundary layers on a flat plate with different freestream turbulence intensities. The intermittency profiles across the transitional boundary layers were reasonably obtained from the conditional sampling technique. An empirical correlation between the representative intermittency near the wall and free parameter K$_1$of the extended wall function of CPM has been newly proposed using the present and other experimental data. The CPM has been verified as a useful tool to measure the wall shear stress in transitional boundary layer with reasonable accuracy.

• 대한기계학회논문집B
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• 제20권5호
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• pp.1689-1699
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• 1996

A study on the transitional boundary layer with arbitrary pressure gradient under various upstream conditions is very important for engineering applications like the performance predictions of the turbomachineries under various and strong disturbances. Experimental data on the transitional boundary layer for real cascades of the turbomachinery are rare because of difficulties in boundary layer experiments. Flow on NACA0012 airfoil is more similar to the real case than that on the flat plate with which many researches are done. The data of the transitional flow on the airfoil could be used to verify or to develop a turbulence model for numerical simulations. The experiment was performed with two cases of Reynolds number at a=0$^{0}$ and one case of Reynolds number at a=5$^{0}$ . The measured data are the transition length and the wall shear stresses. These two characteristic values are measured within 25%~90% of the airfoil chord by Computation Preston tube Method(CPM) proposed by Nitsche et al.(1983). At a=0$^{0}$ , transition occured at 70% and 55% of chord length when R $e_{c}$=6*10$^{5}$ and 8* 10$^{5}$ , respectively. It started when R {\theta}=500 regardless of R $e_{c}$, and ended when R {\theta}=750, and 850 respectively. The transition length was 15~20% of the chord length. At a=5$^{0}$ (R $e_{c}$=6*10$^{5}$ ), boundary layer on the pressure side does not undergo transition, but on the suction side transition occured at .chi.$_{c}$/c=0.16 and ended at .chi.$_{c}$/c=0.22.c//c=0.22./c=0.22.c//c=0.22.

• 대한기계학회논문집B
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• 제22권9호
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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.

• 대한기계학회논문집B
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• 제26권5호
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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.