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안준선(Junsun Ahn),이진영(Jinyoung Lee),황진율(Jinyul Hwang) 한국가시화정보학회 2021 한국가시화정보학회지 Vol.19 No.2
The spanwise growth of turbulence structures in turbulent pipe flow was investigated using the direct numerical simulation data of Re<SUB>τ</SUB> = 544, 934 and 3008. Two-point correlations and pre-multiplied energy spectra of streamwise velocity fluctuations were examined along the spanwise direction. The arclength direction is defined as rθ, which is useful for an analogy with the spanwise direction for channels or boundary layers; here, r and θ are the radial distance from the core and the azimuthal angles, respectively. Both analyses showed that the arclength scales increased with increasing the wall-normal distance. It showed that the coherent structures were confined in the core region due to the crowding effect of a circular pipe geometry. The pipe flow simulation could describe a realistic geometrical flow along the azimuthal direction, unlike the simulations of turbulent channel or boundary layer flow using periodic boundary conditions along the spanwise direction. The present results provided the spanwise organization of energy-containing motions over a broad range of scales in turbulent pipe flow.
3 차원 입방형 표면조도를 갖는 난류경계층에 대한 수치적 연구
안준선(Junsun Ahn),이재화(Jae Hwa Lee),성형진(Hyung Jin Sung) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.10
Direct numerical simulations of turbulent boundary layers over the cube-roughened walls were performed with varying spanwise extent. Recent work by Lee et al. (2011) showed that the inner layer statistics are influenced by the streamwise pitch over the 3-D square-edged roughness and these have the maximum at p<SUB>x</SUB>/k=4 with p<SUB>z</SUB>/k=2. However, the present simulations showed that the surface drag, including form drag and roughness function, etc., is also affected by spanwise spacing and the maximum contribution occurs at p<SUB>z</SUB>/k=3 with p<SUB>x</SUB>/k=3 which has the roughness density, 0.111. This result might indicate that the main parameter for the wall friction is roughness density rather than roughness configurations. In addition, it was shown that outer peaks of the Reynolds stresses are increased with increasing spanwise length, implying stronger inner and outer layer interactions at larger p<SUB>z</SUB>/k, which is consistent with those of streamwise spacing. This signifies streamwise and spanwise spacing as the principal variable in the outer layer.
안준선(Junsun Ahn),황진율(Jinyul Hwang) 한국가시화정보학회 2020 한국가시화정보학회지 Vol.18 No.3
Five layers in mean flow are proposed by using the direct numerical simulation data of turbulent pipe flow up to Re<SUB>τ</SUB> = 3008. Viscous sublayer, buffer layer, mesolayer, log layer and core region are investigated. In the buffer layer, the viscous force is counterbalanced by the turbulent inertia from the streamwise mean momentum balance, and a log law occurs here. The overlap layer is composed of the mesolayer and the log layer. Above the buffer layer, the non-negligible viscous force causes the power law, and this region is the mesolayer, where it is the lower part of the overlap layer. At the upper part of the overlap layer, where the viscous force itself becomes naturally negligible, the log layer will appear due to that the acceleration force of the large-scale motions increases as the Reynolds number increases. In the core region, the velocity-defect form is satisfied with the power-law scaling.
이진영(Jinyoung Lee),안준선(Junsun Ahn),성형진(Hyung Jin Sung) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Azimuthal features of fully developed turbulent pipe flow are investigated by using direct numerical simulations at Re<SUB>τ</SUB> = 544 and 934. Scale growth along the azimuthal direction is analyzed by two-point correlations and energy spectra. Two-point correlations show that in the outer region the outer-scaled spanwise widths of turbulence structures are similar irrespective of the Reynolds numbers, and they converge due to the space limitation of the pipe geometry. In the inner region, however, the spanwise widths of Re<SUB>τ</SUB> = 544 are shorter than those of Re<SUB>τ</SUB> = 934. This behavior comes from the less effective footprints of very-large-scale motions at Re<SUB>τ</SUB> = 544 than those at Re<SUB>τ</SUB> = 934, which is shown in the energy spectra. In addition, the scale growths from the two-point correlations and the energy spectra are different because the energy spectra peaks reflect both the popularity and energy contained in each scale of structures.