Higher-order and length-scale statistics of velocity and temperature fluctuations in turbulent boundary layer along a heated vertical flat plate

Abedin, Mohammad Zoynal,Tsuji, Toshihiro,Kim, Nae-Hyun Elsevier 2017 The International journal of heat and fluid flow Vol.65 No.-

<P><B>Abstract</B></P> <P>Time-developing direct numerical simulation (DNS) was performed to clarify the higher-order turbulent behaviors in the thermally-driven boundary layers both in air and water along a heated vertical flat plate. The predicted statistics of the heat transfer rates and the higher-order turbulent behaviors such as skewness factors, flatness factors and spatial correlation coefficients of the velocity and temperature fluctuations in the natural-convection boundary layer correspond well with those obtained from experiments for space-developing flows. The numerical results reveal that the turbulent structures of the buoyancy-driven boundary layers are mainly controlled by the fluid motions in the outer region of the boundary layer, and these large-scale structures are strongly connected with the generation of turbulence in the thermally-driven boundary layers, in accordance with the actual observations for space-developing flows. Moreover, to specify the turbulence structures of the boundary layers, the cross-correlation coefficients and the characteristic length scales are examined for the velocity and thermal fields. Consequently, it is found that with a slight increase in freestream velocity, the cross-correlation coefficient for the Reynolds shear stress and turbulent heat flux increases for opposing flow and decreases for aiding flow, and the integral scales for the velocity and temperature fields become larger for opposing flow and smaller for aiding flow compared with those for the pure natural-convection boundary layer.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Temporal DNS is done for thermally-driven boundary layer along a heated vertical flat plate. </LI> <LI> Turbulent structures are mainly controlled by fluid motions in the outer boundary layer. </LI> <LI> Large-scale structures are strongly connected with the generation of turbulence in the boundary layer. </LI> <LI> Cross-correlation coefficient for velocity and temperature fluctuations increases for opposing flow and decreases for aiding flow compared with the natural-convection boundary layer. </LI> <LI> Integral scales for velocity and temperature fields become larger for opposing flow and smaller for aiding flow compared with the natural-convection boundary layer. </LI> </UL> </P>

Depth-dependent seismicity and crustal heterogeneity in South Korea

Chung, Tae Woong,Iqbal, Muhammad Zafar,Lee, Youngmin,Yoshimoto, Kazuo,Jeong, Jina Elsevier 2018 Tectonophysics Vol.749 No.-

<P><B>Abstract</B></P> <P>Depth-dependent seismicity has been suggested in the seismogenic layer, which is divided into upper and lower layers by its seismicity boundary. Since large earthquakes tend to occur in the lower layer, the structure of the seismogenic layer, which includes the seismicity boundary, provides important information for seismic hazard assessment. Using 107 land-based earthquakes, the structure of the seismogenic layer in South Korea was studied with focal depth determination by identifying the minimum residuals between the observations and several models. The same procedure was done for the depth relocation of 138 events associated with the Gyeongju earthquake (M<SUB> <I>L</I> </SUB> 5.8), the largest event in South Korea in the last two centuries. The geothermal temperature at the focal depth was also estimated for all earthquakes, including the Gyeongju earthquake cluster and the Pohang earthquake (M<SUB> <I>L</I> </SUB> 5.4), which caused more damage than did the Gyeongju earthquake. As observed in a large-scale averaging of mainland China, a 12-km seismicity boundary was suggested due to the frequent seismicity in the upper layer. At 12-km depth at the Gyeongju earthquake site, the geothermal temperature reached 300 °C, and the occurrence of aftershocks was limited below that depth. Based on the rigorously estimated depth, we separated the scattering attenuation, <I>Q</I> <SUB> <I>s</I> </SUB> <SUP>−1</SUP>, by a multiple lapse time window analysis using individual earthquakes and found higher and lower values for events in the upper and lower layers, respectively. This depth-dependent observation suggests differences in heterogeneity between the upper and lower seismogenic layers and supports the hypothesis that the seismicity of a depth-dependent seismogenic layer is related to cracks, which diminish with depth. Our results suggest that the upper limit of the focal depth can be predicted from the depth of the seismicity boundary for normal large earthquakes.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The seismicity boundary of seismogenic layer in South Korea was identified. </LI> <LI> Crustal seismic-wave scattering strength exhibits depth-dependent heterogeneity between the two layers. </LI> <LI> The upper limit of the focal depth of large earthquakes can be predicted from the depth of the seismicity boundary. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Focal depths versus number of earthquakes for South Korea, and the Gyeongju earthquake cluster. The seismogenic layer (<I>S.L.</I>) consists of upper and lower layers divided at 12-km depth. The histograms with orange fill denote focal depth temperatures higher than 300 °C. Red and blue stars denote the ML 5.8 and 5.1 events, respectively. In the right graph, the scattering Q<SUP>−1</SUP> values with frequencies are shown as connected lines of events exclusively included in the upper and lower layers, respectively.</P> <P>[DISPLAY OMISSION]</P>

극초음속 경계층의 안정화 및 불안정화에 미치는 Hump의 영향

박동훈,박승오 한국항공우주학회 2013 한국항공우주학회 학술발표회 논문집 Vol.2013 No.11

2 차원의 매끄러운 hump 가 극초음속 경계층의 선형 안정성에 미치는 영향을 연구하였다. Hump를 지나는 마하수 4.5 와 5.92 의 평판 경계층과 마하수 7.1 의 원뿔 경계층에 대해 모드 S 의 선형 안정성을 포물형 안정성 방정식(PSE)을 사용하여 해석하였다. 안정성 해석을 위한 평균유동은 포물화된 Navier-Stokes(PNS) 방정식의 계산으로부터 얻었다. Hump 의 높이는 경계층의 두께보다 작은 경우를 고려하였다. 평판 경계층 해석 결과로부터, hump 가 동기화 지점을 기준으로 상류와 하류에 위치할 때 각각 불안정화와 안정화 효과를 발생시킴을 확인하였다. 원뿔 경계층의 해석에서도 hump 에 의해 특정 주파수 범위의 모드 S 가 안정화 됨을 확인하였다. Hump 에 의한 모드 S 의 안정화 효과는 기존의 실험연구들로부터 관찰된 극초음속 경계층의 천이 지연의 가능한 원인으로 판단할 수 있다. Effect of a two-dimensional smooth hump on linear instability of hypersonic boundary layer is studied. Parabolized stability equations (PSE) is employed to analyze the linear evolution of mode S over a hump in Mach 4.5 and 5.92 flat plate and Mach 7.1 sharp cone boundary layers. Mean flow for stability analysis is obtained by solving the parabolized Navier-Stokes (PNS) equations. Hump with height smaller than local boundary layer thickness is used. For flat plate boundary layers, destabilization and stabilization effect of hump located at upstream and downstream of synchronization point are confirmed. For sharp cone boundary layer, stabilization influence of hump is also identified for a specific range of frequency. Stabilization influence of hump on convective instability of mode S is found to be a possible cause of previous experimental observations of delaying transition in hypersonic boundary layers.

전산점근해석기법과 고유벡터를 이용한 복합재료 보의 경계층 응력 해석

김신호,김준식 한국전산구조공학회 2024 한국전산구조공학회논문집 Vol.37 No.1

본 논문에서는 전산점근해석기법을 사용하여 복합재료 보에 대한 경계층 해를 계산하고, ANSYS 결과와 비교 검증하였다. 경계층해는 내부해와 순수 경계층 효과의 합으로 표현되기 때문에, 내부 및 경계층에 대한 수학적으로 엄밀한 정식화를 요구한다. 전산점근 해석기법은 수학적으로 매우 강력한 기법으로, 이러한 문제에 유용하다. 그러나 경계층과 내부 해들의 연결을 시키기 쉽지 않은데, 본 연구에서는 가상일의 원리를 통해 생브낭의 원리와 내부 및 경계층 문제를 체계적으로 분리하였다. 경계층 해는 팝코비치-패들 고유벡터를 계산하여, 실수부와 허수부 벡터들의 선형 조합으로 표현하고, 내부 해의 워핑 함수들을 보상할 수 있도록 최소오차 자승법을 적용하였다. 계산된 해들은 2차원 유한요소 해석 결과와 비교하여 정성적일 뿐만 아니라 정량적으로도 잘 일치하는 결과를 얻었다. This paper utilizes computational asymptotic analysis to compute the boundary layer solution for composite beams and validates the findings through a comparison with ANSYS results. The boundary layer solution, presented as a sum of the interior solution and pure boundary layer effects, necessitates a mathematically rigorous formalization for both interior and boundary layer aspects. Computational asymptotic analysis emerges as a robust technique for addressing such problems. However, the challenge lies in connecting the boundary layer and interior solutions. In this study, we systematically separate the principles of virtual work and the principles of Saint-Venant to tackle internal and boundary layer issues. The boundary layer solution is articulated by calculating the Papkovich-Fadle eigenfunctions, representing them as linear combinations of real and imaginary vectors. To address warping functions in the interior solutions, we employed a least squares method. The computed solutions exhibit excellent agreement with 2D finite element analysis results, both quantitatively and qualitatively. This validates the effectiveness and accuracy of the proposed approach in capturing the behavior of composite beams.

UHF 레이더를 이용한 대류 경계층 고도의 추정

허복행 ( Bok Haeng Heo ),김경익 ( Kyung Eak Kim ) 大韓遠隔探査學會 2001 大韓遠隔探査學會誌 Vol.17 No.1

굴절 지수 구조 매개 변수(refractive index structure parameter) Cn2의 증가는 보통 가온위(virtual potential temperature) θv와 혼합비(mixing ratio) q의 연직 기울기가 최대가 되는 고도에서 발생하며, 대류 경계층(convective toundaly layer)의 고도를 추정하는데 있어서 매우 유용한 매개 변수로 사용된다. 이 연구에서는 대류 경계층 고도의 추정에 이용되는 Cn2 첨두의 발생 특성이 조사되었으며, 또한 UHF 레이더로 관측된 C2n와 연직속도의 분산 σw 자료를 이용하여 대류 경계층 고도를 객관적으로 추정하는 방법이 제시되었다. UHF 레이더의 Cn2 연직 분포에서 첨두는 대류 경계층의 정상부뿐만 아니라 잔류층의 정상부나 구름층에서도 발생하였다. 약한 태양복사로 연직 혼합이 뚜렷하지 않는 경우에 대류 경계층 고도에 상응하는 Cn2 첨두는 레윈존데(rawinsonde) 관측 자료로부터 추정된 대류 경계층 고도보다 약간 낮았다. 반면에, 강한 태양 복사에 의해 연직혼합이 강하고 유입대에서 θv와 q의 연직 기울기가 매우 클 경우에 대류 경계층 고도에 상응하는 Cn2 첨두는 레원존데 관측 자료로부터 추정된 대류 경계층 고도와 잘 일치하였다. Cn2 첨두의 고도를 대류 경계층 고도로 결정하는 최대 후방 산란 강도 방법(maximum backscatter intensity method)은 Cn2 연직 분포에서 하나의 첨두가 있을 경우에는 오류 없이 대류 경계층 고도를 추정하였지만 대류 경계층 고도 위에 잔류층이나 구름층이 있을 경우에는 대류 경계층 고도를 잘못 추정하였다. 본 연구에서 새로이 제시된 방법은 UHF 레이더의 Cn2와 σw자료를 이용하여 대류 경계층 고도로부터 오는 Cn2 첨두를 잔류층이나 구름층으로부터 오는 Cn2 첨두로부터 구별하여 오류 없이 대류 경계층 고도를 추정하였다. 또한 이 방법은 대류 경계층 고도의 일변화 추정에 적용되었으며, 후방 산란 강도의 연적 분포에서 부 개의 첨두가 존재할 경우에도 더욱 선뢰성 있고 안정되게 대류 경계층 고도를 실시간으로 추정하였다. The enhancement of the refractive index structure parameter Cn2 often occurs where vertical gradients of virtual potential temperature θv and mixing ratio q have their maximum values. The Cn2 can be a very useful parameter for estimating the convective boundary layer(CBL) height. The behavior of Cn2 peaks, often used to locate the height of mixed layer, was investigated in the present study. In addition, a new method to determine the CBL height objectively using both Cn2 and vertical air velocity variance σw data of UHF radar was also suggested. The present analysis showed that the Cn2 peaks in the backscatter intensity profiles often occurred not only at the top of the CBL but also at the top of a residual layer or at a cloud layer. The Cn2 peaks corresponding to the CBL heights were slightly lower than the CBL heights derived from rawinsonde sounding data when vertical mixing owing to weak solar heating was not significant and the heights of strong vertical θv gradients were not consistent with that of strong vertical q gradients. However, the Cn2 peaks corresponding to the CBL heights were in good agreement with the rawinsonde-estimated CBL heights when vertical mixing owing to solar heating was significant and the vertical gradient of both θv and q in the entrainment zone was very strong. The maximum backscatter intensity method, which determines the height of Cn2 peak as the CBL height, correctly estimated the CBL height when the Cn2 profile had single peak, but this method erroneously estimated the CBL height when there was a residual layer or a cloud layer over the top of the CBL. The new method distinguished the peak by the CBL height from the peak due to a cloud layer or a residual layer using both Cn2 and σw data, and correctly estimated the CBL height. As for estimation of diurnal variation of the CBL height, the new method provided more stable and reliable estimations of the CBL heights in real time than the maximum backscatter intensity method even if the vertical profile of backscatter intensity had two peaks from the CBL height and a residual layer or a cloud layer.

Super-resolution Reconstruction of Transitional Boundary Layers Using a Deep Neural Network

전영민,유동현 한국항공우주학회 2023 International Journal of Aeronautical and Space Sc Vol.24 No.4

Numerical simulations of turbulent flow require excessive computational resources due to the multi scale characteristics of turbulence. Thus, a technique to reconstruct a high-resolution flow field from a coarse flow data can be helpful. For this purpose, various artificial neural-network-based super-resolution methods have been developed in recent years. Although previous studies reported that the super-resolution methods show remarkable performance for turbulent channel flow and homogeneous isotropic turbulence, its application for spatially developing flow with laminar, transitional, and turbulent characteristics has not been reported. In the present study, a super-resolution reconstruction method applicable for spatially developing laminar-transition-turbulent flow is developed by training the network for boundary layer flow with bypass transition. In addition, the generalization of the network for boundary layer flow with natural transition and for fully turbulent boundary layer flow is attempted. A super-resolution method based on a generative adversarial network (GAN) is employed for the study as it shows the best performance among tested network models. It is found that the developed method successfully reconstructs flow structures in transitional and early turbulent regions. In addition, statistics such as the mean velocity and the power spectral density of velocity from recovered fields show good agreement to those of DNS. Notably, the GAN model which is only trained for the bypass transition is also found to be applicable to boundary layer flow with K-type natural transition and fully developed turbulent boundary layers.

RANS simulation of secondary flows in a low pressure turbine cascade: Influence of inlet boundary layer profile

Michele, Errante,Andrea, Ferrero,Francesco, Larocca Techno-Press 2022 Advances in aircraft and spacecraft science Vol.9 No.5

Secondary flows have a huge impact on losses generation in modern low pressure gas turbines (LPTs). At design point, the interaction of the blade profile with the end-wall boundary layer is responsible for up to 40% of total losses. Therefore, predicting accurately the end-wall flow field in a LPT is extremely important in the industrial design phase. Since the inlet boundary layer profile is one of the factors which most affects the evolution of secondary flows, the first main objective of the present work is to investigate the impact of two different inlet conditions on the end-wall flow field of the T106A, a well known LPT cascade. The first condition, labeled in the paper as C1, is represented by uniform conditions at the inlet plane and the second, C2, by a flow characterized by a defined inlet boundary layer profile. The code used for the simulations is based on the Discontinuous Galerkin (DG) formulation and solves the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the Spalart Allmaras turbulence model. Secondly, this work aims at estimating the influence of viscosity and turbulence on the T106A end-wall flow field. In order to do so, RANS results are compared with those obtained from an inviscid simulation with a prescribed inlet total pressure profile, which mimics a boundary layer. A comparison between C1 and C2 results highlights an influence of secondary flows on the flow field up to a significant distance from the end-wall. In particular, the C2 end-wall flow field appears to be characterized by greater over turning and under turning angles and higher total pressure losses. Furthermore, the C2 simulated flow field shows good agreement with experimental and numerical data available in literature. The C2 and inviscid Euler computed flow fields, although globally comparable, present evident differences. The cascade passage simulated with inviscid flow is mainly dominated by a single large and homogeneous vortex structure, less stretched in the spanwise direction and closer to the end-wall than vortical structures computed by compressible flow simulation. It is reasonable, then, asserting that for the chosen test case a great part of the secondary flows details is strongly dependent on viscous phenomena and turbulence.

Effect of Riblets on the Streaky Structures Excited by Free Stream Tip Vortices in Boundary Layer

Audrey V. Boiko,Kwang Hyo Jung,Ho Hwan Chun,Inwon Lee 대한기계학회 2007 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.21 No.1

In this study, experimental investigations were made regarding the effect of riblets on the streak instability in boundary layer. The streak instability is now regarded as a major source of the self-regeneration mechanism for the hairpin type coherent structures in turbulent boundary layer flow. Thus, it is important to control the instability to suppress the drag-inducing vortical structure in terms of drag reduction. Toward enhancing the measurement accuracy and spatial resolution, an enlarged version of riblets was applied to a streak which was artificially induced by a micro wing in a laminar boundary layer. It is found that the rib lets have attenuation effect on the streak instability, i.e., to reduce the span wise velocity gradient of the quasi-streamwise streak in boundary layer.

Insights of Boundary Layer Turbulence Over the Complex Terrain of Central Himalaya from GVAX Field Campaign

Rajput Akanksha,Singh Narendra,Singh Jaydeep,Rastogi Shantanu 한국기상학회 2024 Asia-Pacific Journal of Atmospheric Sciences Vol.60 No.2

Limited observations hinder understanding of turbulent characteristics in mountainous terrain resulting from heating or cooling of slopes, wind, vertical motions, and heat or moisture advection, which disperse aerosols and other pollutants over the region. In this study, the 1290 MHz radar wind profiler data are utilized to compute the boundary layer height (BLH), the refractive index structure constant (Cn 2), and the energy dissipation rate (ɛ) over the central Himalayan site for the period of November 2011 to March 2012, from the intense Ganges Valley Aerosol Experiment (GVAX) field measurements. The radar wind profiler (RWP) based estimation of BLH and ɛ is validated against the radiosonde, representing the effectiveness of the datasets for further investigation. The strong seasonal variation of log Cn 2 and log ɛ, with average values of ≈ -12 m−2/3 and -2 m2 s−3, respectively, is associated with the mountain-induced local circulations and stability in the atmospheric boundary layer. The weak stratification during weak flow is found to be responsible for deep mixing, particularly in the nocturnal boundary layer in spring. Furthermore, the level of cloud cover significantly impacts the strength of turbulence, with the highest cloud cover resulting in a substantial increase in log Cn 2 (approximately -11 m−2/3) due to intense updraft and downdraft motions compared to clear skies. Additionally, the distribution of aerosol loading across the site, coupled with the behavior of BLH, atmospheric stability, and orographic-induced circulations, implies distinctive seasonal mechanisms for transporting aerosols toward the mountains. This study offers valuable insights into the diurnal and seasonal patterns of turbulent mixing and the mechanisms behind the transport of pollutants through boundary layer processes over the region.

RANS simulation of secondary flows in a low pressure turbine cascade: Influence of inlet boundary layer profile

Michele, Errante,Andrea, Ferrero,Francesco, Larocca Techno-Press 2022 Advances in aircraft and spacecraft science Vol.9 No.5

Secondary flows have a huge impact on losses generation in modern low pressure gas turbines (LPTs). At design point, the interaction of the blade profile with the end-wall boundary layer is responsible for up to 40% of total losses. Therefore, predicting accurately the end-wall flow field in a LPT is extremely important in the industrial design phase. Since the inlet boundary layer profile is one of the factors which most affects the evolution of secondary flows, the first main objective of the present work is to investigate the impact of two different inlet conditions on the end-wall flow field of the T106A, a well known LPT cascade. The first condition, labeled in the paper as C1, is represented by uniform conditions at the inlet plane and the second, C2, by a flow characterized by a defined inlet boundary layer profile. The code used for the simulations is based on the Discontinuous Galerkin (DG) formulation and solves the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the Spalart Allmaras turbulence model. Secondly, this work aims at estimating the influence of viscosity and turbulence on the T106A end-wall flow field. In order to do so, RANS results are compared with those obtained from an inviscid simulation with a prescribed inlet total pressure profile, which mimics a boundary layer. A comparison between C1 and C2 results highlights an influence of secondary flows on the flow field up to a significant distance from the end-wall. In particular, the C2 end-wall flow field appears to be characterized by greater over turning and under turning angles and higher total pressure losses. Furthermore, the C2 simulated flow field shows good agreement with experimental and numerical data available in literature. The C2 and inviscid Euler computed flow fields, although globally comparable, present evident differences. The cascade passage simulated with inviscid flow is mainly dominated by a single large and homogeneous vortex structure, less stretched in the spanwise direction and closer to the end-wall than vortical structures computed by compressible flow simulation. It is reasonable, then, asserting that for the chosen test case a great part of the secondary flows details is strongly dependent on viscous phenomena and turbulence.