• 천문학회보
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• 제39권1호
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• pp.57.2-57.2
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• 2014

Protostellar jets and outflows are signatures of star formation and promising mechanisms for driving supersonic turbulence in molecular clouds. We quantify outflow-driven turbulence through three-dimensional numerical simulations using an isothermal version of the total variation diminishing code. We drive turbulence in real space using a simplified spherical outflow model, analyze the data through density probability distribution functions (PDFs), and investigate density and velocity power spectra. The real-space turbulence-driving method produces a negatively skewed density PDF possessing an enhanced tail on the low-density side. It deviates from the log-normal distributions typically obtained from Fourier-space turbulence driving at low densities, but can provide a good fit at high densities, particularly in terms of mass-weighted rather than volume-weighted density PDF. We find shallow density power-spectra of -1.2. It is attributed to spherical shocks of outflows themselves or shocks formed by the interaction of outflows. The total velocity power-spectrum is found to be -2.0, representative of the shock dominated Burger's turbulence model. Our density weighted velocity power spectrum is measured as -1.6, slightly less that the Kolmogorov scaling values found in previous works.

• Wind and Structures
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• 제24권6호
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• pp.579-611
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• 2017

Bora is a strong, usually dry temporally and spatially transient wind that is common at the eastern Adriatic Coast and many other dynamically similar regions around the world. One of the Bora main characteristics is its gustiness, when wind velocities can reach up to five times the mean velocity. Bora often creates significant problems to traffic, structures and human life in general. In this study, Bora velocity and near-ground turbulence are studied using the results of three-level high-frequency Bora field measurements carried out on a meteorological tower near the city of Split, Croatia. These measurements are analyzed for a period from April 2010 until June 2011. This rather long period allows for making quite robust and reliable conclusions. The focus is on mean Bora velocity, turbulence intensity, Reynolds shear stress and turbulence length scale profiles, as well as on Bora velocity power spectra and thermal stratification. The results are compared with commonly used empirical laws and recommendations provided in the ESDU 85020 wind engineering standard to question its applicability to Bora. The obtained results report some interesting findings. In particular, the empirical power- and logarithmic laws proved to fit mean Bora velocity profiles well. With decreasing Bora velocity there is an increase in the power-law exponent and aerodynamic surface roughness length, and simultaneously a decrease in friction velocity. This indicates an urban-like velocity profile for smaller wind velocities and a rural-like velocity profile for larger wind velocities. Bora proved to be near-neutral thermally stratified. Turbulence intensity and lateral component of turbulence length scales agree well with ESDU 85020 for this particular terrain type. Longitudinal and vertical turbulence length scales, Reynolds shear stress and velocity power spectra differ considerably from ESDU 85020. This may have significant implications on calculations of Bora wind loads on structures.

• Wind and Structures
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• 제39권2호
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• pp.111-123
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• 2024

The atmospheric turbulence characteristics measured at a meteorological station in northwest part of the Czech Republic are presented for selected time periods in the year 2017. The terrain of this region is influenced by surface coal mining and the related industry. The datasets used in this study were measured using four ultrasonic anemometers installed on an 80 m high meteorological mast at heights of 20, 40, 60 and 80 m, respective. From the primary high-frequency datasets, time intervals in order of hours were selected and integral turbulence characteristics (ITCs), turbulence intensities and turbulence spectra were analyzed. The time intervals were selected with respect to atmospheric stability parameter, known as Obukhov number. We concentrated on the days with higher wind velocity and neutral atmospheric stratification. The wind characteristics investigated in this study include the wind speed, wind direction and its histograms, turbulence intensity, friction velocity and wind power spectra. The ITCs and spectral characteristics were compared with the theoretical models and values from the literature. The resulting ITCs showed the values for urban locations similar to those found in other studies and can be used in practical design. The computed turbulence spectra followed the shape of theoretical spectra of turbulence for both horizontal and vertical velocity components. The computed integral length scales have shown to be unsuitable for further use due to their highly scattered values.

• 터널과지하공간
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• 제21권6호
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• pp.471-479
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• 2011

최근 한반도 및 주변해역에서 발생한 규모 4.8 이상의 5개 중규모 지진으로부터 관측된 속도 지반운동 파형을 이용하여 수평 응답스펙트럼을 분석하고 결과를 우선 가속도 지반운동을 이용하여 얻어진 수평 응답스펙트럼, 국내 원자력 관련 구조물의 내진설계 기준, 마지막으로 국내 일반 구조물 및 건축물 내진설계기준과 각각 비교하였다. 연구에 이용된 지반운동은 수평성분 102개(NS 및 EW 성분 포함)이며 고유진동수에 따른 응답을 구하고 각각의 최대 지반 속도 값을 이용하여 정규화 분석을 수행하였다. 첫째, 가속도 응답스펙트럼과 비교한 결과 속도 응답스펙트럼 값은 특히 중간주기에서 높은 응답을 보여 주었고 이에 비해 가속도 응답스펙트럼은 특히 단주기 즉 높은 고유진동수 영역에서 높은 응답을 보여 주었다. 둘째, 국내 원자력시설물의 내진기준으로 이용되고 있는 Reg. Guide 1.60과 비교한 결과 속도 응답스펙트럼 값은 약 6-7Hz를 시작점으로 보다 낮은 장주기 영역에서 기준값을 초과하는 현상을 보여 주었다. 셋째, 500년 재래주기에 해당하는 국내 일반 구조물 및 건축물 내진설계기준인 표준 설계응답스펙트럼을 SC, SD 및 SE지반 조건과 같은 3개 지반조건과 동시에 비교한 결과 차례로 약 1.5초, 2초 및 3초에서 시작하여 보다 장주기 영역에서 국내 일반 구조물 표준 설계 응답스펙트럼값을 초과하였다. 동일한 부지에서 일반적으로 가속도 응답스펙트럼은 단주기에서 가장 큰 값을 나타내며, 속도 응답 스펙트럼은 중간주기에서 가장 크며, 마지막으로 변위 응답스펙트럼은 장주기에서 가장 큰 값을 가진다는 국외 연구결과가 국내 지반운동을 이용한 결과에서 역시 적용가능하다는 점을 확인시켜 주었다. 최근 국내에서도 건축물의 초고층화 등으로 구조물의 디자인이 기존의 단주기에 비해 중간주기 및 장주기 영역이 상대적으로 강조되고 있어 이러한 중간주기영역에서 수평 응답스펙트럼의 정보는 향후 대단히 중요하다고 할 수 있다.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 1992년도 한국우주과학회보 제2권1호
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• pp.16-16
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• 1993

The Cosmic Mach number M is the ratio of the bulk flow velocity of the galaxrvelocity field on some scale R to the unall scale velocity dispersion within refcions of scale R. Because M is the ratio of two velocities, it is inn-dimansionat and the Here, independent of the amplitude of the power specHim and of the biasplnmeter in the linear theory. We have measured the Mach rnlmber for two observational samples: a spiral galaxy sample(AHM) of Aaronson and hiscoBlaborators with absolute distances measured by the infrared Ttillr-Fisher relatioa and an elliptical galaxy sample(EGALS) of Faber or 0, with distances determined by the relation. The effective depths distances of galaxies from the Local Group of these samples are 1639 km/s and 2862 e/s, respectivelr. The Machnumbers from these observed peculiar velocity Selds He fund as M=0.95 for AHMand M=0.59 for EGALS. We comPBre these calculated Mach numbers with thosefrom meck surweys drawn fuom three cosnulogical medels: the stand8rd biased nh=0.5 CDM modet an open CDM rrudel with gh=0.2, and a medd with thepower-law power specelm P(k)-k-1 and n=1. The Mach rnlmber test can give robust constraints on these cosmelogical nudels whose power spectra have very different shapes at large scales.

• 대한기계학회논문집
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• 제18권5호
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• pp.1253-1263
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• 1994

The experimental study is carried out to identify the combustion generated noise mechanism in free turbulent jet diffusion flames. Axial mean fluctuating velocities in cold and reacting flow fields were measured using hot-wire anemometer and LDv.The overall sound pressure level and their spectral distribution in far field with and without combustion were also measured in an anechoic chamber. The axial mean velocity is 10-25% faster and turbulent intensities are about 10 to 15% smaller near active reacting zone than those in nonreacting flow fields. And sound pressure level is about 10-20% higher in reacting flow fields. It is also shown that the spectra of the combustion noise has lower frequency characteristics over a broadband spectrum. These results indicate that the combustion noise characteristics in jet diffusion flames are dominated by energy containing large scale eddies and the combusting flow field itself. Scaling laws correlating the gas velocity and heat of combustion show that the acoustic power of the combustion noise is linearly proportional to the 3.8th power of the mean axial velocity rather than 8th power in nonreacting flow fields, and the SPL increases linearly with logarithmic 1/2th power of the heat of combustion.

• 천문학회지
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• 제37권5호
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• pp.563-570
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• 2004

Turbulence is a crucial component of dynamics of astrophysical fluids dynamics, including those of ISM, clusters of galaxies and circumstellar regions. Doppler shifted spectral lines provide a unique source of information on turbulent velocities. We discuss Velocity-Channel Analysis (VCA) and its offspring Velocity Coordinate Spectrum (VCS) that are based on the analytical description of the spectral line statistics. Those techniques are well suited for studies of supersonic turbulence. We stress that a great advantage of VCS is that it does not necessary require good spatial resolution. Addressing the studies of mildly supersonic and subsonic turbulence we discuss the criterion that allows to determine whether Velocity Centroids are dominated by density or velocity. We briefly discuss ways of going beyond power spectra by using of higher order correlations as well as genus analysis. We outline the relation between Spectral Correlation Functions and the statistics available through VCA and VCS.

• 한국자기학회지
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• 제7권5호
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• pp.232-236
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• 1997

Mossbauer 분광계의 사인파속도 Mode에서 Fe-foil을 사용한 속도눈금매기기를 상세하게 소개하였다. 사인 파속도형 Mossbauer 분광계의 장점은 스프링의 자연진동에 의한 것으로 속도가 안정됨과 정밀함에 있다. Fe$_{3}$O$_{4}$와 U-Fe$_{3}$O$_{4}$(unbalanced Fe$_{3}$O$_{4}$)의 Mossbauer 스펙트럼을 분석하여 사인파속도 mode에서의 성능을 평가하였다.

• Wind and Structures
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• 제22권2호
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• pp.235-252
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• 2016

A wind velocity power spectrum (WVPS) with high fidelity is extremely important for accurate prediction of structural buffeting response. WVPS heavily depends on the geographical locations, local terrains and topographies. Hence, field measurement of wind characteristics may be the unique way to obtain the accurate WVPS for a specific region. In this paper, a systematic analysis and discussions of existing WVPSs were performed. Six recorded strong wind data from the structural health monitoring systems (SHMS) of Runyang Suspension Bridge (RSB) and Sutong Cable-stayed Bridge (SCB) in Jiangsu Province of China were selected for analysis. The measured and pre-processed wind velocity data was first transformed from time domain to frequency domain to obtain the measured spectrum. The spectrum for each strong wind was then fitted using the nonlinear least square method and compared with both the fitted spectrum from statistical analysis and the recommended spectrum in specifications. The modified Kaimal spectrum was proved to be the "best" choice for the coastal area of East Jiangsu Province. Finally, a suitable WVPS formula fit for the coastal area of East Jiangsu Province was presented based on the modified Kaimal spectrum. Results in this study provide a more accurate and reliable WVPS for wind-resistant design of engineering structures in the coastal area of East Jiangsu Province.

• 천문학회보
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• 제46권2호
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• pp.43.5-44
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• 2021

In the local Universe, the gravitational effects of mass density fluctuations exert perturbations on galaxies' redshifts on top of Hubble's Law, called 'peculiar velocities'. These peculiar velocities provide an excellent way to test the cosmological model in the nearby Universe. In this talk, we present new cosmological constraints using peculiar velocities measured with the 2MASS Tully-Fisher survey (2MTF), 6dFGS peculiar-velocity survey (6dFGSv), the Cosmicflows-3 and Cosmicflows-4TF compilation. Firstly, the dipole and the quadrupole of the peculiar velocity field, commonly named 'bulk flow' and 'shear' respectively, enable us to test whether our cosmological model accurately describes the motion of galaxies in the nearby Universe. We develop and use a new estimators that accurately preserves the error distribution of the measurements to measure these moments. In all cases, our results are consistent with the predictions of the Λ cold dark matter model. Additionally, measurements of the growth rate of structure, fσ8 in the low-redshift Universe allow us to test different gravitational models. We developed a new estimator of the "momentum" (density weighted peculiar velocity) power spectrum and use joint measurements of the galaxy density and momentum power spectra to place new constraints on the growth rate of structure from the combined 2MTF and 6dFGSv data. We recover a constraint of fσ8=0.404+0.082-0.081 at an effective redshift zeff=0.03. This measurement is also fully consistent with the expectations of General Relativity and the Λ Cold Dark Matter cosmological model.