• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.6
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• pp.132-141
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• 1998

Particle image velocimetry(PIV), a planar measuring technique, is an efficient tool for studying the complicated flow field such as in-cylinder flow, and intake port flow. PIV can be also used for analyzing the integral length scale of turbulence, which is a measure of the size of the large eddies that contain most of the turbulence kinetic energy. In this study, dual color scanning PIV was designed and demonstrated by using a rotating mirror and a beam splitter. This PIV system allowed enlargement of flexibility in the intensity of vectors to be calculated by spatial filtering technique, even in combustion chamber with high velocity gradient and high vorticity$({\sim}1000s^{-1})$. A new color image processing algorithm was developed, which was used to find the direction of particle movement directly from the digital image. These measuring techniques were successfully applied to obtaining the turbulence intensity (~0.1m/s) and the turbulent integral length scale of vorticity(~1mm).

• Transactions of the Korean Society of Mechanical Engineers
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• v.9 no.6
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• pp.732-742
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• 1985

Measurements of mean velocity and turbulence characteristics are obtained with a linearized constant temperature hot-wire anemometer in a two-dimensional turbulent jet discharging parallel to a flate. Wall static pressure distribution is also measure. The Reynolds number based on the jet nozzle width (D) is about 42,000 and the step height is 2.5D. The reattachment length is found to be 7.5D by using both wool tuft and oil methods. Upstream of the reattachment point, there exist double coherent structures and mean velocity, Reynolds stresses and triple product profiles are asymmetric about jet center line due to the influence of streamline curvature and recirculating flow region. Near the reattachment point, wall static pressure and turbulence quantities change its shape rapidly because of the large eddies by the solid wall. Especially, turbulence intensity has a maximum value in the reattachment regin, then decreases slowly in the redeveloping wall jet ragion. Downstream of X/D=14, a single large scale eddy structure is formed. Far downstream affer the reattachment(X/D.geq.18) mean velocity profile, the decay of maximum velocity and the variation of jet half width are nearly similar to those of plane wall jet, but the Reynolds stresses are higher than those of the latter.

• Journal of the Korean Society of Combustion
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• v.14 no.3
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• pp.29-36
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• 2009

Simulation is performed to analyze the characteristics of turbulent spray combustion in a diesel engine condition. An extended Conditional Moment Closure (CMC) model is employed to resolve coupling between chemistry and turbulence. Relevant time and length scales and dimensionless numbers are estimated at the tip and the mid spray region during spray development and combustion. The liquid volume fractions are small enough to support validity of droplets assumed as point sources in two-phase flow. The mean scalar dissipation rates (SDR) are lower than the extinction limit to show flame stability throughout the combustion period. The Kolmogorov scales remain relatively constant, while the integral scales increase with decay of turbulence. The chemical time scale decreases abruptly to a small value as ignition occurs with subsequent heat release. The Da and Ka show opposite trends due to variation in the chemical time scale. More work is in progress to identify the spray combustion regimes.

• Wind and Structures
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• v.24 no.6
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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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• v.25 no.2
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• pp.195-214
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• 2017

Full-scale wind characteristics based on the field measurements is an essential element in structural wind engineering. Statistical analysis of the wind characteristics at Sutong Cable-stayed Bridge (SCB) site is conducted in this study with the recorded long-term wind data from structural health monitoring system (SHMS) between 2008 and 2015. Both the mean and turbulent wind characteristics and power spectra are comprehensively investigated and compared with those in the current codes of practice, such as the measured wind rose diagram, monthly maximum mean wind speed, turbulence intensity, integral length scale. Measurement results based on the monitoring data show that winds surrounding the SCB site are substantially influenced by the southeast monsoon in summer and strong northern wind in winter. The measured turbulence intensity is slightly higher than the recommended values in specifications, while the measured ratio of lateral to longitudinal turbulence intensity is slightly lower. An approximately linear relationship between the measured turbulence intensities and gust factors is obtained. The mean value of the turbulence integral length scale is smaller than that of typical typhoon events. In addition, it is found that the Kaimal spectrum is suitable to be adopted as the power spectrum for longitudinal wind component at the SCB site. This contribution would provide important wind characteristic references for the wind performance evaluation of SCB and other civil infrastructures in adjacent regions.

• Wind and Structures
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• v.15 no.1
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• pp.1-15
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• 2012

High-resolution wind data were acquired from a 100-m high offshore tower during the passage of Typhoon Hagupit in September, 2008. The meteorological tower was equipped with an ultrasonic anemometer and a number of cup anemometers at heights between 10 and 100 m. Wind characteristics of the strong typhoon, such as mean wind speed and wind direction, turbulence intensity, turbulence integral length scale, gust factor and power spectra of wind velocity, vertical profiles of mean wind speed were investigated in detail based on the wind data recorded during the strong typhoon. The measured results revealed that the wind characteristics in different stages during the typhoon varied remarkably. Through comparison with non-typhoon wind measurements, the phenomena of enhanced levels of turbulence intensity, gust factors, turbulence integral length scale and spectral magnitudes in typhoon boundary layer were observed. The monitored data and analysis results are expected to be useful for the wind-resistant design of offshore structures and buildings on seashores in typhoon-prone regions.

• Journal of Mechanical Science and Technology
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• v.15 no.3
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• pp.392-402
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• 2001

The paper reports a multiple source modeling of low-Reynolds-number dissipation rate equation with aids of DNS data. The key features of the model are to satisfy the wall limiting conditions of the individual source terms in the exact dissipation rate equation using the wall damping functions. The wall damping functions are formulated in term of dimensionless dissipation length scale ι(sup)+(sub)D(≡ι(sub)D($\upsilon$$\xi$)(sup)1/4/$\upsilon$) and the invariants of small and large scale turbulence anisotropy tensors. $\alpha$(sub)ij(=$\mu$(sub)i$\mu$(sub)j/$\kappa$-2$\delta$(sub)ij/3) and e(sub)ij(=$\xi$(sub)ij/$\xi$-2$\delta$(sub)ij/3). The model constants are optimized with aids of DNS data in a plane channel flow. Adopting the dissipation length scale as a parameter of damping function, the applicabilities of $\kappa$-$\xi$ model are extended to the turbulent flow calculation of complex flow passages.

• Journal of Advanced Marine Engineering and Technology
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• v.32 no.4
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• pp.498-505
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• 2008

Many researches have been studied on in-cylinder flow as one of dominant effects for an engine combustion. The combustion phenomena of reciprocating engine is one of the most important processes affecting performance and emissions. One effective way to improve the engine combustion is to control the motion of the charge inside a cylinder by means of optimum induction system design. It is believed that the tumble and swirl motion generated during intake breaks down into small-scale turbulence in the compression stroke of the cycle. However, the exact nature of their relationship is not well known. To know this relationship definitely, this paper describes analytical results of the tumble motion, swirl motion, turbulence intensity, turbulence inside the cylinder of marine engine. 3-D computation has been performed by using STAR-CD solver and es-ice.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.85.2-85.2
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• 2011

We investigate the driven magnetohydrodynamic (MHD) turbulence by including the effect of the expansion and collapse of background medium. The main goal is to quantify the evolution and saturation of the strength and characteristic length scales of magnetic fields in expanding and collapsing media. Our findings are as follows. First, with the expansion and collapse of background medium, the time evolution of the magnetic and kinetic energy densities depends on the nature of forcing as well as the rate of expansion and collapse. Second, at scales close to the energy injection (or driving) scale, the slope of magnetic field power spectrum shallows with expansion but steepens with collapse. Third, various characteristic length scales, relative to the energy injection scale, decrease with expansion but increase with collapse. We discuss the astrophysical implications of our results.

• Transactions of the Korean Society of Mechanical Engineers
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• v.7 no.2
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• pp.193-203
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• 1983

The mixed convective heat transfer problems are characterized by the relatively significant contribution of buoyancy force to the transport processes of momentum and heat. Past analytical studies on this kind of problems have been carried out by employing either the conventional R-.epsilon. turbulence model which includes constant turbulent Prandtl number .sigma.$_{+}$ 1 or an extended R-.epsilon. turbulence model which takes account of the buoyancy effect in appropriate length scale equations. But in the latter case, the temperature variance .the+a.$^{2}$ over bar is approximated by a model under local equilibrium condition and the time scale ratio between velocity and temperature is assumed to be constant. These approximation is known to break down when the buoyancy effect is dominant. The present study is aimed at development of new computational turbulence closure level which can be applied to this rather complex turbulent process. The temperature variance is obtained directly by solving its dynamic transport equation and the time scale ratio which is variable in space is computed by a solution of a dynamic equation for the rate of scalar dissipation .epsilon.$_{\thetod}$ It was found that the computational results are in good agreement with available experimental data of wide range of unstable conditions.