• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.10
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• pp.1223-1228
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• 1999

As part of study on separated flow using the vortex particle method, stability of the method and particle redistribution were scrutinized. Stability was investigated by choosing different combination of numerical parameters. The Gaussian vortex was considered to make the problem simple by eliminating the complexity due to presence of walls. It was shown that the numerical method was stable when $v{\Delta}t/h^2{\leq}0.5$. In all the stable cases the circulation and the linear momentum were conserved. Without the particle redistribution, the angular momentum was severely attenuated.

• Wind and Structures
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• v.21 no.1
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• pp.1-23
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• 2015

Wind tunnel testing of a low-rise building with openings (holes) of different sizes and shapes on a roof corner is conducted to measure the internal and external pressures from the building model. Detailed analysis of the testing data is carried out to investigate the characteristics of the internal and external pressures of the building with different openings' configurations. Superimposition of the internal and external pressures makes the emergence of positive net pressures on the roof. The internal pressures demonstrate an overall uniform distribution. The probability density function (PDF) of the internal pressures is close to the Gaussian distribution. Compared with the PDF of the external pressures, the non-Gaussian characteristics of the net pressures weakened. The internal pressures exhibit strong correlation in frequency domain. There appear two humps in the spectra of the internal pressures, which correspond to the Helmholtz frequency and vortex shedding frequency, respectively. But, the peak for the vortex shedding frequency is offset for the net pressures. Furthermore, the internal pressure characteristics indirectly reflect that the length of the front edge enhances the development of the conical vortices.The objective of this study aims to further understanding of the characteristics of internal, external and net pressures for low-rise buildings in an effort to reduce wind damages to residential buildings.

• Journal of Korea Water Resources Association
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• v.33 no.2
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• pp.207-217
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• 2000

A numerical model is applied to analyze the mixing characteristics of an axisymmetric turbulent buoyant jet discharged into flowing stratified ambients. The numerical model is a Gaussian-vortex model which incorporates the effects of the vortex pair known as the representative characteristics of far-field in flowing ambients. Six ocean outfalls that have field data for the initial dilution at the water surface are selected for testing the applicability of the developed numerical model. The comparisons of the observed initial dilutions and the simulated ones show that the developed numerical model could be used for the analyses of the initial mixings induced by the sewage diffuser discharged into the ocean.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.5
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• pp.661-669
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• 1999

The evolutionary structure of a tip vortex in the initial period have been investigated by the two-dimensional LDV system. Circumferential and axial components of mean velocities, their turbulences and Reynolds stresses were measured by the phase averaging technique at seven different wake ages within one revolution of the rotor. Core growth was also analyzed. It was resulted that circumferential velocity components showed a Rankine combined vortex shape and their circulation profiles viewed in the radial direction were close to the n = 2 model of Vatistas' algebraic formula, while axial velocity components seemed to have the Gaussian profiles In these measured ranges with the base width of three times of core radii. Peaks of circumferential velocities and core radii showed distinct asymmetric behaviors before the wake age of $150^{\circ}$ over inboard and outboard sides of the slipstream, but they became symmetric afterwards. Turbulence profiles which had two peaks Inside the core radii in the earlier wake age were also changed to single peaks after $150^{\circ}$. These trends imply that the tip vortex was barely mature at this wake age.

• Journal of the Korean Society of Visualization
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• v.16 no.2
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• pp.23-30
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• 2018

The present study compares flow fields reconstructed by data assimilation method with different combinations of parameters. As a data assimilation method, Vortex-in-Cell-sharp (VIC#), which supplements additional constraints and multigrid approximation to Vortex-in-Cell-plus (VIC+), is used to reconstruct flow fields from scattered particle tracks. Two parameters, standard deviation of Gaussian radial basis function (RBF) and grid spacing, are mainly tested using artificial data sets which contain few particle tracks. Consequent flow fields are analyzed in terms of flow structure sizes. It is demonstrated that sizes of the flow structures are proportional to an actual scale of the standard deviation of RBF. It implies that a combination of larger grid spacing and smaller standard deviation which preserves the actual standard deviation is able to save computational resources in case of a low track density. In addition, a simple comparison using an experimental data filled with dense particle tracks is conducted.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.7
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• pp.709-715
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• 2011

In order to observe the wake interaction between tip vortices generated by a two-bladed rotor with slightly different pitch angles, the velocity components of the tip vortices were measured by using a two-dimensional LDV system. It was observed that the swirl velocity components of the ensuing blade deviated from the Vatistas' n = 2 vortex model and the axial velocity components of the preceding blade deviated from the Gaussian profile. It was also found that in the wake-age range of $200^{\circ}$ to $240^{\circ}$, the filament of the ensuing blade tip vortex was stretched as result of the closing in of two vortices. The results from these observations suggest the possibility that a similar wake interaction is generated in actual rotor blades, especially, in the ones with articulated hubs.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.3
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• pp.210-217
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• 2011

Evolution of tip vortex generated by a model rotor blade which has a symmetric blade section has been investigated by use of the laser doppler anemometry. Swirl and axial velocity components of tip vortex were measured by the phase averaging technique within one revolution of a rotor blade. It was found that tip vortex becomes matured until 27 degrees and diffuses afterwards with diffusing rate becoming slower compared to the case of the asymmetric blade section, but the tip loss was expected to become more substantial. Swirl velocity components were well fit to n=2 model of Vatistas within measured wake ages, showing the self-similarity exists for the swirl velocity components. The axial components were followed with Gaussian profiles, but had much higher peak values than those of the symmetric blade section.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.4 no.4
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• pp.201-207
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• 1992

The averaged non-linear term in the turbulence equations, suggested by Yeo (1987), is analyzed theoretically and experimentally. It was formulated by applying the filtering concepts to the convolution integral average definition with the Gaussian response function. This filtering approach seems to be superior to the conventional averaging methods in which all four terms at the doubly average vol must be defined separately, and it also gives a very useful tool in understanding the turbulence structures. By theoretically analyzing the newly derived description for the averaged non-linear terms, it is found that the vortex stretching can be explicitly accounted for. Furthermore, comparisons of the correlation coefficients based on the experimental data show that the vortex stretching acts most significantly on the turbulence residual stress. Thus, it strongly supports the claim that the vortex stretching is essential in the transfer of turbulence. In addition. a general form of turbulent energy models in LES is derived, by which it is recognized that the Smagorinsky, the vorticity and the SGS energy models are not distinctive.

• International Journal of Aeronautical and Space Sciences
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• v.18 no.3
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• pp.474-484
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• 2017

This paper studies the applicability of an efficient numerical model based on artificial neural networks (ANNs) to predict the dynamic responses of the wing structure of an airplane due to atmospheric turbulence in the time domain. The turbulence velocity is given in the form of a stationary Gaussian random process with the von Karman power spectral density. The wing structure is modeled by a classical beam considering bending and torsional deformations. An unsteady vortex-lattice method is applied to estimate the aerodynamic pressure distribution on the wing surface. Initially, the trim condition is obtained, then structural dynamic responses are computed. The numerical solution of the wing structure's responses to a random turbulence profile is used as a training data for the ANN. The current ANN is a three-layer network with the output fed back to the input layer through delays. The results from this study have validated the proposed low-cost ANN model for the predictions of dynamic responses of wing structures due to atmospheric turbulence. The accuracy of the predicted results by the ANN was discussed. The paper indicated that predictions for the bending moments are more accurate than those for the torsional moments of the wing structure.

• 한국초전도학회:학술대회논문집
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• v.9
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• pp.251-255
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• 1999

Local field distribution in the mixed state of type II superconductors has been numerically calculated and compared with $^{11}$B NMR spectra for YNi$_2B_2C$ single crystals. We find that only small distortion of vortex positions from the perfect lattice points is enough to smear out the low frequency shoulder. As the vortices are further distorted, the line shape changes from an asymmetric shape with a high frequency tail to a symmetric Gaussian line shape. It is found that the second moment of the field distribution has a major contribution from the high frequency tail. Consequently, a linewidth of the full width at half maximum calculated from the second moment assuming for a Gaussian line shape is overestimated.