• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.1
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• pp.41-52
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• 2021

During atmospheric ascent of a launch vehicle, airborne acoustic loads act on the vehicle and its effect becomes pronounced at transonic speed. In the present study, acoustic loads acting on a hammerhead launch vehicle at a transonic speed have been analyzed using ��-ω SST based IDDES and the results including mean Cp, Cprms, and PSD are compared to available wind-tunnel test data. Mesh dependency of IDDES results has been investigated and it has been concluded that with an appropriate turbulence scale-resolving computational mesh, the characteristic flow features around a transonic hammerhead launch vehicle such as separated shear flow at fairing shoulder and its reattachment on rear body as well as large pressure fluctuation in the region of separated flow behind the boat-tail can be predicted with reasonable accuracy for engineering purposes.

• Wind and Structures
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• v.37 no.4
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• pp.303-314
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• 2023

In this study, two steady RANS turbulence models (SST k-ω and Realizable k-ε) and four unsteady turbulence models (URANS SST k-ω and Realizable k-ε, SST-SAS, and SST-IDDES) are evaluated with respect to their capacity to predict crosswind characteristics on high-speed trains (HSTs). All of the numerical simulations are compared with the wind tunnel values and LES results to ensure the accuracy of each turbulence model. Specifically, the surface pressure distributions, time-averaged aerodynamic coefficients, flow fields, and computational cost are studied to determine the suitability of different models. Results suggest that the predictions of the pressure distributions and aerodynamic forces obtained from the steady and transient RANS models are almost the same. In particular, both SAS and IDDES exhibits similar predictions with wind tunnel test and LES, therefore, the SAS model is considered an attractive alternative for IDDES or LES in the crosswind study of trains. In addition, if the computational cost needs to be significantly reduced, the RANS SST k-ω model is shown to provide relatively reasonable results for the surface pressures and aerodynamic forces. As a result, the RANS SST k-ω model might be the most appropriate option for the expensive aerodynamic optimizations of trains using machine learning (ML) techniques because it balances solution accuracy and resource consumption.

• Journal of Korean Association for Spatial Structures
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• v.19 no.4
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• pp.95-102
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• 2019

In this study, the reliability of the analysis is evaluated by comparing the average wind pressure coefficient, RMS wind pressure coefficient and wind pressure spectrum with same condition of wind tunnel test which are calculated in the high-Reynolds number range of 1.2×10⁶, 2.0×10⁶ each for the typical curved shape dome structure. And it is examined by the reliability of analysis through Improved delayed detached Eddy Simulation(IDDES), which is one of the hybrid RANS/LES techniques that can analyze the realistic calculation range of high Reynolds number. As a result of the study, it was found that IDDES can be predicted very similar to the wind tunnel test. The distribution pattern of the wind pressure coefficient and wind pressure spectrum showed a similar compared with wind tunnel test.

• International Journal of High-Rise Buildings
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• v.11 no.4
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• pp.287-300
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• 2022

Unconventional structures are getting more popular in recent days. Large-span roofs are used for many structures, such as airports, stadiums, and conventional halls. Identifying the pressure distribution and wind load acting on those structures is essential. This paper offers a collaborative study of computational fluid dynamics (CFD) simulations and wind tunnel tests for assessing wind pressure distribution for a building with a combined slender curved roof. The hybrid turbulence model, Improved Delayed Detached Eddy Simulation (IDDES), simulates the open terrain turbulent flow field. The wind-induced local pressure coefficients on complex roof structures and the turbulent flow field around the structure were thus calculated based upon open terrain wind flow simulated with the FLUENT software. Local pressure measurements were investigated in a boundary layer wind tunnel simultaneous to the simulation to determine the pressure coefficient distributions. The results predicted by CFD were found to be consistent with the wind tunnel test results. The comparative study validated that the recommended IDDES model and the vortex method associated with CFD simulation are suitable tools for structural engineers to evaluate wind effects on long-span complex roofs and plan irregular buildings during the design stage.

• Wind and Structures
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• v.35 no.1
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• pp.55-66
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• 2022

Tall buildings are often subjected to steady and unsteady forces due to external wind flows. Measurement and mitigation of these forces becomes critical to structural design in engineering applications. Over the last few decades, many approaches such as modification of the external geometry of structures have been investigated to mitigate wind-induced load. One such proven geometric modification involved the rounding of sharp corners. In this work, we systematically analyze the impact of rounded corner radii on the reducing the flow-induced loading on a square cylinder. We perform 3-Dimensional (3D) simulations for high Reynolds number flows (Re=1 × 105) which are more likely to be encountered in practical applications. An Improved Delayed Detached Eddy Simulation (IDDES) method capable of capturing flow accurately at large Reynolds numbers is employed in this study. The IDDES formulation uses a k-ω Shear Stress Transport (SST) model for near-wall modelling that prevents mesh-induced separation of the boundary layer. The effects of these corner modifications are analyzed in terms of the resulting variations in the mean and fluctuating components of the aerodynamic forces compared to a square cylinder with no geometric changes. Plots of the angular distribution of the mean and fluctuating coefficient of pressure along the square cylinder's surface illustrate the effects of corner modifications on the different parts of the cylinder. The windward corner's separation angle was observed to decrease with an increase in radius, resulting in a narrower and longer recirculation region. Furthermore, with an increase in radius, a reduction in the fluctuating lift, mean drag, and fluctuating drag coefficients has been observed.

• Wind and Structures
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• v.35 no.6
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• pp.405-418
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• 2022

The aerodynamic behaviour of a CRH high-speed train under three infrastructure scenarios (flat ground, embankment, and viaduct) in the presence of a crosswind was simulated using a 1/8th scaled train model with three cars and the IDDES framework. The time-averaged and instantaneous flow field around the model were examined. The employed numerical algorithm was verified through a wind tunnel test, and the grid and timestep resolution analyses were conducted to ensure the reliability of the data. It was noted that the flow around the rail line was different under different infrastructure scenarios, especially in the case of the embankment, which degraded the aerodynamic performance of the train under the crosswind. The flow around the train on the flat ground and viaduct was different, although the aerodynamic performance of the train was similar in both cases. Moreover, the viaduct accidents were noted to have the most critical consequences, thereby requiring the most attention. The aerodynamic performance of the train on the windward track of the embankment under the crosswind was worse than that of the train on the leeward track. But for the other two infrastructure scenarios, the aerodynamic performance of the train on the windward track is relatively dangerous, which is mainly caused by the head car. These observations suggest that the aerodynamic behaviour of the train on an embankment under a crosswind must be carefully considered and that certain wind protection measures must be adopted around rail lines in windy areas.

• International Journal of Naval Architecture and Ocean Engineering
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• v.10 no.3
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• pp.270-281
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• 2018

Application of the LeMoS hybrid (LH) URANS/LES method for the wake parameters prediction is considered. The wake fraction coefficient is calculated for inland ship model M1926 under shallow water conditions and compared to results of PIV measurements. It was shown that due to lack of the resolved turbulence at the interface between LES and RANS zones the artificial grid induced separations can occur. In order to overcome this drawback, a shielding function is introduced into LH model. The new version of the model is compared to the original one, RANS $k-{\omega}$ SST and SST-IDDES models. It is demonstrated that the proposed modification is robust and capable of wake prediction with satisfactory accuracy.

• Wind and Structures
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• v.38 no.4
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• pp.295-307
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• 2024

The funnel-shaped vortex structure of tornadoes results in a spatiotemporally varying wind velocity (speed and direction) field. However, very limited full-scale tornado data along the height and radius positions are available to identify and reliably establish a description of complex vortex structure together with the resulting aerodynamic effects on the high-speed train (HST). In this study, the improved delayed detached eddy simulation (IDDES) for flow structures and aerodynamic pressures around an HST under tornado-like winds are conducted to provide high-fidelity computational fluid dynamics (CFD) results. To demonstrate the accuracy of the numerical method adopted in this study, both field observations and wind-tunnel data are utilized to respectively validate the simulated tornado flow fields and HST aerodynamics. Then, the flow structures and aerodynamic pressures (as well as aerodynamic forces and moments) around the HST at various locations within the tornado-like vortex are comprehensively compared to highlight the importance of considering the complex spatiotemporal wind features in the HST-tornado interactions.

• Journal of Korea Water Resources Association
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• v.52 no.10
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• pp.697-706
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• 2019

A multiphase flow modeling approach equipped with a hybrid turbulence modeling method is applied to compute the gravity currents in a rectangular channel. The present multiphase solver considers the dense fluid, the less-dense ambient fluid and the air above free surface as three phases with separate flow equations for each phase. The turbulent effect is simulated by the IDDES (improved delayed detach eddy simulation), a hybrid RANS/LES, approach which resolves the turbulent flow away from the wall in the LES mode and models the near wall flow in RANS mode on moderately fine computational meshes. The numerical results show that the present model can successfully reproduce the gravity currents in terms of the propagation speed of the current heads and the emergence of large-scale Kelvin-Helmholtz type interfacial billows and their three dimensional break down into smaller turbulent structures, even on the relatively coarse mesh for wall-modeled RANS computation with low-Reynolds number turbulence model. The present solutions reveal that the modeling approach can capture the large-scale three dimensional behaviors of gravity current head accompanied by the lobe-and-cleft instability at affordable computational resources, which is comparable to the LES results obtained on much fine meshes. It demonstrates that the multiphase modeling method using the hybrid turbulence model can be a promising engineering solver for predicting the physical behaviors of gravity currents in natural environmental configurations.

• Proceedings of the Korea Water Resources Association Conference
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• 2021.06a
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• pp.244-244
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• 2021

난류흐름 거동은 지형이나 수공구조물과 같은 고체 경계면의 변화에 민감하게 반응하며 특징 또한 다양하다. 보나 여수로 등과 같은 단차 구조물을 통과하는 흐름은 구조물의 모서리 같은 흐름 경계면이 급변하는 지점에서는 흐름분리(flow separation)가 발생하는 것이 특징이다. 이러한 흐름분리로 인해 전단층이 발생하며 흐름 재순환(recirculation)이 구조물 하류부에 형성된다. 이 연구에서는 낙차공 형식의 단차 구조물 하류부에서의 흐름 거동을 이해하기 위해 CFD모델링을 통하여 계산된 3차원 유동장을 분석한다. 난류 모의는 하이브리드 LES(large-eddy simulation)/RANS 계산 기법인 IDDES(improved delayed detached-eddy simulation)기법을 적용한다. IDDES의 기본 모형으로는 k-ω SST모형과 Spalart-Allmaras모형을 이용하여 두 모형의 성능을 평가한다. 자유수면의 변동은 VoF(volume of fluid)기법을 이용하여 계산하며, 각 지배방정식은 최소의 수치분산을 유지하면서 수치해의 안정성을 확보할 수 있는 2차 정확도의 유한체적법을 이용하여 이산화하였다. 수치해석 결과는 레이놀즈수 23,400과 후르드수 0.22의 조건에서 기존에 계측된 자료와 비교하여 수치모형의 정확도를 평가하고 하상 단차 하류부에서의 흐름 거동 특성을 분석한다. 계산 결과는 공학적으로 널리 사용되는 RANS 수치모의에서 볼 수 없는 전단층과 난류구조의 동적 거동 특성과 이에 따른 레이놀즈 응력분포의 특성을 설명해준다.