• Wind and Structures
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• v.11 no.4
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• pp.257-273
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• 2008

A number of passive aerodynamic drag reduction methods were applied separately and then in different combinations on an intercity bus model, through wind tunnel studies on a 1:20 scale model of a Mercedes Benz Tourismo 15 RHD intercity bus. Computational fluid dynamics (CFD) modelling was also conducted in parallel to assist with flow visualisation. The commercial CFD package $CFX^{TM}$ was used. It has been found that dramatic reductions in coefficient of drag ($C_D$) of up to 70% can be achieved on the model using tapered and rounded top and side leading edges, and a truncated rear boat-tail. The curved front section allows the airflow to adhere to the bus surfaces for the full length of the vehicle, while the boat-tails reduce the size of the low pressure region at the base of the bus and more importantly, additional pressure recovery occurs and the base pressures rise, reducing drag. It is found that the CFD results show remarkable agreement with experimental results, both in the magnitude of the force coefficients as well as in their trends. An analysis shows that such a reduction in aerodynamic drag could lead to a significant 28% reduction in fuel consumption for a typical bus on intercity or interstate operation. This could translate to a massive dollar savings as well as significant emissions reductions across a fleet. On road tests are recommended.

• Advances in aircraft and spacecraft science
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• v.4 no.6
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• pp.651-677
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• 2017

For the past ten years' efforts have been made to introduce environmentally-friendly "green" electric-taxi and maneuvering airplane systems. The stated purpose of e-taxi systems is to reduce the taxiing fuel expenses, expedite pushback procedures, reduce gate congestion, reduce ground crew involvement, and reduce noise and air pollution levels at large airports. Airplane-based autonomous traction electric motors receive power from airplane's APU(s) possibly supplemented by onboard batteries. Using additional battery energy storages ads significant inert weight. Systems utilizing nose-gear traction alone are often traction-limited posing serious dispatch problems that could disrupt airport operations. Existing APU capacities are insufficient to deliver power for tractive taxiing while also providing for power off-takes. In order to perform comparative and objective analysis of taxi tractive requirements a "standard" taxiing cycle has been proposed. An analysis of reasonably expected tractive resistances has to account for steepest taxiway and runway slopes, taxiing into strong headwind, minimum required coasting speeds, and minimum acceptable acceleration requirements due to runway incursions issues. A mathematical model of tractive resistances was developed and was tested using six different production airplanes all at the maximum taxi/ramp weights. The model estimates the tractive force, energy, average and peak power requirements. It has been estimated that required maximum net tractive force should be 10% to 15% of the taxi weight for safe and expeditious airport movements. Hence, airplanes can be dispatched to move independently if the operational tractive taxi coefficient is 0.1 or higher.

• Journal of computational fluids engineering
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• v.17 no.3
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• pp.68-74
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• 2012

Near-wall effect on wakes behind particles is one of the important factors in precise tracking of particles in turbulent flows. However, most aerodynamic force models for particles did not fully consider the wall effect. In the present study, we focused on changes of hydrodynamic forces acting on a particle depending on wall proximity. To this end, we developed an immersed boundary method with multi-direct forcing incorporated to a fully implicit decoupling procedure for incompressible flows. We validate the present immersed boundary method through two-dimensional simulations of flow over a circular cylinder. Comprehensive parametric studies on the effect of the wall proximity on the drag and lift forces acting on an immersed circular cylinder in a channel flow are performed in order to investigate general flow patterns behind the circular cylinder for a wide range of Reynolds number (0.01 ${\leq}$ Re ${\leq}$ 200). As the cylinder is closer to the wall, the drag coefficient decreases while the lift coefficient increases with a local maximum. Maximum drag and lift coefficients for different wall proximities decrease with increment of Reynolds number. Normalized drag and lift coefficients by their maximum values show universal correlations between the coefficients and wall proximity in a low Reynolds number regime (Re ${\leq}$ 1).

• Wind and Structures
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• v.30 no.6
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• pp.617-631
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• 2020

Wake-induced aerodynamics of yawed circular cylinders with smooth and grooved surfaces in a tandem arrangement was studied. This pair of cylinders represent sections of stay-cables with smooth surfaces and high-voltage power conductors with grooved surfaces that are vulnerable to flow-induced structural failure. The study provides some insight for a better understanding of wake-induced loads and galloping problem of bundled cables. All experiments in this study were conducted using a pair of stationary section models of circular cylinders in a wind tunnel subjected to uniform and smooth flow. The aerodynamic force coefficients and vortex-shedding frequency of the downstream model were extracted from the surface pressure distribution. For measurement, polished aluminum tubes were used as smooth cables; and hollow tubes with a helically grooved surface were used as power conductors. The aerodynamic properties of the downstream model were captured at wind speeds of about 6-23 m/s (Reynolds number of 5×10⁴ to 2.67×10⁵ for smooth cable and 2×10⁴ to 1.01×10⁵ for grooved cable) and yaw angles ranging from 0° to 45° while the upstream model was fixed at the various spacing between the two model cylinders. The results showed that the Strouhal number of yawed cable is less than the non-yawed case at a given Reynolds number, and its value is smaller than the Strouhal number of a single cable. Additionally, compared to the single smooth cable, it was observed that there was a reduction of drag coefficient of the downstream model, but no change in a drag coefficient of the downstream grooved case in the range of Reynolds number in this study.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.6
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• pp.533-539
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• 2015

Recently, guided missiles applies folding wings to save space. During wing deployment, aero force acting on wing effects significantly on deployment performance, usually aerodynamic coefficient are calculated by CFD analysis. However, Missile Datcom can calculates estimated aerodynamic coefficient very quickly by assuming wing deployment motions as dihedral angle of wing. If missile has external store, wings may need to be folded on top of each other. In this case, one of wing help or interrupt other wing deployment, locking effect. In this study, both effects were included on wing deployment performance analysis to criteria for wings locked condition and formulated wing deploy performance, and compared with wind tunnel test data. Analysis predicted vulnerable wind direction of wing deployment very well.

• 한국전산유체공학회:학술대회논문집
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• 2001.10a
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• pp.70-75
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• 2001

This paper presents the results of numerical simulation of wind environment and wind coefficient around super high-rise building. The analysis of aerodynamic response due to wind-induced forces and wind effect to surrounding buildings is important to high-rise building. This paper simulates the wind force to the high-rise building and wind flow pattern around the high-rise building, and shows the usability of CFD analysis to design process of high-rise building. A Navier-Stokes-Solver (FLUENT) with Quick spatial discretization scheme and RNG $\kappa-\epsilon$ turbulence model has been applied to the computation of the three dimensional turbulent flow.

• 한국전산유체공학회:학술대회논문집
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• 2005.04a
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• pp.109-113
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• 2005

Recently, the weight of train is decreased by using the light material for improvement in energy efficiency. And the length of whole train is more increased for mass transportation of passengers and cargo. However, decrease of the weight and increase of the length of train can cause the train to be overturned or derailed by strong crosswind. In case of Korean Tilting Train eXpress (TTX), the situation can be more severe. TTX will be developed for a quasi-high speed train at 200km/h speed rate and operated on the existing tracks. Moreover, the weight of TTX will be much less than that of conventional train. It is supposed that TTX will be very sensitive to crosswind. In this paper, numerical analysis is used to investigate aerodynamic characteristics around TTX and obtain the induced lateral force by crosswind. After calculating derailment coefficient and overturning coefficient using numerical results, the crosswind safety of TTX is judged. This paper will be good data for judging crosswind safety of TTX.

• Journal of the Korea Society for Simulation
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• v.28 no.2
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• pp.23-33
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• 2019

A sub-munition which has low aspect ratio does not have flight stability and control of drag force under free-fall condition. In order to satisfy those problems, fin, which is called grid-fin, is designed instead of conventional flight fins and adapted to the sub-munition. The base model of the sub-munition is firstly set and numerical simulation of the model is conducted under transonic condition that is free-fall range of the sub-munition. Wind test is secondly performed to verify the simulation result. The result shows that grid fin adapted sub-munition has high drag force, but the flight stability is still needed. In order to enhance the flight stability, two additional grid-fins are designed which modify web-thickness and numerical simulations of modified models are conducted. As the results, the thinnest web-thickness grid-fin has the highest flight stability and still maintains high drag coefficient. Based on these results, design of grid-fin adapted sub-munition is completed, the path trajectory of the sub-munition can be predicted with acquired aerodynamic datum and it is expected that grid fin can be used to various shape of the flight vehicle and bomb.

• Wind and Structures
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• v.23 no.6
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• pp.559-575
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• 2016

In order to investigate the influence of different blade positions on aerodynamic load and wind loads and load-effects of large scale wind turbine tower under the halt state, we take a certain 3 MW large scale horizontal axis three-blade wind turbine as the example for analysis. First of all, numerical simulation was conducted for wind turbine flow field and aerodynamic characteristics under different halt states (8 calculating conditions in total) based on LES (large eddy simulation) method. The influence of different halt states on the average and fluctuating wind pressure coefficients of turbine tower surface, total lift force and resistance coefficient, circular flow and wake flow characteristics was compared and analysed. Then on this basis, the time-domain analysis of wind loads and load-effects was performed for the wind turbine tower structure under different halt states by making use of the finite element method. The main conclusions of this paper are as follows: The halt positions of wind blade could have a big impact on tower circular flow and aerodynamic distribution, in which Condition 5 is the most unfavourable while Condition 1 is the most beneficial condition. The wind loads and load-effects of disturbed region of tower is obviously affected by different halt positions of wind blades, especially the large fluctuating displacement mean square deviation at both windward and leeward sides, among which the maximum response occurs in $350^{\circ}$ to the tower top under Condition 8; the maximum bending moment of tower bottom occurs in $330^{\circ}$ under Condition 2. The extreme displacement of blade top all exceeds 2.5 m under Condition 5, and the maximum value of windward displacement response for the tip of Blade 3 under Condition 8 could reach 3.35 m. All these results indicate that the influence of halt positions of different blades should be taken into consideration carefully when making wind-resistance design for large scale wind turbine tower.

• Journal of the Korean Society of Visualization
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• v.18 no.2
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• pp.59-65
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• 2020

Vertical axis wind energy systems including 3 and 4 blades are numerically investigated in a two-dimensional (2D) computational domain. The power coefficient (C_p) is adopted to measure the efficiency of the system and the effect of the rotating velocity on the power coefficient is analyzed for the two different systems. The rotating velocity varies from 30 rad/s to 90 rad/s, which corresponds to the tip speed ratio (T.S.R) of 0.5 to 1.5. The torque exerted on the blades is mainly determined by the aerodynamic force in the x-direction and maximized when the blade is positioned at around θ = 186°. The efficiency of the 4-blade system is higher than that of the 3-blade system within the tip speed ratio range between 0.5 and 0.67, besides where the 3-blade system shows a better performance. For the 3-blade system, the maximum efficiency is reached to 0.082 at the tip speed ratio of 1.083. The maximum efficiency of the 4-blade system is 0.071 at T.S.R. = 0.92. The velocity fields in the x-direction, pressure fields, and the vorticity magnitude are analyzed in detail for the optimal cases of the 3- and 4-blades systems, respectively.