• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.2
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• pp.479-491
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• 1994

Discrete vortex method was applied for simulating an active control of turbulent leading- edge separation bubble. The leading-edge separation zone was perturbed by a time-dependent sinusoidal perturbation of different frequencies and levels. In order to describe the local sinusoidal perturbation at the separation point, a source pulsation vortex technique was proposed. The present two-dimensional vortex simulations were qualitatively compared with the experimental results for a blunt circular cylinder, where perturbation was introduced along the square-cut leading edge of the cylinder $(Kiya et al.^{(6,7)}).$ It was found that the reattachment length attained a minimum point at low levels of perturbation and two minima at a moderate higher perturbation frequency. The effects of local perturbation on the evolution of leading-edge separation bubble were scrutinized by comparing the perturbed flow with the natural flow. These comparisons were made for the distributions of mean velocity and its velocity fluctuations, intermittency and wall velocity. The motions of instantaneous reattachment in the space-time domain were demonstrated, which were also compared with the experimental findings. In order to investigate the reduction mehanism of reattachment length in the separation bubble, various cross-correlations for velocity and pressure and the relevant convection velocities were evaluated. It was observed that the convection velocity was closely associated with its corresponding pulsationg frequency.

• International Journal of Fluid Machinery and Systems
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• v.4 no.1
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• pp.199-208
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• 2011

This work presents numerical results, using OpenFOAM, of the flow in the swirl flow generator test rig developed at Politehnica University of Timisoara, Romania. The work shows results computed by solving the unsteady Reynolds Averaged Navier Stokes equations. The unsteady method couples the rotating and stationary parts using a sliding grid interface based on a GGI formulation. Turbulence is modeled using the standard k-${\varepsilon}$ model, and block structured wall function ICEM-Hexa meshes are used. The numerical results are validated against experimental LDV results, and against design velocity profiles. The investigation shows that OpenFOAM gives results that are comparable to the experimental and design profiles. The unsteady pressure fluctuations at four different positions in the draft tube is recorded. A Fourier analysis of the numerical results is compared whit that of the experimental values. The amplitude and frequency predicted by the numerical simulation are comparable to those given by the experimental results, though slightly over estimated.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.05a
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• pp.14-17
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• 2010

To identify the turbulent flow characteristics resulted from the swirl direction of a combustor with seven swirl injectors, a 3D Large Eddy Simulation(LES) was implemented. The combustor of concern is the LRE combustor, designed by Aerospace Combustion Laboratory of Georgia Institute of Technology. The seven-clockwise-swirl-injectors combustor produces stronger flow interference among injectors, specially obvious tangential velocity near the wall, than the combustor with four-clockwise and three-counterclockwise swirl injectors. In addition, pressure fluctuations in the combustor with seven-clockwise-swirl-injectors was more amplified.

• Journal of Energy Engineering
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• v.21 no.3
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• pp.237-242
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• 2012

This study presents numerical solutions of the two-dimensional Navier-Stokes equations for supersonic unsteady flow in a convergent-divergent nozzle with a isolator. The TVD scheme in generalized coordinates is employed in order to calculate the moving shock waves caused by thermal choking. We discuss on transient characteristics, unstart phenomena, fluctuations of specific thrust caused by thermal choking and effects of isolator. The adverse pressure gradient caused by heat addition brings about separation of the wall boundary layers and formation of the oblique shock wave that proceed to upstream. The proceeding speed of the oblique shock wave to upstream direction for the convergent-divergent nozzle with isolator is lower than that for the nozzle without isolator.

• Advances in aircraft and spacecraft science
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• v.4 no.2
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• pp.145-167
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• 2017

The presented paper gives an overview of several projects addressing the experimental characterization and control of the buffet phenomenon on 3D turbulent wings in transonic flow conditions. This aerodynamic instability induces strong wall pressure fluctuations and therefore limits flight domain. Consequently, to enlarge the latter but also to provide more flexibility during the design phase, it is interesting to try to delay the buffet onset. This paper summarizes the main investigations leading to the achievement of open and closed-loop buffet control and its experimental demonstration. Several wind tunnel tests campaigns, performed on a 3D half wing/fuselage body, enabled to characterize the buffet aerodynamic instability and to study the efficiency of innovative fluidic control devices designed and manufactured by ONERA. The analysis of the open-loop databases demonstrated the effects on the usual buffet characteristics, especially on the shock location and the separation areas on the wing suction side. Using these results, a closed-loop control methodology based on a quasi-steady approach was defined and several architectures were tested for various parameters such as the input signal, the objective function, the tuning of the feedback gain. All closed-loop methods were implemented on a dSPACE device able to estimate in real time the fluidic actuators command calculated mainly from the unsteady pressure sensors data. The efficiency of delaying the buffet onset or limiting its effects was demonstrated using the quasi-steady closed-loop approach and tested in both research and industrial wind tunnel environments.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.3
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• pp.232-241
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• 2011

The hybrid rocket shows interesting characteristics of complicated mixing layer developed by the interaction between turbulent oxidizer flow and injected surface mass flow from fuel vaporization. In this study, the compressible LES was conducted to explore the physical phenomena of surface oscillatory flow induced by the flow interferences in a duct domain. From the numerical results, the wall injection generates the stronger streamwise vorticites and the negative components of axial velocity accompanied with the azimuthal vorticity near the surface. And the vortex shedding with a certain time scale was found to be developed by hydrodynamic instability in the mixing layer. The pressure fluctuations in this calculation exhibit a peculiar peak at a specific angular frequency($\omega$=8.8) representing intrinsic oscillation due to the injection.