• Journal of the Korean Society of Visualization
• /
• v.22 no.2
• /
• pp.44-54
• /
• 2024

This study investigates the effectiveness of using a plasma actuator for active control of turbulent flow around a finite square cylinder. The primary objective is to analyze the impact of plasma actuators on flow separation and wake region characteristics, which are critical for reducing drag and suppressing vortex-induced vibrations. Direct Numerical Simulation (DNS) was employed to explore the flow dynamics at various operational parameters, including different actuation frequencies and voltages. The proposed methodology employs a neural network trained using the Proximal Policy Optimization (PPO) algorithm to determine optimal control policies for plasma actuators. This network is integrated with a computational fluid dynamics (CFD) solver for real-time control. Results indicate that this deep reinforcement learning (DRL)-based strategy outperforms existing methods in controlling flow, demonstrating robustness and adaptability across various flow conditions, which highlights its potential for practical applications.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.32 no.8
• /
• pp.37-46
• /
• 2004

Several notes on ground effects drawn from Navier-Stokes analyses and their aerodynamic interpretations were addressed here; For two-dimensional ground effect, the change of surface pressure due to image vortex, the venturi effect due to thickness and the primary inviscid flow phenomena of ground effect, and for three-dimensional ground effect, strengthened wing tip vortices, increased effective span and the outward drift of trailing vortices. Irodov's criteria were evaluated to investigate the static longitudinal stability of conventional NACA 6409 and DHMTU 8-30 airfoils. The analysis results demonstrated superior static longitudinal stability of DHMTU 8-30 airfoil. The DHMTU airfoil has quite lower value of lrodov's criterion than the conventional NACA airfoil, which require much smaller tail volume to stabilize the whole WIG-craft at its design stage.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.6 no.2
• /
• pp.12-20
• /
• 1998

Exter ballistics of a typical high-speed projectile is studied through a flow-visualization experiment and an unstructured grid Navier-Srokes computation. Experiment produced a schlieren photograph that adequately shows the characteristic features of this complex flow, namely two kinds of oblique cone shocks and turbulent wake developing into the downstream. A hybrid scheme of finite volume-element method is used to simulate the compressible Reynolds-Averaged Navier-Stok- es solution on unstructured grids. Osher's approximate Riemann solver is used to discretize the cinvection term. Higher-order spatial accuracy is obtained by MUSCL extension and van Albada ty- pe flux limiter is used to stabilize the numerical oscillation near the solution discontinuity. Accurate Gakerkin method is used to discretize the viscous term. Explict fourth-order Runge-Kutta method is used for the time-stepping, which simplifies the application of MUSCL extension. A two-layer k-$\varepsilon$ turbulence model is used to simulate the turbulent wakes accurately. Axisymmetric folw and two-dimensional flow with an angle of attack have been computed. Grid-dependency is also checked by carrying out the computation with doubled meshes. 2-D calculation shows that effect of angle of attack on the flow field is negligible. Axi-symmetric results of the computation agrees well with the flow visualization. Primary oblique shock is represented within 2-3 meshes in numerical results, and the varicose mode of the vortex shedding is clearly captured in the turbulent wake region.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.42 no.1
• /
• pp.29-36
• /
• 2014

Hybrid rocket displays many different low frequency pressure oscillations during combustion. Thermal lag between solid and gas phase is the primary mechanism to trigger low frequency pressure oscillations of around 10Hz, and Helmholtz or $L^*$ mode also produces other types of low frequency oscillations above 10 Hz which is associated with the change in combustion volume. Since the flow characteristics in hybrid rocket is very similar to those in solid rocket combustion, it is not surprising to observe similar pressure oscillation behaviors. Experimental test shows that combustion pressure suddenly turns into to a big amplitude oscillation around 10Hz then followed by returning to an original pressure level after a short period combustion. Further investigations show that this instability is independent of the change in O/F ratio at all. One of the possible candidates is the vortex shedding dynamics over the backward step in the post combustion chamber. It is required to investigate the low frequency oscillation mechanism in the future study.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.35 no.1
• /
• pp.10-17
• /
• 2007

Numerical results from the "figure-of-eight" motion of Phormia-Regina in Part 1 indicate that vortical structure and vortex dynamics do play a critical role in lift and thrust generation. The aerodynamic force generation of insects' wing could be governed by aerodynamic parameters such as Reynolds number; kinematic parameters such as frequency, amplitude, and component of the figure of eight motion; and morphological parameters such as wing shape and the number of wing. In the present work, the effects of Reynolds number, reduced frequency and motion component are investigated in detail to clarify aerodynamic characteristics of insect wing. Through numerical results and their physical interpretation, the mechanism of aerodynamic force generation is presented more clearly. Rotation turns out to be the most important component in thrust generation and subsequent counterclockwise rotational circulation is closely related with thrust generation.

• Journal of Korean Society of Water and Wastewater
• /
• v.19 no.2
• /
• pp.117-124
• /
• 2005

Appropriate removal of pollutants from combined sewer overflows(CSOs) and stormwater runoff is of primary concern to watershed managers trying to meet water quality standards even under a wet weather condition. Harmful substances associated with particles besides TSS and BOD are subjected to removal prior to discharge into the natural waters. Effectiveness of five major hydrodynamic separation technologies, Vortechs, Downstream Defender including Storm King for CSOs control, CDS, Stormceptor, and IHS, were evaluated in this study. There is not sufficient information for accurate evaluation of the removal efficiency for the pollutants from the stormwater runoff and CSOs. Based upon limited engineering data, however, all technologies were found to be effective in separation of heavy particles and floating solids. Technologies utilizing screens seem to have advantage in the treatment capacity than the other technologies relied fully on hydrodynamic behavior. The IHS system seems to have a strong potential in application for control of CSOs because of unique hydrodynamic behavior as well as a flexibility in opening size of the screens. Size of the particulate matter in the CSOs and stormwater runoff is found to be the most important parameter in selection of the type of the hydrodynamic separators. There exists an upper limit in the solids removal efficiency of a hydrodynamic separator, which is strongly dependent upon the particle size distribution of the CSOs and stormwater runoff.