Tae-Hwan Cho

Downstream flow condition effects on the RR → MR transition of asymmetric shock waves in steady flows

In this paper, the regular reflection (RR) to Mach reflection (MR) transition of asymmetric shock waves is theoretically studied by employing the classical two- and three-shock theories. Computations are conducted to evaluate the effects of expansion fans, which are inherent flow structures in asymmetric reflection of shock waves, on the RR → MR transition. Comparison shows good agreement among the theoretical, numerical and experimental results. Some discrepancies between experiment and theory reported in previous studies are also explained based on the present theoretical analysis. The advanced RR → MR transition triggered by a transverse wave is also discussed for the interaction of a hypersonic flow and a double-wedge-like geometry.

An Aerodynamic Shape Optimization Study to Maximize the Range of a Guided Missile

This paper describes a research of a shape optimization study to maximize a range of a guided missile with canards and tailfins. To design a guided missile for the maximum range, a shape optimization system is incorporated with a trajectory analysis and an optimization technique. In the trajectory analysis part, a component build-up method is directly connected to the equation of motion to calculate aerodynamic coefficients at every time step. In the optimization part, real coded adaptive range genetic algorithm was adopted to find out an optimum shape of the global maximum range. The shape optimization system of a guided missile for the maximum range can maximize the range of a guided missile and yield the optimum shape of canards and tailfins. The analysis results confirmed that the optimum shape thus derived extended the range of the base shape by 5.8% for the unguided case and by 21.4% for the guided case.

Development of Eulerian Droplets Impingement Model Using HLLC Riemann Solver and POD-Based Reduced Order Method

A finite volume method based on approximate Riemann solver is proposed as a basic building block for computing the Eulerian droplet impingement model. The HLLC approximate Riemann solver, which is originally developed for shallow water equations of the fluid depth and velocity field, is applied to the Eulerian droplet model. The positivity condition in liquid water contents is achieved through the HLLC solver. Then, a proper orthogonal decomposition method, a reduced order model that optimally captures the energy content from a large multi-dimensional data set, is utilized to efficiently predict the collection efficiency on an airfoil. It is shown that the collection efficiency reconstructed by utilizing four modes of the POD is in very close agreement with the original value.

A Study on Truncated Flapped Airfoil for Efficient Icing Wind Tunnel Test

The evaluation of supercooled water droplet impingement characteristics of full-scale aircraft components in wind tunnels under icing conditions has been severely limited by the relative size of the component and the test facility. The concept of truncated airfoil sections has been suggested in order to extend the operational range of icing tunnels. With proper deflection of the small trailing-edge flap on the truncated airfoil the local pressure distribution may remain very close to that of the full-scale airfoil. In this study the shape of a truncated flapped airfoil is investigated for various deflection angles. To validate the truncated flapped airfoils, air flow and collection efficiency over the truncated airfoil are compared with the results of the full-scale airfoil obtained from the state-of-the-art icing simulation code.

Development of a Computational Electromagnetics Code for Radar Cross Section Calculations of Flying Vehicles

The ability to predict radar return from flying vehicles becomes a critical technology issue in the development of stealth configurations. Toward developing a CEM code based on Maxwell`s equations for analysis of RCS reduction schemes, an explicit upwind scheme suitable for multidisciplinary design is presented. The DFFT algorithm is utilized to convert the time-domain field values to the frequency-domain. A Green`s function based on near field-to-far field transformation is also employed to calculate the bistatic RCS. To verify the numerical calculation the two-dimensional field around a perfectly conducting cylinder is considered. Finally results are obtained for the scattering electromagnetic field around an airfoil in order to illustrate the feasibility of applying CFD based methods to CEM.

Two Dimensional Numerical Study in Gangway of Next Generation High Speed Train For Reduction of Aero-acoustic Noise

As the preceding research for the design of gangway in the next generation high speed train, the aero-acoustic noise at the gangway is calculated. For this purpose, the shape of gangway with mud flaps is assumed as the two-dimensional cavity. Then, 5 gap sizes between mud flaps of gangway are selected and parametric study is performed according to the gap sizes. From this study, the aerodynamic features such as vortex shedding, pressure, etc. are computed. Also, the aero-acoustic properties of tonal noise and overall noise are analyzed at the 3 locations of microphone and the relation between the gap size of mud flap and the noise level is assessed. Through this study, it is shown that the noise characteristics of base and specific models are better than those of other models.

Calibration of a Five-Hole Multi-Function Probe for Helicopter Air Data Sensors

In the flight of air vehicles, accurate air data information is required to control them effectively. Especially, helicopters are often put in drastic motion involved with high angle of attacks in order to perform difficult missions. Among various sensors, the multi function probe (MFP) has been used in the present study mainly owing to its advantages in structural simplicity and capability of providing various information such as static and total pressure, speed, and pitch and yaw angles. In this study, a five-hole multi-function probe (FHMFP) is developed and its calibration is conducted using multiple regressions. In this work a calibration study on the FHMFP, an air data sensor for helicopters, is reported. It is shown that the pitch and yaw angles’ accuracy of calibration is ± 0.91° at a cone angle of 0° ~ 30° and ± 2.0° at 30°~ 43°, respectively, which is summarized in table 3.

Design of Pitot-Tube Configuration Using CFD Analysis and Optimization Techniques

Accurate measurement of speed and altitude of flying vehicles in air data system remains a critical technical issue. A highly reliable Pitot-static probe is required to obtain air data such as total pressure and static pressure. In this study, an analysis of the characteristics of flowfield around the Pitot-static probe was performed by using a Navier-Stokes CFD code. In addition, for the purpose of finding an optimal configuration, a technique based on the response surface method is applied to the problem with design parameters including shape of the nose section and cone angle. It is shown that the optimal configuration fulfills the MIL specification in wider range of high angles of attack.

Performance Evaluation of Propeller for High Altitude by using Experiment and Computational Analysis

Wind tunnel experiment and computational analysis have been carried out to evaluate the performance of propeller for scale electric-powered HALE UAV, named EAV-2H+. Performance curves are measured for three propellers and their adequacy for EAV-2H+ installation is examined through consideration of operating conditions. Decline in performance coefficients is observed in low rpm region. Also, the effect of transition tape on propeller performance is measured and analyzed. The computational performance analyses are carried out by using commercial CFD program. The thrust and power coefficient from computations show good agreement with experimental results. Performance coefficients are compared and the influence of measurement device which contributes to discrepancy of the results is examined. Transition SST model is confirmed to yield the tendency of performance decline in low rpm range, similar to experimental observation. The decrease in aerodynamic performance of blade element due to low Reynolds number is identified to cause the decline in propeller performance. Analyses for high altitude conditions confirms degradation in propeller performance.

Numerical study of shock interactions in viscous, hypersonic flows over double-wedge geometries

The characteristics of the boundary layer in a supersonic flow can be drastically altered by a shock wave. The interaction between shock wave and boundary layer plays an important role in the design and operation of high-speed vehicles and propulsion systems. In this paper, the shock/shock interaction and the shock/boundary interaction over a compression ramp and a double-wedge structure in supersonic/hypersonic flows were studied numerically by solving the Favre-averaged Navier-Stokes equations. The numerical simulations show that the viscosity effect results in different interaction transition point, and induce more complex flow patterns, such as the separation shock, shocklets, shear unsteadiness, and complex interactions among them. With the presence of viscous effects the pressure load imposed on the wedge surface can be changed a great deal comparing to the inviscid flow simulations.