Jinsoo Cho

Unsteady Trailing Vortex Evolution Behind a Wing in Ground Effect

The unsteady evolution of trailing vortex sheets in ground effect is simulated by the use of a discrete vortex method. The ground effect is included by image method. Two cases of unsteady vortex evolution behind lifting lines (an elliptic loading and a fuselage/flap-wing configuration) are simulated for several ground heights. The present method is validated by comparison of the simulated wake roll-up shapes to published numerical results. Fo ra lifting line with an elliptic loading, the ground has the effect of moving the wingtip vortices laterally outward and suppressing the development of the vortex. An increase in the wing loading has the effect of moving the wingtip vortex more laterally outward. The rotation of the wake vortices behind a fuselage/part-span flap configuration in ground effect is less than the case of flight out of the ground effect. Nomenclature b = span C(�, t) = point on a lifting line in the complex plane C � (� o , t) = point on an image lifting line in the complex plane h = ground height; distance from a lifting line to the ground N = number of point vortices Re =v ortex Reynolds number rc =v ortex core radius

Unsteady aerodynamic analysis of ducted fans

A steady and unsteady aerodynamic analysis of ducted fans has been developed using a frequency domain panel method based on three-dimensional linear compressible lifting surface theory. The duct is assumed to be a finite-length right-circular cylinder concentric with the rotor. Both the duct and rotor blades are modeled by simple harmonic rotating doublet sheets. The model spans a single reference passage with the influence of the rest of the configuration included by symmetry. Results for the steady state performance characteristics of a ducted rotor are compared with an Euler calculation. The effect of the duct on the unsteady aerodynamic forces induced by blade vibration is examined, and comparisons are made with two-dimensional unsteady cascade theory* Finally, it is shown that the duct has an adverse effect on the aeroelastic stability of the rotor.

Propeller-wing interaction using a frequency domain panel method

The unsteady aerodynamic coupling between a propeller and a wing is analyzed using linear compressible aerodynamic theory. The periodic loads are decomposed into harmonics, and the harmonic amplitudes are found iteratively. Each stage of the iteration involves the solution of an isolated propeller or wing problem; the interaction is done through the Fourier transform of the induced velocity field. The method was validated by comparing the predicted velocity field about an isolated propeller with detailed laser Doppler velocimeter measurements and by comparison with mean loads measured in a wing-propeller experiment. Comparisons have also been made between the fluctuating loads predicted by the present method and a quasisteady vortex lattice scheme.

Propeller blade shape optimization for efficiency improvement

Abstract A numerical optimization technique has been developed to determine the optimum propeller blade shape for efficiency improvement. The method satisfies the constraints of the constant power coefficient and the activity factor. A lifting line theory (vortex lattice method) and a lifting surface theory (3-D panel method) are used to calculate aerodynamic performance parameters of propellers. Both lifting theories use rigid helical wake models. The design variables are twist angle and chord length at mid points of vortex lattices for vortex lattice method and nodes of panels for the 3-D panel method. The optimization code is validated by comparing the results with other numerical schemes. Twist angle and chord length distributions are optimized for various propellers. SR-3 and SR-7 propfan blade shapes are also optimized using the 3-D panel method for aerodynamic load calculation.

Design of an Aerolevitation Electric Vehicle for High-Speed Ground Transportation System

An aerolevitation electric vehicle, acting as a tracked wing-in-ground-effect vehicle, is conceptually designed to match the design requirements. The aerodynamic interaction between the vehicle and its track is investigated using a combination of approaches. A boundary-element method is used to study the effect of steady, nonplanar ground effect on the vehicle. The more complicated flow characteristics are investigated using a Navier-Stokes computation. The data obtained from the numerical simulations are compared with the data measured from wind-tunnel tests. The results computed using the boundary-element method agree with the measured data. The longitudinal and lateral stability derivatives are estimated, and a guidance and control system is designed using intelligent techniques based on the estimated stability derivates.

Study on the Aerodynamic Characteristics of Wings Flying Over the Nonplanar Ground Surface

Aerodynamic analysis of NACA wings moving with a constant speed over guideways are performed using an indirect boundary element method (potential-based panel method). An integral equation is obtained by applying Greens theorem on all surfaces of the fluid domain. The surfaces over the wing and the guideways are discretized as rectangular panel elements. Constant strength singularities are distributed over the panel elements. The viscous shear layer behind the wing is represented by constant strength dipoles. The unknown strengths of potentials are determined by inverting the aerodynamic influence coefficient matrices constructed by using the no penetration conditions on the surfaces and the Kutta condition at the trailing edge of the wing. The aerodynamic characteristics for the wings flying over nonplanar ground surfaces are investigated for several ground heights.

Quasi‐steady aerodynamic analysis of propeller–wing interaction

A 3- dimensional quasi - steady aerodynamic interaction between the propeller and wing is analyzed using vortex ring elements. The flow is assumed to be inviscid and the linear compressibility is assumed. The quasi - steady interaction is computed for one complete rotation of a propeller in 10 degree phase angle increments with respect to the wing reference axis. The computed results are compared with the quasi - steady results using a lifting line theory and with the full unsteady results using a frequency domain panel method.

Counter-rotating propellant analysis using a frequency domain panel method

The unsteady aerodynamic coupling between the front and rear rotors in a counter-rotating propeller system is analyzed using a frequency-domain panel method based on linear compressible aerodynamic theory. The periodic loads are decomposed into harmonics, and the harmonic amplitudes are found iteratively. Each stage of the interation involves the solution of an isolated propeller problem, the interaction being done through the Fourier transform of the induced velocity field. The method was validated by comparing mean performance parameters with measured data and by comparing the predicted velocity field with detailed laser Doppler velocimeter measurements. Comparisons have also been made between the fluctuating loads predicted by the present method and a time-domain panel method.

Unsteady Aerodynamic Analysis of Tandem Flat Plates in Ground Effect

Unsteady aerodynamic analysis of flat plates in tandem configuration flying near the ground is done using a discrete vortex method. The vortex core modeling and the core addition scheme are needed for the better prediction of the unsteady downwash on the flat plates and coupled aerodynamic interference between the plates. For the validation of the present method, the computed wake shapes of both single flat plate and flat plates in tandem configuration are compared with flow visualization and other numerical result. The predicted wake shapes and the aerodynamic characteristics of the flat plates in tandem configuration show that the unsteady ground effect can be of considerable importance in the performance of wings in tandem configuration.

Numerical and Experimental Analyses for the Aerodynamic Design of High Performance Counter-Rotating Axial Flow Fans

A study was done on the numerical and experimental analyses for the aerodynamic design of high performance of the counter rotating axial fan (CRF). Front rotor and rear rotor blades of a counter rotating axial fan are designed using the simplified meridional flow analysis method with the radial equilibrium equation and the free vortex design condition, according to design requirements. The through-flow fields and the aerodynamic characteristics of the designed rotor blades are analyzed by the matrix method and the frequency domain panel method. Fan performance curves are measured by following the standard fan testing method, KS B 6311. Three-dimensional flow fields in the CRF are analyzed by using the prism type five-hole probe. Performance characteristics of a counter-rotating axial flow fan are estimated for the variation of design parameters such as the hub to tip ratio, the taper ratio and the solidity. The effect of the hub to tip ratio on the fan efficiency is significant compared with the effects of other design parameters such as the solidity and the taper ratio. The fan efficiency is peak at the hub to tip ratio of 0.4, which is almost same point for the front rotor efficiency and rear rotor efficiency. The magnitudes of the meridional and relative velocities on the front and rear rotors are increased with the radial direction from hub to tip. This results in the reverse pressure gradient at the blade leading edges of both the front rotor and the rear rotor. Axial velocities of the CRF, which are measured by the prism type five-hole probe, are gradually increased at the mean radius due to the flow contraction effect. At the hub region, axial velocity is gradually decreased due to the flow separation and the hub vortex compare with design results. This result induces the increment of the incidence angle and the diffusion factor of the front rotor and the rear rotor.Copyright