Soo Hyung Park

Improvement of the aerodynamic performance by wing flexibility and elytra--hind wing interaction of a beetle during forward flight.

In this work, the aerodynamic performance of beetle wing in free-forward flight was explored by a three-dimensional computational fluid dynamics (CFDs) simulation with measured wing kinematics. It is shown from the CFD results that twist and camber variation, which represent the wing flexibility, are most important when determining the aerodynamic performance. Twisting wing significantly increased the mean lift and camber variation enhanced the mean thrust while the required power was lower than the case when neither was considered. Thus, in a comparison of the power economy among rigid, twisting and flexible models, the flexible model showed the best performance. When the positive effect of wing interaction was added to that of wing flexibility, we found that the elytron created enough lift to support its weight, and the total lift (48.4 mN) generated from the simulation exceeded the gravity force of the beetle (47.5 mN) during forward flight.

Effect of Chord Flexure on Aerodynamic Performance of a Flapping Wing

Inspired by the fact that a high flexible wing in nature generates high aerodynamic performance, we investigated the aerodynamic performance of the flapping wing with different chord flexures. The unsteady, incompressible, and viscous flow over airfoil NACA0012 in a plunge motion was analyzed by using Navier-Stokes equation. Grid deformation, in which finite element and interpolation ideas are mixed, was introduced for computing large grid deformation caused by the chord flexures. We explored the optimal phase angle for thrust force and propulsive efficiency by varying the chord flexure from 0.05 to 0.7 when reduced frequency and plunge amplitude were fixed. Throughout parametric study on the phase angle and chord flexure amplitude, the maximum thrust force is achieved near at 0° in all given conditions, meanwhile, it is found that the optimal phase angle has dependency of chord flexure amplitude, which achieves higher aerodynamic performance compared to previous studies. These findings will provide a useful guideline for determining wing flexibility in design of a bio-mimetic air vehicle.

How Could Beetle's Elytra Support Their Own Weight during Forward Flight?

The aerodynamic role of the elytra during a beetle’s flapping motion is not well-elucidated, although it is well-recognized that the evolution of elytra has been a key in the success of coleopteran insects due to their protective function. An experimental study on wing kinematics reveals that for almost concurrent flapping with the hind wings, the flapping angle of the elytra is 5 times smaller than that of the hind wings. Then, we explore the aerodynamic forces on elytra in free forward flight with and without an effect of elytron-hind wing interaction by three-dimensional numerical simulation. The numerical results show that vertical force generated by the elytra without interaction is not sufficient to support even its own weight. However, the elytron-hind wing interaction improves the vertical force on the elytra up to 80%; thus, the total vertical force could fully support its own weight. The interaction slightly increases the vertical force on the hind wind by 6% as well.

KFLOW Results of Airloads on HART-II Rotor Blades with Prescribed Blade Deformation

A three-dimensional compressible Navier-Stokes solver, KFLOW, using overlapped grids has recently been developed to simulate unsteady flow phenomena over helicopter rotor blades. The blade-vortex interaction is predicted for a descending flight using measured blade deformation data. The effects of computational grid resolution and azimuth angle increments on airloads were examined, and computed airloads and vortex trajectories were compared with HART-II wind tunnel data. The current method predicts the BVI phenomena of blade airloads reasonably well. It is found from the present study that a peculiar distribution of vorticity of tip vortices in an approximate azimuth angle range of 90 to 180 degrees can be explained by physics of the shear-layer interaction as well as the dissipation of numerical schemes.

Selection of Rotorcraft Models for Application to Optimal Control Problems

The maneuver characteristics of rotorcraft are analyzed using a nonlinear optimal control theory. The flight path deviations from a prescribed maneuver trajectory are penalized in the optimal control formulation to avoid numericaldifficulties.Thesystemoptimality isrepresented byatwo-point boundaryvalue problemandsolved viaa multiple-shooting method. The focus of this paper is on the model-selection strategies for resolving the problems of numerical instability and high computational overhead when complex rotor dynamics are included in the mathematical model. Four different types of rotorcraft models are identified, two of which are linear models with or without rotor dynamics, as well as two models that include nonlinear dynamics for the rotor in its formulation. The effect each modelwas found to impart on the numerical analysis isreported. Therelative computational efficiency is assessed in terms of computation time and the number of function calls for each model. The applications encompass the analyses for bob-up, turn, and slalom maneuvers and the results are used as guidelines for the selection of appropriate rotorcraft models.

Loose Fluid-Structure Coupled Approach for a Rotor in Descent Incorporating Fuselage Effects

A loose coupling approach is used to combine a comprehensive structural dynamics code CAMRAD II and a computational fluid dynamics solver KFLOW to validate the data and to identify the effect of fuselage on the aeroelastic behavior of the second higher harmonic rotor acoustic test rotor. The computations are made using isolated rotor and rotor–fuselage models. To demonstrate the effect of a fuselage, the shaft tilt angles remain identical for both configurations. A good correlation has been obtained with the present computational fluid dynamics/comprehensive structural dynamics method. It is observed that a rotor–fuselage model improves the correlation significantly in terms of magnitudes and phases of the airloads solution. All the blade–vortex interaction peaks are captured accurately, and the phase shift in the section normal forces improves significantly, with the inclusion of a fuselage. The sources of improvements are investigated, considering the vorticity distributions and induced velocity fields ...

Two- and Three-Dimensional Simulations of Beetle Hind Wing Flapping during Free Forward Flight

Aerodynamic characteristic of the beetle, Trypoxylus dichotomus, which has a pair of elytra (forewings) and hind wings, is numerically investigated. Based on the experimental results of wing kinematics, two-dimensional (2D) and three-dimensional (3D) computational fluid dynamic simulations were carried out to reveal aerodynamic performance of the hind wing. The roles of the spiral Leading Edge Vortex (LEV) and the spanwise flow were clarified by comparing 2D and 3D simulations. Mainly due to pitching down of chord line during downstroke in highly inclined stroke plane, relatively high averaged thrust was produced in the free forward flight of the beetle. The effects of the local corrugation and the camber variation were also investigated for the beetle’s hind wings. Our results show that the camber variation plays a significant role in improving both lift and thrust in the flapping. On the other hand, the local corrugation pattern has no significant effect on the aerodynamic force due to large angle of attack during flapping.

RANS Simulation of a Synthetic Jet in Quiescent Air

Synthetic jet flows from piezoelectric actuators have been investigated for flow control in the field of fluid dynamics. Numerical simulation for a single diaphragm piezoelectric actuator operating in quiescent air is performed to investigate the complex flow field around the slot exit. A periodic velocity transpiration condition is used to simulate the effect of the moving diaphragm. Numerical aspects of the artificial dissipation and time integration schemes are investigated. Results for the flow field around the slot exit agree well with the experimental data. The k-ω shear stress transport model gives superior results to the Spalart-Allmaras model and a linear version of the k-e Craft model.

An Efficient Multigrid Method for Overlapped Grid System to Integrated System Analysis

A technique for the generation of overlapped grid using a simple shooting method, that is a cut-paste algorithm, is presented. It makes possible to generate overlapping grids with moderate mesh interface region. To generate overlapped grids with minimal amount of user inputs, the advancing-front technique using the fringe points as facets is used. To remove hole points initially, fronts using solid bodies and the zones of interference algorithm are used. The overlapped grids are generated for several cases using present algorithms and Euler equations are solved for two/three-dimensional steady state flow fields. To demonstrate the capability of present method, the two dimensional store separation with trajectory mode of three-DOF is computed in an unsteady flow field.

Modeling and Prediction of the Crossflow Transition Using Transition Transport Equations

In this paper, the Reynolds-Averaged Navier-Stokes (RANS) equations are coupled with transition transport equations to predict the natural transition. The present γ-Reθ transition model has been developed without using boundary layer integral parameters and this model is based on two additional transport equations, one for the intermittency and one for the transition-onset momentum thickness Reynolds number. An engineering transition model for crossflow instability is proposed based on the philosophy of the local-correlation transition transport. The present transition model is applied to a variety of different complex applications. The main goal of the present paper is to validate the prediction capability for aeronautical flows over three-dimensional configurations using the γ-Reθ transition transport model considering crossflow effect. Validations of the transition model are conducted for an infinite NLF(2)-0415 swept wing, a finite ONERA M6 wing, and 6:1 inclined prolate spheroid configuration. In all cases numerical results considering the crossflow effect are in good agreement with the available experimental data and prove that the proposed crossflow transition model is well validated.