Cheolheui Han

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

Actuator-Work Concepts Applied to Unconventional Aerodynamic Control Devices

This paper investigates the resistance to a change in wing shape due to the aerodynamic forces. In particular, the work required by an airfoil to overcome the aerodynamic forces and produce a change in lift is examined. The relationship between this work and the total aerodynamic energy balance is shown to have significant consequences for transient changes in airfoil shape. Specification of the placement of the actuators and the actuator energetics is shown to be required for the determination of the airfoil shape change, requiring minimum energy input. A general simplified actuator model is adopted in this study, which assigns different values of actuator efficiency for negative and positive power output. Unsteady thin airfoil theory is used to analytically determine the pressure distribution and aerodynamic coefficients as a function of time for a ramp input of control deflection. This allows the required power and work to overcome the aerodynamic forces to be determined for a prescribed change in the airfoil camberline. The energy required for a pitching flat plate, conventional flap, conformal flap, and two variable camber configurations is investigated. For the pitching flat plate, the minimum energy pitching axis is shown to be dependent on the pitch rate and the initial angle of attack. The conformal flap is shown to require less actuator energy than the conventional flap to overcome the aerodynamic forces for a prescribed change in lift. The energy requirements of a variable camber configuration are shown to be sensitive to the layout of the variable camber device. The present analysis shows that the unsteady aerodynamic influence is important only for τ* values less than five. For τ* values larger than this, the present analysis reduces to the steady airfoil results of past studies.

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.

Thickness Effect on the Thrust Generation of Heaving Elliptic Airfoils

The influence of the Reynolds number on the propulsive characteristics of insect wings is investigated by focusing on the aerodynamic transition from drag to thrust. The effect of both the thickness ratio and the Reynolds number on the thrust generation of elliptic airfoils is investigated using a lattice Boltzmann method. Three Reynolds numbers (Re = 50,100, and 185) and two Strouhal numbers (Sr = 0.2 and 0.4) are treated, while the thickness ratio is varied from 0.05 to 1.0. For all investigated Reynolds numbers, it is found that the airfoils oscillating at Sr = 0.2 do not produce thrust. For Sr = 0.4, it is found that thrust is produced at Re = 185. It is found that an airfoil of approximately 10% thickness produces maximum thrust. Thus, it can be said that, for thrust generation, there exists critical Reynolds and Strouhal numbers, and the thickness ratio is also a crucial parameter. By investigating the vortex pattern, velocity profiles, and vorticity intensities of the vortex cores behind the oscillating airfoils, it is found that 1) mushroom vortex patterns indicative of thrust do not completely guarantee the thrust generation, and 2) a phase lag between the thrust-force indicating vortex pattern and the resulting thrust production is observed. The present investigation shows that thrust-producing airfoils should produce strong vortices, enough to overcome the momentum deficit due to the boundary layer. Present results are obtained within the limitation of the laminar flow assumption.

Inviscid Wing-Tip Vortex Behavior Behind Wings in Close Formation Flight

Nomenclature bH = nondimensionalized half-span of the hitchhiker aircraft by the half-span of the hitchhiker aircraft, Eq. (1) bM = half-span of the mothership divided by the half-span of the hitchhiker aircraft dt = time step, s dy = distance between the mothership and the hitchhiker divided by the half-span of the hitchhiker aircraft dz = relative height between the mothership and the hitchhiker divided by the half-span of the hitchhiker aircraft N = total number of elements representing each wing (u, v) H j = induced velocity at a wake vortex j on the hitchhiker aircraft by other vortices on the hitchhiker aircraft (u, v) M j = induced velocity at a wake vortex j on the hitchhiker aircraft by vortices from the mothership aircraft (u, v) H j = induced velocity at a wake vortex j on the mothership aircraft by vortices on the hitchhiker aircraft (u, v) M j = induced velocity at a wake vortex j on the mothership aircraft from other vortices on the mothership aircraft x = downstreamwise distance from a wing’s trailing edge H 0 = maximum circulation on the hitchhiker for an elliptic load distribution M 0 = maximum circulation on the mothership for an elliptic load distribution δ = smoothing factor, 0.1

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.

Experimental Study of a Telescopic Morphing Aspect Ratio Wing inside a Channel

W ING-IN-GROUND (WIG) effect vehicles use the wellknown increase in the lift-to-drag ratio of a wing near the ground. Most WIG vehicles developed or under construction are water based or amphibious. Ollia [1] and Hooker [2] published historical reviews and technological knowledge of water-basedWIG vehicles. Rozhdestvensky [3] presented an extensive literature review on water-based WIG vehicle developments. Conceptual land-based WIG vehicles have also been proposed. These are designed to move faster and to consume less energy than current conventional ground transporters [4–9]. A novel form of high-speed ground transporter, called an aerolevitation electric vehicle (AEV), has been proposed in South Korea [8,9]. This is an over-the-ground-surface tracked WIG vehicle (TWIG) [4], which flies above or within the guideways of the track. It has the advantage of being able to fly faster and in close proximity to the rigid ground surface, as compared to the water-based WIG vehicles. The TWIG has a relatively small wing area (ormore precisely, a low aspect ratio) because of the large lift augmentation resulting from the ground effect. However, the small wing area optimized for a design cruise condition requires some special high-lift concepts for takeoff because of the limited capability of flaps [9]. The AEV also requires effective roll control assistance when it runs inside the winding channel-type guideway. The variable-spanmorphing (telescopic) wing can change itswing area symmetrically to obtain an optimally performing wing configuration at each given flight condition. It changes its wing area asymmetrically to swiftly control the roll motion of the vehicle. Tidwell et al. [10] compared various morphing strategies while demonstrating the impact of the morphing aircraft on aircraft performance. They showed that planform morphing improves the performance significantly more than that provided by airfoil morphing alone. Blondeau et al. [11] discussed the design and testing of a telescopic wing. They tested a small scale telescopic wingmodel within theReynolds number range of 182,000–454,000. It was found in their study that the telescoping wing at maximum deployment did incur a slightly larger drag penalty and a reduced lift-to-drag ratio. Neal [12] used the vortex-lattice method and performed the wind tunnel testing to model the aerodynamics on the morphing aircraft and to evaluate the performance and control of the morphing aircraft maneuvering. Neal et al. [13] designed and tested a fully adaptive aircraft configuration to investigate morphing for multimission unmanned aerial vehicles (UAVs). Wind tunnel tests of five independent planform changes along with independent twist control for each wing showed that different configurations produce minimum drag over a range of flight conditions. The present study is focused on applying the variable-span morphingwing concept [10–12] to the design of the land-basedWIG vehicles [6,9] with effective roll control. Thus, it is the aim of this paper to investigate the basic aerodynamic characteristics of a telescopic wing inside of a channel guideway. The effects of the ground and sidewall (GE and SE) on the steady aerodynamic characteristics of the telescopic wing are investigated by changing the ground height and the gap between wing tips and sidewalls.

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.

Wake Shapes Behind Wings in Close Formation Flight Near the Ground

The unsteady evolution of trailing vortex sheets behind wings in close formation flight near the ground is simulated using a discrete vortex method The ground effect is included by an image method The method is validated by comparing computed results with other numerical results. For a lifting line with an elliptic loading, the ground has an effect of moving wingtip vortices laterally outward and suppressing the development of vortex evolution The gap between wings in close formation flight has an effect of moving up wingtip vortices facing each other For wings flying in parallel, the ground effect causes the wingtip vortices facing each other to move up, and it makes the opposite wing tip vortices to move laterally outward When there is a relative height between the wings in ground effect, right-hand side wingtip vortices from a mothership move laterally inward.

Steady aerodynamic characteristics of a wing flying over a nonplanar ground surface. Part II: Channel

The aerodynamic interaction between a wing and a rail is investigated using a boundary-element method. The source and doublet singularities are distributed on the wing and its guideway rail surface. The unknown strengths of the singularities are determined by inverting the aerodynamic influence coefficient matrices. Present method is validated by comparing computed results with the other numerical data. Rail width and rail height affect the aerodynamic characteristics of the wing only if the rail is narrower than the wing span. Although the present results are limited to the inviscid, irrotational flows, it is believed that the present method can be applied to the conceptual design of the high speed ground transporters moving over the rail.