Session LX: Aerodynamics IV

Aerodynamic shape optimization of Airfoils in 2-D incompressible flow

An optimization framework was developed for maximizing the region of 2-D airfoil immersed in laminar flow with enhanced aerodynamic performance. It uses genetic algorithm over a population of 125, across 1000 generations, to optimize the airfoil. On a stand-alone computer, a run takes about an hour to obtain a converged solution. The airfoil geometry was generated using two Bezier curves; one to represent the thickness and the other the camber of the airfoil. The airfoil profile was generated by adding and subtracting the thickness curve from the camber curve. The coefficient of lift and drag was computed using potential velocity distribution obtained from panel code, and boundary layer transition prediction code was used to predict the location of onset of transition. The objective function of a particular design is evaluated as the weighted-average of aerodynamic characteristics at various angles of attacks. Optimization was carried out for several objective functions and the airfoil designs obtained were analyzed.   

Aerodynamic pitching damping of vehicle-inspired bluff bodies

Aerodynamic damping mechanism of road vehicles subjected to pitching oscillation was investigated by using large-eddy simulation technique. The study was based on two kinds of simplified vehicle models, which represent real sedan-type vehicles with different pitching stability in the on-road test. The simplified vehicle modes were developed so as to reproduce the characteristic flow structures above the trunk deck of the real vehicles measured in a wind-tunnel at the static case without oscillation. The forced sinusoidal pitching oscillation was imposed on the models and their pitching damping factors were evaluated through the phase-averaged pitching moment. Then flow structures in the wake of the models were extracted and its contribution to the damping mechanism was discussed. It was found that slight difference of the front and rear pillars' shape drastically affects the flow structures in the wake of the models, which enhance or restrain the vehicles' pitching instability.   

Aerodynamics of a Dimpled Vehicle

Automobiles consume approximately two billion barrels of fuel each year throughout the United States. A significant portion of this fuel is used to overcome aerodynamic drag at highway speeds. As a result, even small improvements made to the aerodynamics of automobiles can result in sizeable fuel savings. Since the shape of a vehicle is often dictated by design, economics, and function, aerodynamic improvements by means of obvious body streamlining are not always possible. However, minor modifications can be made to the vehicle, such as changing the behavior of the boundary layer to delay flow separation or installing small components either to reduce underbody flow or to mitigate induced drag. In this study, we examine the effect that dimples have upon the aerodynamics of a simplified vehicle. Reynolds-averaged Navier-Stokes simulations are performed on a full-scale Ahmed body at a Reynolds number of 9.5e6 based upon the vehicle length. The dimples, which have a uniform diameter of 0.1 m and a dimple depth-to-diameter ratio of 0.14, are distributed across the vehicle surface. The results of the simulations demonstrate that the dimples modify both the recirculation zone and the strength and location of the counter-rotating vortex pair in the vehicle wake. Although an increase in base pressure can occur for a dimpled configuration, the net drag change is sensitive to both the number and placement of the dimples on the vehicle body.   

A computational study of golfball aerodynamics: effects of rotation

An efficient finite-difference Navier-Stokes solver is used to carry out a series of simulations of a spinning golfball at three distinct flow regimes: subcritical, critical and super-critical. The golfball is treated using an embedded boundary formulation, where the velocity near the surface is locally reconstructed to satisfy the proper boundary conditions. All scales down to the dimples are resolved by means of direct numerical simulations. Results exhibit all the qualitative flow features that are unique in each regime, namely the drag crisis and the alternation of the Magnus effect. In particular, the key features in each regime are captured and the correct trends are reproduced in all cases, namely a significant drop in the drag coefficient from the sub-critical to the critical regime and a subsequent drop as the Reynolds number gets into the super- critical regime. In addition, the lift exhibits a change in sign from positive to negative values when the Reynolds number increases from sub-critical to critical values. These phenomena are explained in terms of the distinct boundary layer dynamics present in each regime and are further illuminated by flow visualizations.   

A Panel-Particle Method for Studies of Maneuvering Aircraft Formations

It is well established that flying a set of aircraft in formation leads to improvements in overall aerodynamic efficiency through the reduction of induced drag. Though many efforts have been made on controlling and optimizing such formations, these analyses have traditionally been restricted to quasi-steady aerodynamic models with flat wakes. Such models, though insightful, fail to capture essential wake dynamics during maneuvers. Here, a hybrid panel-particle method is presented to introduce unsteady wake effects in the study of formation flight systems. A fast-multipole algorithm is used for numerical speed-up. This approach is less computationally expensive than high-fidelity CFD, but is still able to capture essential wake physics lacking in quasi-steady approaches. Moreover, the representation of the wake as vortex particles allows for studies of wake-body interactions to be handled with greater ease. This presentation will introduce the method and demonstrate its merits in simulating single and multiple aircraft undergoing maneuvers.   

Simulation of a Plunging Airfoil with a Flexible Tail

The fluid motion of an airfoil with a flexible tail is simulated using an unsteady panel method with diffusive wake modeling. The fluid simulation is coupled with a CSD solver to simulate the deflection of the flexible tail due to both inertial and aerodynamic forces. The modal equations were used to calculate the structural deformation under an aerodynamic load. Computations with varying stiffness coefficient and reduced frequencies were performed to produce a performance map of a plunging airfoil with a flexible tail. The results showed a range of reduced frequencies and tail stiffness that increased the thrust produced by the plunging motion by as much as 45{\%} when compared with the same airfoil that has a rigid tail. The propulsive efficiency with a flexible tail increased slightly as well. The upper limit of the thrust enhancement is bounded by the first natural frequency of the flexible tail. A stiffer tail is shown to be most beneficial, but the minimum reduced frequency where thrust is improved increases with increasing stiffness. Spectral analysis of the unsteady forces and wake velocities showed that the increase in thrust can be directly attributed to the effect the flexible tail has on the wake vorticies.   

Flow Structure on a Rotating Plate

The flow structure on a rotating (revolving) plate with an aspect ratio of one is considered for the case of steady plate rotation, well after the transient startup. Techniques of particle image velocimetry lead to patterns of vorticity in relation to the sectional streamline topology. Values of Reynolds number, attained by variation of the angular velocity of the plate, range from approximately 3,000 to 13,000. The observed patterns are relatively insensitive to Reynolds number. A well-defined leading-edge vortex, which remains in a stable position, is attainable over a wide range of effective angle of attack, up to 75 degrees. These quasi-two-dimensional features are intimately related to three-dimensional characteristics of the flow, which involves spanwise-oriented patterns of velocity that vary along the chord of the plate, as well as distinctive patterns in the vicinity of the root and the tip of the rotating plate. The flow structure on the corresponding plate undergoing steady, purely translational motion is directly compared with that along the rotating plate. The flow pattern is fundamentally altered in absence of rotation, with dominance of large-scale stall.   

An investigation of the influence of freestream turbulence on the laminar separation bubble of an SD7003 airfoil at low Reynolds number

There is considerable discrepancy in the literature regarding the location of separation and reattachment points on the steady SD7003 airfoil obtained in different experimental and computational studies. Among several factors that could lead to this discrepancy in experiments, the facility's freestream turbulence level is believed to be important. Freestream turbulence acts as an excitation source that can influence the evolution of the boundary layer and the separated shear layer. The current investigation tries to quantify this influence by deliberately modifying the freestream turbulence using a turbulence-generating grid upstream of the airfoil. Multiline single-component Molecular Tagging Velocimetry (MTV) with its high resolution near-wall measurement capability (approximately 10 times better cross-stream resolution than recent PIV studies) is utilized. Results with/without the grid are compared with computations and other experiments in different facilities.