Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G16: Aerodynamics: Airfoils, Wings and Bodies 
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Chair: Azar EslamPanah, Pennsylvania State University, Berks Room: Georgia World Congress Center B303 
Monday, November 19, 2018 10:35AM  10:48AM 
G16.00001: Prandtl second optimal wing: model design and testing with near wake PIV, and 3D simulation Gregoire Winckelmans, Jonathan Toussaint, Philippe Chatelain, DenisGabriel Caprace In 1933, Prandtl proposed a second and superior optimal wing, also much closer to what birds do: the wing with minimum induced drag for fixed lift and fixed amount of structural material. The obtained lift distribution is then bell shaped (not elliptical as the 1922 optimal wing). This design has regained much interest, also by NASA. As the distribution has a zero slope at the wing tip, the near wake doesn’t display a tip vortex. There is also an upwash velocity in the near tip region, which allows for efficient maneuvers without vertical tail. For a chosen geometry (here a linearly tapered wing) and cruise lift coefficient (here 0.4), the twist distribution is then tailored to obtain the desired lift distribution. A wing model was designed, built and tested in a small wind tunnel. The near wake was measured at stations downstream using PIV. It is seen that the near wake doesn’t really roll up. Hence, the lift distribution can also be recovered by integrating near field PIV measurements. PIV results will be shown at cruise condition and also at offdesign (from 0.1 to 0.6). The recovered lift distribution will also be compared to that provided by the lifting line theory. Results of a near wake simulation performed using the 3D Vortex ParticleMesh method will also be presented. 
Monday, November 19, 2018 10:48AM  11:01AM 
G16.00002: Tip effects on threedimensional wake dynamics over highincidence finiteaspectratio wings Kai Zhang, Kunihiko Taira Separated flows over a canonical unswept finiteaspectratio wing exhibits complex wake dynamics due to its tip effects. To reveal the fundamental physics of threedimensional separation, we employ direct numerical simulation over a range of Reynolds numbers, aspect ratios, and angle of attacks for a NACA0015 airfoil. We identify three key regions in the wake based on its dynamics, namely (1) the tip, (2) spanwise shedding, and (3) interacting regions. The dynamics observed in these regions not only show distinct vortex dynamics but also exhibit unique separation patterns over the wing surface. Furthermore, we examine the influence of these dynamics on the sectional forces on the wing. Based on the present parametric study, we map out the behavior of the wake over the angle of attack and aspect ratio. 
Monday, November 19, 2018 11:01AM  11:14AM 
G16.00003: Abstract Withdrawn

Monday, November 19, 2018 11:14AM  11:27AM 
G16.00004: Aerodynamic interaction of collective plates in sidebyside arrangement Dongwook Kim, Seung Hun Lee, Daegyoum Kim In tipreversal upstrokes of bird flight, primary feathers twist individually and make gaps between them. Although this behavior was reported to allow individual feathers to function as individual airfoils, lift contribution of each feather and interaction of flow between feathers with a gap still remain unclear. To elucidate aerodynamic characteristics of each feather in tipreversal upstrokes, we conduct numerical simulation at Reynolds numbers 100 and 1000. We model feathers as collective plates. The angle of attack of the collective plates is fixed at 90°, and the gap distance and the angle of attack of each plate are varied. Lift of the plates generally increases from the bottom plate to the top one. In some specific cases, lift coefficient and lifttodrag ratio of the collective plates are higher than those of a single plate. These cases also exhibit low unsteadiness of the flow and low fluctuation of the lift. The model of our study may be able to be applied to micro air vehicle in that collective plates can have the better aerodynamic performance than that of a single plate. In addition, this study can give insights into understanding the principles of active flow control using multibody system for the purpose of enhancing maneuverability. 
Monday, November 19, 2018 11:27AM  11:40AM 
G16.00005: TurbulenceDriven Reverse Lift on 2D and 3D Bodies with Deflected Tails Yaqing Jin, Shifeng Fu, Sheikh Tahmid Farhan, Leonardo P. Chamorro Deflection of tails is a basic strategy used by many species and manmade structures to induce drift. This process is often performed under flows with a variety of turbulence levels, which may induce distinctive force. Using laboratory experiments and simplified theoretical arguments, we show that the level of turbulence may reverse the direction of the mean lift on two and threedimensional structures with relatively short, deflected tails. Planar particle image velocimetry and a highresolution load cell were used to characterize the near wake region and the instantaneous lift of the bodytail assembly for various geometric configurations, tail angles and turbulence levels at Reynolds number Re=2 X 10^4 based on the body width. The possibility of reverse lift may occur within a critical deflection angle depending on the tail length and turbulence level, which induced changes in the backwardflow region. This particular phenomenon is explained quantitatively with a simple formulation that accounts for the effects of the body geometry and turbulence. The uncovered phenomenon offers insight for optimum design of vehicles in various flows. 
Monday, November 19, 2018 11:40AM  11:53AM 
G16.00006: Effect of ThicknesstoChord Ratio on Flow Structure of Nonslender Delta Wing Burak Gulsacan, Mohammad Sharifi Ghazijahani, Gizem Sencan, Mehmet Metin Yavuz The effects of thicknesstochord (t/C) ratio on flow structures of nonslender delta wings with sweep angles of 35 and 45 degrees are characterized in a lowspeed wind tunnel using velocity, pressure, and force measurements. The delta wings with t/C ratios varying from 2 % to 19 % are tested at broad ranges of attack angles and Reynolds numbers. The results indicate that the effect of t/C ratio on flow structure is quite substantial. Considering the low angles of attack where the wings experience leading edge vortex structure, the strength of the vortex structure increases as the t/C ratio increases. However, the wing with lowest t/C ratio has pronounced surface separation at significantly higher angle of attack compared to the wing with highest t/C ratio, which indicates that lowest t/C ratio wing might be more resistive to the stall condition. These results are well supported by the force measurements such that high t/C ratio induces higher lift coefficient, C_{L}, at low attack angles, whereas maximum C_{L} values that can be reached are significantly higher and appears at significantly higher angle of attack for low t/C ratio wings. Considering the C_{L}/C_{D} ratio, increase in t/C ratio induces remarkable drop in C_{L}/C_{D} values. 
Monday, November 19, 2018 11:53AM  12:06PM 
G16.00007: Vortex Interactions on a Long Slender Body at High Angles of Incidence Tufan k Guha, Roopesh Kumar, Rajan Kumar A slender body of revolution experiences large side forces and yaw moments due to flow asymmetry at high angles of incidence. The flow asymmetry is characterized by a pair of asymmetric vortices that form near the nose region; interact with each other and the surfaces downstream. The goal of the present experimental study is to better understand the flow physics associated with vortex asymmetry on long slender bodies at high incidence particularly the initiation, growth, and interaction of vortices. Measurements were carried out on a conecylinder body of semiapex angle 12° and slenderness ratio of 8. Particle Image Velocimetry results show that the vortices which are symmetric over the cone can develop asymmetry at the conecylinder junction. The primary counterrotating vortex pair initiates at the conetip whereas several secondary vortices emanate from the shear layer along the cylindrical body and merge with the primary vortex pair. Various stages of vortex merging were captured in the present study on a long slender body at high angles of incidence. 
Monday, November 19, 2018 12:06PM  12:19PM 
G16.00008: Investigation of the Sweep Independence Principle for transitional regime of the flow past an aerofoil Carlo Suardi, Mohammad Omidyeganeh, Alfredo Pinelli Highly resolved incompressible Large Eddy Simulations of the flow past a NACA4412 aerofoil have been carried out for a chordbased Reynolds number of 50.000 at 5 degrees incidence considering both straight and 30 degrees sweptback infinite wings. Furthermore, two different sets of incoming flow conditions have been studied: one with an incoming laminar flow that naturally transitions to turbulence on the profile, the other with grid generated turbulence superimposed on the incoming flow. The main objective of the study is the verification of the socalled ''independence sweep principle''. This principle suggests that the boundary layer on a swept wing can be analysed using a linear superposition of the straight wing flow with the one generated by the mean spanwise velocity. Detailed results from the transitional laminar case and the one carrying upstream induced turbulence have allowed to establish the validity range of the principle in these two regimes. In particular, the flow with incoming turbulence delivers results on separation, reattachment, mean profiles and Reynolds stresses that can be extrapolated to higher Reynolds number flows. 
Monday, November 19, 2018 12:19PM  12:32PM 
G16.00009: PIV study on flow around leadingedge slat of 30P30N airfoil Yusaku Onishi, Jun Sakakibara We acquired the flow field around a leadingedge slat using PIV and flow visualization. Simultaneously, sound measurement using a microphone was also performed. The leadingedge slat and main wing model was placed in the tunnel. The angle of attack was varied from 4 to 10 degrees, and the free stream velocity was set at 15m/s in maximum. At the slat cusp, a shear layer and vortices are formed. When the vortices convect along the shear layer towards the slat trailing edge, they split finely and then impinge on the slat inner wall. The magnitude of the correlation coefficient between the sound and the vorticity takes the maximum value near the vortex impingement region. In FFT analysis, peak frequencies in the sound spectrum were agreed well with that in the vorticity spectrum and the spectrum of the brightness in the images of the smoke visualization. The first peak frequency is agreed with the natural frequency of the model. The second and third ones are agreed with the movement of the vortices in the slat cove. By reconstruction of the vorticity fluctuation from several POD modes, the trajectory of the vortices is considered to correlate with the peak frequency in the sound spectrum. 
Monday, November 19, 2018 12:32PM  12:45PM 
G16.00010: Mechanism of Transonic Buffet on Symmetric Airfoils at Zero Incidence Angle Guangzhao Zhou, Feng Liu The buffet phenomenon suffered by airfoils in transonic flow has drawn much attention of researchers in the recent decades. For an airfoil with a fixed shape, the transonic buffet occurs at proper Mach numbers, Reynolds numbers, and incidence angles. In this paper, we focus on the mechanism of the buffet in transonic flows around the NACA 0012 airfoil at zero incidence angle, in which case the unsteady flow structures repeat alternatively on the upper and lower surfaces of the airfoil. The discussions are based on computational results obtained with an inhouse code for solving the unsteady Reynoldsaveraged NavierStokes equations (URANS) with a secondorder gaskinetic scheme. The dynamic process in a buffet cycle is presented and analyzed. In addition to the interaction between the shock wave and the boundary layer separation induced by it, the bifurcation of the topological structure composed by the trailing edge and the saddle point around it is found to be responsible for the occurrence and sustaining of the buffet phenomenon. The conclusions can be qualitatively extended to the buffet on other symmetric airfoils at zero or very small incidence angles. 
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