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 M14: Aerodynamics: General |
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Chair: Vibhav Durgesh, California State University, Northridge Room: Georgia World Congress Center B301 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M14.00001: Surprises and disagreements in wing performance at moderate Reynolds number Geoffrey Spedding, Joseph D Tank, Bjoern Klose, Gustaaf Jacobs Due to the extreme sensitivity of the global flow state to small differences in geometry and/or environmental conditions, the state of the current technical literature in moderate Reynolds number aerodynamics can most charitably be described as in flux. Reference data from either experiment or computation are not readily available, and in many cases do not agree. Here we focus attention on a single case of the NACA 65(1)-412 airfoil at chord Reynolds number of 20 000. A combination of information from DNS, wind tunnel, and water-channel experiments will be described. Abrupt transitions between attracting flow states open the possibility of efficient control with large gain from small input disturbance amplitudes. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M14.00002: The underlying mechanism behind the bursting and reformation cycle of the laminar separation bubble over a NACA-0012 at the inception of stall Eltayeb M. ElJack, Julio Soria The mechanism behind the bursting and reformation cycle of the laminar separation bubble (LSB) over a NACA-0012 at Rec = 5 x 104, Ma∞ = 0.4, and various angles of attack at the inception of stall is investigated numerically using Large Eddy Simulation (LES). It is shown that a triad of three vortices is behind the quasi-periodic self-sustained bursting and reformation of the LSB and its associated low-frequency flow oscillation (LFO). A global oscillation in the flow-field around the aerofoil is observed in all of the investigated angles of attack. The flow switches between an attached-phase against an adverse pressure gradient (APG) and a separated-phase despite a favourable pressure gradient (FPG) in a periodic manner with some disturbed cycles. When the direction of the oscillating-flow is clockwise, it adds momentum to the boundary layer and helps it to remain attached against the APG and vice versa. The dynamics of the flow and how the triad of vortices drives and sustains the global oscillation in the flow-field are discusseda. aarXiv:1807.09199 |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M14.00003: Efficiency enhancement of a self-propelled pitching profile using non-sinusoidal trajectories M. Mekadem, E. Chihani, H. Oualli, S. Hanchi, A. Bouabdallah, M. Gad-el-Hak A symmetrical profile is subjected to non-sinusoidal pitching motion. The airfoil has a chord length c=0.006 m and a semi-circular leading edge with a diameter of d=0.001 m. The extrados and intrados are two straight lines that intersect at a tapered trailing edge, and the pitching pivot point is positioned at the leading edge. The pitching frequency is in the range of 1<f<190 Hz, while the tangential amplitude of the flapping trailing edge varies from 18% to 114% of the foil cord. To improve the airfoil propulsive performance, two-dimensional numerical simulations are implemented on FLUENT. The Reynolds number based upon the maximum profile thickness t varies in the range of 35<Re<210, comparable to insect’s Reynolds numbers. The foil movement is executed using a dynamic mesh technique and a user defined function (UDF). The adopted mesh has 70,445 nodes with 5,1960 quadrilateral cells. The results are in good agreement with prior experiments, and, compared to sinusoidal oscillations, show that non-sinusoidal flapping trajectories lead to advancing velocity increase of 550%. Additionally, if improved propulsive efficiency is sought, non-sinusoidal flapping lead to better thrust. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M14.00004: Experimental Study of Flow Separation over a NACA 0012 Airfoil at Low Reynolds Number with Freestream Turbulence Ja'Kira Jackson, John Hrynuk The influence of freestream turbulence on flow separation over a NACA0012 airfoil at low Reynolds number (Re=12,000) was investigated. Particle Image Velocimetry (PIV) experiments were performed at an angle of attack 0˚ ≤ α ≤ 15˚. Four grid turbulence generators of different sizes were used to introduce turbulence into the flow. PIV was used to roughly characterize turbulence levels of the flow. Results have shown there to be approximately 2%-4% freestream turbulence intensity provided by the turbulence generators. The wind tunnel baseline turbulence was approximately 0.1-0.3%. Results have shown that by introducing turbulence, vorticity in the boundary layer increased and stall was delayed. Flow fields for the baseline and different turbulence generators will be compared with the wing at varying angle of attack. Preliminary results have shown the flow around the wing in the no-turbulence case was separated at 8º, but the flow remained attached when a small amount of turbulence was introduced. These results have implications for the separation behavior and stall characteristics for small UAVs flying at low Reynolds numbers. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M14.00005: Aerodynamic Performance of NACA-0012 Airfoil at Low Reynolds Numbers in Wake of Bluff Body Vibhav Durgesh, Rodrigo Padilla, Hamid Johari The aerodynamic performance of airfoils is adversely impacted when operating in a bluff-body wake. This investigation focused on understanding the impact of larger-scale structures on the aerodynamic performance of the NACA-0012 airfoil when operating in the wake of a rectangular bluff body. The measurements for this study were performed in the water tunnel facility with a Reynolds number of 40,000. A 6-axis force/torque transducer was used for measuring force and moment on the airfoil, and Particle Image Velocimetry was used to quantify the inflow velocity field. The results indicated that there is a significant reduction in lift coefficients with an increase in the drag coefficients experienced by the airfoil when operating in the wake of bluff body. Moreover, the vortex shed by the bluff body also impacts the lift force spectra, showing a correlation between the observed flow structures and instantaneous load on the airfoil. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M14.00006: Laminar separation delay by optimal streaks Michael Karp, M. J. Philipp Hack Boundary layers are prone to separation when subjected to adverse pressure gradients. Flow separation often reduces the performance of aerodynamic and hydrodynamic vehicles. Vortex generators have been utilized successfully as passive flow control devices; however they often trigger turbulence, which increases drag and heat transfer. Moreover, finding the optimal parameters for their design is oftentimes based on trial and error. In this study we seek by means of nonlinear optimization the velocity disturbance at a given upstream position, which maximizes the downstream location of the separation point. Our approach leverages insights gained from the non-modal growth of disturbances in boundary layers and their role in generating a mean-flow distortion which augments the shear at the wall. Optimal laminar separation delay is achieved for a disturbance described by counter-rotating vortices, which generate streamwise streaks downstream. The optimal spanwise wavelength of the vortices is found and its connection to the mean-flow distortion is discussed. |
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M14.00007: Abstract Withdrawn
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Tuesday, November 20, 2018 9:18AM - 9:31AM |
M14.00008: Drag on flat plates of arbitrary porosity Konstantinos Steiros, Marcus Hultmark An analytical model for the prediction of the drag on a plate of arbitrary porosity (actuator disk) is introduced. The flow kinematics are determined using potential flow theory, along with several conditions to render the flow field realistic. Drag is then calculated under the constraints of mass, momentum and energy balance. The predictions exhibit convincing agreement with experimental observations over a wide range of porosities, including the solid plate case, as long as shedding is absent or suppressed. The developed model offers better predictions of the drag and wake velocities compared to the classical Betz model, and therefore can replace the latter in blade element momentum theory (BEM) algorithms, widely used for the prediction of wind-turbine loads and power generation. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M14.00009: Cylinder Ice Accretion with Application to UAS Alyssa Avery, Jamey D Jacob As unmanned aircraft begin to fill the roles for many flight needs, the impact of icing on unmanned aircraft (UA) needs to be explored. Small UAS (SUAS) are usually incapable of supporting anti-aircraft systems and are may be required to fly in hazardous icing regions. In order to further the understanding of the ice accretion at the range of Reynolds numbers, a simple model is created specifically suited to the conditions expected for SUAS in icing conditions. The code calculates heat flux based on its atmospheric conditions and uses empirical equations to get collection efficiency based on the flow’s amount of deviation from Stoke’s Law, CD Re/24 = 1. For this range of Re, Nusselt number is uncertain. The flow is not yet transcritical and lands in an unpredictable range of results. Simulations use both the smooth cylinder expression and the pre-critical Nusselt number equation for comparison. This study will run an exhaustive set of simulations within the Weber and Reynolds numbers prescribed, 0.2(106)- 0.5(106) and 0.3 (106)- 0 0.8(106) respectively, and the environmental conditions that are known to create aircraft icing.
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Tuesday, November 20, 2018 9:44AM - 9:57AM |
M14.00010: Straight flight of meteorites Khunsa Amin, Jinzi Mac Huang, Jun Zhang, Leif Ristroph Meteorites are fascinating for the information they bring as alien visitors to Earth, and their shapes also give clues to their fiery flight through the atmosphere. Meteors are melted and reshaped by aerodynamic heating, and while most of the resulting meteorites are rather arbitrarily shaped, a surprisingly large number resemble nearly prefect cones. Using lab experiments on model meteorites of various shapes, we show that these conical forms are distinguished by their robust stability during straight flight through a fluid. We use our findings to suggest possible origin stories for conical meteorites, whose final shapes reflect the simultaneous aerodynamic processes of reshaping and reorientation during flight. |
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