Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session P17: Aerodynamics: General |
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Chair: Haithem Taha, UC Irvine Room: North 131 AB |
Monday, November 22, 2021 4:05PM - 4:18PM Not Participating |
P17.00001: Blade tip geometry modifications to energize the centerline flow through the Rim Driven Thruster Agata Grzyb, Alexander Sullivan, Oleg Goushcha Unlike a conventional thrust-producing fan, which is driven by a shaft located along the centerline, rim driven thruster blades are actuated into a rotational motion at their outer radius. In such a configuration, blades are attached to the outer rim and extend toward the center leaving an unobstructed flow through the centerline region. Previous investigations of the velocity field downstream of a rim driven thruster prototype showed the existence of an accelerated fluid stream in the centerline. In the current study, a numerical simulation is performed to confirm the presence of the accelerated flow region. An attempt is made to modify the blade tip geometry to further energize the flow in the centerline and increase thrust. |
Monday, November 22, 2021 4:18PM - 4:31PM |
P17.00002: Smoke Visualization Over Spinning Cones at Angle of Attack in Axial Flow Abdullah Kuraan, Omer Savas Smoke visualizations over spinning cones with various half angles at angles of attack are carried out to study qualitatively the salient features of flows over spin stabilized and axially symmetric bodies. A smoke wire technique is employed while images are captured with a high speed camera. Flow patterns are studied over a range of spinning cones with half angles θc= 10◦,15◦,22.5◦,30◦,45◦,and 90◦. The lengths of the cones are fixed to 20 cm. The rotational speed ratio, S= ΩRbase/U∞, of the cones are fixed to S= 2, where Ω is the rotational speed of the cone,U∞ is the uniform axial flow, and Rbase is the radius of the base of each cone. A range of angles of attack is studied, 0≤α≤36◦. Dictated by the dimensions of the cones, the Reynolds number is on the order of 10^4 for all cases. The potential flow over all nonrotating cones at zero incidence are reviewed for comparison of the flow field inside and outside of the boundary layer. Good agreement between experiment and theory is observed near the tip of the cone at zero incidence for all cases. Signatures of tip vortices are present in the streakline images captured for shallow cones at high angles of attack. In these cases, the influence of rotation on the vortices are observed. |
Monday, November 22, 2021 4:31PM - 4:44PM |
P17.00003: Experimental Observations of the Boundary Layer and Movement in Varying Topography with Unmanned Aircraft Kerrick Ray, Victoria A Natalie, Jamey D Jacob Desert dunes offer unique geophysical interactions, and these aeolian dominant interactions occur over a significant portion of earth surfaces. We pursued whether the movement of desert dunes can be accurately measured using unmanned aerial vehicle (UAV) data and structure-from-motion as well as tracking boundary layer propagation and change in the topography during the boundary layer interaction. This volatile landscape makes an ideal setup for tracking imagery combined with the effect of atmospheric data. |
Monday, November 22, 2021 4:44PM - 4:57PM |
P17.00004: Quantitative and Qualitative Testing on a Model of the Prandtl-D Research Glider to Validate its Geometry Bradley J Zelenka, Xiaofeng Liu, Erik D Olson As part of an effort to elucidate the mechanisms behind the Prandtl-D research glider's novel aerodynamic behaviors, an experimental study has been designed and is being implemented. The initial test model was found not to behave as was numerically predicted, so the model was rebuilt, and the test setup was carefully and thoroughly recalibrated. The data acquisition system was updated to new hardware and software and the external force balance used for aerodynamic force measurements was recalibrated using a carefully designed, 824-point static calibration. Tests on the rebuilt test model included aerodynamic force measurements, the use of trailing edge streamers, and a series of surface oil flow visualization tests. Force measurements showed that the rebuilt test model behaves very similarly to the original model, having a maximum lift coefficient deficit of greater than 10% as compared to NASA simulations, among other discrepancies. Trailing edge streamers clearly show the position of the trailing vortices on the test model moved inboard precisely at the predicted angle of attack. Ongoing surface oil flow visualizations are being carried out to examine the stalling characteristics of the test model. |
Monday, November 22, 2021 4:57PM - 5:10PM |
P17.00005: Surrogate Modeling of Aerodynamic Flows using Implicit Neural Representations James Duvall, Karthik Duraisamy A framework for surrogate modeling of parametric problems built upon implicit neural representations is applied to the prediction of aerodynamic flow fields. Implicit neural representations parameterize an unknown function, implicitly defined by some set of relations, using neural networks and recent advances in computer graphics have shown the viability of such an approach for geometric shape representations and modeling of boundary value problems. The solution to partial differential equations may be viewed through this lens and the non-linear independent dual system (NIDS) framework leverages these concepts. The method allows for continuous prediction of field variables, independent of discretization and topology. This is in contrast to other methods using convolutional neural networks or snapshot based decomposition, which are mesh dependent, provide predictions only at discrete locations, and may require interpolation. The NIDS framework is used to construct surrogate models for two dimensional RANS flows around bodies of varying geometry. Such models ability to accurately predict flow fields and compute aerodynamic quantities of interest from predictions on seen and unseen geometries and mesh topologies is investigated. |
Monday, November 22, 2021 5:10PM - 5:23PM |
P17.00006: A Variational Theory of Lift Haithem E Taha, Cody Gonzalez In this work, we consider one of the most fundamental questions in fluid mechanics and aerodynamics: lift generation over an airfoil. We exploit a special, less-common, variational principle in analytical mechanics (Hertz' principle of least curvature) to develop a variational analogue of Euler's equations for the dynamics of an ideal fluid. We apply this variational formulation to the classical problem of the flow over an airfoil, which presents, for the first time, an airfoil theory that dispenses with the Kutta condition. The developed variational principle reduces to the Kutta-Zhukovsky condition in the special case of a sharp-edged airfoil, which challenges the accepted wisdom about the Kutta condition being a manifestation of viscous effects. Rather, we found that it represents conservation of momentum. Moreover, the developed variational principle provides, for the first time, a theoretical model for lift over smooth shapes without sharp edges where the Kutta condition is not applicable. It is quite a fundamental result that generalizes a 120 years old theory by developing an original framework of the aerodynamics of lifting bodies into a theoretical mechanics formulation, exploiting and reviving forgotten tools from the history of mechanics (Hertz' principle of least curvature). |
Monday, November 22, 2021 5:23PM - 5:36PM |
P17.00007: Effects of Axial Slots on Airfoils - First Results Mark G Mungal, Noah K Villar Most two-dimensional airfoils typically increase lift and delay stall by use of trailing edge flaps or leading edge slats, or a combination of both. These devices actively open slots along the span of the airfoil allowing flow from the pressure side to the suction side which energizes the boundary layer and prevents or delays separation. In this study we investigate an alternate passive approach which uses axial slots beginning at the sharp trailing edge and extending forward through the airfoil, while repeating along the span. Such slots allow flow from the pressure side to the suction side as angle of attack increases, with each slot producing a pair of counter-rotating vortices on the suction side, thus affecting overall flow separation. Results are obtained by use of STAR-CCM+ computations on a NACA 0012 airfoil at Reynolds numbers of 1.8 million. The code first reproduces existing experimental results (NACA, 1955) on unslotted airfoils from 0 degrees angle of attack (rounded leading edge, sharp trailing edge) to 180 degrees (sharp leading edge, rounded trailing edge), showing the two stall behaviors seen experimentally, then produces new results on axially slotted airfoils. We report the changes to the lift and drag over the same range of angle of attack, the flow separation characteristics, and the effects of varying the length, width and spacing of the axial slots. |
Monday, November 22, 2021 5:36PM - 5:49PM Not Participating |
P17.00008: Multivariate Design Optimization using the Proper Orthogonal Decomposition Method applied to a Low Reynolds Number Airfoil Elizabeth H Krath, Brent C Houchens A multivariate design optimization that utilizes the Proper Orthogonal Decomposition (POD) method is presented and applied to the design of a low Reynolds number airfoil. The airfoil shape is described by seven independent design variables. The optimization procedure modifies the airfoil shape such that lift-over-drag is maximized. To reduce the computational expense of the optimization process, the POD method is used. In the POD method, each state variable is approximated with a linear combination of modal coefficients, which are functions of the design variables, and basis functions, which are functions of space. The optimization method is as follows. First, reference solutions using an appropriate CFD solver are computed across the range of design variables at each reference point. The reference points are placed using Latin Hypercube sampling (LHS). Then, a global POD basis is created from all reference points. Finally, the modal coefficients are computed using multivariate interpolation from neighboring points. The final solution is reconstructed using the global POD basis and the interpolated modal coefficients and the lift-over-drag is calculated. The interpolation process is iterated until an optimum is reached. |
Monday, November 22, 2021 5:49PM - 6:02PM |
P17.00009: Aerodynamic Modeling of Leading-Edge Slat Morphing in Low Reynolds Number Flow Benjamin E Diaz Villa Atmospheric flight is becoming more prevalent and congested from the use of drones for delivery of goods to the increased accessibility of international travel in jumbo jets. Current methods of flight control consist of discrete flaps and slats that decrease flight efficiency and consequently consume more energy and fuel than in their non-deployed state. Wing morphing allows for a non-abrupt motion and smooth contours of the control surface that proves to be a solution to this dilemma. This thesis examines two novel morphing techniques in the leading-edge of a NACA0012 airfoil compared to a conventional slat deployment. Both the second order (quadratic) and third order (cubic) morphing proved to have better aerodynamic performance than the conventional rigid slat between angles of attack (AOA) of 0-14 degrees. Their smooth shape outline that connected to the rest of the airfoil maintained the flow separation point further downstream than their discrete counterpart. However, between the two types of morphing, each excelled at different regimes of AOAs. In addition, this thesis also laid the framework for future analysis in the unsteady motion of these control surfaces, as it proved that the distance at which the far-field boundaries are located from the airfoil plays an important role in the accuracy of the results. In addition, the constants used in the quadratic and cubic deformations have a significant impact on the time-step size by which the unsteady motion of the slats is analyzed. The work on this thesis shows that CFD can be an effective research tool in optimizing the performance of morphing airfoils. |
Monday, November 22, 2021 6:02PM - 6:15PM |
P17.00010: An empirical extension to free-streamline methods: Modeling leading-edge separation Cody Gonzalez, Haithem E Taha Potential-flow methods underpin the classical theory of aerodynamics, yet this under-determined, kinematic framework relies on additional constraints in order to provide physically-correct flowfields. Most famously, the Kutta-Joukowsky condition is applied for attached flows at moderate angles of attack, yet there does not exist a classical method which may predict leading-edge separation. If a separation point is chosen a priori, methods for modeling leading-edge separation in potential flows via free streamlines trace back to the works of Helmholtz, Kirchhoff, Riabouchinsky and others. We reconsider this classical problem, without ad hoc consideration of the trailing-edge and leading-edge geometry, by augmenting the potential-flow solutions with a variational framework providing dynamics. We employ an empirically-derived upper-bound to streamlines of the flow, aided by the maximum modulus principle, in order to establish where the immersed body cannot be a streamline, and likewise use this condition to modify the conformal mapping to one which represents a physically-realizable outer solution. The NACA 0012 and the canonical flat-plate airfoil are analyzed in this framework, and contrasted with classical, empirical, and computational solutions. |
Monday, November 22, 2021 6:15PM - 6:28PM |
P17.00011: Shape optimization based on physics-informed neural networks Yubiao Sun, Ushnish Sengupta, Matthew P Juniper Shape optimization, which involves modelling and optimization of a designed geometry to achieve targeted goals, is a prominent but challenging topic. The complexity and high dimensionality of the search space make some existing methods computationally expensive. In this talk, we propose a physics-informed neural networks (PINN) as a solver for the flow around an object and also a provider of gradient information for shape optimization. In this study a PINN is employed to solve the flow around a cylinder and to optimize the cylinder shape in order to minimize its drag. The point cloud used for training the PINN is adapted using the gradient of the objective functions so that accurate flow fields can be obtained for geometries closer to optimal shape. The proposed model, given adequate training, is capable of efficiently exploring a high dimensional shape space generate optimal shapes without any prior knowledge and in a reasonable cost. We illustrate this for shape optimization to reduce drag around a 2D cylinder at Reynolds number 20. |
Monday, November 22, 2021 6:28PM - 6:41PM |
P17.00012: Simulation of a Wing with Wingtip Devices Using the Actuator Line Method Vitor G Kleine, Ardeshir Hanifi, Dan S Henningson The actuator line method (ALM) was developed to represent blades or wings as lifting lines in numerical simulations of the Navier-Stokes equations. In order to accurately calculate the forces, imposed as body forces along the line, a tip correction is usually applied. Recent advances on the understanding of the vorticity created by the smeared actuator line led to corrections based on complementing the velocity with the difference between the velocity induced by a Lamb-Oseen and a singular vortex. Since these corrections are more physically meaningful, they offer the possibility of simulating wingtip devices. In the present work, we simulate a straight wing with winglets using the code Nek5000 with ALM. It is shown that these corrections miss a relevant effect for this case, even when including the correction due to the bound vortex: when imposing the corrected lift coefficient in the numerical domain, the local generated circulation is inaccurate, due to the difference between the velocity measured in the numerical domain and the corrected velocity. We propose a correction that takes this difference into account. The results of both methods are compared to the lifting line method and the presently proposed correction shows better agreement. |
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