The Belgian National Committee for Theoretical and Applied Mechanics (NCTAM) is pleased to announce that the 2020 edition of the Award distinguished a young scientist from iMMC (UCLouvain) who has recently completed a PhD thesis in the field of Mechanics.
Denis-Gabriel Caprace received the NCTAM award with his PhD thesis entitled "Modeling of lifting-dragging devices for large eddy simulation of space-developing wakes : application to wings, rotors and formation flight”(prom. : Winckelmans, Grégoire ; Chatelain, Philippe, 02/09/2020).Image source (The appearance of U.S. Department of Defense (DoD) visual information does not imply or constitute DoD endorsement)
Summary of the thesis
When an object generates lift as a result of its motion through the air, it leaves a typical signature in its wake invariably featuring two parallel vortices. These structures are of great interest: (1) fundamentally considering their prevalence in fluid dynamics, (2) industrially for the critical impact they have on current air traffic, and (3) technologically for the challenge they pose in the development of urban air mobility. This thesis is devoted to the study of the formation of such vortices in the wake of wings and helicopter rotors, and to the development of the numerical methods upon which the related analyses depend.
Wake vortices generally result from the roll-up of vortex sheets. We aim to simulate this entire process over space and time, starting with the shedding of vorticity from the aerodynamic devices (wings and blades). The ability to simulate the wake roll-up in full is here crucial, as much of the phenomena under investigation will occur during that phase. To this end, we thus work in a space-developing context, and we aim to develop and to validate a novel mollified immersed lifting-dragging line model, implemented in a vortex particle-mesh method which provides the necessary large eddy simulation capabilities.
Taking advantage of this computational tool, we then address the following research questions:
- What are the fundamental similarities and dissimilarities between the wake vortices shed by a wing and by a helicopter rotor in advancing flight? How to characterize them?
- How do the vortex instabilities of the near wake (should they originate in parasitic drag or in other complex 3-D phenomena) affect the roll-up process and the state of the far wake? Precisely, what is the role of the parasitic (i.e. profile) drag of the airfoil in this process?
- Can we design an efficient, reliable, affordable, stand-alone approach for an aircraft to sense the wake vortices shed by another aircraft?
- What are the various effects of the mollification in a numerical method based on a lifting or an actuator line model?
The originality of the computational framework developed in this thesis resides in its hybrid Lagrangian-Eulerian character which proves adequate for the study of large advection-dominated problems, on massively parallel computational architectures. One major methodological enabler that we brought in this work is the extension of the Prandtl Lifting Line to account for a mollification parameter, and for the parasitic drag of the airfoil, as we foresaw their role in the answer to some of the above questions. The resulting mollified immersed lifting and dragging line takes care of the shedding of the lift and drag-related vorticity in the vortex sheets and, most importantly, preserves the Lagrangian property of the method. Additionally, we developed, tested, and deployed a new software library (called FLUPS) to efficiently compute the solution of Poisson equations with unbounded boundary conditions, a necessary ingredient to our simulation method.
Despite their inherently different generation processes, the far wakes of wings and rotors appear similar. Exploiting our simulation results, we investigated their universal character, unveiling some intriguing specificities which depend on the device operating conditions. Focusing on the three-dimensional effects affecting the wake development, we emphasized the important role of vortex interactions and instabilities in the alignment and in the establishment of a turbulent two-vortex system statistically at equilibrium (such as the one visible in Fig. 1, in the far wake). We also identified the airfoil parasitic drag as the leading cause of vortex wandering, one of such instabilities. Finally, taking advantage of the present numerical framework, we designed multiple wake sensing strategies for airplane formation flight, a technique that could one day help commercial airliners save energy.