Aerodynamics | EPFL Graph Search

Aerodynamics (ἀήρ aero (air) + δυναμική (dynamics)) is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an important domain of study in aeronautics. The term aerodynamics is often used synonymously with gas dynamics, the difference being that "gas dynamics" applies to the study of the motion of all gases, and is not limited to air. The formal study of aerodynamics began in the modern sense in the eighteenth century, although observations of fundamental concepts such as aerodynamic drag were recorded much earlier. Most of the early efforts in aerodynamics were directed toward achieving heavier-than-air flight, which was first demonstrated by Otto Lilienthal in 1891. Since then, the use of aerodynamics through mathematical analysis, empirical approximations, wind tunnel experimentation, and computer simulations has formed a rational basis for the development of heavier-than-air flight and a number of other technologies. Recent work in aerodynamics has focused on issues related to compressible flow, turbulence, and boundary layers and has become increasingly computational in nature. History of aerodynamics Modern aerodynamics only dates back to the seventeenth century, but aerodynamic forces have been harnessed by humans for thousands of years in sailboats and windmills, and images and stories of flight appear throughout recorded history, such as the Ancient Greek legend of Icarus and Daedalus. Fundamental concepts of continuum, drag, and pressure gradients appear in the work of Aristotle and Archimedes. In 1726, Sir Isaac Newton became the first person to develop a theory of air resistance, making him one of the first aerodynamicists. Dutch-Swiss mathematician Daniel Bernoulli followed in 1738 with Hydrodynamica in which he described a fundamental relationship between pressure, density, and flow velocity for incompressible flow known today as Bernoulli's principle, which provides one method for calculating aerodynamic lift.

Aeroelastic characterisation of a bio-inspired flapping membrane wing

Karen Ann J Mulleners, Alexander Gehrke, Esra Uksul, Jules Ange Philippe Richeux

Natural fliers like bats exploit the complex fluid-structure interaction between their flexible membrane wings and the air with great ease. Yet, replicating and scaling the balance between the structural and fluid-dynamical parameters of unsteady membrane wings for engineering applications remains challenging. In this study, we introduce a novel bio-inspired membrane wing design and systematically investigate the fluid-structure interactions of flapping membrane wings. The membrane wing can passively camber, and its leading and trailing edges rotate with respect to the stroke plane. We find optimal combinations of the membrane properties and flapping kinematics that out-perform their rigid counterparts both in terms of increased stroke-average lift and efficiency, but the improvements are not persistent over the entire input parameter space. The lift and efficiency optima occur at different angles of attack and effective membrane stiffnesses which we characterise with the aeroelastic number. At optimal aeroelastic numbers, the membrane has a moderate camber between 15% and 20% and its leading and trailing edges align favourably with the flow. Higher camber at lower aeroelastic numbers leads to reduced aerodynamic performance due to negative angles of attack at the leading edge and an over-rotation of the trailing edge. Most of the performance gain of the membrane wings with respect to rigid wings is achieved in the second half of the stroke when the wing is decelerating. The stroke-maximum camber is reached around mid-stroke but is sustained during most of the remainder of the stroke which leads to an increase in lift and a reduction in power. Our results show that combining the effect of variable stiffness and angle of attack variation can significantly enhance the aerodynamic performance of membrane wings and has the potential to improve the control capabilities of micro air vehicles.

IOP Publishing Ltd2022

Effects of wing kinematics, corrugation, and clap-and-fling on aerodynamic efficiency of a hovering insect-inspired flapping-wing micro air vehicle

Hoang Vu Phan, Khanh Nguyen

A flapping-wing micro air vehicle (FW-MAV) operating with aerodynamically optimal wing configuration and kinematics may save energy and thus prolong flight time. In this work, we use a computational-fluid dynamic method to investigate the effects of wing kinematics, corrugation structures, and clap-and-fling on the aerodynamic efficiency of our hovering two-winged FW-MAV (KUBeetle). From the measured reference wing kinematics, we generated several different wing kinematics, considering the effect of spanwise twist and chordwise camber that produce high lift-to-drag ratio (L/D). Among the investigated cases, the modified wing kinematics version 3, which includes both camber and twist with an average angle of attack of about 37 degrees, was selected, because of its similar to 24% improvement of L/D, while maintaining similar lift to the measured reference wing kinematics. The results also showed that the camber plays a role in the improvement of both lift and L/D, which improvements are approximately (16.7 and 10.6)%, respectively. We then used wing kinematics version 3 to investigate the effects of various leading-edge corrugation structures. Based on the results of lift and L/D, we proposed a wing with distributed wing corrugations along the wingspan, which slightly augments the L/D by 2%. In addition, to see how the clap-and-fling behavior contributes to the aerodynamic efficiency, its effects on lift and drag generation were examined. We found that the clap-and-fling enhanced lift by 5%, but increased drag by 9%, resulting in a 4% reduction of the L/D for both the measured and the modified wing kinematics. Thus, the lift-augmented clap-and-fling is inefficient for FW-MAVs. Finally, the study confirmed that the wing with distributed wing corrugations using wing kinematics version 3 without clap-and-fling presented at the stroke reversals is preferable for the high aerodynamic efficiency of the KUBeetle robot, with 31% improvement in L/D. (C) 2021 Elsevier Masson SAS. All rights reserved.

ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER2021

Accurate buoyancy and drag force models to predict particle segregation in vibrofluidized beds

Mehrdad Kiani Oshtorjani

The segregation of large intruders in an agitated granular system is of high practical relevance, yet the accurate modeling of the segregation (lift) force is challenging as a general formulation of a granular equivalent of a buoyancy force remains elusive. Here, we critically assess the validity of a granular buoyancy model using a generalization of the Archimedean formulation that has been proposed very recently for chute flows. The first model system studied is a convection-free vibrated system, allowing us to calculate the buoyancy force through three different approaches, i.e., a generalization of the Archimedean formulation, the spring force of a virtual spring, and through the granular pressure field. The buoyancy forces obtained through these three approaches agree very well, providing strong evidence for the validity of the generalization of the Archimedean formulation of the buoyancy force which only requires an expression for the solid fraction of the intruder, hence allowing for a computationally less demanding calculation of the buoyancy force as coarse graining is avoided. In a second step, convection is introduced as a further complication to the granular system. In such a system, the lift force is composed of granular buoyancy and a drag force. Using a drag model for the slow-velocity regime, the lift force, directly measured through a virtual spring, can be predicted accurately by adding a granular drag force to the generalization of the Archimedean formulation of the granular buoyancy. The developed lift force model allows us to rationalize the dependence of the lift force on the density of the bed particles and the intruder diameter, the independence of the lift force on the intruder diameter, and the independence of the lift force on the intruder density and the vibration strength (once a critical value is exceeded).

AMER PHYSICAL SOC2021