Session A04: Aerodynamics: Fluid-Structure Interactions, General

On vortex rings and Vogel exponents

In heavy winds, plants bend and align themselves with the flow. These passive behaviors allow plants to resist being uprooted by reducing their drag. Here, we study the drag reduction of “leaves”, approximated by flexible disks with radial cuts. The disks are translated vertically upwards through water and bend axisymmetrically in response to the motion. We measure the drag on the deforming disks using a high precision load cell. Existing models that predict the drag on flexible disks assume a uniform pressure distribution in their wake. Spatially resolved velocity field measurements show that the pressure distribution is not uniform, as an axisymmetric vortex ring forms beneath the deforming disks. The vortex size, circulation, and non-dimensional energy, which is a proxy for its stability, are modified by the disk’s deformation during the motion and directly influence the drag. Using these time-varying quantities, we derive an improved model for the drag reduction experienced by flexible disks. We further extend our model to disks with varying numbers of radial cuts, which fold and bend differently during the same motion. The vortex-structure interactions highlighted in our work can guide the design of robust, flexible components for drones and other aerial vehicles.   

On the turbulent flow dynamics in flexible emergent canopies

Experimental investigations concerning the turbulent characteristics of flow within flexible emergent canopies were conducted in the water channel facility in UT Dallas. The canopies were mimicked through a staggered arrangement of single flexible blades of 5 mm width and frontal area per unit volume, a varying from 0.02 to 0.08 cm^-1. The relative fluid forcing to the restoring forces in the blade was achieved by varying the channel-to-canopy height ratio and incoming velocities. The dynamics of fluid flow was captured using time-resolved particle image velocimetry and the plate deformation was characterized through high-speed cameras. The results indicate that the deformation of flexible vegetation highly modulated form drag, turbulence intensity levels and turbulent eddy scales, and the velocity statistics exhibited a distinctive variation when the vegetation posture altered from emergent to submerged conditions with the growth of water flow speed. A revised Cauchy number was developed to incorporate the emergent canopy effects on blade posture.

    

Building in reverse: a design method for highly deformable fluid-loaded structures

Advances in energy storage, navigation, and computational power enable smaller and highly capable unmanned systems. As system size decreases, highly deformable subsystems, whose morphology changes with the operating point of the system become useful. Tolerance and clever use of flexibility can allow us to build lighter wings, passively morphing rotorcraft, and better control surfaces. Designing highly flexible craft tends to rely on the repeated solution of the forward problem, coupled to some optimization. As the dimension of the search space increases, this technique becomes more costly, and the solution is constrained by the parameterization chosen by the designer. In this talk, we show a design method based on analyzing a closed inverse problem, which produces interesting solutions to this design problem on a number of sample subsystems: flexible multirotor drones, fish-like propulsors, and deformable fins.   

Dynamics of a passively flapping paper airplane

Paper airplanes are commonly made to be good gliders. Interestingly, some can also be made to flap while flying. We study the dynamics of such a flapping paper airplane by John Collins named the “Bat Plane”. Flight experiments were recorded with 2 high speed cameras for 3D reconstruction of the paper airplanes in flight by tracking marker points. The paper airplanes were observed to have a periodic body pitch angle in sync with the flapping motion during flight. We propose displacement of the center of pressure (CoP) location about the center of mass (CoM) causing change in the direction of the aerodynamic moment about CoM. Steady state measurements of lift, drag and aerodynamic moments in a wind tunnel are used to explain this hypothesis. Moreover, the flapping frequency of these paper airplanes were observed to increase upon shortening wing dimensions (chord and span) and increasing material thickness. The flapping frequencies do not match with the corresponding natural frequency of the wing. Additionally, paper airplanes made with sufficiently thick paper and reduced wing dimensions were observed to transition to gliding. We attempt a theoretical model to understand the difference in the frequencies and the gliding transition.   

Unraveling the Complexity of Flapping Flight in Bats via High-Fidelity Fluid-Structure Interaction Modeling

Bats are amongst the most agile of natural flyers, and this agility can be attributed to the anatomy and kinematics of their wings. The wing consists of a highly elastic membrane stretched upon a skeleton with multiple joints, and the degree of articulation in the bat wing is incomparable amongst extant natural flyers, allowing for highly complex maneuvers. Previous studies have noted a high degree of dimensionality in the wing joint kinematics across bat species and flight conditions, and understanding this complexity and its implications for aerodynamics and aerodynamic force generation could be useful in developing bat-inspired micro aerial vehicles. In this work, we use modal decomposition to extract the spatio-temporal patterns from a collection of experimentally measured joint kinematics for different bat species and flight conditions. The extracted patterns are used to develop a data-driven model to generate representative joint kinematics for various flight conditions. The model serves as input to our coupled fluid-structure interaction solver, ViCar3D, and the force partitioning method (FPM) to quantify the role of wing kinematics and vortex dynamics on the aerodynamic forces in bat flight. By combining modal decomposition, data-driven modeling, fluid-structure interaction, and force partitioning techniques, our research aims to deepen our understanding of bat flight mechanics and pave the way for developing advanced biomimetic micro aerial vehicles.   

Full-Scale Flow Visualization of the Maneuvering Racing Sailboat

While turning a sailboat through the wind, Olympic sailors will intentionally roll the boat to the point of near capsize – either executing a “roll tack” when sailing upwind or a “roll gybe” when sailing downwind. To modulate heel angle on the boat and avoid capsize, the sailor will shift their bodyweight to the opposite side of the boat’s longitudinal axis during the motion. These rolling maneuvers effectively “whip” the mast and sail across the boat, producing a forward propulsive force. In generally light conditions, the proper execution of these techniques allows the sailboat to propel itself faster than under steady wind alone – a critical advantage while racing. On Lake Cayuga’s waters, we study the kinematics underpinning these maneuvers by outfitting an Olympic Laser sailboat with IMU and GPS sensors and an array of GoPro cameras. Comparisons are made between the performance of the steady sailing boat and the flat or rolling tack and gybe. The vortex dynamics associated with the sail’s unsteady whipping motion is visualized at full-scale on the water by sailing the boat within a cloud of smoke produced by smoke canister and captured in footage via flying drone camera.   

Structural Design of Composite Aircraft Based on Static Aeroelastic Analysis

In this research article, the structural sizing and weight reduction of an airplane structure have been effectively carried out using a Two-way FSI analysis and carbon fiber-reinforced plastic (CFRP) material. Since the maximum percentage of an airplane’s weight is carried by the fuselage and main wing so in this manuscript the structural sizing and weight reduction is mainly conducted for these parts. Here the model assumes to be B737 and the composite material (T800S/3900-2B) used as a structural material is similar to the B787. In two-way FSI analysis, coupled-field method is used to exchange of information (load and deformation) between the fluid (CFD) and structural (CSD) solvers. CFD analysis has been conducted to estimate the aerodynamic load on the structure by the main wing while structural sizing has been carried out on the fuselage skin thickness using inertia relief with buckling analysis. In this model, the load between the main wing and fuselage is transferred by the RBE3 element. CROD and CBEAM element has been used for the stringer and frame while CSHELL element is used for the spar, floor beam, skin, keel beam, and bulkhead. In previous studies, CFRPs are applied for a maximum of 50% of the whole structure while in this study CFRP has been employed for the whole structure. The outcome of this study expedites that the weight of the airplane design can be reduced by 20-25%.    

Examining the interplay between body shape and stiffness for improving water entry

When a high-velocity rigid body breaches the water surface, the resulting impulse force, while brief,

is significantly high, and often damaging to the body. Here, experimental results show that a segmented

body with a single programmable stiffness element coupled with a nose cone and cylindrical after-body, can

mitigate high-velocity impact forces. Programming the stiffness allows the body to increase the impulse force

duration, and thus, alter the severity of the impact. Utilizing experimental prototypes with 25°, 45°, and

60°half-angle cone shapes, our findings show that the impulse force time decay and cavity shape evolution

can be manipulated directly by varying the stiffness between the segments. The study also demonstrates

the importance of the interplay between the cone shape and body stiffness. Notably, a body with 60°cone

exhibits the highest reduction in maximal impact acceleration, approximately 60% from the initial value of

17.0 g to 6.5 g for the rigid and a spring case, respectively. A preliminary discrete dynamic model is provided

for estimating the impulse forces for bodies with and without segmentation. The model shows reasonable

agreement with the experimental results.   

Statistical properties of the low-frequency flow oscillations over an airfoil near stall

We investigate the stochastic behavior of the aerodynamic force fluctuations on an LRN(1)-1007 airfoil operating at a chord-based Reynolds number of 10⁵. Our focus is on angles of attack near stall conditions, where low-frequency flow oscillations can occur. We find that the conditional probability distribution of the lift fluctuations obeys the Chapman–Kolmogorov equation. We confirm through a three-point joint probability check that the lift fluctuations follow a Markov process, and we estimate the Markov time scale using the chi-squared test.  The Kramers–Moyal coefficients of the master equation indicate that only the drift and diffusion terms contribute significantly to the stochastic processes in this system, implying that the Fokker–Planck equation governs the evolution of the probability density functions. Our results show that the stationary solution of the Fokker–Planck equation can accurately predict the empirical probability density functions. These findings provide insight into the stochastic behavior of airfoil systems and may have practical implications for the design and optimization of aircraft.