Computational & Theoretical Multiphysics Laboratory

The nucleate boiling process is essential to achieve extreme heat flux in heat exchangers and cooling systems. We have proposed using active vortex generation to manipulate the boiling process dynamics. We have simulated the response of the boiling process in a heat exchanger channel to an oscillating flexible/rigid/hybrid plates and found out the extent of thermal enhancement and effects on dynamics of the vapor bubble. Our preliminary results show that surprisingly, a specific type of active vortex generators may enhance the thermal heat transfer by 500%-1000% much more than other proposed techniques. Considering the impact of active vortex generators compared to the crossflow-only case, we found out that by using a flexible insert, one can reach a 200%-250% increase in the coefficient of performance. The initial results suggest a promising technique to have a paradigm-shift heat transfer enhancement methodology especially for boiling heat transfer in microchannels using minimally invasive piezoelectric or magnetic vortex generators. Further studies will increase our understanding of the critical features of this process, also gives the suitable parameter regimes for experimental studies and practical applications. This may lead to an economical thermal management procedure using a passive system for the real-world applications.

The face mask “fit” affects the mask’s efficacy in preventing airborne transmission. To date, research on the face mask fit has been conducted mainly using experiments on limited subjects. The limited sample size in experimental studies makes it hard to reach a statistical correlation between mask fit and facial features in a population. Here, we employ a novel framework that utilizes a morphable face model and mask deployment simulation to test mask fit for many facial characteristics and mask designs. The proposed technique is an important step toward enabling personalized mask selection with maximum efficacy for society members.

When a shock wave comes into contact with a boundary layer flow, the large adverse pressure gradient associated with the shock wave can cause the flow to separate from the surface. When this happens, a recirculation bubble forms close to the wall, and significantly alters the stability and dynamics of the flow. This shock typically bends as it encounters lower Mach numbers inside the boundary layer and ultimately breaks up into a compression fan and a reflected shock develops. We are doing research on SWBLI with flexible structure focusing on structural load minimization and flow control. Panel dynamics can help us to find a potential use of an aeroelastically tailored flexible panel as a means of passive flow control. Cavity pressure underneath the panel can also create forced panel oscillations which may reduce separation in the interaction zone.

The canonical problem of flow-induced flutter of a thin flexible plate is revisited, with an emphasis on the thermally induced buoyancy effects on the dynamics and thermal characterization of the system. An immersed boundary method is used to simulate mixed convection of a heated 2D inextensible and flexible thin plate. The bending stiffness, Richardson number, and Reynolds number are chosen as the characteristic parameters of the system. The dynamic and thermal responses of the plate are examined over a wide range of the characteristic parameters, and it is shown that the stability boundary growth rate of the flapping dynamics dramatically increases after a particular threshold Richardson number due to the mode switching behavior. The appearance of higher oscillatory modes and a shift in the nodes of the dominant oscillatory mode are found to also be correlated to the observed higher Nusselt numbers.

Wind-induced stress is the major mechanical cause of tree failures. Among different factors, the branching mechanism plays a central role in the stress distribution and stability of trees in windstorms.  The recent study by Eloy showed that Leonardo da Vinci’s original observation stating the total cross-section of branches is conserved across branching nodes is the optimal configuration for resisting wind-induced damage in rigid trees. However, the breaking risk and the optimal branching pattern of trees are also a function of their reconfiguration capabilities and the processes they employ to mitigate high wind-induced stress hotspots. In this study, using an efficient numerical model of rigid and flexible branched trees, we explore the role of flexibility and branching pattern of trees on their reconfiguration and stress mitigation capabilities. We identify the optimal power-law branching mechanism that is robust for a large tree flexibility range. Our results show that the probability of a tree breaking at each level of branching from the stem to terminal foliage depends strongly on both the cross-section changes in the branching nodes, the overall tree geometry, and the level of tree flexibility. It is found that the optimal branching of trees is a function of its deformability and could be different from the rigid trees.

A computational model is used to study the effect of wave-induced motion on the aerodynamics of compliant offshore wind turbines. The wake response of two promising offshore platform concepts, Spar buoy and Barge type turbines were studied in details and their aerodynamic, power and wake characteristics were compared with a stationary wind turbine case. Results obtained from this study indicates that surprisingly the wake response of the oscillating wind turbine recovers faster compared to the stationary turbine, with a 50%wake recovery in a distance that is 33% shorter than the static counterpart.

Aeroelastic effect plays an important role in various research topics including aero vehicle stability, renewable energy extraction, and animal locomotion. Active and passive control methods have been proposed to control the flutter phenomenon of the airfoil. For example, the EET high-lift flexible wing with actively bending flaps provides an active actuator that can modify the flow around the airfoil. Through the use of a high-fidelity fluid-structure interaction algorithm we can investigate the effect on the aeroelastic motion of the EET airfoil over a wide range of parameters. Preliminary results show that the active flap is capable of regulating the oscillation period of the airfoil. The simulations can provide physical insight behind the highly nonlinear motion, and eventually derive the control law to regulate the oscillation.

Cryogenic fluids are one of the critical components of current and future space exploration, and a better understanding of cryogenic flow is necessary for safe and efficient transport and storage of cryogenic fluids. This project aims to develop and employ novel modeling and analysis tools for capturing the flow physics and thermodynamics of cryogenic flows in storage vessels in both normal and microgravity conditions. The flow and thermal interaction of cryogenic systems with three phases of the flow, gas, and solid boundaries can generate a rich spectrum of phenomena. To accurately model the system while keeping the running time much lower than the conventional CFD approaches, the block-structured adaptive mesh refinement (AMR) is using. This project's main innovation is the development of an AMR-based computational tool based on the integration of the continuum multiphase-phase model of the cryogenic and the multi-node model of the system. The approach provides high-fidelity modeling of the complex coupled dynamics while maintaining the computational efficiency of nodal models.

Traditional structural health monitoring (SHM) techniques are based on the strong assumption that the acting loads are either absent or stationary. In many high-speed applications, these criteria are not met, and a more versatile SHM method is required for their monitoring. In this work, we perform extended wavelet-based structural health monitoring using time histories of the embedded impedance-based piezoelectric sensors and the physics-based identification of the causal environmental loads. This technique is the first effort to extend the health monitoring to unsteady short-time load scenarios and use the physics-based force-partitioning technique for SHM under complex loading conditions. To perform SHM in the complex loading condition using impedance-based techniques, we include the mechanistic causal model of the flow forces in the identification procedure. This project breaks new ground in developing and employing a novel multi-physical modeling framework in which both the load conditions and structural responses are monitored simultaneously. Its success in capturing the structural damage is assessed numerically and experimentally for two types of damages, cracking and delamination, under two distinct loading conditions, high thermal loading and shock impingement.

Long slender organisms demonstrate remarkable proficiency in varied terrain and especially water. The success of these organisms has proven a source of bio-inspired design in various fields especially robotics. Recent advances in robotic fabrication techniques have led to a new class of "soft robots". Soft robots are made using highly compliant materials and actuated using pneumatics, electromagnetism and tendons. Because of the low elastic modulus for the materials used to construct soft robots, they are inherently back drivable and compatible for interacting with humans and animals. These same properties, many degrees of freedom and under-actuation, create challenges in the area of modeling and control of soft robots. We address these challenges by modeling a long, slender, soft robotic swimmer as a visco-elastic rod whose motion is governed by the partial differential equations of the same. We use Lighthill's High Amplitude Elongated body theory to model the fluid structure interaction with high accuracy and a relatively low computational cost. We model the actuation of swimmers body using a parameterized internal torque linear density, and co-optimize the design and control of the soft swimming robot in terms of locations, size and number of actuators.

The transformation of wind energy into low-power electricity using piezoelectric materials enables the possibility of powering wireless electronic components especially in high wind areas. Here, we investigate the piezoelectric energy harvesting performance of inverted flags with different aspect ratio subject to unidirectional flow. Flags with different aspect ratios were studied both numerically and experimentally to explore the different oscillatory modes of the system and their different energy harvesting capability. Each flag is intrinsically coupled with the piezoelectric patches attached to its surfaces. As the piezo patches deform with the inverted flag, they generate electrical power which is dependent on the flow, structural and electrical parameters of the problem. Experiments on flags made of spring steel were conducted in a wind tunnel, where the wind speed was swept up through the various vibration modes of the inverted flags. The roles of flow conditions, structural parameters and electric setup on the oscillatory behavior and power capturing efficiency of the inverted flag were assessed and preliminary results show that the aspect ratio of the flag can be leveraged to increase the energy harvesting attainable during large amplitude two-sided flapping modes.

Inspired by the tree leaves' problem, the flexible flags problem is studied. We run a series of simulation cases for the FSI problem. The computational results are first compared to the wind tunnel experiment that uses stainless steel sheets as flags and then is employed to quantify how the flag aspect ratio and separation distance between the flags affect the flapping behavior. Also place the flags in a star pattern by rotating them from a center. The frequency, Reynolds number, drag, side, and lift forces are important parameters during the observation. Compared with the 2-D model, it appears more easily to collect results on the amplitude and rotation angle on a particular point of the flags and the vortex distribution with the 3D model. The finding is important when discussing the possibility of using a tree morphology with many flexible piezoelectric leaves to harvest energy from the wind collectively.