Jonghoon Bin

High-Fidelity Numerical Simulation of a Chevron Nozzle Jet Flow

We report results from the simulation of a moderate Reynolds number cold jet flow exhausting from a chevron nozzle with six symmetric chevrons that have an approximately 18-degree penetration angle. The flow inside the nozzle geometry and the free jet flow outside are computed simultaneously by a high-order accurate, multi-block, large eddy simulation code with overset grid capability. The simulation is performed on 400 million grid points using 2048 processor cores in parallel. The main emphasis of the simulation is to compute the jet flow in maximum detail possible and accurately capture the physical processes that lead to noise generation. It is especially critical to capture the enhanced shear layer mixing due to chevrons that takes place within the first few diameters downstream of the nozzle exit. Our calculations resolve the jet flow field at an unprecedented level of detail. Despite some issues such as an order of magnitude lower simulation Reynolds number (relative to experimental value) and unknown turbulence intensity levels within the experimental nozzle, it is shown that both the near-field jet turbulence and far-field noise predictions are in good agreement with the experimental measurements. Because of the enormous number of grid points required to resolve the near-nozzle region, the computational domain size is constrained to ten nozzle exit diameters downstream of nozzle exit by the limited computational resources. As a result, not all of the low frequency generating noise sources are resolved in the calculation, resulting in some errors in the low frequency range of the predicted noise spectrum. Nevertheless, it is shown that the high frequency noise generation in the near-nozzle region is predicted quite well.

A Unified Material Description for Light Induced Deformation in Azobenzene Polymers.

Complex light-matter interactions in azobenzene polymers have limited our understanding of how photoisomerization induces deformation as a function of the underlying polymer network and form of the light excitation. A unified modeling framework is formulated to advance the understanding of surface deformation and bulk deformation of polymer films that are controlled by linear or circularly polarized light or vortex beams. It is shown that dipole forces strongly respond to polarized light in contrast to higher order quadrupole forces that are often used to describe surface relief grating deformation through a field gradient constitutive law. The modeling results and comparisons with a broad range of photomechanical data in the literature suggest that the molecular structure of the azobenzene monomers dramatically influences the photostrictive behavior. The results provide important insight for designing azobenzene monomers within a polymer network to achieve enhanced photo-responsive deformation.

Broadband impedance boundary conditions for the simulation of sound propagation in the time domain.

An accurate and practical surface impedance boundary condition in the time domain has been developed for application to broadband-frequency simulation in aeroacoustic problems. To show the capability of this method, two kinds of numerical simulations are performed and compared with the analytical/experimental results: one is acoustic wave reflection by a monopole source over an impedance surface and the other is acoustic wave propagation in a duct with a finite impedance wall. Both single-frequency and broadband-frequency simulations are performed within the framework of linearized Euler equations. A high-order dispersion-relation-preserving finite-difference method and a low-dissipation, low-dispersion Runge-Kutta method are used for spatial discretization and time integration, respectively. The results show excellent agreement with the analytical/experimental results at various frequencies. The method accurately predicts both the amplitude and the phase of acoustic pressure and ensures the well-posedness of the broadband time-domain impedance boundary condition.

Thermoacoustic modeling and uncertainty analysis of two-dimensional conductive membranes

A model for two-dimensional graphene-based thermoacoustic membranes is investigated analytically and numerically validated using Bayesian statistics in this study. The temperature and the pressure variables are first analytically determined in one-dimension by noticing that the magnitude of the pressure time derivative is small in the heat transfer equations and by taking advantage of the large disparity between the length scales. The one-dimensional findings are then extended to three-dimensions, where pressure fluctuation produced by the surface temperature variation is determined using an acoustic piston model. Through the one and three-dimensional model analysis, the dependence of acoustic pressure as a function of frequency is studied. The acoustic response with respect to the frequency shows different characteristics when assuming Dirichlet (temperature) or Neumann (heat flux) boundary conditions. The thermoacoustic model is validated with a graphene-on-paper loudspeaker using Bayesian statistical m...

Adaptive mesh redistribution method for domains with complex boundaries

An adaptive structured mesh redistribution method (ASMRM) that permits smooth transition from non-uniformly distributed boundary points to solution-adaptive interior points and enables the resolution of complex flow in the complex boundary region as well as away from the boundary is proposed. It is a variant of the traditional variational technique. It involves a combination of static and dynamic monitor functions, the former for mesh distribution in the vicinity of a complex boundary and the latter for mesh adaption with the evolving solution elsewhere. Its effectiveness is demonstrated on some example problems, and it is then applied to a chevron nozzle. The proposed method is shown to be capable of generating a mesh with a good balance of orthogonality and smoothness in the entire domain.

Thermodynamics and nonlinear mechanics of materials with photoresponsive microstructure

The ability to directly convert visible light radiation into useful mechanical work provides many opportunities in the field of smart materials and adaptive structures ranging from biomedical applications to control of heliostat mirrors for solar harvesting. The complexities associated with coupling time-dependent Maxwell’s equations with linear momentum and mechanics is discussed by introducing a set of electronic order parameters that govern the coupling between electromagnetic radiation and mechanics of a deformable solid. Numerical examples are given illustrating how this methodology is applied to a special class of liquid crystal polymer networks containing azobenzene. The dynamics associated with light absorption and its effect on deformation of the polymer are solved in three dimensions using finite difference methods and compared to experimental results. Particular emphasis is placed on the effect of polarized light on microstructure evolution and stresses that occur during photoisomerization of the optically active microstructure.

Simulation and Analysis of Noise Associated with Muzzle Flow

A numerical study of a muzzle blast flow-field is performed and analyzed to understand the evolution of flow structures and to examine the sound wave generation mechanisms in the near field. The analysis of vortex dynamics based on the vorticity transport equation shows that the dilatation term contributes more than the baroclinic term to vorticity generation and deformation. The motion of the vortex structures is found to be similar in the cases studied here: the main vortex formation, additional vortex generation and their interactions. The Helmholtz decomposition and acoustic perturbation equations are used to analyze the sound generation mechanism in the muzzle flow. The most significant sound source term is identified and the dominant sound generation phenomenon is shown to occur near the vortex ring region and not in the shocked jet flow.

Fluid-structure interactions of fast photomechanical liquid crystal elastomers driven by light

A new class of photomechanical liquid crystal elastomers (LCE) has emerged, which generate large bending deformation and fast response times that scale with the resonance of the elastomer films. These films are classified as glassy elastomers (modulus ~1GPa) and are doped with photoresponsive azobenzene liquid crystals to provide novel light induced deformation. These materials are promising for developing propulsions systems for insect size aircraft and microfluidic devices, for example. The photomechanical efficiency of these materials in a fluid medium is of high interest to understand the performance attributes of this class of smart materials. Here, a numerical study is presented that describes the photomechanical structural dynamic behaviour in a fluid medium. We simulate the oscillation of photomechanical cantilevers excited by light while simultaneously modeling the effect of the surrounding fluid at different ambient pressures. The photoelastomer structure is modeled as a thin plate and coupled with photomechanical constitutive relations to compute the transverse displacement. For the fluid, three dimensional unsteady incompressible Navier-Stokes equations using the arbitrary Lagrangian Eulerian (ALE) form are used to consider dynamic mesh movement on a local mesh and boundary conditions on the elastomer material interface. The fluid equations are discretized using a conventional finite volume method (FVM) on a structured curvilinear coordinate system. Numerical examples are given which provide new insight into photomechanical material efficiencies in a fluid medium as a function of ambient pressure.

Numerical Study of Sound Generation Mechanism by a Blast Wave

The goal of this paper is to investigate the generation characteristics of the main impulsive noise sources generated by the supersonic flow discharging from a muzzle. For this, this paper investigates two fundamental mechanisms to sound generation in shocked flows: shock motion and shock deformation. Shock motion is modeled numerically by examining the interaction of a sound wave with a shock. The numerical approach is validated by comparison with results obtained by linear theory for a small disturbance case. Shock deformations are modeled numerically by examining the interaction of a vortex ring with a blast wave. A numerical approach of a dispersion-relation-preserving(DRP) scheme is used to investigate the sound generation and propagation by their interactions in near-field.

Unifying relations in polymer photomechanics

Photoresponsive polymers offer novel methods for morphing applications due to its unique ability to control shape spatially and temporally with light. The constitutive behavior of these materials is complicated by the interactions of time-dependent light fields and molecular conformation changes within the polymer network. This requires applications in non-equilibrium thermodynamics, nonlinear photomechanics, and high fidelity numerical simulations using finite difference/finite element methods. The proposed approach utilizes a set of electronic order parameters to represent light driven molecular conformation changes which are coupled to mechanics of a continuum scale polymer network and time-dependent electromagnetics. The model is applied to explain photoisomerization of azobenzene as it deforms a polymer during different types of light excitation. We consider local surface deformation from laser beams including linearly and circularly polarized lights where the azobenzene liquid crystal microstructure couples to affine deformation of the host polymer network. This local deformation from a laser beam is compared to homogeneous polarized light across the surface of a cantilever film. Non-trivial deformation is predicted and the internal mechanisms associated with bending in different directions is discussed.