Xun Li


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Li

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Xun

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0000-0001-9203-4205

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2019 - 201912020 - 202312
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Publications 1 - 10 of 13
  • Elastic immersive wave experimentation
    Item type: Journal Article
    Li, Xun; Robertsson, Johan; van Manen, Dirk-Jan (2023)
    Geophysical Journal International
    We describe an elastic wave propagation laboratory that enables a solid object to be artificially immersed within an extended (numerical) environment such that a physical wave propagation experiment carried out in the solid drives the propagation in the extended (numerical) environment and vice versa. The underlying method of elastic immersive wave experimentation for such a laboratory involves deploying arrays of active multicomponent sources at the traction-free surface of the solid (e.g. a cube of granitic rock). These sources are used to accomplish two tasks: (1) cancel outgoing waves and (2) emit ingoing waves representing the first-order interactions between the physical and extended domains, computed using, for example, a finite-difference (FD) method. Higher-order interactions can be built by alternately carrying out the processes for cancelling the outgoing waves and the FD simulations for generating the ingoing waves. We validate the proposed iterative scheme for realizing elastic immersive wave experimentation using 2-D synthetic wave experiments.
  • Compensating for source directivity in immersive wave experimentation
    Item type: Journal Article
    Li, Xun; Robertsson, Johan O.A.; Curtis, Andrew; et al. (2019)
    The Journal of the Acoustical Society of America
  • Immersive Wave Control Experiments Using Non-isotropic Sources: Laboratory Applications
    Item type: Conference Paper
    Li, Xun; Becker, Theodor; Börsing, Nele; et al. (2020)
    Forum Acusticum
    A physical experiment can be fully linked and immersed within a virtual (numerical) domain through the real-time exchange of boundary conditions between the experiment and the numerical simulation. Such immersive wave control experimentation relies on active sources deployed around a physical domain (e.g., a water tank) to change its boundary condition. However, in theory, the control algorithm requires sources with an isotropic radia tion pattern, while physical sources typically exhibit angle dependent radiation characteristics. This discrepancy can be overcome by a processing method carried out in the frequency-wavenumber (f-k) domain if the source radiation pattern is known. Here we show the measured radiation pattern of a custom-built Bender-mode X-spring (BMX) type piezoelectric source that will be used in under water acoustic immersive wave experimentation. We measure the half-space radiation pattern of the BMX source that is mounted at the center of a steel plate with a transversely deployed planar receiver surface. The acquired data is redatumed to that corresponding to receivers deployed along a semi-circle and the source at the origin. The BMX source shows a non-negligible radiation pattern with the highest energy at normal and decreasing power gradually to its perpendicular. The measured radiation pattern will be incorporated into the 3-D physical implementation of immersive wave experimentation.
  • Multidimensional Deconvolution for Boundary Reflection Removal and Complete Scattering Characterization in Physical Acoustics Experiments
    Item type: Conference Paper
    Li, Xun; Becker, Theodor; Ravasi, Matteo; et al. (2021)
    In acoustic wave propagation experiments, waves re ected from the bound- aries of the experimental setup often contaminate recorded data. We propose using multidimensional deconvolution (MDD) to post-process laboratory data such that the boundary-related scattering component is completely removed from recorded data and only the Green's functions associated with a scattering object of interest are obtained. The obtained Green's functions between any pair of points on a closed recording surface completely characterize an unknown scattering object inside the recording surface.
  • Elastic immersive wave experimentation
    Item type: Doctoral Thesis
    Li, Xun (2022)
    Elastic wave propagation in earth materials is often studied through physical experiments in a size-limited laboratory. Such studies have strong limitations because waves reflect and mode convert at the closed boundary of an experiment and mask the waves related to the scattering within the materials. This problem is commonly mitigated by using waves at high frequencies (e.g., >1 MHz) in conventional laboratories. However, such an approach is not useful for studying wave phenomena that strongly depend on frequency. This thesis outlines a novel method of elastic immersive wave experimentation to overcome the boundary-related challenges in 3D elastic wave experiments. The boundary reflections and mode conversions are canceled by arrays of active sources deployed at the traction-free surface of the object. Furthermore, these sources can emit waves, which represent the interactions of the physical medium with a virtual exterior. Elastic immersive wave experimentation paves the way to the next generation of laboratories, in which wave experiments can be carried out with (1) an extended volume, and (2) significantly lower frequencies (1-20 kHz) than commonly used in conventional laboratories (> 1 MHz). Due to the inaccessibility of the interior of a solid, the desired time signatures of the boundary sources in an elastic immersive wave experiment have to be sought from the wavefield recordings made at the boundaries. I propose an iterative method, which involves alternating a physical experiment for recording free-surface data, a numerical simulation in which the data are injected along a transparent surface for separating outgoing wavefield constituents, and another numerical simulation for extrapolating the separated outgoing wavefield into a virtual environment. These two numerical simulations, implemented through a finite-difference (FD) method, are used to calculate source signatures corresponding to canceling boundary reflections and generating physical-to-virtual interactions. I propose a method of multiple point sources to implement FD-based wavefield injection and extrapolation in elastic media. I propose internal absorbing boundary conditions, which are incorporated into both the FD simulations to allow the physical experiment to involve an unknown interior medium. When using the iterative method, unwanted boundary reflections are canceled by active sources during a physical experiment. The same effect can be achieved by removing the imprints of the boundary reflections from recorded data afterward using a method of multidimensional deconvolution, which is applied to a two-dimensional acoustic waveguide experiment for a proof of concept. This work provides an insight into realizing elastic immersive wave experimentation by post-processing data recorded in physical experiments. Apart from developing the theories for immersive wave experimentation, I propose a method to compensate for physical source directivity in acoustic immersive wave experiments. Such a method will also help understand and suppress the effect of using imperfect sources in an elastic immersive wave experimentation laboratory.
  • Exact extrapolation and immersive modelling with finite-difference injection
    Item type: Journal Article
    van Manen, Dirk-Jan; Li, Xun; Vasmel, Marlies; et al. (2020)
    Geophysical Journal International
    In numerical modelling of wave propagation, the finite-difference (FD) injection method enables the re-introduction of simulated wavefields in model subdomains with machine precision, enabling the efficient calculation of waveforms after localized model alterations. By rewriting the FD-injection method in terms of sets of equivalent sources, we show how the same principles can be applied to achieve on-the-fly wavefield extrapolation using Kirchhoff–Helmholtz (KH)-like integrals. The resulting extrapolation methods are numerically exact when used in conjunction with FD-computed Green’s functions. Since FD injection only relies on the linearity of the wave equation and compactness of FD stencils in space, the methods can be applied to both staggered and non-staggered discretizations with arbitrary-order spatial operators. Examples for both types of discretizations show how these extrapolators can be used to truncate models with exact absorbing or immersive boundary conditions. Such immersive modelling involves the evaluation of KH-type extrapolation and representation integrals in the same simulation, which include the long-range interactions missing from conventional FD injection.
  • Internal absorbing boundary conditions for closed-aperture wavefield decomposition in solid media with unknown interiors
    Item type: Journal Article
    Li, Xun; Robertsson, Johan O.A.; Curtis, Andrew; et al. (2022)
    The Journal of the Acoustical Society of America
    We present a method to create an internal numerical absorbing boundary within elastic solid media whose properties are largely unknown and use it to create the first wavefield separation method that retrieves all orders of outgoing elastic wavefield constituents for real data recorded on a closed free surface. The recorded data are injected into a numerical finite-difference (FD) simulation along a closed, transparent surface, and the new internal numerical absorbing boundary condition achieves high attenuation of the ingoing waves radiated from the injection surface. This internal wave absorption enables the data injection to radiate all outgoing waves for experimental domains that include arbitrary unknown scatterers in the interior. The injection-absorption-based separation scheme is validated using three-dimensional (3D) synthetic modeling and a real data experiment acquired using a 3D laser Doppler vibrometer on a granite rock. The wavefield separation method forms a key component of an elastic immersive wave experimentation laboratory, and the ability to numerically absorb ingoing scattered energy in an uncharacterized medium while still radiating the true outgoing energy is intriguing and may lead to other development and applications in the future.
  • Broadband acoustic invisibility and illusions
    Item type: Journal Article
    Becker, Theodor S.; van Manen, Dirk-Jan; Haag, Thomas; et al. (2021)
    Science Advances
    Rendering objects invisible to impinging acoustic waves (cloaking) and creating acoustic illusions (holography) has been attempted using active and passive approaches. While most passive methods are inflexible and applicable only to narrow frequency bands, active approaches attempt to respond dynamically, interfering with broadband incident or scattered wavefields by emitting secondary waves. Without prior knowledge of the primary wavefield, the signals for the secondary sources need to be estimated and adapted in real time. This has thus far impeded active cloaking and holography for broadband wavefields. We present experimental results of active acoustic cloaking and holography without prior knowledge of the wavefield so that objects remain invisible and illusions intact even for broadband moving sources. This opens previously inaccessible research directions and facilitates practical applications including architectural acoustics, education, and stealth.
  • Unbounded full-aperture acoustic wave experimentation using multidimensional deconvolution
    Item type: Other Journal Item
    Li, Xun; Becker, Theodor; Ravasi, Matteo; et al. (2021)
    The Journal of the Acoustical Society of America
  • Closed-aperture unbounded acoustics experimentation using multidimensional deconvolution
    Item type: Journal Article
    Li, Xun; Becker, Theodor; Ravasi, Matteo; et al. (2021)
    The Journal of the Acoustical Society of America
    In physical acoustic laboratories, wave propagation experiments often suffer from unwanted reflections at the boundaries of the experimental setup. We propose using multidimensional deconvolution (MDD) to post-process recorded experimental data such that the scattering imprint related to the domain boundary is completely removed and only the Green's functions associated with a scattering object of interest are obtained. The application of the MDD method requires in/out wavefield separation of data recorded along a closed surface surrounding the object of interest, and we propose a decomposition method to separate such data for arbitrary curved surfaces. The MDD results consist of the Green's functions between any pair of points on the closed recording surface, fully sampling the scattered field. We apply the MDD algorithm to post-process laboratory data acquired in a two-dimensional acoustic waveguide to characterize the wavefield scattering related to a rigid steel block while removing the scattering imprint of the domain boundary. The experimental results are validated with synthetic simulations, corroborating that MDD is an effective and general method to obtain the experimentally desired Green's functions for arbitrary inhomogeneous scatterers.
Publications 1 - 10 of 13