7 research outputs found
A Review of Time Relaxation Methods
The time relaxation model has proven to be effective in regularization of Navier–Stokes Equations. This article reviews several published works discussing the development and implementations of time relaxation and time relaxation models (TRMs), and how such techniques are used to improve the accuracy and stability of fluid flow problems with higher Reynolds numbers. Several analyses and computational settings of TRMs are surveyed, along with parameter sensitivity studies and hybrid implementations of time relaxation operators with different regularization techniques
In-Situ Visualization of Long-Range Defect Interactions at the Edge of Melting
Connecting a bulk material's microscopic defects to its macroscopic
properties is an age-old problem in materials science. Long-range interactions
between dislocations (line defects) are known to play a key role in how
materials deform or melt, but we lack the tools to connect these dynamics to
the macroscopic properties. We introduce time-resolved dark-field X-ray
microscopy to directly visualize how dislocations move and interact over
hundreds of micrometers, deep inside bulk aluminum. With real-time movies, we
reveal the thermally-activated motion and interactions of dislocations that
comprise a boundary, and show how weakened binding forces inhomogeneously
destabilize the structure at 99% of the melting temperature. Connecting
dynamics of the microstructure to its stability, we provide important
opportunities to guide and validate multiscale models that are yet untested
An Online Dynamic Amplitude-Correcting Gradient Estimation Technique to Align X-ray Focusing Optics
High-brightness X-ray pulses, as generated at synchrotrons and X-ray free
electron lasers (XFEL), are used in a variety of scientific experiments. Many
experimental testbeds require optical equipment, e.g Compound Refractive Lenses
(CRLs), to be precisely aligned and focused. The lateral alignment of CRLs to a
beamline requires precise positioning along four axes: two translational, and
the two rotational. At a synchrotron, alignment is often accomplished manually.
However, XFEL beamlines present a beam brightness that fluctuates in time,
making manual alignment a time-consuming endeavor. Automation using classic
stochastic methods often fail, given the errant gradient estimates. We present
an online correction based on the combination of a generalized finite
difference stencil and a time-dependent sampling pattern. Error expectation is
analyzed, and efficacy is demonstrated. We provide a proof of concept by
laterally aligning optics on a simulated XFEL beamline
Simultaneous Bright- and Dark-Field X-ray Microscopy at X-ray Free Electron Lasers
The structures, strain fields, and defect distributions in solid materials
underlie the mechanical and physical properties across numerous applications.
Many modern microstructural microscopy tools characterize crystal grains,
domains and defects required to map lattice distortions or deformation, but are
limited to studies of the (near) surface. Generally speaking, such tools cannot
probe the structural dynamics in a way that is representative of bulk behavior.
Synchrotron X-ray diffraction based imaging has long mapped the deeply embedded
structural elements, and with enhanced resolution, Dark Field X-ray Microscopy
(DFXM) can now map those features with the requisite nm-resolution. However,
these techniques still suffer from the required integration times due to
limitations from the source and optics. This work extends DFXM to X-ray free
electron lasers, showing how the photons per pulse available at these
sources offer structural characterization down to 100 fs resolution (orders of
magnitude faster than current synchrotron images). We introduce the XFEL DFXM
setup with simultaneous bright field microscopy to probe density changes within
the same volume. This work presents a comprehensive guide to the multi-modal
ultrafast high-resolution X-ray microscope that we constructed and tested at
two XFELs, and shows initial data demonstrating two timing strategies to study
associated reversible or irreversible lattice dynamics
An automated approach to the alignment of compound refractive lenses
Compound refractive lenses (CRLs) are established X-ray focusing optics, and are used to focus the beam or image the sample in many beamlines at X-ray facilities. While CRLs are quite established, the stack of single lens elements affords a very small numerical aperture because of the thick lens profile, making them far more difficult to align than classical optical lenses that obey the thin-lens approximation. This means that the alignment must be very precise and is highly sensitive to changes to the incident beam, often requiring regular readjustments. Some groups circumvent the full realignment procedure by using engineering controls (e.g. mounting optics) that sacrifice some of the beam’s focusing precision, i.e. spot size, or resolution. While these choices minimize setup time, there are clear disadvantages. This work presents a new automated approach to align CRLs using a simple alignment apparatus that is easy to adapt and install at different types of X-ray experiments or facilities. This approach builds on recent CRL modeling efforts, using an approach based on the Stochastic Nelder–Mead (SNM) simplex method. This method is outlined and its efficacy is demonstrated with numerical simulation that is tested in real experiments conducted at the Advanced Photon Source to confirm its performance with a synchrotron beam. This work provides an opportunity to automate key instrumentation at X-ray facilities