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1. Turbulence Model Assessment in Compressible Flows around Complex Geometries with Unstructured Grids

   https://doi.org/10.3390/fluids4020081  

   Araya, Guillermo ( June 2019 , Fluids)

   One of the key factors in simulating realistic wall-bounded flows at high Reynolds numbers is the selection of an appropriate turbulence model for the steady Reynolds Averaged Navier–Stokes equations (RANS) equations. In this investigation, the performance of several turbulence models was explored for the simulation of steady, compressible, turbulent flow on complex geometries (concave and convex surface curvatures) and unstructured grids. The turbulence models considered were the Spalart–Allmaras model, the Wilcox k- ω model and the Menter shear stress transport (SST) model. The FLITE3D flow solver was employed, which utilizes a stabilized finite volume method with discontinuity capturing. A numerical benchmarking of the different models was performed for classical Computational Fluid Dynamic (CFD) cases, such as supersonic flow over an isothermal flat plate, transonic flow over the RAE2822 airfoil, the ONERA M6 wing and a generic F15 aircraft configuration. Validation was performed by means of available experimental data from the literature as well as high spatial/temporal resolution Direct Numerical Simulation (DNS). For attached or mildly separated flows, the performance of all turbulence models was consistent. However, the contrary was observed in separated flows with recirculation zones. Particularly, the Menter SST model showed the best compromise between accurately describing the physicsmore »of the flow and numerical stability.« less
2. Crustal and Upper Mantle Structure beneath the Wilkes and Aurora Subglacial Basins, East Antarctica from Full-Waveform Ambient Noise Tomography

   Kumar, A. ; Emry, E. ; Hansen, S. ( December 2019 , American Geophysical Union)

   East Antarctica is covered by thick sheets of ice and is underlain by stable cratonic lithosphere, extensive mountain ranges, and subglacial basins. The sparse seismic coverage in this region makes it difficult to assess the crustal and mantle structure, which are important to understanding the tectonic evolution of the continent as well as the behavior of the overlying ice sheets. Present tomographic models lack resolution and are often inconsistent with one another; therefore, delineating sub-surface characteristics associated with old rift systems or structures that would allow us to assess the origins of the Wilkes and Aurora subglacial basins, for instance, becomes challenging. To overcome these limitations, we are using a full-waveform tomography method to model the crustal and upper mantle structure in East Antarctica. We have used a frequency-time normalization approach to extract empirical Green’s functions (EGFs) from ambient seismic noise, between periods of 15-340 seconds. The ray path coverage of the EGFs is dense throughout East Antarctica, indicating that our study will provide new, high resolution imaging of this area. Synthetic waveforms are simulated through a three-dimensional heterogeneous Earth model using a finite-difference wave propagation method with a grid spacing of 0.025º (~ 2.25 km), which accurately reproduce Rayleighmore »waves at 15+ seconds. Following this, phase delays are measured between the synthetics and the data, sensitivity kernels are constructed using a scattering integral approach, and we invert using a sparse, least-squares method. The resulting shear-wave velocity model will be used to assess crustal and upper mantle features, ultimately aimed at resolving whether old rift systems exist within East Antarctica in relation to prominent subglacial basins. Preliminary results will be shared.« less
3. The dependence of planetary tectonics on mantle thermal state: Applications to early Earth evolution

   https://doi.org/10.1098/rsta.2017.0409  

   Foley, Bradford J. ( October 2018 , Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences)

   For plate tectonics to operate on a planet, mantle convective forces must be capable of forming weak, localized shear zones in the lithosphere that act as plate boundaries. Otherwise, a planet's mantle will convect in a stagnant lid regime, where subduction and plate motions are absent. Thus, when and how plate tectonics initiated on Earth is intrinsically tied to the ability of mantle convection to form plate boundaries; however, the physics behind this process are still uncertain. Most mantle convection models have employed a simple pseudoplastic model of the lithosphere, where the lithosphere "fails" and develops a mobile lid when stresses in the lithosphere reach the prescribed yield stress. With pseudoplasticity high mantle temperatures and high rates of internal heating, conditions relevant for the early Earth, impede plate boundary formation by decreasing lithospheric stresses, and hence favor a stagnant lid for the early Earth. However, when a model for shear zone formation based on grain size reduction is used, early Earth thermal conditions do not favor a stagnant lid. While lithosphere stress drops with increasing mantle temperature or heat production rate, the deformational work, which drives grain size reduction, increases. Thus the ability of convection to form weak plate boundariesmore »is not impeded by early Earth thermal conditions. However, mantle thermal state does change the style of subduction and lithosphere mobility; high mantle temperatures lead to a more sluggish, drip-like style of subduction. This "sluggish lid" convection may be able to explain many of the key observations of early Earth crust formation processes preserved in the geologic record. Moreover, this work highlights the importance of understanding the microphysics of plate boundary formation for assessing early Earth tectonics, as different plate boundary formation mechanisms are influenced by mantle thermal state in fundamentally different ways.« less
4. Shear-wave Velocity Structure of the Crust and Upper Mantle beneath East Antarctica from Full-Waveform Ambient Noise Tomography

   Kumar, Ashish ; Emry, Erica ; Hansen, Samantha ( July 2020 , Scientific Committee on Antarctic Research)

   The origin and tectonic evolution of various features in East Antarctica, such as the Wilkes Subglacial Basin (WSB), Aurora Subglacial Basin (ASB), Transantarctic Mountains (TAMs), and Gamburtsev Subglacial Mountains (GSM), are unconstrained due to thick ice coverage and a lack of direct geologic samples. We are modeling the crustal and upper mantle structure beneath these areas using a full-waveform tomography method to further our understanding the tectonic evolution of the continent as well as the behavior of the overlying ice sheet. A frequency-time normalization approach is employed to extract empirical Green’s functions (EGFs) from ambient seismic noise, between periods of 15-340 seconds. EGF ray path coverage is dense throughout East Antarctica, indicating that our study will provide new, high resolution imaging of this area. Synthetic waveforms are simulated through a three-dimensional heterogeneous Earth model using a finite-difference wave propagation method with a grid spacing of 0.025º, which accurately reproduces Rayleigh waves at 15+ seconds. Following this, phase delays are measured between the synthetics and the data, sensitivity kernels are constructed using the scattering integral approach, and we invert using a sparse, least-squares method. Preliminary results show that slow velocities are present beneath both the WSB and ASB, possibly indicating oldmore »rift systems or other inherited tectonic structures. The transition from slow to fast velocities beneath the Northern Victoria Land section of the TAMs is consistent with thermal loading beneath the mountain range. The presence of slow velocities near the GSM may be associated with rifting along the Lambert Rift System.« less
5. Shear-wave Velocity Structure of the Crust and Upper Mantle beneath East Antarctica from Full-Waveform Ambient Noise Tomography

   Kumar, Ashish ; Emry, Erica L ; Hansen, Samantha E ( June 2020 , Scientific Committee on Antarctic Research, Summer 2020 Meeting)

   The origin and tectonic evolution of various features in East Antarctica, such as the Wilkes Subglacial Basin (WSB), Aurora Subglacial Basin (ASB), Transantarctic Mountains (TAMs), and Gamburtsev Subglacial Mountains (GSM), are unconstrained due to thick ice coverage and a lack of direct geologic samples. We are modeling the crustal and upper mantle structure beneath these areas using a full-waveform tomography method to further our understanding the tectonic evolution of the continent as well as the behavior of the overlying ice sheet. A frequency-time normalization approach is employed to extract empirical Green’s functions (EGFs) from ambient seismic noise, between periods of 15-340 seconds. EGF ray path coverage is dense throughout East Antarctica, indicating that our study will provide new, high resolution imaging of this area. Synthetic waveforms are simulated through a three-dimensional heterogeneous Earth model using a finite-difference wave propagation method with a grid spacing of 0.025º, which accurately reproduces Rayleigh waves at 15+ seconds. Following this, phase delays are measured between the synthetics and the data, sensitivity kernels are constructed using the scattering integral approach, and we invert using a sparse, least-squares method. Preliminary results show that slow velocities are present beneath both the WSB and ASB, possibly indicating oldmore »rift systems or other inherited tectonic structures. The transition from slow to fast velocities beneath the Northern Victoria Land section of the TAMs is consistent with thermal loading beneath the mountain range. The presence of slow velocities near the GSM may be associated with rifting along the Lambert Rift System.« less