### Recent Submissions

• #### Runtime Abstraction for Autonomous Adaptive Systems on Reconfigurable Hardware

(IEEE, 2021-02-01) [Conference Paper]
Autonomous systems increasingly rely on on-board computation to avoid the latency overheads of offloading to more powerful remote computing. This requires the integration of hardware accelerators to handle the complex computations demanded by date-intensive sensors. FPGAs offer hardware acceleration with ample flexibility and interfacing capabilities when paired with general purpose processors, with the ability to reconfigure at runtime using partial reconfiguration. Managing dynamic hardware is complex and has been left to designers to address in an ad-hoc manner without first-class integration in autonomous software frameworks. This paper presents an abstracted runtime for managing adaptation of FPGA accelerators, including partial reconfiguration and parametric changes, that presents as a typical interface used in autonomous software systems. We present a demonstration using the Robot Operating System (ROS), showing negligible latency overhead as a result of the abstraction.
• #### A Visual Analytics Based Decision Making Environment for COVID-19 Modeling and Visualization

(IEEE, 2021-02-01) [Conference Paper]
Public health officials dealing with pandemics like COVID-19 have to evaluate and prepare response plans. This planning phase requires not only looking into the spatiotemporal dynamics and impact of the pandemic using simulation models, but they also need to plan and ensure the availability of resources under different spread scenarios. To this end, we have developed a visual analytics environment that enables public health officials to model, simulate, and explore the spread of COVID-19 by supplying county-level information such as population, demographics, and hospital beds. This environment facilitates users to explore spatiotemporal model simulation data relevant to COVID-19 through a geospatial map with linked statistical views, apply different decision measures at different points in time, and understand their potential impact. Users can drill-down to county-level details such as the number of sicknesses, deaths, needs for hospitalization, and variations in these statistics over time. We demonstrate the usefulness of this environment through a use case study and also provide feedback from domain experts. We also provide details about future extensions and potential applications of this work.
• #### DOA Estimation with a Rank-deficient Covariance matrix: A Regularized Least-squares approach

(IEEE, 2021-01-18) [Conference Paper]
DOA estimation in the presence of coherent sources using a small number of snapshots faces the challenge of rank deficiency of the received signal covariance matrix. When the covariance matrix is rank deficient, only the pseudo inverse of the covariance matrix can be computed, which can give undesirable results. Traditionally, regularized least-squares (RLS) algorithms are used to tackle estimation problems in systems with ill-conditioned or rank deficient matrices. In this work, we combine the Capon beamformer with the RLS framework to develop a DOA estimation method for scenarios with rank deficient covariance matrices. Simulation results demonstrate the effectiveness of the proposed approach.
• #### A Machine Learning-Based Microwave Device Model for Fully Printed VO2 RF Switches

(IEEE, 2021-01-12) [Conference Paper]
Fully printed vanadium dioxide (VO2) based Radio Frequency (RF) switches have been recently developed for advanced frequency-reconfigurable RF electronics. A reliable and versatile model for the VO2 switches is required for design and simulations in the modern Computer-Aided Design (CAD) tools. This paper proposes a machine learning (ML) based model for VO2 RF switches, which is much more time and resource efficient as compared to the traditional device models. The computational efficiency, accuracy and robustness of the proposed model over a frequency range of 30 GHz is demonstrated through an excellent agreement between the modelled and measured results. The comparison between the measured and modelled results demonstrate a mean-square error (MSE) of lower than 5 x 10-4 and 5 x10-3 for the magnitude and phase values over the complete frequency range.
• #### Volume of fluid based model of heavy fuel oil droplet evaporation and combustion

(American Society of Mechanical Engineers, 2021-01-11) [Conference Paper]
Modern CFD simulations of combustion resolve simplified sub-models for droplet evaporation and combustion within a Lagrangian framework. Break-up effects like puffing and micro-explosions are usually neglected but, they eventually influence the evaporation and combustion behaviour of heavy fuel sprays. We are developing a Volume of Fluid (VoF)-based CFD solver that allows us to model single droplet differential evaporation with the break-up effects. Puffing/micro-explosion and droplet ejection proceed in three steps: nucleation of a bubble of a light component within the droplet, expansion/coalescence of the bubbles and finally eruption with sub-droplets formation. Our goal is to individually model each event and then combine them in a composite simulation. Henceforth we can get data in realistic conditions to be used in Lagrangian spray simulations. We identified three relevant features that are necessary to create a reliable representation: interface tracking, differential evaporation, and compressibility effect. The solver is based on the Volume of Fluid (VoF) technique and coded within the open-source OpenFOAM framework. VoF technique consists of transporting the volume fraction of one of the two phases (liquid or gas). The Navier-Stokes equations are solved for a single-phase but adapting the physical properties to the volume fraction value. A state-of-the-art method called iso-Advector was adopted to reconstruct the interface from the volume fraction field. The evaporation was implemented as a source term in the volume fraction equation, and the conservation equations were modified accordingly. In order to calculate the vapour and liquid physical properties, RKS equation of state (EOS) was implemented. The droplet was assumed to have 2 phases: light and heavy, having physical properties comparable to water and heavy fuel oil (HFO), respectively. The pressure closure equation was modified to handle large pressure differences during the internal evaporation of light components. The validation of the solver was performed through benchmark cases as multiphase shock-tube, droplet oscillation and boiling interface either with experimental works and analytical solutions. A single suspended droplet experiment was performed to measure the velocity of an ejected micro-droplet during puffing using a shadowgraphy technique. The code is able to predict ejection velocity within a 15% error, which seems to be promising. The present article documents part of the algorithm development and its validation. In particular, the step describing the ejection of the inner vapours is described.
• #### Feasibility of innovative solar-thermo-acoustic power conversion cycles

(American Society of Mechanical Engineers, 2021-01-11) [Conference Paper]
• #### An Assessment of Unmanned Aircraft System Operations with the Extensible Trajectory Optimization Library

(American Institute of Aeronautics and Astronautics, 2021-01-04) [Conference Paper]
In the not too distant future, Unmanned Aircraft Systems (UAS) will be a source of economic power in urban environments. These systems will be used for package delivery, building inspections, filming, and many other tasks. However, economic benefits should not outweigh public safety. These systems must not collide into buildings, and they must maintain a safe separation distance from neighboring systems. In other words, UAS operations should be assessed for safety. This paper assesses how safety requirements in the urban environment influence UAS operations. It presents a novel way of performing this assessment by using the Extensible Trajectory Optimization Library (ETOL) to continuously solve a vehicle guidance problem (VGP) in a multi-agent robot simulator. This paper introduces the common structure of a VGP, along with a VGP formulation for a UAS in an urban environment. In addition, a platform for safety assessments is presented, along with recommendations for improving the safety of UAS operations.
• #### Simulation of Turbulent Flows Using a Fully Discrete Explicit hp-nonconforming Entropy Stable Solver of Any Order on Unstructured Grids

(American Institute of Aeronautics and Astronautics, 2021-01-04) [Conference Paper]
We report the numerical solution of two challenging turbulent flow test cases simulated with the SSDC framework, a compressible, fully discrete hp-nonconforming entropy stable solver based on the summation-by-parts discontinuous collocation Galerkin discretizations and the relaxation Runge—Kutta methods. The algorithms at the core of the solver are systematically designed with mimetic and structure-preserving techniques that transfer fundamental properties from the continuous level to the discrete one. We aim at providing numerical evidence of the robustness and maturity of these entropy stable scale-resolving methods for the new generation of adaptive unstructured computational fluid dynamics tools. The two selected turbulent flows are i) the flow past two spheres in tandem at a Reynolds number based on the sphere diameter of ReD = 3.9 × 103 and 104, and a Mach number of Ma∞ = 0.1, and ii) the NASA junction flow experiment at a Reynolds number based on the crank chord length of Reℓ = 2.4×106 and Ma∞ = 0.189.
• #### Compressibility effects on homogeneous isotropic turbulence using Schur decomposition of the velocity gradient tensor.

(American Institute of Aeronautics and Astronautics, 2021-01-04) [Conference Paper]
The study of compressibility effects on the dynamics and the structure of turbulence is an important, but difficult, topic in turbulence modeling. Taking advantage of a recently proposed Schur decomposition approach (Keylock, C. J., The Schur decomposition of the velocity gradient tensor for turbulent flows, Journal of Fluid Mechanics, 2018) to decompose the velocity gradient tensor into its normal and non-normal parts, here we evaluate the influence of the compressibility on some statistical properties of the turbulent structures. We perform a set of direct numerical simulations of decaying compressible turbulence at six turbulent Mach numbers between Mt = 0.12 and Mt = 0.89 and a Reynolds number based on the Taylor micro-scale of Ret = 100. All the simulations have been carried out using an improved seventh-order accurate WENO scheme to discretize the non-linear advective terms and an eight-order accurate centered finite difference scheme is retained for the diffusive terms. In the double decomposition, the normal parts of the velocity gradient tensor (represented by the eigenvalues) are separated explicitly from non-normal components. The two-dimensional space defined by the second and third invariants of the velocity gradient tensor is subdivided into six regions and the contribution of each regional term to the Schur decomposition of the velocity gradient tensor is analyzed. Our preliminary findings show the difficulty of understanding the non-local effects without taking into account both the normal contribution (represented by the eigenvalues) and the non-normal component computed with of the Schur decomposition.
• #### A Computational Study of Ammonia Combustion in MILD Conditions

(American Institute of Aeronautics and Astronautics, 2021-01-04) [Conference Paper]
o provide fundamental insights into the heat release and emission characteristics of ammonia flames in moderate or intense low-oxygen dilution (MILD) conditions, numerical simulations in one-dimensional laminar and two-dimensional turbulent flame configurations are conducted. The analysis shows that for the ammonia MILD flame, lower NOx emissions can be achieved while improving flame stability characteristics. In addition, the propagation of ammonia flames into forced turbulent flows in a channel is examined in terms of turbulent flame speed and surface area enhancement. The normalized flame speed and flame surface area are well correlated and show qualitatively similar behavior for the MILD and non-MILD flames.