Efficient Certified RAT Verification

Clausal proofs have become a popular approach to validate the results of SAT solvers. However, validating clausal proofs in the most widely supported format (DRAT) is expensive even in highly optimized implementations. We present a new format, called LRAT, which extends the DRAT format with hints that facilitate a simple and fast validation algorithm. Checking validity of LRAT proofs can be implemented using trusted systems such as the languages supported by theorem provers. We demonstrate this by implementing two certified LRAT checkers, one in Coq and one in ACL2

A framework to evaluate the viability of robotic process automation for business process activities

Robotic process automation (RPA) is a technology for centralized automation of business processes. RPA automates user interaction with graphical user interfaces, whereby it promises efficiency gains and a reduction of human negligence during process execution. To harness these benefits, organizations face the challenge of classifying process activities as viable automation candidates for RPA. Therefore, this work aims to support practitioners in evaluating RPA automation candidates. We design a framework that consists of thirteen criteria grouped into five perspectives which offer different evaluation aspects. These criteria leverage a profound understanding of the process step. We demonstrate and evaluate the framework by applying it to a real-life data set.Comment: This is an accepted manuscript for the "RPA Forum" at the "Int. Conference on Business Process Management (BPM 2020)". The final authenticated version is available online at https://doi.org/10.1007/978-3-030-58779-6_1

Model Checking Boot Code from AWS Data Centers

This paper describes our experience with symbolic model checking in an industrial setting. We have proved that the initial boot code running in data centers at Amazon Web Services is memory safe, an essential step in establishing the security of any data center. Standard static analysis tools cannot be easily used on boot code without modification owing to issues not commonly found in higher-level code, including memory-mapped device interfaces, byte-level memory access, and linker scripts. This paper describes automated solutions to these issues and their implementation in the C Bounded Model Checker (CBMC). CBMC is now the first source-level static analysis tool to extract the memory layout described in a linker script for use in its analysis

Moving mesh strategy for simulating sliding and rolling dynamics of droplets on inclined surfaces with finite element method

[[abstract]]Finite element method (FEM) moving mesh strategy for simulating dynamics of droplets on inclined surfaces is designed within arbitrary Lagrangian Eulerian (ALE) framework. Depending on the physical and chemical properties of the fluid and the supporting surface, resulting droplet dynamics can exhibit sliding and/or rolling regimes. Exploiting the full potential of the ALE framework, moving mesh strategy designed in this work is capable of tracking the droplet evolution regardless of the flow regime and without the need for frequent mesh adaptation. Additional attention is invested into the discrete energy balance: possible sources of spurious energy due to the mesh motion are identified and investigated. The overall strategy exhibits a good tradeoff between the stability and efficiency, and demonstrates the ability to perform long-time simulations. The capabilities of the proposed strategy are demonstrated on a couple of fairly complex (3D) scenarios motivated by the industrial applications. In particular, droplet dynamics is simulated for the case of inclined and heterogeneous supporting surfaces, which are designed with the aim to manipulate the droplet motion. Complex dynamics, including sliding, rolling and change in movement direction and wetting area, is successfully captured by the numerical simulation

Elimination of spurious velocities generated by curvature dependent surface force in finite element flow simulation with mesh-fitted interface

[[abstract]]It is known that spurious non-physical velocities can occur when one employs the finite element method for simulation of incompressible flows subjected to external forces. In presence of external body forces, the main reason for this is the incompressibility constraint that is satisfied only in a weak sense against test functions from the pressure function space. In case of the two-phase (incompressible) immiscible flow, a surface force, which is a function of the interface curvature, arises and introduces additional problematics to the finite element model. Due to discrete representation of the interface, the question arises on how to approximate the curvature. A particularly natural approach for the finite element method employs the Laplace–Beltrami operator which allows to express the mean curvature in a weak sense. However, once incorporated into the equations governing the fluid flow, Laplace–Beltrami-reconstructed curvature may introduce spurious non-physical forces at the interface if finite element spaces are chosen arbitrarily. The reason for this is that the test space used for curvature calculation is the test space associated with the velocity field. We show that it is necessary for the function space used for the geometry construction to be of the order equal to or higher than the order of the test space involved in curvature evaluation. This leaves two possibilities for practical fluid flow problems: use the same function spaces for the mesh geometry and the velocity field (isoparametric concept) or decouple the curvature calculation from the main problem

Energy stable finite element strategy for simulating spreading, sliding and rolling flow dynamics of viscoelastic droplets

[[abstract]]Moving mesh finite element method (FEM) for simulating dynamics of viscoelastic droplets on inclined surfaces is proposed. Viscoelasticity is incorporated into the governing system employing the Oldroyd-B constitutive model. The supporting (inclined) surface is allowed to have non-homogeneous properties incorporated into the mathematical model through the generalized Navier boundary conditions (GNBC). The droplet motion (sliding and/or rolling) is handled by employing arbitrary Lagrangian Eulerian (ALE) framework. Energy balance is derived from the governing system and it is a starting point in the derivation of the numerical scheme. The overall numerical strategy is designed in such a way that a counterpart of the (continuous) energy balance holds on the discrete level. This ensures that no spurious energy is introduced into the discrete system which, in turn, guarantees the stability of the scheme in the energy norm. The framework proposed is very general and encapsulates both two and three dimensional scenarios. Numerical studies in this work focus primarily on the three dimensional scenarios since such are significantly more challenging and seem to be much scarcer in the literature. The newly proposed numerical strategy is validated on several examples to confirm the theoretical predictions. The role of viscoelasticity in the overall droplet dynamics is briefly investigated and the behaviors of Newtonian and non-Newtonian droplets are compared

Energy stable arbitrary lagrangian eulerian finite element scheme for simulating flow dynamics of droplets on non–homogeneous surfaces

[[abstract]]An energy stable finite element scheme within arbitrary Lagrangian Eulerian (ALE) framework is derived for simulating the dynamics of millimetric droplets in contact with solid surfaces. Supporting surfaces considered may exhibit non–homogeneous properties which are incorporated into the governing system through generalized Navier boundary conditions (GNBC). Numerical scheme is constructed such that the counterpart of (continuous) energy balance holds on the discrete level. This ensures that no spurious energy is introduced into the discrete system, i.e. the discrete formulation is stable in the energy norm. The newly proposed scheme is numerically validated to confirm the theoretical predictions. Of a particular interest is the case of droplet on a non–homogeneous inclined surface. This case shows the capabilities of the scheme to capture the complex droplet dynamics (sliding and rolling) while maintaining stability during the long time simulation

The free surface effect on a chemotaxis–diffusion–convection coupling system

[[abstract]]Suspension of an oxytactic bacteria (e.g. the species Bacillus subtilis) placed in a container with its upper surface open to the atmosphere results in the formation of complex bioconvection patterns. The bacteria consume the oxygen diluted in the water, thereby causing the decrease of oxygen concentration everywhere except on the free surface. Through the free surface, which is in direct contact with the air, oxygen diffuses into the water. Slightly denser than water, the oxytactic bacteria are able to swim towards the higher concentration of oxygen (i.e. upwards) and they concentrate in a thin layer below the free surface. This causes the change of the suspension density and Rayleigh–Taylor type instabilities to occur. The chemotaxis phenomenon has been successfully modeled within continuum mechanics approach under certain simplifications. The set of (non-linearly) coupled equations describing the process involves the Boussinesq approximation of the Navier–Stokes equations governing the fluid motion and two convection–diffusion type equations governing the bacteria and oxygen concentrations. One of the simplifications that might significantly influence numerical simulations is the boundary condition for fluid equation on the free surface. This condition ensures that the vertical component of the velocity is zero, thus keeping the position of free surface fixed. This assumption significantly simplifies numerical procedure since the non-linearly coupled system can then be solved on stationary grid. However, allowing the motion of the free surface and completing the system with appropriate boundary conditions on contact line (liquid–solid–gas interface), a more realistic model is derived and new insights on nonlinear dynamics of the chemotaxis phenomenon are obtained. Our aims in this paper are to upgrade the currently available model into a more realistic one in both two and three dimensions, to propose a numerical procedure to deal with the new system (now posed on time-dependent domain) and, finally, to show the difference between this new model and the previous simplified one