Rainald Lohner

• George Mason University

A technique to combine codes to solve barely coupled multiphysics problems has been developed. Each field is advanced separately until a stop is triggered. This could be due to a preset time increment, a preset number of timesteps, a preset decrease of residuals, a preset change in unknowns, a preset change in geometry, or any other physically mean...

This work focuses on solving the system identification problems of the high-fidelity digital twin based on the measured data via an optimization process. The cost function formulation is based on the aggregated error between measured and computed displacements in different locations. The work aims to test different optimization algorithms with the...

An adjoint-based procedure to determine weaknesses, or, more generally, the material properties of structures is improved by implementing two techniques to progressively reduce the regions considered as weakened. In this way, the number of degrees of freedom that are being optimized is reduced, leading to faster convergence and a better definition...

As the number of cerebral aneurysms treated with flow diverters continues to increase, it is important to understand what factors influence not only thrombus formation within the aneurysm cavity but also fibrin accumulation across the device and its associated disruption and blockage of the inflow stream. Both processes contribute to the eventual o...

A finite-element method dependant adjoint-based procedure to determine the temperature field of structures based on measured displacements or strains and a set of standard loads is developed and tested. Given a series of force and deformation measurements, the temperature field is obtained by minimizing the adequately weighted differences between t...

The mechanisms leading to aneurysm occlusion after treatment with flow‐diverting devices are not fully understood. Flow modification induces thrombus formation within the aneurysm cavity, but fibrin can simultaneously accumulate and cover the device scaffold, leading to further flow modification. However, the interplay and relative importance of th...

The nudging data assimilation algorithm is a powerful tool used to forecast phenomena of interest given incomplete and noisy observations. Machine learning is becoming increasingly popular in data assimilation given its ease of computation and forecasting ability. This work proposes a new approach to data assimilation via machine learning where Dee...

We present a family of fast explicit time integration schemes of first, second and third order accuracy for parabolic problems in mechanics solved via standard numerical methods that have considerable higher computational efficiency versus existing explicit methods of the same order. The derivation of the new explicit schemes is inspired on the fin...

The goal of this study was to test if CFD-based virtual angiograms could be used to automatically discriminate between intracranial aneurysms (IAs) with and without flow stagnation. Time density curves (TDC) were extracted from patient digital subtraction angiography (DSA) image sequences by computing the average gray level intensity inside the ane...

Deep neural network (DNN) architectures are constructed that are the exact equivalent of explicit Runge–Kutta schemes for numerical time integration. The network weights and biases are given, that is, no training is needed. In this way, the only task left for physics-based integrators is the DNN approximation of the right-hand side. This allows to...

An adjoint-based procedure to determine weaknesses, or, more generally the material properties of structures is developed and tested. Given a series of force and deformation/strain measurements, the material properties are obtained by minimizing the weighted differences between the measured and computed values. Several examples with truss, plain st...

Purpose This study aims to evaluate blast loads on and the response of submerged structures. Design/methodology/approach An arbitrary Lagrangian–Eulerian method is developed to model fluid–structure interaction (FSI) problems of close-in underwater explosions (UNDEX). The “fluid” part provides the loads for the structure considers air, water and h...

A finite volume method with five-equation model for multi-fluid is implemented in a three-dimensional unstructured vertex centered code, FEFLO. Air, water, and high explosive (HE) material are considered as compressible fluids in the numerical model. The equations of state of the considered fluids are written in a general formulation. First, one-di...

The use of adjoint solvers is considered in order to obtain the sensitivity of clinical measures in aneurysms to incomplete (or unknown) boundary conditions and/or geometry. It is shown that these techniques offer interesting theoretical insights and viable computational tools to obtain these sensitivities.

Deep neural network (DNN) architectures are constructed that are the exact equivalent of explicit Runge-Kutta schemes for numerical time integration. The network weights and biases are given, i.e., no training is needed. In this way, the only task left for physics-based integrators is the DNN approximation of the right-hand side. This allows to cle...

The five-equation model for compressible two-phase flows has been extended to unstructured grids in order to model underwater explosions (UN-DEX) close to complex geometries. The ideal equation of state (EOS) is used for air. The stiffened gas EOS is used for water. The Johnes-Wilkins-Lee (JWL) EOS is used for the high-explosive (HE) material to de...

View Video Presentation: https://doi.org/10.2514/6.2022-3225.vid An efficient numerical modeling of multi-fluid flows is used to study the motion of large underwater bubbles. The considered fluids include air, water and high explosive material. The numerical solvers employed follow the physics of these problems. During detonation and recompression...

The methodology previously proposed by the authors to solve particle-laden turbulent flows through a multiscale approach is extended by introducing a continuous function for the dispersed phase concentration. The proposed continuous model is especially useful for studying the motion of particle streams in which gravitational and inertial effects ca...

A robust and efficient finite volume method with interface sharpening technique has been developed to solve the six-equation multi-fluid single-pressure model for compressible two-phase flows. The numerical method is implemented in a three-dimensional vertex-centered code. A least-squares reconstruction with Kuzmin's vertex-based (VB) limiter is im...

A deterministic pathogen transmission model based on high-fidelity physics has been developed. The model combines computational fluid dynamics and computational crowd dynamics in order to be able to provide accurate tracing of viral matter that is exhaled, transmitted and inhaled via aerosols. The examples shown indicate that even with modest compu...

New algorithms called nudging induced neural networks (NINNs), to control and improve the accuracy of deep neural networks (DNNs), are introduced. The NINNs framework can be applied to almost all pre-existing DNNs, with forward propagation, with costs comparable to existing DNNs. NINNs work by adding a feedback control term to the forward propagati...

A deterministic pathogen transmission model based on high-fidelity physics has been developed. The model combines computational fluid dynamics and computational crowd dynamics in order to be able to provide accurate tracing of viral matter that is exhaled, transmitted and inhaled via aerosols. The examples shown indicate that even with modest compu...

View Video Presentation: https://doi.org/10.2514/6.2022-0334.vid The flow inside a typical narrow-body airplane cabin was computed. The highly turbulent flowfield was of the order of 20-30~cm/sec at the level of passenger heads, not too strong, but giving the feeling of an air-conditioned environment. Different `sneezing positions' were considered...

View Video Presentation: https://doi.org/10.2514/6.2022-0352.vid A numerical model for three-fluid flow including air, water and high explosive (HE) material is extended to fluid-structure interaction (FSI) problems. The aim is to study the complex interactions between compressible multi-phase flows and deformable structures in near field underwate...

View Video Presentation: https://doi.org/10.2514/6.2022-1076.vid This article continues the program initiated by the authors in [1] with an eventual goal to replace the existing chemistry software packages by Deep Neural Networks (DNNs). Chemically reacting flows are common in engineering, such as hypersonic flow, combustion, explosions, manufactur...

After a brief historical review of unstructured grid generation methods the two ‘breakthrough workarounds’ that made these methods reliable industrial tools are discussed. In many previous publications these important ‘workarounds’ were never mentioned. Yet without them computational science would not have become the third pillar of the empirical s...

For any given neural network architecture a permutation of weights and biases results in the same functional network. This implies that optimization algorithms used to 'train' or 'learn' the network are faced with a very large number (in the millions even for small networks) of equivalent optimal solutions in the parameter space. To the best of our...

The nudging data assimilation algorithm is a powerful tool used to forecast phenomena of interest given incomplete and noisy observations. Machine learning is becoming increasingly popular in data assimilation given its ease of computation and forecasting ability. This work proposes a new approach to data assimilation via machine learning where Dee...

Chemically reacting flows are common in engineering, such as hypersonic flow, combustion, explosions, manufacturing processes and environmental assessments. For combustion, the number of reactions can be significant (over 100) and due to the very large CPU requirements of chemical reactions (over 99%) a large number of flow and combustion problems...

This paper presents a review of research on underwater explosions (UNDEX) with a focus on the structural response of concrete or reinforced concrete (RC) structures. First, the physical phenomena of UNDEX and its effects are discussed describing both the theory and considerations of the event. Then a brief description of the standard UNDEX experime...

View Video Presentation: https://doi.org/10.2514/6.2021-2858.vid A finite volume method with a multi-fluid single velocity and single pressure model is implemented in a three-dimensional vertex centered code, FEFLO. The considered fluids include air, water and high explosive material. The ideal equation of state is used for air. The stiffened gas e...

An overview of high-fidelity modeling of pathogen propagation, transmission and mitigation in the built environment is given. In order to derive the required physical and numerical models, the current understanding of pathogen, and in particular virus transmission and mitigation is summarized. The ordinary and partial differential equations that de...

For any given neural network architecture a permutation of weights and biases results in the same functional network. This implies that optimization algorithms used to 'train' or 'learn' the network are faced with a very large number (in the millions even for small networks) of equivalent optimal solutions in the parameter space. To the best of our...

A multiscale approach for the detailed simulation of water droplets dispersed in a turbulent airflow is presented. The multiscale procedure combines a novel representative volume element (RVE) with the Pseudo Direct Numerical Simulation (P-DNS) method. The solution at the coarse-scale relies on a synthetic model, constructed using precomputed offli...

The aim of this work is a fair and unbiased comparison of a lattice Boltzmann method (LBM) against a finite difference method (FDM) for the simulation of fluid flows. Rather than reporting metrics such as floating point operation rates or memory throughput, our work considers the engineering quest of reaching a desired solution quality with the lea...

Purpose The purpose of this study is to compare interpolation algorithms and deep neural networks for inverse transfer problems with linear and nonlinear behaviour. Design/methodology/approach A series of runs were conducted for a canonical test problem. These were used as databases or “learning sets” for both interpolation algorithms and deep neu...

A finite volume method with a six-equation multi-fluid single-pressure model is implemented in a three-dimensional vertex centered code, FEFLO. The six-equation was developed from the stratified flow model. A least-squares reconstruction with Kuzmins vertex-based (VB) limiter is implemented for the volume fraction and a set of primitive variables i...

Fault-tolerant computing options based on the use of restart information stored on and off node and the use of reserve processes have been developed, implemented and tested in a large-scale, production field solver taken from the domain of computational fluid dynamics. The tests conducted to date have shown good results, with recovery rates approac...

Experiences gained when porting a production finite difference code written in Fortran to GPUs using OpenACC are summarized. While the original code was written taken into consid- eration vectorization and shared-memory parallelization via OpenMP, the loop structures and the proper placement of directives had to be analyzed in depth, leading to a p...

Engineers create new things and hence always have to deal with incomplete information. A critical review is made of the accuracy of the available physical and modeling parameters. It shows that in many cases key physical and modeling parameters such as viscosities, boundary conditions, geometry, and even basic physics are not known to within 1%. Th...

A high fidelity model for the propagation of pathogens via aerosols in the presence of moving pedestrians is proposed. The key idea is the tight coupling of computational fluid dynamics and computational crowd dynamics in order to capture the emission, transport and inhalation of pathogen loads in space and time. An example simulating pathogen prop...

This paper introduces a novel algorithmic framework for a deep neural network (DNN), which in a mathematically rigorous manner, allows us to incorporate history (or memory) into the network—it ensures all layers are connected to one another. This DNN, called Fractional-DNN, can be viewed as a time-discretization of a fractional in time non-linear o...

A summary is given of the mechanical characteristics of virus contaminants and the transmission via droplets and aerosols. The ordinary and partial differential equations describing the physics of these processes with high fidelity are presented, as well as appropriate numerical schemes to solve them. Several examples taken from recent evaluations...

A high fidelity model for the propagation of pathogens via aerosols in the presence of moving pedestrians is proposed. The key idea is the tight coupling of computational fluid dynamics and computational crowd dynamics in order to capture the emission, transport and inhalation of pathogen loads in space and time. An example simulating pathogen prop...

We adopted the Ensemble Kalman Filter (EnKF) methodology in our computational simulation code for pedestrian flows. The EnKF, which is a type of data assimilation methodology, has been developed in the field of weather forecast where the atmospheric condition varies hour by hour. The EnKF estimates the parameters or boundary/initial conditions in t...

Velocity is a fundamental property of foot traffic flow. Monitoring the change of velocity patterns at high pedestrian densities may provide valuable insights on foot traffic dynamics. In this paper, a closer look is taken to explore the capability of the Particle Image Velocimetry (PIV) technique on extracting crowd velocities from surveillance ca...

Two models for the motion in the Mataf of the Holy Mosque of Makkah that are applicable to any so-called social force or agent-based model are presented. The first model assigns a desired distance to the Kaaba to each pilgrim. The second model assumes that pilgrims desire to get as close as possible to the Kaaba until a tolerable density is reached...

Explicit two-step Runge-Kutta (TSRK) methods offer an efficient alternative to traditional explicit Low-Storage Runge-Kutta (LSRK) schemes for solving the Navier-Stokes equations. A special class of TSRK methods that reduce requirement compared to previous TSRK schemes are derived. Schemes of fourth, fifth and sixth order are implemented and tested...

Computational fluid dynamics (CFD) has emerged as a successful tool for industry applications and basic science during the last decades. However, accurate solutions involving vortex propagation, and separated and turbulent flows, are still associated with high computing costs. In particular, large eddy simulations (LES) of complex geometries, such...

A summary is given of the mechanical characteristics of virus contaminants and the transmission via droplets and aerosols. The ordinary and partial differential equations describing the physics of these processes with high fidelity are presented, as well as appropriate numerical schemes to solve them. Several examples taken from recent evaluations...

Fault-tolerant computing options based on the use of restart information stored on and off node and the use of reserve processes have been developed, implemented and tested in a large-scale, production field solver taken from the domain of computational fluid dynamics. The tests conducted to date have shown good results, with recovery rates approac...

Purpose The purpose of this paper is to shed some light into the equivalency of grid size and spatial and temporal orders of accuracy for turbulent flows. Design/methodology/approach This paper compared several finite difference schemes on various meshes and various orders of accuracy for the canonical Taylor–Green vortex testcase. Findings A rem...

This paper introduces a novel algorithmic framework for a deep neural network (DNN), which in a mathematically rigorous manner, allows us to incorporate history (or memory) into the network -- it ensures all layers are connected to one another. This DNN, called Fractional-DNN, can be viewed as a time-discretization of a fractional in time nonlinear...

An automated approach to explore the fundamental properties of high-density pedestrian traffic is outlined. The framework operates on video or time lapse images captured from surveillance cameras. For pedestrian velocity extraction, the framework incorporates cross-correlation based Particle Image Velocimetry (PIV) techniques. For pedestrian densit...

Purpose The purpose of this study is to determine the possibility of an accurate assessment of the spatial distribution of material properties such as conductivities or impedances from boundary measurements when the governing partial differential equation is a Laplacian. Design/methodology/approach A series of numerical experiments were carefully...

A finite element code for heat conduction, together with an adjoint solver and a suite of optimization tools was applied for the solution of Calderon's problem. One of the questions whose answer was sought was whether the solution to these problems is unique and obtainable. The results to date show that while the optimization procedure is able to o...

This work presents a novel framework for automated monitoring of high density crowds from closed circuit television (CCTV) image data. The framework obtains pedestrian velocities from particle image velocimetry (PIV) technique and densities from a boosted ferns machine learning model. A pinhole camera based perspective correction scheme is employed...

The discrete element method (DEM) has been implemented to model abrasive wear on steel plates that interact with bulk solids, such as crushed copper ore. The Archard model was used to predict the rate and the wear pattern on a structural steel plate A37-24 ES under different types of copper ore. One of the key steps of the methodology consists of t...

Pedestrian traffic is an important subject of surveillance to ensure public safety and traffic management, which may benefit from intelligent and continuous analysis of pedestrian videos. State-of-the-art methods for intelligent pedestrian surveillance have a number of limitations in automating and deriving useful information of high-density pedest...

Industrial large-eddy simulation (LES), i.e. overnight runs with O(109) degrees of freedom (DOFs) and O(107) timesteps, has been one of the top priorities of CFD research over the last two decades. Current network and solver timings indicate that the required target of 5 ms/timestep is within reach: some cumulant Lattice Boltzmann Method (LBM) code...

A complex numerical simulation where blast waves interacted with a liquid, burnable simulant inside a chamber was conducted. The liquid simulant was dispersed by a small high explosive (HE) charge and then impacted by the blast wave produced by a significantly larger HE charge. The physics of the problem required numerical tools that allowed for (1...

The analysis of velocities from high-density pedestrian events may provide more information on pedestrian flow dynamics. A framework based on particle image velocimetry (PIV)--a technique commonly used in experimental fluid dynamics--has been developed to evaluate the pedestrian velocities from high-density pedestrian events. The framework takes a...

This paper describes the application of a coupled CFD and CSD methodology to the simulation of a thick-cased explosive-filled cylinder placed inside a pipe composed of either steel or various strength stones. The objective is to better understand the energy exchange mechanisms between the impacting case debris produced by either stationary or movin...

Purpose It is of paramount importance to ensure safe and fast evacuation routes in cities in case of natural disasters, environmental accidents or acts of terrorism. The same applies to large-scale events such as concerts, sport events and religious pilgrimages as airports and to traffic hubs such as airports and train stations. The prediction of...

Nested cartesian grid systems by design require interpolation of solution fields from coarser to finer grid systems. While several choices are available, preserving accuracy, stability and efficiency at the same time require careful design of the interpolation schemes. Given this context, a series of interpolation algorithms based on post processin...

We present recent developments in the coupling of the finite element method (FEM) and the discrete element method (DEM) for the analysis of rock blasting operations in tunnels. The coupled FEM-DEM technique has been proven to be an efficient procedure for predicting the multiple fractures of rock induced by the loads generated in blasting. The coup...

This work investigates the application and interaction of optimization techniques and performance models in a computational fluid dynamics (CFD) approach employing an OpenMP parallelized, explicit, weakly compressible, finite difference–based solver for the incompressible Navier–Stokes equations using a five-point wide stencil. The presented loop a...

An experimental campaign was undertaken to measure the pedestrian flow in the region close to the Kaaba during the Hajj pilgrimages of 2014 and 2015. High resolution video and photographs were used. The space was divided into areas of 10 sqm. The pedestrians were counted, and the velocity measured from video clips. The results were surprising: the...

The role of computational modeling for biomechanics will be increasingly prominent. In computational biomechanics, model sharing can facilitate assessment of reproducibility, and can provide an opportunity for repurposing and reuse, and a venue for medical training. The community's desire to investigate biological and biomechanical phenomena crossi...

Purpose A common observation made when computing chemically reacting flows is how CPU intensive these are in comparison to cold flow cases. The update of tens or hundreds of species with hundreds or thousands of reactions can easily consume more then 95\% of the total CPU time. In many cases the region where reactions (combustion) are actually tak...

Intel's latest Xeon Phi processor, Knights Landing (KNL), has the potential to provide over 2.6 TFLOPS. However, to obtain maximum performance on the KNL, significant refactoring and optimization of application codes is still required to exploit key architectural innovations that KNL features – wide vector units, many-core node design, and deep mem...

A methodology to integrate geographical information system (GIS) data with large-scale pedestrian simulations has been developed. Advances in automatic data acquisition and archiving from GIS databases, automatic input for pedestrian simulations, as well as scalable pedestrian simulation tools have made it possible to simulate pedestrians at the in...

Currently, several challenges prevent poly(lactic-co-glycolic acid) (PLGA) particles from reaching clinical settings. Among these is a lack of understanding of the molecular mechanisms involved in the formation of these particles. We have been studying in depth the formation of patchy polymeric particles. These particles are made of PLGA and lipid-...

Purpose – The purpose of this paper is to determine the reason for the discrepancy in estimated and observed damage caused by fragmenting charges in closed environments. Design/methodology/approach – A series of carefully conducted physical and numerical experiments was conducted. The results were analyzed and compared. Findings – The analysis sh...

Purpose – The purpose of this paper is to develop a first-principles model for the simulation of pedestrian flows and crowd dynamics capable of computing the movement of a million pedestrians in real-time in order to assess the potential safety hazards and operational performance at events where many individuals are gathered. Examples of such situa...

Patchy polymeric particles have anisotropic surface domains that can be remarkably useful in diverse medical and industrial fields because of their ability to simultaneously present two different surface chemistries on the same construct. In this article, we report the mechanisms involved in the formation of novel lipid-polymeric hollow patchy part...