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Documents published in Scipedia

• Strategy for modeling flow diverters in cerebral aneurysms as a porous medium

  M. Raschi, F. Mut, R. Lohner, J. Cebral

  Int. J. Numer. Meth. Bio. Engng. (2014). Vol. 30 (9), pp. 909-925

  
  

  Abstract

  Simulations using the patient‐specific geometry of the aneurysm may help in a better planning of the treatment and in a consequent reduction of the associated risks. We propose, evaluate, and implement a methodology for the simulation of flow diverter (FD) devices in intracranial aneurysms by using a porous medium method (PMM), which greatly reduces the computational cost of these simulations compared with immersed method (IMM) approaches used to model complex FDs. The method relies on parameters from an empirical correlation derived from experimental observations in wire screens, consistent with CFD simulations. The verification of our PMM strategy was carried out by comparing the results of simulations in different (patient‐specific) geometries and FDs, to those obtained under identical conditions by the IMM. Overall, both quantitative and qualitative results are consistent between IMM and PMM in cases where the local porosity remains roughly uniform throughout the neck, with differences in the reduction of the observables lower than 10%. This PMM strategy is up to 10 times faster than the IMM, which allows for a runtime of hours instead of days, bringing it closer for its application in the clinic.

  Abstract
  Simulations using the patient‐specific geometry of the aneurysm may help in a better planning of the treatment and in a consequent reduction of the associated risks. We [...]

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• Verification of a Pedestrian Simulation Tool Using the NIST Recommended Test Cases

  M. Isenhour, R. Lohner

  Transportation Research Procedia (2014). Vol. 2, pp. 237-248

  
  

  Abstract

  In an attempt to develop a verification and validation standard for building fire evacuation models, Ronchi et al. (2013) at the United States’ National Institute of Standards and Technology (NIST) recommended a set of seventeen verification tests. We found that the application of these verification tests allowed us to make rather significant improvements to our simulation code (PEDFLOW) for approximately half of the recommended tests (Table 1). In some cases, we added capabilities that did not exist before. In other cases, we found anomalous behaviors and adjusted the existing code to remove these unexplained behaviors. This paper summarizes the work on the verification tests, highlighting the lessons learned and modifications made. We also discuss some modifications we recommend to the NIST verification tests, as well as demonstrate how to make these tests suitable for all pedestrian flow models (not just building fire evacuation).

  Abstract
  In an attempt to develop a verification and validation standard for building fire evacuation models, Ronchi et al. (2013) at the United States’ National Institute [...]

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• Validation of a Pedestrian Simulation Tool Using the NIST Stairwell Evacuation Data

  M. Isenhour, R. Lohner

  Transportation Research Procedia (2014). Vol. 2, pp. 739-744

  
  

  Abstract

  Researchers at the United States National Institute of Standards and Technology (NIST) collected occupant egress data in the stairwells of several high-rise buildings for potential use in quantitative and qualitative validation of evacuation simulation tools (Kuligowski and Peacock (2010)). We found this data suitable for establishing occupant initial locations, pre-evacuation time distributions, and other parametric inputs for our simulation code (PEDFLOW). With this data set, we were able to validate several core behavioral components of PEDFLOW by directly comparing actual versus predicted values for occupant speed on stairs and building total evacuation times. This paper summarizes our work on the stairwell data sets, highlighting the methodology behind the extraction of values for the parametric inputs, and demonstrating the results obtained for one specific 10-story high-rise building data-set.

  Abstract
  Researchers at the United States National Institute of Standards and Technology (NIST) collected occupant egress data in the stairwells of several high-rise buildings for [...]

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• On critical densities and velocities for pedestrians entering a crowd

  R. Lohner, E. Haug

  Transportation Research Procedia (2014). Vol. 2, pp. 394-399

  
  

  Abstract

  The problem of pedestrians trying to enter and/or advance against an incoming crowd is studied. The analysis shows that starting with a very limited set of assumptions (elliptical cross-section of pedestrians; constant ratio of forward to lateral separation) one is able to derive from purely kinematic considerations critical densities beyond which it is impossible for pedestrians to enter and/or advance into an incoming crowd. The results obtained indicate that for the common pedestrian size of a = 0.5 m, b = 0.3 m, the limit densities range from ρ = O(5.0 – 6.06) [p/m2], in good agreement with empirical observations.

  Abstract
  The problem of pedestrians trying to enter and/or advance against an incoming crowd is studied. The analysis shows that starting with a very limited set of assumptions (elliptical [...]

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• On the Influence of columns in densely populated corridors

  M. Gdoura, R. Lohner, E. Haug, B. Gawenat

  Transportation Research Procedia (2014). Vol. 2, pp. 2-9

  
  

  Abstract

  An experimental and computational campaign was undertaken to measure the influence of columns in densely populated corridors. undertaken. A instrumented corridor of 5x12.54 m with two columns was enclosed by walls, and several hundred people were asked to walk repeatedly through the enclosed region. The maximum density achieved was of the order of 6 people/sqm, higher than what is deemed safe for many operating standards. The experiments were run with and without columns to see their influence. Remarkably, even at high densities, the columns had no noticeable influence on the pedestrian motion. Furthermore, the forces measured on the columns were very small (noise-level).

  Abstract
  An experimental and computational campaign was undertaken to measure the influence of columns in densely populated corridors. undertaken. A instrumented corridor of [...]

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• Quantitative Validation of PEDFLOW for Description of Unidirectional Pedestrian Dynamics

  J. Zhang, D. Britto, M. Chraibi, E. Haug, B. Gawenat, R. Lohner

  Transportation Research Procedia (2014). Vol. 2, pp. 733-738

  
  

  Abstract

  The results of a systematic quantitative validation of PEDFLOW based on the experimental data from FZJ are presented. Uni- directional flow experiments, totaling 28 different combinations with varying entry, corridor and exit widths, were considered. The condition imposed on PEDFLOW was that all the cases should be run with the same input parameters. The exit times and fundamental diagrams for the measuring region were evaluated and compared. This validation process led to modifications and enhancements of the model underlying PEDFLOW. The preliminary conclusions indicate that the results agree well for densities smaller than 3 m-2 and a good agreement is observed even at high densities for the corridors with bcor = 2.4 m, and bcor = 3.0 m. For densities between 1 and 2 m-2 the specific flow and velocities are underpredicted by PEDFLOW.  

  Abstract
  The results of a systematic quantitative validation of PEDFLOW based on the experimental data from FZJ are presented. Uni- directional flow experiments, totaling 28 [...]

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• Mechanisms Involved in the Formation of Biocompatible Lipid Polymeric Hollow Patchy Particles

  N. Rasheed, A. Khorasani, J. Cebral, C. Salvador-Morales, F. Mut, R. Lohner

  Langmuir (2015). Vol. 31 (24), pp. 6639-6648

  
  

  Abstract

  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 particles during their synthesis. By cross-sectioning the patchy particles, we found that a phase segregation phenomenon occurs between the core, shell, and patch. Importantly, we found that the shear stress that the polymer blend undergoes during the particle synthesis is the most important parameter for the formation of these patchy particles. In addition, we found that the interplay of solvent–solvent, polymer–solvent, and polymer–polymer–solvent interactions generates particles with different surface morphologies. Understanding the mechanisms involved in the formation of patchy particles allows us to have a better control on their physicochemical properties. Therefore, these fundamental studies are critical to achieve batch control and scalability, which are essential aspects that must be addressed in any type of particle synthesis to be safely used in medicine.

  Abstract
  Patchy polymeric particles have anisotropic surface domains that can be remarkably useful in diverse medical and industrial fields because of their ability to simultaneously [...]

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• Recent advances in computational wind engineering and fluid-structure interaction

  R. Lohner, E. Haug, A. Michalski, B. Muhammad, A. Drego, R. Nanjundaiah, R. Zarfam

  Journal of Wind Engineering and Industrial Aerodynamics (2015). Vol. 144, pp. 14-23

  
  

  Abstract

  Recent developments that are pertinent to the particular field of computing lightweight structures exposed to windloads are described. The topics covered include computational fluid dynamics (CFD), computational structural dynamics (CSD) and fluid–structure interaction (FSI) for wind vs. aerospace engineering, recent hardware and software trends, butterfly effects, rogue loads, and adjoint estimation of boundary conditions.

  Abstract
  Recent developments that are pertinent to the particular field of computing lightweight structures exposed to windloads are described. The topics covered include computational [...]

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• Real-time micro-modelling of a million pedestrians

  R. Lohner, M. Baqui, E. Haug, B. Muhamad

  Engineering Computations (2016). Vol. 33 (1), pp. 217-237

  
  

  Abstract

  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 situations are sport and music events, cinemas and theatres, museums, conference centres, places of pilgrimage and worship, street demonstrations, emergency evacuation during natural disasters. Design/methodology/approach - The model is based on a series of forces, such as: will forces (the desire to reach a place at a certain time), pedestrian collision avoidance forces, obstacle/wall avoidance forces; pedestrian contact forces, and obstacle/wall contact forces. In order to allow for general geometries a so-called background triangulation is used to carry all geographic information. At any given time the location of any given pedestrian is updated on this mesh. The model has been validated qualitatively and quantitavely on repeated occasions. The code has been ported to shared and distributed memory parallel machines. Findings - The results obtained show that the stated aim of computing the movement of a million pedestrians in real-time has been achieved. This is an important milestone, as it enables faster-thanreal- time simulations of large crowds (stadiums, airports, train and bus stations, concerts) as well as evacuation simulations for whole cities. Research limitations/implications - All models are wrong, but some are useful. The same applies to any modelling of pedestrians. Pedestrians are not machines, so stochastic runs will be required in the future in order to obtain statistically relevant ensembles. Practical implications - This opens the way to link real-time data gathering of crowds (i.e. via cameras) with predictive calculations done faster than real-time, so that security personnel can be alerted to potential future problems during large-scale events. Social implications - This will allow much better predictions for large-scale events, improving security and comfort. Originality/value - This is the first time such speeds have been achieved for a micro-modelling code for pedestrians.

  Abstract
  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 [...]

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• The simulation of dust effects from fragmenting charges

  O. Soto, J. Baum, F. Togashi, R. Lohner, R. Frank, A. Amini

  Int. J. Numer. Meth. Heat and Fluid Flows (2016). Vol. 26 (3/4), pp. 999-1026

  
  

  Abstract

  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 shows that for fragmenting charges in closed environments, dust plays a far larger role than previously thought, leading to much lower pressures and damage. Research limitations/implications - In light of these findings, many assumptions and results for fragmenting charges in closed environments need to be reconsidered. Practical implications - This implies that for a far larger class of problems than previously estimated it is imperative to take into consideration dust production and its effect on the resulting pressures. Originality/value - This is the first time such a finding has been reported in this context.    

  Abstract
  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. [...]

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