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Article

Rainald Löhner, Fumiya Togashi and Joseph David Baum

A common observation made when computing chemically reacting flows is how central processing unit (CPU)-intensive these are in comparison to cold flow cases. The update of…

Abstract

Purpose

A common observation made when computing chemically reacting flows is how central processing unit (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 than 95% of the total CPU time. In many cases, the region where reactions (combustion) are actually taking place comprises only a very small percentage of the volume. Typical examples are flame fronts propagating through a domain. In such cases, only a small fraction of points/cells needs a full chemistry update. This leads to extreme load imbalances on parallel machines. The purpose of the present work is to develop a methodology to balance the work in an optimal way.

Design/methodology/approach

Points that require a full chemistry update are identified, gathered and distributed across the network, so that work is evenly distributed. Once the chemistry has been updated, the unknowns are gathered back.

Findings

The procedure has been found to work extremely well, leading to optimal load balance with insignificant communication overheads.

Research limitations/implications

In many production runs, the procedure leads to a reduction in CPU requirements of more than an order of magnitude. This allows much larger and longer runs, improving accuracy and statistics.

Practical implications

The procedure has allowed the calculation of chemically reacting flow cases that were hitherto not possible.

Originality/value

To the authors’ knowledge, this type of load balancing has not been published before.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 27 no. 12
Type: Research Article
ISSN: 0961-5539

Keywords

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Article

Fumiya Togashi, Takashi Misaka, Rainald Löhner and Shigeru Obayashi

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…

Abstract

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 pedestrian is notoriously difficult because it varies depending on circumstances (age group, cultural characteristics, etc.). In this study, the Ensemble Kalman Filter (EnKF) data assimilation technique, which uses the updated observation data to improve the accuracy of the simulation, was applied to improve the accuracy of numerical simulations of pedestrian flow.

Design/methodology/approach

The EnKF, one of the data assimilation techniques, was applied to the in-house numerical simulation code for pedestrian flow. Two cases were studied in this study. One was the simplified one-directional experimental pedestrian flow. The other was the real pedestrian flow at the Kaaba in Mecca. First, numerical simulations were conducted using the empirical input parameter sets. Then, using the observation data, the EnKF estimated the appropriate input parameter sets. Finally, the numerical simulations using the estimated parameter sets were conducted.

Findings

The EnKF worked on the numerical simulations of pedestrian flow very effectively. In both cases: simplified experiment and real pedestrian flow, the EnKF estimated the proper input parameter sets which greatly improved the accuracy of the numerical simulation. The authors believe that the technique such as EnKF could also be used effectively in other fields of computational engineering where simulations and data have to be merged.

Practical implications

This technique can be used to improve both design and operational implementations of pedestrian and crowd dynamics predictions. It should be of high interest to command and control centers for large crowd events such as concerts, airports, train stations and pilgrimage centers.

Originality/value

To the authors’ knowledge, the data assimilation technique has not been applied to a numerical simulation of pedestrian flow, especially to the real pedestrian flow handling millions pedestrian such as the Mataf at the Kaaba. This study validated the capability and the usefulness of the data assimilation technique to numerical simulations for pedestrian flow.

Details

Engineering Computations, vol. 35 no. 7
Type: Research Article
ISSN: 0264-4401

Keywords

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Article

Alejandro Figueroa and Rainald Löhner

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.

Abstract

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 remarkable empirical result observed is that the accuracy of eighth-order schemes on grids of size 2 h was equivalent to second-order schemes on grids of size h. This was the case when going from 1003 to 2003 as well as from 2003 to 4003. In all these cases, kinetic energy, vorticity and numerical dissipation were compared in detail.

Research limitations/implications

The results obtained were carried out using finite difference solvers. It remains to be seen if similar behaviors are also observed for other numerical schemes.

Originality/value

These (surprising) findings should be of interest to the larger community.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 31 no. 1
Type: Research Article
ISSN: 0961-5539

Keywords

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Article

Rainald Löhner, Harbir Antil, Hamid Tamaddon-Jahromi, Neeraj Kavan Chakshu and Perumal Nithiarasu

The purpose of this study is to compare interpolation algorithms and deep neural networks for inverse transfer problems with linear and nonlinear behaviour.

Abstract

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 neural networks. A second set of runs was conducted to test the prediction accuracy of both approaches.

Findings

The results indicate that interpolation algorithms outperform deep neural networks in accuracy for linear heat conduction, while the reverse is true for nonlinear heat conduction problems. For heat convection problems, both methods offer similar levels of accuracy.

Originality/value

This is the first time such a comparison has been made.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. ahead-of-print no. ahead-of-print
Type: Research Article
ISSN: 0961-5539

Keywords

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Article

Rainald Löhner and Harbir Antil

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…

Abstract

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 performed. The results were analyzed and compared.

Findings

The results to date show that while the optimization procedure is able to obtain spatial distributions of the conductivity k that reduce the cost function significantly, the resulting conductivity k is still significantly different from the target (or real) distribution sought. While the normal fluxes recovered are very close to the prescribed ones, the tangential fluxes can differ considerably.

Research limitations/implications

At this point, it is not clear why rigorous mathematical proofs yield results of convergence and uniqueness, while in practice, accurate distributions of the conductivity k seem to be elusive. One possible explanation is that the spatial influence of conductivities decreases exponentially with distance. Thus, many different conductivities inside a domain could give rise to very similar (infinitely close) boundary measurements.

Practical implications

This implies that the estimation of field conductivities (or generally field data) from boundary data is far more difficult than previously assumed when the governing partial differential equation in the domain is a Laplacian. This has consequences for material parameter assessments (e.g. for routine maintenance checks of structures), electrical impedance tomography, and many other applications.

Originality/value

This is the first time such a finding has been reported in this context.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 30 no. 11
Type: Research Article
ISSN: 0961-5539

Keywords

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Article

Orlando A. Soto, Joseph D. Baum, Fumiya Togashi, Rainald Löhner, Robert A. Frank and Ali Amini

– 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.

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.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 26 no. 3/4
Type: Research Article
ISSN: 0961-5539

Keywords

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Article

Rainald Löhner and Joseph Baum

Limitations in space and city planning constraints have led to the search for alternative shock mitigation devices that are architecturally appealing. The purpose of this…

Abstract

Purpose

Limitations in space and city planning constraints have led to the search for alternative shock mitigation devices that are architecturally appealing. The purpose of this paper is to consider a compromise solution which consists of partially open, thick, bending-resistant shapes made of acrylic material that may be Kevlar- or steel-reinforced. Seven different configurations were analyzed numerically.

Design/methodology/approach

For the flow solver, the FEM-FCT scheme as implemented in FEFLO is used. The flowfields are initialized from the output of highly detailed 1-D (spherically symmetric) runs. Peak pressure and impulse are stored and compared. In total, seven different configurations were analyzed numerically.

Findings

It is found that for some of these, the maximum pressure is comparable to usual, closed walls, and the maximum impulse approximately 50 percent higher. This would indicate that such designs offer a blast mitigation device eminently suitable for built-up city environments.

Research limitations/implications

Future work will consider fully coupled fluid-structure runs for the more appealing designs, in order to assess whether such devices can be manufactured from commonly available materials such as acrylics or other poly-carbonates.

Practical implications

This would indicate that such designs offer a blast mitigation device eminently suitable for built-up city environments.

Originality/value

This is the first time such a semi-open blastwall approach has been tried and analyzed.

Details

Engineering Computations: International Journal for Computer-Aided Engineering and Software, vol. 29 no. 1
Type: Research Article
ISSN: 0264-4401

Keywords

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Article

Rainald Löhner and Fernando Camelli

Develop a method for the optimal placement of sensors in order to detect the largest number of contaminant release scenarios with the minimum amount of sensors.

Abstract

Purpose

Develop a method for the optimal placement of sensors in order to detect the largest number of contaminant release scenarios with the minimum amount of sensors.

Design/methodology/approach

The method considers the general sensor placement problem. Assuming a given number of sensors, every release scenario leads to a sensor input. The data recorded from all the possible release scenarios at all possible sensor locations allow the identification of the best or optimal sensor locations. Clearly, if only one sensor is to be placed, it should be at the location that recorded the highest number of releases. This argument can be used recursively by removing from further consideration all releases already recorded by sensors previously placed.

Findings

The method developed works well. Examples showing the effect of different wind conditions and release locations demonstrate the effectiveness of the procedure.

Practical implications

The method can be used to design sensor systems for cities, subway stations, stadiums, concert halls, high value residential areas, etc.

Originality/value

The method is general, and can be used with other physics‐based models (puff, mass‐conservation, RANS, etc.). The investigation also shows that first‐principles CFD models have matured sufficiently to be run in a timely manner on PCs, opening the way to optimization based on detailed physics.

Details

Engineering Computations, vol. 22 no. 3
Type: Research Article
ISSN: 0264-4401

Keywords

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Article

Rainald Lohner, Dominic Britto, Alexander Michailski and Eberhard Haug

During a routine benchmarking and scalability study of CFD codes for typical large-scale wind engineering runs, it was observed that the resulting loads for buildings…

Abstract

Purpose

During a routine benchmarking and scalability study of CFD codes for typical large-scale wind engineering runs, it was observed that the resulting loads for buildings varied considerably with the number of parallel processors employed. The differences remained very small at the beginning of a typical run, and then grew progressively to a state of total dissimilitude. A “butterfly-effect” for such flows was suspected and later confirmed. The paper aims to discuss these issues.

Design/methodology/approach

A series of numerical experiments was conducted for massively separated flows. The same geometry – a cube in front of an umbrella – was used to obtain the flowfields using different grids, different numbers of domains/processors, slightly different inflow conditions and different codes.

Findings

In all of these cases the differences remained very small at the beginning of a typical run, they then grew progressively to a state of total dissimilitude. While the mean and maximum loads remained similar, the actual (deterministic) instantiations were completely different. The authors therefore suspect that for flows of this kind a “butterfly effect” is present, whereby even very small (roundoff) errors can have a pronounced effect on the actual deterministic instantiation of a flowfield.

Research limitations/implications

This implies that for flows of this kind the CFD runs have to be carried out to much larger times than formerly expected (and done) in order to obtain statistically relevant ensembles.

Practical implications

For practical calculations this implies running to much larger times in order to reach statistically relevant ensembles, with the associated much higher CPU time requirements.

Originality/value

This is the first time such a finding has been reported in the numerical wind engineering context.

Details

Engineering Computations, vol. 31 no. 4
Type: Research Article
ISSN: 0264-4401

Keywords

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Article

Ralf Tilch, Ali Tabbal, Ming Zhu, Friedhelm Decker and Rainald Löhner

This paper seeks to reduce the time it takes to perform external aerodynamic simulations without compromising accuracy. At present, cleaning up CAD data sets, in…

Abstract

Purpose

This paper seeks to reduce the time it takes to perform external aerodynamic simulations without compromising accuracy. At present, cleaning up CAD data sets, in particular for undercarriage parts, takes several man‐weeks.

Design/methodology/approach

Body‐fitted and embedded mesh techniques are combined to obtain accurate external aerodynamic solutions for realistic car geometries with minimal user intervention. The key idea is to mesh with typical body‐fitted RANS grids the external shape of the vehicle, which is smooth and requires detailed physical modeling. The underhood and undercarriage are treated as embedded surfaces. The flow in this region is massively separated, requiring LES runs and isotropic grids. This makes it a suitable candidate for embedded grids.

Findings

Comparisons with body‐fitted and experimental data for a typical car show that this approach can yield drag predictions with an error less than 5 percent.

Practical implications

The present approach reduces turnaround times for complete car geometries to one to two days, without compromising accuracy.

Originality/value

To the authors' knowledge, this is the first time such an approach has been tried and validated for external aerodynamics.

Details

Engineering Computations, vol. 25 no. 1
Type: Research Article
ISSN: 0264-4401

Keywords

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