Optimal placement of sensors for contaminant detection based on detailed 3D CFD simulations

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

Dynamic deactivation for advection‐dominated contaminant transport

A simple dynamic deactivation procedure for advection‐dominated contaminant transport is presented. The key idea is to avoid any work in regions where the solution cannot change, i.e. where sources vanish and unknowns do not exhibit any spatial change. Saving factors in CPU of 1: 3–1: 10 are commonly achieved. The procedure is general and simple to implement, and should be applicable to a large number of codes and transport problems

Assessing maximum possible damage for contaminant release events

The combined use of damage criteria, genetic algorithms and advanced CFD solvers provides an effective strategy to identify locations of releases that produce maximum damage. The implementation is simple and does not require any change to flow solvers. A rather general criterion has been formulated to determine the damage inflicted by the intentional or unintentional release of contaminants. Results of two typical cases show that damage can vary considerably as a function of release location, implying that genetic algorithms are perhaps the only techniques suited for this type of optimization proble

Tightly coupled computational fluid and crowd dynamics via immersed boundary methods

A methodology to couple computational fluid and computational crowd dynamics (CFD, CCD) has been developed. Technological advances that made this possible include: a) Mature CFD and CCD codes/solvers; b) Development of immersed boundary methods for moving bodies (CFD); c) Strong scaling to tens of thousands of cores (CFD); and d) Implementation of fast search techniques for information transfer between codes (CFD, CCD). We consider that tightly coupled simulations such as the ones presented here will lead to more realistic evacuation studies where fire, smoke, visibility and inhalation of toxic materials influence the motion of people, and where a large crowd can block or influence the flow in turn. Cases where this may occur are metro-stations, high-rise buildings and indoor sports arenas, where a crowd can block a considerable portion of the passage area, thereby influencing the flow

Simulation of the must field experiment using the FEFLO-URBAN CFD model

The Setting Test (MUST) experiment, that was carried out at Dugway Proving Ground, was analyzed using Very Large Eddy Simulation (VLES). The CFD approaches used to simulate transport and dispersion in the atmosphere at the urban scale included Reynold Average Navier-Stokes (RANS) and LES. Improvements of the vertical turbulence production were introduced using the geometrical roughness applied to the ground surface. The results show that small changes in wind direction can produce large localized changes in concentration levels

Semi‐automatic porting of a large‐scale Fortran CFD code to GPUs

The development of automatic techniques to port a substantial portion of FEFLO, a general‐purpose legacy CFD code operating on unstructured grids, to run on GPUs is described. FEFLO is a typical adaptive, edge‐based finite element code for the solution of compressible and incompressible flows, which is primarily written in Fortran 77 and has previously been ported to vector, shared memory parallel and distributed memory parallel machines. Owing to the large size of FEFLO and the likelihood of human error in porting, as well as the desire for continued development within a single codebase, a specialized Python script, based on FParser (Int. J. Comput. Sci. Eng. 2009; 4 :296–305), was written to perform automated translation from the OpenMP‐parallelized edge and point loops to GPU kernels implemented in CUDA, along with GPU memory management. The results of verification benchmarks and performance indicate that performances achieved by such a translator can rival those of codes rewritten by specialists. The approach should be of general interest, as how best to run on GPUs is being presently considered for many so‐called legacy codes

A Linelet preconditioner for incompressible flows

A parallel linelet preconditioner has been implemented to accelerate finite element (FE) solvers for incompressible flows when highly anisotropic meshes are used. The convergence of the standard preconditioned conjugate gradient (PCG) solver that is commonly used to solve the discrete pressure equations, greatly deteriorates due to the presence of highly distorted elements, which are of mandatory use for high Reynolds-number flows. The linelet preconditioner notably accelerates the convergence rate of the PCG solver in such situations, saving an important amount of CPU time. Unlike other more sophisticated preconditioners, parallelization of the linelet preconditioner is almost straighforward. Numerical examples and some comparisons with other preconditioners are presented to demonstrate the performance of the proposed preconditioner

A feature‐preserving volumetric technique to merge surface triangulations

Several extensions and improvements to surface merging procedures based on the extraction of iso-surfaces from a distance map defined on an adaptive background grid are presented. The main objective is to extend the application of these algorithms to surfaces with sharp edges and corners. In order to deal with objects of different length scales, the initial background grids are created using a Delaunay triangulation method and local voxelizations. A point enrichment technique that introduces points into the background grid along detected surface features such as ridges is used to ensure that these features are preserved in the final merged surface. The surface merging methodology is extended to include other Boolean operations between surface triangulations. The iso-surface extraction algorithms are modified to obtain the correct iso-surface for multi-component objects. The procedures are demonstrated with various examples, ranging from simple geometrical entities to complex engineering applications. The present algorithms allow realistic modelling of a large number of complex engineering geometries using overlapping components defined discretely, i.e. via surface triangulations. This capability is very useful for grid generation starting from data originated in measurements or images

Adjoint-Based Design of Shock Mitigation Devices

Unsteady Euler and adjoint Euler solvers have been combined in order to aid in the design of shock mitigation devices. The flowfield is integrated forward in time and stored. The adjoint is then integrated going backwards in time, restoring and interpolating the saved Euler solution to the current point in time. The gradient is obtained from a surface integral formulation during the adjoint run. Comparisons of adjoint‐based and finite‐differencing gradients for different verification cases show less than 10% deviation. The results obtained indicate that this is a very cost‐effective way to obtain the gradients of an objective function with respect to surface design changes. Moreover, as the sensitivity information is determined over a complete surface, the procedure provides considerable insight, and can efficiently facilitate the design of shock mitigation devices such as architecturally appealing blast walls

A stabilized edge-based implicit incompressible flow formulation

An edge-based implementation of an implicit, monolithic, finite element (FE) scheme for the solution of the incompressible Navier–Stokes (NS) equations is presented. The original element formulation is based on the pressure stability properties of an implicit second-order in time fractional step (FS) method, which is conditionally stable. The final monolithic scheme preserves the second-order accuracy of the FS method, and is unconditionally stable. Furthermore, it can be demonstrated that the final pressure stabilizing term is practically the same fourth-order pressure term added by some authors (but following different arguments) to obtain high order accurate results, and that the final discretized convective terms are formally a second-order discretization of the respective continuous one. The development of the edge implementation is supported by two criteria: the properties of the element based one, which has already been extensively tested and for which convergence and stability analysis has already been presented, and on the enforcement of global conservation and symmetry at the discrete level. A monotonicity preserving term which decreases the discretization order in sharp gradient regions to avoid localized oscillations (overshoots and undershoots), is formulated and tested. Some numerical examples and experimental comparisons are presente