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• Parabolic recovery of boundary gradients

  R. Lohner, J. Cebral, S. Appanaboyina

  Communications in numerical methods in engineering (2008). Vol. 24 (12), pp. 1611-1615

  
  

  Abstract

  A parabolic recovery procedure suited for shear stress and heat flux recovery on surfaces from linear element data is proposed. The information required consists of the usual unknowns at points, as well as gradients recovered at the points that are one layer away from the wall. The procedure has been in use for some time and has consistently delivered superior results as compared with the usual wall shear stress and heat flux obtained from linear finite element method shape functions.

  Abstract
  A parabolic recovery procedure suited for shear stress and heat flux recovery on surfaces from linear element data is proposed. The information required consists of the usual [...]

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• Cache‐efficient renumbering for vectorization

  R. Lohner

  International Journal for Numerical Methods in Biomedical Engineering (2008). Vol. 26 (5), pp. 628-636

  
  

  Abstract

  A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring techniques, the new renumbering strategy reduces the spread in point‐data access for edge‐based solvers by more than an order of magnitude. The technique is particularly suited for multicore, cache‐based machines that allow for vectorization or pipelining.

  Abstract
  A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring [...]

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• Combination of Body-Fitted and Embedded Grids for External Vehicle Aerodynamics

  R. Tilch, A. Tabbal, M. Zhu, F. Decker, R. Lohner

  Engineering Computations (2008). Vol. 25 (1), pp. 28-41

  
  

  Abstract

  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 undercar- riage 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. 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%. Thus, the present approach reduces turnaround times for complete car geometries to 1-2 days, without compromising accuracy.

  Abstract
  Body-fitted and embedded mesh techniques are combined to obtain accurate external aerodynamic solutions for realistic car geometries with minimal user intervention. The key [...]

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• On the computation of steady‐state compressible flows using a discontinuous Galerkin method

  H. Luo, J. Baum, R. Lohner

  International Journal for Numerical Methods in Engineering (2008). Vol. 73 (5), pp. 597-623

  
  

  Abstract

  Computation of compressible steady‐state flows using a high‐order discontinuous Galerkin finite element method is presented in this paper. An accurate representation of the boundary normals based on the definition of the geometries is used for imposing solid wall boundary conditions for curved geometries. Particular attention is given to the impact and importance of slope limiters on the solution accuracy for flows with strong discontinuities. A physics‐based shock detector is introduced to effectively make a distinction between a smooth extremum and a shock wave. A recently developed, fast, low‐storage p‐multigrid method is used for solving the governing compressible Euler equations to obtain steady‐state solutions. The method is applied to compute a variety of compressible flow problems on unstructured grids. Numerical experiments for a wide range of flow conditions in both 2D and 3D configurations are presented to demonstrate the accuracy of the developed discontinuous Galerkin method for computing compressible steady‐state flows.

  Abstract
  Computation of compressible steady‐state flows using a high‐order discontinuous Galerkin finite element method is presented in this paper. An accurate representation of [...]

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• Adaptive methodology for meshless finite point method

  F. Perazzo, R. Lohner, L. Perez-Pozo

  Advances in Engineering Software (2008). Vol. 39 (3), pp. 156-166

  
  

  Abstract

  In this work, a posteriori error estimator and an adaptive refinement process for the meshless finite point method (FPM), which is based on point collocation, are presented. The error indicator is formulated by the least-squares functional evaluation, used in the shape function development. New degrees of freedom or additional points can be incorporated without difficulty, in zones where the error estimator presents a high value, by means of h–p refinement processes. The validity of the proposed error estimator can be demonstrated by developments of numerical problems in mechanics of solids, using an adaptive refinement process of the solution.

  Abstract
  In this work, a posteriori error estimator and an adaptive refinement process for the meshless finite point method (FPM), which is based on point collocation, are presented. [...]

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• A Fast p-Multigrid Discontinuous Galerkin Method for Compressible Flows at All Speeds

  H. Luo, J. Baum, R. Lohner

  AIAA Journal (2008). Vol. 46 (3), pp. 635-652

  
  

  Abstract

  A p-multigrid (p=polynomial degree) discontinuous Galerkin method is presented for the solution of the compressible Euler equations on unstructured grids. The method operates on a sequence of solution approximations of different polynomial orders. A distinct feature of this p-multigrid method is the application of an explicit smoother on the higher level approximations (p > 0) and an implicit smoother on the lowest level approximation (p = 0), resulting in a fast as well as low storage method that can be efficiently used to accelerate the convergence to a steady state solution. Furthermore, this p-multigrid method can be naturally applied to compute the flows with discontinuities, where a monotonic limiting procedure is usually required for discontinuous Galerkin methods. An accurate representation of the boundary normals based on the definition of the geometries is used for imposing slip boundary conditions for curved geometries. A variety of compressible flow problems for a wide range of flow conditions, from low Mach number to supersonic, in both 2D and 3D configurations are computed to demonstrate the performance of this p-multigrid method. The numerical results obtained strongly indicate the order independent property of this p-multigrid method and demonstrate that this method is orders of magnitude faster than its explicit counterpart. The performance comparison with a finite volume method shows that using this p-multigrid method, the discontinuous Galerkin method provides a viable, attractive, competitive and probably even superior alternative to the finite volume method for computing compressible flows at all speeds.

  Abstract
  A p-multigrid (p=polynomial degree) discontinuous Galerkin method is presented for the solution of the compressible Euler equations on unstructured grids. The method operates [...]

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• The backwater effect as a tool to assess formative long-term flood regimes

  F. Castelltort, E. Bladé, J. Balasch, M. Ribe

  Quaternary international (2020). Vol. 583, pp. 29-43

  
  

  Abstract

  The Ter River drains the south-eastern Pyrenees and flows into the Mediterranean Sea. A lithological constriction affects the normal water flow in the La Plana de Vic area. As a consequence of this disturbance in the flow, the shrinkage in the bedrock activates a backwater effect. Systematic water retention during extreme events has formative consequences. The process involves the creation of a helical flow for the structural misalignment of the channel at the point of narrowness. The backwater effect transmits the secondary currents backwards, resulting in the creation of a sinuous pattern upstream from the shrinkage. A tributary, the Gurri River, flows into the main river just before the constriction, and this too has been affected by the process of water storage and channel pattern change. A two-dimensional numerical flow model (Iber) has modelled several hypothetical cases of flooding. This modelling aims to test the reach of the hydraulic influence upstream from the constriction, both in the main river and its tributary, due to the backwater effect. Moreover, it sought to find the best balance of discharge between the two streams. The upstream reach of the backwater effect was considered as its endpoint. During the flooding, the system reached hydraulic equilibrium between the constriction and the two endpoints when both were at the same water level, and the flow regime was subcritical everywhere. The hydraulic conditions that drove the water flow to the equilibrium are thought to be the ones that promoted formative processes in a sinuous pattern on a long-term basis. The water discharge values obtained from this procedure are, in general terms, 50% above those considered to be a peak flood with a recurrence time of 500 years (Q500), and they accomplish the conditions of extreme events. Thus classified, the calculated discharges can be helpful for comparison with those measured in historical and systematic records, because a water discharge like the one calculated has never been measured at the Ter River.

  Abstract
  The Ter River drains the south-eastern Pyrenees and flows into the Mediterranean Sea. A lithological constriction affects the normal water flow in the La Plana de Vic area. [...]

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• Evaluation of two hydrometeorological ensemble strategies for flash-flood forecasting over a catchment of the eastern Pyrenees

  H. Roux, A. Amengual, R. Romero, E. Bladé, M. Sanz-Ramos

  Natural hazards and earth system sciences (2020). Vol. 20 (2), pp. 425-450

  
  

  Abstract

  This study aims at evaluating the performances of flash flood forecasts issued from deterministic and ensemble meteorological prognostic systems. The hydro-meteorological modeling chain includes the Weather Research and Forecasting model (WRF) forcing the rainfall-runoff model MARINE dedicated to flash flood. Two distinct ensemble prediction systems accounting for (i) perturbed initial and lateral boundary conditions of the meteorological state and (ii) mesoscale model physical parameterizations, have been implemented on the Agly catchment of the Eastern Pyrenees with three sub-catchments exhibiting different rainfall regimes. Different evaluations of the performance of the hydrometeorological strategies have been performed: (i) verification of short-range ensemble prediction systems and corresponding stream flow forecasts, for a better understanding of how forecasts behave, (ii) usual measures derived from a contingency table approach, to test an alert threshold exceedance, and (iii) overall evaluation of the hydro-meteorological chain using the Continuous Rank Probability Score, for a general quantification of the ensemble performances. Results show that the overall discharge forecast is improved by both ensemble strategies with respect to the deterministic forecast. Threshold exceedance detections for flood warning also benefit from large hydro-meteorological ensemble spread. There are no substantial differences between both ensemble strategies on these test cases in terms both of the issuance of flood warnings and the overall performances, suggesting that both sources of external-scale uncertainty are important to take into account

  Abstract
  This study aims at evaluating the performances of flash flood forecasts issued from deterministic and ensemble meteorological prognostic systems. The hydro-meteorological [...]

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• Las ecuaciones de Saint Venant para la modelización de avalanchas de nieve densa

  M. Sanz-Ramos, E. Bladé, A. Torralba, P. Oller

  Ingeniería del agua (2020). Vol. 24 (1), pp. 65-79

  
  

  Abstract

  La creciente preocupación por los riesgos naturales, como las avalanchas de nieve, ha propiciado el desarrollo de modelos numéricos ad hoc como una herramienta de soporte para su análisis y evaluación. Los modelos existentes para simulación de aludes se basan en la conservación de la masa y de la cantidad de movimiento, que son unas ecuaciones similares a las ecuaciones de Saint Venant para agua con diferencias sólo en los términos de fricción (modelo reológico). Este documento muestra las posibilidades de estas ecuaciones para simular avalanchas de placa-densa y el tratamiento numérico realizado en Iber. Se ha empleado una nueva metodología para equilibrar el término fuente y el vector de flujo evitando así oscilaciones espurias y movimientos no reales, y que modifica la pendiente de fondo en base a los parámetros del fluido y así detener su movimiento. La herramienta se ha probado en dos casos de estudio para analizar el comportamiento del fluido en función de los parámetros del modelo reológico.

  Abstract
  La creciente preocupación por los riesgos naturales, como las avalanchas de nieve, ha propiciado el desarrollo de modelos numéricos ad hoc como una herramienta [...]

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• Hydrodynamic determination of the kinematic viscosity of waste brines

  M. Sánchez-Juny, A. Triadú, A. Andreu, E. Bladé

  ACS Omega (2019). Vol. 4 (25), pp. 20987-20999

  
  

  Abstract

  Wastewater from a potash mine in the central region of Catalonia is transported by means of a collector that runs more than 100 km, spilling into the sea on the Catalan central coast. To analyze the hydraulics of this infrastructure, the values of the basic parameters that condition the flow, such as the absolute roughness of poly(vinyl chloride) (PVC) pipes and the viscosity of the transported brine mixtures, must be characterized. There exists uncertainty about the value of absolute roughness of a PVC pipe as described in the literature; nevertheless, if the pipe is smooth, the influence of the absolute roughness in the hydraulic determination of viscosity will not be significant. In this work, an experimental procedure based on a hydraulic analysis was applied to estimate the kinematic viscosity of a brine mixture, depending on its temperature and concentrations of salts and fines. The results obtained were compared with the results from experiments using an Ostwald viscometer.

  Abstract
  Wastewater from a potash mine in the central region of Catalonia is transported by means of a collector that runs more than 100 km, spilling into the sea on the Catalan central [...]

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