Dynamic Model for LES Without Test Filtering: Quantifying the Accuracy of Taylor Series Approximations

The dynamic model for large-eddy simulation (LES) of turbulent flows requires test filtering the resolved velocity fields in order to determine model coefficients. However, test filtering is costly to perform in large-eddy simulation of complex geometry flows, especially on unstructured grids. The objective of this work is to develop and test an approximate but less costly dynamic procedure which does not require test filtering. The proposed method is based on Taylor series expansions of the resolved velocity fields. Accuracy is governed by the derivative schemes used in the calculation and the number of terms considered in the approximation to the test filtering operator. The expansion is developed up to fourth order, and results are tested a priori based on direct numerical simulation data of forced isotropic turbulence in the context of the dynamic Smagorinsky model. The tests compare the dynamic Smagorinsky coefficient obtained from filtering with those obtained from application of the Taylor series expansion. They show that the expansion up to second order provides a reasonable approximation to the true dynamic coefficient (with errors on the order of about 5 % for c_s^2), but that including higher-order terms does not necessarily lead to improvements in the results due to inherent limitations in accurately evaluating high-order derivatives. A posteriori tests using the Taylor series approximation in LES of forced isotropic turbulence and channel flow confirm that the Taylor series approximation yields accurate results for the dynamic coefficient. Moreover, the simulations are stable and yield accurate resolved velocity statistics.Comment: submitted to Theoretical and Computational Fluid Dynamics, 20 pages, 11 figure

HyperCODA - Towards high-performing time-resolving flow simulations

The present work focuses on the performance analysis of the DG-SEM implementation of the CFD solver CODA. The turbulent Taylor-Green vortex is employed as a simple testcase for scaling behavior, while for a more detailed node-level performance analysis more granular kernel benchmarks are used. Bottlenecks in the implementation are highlighted and possible solutions proposed

Leveraging Interactivity and MPI for Environmental Applications

This paper describes two different approaches to exploiting interactivity and MPI support available in the Interactive European Grid project.The first application is an air pollution simulation using Lagrangian trajectory model to simulate the spread of pollutant particles released into the atmosphere. The performance of the sequential implementation of the application was not satisfactory, therefore a parallelization was planned. The MPI programming model was used because of some previous experience with it and its support in the grid infrastructure to be used. Then the interactivity enabling the user to receive visualizations of simulation steps and to exercise control over the application running in the grid was added. The user interface for interacting with the application was implemented as a plug-in into the Migrating Desktop user interface client platform. The other application is an interactive workflow management system, which is a modification of a previously developed system for management of applications composed of web and grid services. It allows users to manage more complex jobs, composed of several program executions, in an interactive and comfortable manner. The system uses the interactive channel of the project to forward commands from a GUI to the on-site workflow manager, and to control the job during execution. This tool is able to visualize the inner workflow of the application. User has complete in-execution control over the job, can see its partial results, and can even alter it while it is running. This allows not only to accommodate the job workflow to the data it produces, extend or shorten it, but also to interactively debug and tune the job

Surface Water Quality Modelling Considering Riparian Wetlands

Riparian wetlands are believed to play an important role in mitigating non-point source pol- lution, acting as physical and biochemical buffers between diffuse pollution sources and receiving waters. Many studies examined riparian wetlands at the field scale, but there is a dearth of re- search at the watershed scale, particularly in the region of Southern Ontario, where agricultural land use predominates. This study examined the impacts of riparian wetlands on surface water quality at the water- shed scale. A field study was conducted on two sub-watersheds at the northern headwaters of the Canagagigue Creek within the Grand River Watershed in Southern Ontario. The two watersheds were similar in area and land use but with differing riparian wetland extent adjacent to the sub- watershed main channels. A two-year study was conducted examining the hydrology, hydraulics, water quality and nutrient fluxes from the two sub-basins. Water quality data were obtained at the outlet of each sub-basin during base-flow conditions and during 16 rainfall and snow melt runoff events. The hydrology was simulated using the WatFlood model and the water quality (nitrate and total suspended solids) was simulated using an enhancedWatFlood/AGNPS model that was modified to account for continuous simulation, in-stream contaminant fate/transport and riparian wetland influences. The hydraulics and hydrological characteristics of the two basins were distinct. The basin without riparian wetland protection (“West Basin”) exhibited ephemeral tendencies, going dry for several months in the summer, whereas the basin with extensive riparian wetland protection (“East Basin”) showed a persistent base-flow throughout the year with a consistently more rapid hydrological response. This study showed higher nutrient concentrations including nitrate, total nitrogen (TN), and total phosphorus (TP) in the West basin than the East basin, attributed to the lack of riparian wetland protection in the West sub-basin. Total Suspended Solids (TSS) concentration were higher in the east sub-basin than the west sub-basin attributed to differences in sediment grain size distributions and differences in local stream bed slope. Constituent loading estimates from the two sub-basins were conducted on an event-basis and on an average monthly load basis. This study showed that during events most constituents (Nitrate, TP, and TSS) were discharged in greater quantities from the East sub-basin than the West sub-basin for both rainfall and snowmelt events. Event-based TN loading was also higher for the East sub-basin but the difference was not statistically significant. Monthly average loading was significantly higher in the East sub-basin than the West sub-basin for Nitrate, TN and TSS. Monthly average loading was higher in the East basin than the West basin for TP as well, but the difference was not statistically significant. In spite of the generally higher nutrient concentrations in the West sub- basin, the east sub-basin exhibits higher loads due to the differing hydrological conditions in that basin. The persistent stream flow in the East basin continuously transports nutrients of a lower concentration than the West, but the consistent flow dominates the loading calculations resulting in a greater constituent mass transported. The modelling of sediment and nitrogen loading was conducted over the study period. Sedi- ment modelling results showed that the dominant process in the model was in-channel transport with the calibrated model showing very little sensitivity to overland transport parameters and riparian wetland retention. The ability to hydrologically model the basin accurately dictated the performance of the sediment transport model. Nitrogen modelling results demonstrated an ability to generally simulate the nitrogen profiles trends during storm events. However, the WatFlood groundwater storage model provided limitations in terms matching the nutrient concentration variability observed in the measured data. The processes that dominated model performance were fertilizer loading and nitrogen mineralization coefficients, with the riparian wetlands playing a small role in nitrogen removal in the calibrated model

THREE-DIMENSIONAL FREE SURFACE NON-HYDROSTATIC MODELING OF PLUNGING WATER WITH TURBULENCE AND AIR ENTRAINED TRANSPORT

The advance in computational fluid dynamics in recent years has provided the opportunity for many fluid dynamic problems to be analyzed numerically. One such problem concerns the modeling of plunging water into a still water body, often encountered in pump stations. Air bubbles introduced into the system by the plunging jet can be a significant problem, especially when consumed into operating pumps. The classical approach to investigate the hydrodynamics of plunging jet in pump stations is by physical model studies. This approach is time consuming, tedious and costly. The availability of computational power today, along with appropriate numerical techniques, allows such phenomenon to be studied in a greater level of detail and more cost efficient. Despite the advantages of numerical studies, little attention has been devoted to solve the plunging jet and air transport problem numerically. In this current work, a 3-dimensional finite volume, Large Eddy Simulation (LES) code is developed to simulate these flow conditions. For turbulent flow, the large scale quantities were numerically resolved while the dynamic sub-grid scale model is used to model the small scale energy dissipations. The code also has the capability to handle free surface deformation, an important aspect in simulating the impact section of an impinging jet. Modeling of the air entrainment is performed numerically utilizing the information obtained from the hydrodynamics. Migration of air bubbles is modeled using the scalar transport equation, modified to account for the buoyancy of the bubbles. Instead of the typical Lagrangian schemes, which track individual air bubbles, air bubble dynamics are modeled in the form of concentrations. Modeling air bubbles in this manner is computational efficient and simpler to implement. For the air entrainment simulations, standard numerical boundaries conditions and empirical entrainment equations are used to provide the necessary boundary conditions. The developed model is compared with the literature, producing satisfactory results, suggesting that the code has an excellent potential of extending its application to practical industry practices