NICE-SLAM with Adaptive Feature Grids

NICE-SLAM is a dense visual SLAM system that combines the advantages of neural implicit representations and hierarchical grid-based scene representation. However, the hierarchical grid features are densely stored, leading to memory explosion problems when adapting the framework to large scenes. In our project, we present sparse NICE-SLAM, a sparse SLAM system incorporating the idea of Voxel Hashing into NICE-SLAM framework. Instead of initializing feature grids in the whole space, voxel features near the surface are adaptively added and optimized. Experiments demonstrated that compared to NICE-SLAM algorithm, our approach takes much less memory and achieves comparable reconstruction quality on the same datasets. Our implementation is available at https://github.com/zhangganlin/NICE-SLAM-with-Adaptive-Feature-Grids.Comment: Course project of 3D Vision at ETH Zuric

Hybrid eulerian-lagrangian approach for dense spray simulations

In this work, a hybrid Euler-Lagrangian solver for dense spray systems is developed specifically for cases where film creation by accumulation of liquid droplets at the walls plays a crucial role. EulerLagrangian solvers are commonly used to describe the spray with predefined spray characteristics. The Lagrangian particles represent liquid drops moving along the continuous gaseous phase. This approach assumes a small particle size compared to the cell size and it is unable to capture the breakup behavior of liquid jets in the presence of instabilities. VOF methods, on the other hand, are not a computationally feasible option when it comes to small droplet sizes as a result of liquid atomization because they have to be fully resolved by the computational mesh. Hence, multiscale simulations are required to bridge the gap between the two methods combining subgrid droplets in Lagrangian approaches and large liquid structures in VOF methods. In the present work, a multiscale approach is developed where Lagrangian particles representing small droplets are tracked through the continuous phase until they hit a wall or a liquid-gas interface represented by a continuous VOF field. At the time of impact, the Lagrangian particles are removed and the mass and momentum of these particles are transferred to the VOF field. This allows having a spray consisting of subgrid droplets computed with a Lagrangian particle tracking (LPT) approach and liquid films at the walls with VOF method. The method represents a one-way coupling, converting Lagrangian particles to Eulerian liquid phase (VOF) and has been implemented into the open-source CFD code OpenFOAM. Subsequently, the solver has been tested in different scenarios to ensure mass and momentum conservation. Positive test results encouraged its use to gain insight on the fluid flow in a real cylindrical compression-dissolution unit where the liquid is sprayed from the top while simultaneously the gas is compressed from the bottom. Dynamic mesh technique is used to account for the compression with a moving piston

Detailed Transport and Performance Optimization for Massively Parallel Simulations of Turbulent Combustion with OpenFOAM

This work describes the implementation of two key features for enabling high performance computing (HPC) of highly resolved turbulent combustion simulations: detailed molecular transport for chemical species and efficient computation of chemical reaction rates. The transport model is based on an implementation of the thermo-chemical library Cantera [1] and is necessary to resolve the inner structure of flames. The chemical reaction rates are computed from automatically generated chemistry-model classes [2], which contain highly optimized code for a specific reaction mechanism. In combination with Sundials’ [3] ODE solver, this leads to drastic reductions in computing time. The new features are validated and applied to a turbulent flame with inhomogeneous mixing conditions on a grid with 150 million cells. The simulation is performed on Germany’s fastest supercomputer “Hazel Hen” [4] on 28,800 CPU cores, showing very good scalability. The good agreement with experimental data shows that the proposed implementations combined with the capabilities of OpenFOAM are able to accurately and efficiently simulate even challenging flame setups

The Eulerian Stochastic Fields Method Applied to Large Eddy Simulations of a Piloted Flame with Inhomogeneous Inlet

Large Eddy Simulations of the Sydney mixed-mode flame with inhomogeneous inlet (FJ200-5GP-Lr75-57) are performed using the Eulerian Stochastic Fields (ESF) transported probability functions method to account for the sub-grid scale turbulence–chemistry interaction, to demonstrate the suitability of the ESF method for mixed-mode combustion. An analytically reduced 19-species methane mechanism is used for the description of the chemical reactions. Prior to the reactive case, simulation results of the non-reactive setup with cold and hot pilot stream are presented, which show differences in the jet breakup and radial species mass fluxes. The reactive case simulations are compared to experimental data and a recently conducted model free quasi-DNS (qDNS), showing very good agreement with the qDNS in terms of scatter data and radial mean values of temperature and species distribution, as well as mixture fraction conditional statistics. Further analysis is dedicated to sub-grid scale statistics, showing that mixture fraction and reaction progress variable are strongly correlated in this flame. The impact of the number of stochastic fields on the filtered temperature and species distribution is investigated; it reveals that the ESF method in conjunction with finite-rate chemistry is very insensitive to the number of employed fields to obtain highly accurate simulation results

DNS of Near Wall Dynamics of Premixed CH4_4/Air Flames

This work presents a numerical study on the effect of flame-wall interaction (FWI) from the viewpoint of flame dynamics. For that purpose, direct numerical simulations (DNS) employing detailed calculations of reaction rates and transport coefficients have been applied to a 2D premixed methane/air flame under atmospheric condition. Free flame (FF) and side-wall quenching (SWQ) configurations are realized by defining one lateral boundary as either a symmetry plane for the FF or a cold wall with fixed temperature at 20 oC for the SWQ case. Different components of flame stretch and Markstein number regarding tangential, normal (due to curvature) and total stretch, Ka$_s$ , Ka$_c$ and Ka$_tot$ = Ka$_s$ + Ka$_c$, as well as their correlations with respect to the local flame consumption speed SL have been evaluated. It has been shown that the FWI zone is dominated by negative flame stretch. In addition, S$_L$ decreases with decreasing normal stretch due to curvature Ka$_c$ while approaching the cold wall. However, S$_L$ increases with decreasing Ka$_c$ while approaching the symmetry boundary for the free flame case, leading to an inversion of the Markstein number Ma$_tot$ based on Ka$_tot$ from positive in the free flame case to negative in the SWQ case. The quenching distance evaluated based on wall-normal profiles of S$_L$ has been found to be approximately equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature. The flame speed has been confirmed to scale quasi-linearly with the stretch in the FWI zone. The results reveal a distinct correlation during transition between FWI and FF regarding flame dynamics, which brings a new perspective for modeling FWI phenomena by means of flame stretch and Markstein number. To do this, the quenching effect of the wall may be reproduced by a reversed sign of the Markstein number from positive to negative in the FWI zone and by applying the general linear Markstein correlation (S$_L$/S$_{L,0}$ = 1− Ma · Ka), leading to a decrease of the flame speed or the reaction rate in the near-wall region