scholarly journals Direct numerical simulations of particle-laden density currents with adaptive, discontinuous finite elements

2014 ◽  
Vol 7 (3) ◽  
pp. 3219-3264 ◽  
Author(s):  
S. D. Parkinson ◽  
J. Hill ◽  
M. D. Piggott ◽  
P. A. Allison
Keyword(s):  
High Resolution ◽  
Numerical Simulations ◽  
Turbidity Current ◽  
Direct Numerical Simulations ◽  
Three Dimensions ◽  
Turbidity Currents ◽  
Normal Velocity ◽  
Density Currents ◽  
Conservation Of Energy ◽  
Computationally Expensive

Abstract. High resolution direct numerical simulations (DNS) are an important tool for the detailed analysis of turbidity current dynamics. Models that resolve the vertical structure and turbulence of the flow are typically based upon the Navier–Stokes equations. Two-dimensional simulations are known to produce unrealistic cohesive vortices that are not representative of the real three-dimensional physics. The effect of this phenomena is particularly apparent in the later stages of flow propagation. The ideal solution to this problem is to run the simulation in three dimensions but this is computationally expensive. This paper presents a novel finite-element (FE) DNS turbidity current model that has been built within Fluidity, an open source, general purpose, computational fluid dynamics code. The model is validated through re-creation of a lock release density current at a Grashof number of 5 × 106 in two, and three-dimensions. Validation of the model considers the flow energy budget, sedimentation rate, head speed, wall normal velocity profiles and the final deposit. Conservation of energy in particular is found to be a good metric for measuring mesh performance in capturing the range of dynamics. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. Use of discontinuous discretisations and adaptive unstructured meshing technologies, which reduce the required element count by approximately two orders of magnitude, results in high resolution DNS models of turbidity currents at a fraction of the cost of traditional FE models. The benefits of this technique will enable simulation of turbidity currents in complex and large domains where DNS modelling was previously unachievable.

scholarly journals Direct numerical simulations of particle-laden density currents with adaptive, discontinuous finite elements

2014 ◽  
Vol 7 (5) ◽  
pp. 1945-1960 ◽  
Author(s):  
S. D. Parkinson ◽  
J. Hill ◽  
M. D. Piggott ◽  
P. A. Allison
Keyword(s):  
Numerical Simulations ◽  
Stokes Equations ◽  
Turbidity Current ◽  
Direct Numerical Simulations ◽  
Three Dimensions ◽  
Uniform Mesh ◽  
Normal Velocity ◽  
Density Currents ◽  
Conservation Of Energy ◽  
Computationally Expensive

Abstract. High-resolution direct numerical simulations (DNSs) are an important tool for the detailed analysis of turbidity current dynamics. Models that resolve the vertical structure and turbulence of the flow are typically based upon the Navier–Stokes equations. Two-dimensional simulations are known to produce unrealistic cohesive vortices that are not representative of the real three-dimensional physics. The effect of this phenomena is particularly apparent in the later stages of flow propagation. The ideal solution to this problem is to run the simulation in three dimensions but this is computationally expensive. This paper presents a novel finite-element (FE) DNS turbidity current model that has been built within Fluidity, an open source, general purpose, computational fluid dynamics code. The model is validated through re-creation of a lock release density current at a Grashof number of 5 × 106 in two and three dimensions. Validation of the model considers the flow energy budget, sedimentation rate, head speed, wall normal velocity profiles and the final deposit. Conservation of energy in particular is found to be a good metric for measuring model performance in capturing the range of dynamics on a range of meshes. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. The use of adaptive mesh optimisation is shown to reduce the required element count by approximately two orders of magnitude in comparison with fixed, uniform mesh simulations. This leads to a substantial reduction in computational cost. The computational savings and flexibility afforded by adaptivity along with the flexibility of FE methods make this model well suited to simulating turbidity currents in complex domains.

scholarly journals Turbulent Coagulation of Aerosol Particles: New Insights From Direct Numerical Simulations and Holographic Imaging Experiments

2003 ◽  
Author(s):  
Lance R. Collins ◽  
Hui Meng ◽  
Aruj Ahluwalia ◽  
Lujie Cao ◽  
Gang Pan
Keyword(s):  
Reynolds Number ◽  
Numerical Simulations ◽  
Search Algorithm ◽  
Concentration Field ◽  
Direct Numerical Simulations ◽  
Three Dimensions ◽  
Collision Kernel ◽  
Coagulation Rate ◽  
Subsequent Work ◽  
Turbulent Fluctuations

Particle collisions driven by turbulent fluctuations play a key role in such diverse problems as cloud formation, aerosol powder manufacturing and inhalation drug therapy to name a few. In all of these examples (and many others) turbulent fluctuations increase the rate of collisions relative to the background collision rate driven by Brownian motion. Furthermore, turbulence can spontaneously generate very large fluctuations in the particle concentration field. This “clustering” is caused by the inertial mismatch between the heavy particles and the lighter surrounding gas; vortices in the flow “centrifuge” the heavier particles out of vortex cores and into the straining regions that lie in between the vortices. Because collision is a binary process, concentration fluctuations further enhance the turbulent coagulation rate by as much as two orders of magnitude. An effect of this size must be accounted for in a rational model of turbulent coagulation. Sundaram & Collins (J. Fluid Mech. 1997) showed that the radial distribution function (RDF) of the particle population, evaluated at contact, precisely corrects the collision kernel for clustering. Subsequent work has explored the dependence of the RDF on the system parameters (e.g., particle size, concentration, response time and Reynolds number) using direct numerical simulations. These results have improved our understanding and ability to predict the effect of the first three parameters; however, owing to the limited range of Reynolds number that can be reached in a numerical simulation, questions remain over the scaling of the RDF with Reynolds number. This is a critical issue for high-Reynolds-number applications such as cloud physics, where values of the Reynolds number can be 1–2 orders of magnitude greater than can be simulated. We will present our highest Reynolds number simulations to date and show our attempts to resolve this issue. Recently, the ability to measure three-dimensional particle positions using holography has been realized (e.g., Meng & Pu, J. Opt. Soc. Am. 2003). With holography, the optical image that is produced contains fringes that, upon inverting the laser, reproduce the original image in three dimensions. The hologram can then be scanned using a digital camera to obtain the particle positions. An important consideration with this study is the need to differentiate individual particles. We developed a search algorithm that locates particle centers, even in the presence of optical aberations and speckle noise. The algorithm has been used to obtain the first experimental RDF measurements to date. Thus far we see good agreement between the experimentally obtained RDF and the simulations. Besides validating the simulations, experiments can span a much broader range of Reynolds numbers, providing critical data that may help resolve the open questions associated with this parameter.

Density currents induced by differential cooling in lake

2020 ◽  
Author(s):  
Damien Bouffard ◽  
Cintia Ramón ◽  
Tomy Doda ◽  
Hugo Ulloa
Keyword(s):  
High Resolution ◽  
Numerical Simulations ◽  
Water Column ◽  
Lake Ecosystem ◽  
Density Current ◽  
Field Observations ◽  
Density Currents ◽  
Lake Surface ◽  
Littoral Region ◽  
Negatively Buoyant

<p>Differential cooling occurs in the littoral region of lakes, during calm and cold nights in summer and continuously in fall/ winter. For uniform heat loss over the lake surface, shallower regions cool faster than deeper regions leading to horizontal density gradients. Nearshore waters become negatively buoyant and start to plunge creating a cold downslope density current that can reach the pelagic zone. This cross-shore flow, also referred to as “thermal siphon”, has the potential to transport biogeochemical constituents offshore and deeper into the water column. However, its significance for the lake ecosystem remains unknown. Here we combine high resolution numerical simulations and field observations to evaluate the global significance of lateral boundaries and differential cooling on the lake dynamic. We focus on a small elongated lake: Lake Rotsee (Switzerland) and investigated the dynamic of the density current as well as the consequences of this flow such as the flushing of the littoral water.</p>

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