Diversity and distribution of epiphytic lichens and bryophytes on aspen (Populus tremula) in the middle boreal forests of Republic of Karelia (Russia)

The distribution of epiphytic bryophyte and lichen species growing on aspen in the middle boreal forests was studied in southern Karelia (Russia). These forests varied in time-since-disturbance from 80 to 450 years. Two hundred twenty two species of epiphytes, including 178 lichens, 32 mosses and 12 liverworts, were recorded on 192 aspen trees in forests over 24 ha, in the Karelian part of the Vodlozero National Park, Kivach Strict Nature Reserve, Kizhi Sanctuary and Petrozavodsk City. Arthonia biatoricola, A. excipienda and Biatoridium monasteriense were collected in Karelia for the first time. Eighteen rare species (lichens Anaptychia ciliaris, Arthonia vinosa, Bryoria nadvornikiana, Chaenotheca gracilenta, C. stemonea, Lecidea albofuscescens, Lobaria pulmonaria, Melanelixia subaurifera, Nephroma bellum, N. laevigatum, Phaeocalicium populneum, Ramalina thrausta, Rostania occultata, Scytinium subtile, Usnea barbata, mosses Neckera pennata, Plagiomnium drummondii and liverwort Lejeunea cavifolia) listed in the Red Data Book of Republic of Karelia (2007) were found. Relationships between epiphytic lichen and bryophyte species richness and certain environmental variables (at different trunk heights above ground and time-since-disturbance) were evaluated. Lichens and mosses on aspen trunks often occupy different ecological niches. Cover and diversity of bryophytes was high on trunk bases, while the number of lichen species and their cover were higher at a height of 1.3 m above ground level. The total number of lichen species on aspen increased on average from 40 to 60 species per ha with increasing time-since-disturbance from 100 to 450 years. A stabilization in lichen species number was observed at about 200 years since disturbance. No significant correlation was determined between bryophyte diversity on aspens and the time-since-disturbance.

Viscous-Inviscid Interactions in a Boundary-Layer Flow Induced by a Vortex Array

In this paper we investigate the asymptotic validity of boundary layer theory. For a flow induced by a periodic row of point-vortices, we compare Prandtl's solution to Navier-Stokes solutions at different $Re$ numbers. We show how Prandtl's solution develops a finite time separation singularity. On the other hand Navier-Stokes solution is characterized by the presence of two kinds of viscous-inviscid interactions between the boundary layer and the outer flow. These interactions can be detected by the analysis of the enstrophy and of the pressure gradient on the wall. Moreover we apply the complex singularity tracking method to Prandtl and Navier-Stokes solutions and analyze the previous interactions from a different perspective

CFD validation in OECD/NEA t-junction benchmark.

When streams of rapidly moving flow merge in a T-junction, the potential arises for large oscillations at the scale of the diameter, D, with a period scaling as O(D/U), where U is the characteristic flow velocity. If the streams are of different temperatures, the oscillations result in experimental fluctuations (thermal striping) at the pipe wall in the outlet branch that can accelerate thermal-mechanical fatigue and ultimately cause pipe failure. The importance of this phenomenon has prompted the nuclear energy modeling and simulation community to establish a benchmark to test the ability of computational fluid dynamics (CFD) codes to predict thermal striping. The benchmark is based on thermal and velocity data measured in an experiment designed specifically for this purpose. Thermal striping is intrinsically unsteady and hence not accessible to steady state simulation approaches such as steady state Reynolds-averaged Navier-Stokes (RANS) models.1 Consequently, one must consider either unsteady RANS or large eddy simulation (LES). This report compares the results for three LES codes: Nek5000, developed at Argonne National Laboratory (USA), and Cabaret and Conv3D, developed at the Moscow Institute of Nuclear Energy Safety at (IBRAE) in Russia. Nek5000 is based on the spectral element method (SEM), which is a high-order weighted residual technique that combines the geometric flexibility of the finite element method (FEM) with the tensor-product efficiencies of spectral methods. Cabaret is a 'compact accurately boundary-adjusting high-resolution technique' for fluid dynamics simulation. The method is second-order accurate on nonuniform grids in space and time, and has a small dispersion error and computational stencil defined within one space-time cell. The scheme is equipped with a conservative nonlinear correction procedure based on the maximum principle. CONV3D is based on the immersed boundary method and is validated on a wide set of the experimental and benchmark data. The numerical scheme has a very small scheme diffusion and is the second and the first order accurate in space and time, correspondingly. We compare and contrast simulation results for three computational fluid dynamics codes CABARET, Conv3D, and Nek5000 for the T-junction thermal striping problem that was the focus of a recent OECD/NEA blind benchmark. The corresponding codes utilize finite-difference implicit large eddy simulation (ILES), finite-volume LES on fully staggered grids, and an LES spectral element method (SEM), respectively. The simulations results are in a good agreement with experimenatl data. We present results from a study of sensitivity to computational mesh and time integration interval, and discuss the next steps in the simulation of this problem

CFD benchmark for a heavy liquid metal fuel assembly

As part of a Department of Energy International Nuclear Energy Research Initiative (1-NERI), the Dutch Nuclear Research and consultancy Group (NRG), the Belgian Nuclear Research Centre (SCKCEN), Ghent University (UGent) in Belgium and the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) are collaborating with Argonne National Laboratory (ANL) to perform and compare a series of thermal hydraulic simulations representative of a heavy liquid metal fast reactor fuel assembly. Such a widely spaced wire-wrapped fuel assembly is a complex configuration for which few flow data are available for verification and validation of computational fluid dynamics (CFD) simulations. For this benchmark a 19-pin wire-wrapped rod bundle with characteristics representative of the MYRRHA flexible fast research reactor, under design at SCK'CEN in Belgium, is modeled. The heat conduction in the cladding of the fuel rods and the spacer wires is taken into account by conjugate heat transfer. UGent, ENEA and NRG performed their Reynolds Averaged Navier-Stokes (RANS) simulations with commercially available CFD codes. The high-fidelity ANL Large-Eddy Simulation (LES) was performed with Nek5000, used for CFD in the Simulation-based High-efficiency Advanced Reactor Prototyping (SHARP) suite. The paper will show and discuss the comparison of the thermal and hydraulic RANS results and the reference Nek5000 LES results. The comparison with the LES results will indicate to which extent the current liquid metal modeling methods are sufficient and help to highlight remaining issues. The results of the study are very valuable in the design and licensing process for MYRRHA