Number of found documents: 1187
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Finite element approximation of fluid structure interaction using Taylor-Hood and Scott-Vogelius elements
Vacek, Karel; Sváček, P.
2024 - English
This paper addresses the problem of fluid flow interacting a vibrating solid cylinder described by one degree of freedom system and with fixed airfoil. The problem is described by the incompressible Navier-Stokes equations written in the arbitrary Eulerian-Lagrangian (ALE) formulation. The ALE mapping is constructed with the use of a pseudo-elastic approach. The flow problem is numerically approximated by the finite element method (FEM). For discretization of the fluid flow, the results obtained by both the Taylor-Hood (TH) element and the Scott-Vogelius (SV) finite element are compared. The TH element satisfies the Babuška-Brezzi inf-sup condition, which guarantees the stability of the scheme. In the case of the SV element the mesh, that is created as a barycentric refinement of regular triangulation, is used to satisfy the Babuška-Brezzi condition. The numerical results for two benchmark problems are shown. Keywords: finite element method; arbitrary Lagrangian-Eulerian method; Scott-Vogelius element; Taylor-Hood element Available in digital repository of the ASCR
Finite element approximation of fluid structure interaction using Taylor-Hood and Scott-Vogelius elements

This paper addresses the problem of fluid flow interacting a vibrating solid cylinder described by one degree of freedom system and with fixed airfoil. The problem is described by the incompressible ...

Vacek, Karel; Sváček, P.
Matematický ústav, 2024

Numerical study of the steady airflow in the human respiratory system during inhaling and exhaling
Lancmanová, Anna; Bodnár, Tomáš
2024 - English
This paper presents some of the initial results of the numerical simulations of a steady turbulent flow in human upper airways during inhalation and exhalation. The mathematical model is based on the system of Reynolds-Averaged incompressible Navier-Stokes equations complemented by the SST k − ω turbulence model. The simulations were performed using finite-volume open source solver OpenFOAM on a realistic three-dimensional geometry. The main aim of this particular study is to verify the computational setup with special focus on appropriate choice and implementation of boundary conditions. The prescribed boundary conditions are chosen to mimic the physiological conditions during normal breathing cycle. This study aims to gain an insight into the airflow behavior during the inhalation and exhalation process by comparing the results of two distinct simulations corresponding to two different (opposite) flow rates . The obtained local flow rates and flow fields for both cases are presented and mutually compared. This initial work should serve as a foundation for future more complex simulations that will include the time-dependent and compressible effects. Keywords: human airways; incompressible Navier-Stokes; OpenFOAM Available in digital repository of the ASCR
Numerical study of the steady airflow in the human respiratory system during inhaling and exhaling

This paper presents some of the initial results of the numerical simulations of a steady turbulent flow in human upper airways during inhalation and exhalation. The mathematical model is based on the ...

Lancmanová, Anna; Bodnár, Tomáš
Matematický ústav, 2024

Numerical evaluation of mass-diffusive compressible fluids flows models
Bodnár, Tomáš; Fraunié, P.
2024 - English
This contribution presents first numerical tests of some recently published alternative models for solution of viscous compressible and nearly incompressible models. All models are solved by high resolution compact finite difference scheme with strong stability preserving RungeKutta time stepping. The two simple but challenging computational test cases are presented, based on the double-periodic shear layer and the Kelvin-Helmholtz instability. The obtained time-dependent flow fields are showing pronounced shear and vorticity layers being resolved by the standard as well as by the new mass-diffusive modified models. The preliminary results show that the new models are viable alternative to the well established classical models. Keywords: compressible Navier-Stokes; nearly incompressible flow; mass diffusion; compact finite-difference Available in digital repository of the ASCR
Numerical evaluation of mass-diffusive compressible fluids flows models

This contribution presents first numerical tests of some recently published alternative models for solution of viscous compressible and nearly incompressible models. All models are solved by high ...

Bodnár, Tomáš; Fraunié, P.
Matematický ústav, 2024

Motion of fluids in the moving domain
Nečasová, Šárka
2024 - English
It is a survay paper where the problem of the existence of weak solutions of compressible barotropic solutions in a moving bounded domain is studied. Keywords: compressible fluid; moving domain; weak solutions Available in digital repository of the ASCR
Motion of fluids in the moving domain

It is a survay paper where the problem of the existence of weak solutions of compressible barotropic solutions in a moving bounded domain is studied.

Nečasová, Šárka
Matematický ústav, 2024

TESTING THE METHOD OF MULTIPLE SCALES AND THE AVERAGING PRINCIPLE FOR MODEL PARAMETER ESTIMATION OF QUASIPERIODIC TWO TIME-SCALE MODELS
Papáček, Štěpán; Matonoha, Ctirad
2023 - English
Some dynamical systems are characterized by more than one timescale, e.g. two well separated time-scales are typical for quasiperiodic systems. The aim of this paper is to show how singular perturbation methods based on the slow-fast decomposition can serve for an enhanced parameter estimation when the slowly changing features are rigorously treated. Although the ultimate goal is to reduce the standard error for the estimated parameters, here we test two methods for numerical approximations of the solution of associated forward problem: (i) the multiple time-scales method, and (ii) the method of averaging. On a case study, being an under-damped harmonic oscillator containing two state variables and two parameters, the method of averaging gives well (theoretically predicted) results, while the use of multiple time-scales method is not suitable for our purposes. Keywords: Dynamical system; Singular perturbation; Averaging; Parameter estimation; Slow-fast decomposition; Damped oscillations Fulltext is available at external website.
TESTING THE METHOD OF MULTIPLE SCALES AND THE AVERAGING PRINCIPLE FOR MODEL PARAMETER ESTIMATION OF QUASIPERIODIC TWO TIME-SCALE MODELS

Some dynamical systems are characterized by more than one timescale, e.g. two well separated time-scales are typical for quasiperiodic systems. The aim of this paper is to show how singular ...

Papáček, Štěpán; Matonoha, Ctirad
Ústav teorie informace a automatizace, 2023

Kam a kolik vody odvádí stromy? Modelování vlivu smrkových a bukových porostů na toky vody v půdě během extrémních klimatických podmínek
Zelíková, Nikol; Šípek, Václav
2023 - Czech
Půdní vlhkost propojuje takové procesy, které ovlivňují celý hydrologický cyklus, a tím i dostupnost vodních zdrojů. Jedním z významných faktorů ovlivňující tyto procesy je přítomnost vegetace. Výzkum interakcí mezi vegetací, jejím managementem a procesy ovlivňující toky půdní vody nabývá na významu zejména v době probíhajících klimatických změn i změn krajinného pokryvu. Avšak komplexnost těchto interakcí, ovlivněná navíc jejich rozdílností mezi jednotlivými rostlinnými druhy tento výzkum ztěžuje. Proměna krajinného pokryvu probíhá také v českém lesnictví, konkrétně v podobě obměny smrkových monokultur buky. V této studii je proto zkoumán vliv dvou typů lesních porostů, smrku ztepilého (Picea abies) a buku lesního (Fagus sylvatica), na vodní režim půdy v experimentálním povodí Liz na Šumavě. A to (1) zjištěním rozdílů v množství půdní vláhy na základě dvaceti let naměřených dat a (2) získáním složek vodní bilance (transpirace a perkolace) na těchto dvou kontrastních plochách pomocí modelu bilance půdní vody. Z analýzy dlouhodobých dat půdních vlhkostí vyplývají nepatrně nižší hodnoty půdních vlhkostí pod porosty buku, které zanikají při srovnání průměrných ročních hodnot. Odlišnosti jsou patrné při hodnocení průměrných vlhkostí s hloubkou, kde v nejsvrchnější vrstvě půdního profilu smrk disponuje v průměru o 6 % vyššími zásobami půdní vody než buk. Z pohledu vývoje půdních vlhkostí během vegetační sezóny je půda z počátku sušší pod smrkem, který započíná svou transpiraci dříve. Tento rozdíl je v průběhu sezóny zredukován intenzivní transpirací buku. Výstupy bilančního modelu poukázaly na vyšší míru aktuální evapotranspirace buku a vyšší míru perkolace smrku během každého roku. Tento jev byl zvýrazněn během suchých let, kdežto v letech s dostatkem srážek byly rozdíly minimální. Zvýšení četnosti buku v krajině, tak může ovlivnit míru dotace podzemních vod. Soil moisture links processes that influence the entire hydrological cycle and thus the availability of water resources. One important factor influencing these processes is the presence of vegetation. Research on the interactions between vegetation, its management and the processes affecting soil water fluxes is of particular importance in times of ongoing climate change and land cover changes. However, the complexity of these interactions, further influenced by differences between plant species, makes this research more difficult. Land cover change is also taking place in Czechia, specifically in the replacement of spruce monocultures by beech. Therefore, this study investigates the influence of two types of forest stands, spruce (Picea abies) and beech (Fagus sylvatica), on the soil water regime in the experimental Liz catchment in Bohemian Forest, Czechia. This was performed by (1) evaluating differences in soil moisture based on twenty years of measured data and (2) obtaining the two components of the soil water balance (transpiration and percolation) at two plots (beech and spruce) using a soil water balance model. Analysis of the long-term soil moisture data show slightly lower soil moisture values under the beech stands, which disappear when comparing the annual mean values. Differences are evident when evaluating average soil moisture data with depth, where the topmost layer of the soil profile at the spruce site has on average 6% higher soil water content than beech. At the start of the growing season the soil moisture was initially drier under spruce, due to its earlier start of transpiration. This difference was reduced over the season by the intensive transpiration of beech. The outputs of the balance model indicated a higher rate of actual evapotranspiration of beech and a higher rate of percolation of spruce every year. This effect was more pronounced over the dry years, whereas in years with sufficient rainfall the differences were minimal. Thus, the replacement of spruce trees by beech trees may affect the rate of groundwater recharge. Keywords: soil water balance; transpiration; percolation; land cover change; drought Fulltext is available at external website.
Kam a kolik vody odvádí stromy? Modelování vlivu smrkových a bukových porostů na toky vody v půdě během extrémních klimatických podmínek

Půdní vlhkost propojuje takové procesy, které ovlivňují celý hydrologický cyklus, a tím i dostupnost vodních zdrojů. Jedním z významných faktorů ovlivňující tyto procesy je přítomnost vegetace. Výzkum ...

Zelíková, Nikol; Šípek, Václav
Ústav pro hydrodynamiku, 2023

On the structure and values of betweenness centrality in dense betweenness-uniform graphs
Ghanbari, B.; Hartman, David; Jelínek, V.; Pokorná, Aneta; Šámal, R.; Valtr, P.
2023 - English
Betweenness centrality is a network centrality measure based on the amount ofshortest paths passing through a given vertex. A graph is betweenness-uniform (BUG)if all vertices have an equal value of betweenness centrality. In this contribution, wefocus on betweenness-uniform graphs with betweenness centrality below one. Wedisprove a conjecture about the existence of a BUG with betweenness valueαforany rational numberαfrom the interval (3/4,∞) by showing that only very few be-tweenness centrality values below 6/7 are attained for at least one BUG. Furthermore,among graphs with diameter at least three, there are no betweenness-uniform graphswith a betweenness centrality smaller than one. In graphs of smaller diameter, thesame can be shown under a uniformity condition on the components of the comple-ment. Available in digital repository of the ASCR
On the structure and values of betweenness centrality in dense betweenness-uniform graphs

Betweenness centrality is a network centrality measure based on the amount ofshortest paths passing through a given vertex. A graph is betweenness-uniform (BUG)if all vertices have an equal value of ...

Ghanbari, B.; Hartman, David; Jelínek, V.; Pokorná, Aneta; Šámal, R.; Valtr, P.
Ústav informatiky, 2023

Different Boundary Conditions For LES Solver PALM 6.0 Used for ABL in Tunnel Experiment
Řezníček, Hynek; Geletič, Jan; Bureš, Martin; Krč, Pavel; Resler, Jaroslav; Vrbová, Kateřina; Trush, Arsenii; Michálek, Petr; Beneš, L.; Sühring, M.
2023 - English
We tried to reproduce results measured in the wind tunnel experiment with a CFD simulation provided by numerical model PALM. A realistic buildings layout from the Prague-Dejvice quarter has been chosen as a testing domain because solid validation campaign for PALM simulation of Atmospheric Boundary Layer (ABL) over this quarter was documented in the past. The question of input data needed for such simulation and capability of the model to capture correctly the inlet profile and its turbulence structure provided by the wind-tunnel is discussed in the study The PALM dynamical core contains a solver for the Navier-Stokes equations. By default, the model uses the Large Eddy Simulation (LES) approach in which the bulk of the turbulent motions is explicitly resolved. It is well validated tool for simulations of the complex air-flow within the real urban canopy and also within its reduced scale provided by wind tunnel experiments. However the computed flow field between the testing buildings did not correspond well to the measured wind velocity in some points. Different setting of the inlet boundary condition was tested but none of them gave completely developed turbulent flow generated by vortex generators and castellated barrier wall place at the entrance of the aerodynamic section of the wind tunnel.\n Keywords: large eddy simulation; wind tunnel; atmospheric boundary layer; PALM model; turbulence Fulltext is available at external website.
Different Boundary Conditions For LES Solver PALM 6.0 Used for ABL in Tunnel Experiment

We tried to reproduce results measured in the wind tunnel experiment with a CFD simulation provided by numerical model PALM. A realistic buildings layout from the Prague-Dejvice quarter has been ...

Řezníček, Hynek; Geletič, Jan; Bureš, Martin; Krč, Pavel; Resler, Jaroslav; Vrbová, Kateřina; Trush, Arsenii; Michálek, Petr; Beneš, L.; Sühring, M.
Ústav informatiky, 2023

Zapojení rašelinišť do odtoku z povodí
Vlček, Lukáš; Falátková, Kristýna
2023 - Czech
Příspěvek se věnuje hydrologickému režimu povodí, kde rašeliniště tvoří přibližně polovinu rozlohy. Pomocí modelu byly hodnoceny hydrologické procesy v rašeliništi a okolním lese. Z výsledků vyplývá, že rašeliniště výrazně zvyšuje fluktuaci vodního režimu místních toků. Vodní bilance zase ukazuje snížený ochalozovací efekt rašelinišť bez stromového patra oproti lesům na organominerální půdě. Revitalizace rašelinišť tak může mít kromě pozitivního ekologického aspektu negativní hydroogický dopad na okolní krajinu a místní toky. This research focuses on the basin's hydrological regime, where peatlands make up approximately half of the area. Hydrological processes in the peat bog and the surrounding forest were evaluated using the HBV model. The results show that the peatlands significantly increase the water regime fluctuation of local streams. Moreover, water balance shows a lower cooling effect of tree-less peat bogs than forests on organo-mineral soils. Except for a positive ecological aspect, peatland restoration can have a negative hydrological impact on the surrounding landscape and local streams. Keywords: water balance; peat bog; runoff formation Fulltext is available at external website.
Zapojení rašelinišť do odtoku z povodí

Příspěvek se věnuje hydrologickému režimu povodí, kde rašeliniště tvoří přibližně polovinu rozlohy. Pomocí modelu byly hodnoceny hydrologické procesy v rašeliništi a okolním lese. Z výsledků vyplývá, ...

Vlček, Lukáš; Falátková, Kristýna
Ústav pro hydrodynamiku, 2023

Časová proměnlivost hydraulických vlastností půdy a jejich vliv na odhad půdní vlhkosti
Šípek, Václav; Vlček, Lukáš; Tesař, Miroslav; Zelíková, Nikol; Hnilica, Jan
2023 - Czech
Půdní vlhkost hraje významnou roli v hydrologickém cyklu, protože zásadně ovlivňuje proudění vody v systému voda-půda-rostlina. Studie je zaměřena na celoroční odhad nasycení půdního profilu vodou v zalesněném horském území s využitím jednoduchého modelu založenému na konceptu lineární nádrže. Studie je založena na terénním měření nasycené hydraulické vodivosti a zavedení periodické funkce nasycené hydraulické vodivosti do modelu půdní vlhkosti, která možní její sezónní proměnlivost. \nPrůměrná nasycená hydraulická vodivost měřená jednoválcovým infiltrometrem byla v průběhu dvou let vždy vyšší ve vegetačním období, kdy se pohybovala od 33,6 do 44,8 cm hod-1, oproti zimním období, kdy poklesla do rozmezí 13,2 až 22,3 cm hod-1. Do modelu půdní vlhkosti byla implementována funkce, která umožnila spojitou změnu nasycené hydraulické vodivosti s maximem ve vegetační sezóně a minimum v zimním období. Tato modifikace modelu půdní vlhkosti vedla k poklesu průměrné čtvercové chyby odhadu půdní vlhkosti o 33,2 % (z 0,034 cm3cm-3 na 0,023 cm3cm-3) a nárůstu hodnoty Nash-Sutcliffova koeficientu z 0,34 na 0,68. Výsledky studie přispějí k přesnějšímu celoročnímu odhadu půdní vlhkosti, a tedy i ke zlepšení dlouhodobých simulací vlivu klimatické změny na dostupnost vodních zdrojů.\n The study was focused on the temporal variability of saturated hydraulic conductivity in an experimental plot covered by spruce forest and investigated benefits of its incorporation into soil water balance model. The results showed that the higher saturated hydraulic conductivity (33.6-44.8 cm hr-1) was observed in summer period compared to lower values (13.2-22.3 cm hr-1) in the winter period. The use of seasonally variabile saturated hydraulic conductivity improved the efficiency of soil water balance model in terms of lower root mean square error between observed and simulated volumetric soil water content by 33.2 %. The Nash-Sutcliffe coefficient rose from 0.34 to 0.68. Keywords: soil moisture; soil hydraulic properties; saturated hydraulic conductivity; climate change Fulltext is available at external website.
Časová proměnlivost hydraulických vlastností půdy a jejich vliv na odhad půdní vlhkosti

Půdní vlhkost hraje významnou roli v hydrologickém cyklu, protože zásadně ovlivňuje proudění vody v systému voda-půda-rostlina. Studie je zaměřena na celoroční odhad nasycení půdního profilu vodou v ...

Šípek, Václav; Vlček, Lukáš; Tesař, Miroslav; Zelíková, Nikol; Hnilica, Jan
Ústav pro hydrodynamiku, 2023

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