| 1. |
- Zeli, Velibor, et al.
(författare)
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Analytical and numerical treatment of the heat conduction equation obtained via time-fractional distributed-order heat conduction law
- 2018
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Ingår i: Physica A. - : Elsevier. - 0378-4371 .- 1873-2119. ; 492, s. 2316-2335
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Tidskriftsartikel (refereegranskat)abstract
- Generalization of the heat conduction equation is obtained by considering the system of equations consisting of the energy balance equation and fractional-order constitutive heat conduction law, assumed in the form of the distributed-order Cattaneo type. The Cauchy problem for system of energy balance equation and constitutive heat conduction law is treated analytically through Fourier and Laplace integral transform methods, as well as numerically by the method of finite differences through Adams-Bashforth and Grunwald-Letnikov schemes for approximation derivatives in temporal domain and leap frog scheme for spatial derivatives. Numerical examples, showing time evolution of temperature and heat flux spatial profiles, demonstrate applicability and good agreement of both methods in cases of multi-term and power-type distributed-order heat conduction laws.
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| 2. |
- Zeli, Velibor, et al.
(författare)
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Consistent Boundary-Condition Treatment for Computation of the Atmospheric Boundary Layer Using the Explicit Algebraic Reynolds-Stress Model
- 2019
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Ingår i: Boundary-layer Meteorology. - : SPRINGER. - 0006-8314 .- 1573-1472. ; 171:1, s. 53-77
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Tidskriftsartikel (refereegranskat)abstract
- Standard turbulence models for the atmospheric boundary layer (ABL) typically use boundary conditions based on the Monin-Obukhov similarity theory (MOST). This can lead to inconsistency between the boundary condition and the closure model. Here, we propose a new boundary-condition treatment of the stratified ABL, derived for the so-called explicit algebraic Reynolds-stress model. The boundary conditions correspond to the relations for vanishing buoyancy effects that are valid close to the ground. The solution for the stratified surface layer is in agreement with the surface scaling physics and MOST functions. This was validated in a simulation of an idealized diurnal cycle of the ABL based on the second Global Energy and Water cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS2) case.
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| 3. |
- Zeli, Velibor, et al.
(författare)
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Explicit Algebraic Reynolds-stress Modelling of a Convective Atmospheric Boundary Layer Including Counter-Gradient Fluxes
- 2021
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Ingår i: Boundary-layer Meteorology. - : Springer. - 0006-8314 .- 1573-1472. ; 178:3, s. 487-497
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Tidskriftsartikel (refereegranskat)abstract
- In a recent study (Želi et al. in Bound Layer Meteorol 176:229–249, 2020), we have shown that the explicit algebraic Reynolds-stress (EARS) model, implemented in a single-column context, is able to capture the main features of a stable atmospheric boundary layer (ABL) for a range of stratification levels. We here extend the previous study and show that the same formulation and calibration of the EARS model also can be applied to a dry convective ABL. Five different simulations with moderate convective intensities are studied by prescribing surface heat flux and geostrophic forcing. The results of the EARS model are comparedto large-eddy simulations of Salesky and Anderson (J Fluid Mech 856:135–168, 2018). It is shown that the EARS model performs well and is able to capture the counter-gradient heat flux in the upper part of the ABL due to the presence of the non-gradient term in the relation for vertical turbulent heat flux. The model predicts the full Reynolds-stress tensor and heat-flux vector and allows us to compare other important aspects of a convective ABLsuch as the profiles of vertical momentum variance. Together with the previous studies, we show that the EARS model is able to predict the essential features of the ABL. It also shows that the EARS model with the same model formulation and coefficients is applicable over awide range of stable and moderately unstable stratifications.
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| 4. |
- Zeli, Velibor, et al.
(författare)
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Modelling of Stably Stratified Atmospheric Boundary Layers with Varying Stratifications
- 2020
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Ingår i: Boundary-layer Meteorology. - : Springer. - 0006-8314 .- 1573-1472. ; 176:2, s. 229-249
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Tidskriftsartikel (refereegranskat)abstract
- A recently developed explicit algebraic Reynolds-stress (EARS) model is validated for an idealized representation of the night-time high-latitude stably stratified atmospheric boundary layer. The simulations are made with four surface cooling rates that result in weakly to moderately stratified stable boundary layers. The predictions of the EARS model are compared to high-resolution large-eddy simulations (LES) of Sullivan et al. (J Atmos Sci 73(4):1815–1840, 2016). First- and second-order statistics are shown to be well predicted by the EARS model. The EARS model also predicts the horizontal turbulent fluxes and turbulence anisotropy and these compare well with the LES results. The sensitivity to the model coefficients is studied by comparing the EARS model results with LES results. Finally, we propose a new scaling for the production of turbulence kinetic energy and show that the EARS model captures the essential trends of the LES results for different cooling rates.
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| 5. |
- Zeli, Velibor
(författare)
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Modelling of stably-stratified, convective and transitional atmospheric boundary layers using the explicit algebraic Reynolds-stress model
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The atmospheric boundary layer (ABL) is in continuous turbulent motion. The heating and cooling of the Earth’s surface drives mechanic and thermodynamic processes in the ABL through enhancing and damping of atmospheric turbulence. The surface forcing has a profound effect on the diurnal cycle of temperature,wind and related variables in the ABL. Efforts have been made to model atmospheric turbulence with linear algebraic relations such as the eddy-viscosity hypothesis. Modelling of atmospheric turbulence, however, still remains a great challenge and forms an important problem in the context of numerical weather prediction and climate models. In this thesis a recently developed non-linear turbulence model, the so-called explicit algebraic Reynolds-stress (EARS) model, implemented in the context of a single-column model is used to simulate dry, stratified ABLs.We propose a new boundary-condition treatment in the EARS model. The boundary conditions correspond to the relations for vanishing buoyancy effects that are valid close to the ground. In the simulation of an idealized diurnal cycle the solutions for the stratified surface layer is in agreement with the surface scaling physics and the Monin–Obukhov functions.We have carried out simulations of the ABL with varying levels of stratification using the EARS model implemented in the context of a single-column model. We use the same model formulation and coefficients in these simulations with different thermal stratifications of the ABL. Even in the SCM formulation the EARS model solution produces a full Reynolds-stress tensor and heat flux vector. The set-up of the numerical experiments are taken from previously published large-eddy simulation (LES) studies of ABL.Simulations of stably-stratified ABL show that the EARS model is able to accurately predict the development of a low-level jet and wind turning for different levels of stratification. In addition to first-order statistics, the model also predicts more intricate features of the turbulent ABL such as the relation between vertical and horizontal fluctuations for different stratifications and horizontal heat fluxes caused by wind shear. In the simulations of convective ABL the EARS model correctly predicts the horizontal wind speed and potential temperature profiles. The study also shows that the non-gradient term in the vertical heat flux equation, that naturally appears in the model formulations, gives a large contribution to the heat flux and has a significant influence on the predicted potential temperature profile of the convective ABL. Finally, we study the effects of transitional turbulence in the simulation of diurnal cycle extended to several days. The comparison with the LES shows that the EARS model correctly predicts the mean profiles and surface fluxes at different times of the day, including the low-level jet close to the surface. The model also predicts residual turbulence near the top of the ABL at night. The study demonstrates that the EARS model is able to capture key features of stably-stratified and convective ABLs as well as transitional processes that drive the ABL from one stratification to another.
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| 6. |
- Zeli, Velibor, et al.
(författare)
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Transitional Atmospheric Boundary Layer in GABLS4 Experiment Modelled Using the Explicit Algebraic Reynolds-stress Model
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Tidskriftsartikel (refereegranskat)abstract
- A recently developed so-called explicit algebraic Reynolds-stress (EARS) model is validated in a transitioning atmospheric boundary layer (ABL). The simulations describes a diurnal cycle with a deep convective ABL during daytime and an extremely thin and stably stratified ABL during the nighttime. The predictions of the EARS model are compared to large-eddy simulations (LES) of Couvreux et al. [2019]. The simulation is also extended in time in order to study several consecutive diurnal cycles. The EARS model uses the same parametrization and model coefficients for stable and convective ABL and is applicable over a wide range of thermal stratifications. First-order statistics are shown to be well predicted by the model. We also show that the model is able to predict transitional effects such as residual turbulence as well as horizontal turbulent fluxes, which are an inherent part of the EARS model solution.
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