1,720,995 research outputs found
The effect of numerical dissipation on the predictive accuracy of wall-modelled large-eddy simulation
No full textThe effect of numerical dissipation on the predictive accuracy of wall-modelled large-eddy simulation is investigated via systematic simulations of fully-developed turbulent channel flow. A total of 16 simulations are conducted using the open-source computational fluid dynamics software OpenFOAM\uae. Four densities of the computational mesh are considered, with four simulations performed on each, in turn varying in the amount of numerical dissipation introduced by the numerical scheme used for interpolating the convective fluxes. The results are compared to publicly-available data from direct numerical simulation of the same flow. Computed error profiles of all the considered flow quantities are shown to vary monotonically with the amount of dissipation introduced by the numerical schemes. As expected, increased dissipation leads to damping of high-frequency motions, which is clearly observed in the computed energy spectra. But it also results in increased energy of the large-scale motions, and a significant over-prediction of the turbulent kinetic energy in the inner region of the boundary layer. On the other hand, dissipation benefits the accuracy of the mean velocity profile, which in turn improves the prediction of the wall shear stress given by the wall model. Thus, in the current framework, the optimal choice for the dissipation of the numerical schemes may depend on the primary quantity of interest for the conducted simulation. With respect to the resolution of the grid, the change in the accuracy is much less predictable, and the optimal resolution depends on the considered quantity and the amount of dissipation introduced by the numerical schemes
Modeling Large Equipment Behavior in the Sea Splash-Zone Using Methods of Computational Fluid Dynamics
No full textOil industry uses a complex device, called the "Christmas tree" (XT in the remainder of the thesis) to control the hydrocarbon flow out of the oil well and for various other purposes. In case of off-shore extraction, the device has to be submerged into the water and then delivered further down. The most dangerous part of the delivery process is known to be the splash-zone of the sea.
In order to effectively simulate the XT's behaviour during delivery, a geometrically simplified model of the latter has to be created. Yet this model should posses the hydrodynamical properties of a real XT. In the first part of this thesis, a computational fluid dynamics (CFD) simulation is performed to investigate the results of a previous experimental work conducted on some proposed XT models. Significant disagreement between experimental results and the results of CFD modelling has been observed. Possible explanations of this divergence are given.
In the second part of this work the methodology of modelling the XT's behaviour in the splash-zone using
CFD is developed. The built model allows to simulate different weather conditions and evaluate the forces
acting on the XT. The necessity of including
fluid-structure interaction (FSI) into the model is investigated by comparing results obtained with and without FSI respectively
Modelling Techniques for Large-Eddy Simulation of Wall-Bounded Turbulent Flows
No full textLarge-eddy simulation (LES) is a highly accurate turbulence modelling approach in which a wide range of spatial and temporal scales of the flow are resolved. However, LES becomes prohibitively computationally expensive when applied to wall-bounded flows at high Reynolds numbers, which are typical of many industrial applications. This is caused by the need to resolve very small, yet dynamically important flow structures found in the inner region of turbulent boundary layers (TBLs). To remove the restrictive resolution requirements, coupling LES with special models for the flow in the inner region has been proposed. The predictive accuracy of this promising approach, referred to as wall-modelled LES (WMLES), requires further analysis and validation. In this work, systematic simulation campaigns of canonical wall-bounded flows have been conducted to support the development of a complete methodology for highly accurate WMLES on unstructured grids. Two novel algebraic wall-stress models are also proposed and shown to be more robust and precise than the classical approaches of the same type. For turbulence simulations, it is often challenging to provide accurate conditions at the inflow boundaries of the domain. Here, a novel methodology is proposed for generating an inflow TBL using a precursor simulation of turbulent channel flow. A procedure for determining the parameters of the precursor based on the Reynolds number of the inflow TBL is given. The proposed method is robust and easy to implement, and its accuracy is demonstrated to be on par with other state-of-the-art approaches. To make the above investigations possible, several software packages have been developed in the course of the work on this thesis. This includes a Python package for post-processing the flow simulation results, a Python package for inflow generation methods, and a library for WMLES based on the general-purpose software for computational fluid dynamics OpenFOAM. All three codes are publicly released under an open-source licence to facilitate their use by other research groups
Inflow generation for scale-resolving simulations of turbulent boundary layers
No full textGenerating inflow fields for scale-resolving simulations of turbulent flow is crucial for a wide range of applications and is an active area of research. In this thesis, a method for inflow generation employing a precursor turbulent channel flow simulation is proposed. A procedure for determining the parameters of the precursor simulation based on the properties of the inflow is given. To evaluate the performance of the method, results from a simulation of a flat-plate zero-pressure-gradient turbulent boundary layer are analysed. The adaption length is quantified in terms of the development of integral quantities and the statistical moments of the velocity field. The performance is also compared with that of a state-of-the-art rescaling method for the generation of inflow data. It is shown that both approaches result in adaption lengths of comparable sizes, which makes the proposed method an attractive alternative due to its conceptual simplicity and robustness. As part of the work on inflow generation, a Python package, eddylicious, was developed. The purpose of the package is to be a framework within which various generation methods can be implemented. The package is available online under an open-source license. An overview of the architecture and currently implemented functionality of the package is given in this thesis. Furthermore, the results of a preparatory study on large-eddy simulation of wall-bounded turbulent flows are discussed. Fully-developed turbulent channel flow is used as a model problem, and the general-purpose computational fluid dynamics solver OpenFOAM is employed. The accuracy of the results with respect to the resolution of the computational mesh is analysed. Several modelling approaches for the subgrid scale stresses are considered
Modeling Large Equipment Behavior in the Sea Splash-Zone Using Methods of Computational Fluid Dynamics
No full textOil industry uses a complex device, called the "Christmas tree" (XT in the remainder of the thesis) to control the hydrocarbon flow out of the oil well and for various other purposes. In case of off-shore extraction, the device has to be submerged into the water and then delivered further down. The most dangerous part of the delivery process is known to be the splash-zone of the sea. In order to effectively simulate the XT's behaviour during delivery, a geometrically simplified model of the latter has to be created. Yet this model should posses the hydrodynamical properties of a real XT. In the first part of this thesis, a computational fluid dynamics (CFD) simulation is performed to investigate the results of a previous experimental work conducted on some proposed XT models. Significant disagreement between experimental results and the results of CFD modelling has been observed. Possible explanations of this divergence are given. In the second part of this work the methodology of modelling the XT's behaviour in the splash-zone using CFD is developed. The built model allows to simulate different weather conditions and evaluate the forces acting on the XT. The necessity of including fluid-structure interaction (FSI) into the model is investigated by comparing results obtained with and without FSI respectively
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Flow dynamics in the closure region of an internal ship air cavity
Full text linkAccurate prediction of air leakage is crucial for the design of ship air lubrication systems based on internal cavities. Currently, the flow dynamics that govern the leakage remain largely unexplored, and the goal of this work is to elucidate them by means of numerical simulation. A geometrically simple test cavity is considered, and a simulation of the flow is conducted using large-eddy simulation coupled with a Volume of Fluid interface capturing method. The flow in the closure region is shown to be highly unsteady and turbulent. The cause of this is identified to be the pressure gradient on the beach wall of the cavity, occurring due to the stagnation of the flow. This pressure gradient pushes the air–water interface upwards, making it steeply inclined. As a result, the flow separates from the interface and forms a recirculation zone, in which air and water are mixed by means of overturning waves and turbulent entrainment Swarms of air bubbles leak periodically. Upstream of the closure region, the phase and length of the wave are found to be well-predicted using existing approximations based on linear flow theory. However, for the corresponding prediction of the amplitude of the wave the agreement is worse
Wall-Modelled Les Using High- and Low-Order Cfd Codes: Application to a Flat-Plate Boundary Layer
No full textResults from wall-modelled large-eddy simulations of a zero-pressure-gradient flat-plate turbulent boundary layer in the range Reθ ≈ [4000,12000] are reported. The simulations are performed using a low- and a high-order code: OpenFOAM® and Nek5000, respectively. For the latter, such simulations have previously not been reported in the literature. Structured hexahedral meshes are used, with two levels of refinement. As an important aspect in the wall modelling methodology, we use a temporally varying wall viscosity in order to enforce the wall shear stress. An equivalent inflow generation procedure is used for both codes, allowing for a more fair comparison. Results from Nek5000 simulations are generally more accurate. Both the skin friction and the profiles of velocity statistics are in good agreement with reference data. For Nek5000, this is an important milestone in the development of wall modelling capabilities for this solver. The results from OpenFOAM simulations exhibit a significant over-prediction of the skin friction, which has not been previously reported in the literature. Further investigation of the simulation methodology is necessary to find the cause of the problematic behaviour.</p
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