A hybrid stabilization technique for simulating water wave - Structure interaction by incompressible Smoothed Particle Hydrodynamics (ISPH) method

The Smoothed Particle Hydrodynamics (SPH) method is emerging as a potential tool for studying water wave related problems, especially for violent free surface flow and large deformation problems. The incompressible SPH (ISPH) computations have been found not to be able to maintain the stability in certain situations and there exist some spurious oscillations in the pressure time history, which is similar to the weakly compressible SPH (WCSPH). One main cause of this problem is related to the non-uniform and clustered distribution of the moving particles. In order to improve the model performance, the paper proposed an efficient hybrid numerical technique aiming to correct the ill particle distributions. The correction approach is realized through the combination of particle shifting and pressure gradient improvement. The advantages of the proposed hybrid technique in improving ISPH calculations are demonstrated through several applications that include solitary wave impact on a slope or overtopping a seawall, and regular wave slamming on the subface of open-piled structure

Turbulent Details Simulation for SPH Fluids via Vorticity Refinement

A major issue in Smoothed Particle Hydrodynamics (SPH) approaches is the numerical dissipation during the projection process, especially under coarse discretizations. High-frequency details, such as turbulence and vortices, are smoothed out, leading to unrealistic results. To address this issue, we introduce a Vorticity Refinement (VR) solver for SPH fluids with negligible computational overhead. In this method, the numerical dissipation of the vorticity field is recovered by the difference between the theoretical and the actual vorticity, so as to enhance turbulence details. Instead of solving the Biot-Savart integrals, a stream function, which is easier and more efficient to solve, is used to relate the vorticity field to the velocity field. We obtain turbulence effects of different intensity levels by changing an adjustable parameter. Since the vorticity field is enhanced according to the curl field, our method can not only amplify existing vortices, but also capture additional turbulence. Our VR solver is straightforward to implement and can be easily integrated into existing SPH methods

Vortex Formation and Decay: The Scaling of Vortex-Wall Interaction

Das Ziel der vorliegenden Arbeit ist es den Einfluss von Turbulenz und im speziellen der Reynoldszahl ($Re$) auf die Prozesse der Wirbelformation und des Wirbelzerfalls zu untersuchen. Inspiriert von Wirbeln, die bei der Fortbewegung von schwimmenden und fliegenden Tieren auftreten, werden drei charakteristische Merkmale identifiziert, die durch das Zusammenspiel eines Wirbels mit einem beschleunigten Festkörper (z.B. einem Flügel oder einer Flosse) entstehen: eine gekrümmte freie Scherschicht, der Wirbelkern und die Grenzschicht zwischen dem Wirbel und dem beschleunigten Körper. Im Rahmen der Arbeit werden mehrere Experimente und Simulationen vorgestellt, welche diese Merkmale der Wirbel in vereinfachten Konfigurationen reproduzieren und von anderen Effekten isolieren. Dies ermöglicht es den Einfluss der Turbulenz auf besagte Wirbelmerkmale zu quantifizieren. Zunächst wird die Interaktion eines generischen Wirbels mit einer Wand betrachtet. Hierzu wird ein Zylinder, welcher ein Fluid in Starrkörperrotation enthält, schlagartig angehalten. Das Abklingen und der Zerfall der des Starrkörperwirbels wird mithilfe einer direkten numerischen Simulation (DNS) für Reynoldszahlen im Bereich $Re \leq 2.8 \cdot 10^4$ analysiert. Fünf Stufen des Wirbelzerfalls können aufgrund der zugrundeliegenden Strukturen der Strömung charakterisiert werden. Darüber hinaus liefern die Ergebnisse der DNS empirische Skalierungsgesetze, die verschiedene Stufen den Wirbelzerfalls beschreiben. Die Skalierungsgesetze werden anschließend anhand von zwei experimentellen Kampagnen im moderaten ($Re\leq 5.6\cdot 10^5$) und hohen ($Re\leq 4\cdot 10^6$) Reynoldszahlbereich validiert. Anschließend wird die gekrümmte Scherschicht und der Wirbelkern genauer betrachtet. In Experimenten in einem großskaligen Schleppkanal wird eine runde Platte aus der Ruhe beschleunigt. Hierbei liefern Experimente in einem großen Reynoldszahlbereich Erkenntnisse über die Auswirkungen von kleinskaligen Strukturen auf die Wirbelformation. Weiterhin wird die Turbulenz im Wirbel durch Modifikationen der umlaufenden Plattenkante beeinflusst. Inspiriert von in der Natur auftretenden Flossenformen werden wellenartige Strukturen aufgeprägt, welche Strukturen im Bereich ihrer eigenen Wellenlänge in die Strömung einbringen. Sind diese Wellenlängen größer als die Dicke der gekrümmten Scherschicht, wird das Wirbelwachstum beeinflusst und damit die Kraft, die auf die Platte wirkt, modifiziert

Hydrodynamic Simulation of Cyclone Separators

Cyclone separators are commonly used for separating dispersed solid particles from gas phase. These devices have simple construction; are relatively inexpensive to fabricate and operate with moderate pressure losses. Therefore, they are widely used in many engineering processes such as dryers, reactors, advanced coal utilization such as pressurized and circulating fluidized bed combustion and particularly for removal of catalyst from gases in petroleum refinery such as in fluid catalytic cracker (FCC). Despite its simple operation, the fluid dynamics and flow structures in a cyclone separator are very complex. The driving force for particle separation in a cyclone separator is the strong swirling turbulent flow. The gas and the solid particles enter through a tangential inlet at the upper part of the cyclone. The tangential inlet produces a swirling motion of gas, which pushes the particles to the cyclone wall and then both phases swirl down over the cyclone wall. The solid particles leave the cyclone through a duct at the base of the apex of the inverted cone while the gas swirls upward in the middle of the cone and leaves the cyclone from the vortex finder. The swirling motion provides a centrifugal force to the particles while turbulence disperses the particles in the gas phase which increases the possibility of the particle entrainment. Therefore, the performance of a cyclone separator is determined by the turbulence characteristics and particle-particle interaction.Full Tex

Numerical analysis of axisymmetric turbulent swirling flow in circular pipe

In this paper, turbulent swirling flow in circular pipe is numerically investigated using OpenFOAM, an open-source computational fluid dynamics software. Flow is computed as 2-D axisymmetric, with various turbulent models, but with main accent on computations with Reynolds stress transport models. Two Reynolds stress models were used in computations: Launder-Gibson and Speziale-Sarkar-Gatski models. Previous author's experimental results are used as a validation tool for numerical computations. It was shown that standard two-equation models can not predict the flow in right manner, while the Reynolds stress models give good prediction of mean velocities. As apart of research Speziale-Sarkar-Gatski model is implemented in OpenFOAM code

Numerical analysis of axisymmetric turbulent swirling flow in circular pipe

In this paper, turbulent swirling flow in circular pipe is numerically investigated using OpenFOAM, an open-source computational fluid dynamics software. Flow is computed as 2-D axisymmetric, with various turbulent models, but with main accent on computations with Reynolds stress transport models. Two Reynolds stress models were used in computations: Launder-Gibson and Speziale-Sarkar-Gatski models. Previous author's experimental results are used as a validation tool for numerical computations. It was shown that standard two-equation models can not predict the flow in right manner, while the Reynolds stress models give good prediction of mean velocities. As apart of research Speziale-Sarkar-Gatski model is implemented in OpenFOAM code

Engineering Fluid Dynamics 2019-2020

This book contains the successful submissions to a Special Issue of Energies entitled “Engineering Fluid Dynamics 2019–2020”. The topic of engineering fluid dynamics includes both experimental and computational studies. Of special interest were submissions from the fields of mechanical, chemical, marine, safety, and energy engineering. We welcomed original research articles and review articles. After one-and-a-half years, 59 papers were submitted and 31 were accepted for publication. The average processing time was about 41 days. The authors had the following geographical distribution: China (15); Korea (7); Japan (3); Norway (2); Sweden (2); Vietnam (2); Australia (1); Denmark (1); Germany (1); Mexico (1); Poland (1); Saudi Arabia (1); USA (1); Serbia (1). Papers covered a wide range of topics including analysis of free-surface waves, bridge girders, gear boxes, hills, radiation heat transfer, spillways, turbulent flames, pipe flow, open channels, jets, combustion chambers, welding, sprinkler, slug flow, turbines, thermoelectric power generation, airfoils, bed formation, fires in tunnels, shell-and-tube heat exchangers, and pumps

Investigation of Swirl Flows Applied to the Oil and Gas Industry

Understanding how swirl flows can be applied to processes in the oil and gas industry and how problems might hinder them, are the focus of this thesis. Three application areas were identified: wet gas metering, liquid loading in gas wells and erosion at pipe bends due to sand transport. For all three areas, Computational Fluid Dynamics (CFD) simulations were performed. Where available, experimental data were used to validate the CFD results. As a part of this project, a new test loop was conceived for the investigation of sand erosion in pipes. The results obtained from CFD simulations of two-phase (air-water) flow through a pipe with a swirl-inducing device show that generating swirl flow leads to separation of the phases and creates distinct flow patterns within the pipe. This effect can be used in each of the three application areas of interest. For the wet gas metering application, a chart was generated, which suggests the location of maximum liquid deposition downstream of the swirling device used in the ANUMET meter. This will allow taking pressure and phase fraction measurements (from which the liquid flow rate can be determined) where they are most representative of the flow pattern assumed for the ANUMET calculation algorithms. For the liquid loading application, which was taken as an upscaling of the dimensions investigated for the wet gas metering application, the main focus was on the liquid hold-up. This parameter is defined as the ratio of the flowing area occupied by liquid to the total area. Results obtained with CFD simulations showed that as the water rate increases, the liquid hold-up increases, implying a more effective liquid removal. Thus, it was concluded that the introduction of a swirler can help unload liquid from a gas well, although no investigation was carried out on the persistance of the swirl motion downstream of the device. For the third and final application, the erosion at pipe bends due to sand transport, the main focus was to check the erosion rate on the pipe wall with and without the introduction of a swirler. The erosion rate was predicted by CFD simulations. The flow that was investigated consisted of a liquid phase with solid particles suspended in it. The CFD results showed a significant reduction in erosion rate at the pipe walls when the swirler was introduced, which could translate into an extended working life for the pipe. An extensive literature review performed on this topic, complemented by the CFD simulations, showed the need for a dedicated multiphase test loop for the investigation of sand erosion in horizontal pipes and at bends. The design of a facility of this type is included in this thesis. The results obtained with this work are very encouraging and provide a broad perspective of applications of swirl flows and CFD for the oil and gas industry

A study of the internal flow of dense vapours used in Organic Rankine Cycle (ORC) turbines

An Organic Rankine Cycle (ORC) is a thermodynamic cycle utilizing a heat source at low-temperature. It can be used for the waste heat recovery of vehicle engines, industrial processes, solar thermal power plants, and geothermal power plants, leading to reduction of CO2 emissions. The turbine expander is a key component of this cycle, and its efficiency is critical to the system performance. To improve the design of an ORC turbine, the internal flow of the turbine should be studied. Unlike the fluids in conventional turbines, the fluids used in ORC turbines are dense vapours. These vapours have complex molecules and relatively large molecular weights, and they operate at states close to thermodynamic critical points or saturation line. Therefore, the thermodynamic behaviours of dense vapours are far from those of ideal air. However, the influence of these effects on the internal flows is not well understood. The fundamental flow behaviours of dense vapours, including gasdynamic behaviours in blade-shaped nozzle flows and turbulent behaviours in wall-bounded flows, are the focus of this work. A supersonic cascade using R1233zd(E) as working fluids is designed by the method of characteristics. The designed geometry is able to achieve a nearly uniform outlet flow at about $Ma=2$, which is checked in an RANS simulation. A blade-shaped nozzle is designed using the blade shape of this cascade, and the preliminary test results of this nozzle is presented. This nozzle is tested with both nitrogen and R1233zd(E) as working fluids. In the test with nitrogen, an adverse pressure gradient is measured on both nozzle surfaces downstream of the nozzle throat, and a shock train is observed at the corresponding position. Similar to the nitrogen test, the adverse pressure gradient is also found in the tests with R1233zd(E), but the Schlieren images cannot clearly show the shock train due to the disturbance of two-phase flows. A Direct Numerical Simulation (DNS) method for dense vapours is developed to obtain detailed information on turbulence. A modified Steger-Warming splitting is proposed to consider the strong non-ideal effects of gases, and the Span-Wagner EoS \cite{span2003equations} is used for studied dense vapours. The first studied benchmark case of wall-bounded flow is the supersonic fully-developed channel flow. Both the mean flow fields and the turbulent fluctuation fields are analysed. The mean profile and the fluctuation of thermodynamic properties are significantly affected by both molecular-complexity effects and non-ideal effects, and the sound-wave mode can be the dominant mode for generating fluctuations of thermodynamic properties ($T'$, $\rho'$) in dense vapours. Important modelling issues for dense vapours are also discussed, including the Strong Reynolds Analogy (SRA) and assumptions required for the $k-\varepsilon$ RANS method. The second studied benchmark case of wall-bounded flow is the bypass laminar-turbulent transition over a flat plate under a supersonic incoming flow. At the same incoming non-dimensional numbers ($Ma_\infty$ and $Re_x$), the breakdown of laminar flow starts earlier in dense vapours than in air. The mechanism of breakdown for both dense vapours and air is due to the interaction of two streamwise vortices in opposite rotational directions, leading to Kelvin-Helmholtz (KH) instability in a high shear layer. Proper Orthogonal Decomposition (POD) is also used to support the findings in vortex structure analysis.Open Acces