Statistical Description in the Turbulent Near Wake of a Rotating Circular Cylinder

Turbulence studies were made in the wake of a rotating circular cylinder in a uniform free stream. The interest was to examine the turbulence properties at the suppression of periodicity in vortex formation process. An experimental study of the turbulent near wake of a rotating circular cylinder was made at a Reynolds number of 9000 for velocity ratios, λ between 0 and 2.7. Hot-wire anemometry and particle image velocimetry results indicate that the rotation of the cylinder causes significant changes in the vortical activities. The turbulence quantities are getting smaller as λ increases due to suppression of coherent vortex structures

HEAT TRANSFER ENHANCEMENT IN STIRLING ENGINE USING FINS WITH DIFFERENT CONFIGURATIONS AND GEOMETRIES

The temperature difference across a Stirling engine cylinder is one of the main factors contributing to the power output and efficiency. Stirling engine is a heat engine that operates through expansion and compression cycles through different fluid temperatures. In addition, the engine was noted to have three different configurations, which include the alpha, beta, and gamma types, which are discussed in work. This study aims to investigate the effect of adding various fin geometries on different engine cylinder locations to simulate and visualize heat transfer enhancement across the cylinder. The various configurations were simulated through the ANSYS Fluent commercially available CFD model, which was used to simulate the temperature difference on the hot and cold ends of the tested fins. A validation model of mesh element sizing has also been implemented to determine the accuracy of the achieved numbers. Moreover, Fusion360 has been used to 3D design different geometries, including annular, conical and rectangular fins.Furthermore, lastly, validation of the obtained simulated results will be conducted to monitor the accuracy of the simulation through two different methods. An effectiveness of 8.17 has been obtained through the simulation for annular fins; in addition, it has been determined that annular fins achieve the best temperature distribution among the selection of geometries and configurations simulated which was recorded as 223°C for the hot end and 40°C for the cold end

FSI SIMULATION OF A FLEXIBLE VORTEX GENERATOR AND THE EFFECTS OF VORTICES TO THE HEAT TRANSFER PROCESS

This work numerically investigated the strength of vortices behind a flexible vortex generator (FVG) by calculating the circulation using Fluid-Structural Interaction (FSI) simulation of RANS (SST) k-ω model. The total circulation showed the vortices formed by the FVG was more significant than those originated by the rigid vortex generator (RVG). From the analyses, the case with more significant structural velocity results in a larger shear or circulation. The structural velocity computation proves an excellent consistency with the model predicted values. This has suggested the reliability of the proposed model. Therefore, the performance of FVG in turbulence augmentation has been identified, suggesting enhanced turbulent transport for the greater heat transfer process. Next, the numerical model was extended to examine the heat transfer performance of the FVG. In general, the findings show that the employment of the FVG has a significant positive impact on the heat transfer process in the heat exchanger

DESIGN IMPROVEMENT OF THE VERTICAL AXIS WIND TURBINE WITH APPLIED FLAPS THROUGH CFD ANALYSIS

This research aims to optimise VAWT power generation capabilities through the design alteration of blades by the addition of flaps. The application is aimed to be applied in the United Arab Emirates, specifically at Abu Dhabi University campus in Al Ain to replace a margin of the consumed electrical energy (15%) powered by the typical nonrenewable energy means. To study the effects of design alteration, relevant design parameters on the aerodynamic properties of the wind turbine, a typical standard design with standard dimensions were considered and used as a benchmark for comparison. Two aerodynamic simulation software were adopted, namely ANSYS FLUENT and QBlade, while the designs were drawn through AutoCAD. As per the simulation results, the addition of flaps resulted in an overall increase of 3.11% in power generation. The simulation results were then scaled up using dynamic similarity to obtain a total power consumption of 6 kW for each turbine suggesting that the newly built Al Ain campus would require 43 turbines to cover 15% of the total electricity consumption in a year. The building of a control system for active pitch control was not feasible due to increased complexity

EFFECTS OF GAS VOLUME FRACTIONS ON ELECTRICAL SUBMERSIBLE PUMP PERFORMANCE UNDER TWO-PHASE FLOW

An Electrical Submersible Pump (ESP) is the most commonly used artificial lifting method in the oil and gas industry through conversion of kinetic energy to pump pressure head; however, issues like gas entrainment and shifting production rates tend to cause ESP pressure degradation. Flow behaviours inside the ESP, such as gas pockets or pressure surging tend to diminish the pump pressure head significantly. Observation of gas-liquid flow in the ESP is challenging due to the compact, sophisticated geometry and highly turbulent flow strucutres. This paper observes two-phase flow in the ESP through CFD simulation, and illustrates its pressure degradation through Air Volume Fraction contours showing formations of gas pockets and recirculation bubbles. This research utilized Mixture Flow as its main two-phase model with 1%, 6% and 10% Gas Volume Fraction (GVF) ratios inside the pump at a constant flow rate of 250 L/min. The results show a clear pump head degradation from 3.062 m to 2.251 m overall. The centrifugal pump under two-phase flow is not able to generate the same amount of pressure head as it normally does due to the bubble point pressure and this decrease in pump pressure head is a potentially unstable behaviour, which is acknowledged as pressure surging

CFD ANALYSIS ON THE SINGLE-PHASE FLOW ELECTRICAL SUBMERSIBLE PUMP PERFORMANCE CURVE

Most of the industrial fields, especially oil and gas, involve the application of electrical submersible pump (ESP) for flow transport. The pump performance will deteriorate depending on conditions of usage. The main parameters that affect or influence the pump performance curve are pump types, fluid flow rates, gas fractions, the impellers’ geometry, rotational speeds and fluid properties. In this work, the pump performance curve was studied as the governing parameters such as flow rates, meshing elements and turbulence models were varied. A computational fluid dynamics simulation (CFD) was applied and the findings were compared with the manufacturer’s data for single-phase flows. The main purpose was focused on getting the right technique to investigate this problem. The various turbulence CFD models were analysed and sources of errors were explained. The corresponding pump head losses were discussed and the main improvement or the criteria to obtain the highest efficiency were suggested. The results show that the k-epsilon with enhanced wall treatment is the best CFD technique since it produces the most accurate results and the least errors by allowing flexibility in large pressure gradient and rapid changes in flow properties

SIMULATION STUDY ON VORTEX-INDUCED VIBRATION AIR WAKE ENERGY FOR AIRPORT RUNAWAY APPLICATION: A PRELIMINARY ANALYSIS

The objective of this research paper is to conduct a preliminary analysis of the effects of various cylindrical cross-sectional shapes on vortex-induced vibration for airport runaway air wake energy generation. In the case of the airport runway, vortices are generated from the aeroplane bodies that exit and enter the runway during take-off and landing operations. These oscillations can be utilized to generate power due to the large fluctuations produced by the vortices. Vortexinduced vibration works on the principle of Kármán vortices where a cylindrical or bluff-body shaped object oscillates due to the alternate vortex formation on the boundary layers by adverse fluid pressure. The oscillation depends on the unsteady lift force generated. This mechanical oscillation is later converted to electrical energy. Five cylindrical crosssection cases are investigated by computational fluid dynamics (CFD) k-ω turbulence model to identify the best case that will provide the largest lift force; hence the maximum power output generation. These cases are simulated using typical take-off air speeds for jetliners in order to finalize the overall performance. The observed results show that the elliptical cross-section with 3” height and 2” length provides the best cross-section in producing the greatest amount of energy. The total power generated from a single-cylinder was 707.1 Watts. The system can be optimized for larger aeroplanes and operation frequencies as well as could be expanded for larger energy output

Proper orthogonal decomposition analysis of vortex shedding behind a rotating circular cylinder

Turbulence studies were made in the wake of a rotating circular cylinder in a uniform free stream with the objective of describing the patterns of the vortex shedding up to suppression of the periodic vortex street at high velocity ratios, λ. The results obtained in the present study establish that shedding of Kármán vortices in a rotating circular cylinder-generated wake is modified by rotation of the cylinder. Alternate vortex shedding is highly visible when λ < 2.0 although the strength of the separated shear layers differ due to the rotation of the cylinder. The spectral density in the wakes indicate significant changes at λ = 2.0. The results indicate that the rotation of the cylinder causes significant disruption in the structure of the flow. Alternate vortex shedding is weak, distorted and close to being suppressed at λ = 2.0. It is clear that flow asymmetries will weaken vortex shedding, and when the asymmetries are significant enough, total suppression of a periodic street occurs. Particular attention was paid to the decomposition of the flow using Proper Orthogonal Decomposition (POD). By analyzing this decomposition with the help of Particle Image Velocimetry (PIV )data, it was found that large scales contribute to the coherent motion. Vorticity structures in the modes become increasingly irregular with downstream distance, suggesting turbulent interactions are occurring at the more downstream locations, especially when the cylinder rotates

Particle image velocimetry investigation of steady flow over a backwardfacing step

The backward-facing step (BFS) is a heuristic example, allowing for complex phenomena to arise in a simple geometry. Particle Image Velocimetry (PIV) investigations of mean-velocity distributions of backward-facing step flow with steady inlet condition were carried out and good agreement was obtained between current and previously published results for 50 ≤ Re ≤ 400. This confirms that the current experimental capabilities can provide detailed and accurate velocity information. The flow behaviour downstream the step depends on the strength of separated shear layer, which the circulation depends on the bulk flow, recirculation zone length and vortex formation time. Since the vortex formation process is governed by the circulation flux convected along the wall layer from the step, for Re ≤ 400, all of the circulation contained in the shear layer is drawn into the recirculation region. Thus, in a case where the shear layer characteristics are modified (e.g. in higher Reynolds number and unsteady flows), the balance of circulation is modified that would result in shedding

Particle image velocimetry investigaton of pulsatile folw over a backward-facing step

Bibliography: p. 147-153