Vortex dynamics of in-line twin synthetic jets in a laminar boundary layer

2015-2016 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Flow Characteristics of Self-Oscillating Round and Square Jets in a Confined Cavity

Oscillating jets have many practical applications in industry. The self-oscillating behavior of a jet can be observed when the jet emanates into a confined cavity. In this thesis, a step-by-step approach has been followed to investigate important aspects of self-oscillating turbulent jets. The first step focuses on evaluating the characteristics of self-oscillating square and round jets. The jet exits from a submerged round nozzle or a square nozzle with the same hydraulic diameter into a narrow rectangular cross-section cavity at a Reynolds number of 54,000 based on nozzle hydraulic diameter and average jet exit velocity. A numerical investigation of the three-dimensional self-oscillatory fluid structures in the cavity is conducted by solving the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations using a Reynolds stress turbulence model (RSM). Vortex identification using the λ2-criterion method is used to investigate the flow dynamics. The simulations show that the vortex rings initially have the nozzle shape near the nozzle exit and, for a square nozzle, axis-switching occurs at about 0.7 hydraulic diameters downstream. Furthermore, after impact on the walls, the vortex rings are converted into two tornado-like vortices. The decay rates of both types of self-oscillating jets initially show the same trend as free round and square jets but change significantly as the effects of oscillation and confinement begin to dominate. The results show that the spread and decay rates of the self-oscillating square jet are higher, while the self-oscillating round jet has higher turbulence intensities near the jet center. Moreover, the Reynolds stress profiles of both round and square self-oscillating jets are qualitatively similar and show two peaks on either side of the centerline, which convert to mild peaks at distances farther downstream.The second step focuses on the numerical study of self-oscillating twin jets emanating from round and square cross-section nozzles into a narrow rectangular cavity. The flow characteristics are evaluated at nozzle spacing-to-diameter ratios of 2, 3, 4 and 5 at a jet Reynolds number of 27,000. The effects of nozzle spacing on the frequency of oscillation, mean velocity and turbulence features are examined. The results indicate that increasing the spacing does not have much effect on the frequency of oscillations. For a spacing-to-diameter ratio up to four, the two jets merge in the downstream and oscillate as one. At the largest nozzle spacing, the two jets do not merge but oscillate separately across half of the cavity width. Furthermore, as the nozzle spacing is increased, the profiles of Reynolds shear stress demonstrates that the mixing increases in the inner shear layer region. The last part of the thesis focuses on potential cooling applications of self-oscillating jets. The jet exits from a square cross-section nozzle at a Reynolds number of 54,000. The heated devices are attached externally on the front surface of the cavity. A three-dimensional numerical simulation of the flow is conducted by solving the URANS and energy equations with RSM to assess the thermal features of the flow field. The cooling performance of the self-oscillating jet is compared with the channel flow and the wall jet. The results show that the channel flow has the lowest heat transfer. The heat transfer of wall jets increases around the central region, while the heat transfer of self-oscillating jets is higher farther from the central region. Self-oscillating jets can improve heat transfer over a larger area when the heated elements are in a horizontal arrangement, while the wall jet shows a higher performance for a vertical arrangement of elements

Turbulent drag reduction using surface plasma

An experimental investigation has been undertaken in a wind tunnel to study the induced airflow and drag reduction capability of AC glow discharge plasma actuators. Plasma is the fourth state of matter whereby a medium, such as air, is ionized creating a system of electrons, ions and neutral particles. Surface glow discharge plasma actuators have recently become a topic for flow control due to their ability to exert a body force near the wall of an aerodynamic object which can create or alter a flow. The exact nature of this force is not well understood, although the current state of knowledge is that the phenomenon results from the presence of charged plasma particles in a highly non-uniform electric field. Such actuators are lightweight, fully electronic (needing no moving parts or complicated ducting), have high bandwidth and high energy density. The manufacture of plasma actuators is relatively cheap and they can be easily retrofitted to existing surfaces. The first part of this study aims at characterising the airflow induced by surface plasma actuators in initially static air. Ambient air temperature and velocity profiles are presented around a variety of actuators in order to understand the nature of the induced flow for various parameters such as applied voltage, frequency, actuator geometry and material. It is found that the plasma actuator creates a laminar wall jet along the surface of the material on which it is placed. The second part of the study aims at using plasma actuators to reduce skin-friction drag in a fully developed turbulent boundary layer. Actuators are designed to induce spanwise forcing near the wall, oscillating in time. Thermal anemometry measurements within the boundary layer are presented. These show that the surface plasma can cause a skin-friction drag reduction of up to 45% due to the creation of streamwise vortices which interact with, and disrupt the near-wall turbulence production cycle

Turbulent drag reduction using surface plasma

An experimental investigation has been undertaken in a wind tunnel to study the induced airflow and drag reduction capability of AC glow discharge plasma actuators. Plasma is the fourth state of matter whereby a medium, such as air, is ionized creating a system of electrons, ions and neutral particles. Surface glow discharge plasma actuators have recently become a topic for flow control due to their ability to exert a body force near the wall of an aerodynamic object which can create or alter a flow. The exact nature of this force is not well understood, although the current state of knowledge is that the phenomenon results from the presence of charged plasma particles in a highly non-uniform electric field. Such actuators are lightweight, fully electronic (needing no moving parts or complicated ducting), have high bandwidth and high energy density. The manufacture of plasma actuators is relatively cheap and they can be easily retrofitted to existing surfaces. The first part of this study aims at characterising the airflow induced by surface plasma actuators in initially static air. Ambient air temperature and velocity profiles are presented around a variety of actuators in order to understand the nature of the induced flow for various parameters such as applied voltage, frequency, actuator geometry and material. It is found that the plasma actuator creates a laminar wall jet along the surface of the material on which it is placed. The second part of the study aims at using plasma actuators to reduce skin-friction drag in a fully developed turbulent boundary layer. Actuators are designed to induce spanwise forcing near the wall, oscillating in time. Thermal anemometry measurements within the boundary layer are presented. These show that the surface plasma can cause a skin-friction drag reduction of up to 45% due to the creation of streamwise vortices which interact with, and disrupt the near-wall turbulence production cycle

Flow control for road vehicle drag reduction

This thesis covers topics that span bluff-body aerodynamics, hybrid RANS-LES CFD methods, flow control and model-order reduction. These topics arise from investigating the flow past three geometries: the bullet shaped D-body, the canonical squareback Ahmed body and the commerical Nissan NDP. The study on the D-body was aimed at transitioning the research group from the restrictive block-structured formulated StreamLES solver to the more flexible OpenFOAM code that can use unstructured meshes. Linear feedback control for base pressure increase was applied as was done in the work by Dalla Longa et al. (2017). Identification of the plant, G(s), that represents the wake's response to forcing was completed and correlated well with the results from Dalla Longa et al. (2017). The same can also be said of the sensitivity based designed feedback control law, K(s). When applied in simulation, an attenuation of the base pressure fluctuations was, as desired, achieved, although the base pressure increased by 24.5% as opposed to the 38% achieved by Dalla Longa et al. (2017). In the study on the squareback Ahmed body, wall-resolving (WRLES) and wall-modelled (WMLES) large eddy simulation were successfully applied. First, a simulation setup that is both able to resolve wake bimodality, while remaining reasonable in computational resource use, was created. Subsequently, variants of this setup were used to identify a flow feature that plays a critical role in forcing wake bimodality events. More specifically, a heavily under-resolved WMLES simulation in which both the near-wall and part of the outer-region of the turbulent boundary layer are Reynolds-averaged did not capture the front recirculation bubble near the Ahmed body nose; neither did it resolve a bimodal wake switching event. Meanwhile, the simulations with a more refined near-wall mesh did capture the front separation bubble as well as bimodal switching events of the wake. This front separation bubble sends out powerful hairpin vortices that interact with the rear wake. Specifically, these vortices go on to produce significant amounts of TKE, which, upon convection to the rear of the Ahmed body, ultimately help trigger a bimodal event. The Ahmed body study also involved the application of linear feedback control for drag reduction as was done in the D-body study. In the short term, mean blowing did lead to a base pressure increase, but as the zero-net-mass-flux (ZNMF) jet settled, it oscillated around zero making its effects indiscernible. The final geometry analyzed was the Nissan NDP. This was done by performing benchmark wall-resolving LES (WRLES). First, the benefit of appending a rear cavity to an otherwise "squareback" geometry was assessed. It was concluded that the cavity allows the wake to move more freely about the rear base. Specifically, the wake is freed from its more restricted motion that is present with the "squareback" Nissan NDP. In doing so, the drag reduction achieved with the cavity appendage is about 13.6%. Work on the Nissan NDP also involved an assessment of a moving ground in the simulation. It was concluded that, in the stationary ground simulation, flow detachment at the ground where the flow exits from the underbody has an adverse drag effect. In other words, although moving ground simulations better replicate the real-world conditions, the stationary ground variant is in this case more conservative, as it returns slightly higher drag values.Open Acces

Vortex evolution in the near wake behind polygonal cylinders

The near wake of the polygonal cylinder with the side number N = 3~∞ is systematically studied using particle image velocimetry (PIV) at Re = 1.6 × 104. The proper orthogonal decomposition (POD) analysis is carried out to extract the large-scale coherent vortex structures and their evolution. It has been found that the vortex circulation grows to the maximum at the vortex formation length by entraining the vorticity from the separated shear layer and then undergoes a two-stage decay. The maximum circulation scales with the wake width, defined as the vertical distance between the two peaks of streamwise velocity fluctuation at vortex formation length. The vortex center trajectory indicates that the vortices move towards the centerline first and then away, with the vortex size monotonically increasing over the examined streamwise range. The vortex size at the maximum circulation also scales with the wake width. The vortex convection velocity increases gradually in the streamwise direction, and the ratio of the lateral and streamwise components of the vortex convection velocity, when scaled by wake width and vortex formation length respectively, approaches asymptotically 0.18 in the downstream, irrespective of the cylinder orientation or N

Characterisation of turbulence in an open channel flow and in a fountain with tomographic PIV.

This work aims to improve the understanding of the fundamental characteristics of environmental flows by interpreting the turbulence in a 3D measurement domain. This thesis primarily describes the Tomographic PIV technique and the results of three experimental investigations of environmental flows. Two experiments were conducted in an open channel flow, divided into four sequential, identical pools, by a combination of regular grids. The first set of TPIV measurements were in the water column, while the second set of measurements were made along the channel bottom. The instantaneous structures in the flow were visualised and the turbulent kinetic energy k, energy dissipation ε and vorticity ω were analysed; their decay along the streamwise direction was revealed. Ejections (Q2) and sweeps (Q4) were identified along the channel bottom. A major contribution that resulted from the investigation pertains to the vibration correction of the cameras. TPIV measurements were taken of a regime of turbulent, forced fountain flows. The fountains were created by injecting a salt-water solution through a circular opening into the bottom of a reservoir of a water-ethanol solution, with their refractive indices carefully matched. The evolution of the fountain in its initial stages was captured and described in a series of chronological measurement volumes. Measurements of the fully developed fountains captured the large scale structures and their characteristics were analysed by considering the topology of the invariants of the velocity gradient tensor. The TPIV system was designed and built in-house at the University of Sydney. The experimental investigations described in this work revealed some interesting features of the environmental flows. The applicability and versatility of TPIV for these flows were demonstrated. The measurements allowed for the quantification and visualisation of the turbulence in the flows and hence shed light on the physics behind them

Characterisation of turbulence in an open channel flow and in a fountain with tomographic PIV.

This work aims to improve the understanding of the fundamental characteristics of environmental flows by interpreting the turbulence in a 3D measurement domain. This thesis primarily describes the Tomographic PIV technique and the results of three experimental investigations of environmental flows. Two experiments were conducted in an open channel flow, divided into four sequential, identical pools, by a combination of regular grids. The first set of TPIV measurements were in the water column, while the second set of measurements were made along the channel bottom. The instantaneous structures in the flow were visualised and the turbulent kinetic energy k, energy dissipation ε and vorticity ω were analysed; their decay along the streamwise direction was revealed. Ejections (Q2) and sweeps (Q4) were identified along the channel bottom. A major contribution that resulted from the investigation pertains to the vibration correction of the cameras. TPIV measurements were taken of a regime of turbulent, forced fountain flows. The fountains were created by injecting a salt-water solution through a circular opening into the bottom of a reservoir of a water-ethanol solution, with their refractive indices carefully matched. The evolution of the fountain in its initial stages was captured and described in a series of chronological measurement volumes. Measurements of the fully developed fountains captured the large scale structures and their characteristics were analysed by considering the topology of the invariants of the velocity gradient tensor. The TPIV system was designed and built in-house at the University of Sydney. The experimental investigations described in this work revealed some interesting features of the environmental flows. The applicability and versatility of TPIV for these flows were demonstrated. The measurements allowed for the quantification and visualisation of the turbulence in the flows and hence shed light on the physics behind them

Towards the noise reduction of synthetic jet actuators using lobed orifices

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonWith increasing strain on the civil aviation industry to meet strict targets to reduce the adverse effects aviation has on the environment by 2050, significant advances in aircraft design and research are required. Aerodynamic improvements have been a focus for several decades now, however, current and future civil transport aircraft are based on traditional designs originating from the 1950s. Optimisation of aircraft external geometry for aerodynamic gain is reaching maturity and is becoming increasingly non-cost-effective. New advances in sensor and actuator technology has allowed for the development of active flow control (AFC) devices that have shown promising results in laboratory and even full-scale flight conditions, as seen by the joint NASA-Boeing ecoDemonstrator. One such device is the synthetic jet actuator (SJA), that synthesises periodic jets without the requirement for external air supply, while adding momentum to the surrounding flow. For this reason, SJAs are also referred to as zero-net-mass-flux actuators. There exists extensive work on the use of these devices for flow control applications in a laboratory setting. One of the key issues that remains unresolved, hindering successful aircraft application to-date, is the actuator self-noise generated. The noise level of SJAs can be so severe that they were rejected for application on the ecoDemonstrator in favour of a higher authority, quieter AFC device. SJAs were only considered for use in emergency situations on aircraft. Furthermore, the actuators were also not permitted to operate simultaneously at full power, which may severely limit scope for flow control on aircraft. Other applications that would benefit from SJAs include heat transfer for cooling in electronic devices. Studies in this field identify the same problem with noise levels of up to 73 dB reported. It is clear that work towards the self-noise reduction of SJAs is required to harness the full potential of this actuator technology. In the work presented, passive and active noise control measures in the form of lobed orifices and antiphase operation of two jets, respectively, on the noise reduction of SJAs are ii investigated. Noise sources of synthetic jet actuators include mechanical (diaphragm) and jet induced noise, where the focus of this work is on the latter type. Tests were conducted in quiescent conditions using jet velocity measurements, acoustic measurements, and flow visualisation. Tests were carried out using a single chamber SJA with variable cavity height and both circular and lobed orifices. These tests helped identify a SJA self-noise generation mechanism when using a circular orifice. This mechanism is characterised by a constant frequency behaviour visible in acoustic spectra for a specific jet Reynolds number range of 600< Rej<750 and Strouhal number range of 0.22<St<0.50. The geometries of the lobed orifices used in this work differ in lobe count and penetration. It was shown that a broadband noise reduction is possible with such orifices, with a maximum noise reduction of 14 dB at particular frequencies. The results indicate that a high number of lobes and penetration are preferred for noise reduction, however, at the expense of quickly dissipating downstream jet velocity. Flow visualisation reveals that this adverse effect is caused by enhanced mixing of lobed jets with ambient air that leads to earlier and more aggressive breakup of flow structures. A double chamber SJA is also used to demonstrate the noise attenuation through the antiphase operation of two cavities, caused by the interference pattern of the sound field of each source. The maximum reduction measured using this actuator configuration is 14 dB, depending on directivity.EPSRC & RAe

Experimental, Numerical and Field Approaches to Scour Research

This book presents fourteen state-of-the-art research papers prepared by research scientists and engineers around the world. They explore the subject of scour related to bridge piers, monopiles, propellers, turbines, weirs, dams, grade-control structures, and pipelines. Their works are based on three different research methodologies, namely experimental, numerical, and field approaches