Browsing by Author "Govender, Kessie."

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Digital, statistical and wavelet study of turbulence flow structure in laboratory plunging water waves.
(2014) Mukaro, Raphael.; Govender, Kessie.
This dissertation presents an experimental investigation set out to study the evolution of turbulence in laboratory generated breaking water waves. The waves propagate and break as plunging waves on a 1:20 sloping beach. The aim of the study was to investigate the spatial and temporal evolution of the velocity field and subsequent turbulence structures induced by these plunging waves as they propagate along the flume. Experimental parameters measured included free surface elevations, wave height, mean water levels, wave phase velocity and synoptic measurements of fluid velocities for five accretive runs in the surf zone. Experimental data were analyzed using digital, statistical as well as wavelet approaches, in order to derive other turbulence quantities associated with the flow. The first set of experiments, performed in the vicinity of the break point, involved measuring the external flow characteristics of the breaker. This was done to get prior information about the breaking behaviour of the wave in terms of surface elevation, wave heights and wave velocities. 0.4 Hz monochromatic waves were generated in a glass-walled flume. A set of capacitive wave gauges were calibrated and then employed to record time series measurements of free surface elevations. Mean water levels, wave heights and phase velocities were then determined from the water level time series. Results show that as the wave propagates from deep water towards shallow water, there is an increase in the wave height, reaching a maximum height at the break point, and then decreases sharply thereafter. Cross correlation techniques were then used to determine the phase difference between the wave near the generator and the wave at various points along the flume. The local wave velocity was obtained by taking the phase difference between two points spaced 0.2 m apart. A comparison of the measured wave phase velocity, with linear shallow water and modified linear wave velocity approximations, is made at various points along the flume. The second set of experiments involved measuring internal flow parameters of the fluid. The experimental setup for this included a progressive scan digital camera (connected to a frame grabber inserted inside a computer) that was used to capture images of the breaking wave. The flow was seeded with neutrally buoyant, white polystyrene beads. The wave cycle was imaged using twenty fields of view or phases and 100 image pairs of the flow captured at each phase. A pair of strobe lights was used to illuminate the flow when capturing a pair of images which are spaced a few milliseconds apart. Thus, image pairs of the particle image field are captured with a set time interval. A video technique called digital correlation image velocimetry (DCIV) was used to analyze the images. With this method, image pairs were cross-correlated to determine particle displacements and thereby the instantaneous particle velocities. 100 instantaneous velocity flow fields spanning the entire water column including the aerated region were obtained at each phase. This enabled the quantification of the temporal and spatial evolution of the various turbulence parameters associated with the flow. Measurements were taken at five stations across the entire surf zone. Two-dimensional velocity flow fields are presented for phases where turbulence was observed to be predominant. The instantaneous velocities measured are up to two times the wave phase speed. The instantaneous velocity fields were then processed using phase-ensemble averaging to estimate the phase-averaged horizontal and vertical velocity fields and their corresponding fluctuating parts. Mean flow fields obtained by averaging 100 instantaneous flow fields at each phase, show the evolution of a shear layer between the nearly stagnant underlying fluid and the fast moving crest flow. Evolution of the stream-wise and along-shore profiles of the mean and turbulent quantities such as turbulence intensity, turbulent kinetic energy and vorticity of the flow are also presented. For this breaker, peak phase-ensemble averaged horizontal velocities were observed to be of the order of 250 cm/s while the vertical was about 50 cm/s. Measurements of the forward and reverse mass fluxes indicate a mean relative density for this plunging breaker to be around 0.44. Further statistical analysis yielded time-averaged mean horizontal and vertical velocities, root-mean-square (r.m.s) fluctuating velocities, turbulent kinetic energy and vorticity. Evolution results of these flow pa- rameters are also presented in the form of contour plots. Vertical and cross-variation of these turbulence characteristics are also presented for each of the chosen wave phases. Results show that most turbulence parameters appear to rise steadily from the trough, then rapidly, reaching peak values just above the still water line. They also show that a relatively large turbulent motion is initially organized in the crest of the breaking wave region. As the wave crest passes, this turbulent structure will then stretch downward to the lower interior region of the water column. Measurements of the time-averaged turbulence intensities and kinetic energy reveal that vertical profiles of these parameters increase from the flume bed up to a normalized elevation, z/h = 1.0. Thereafter, these parameters begin to decrease towards the crest. Peak turbulence parameters were observed near the front part of the wave crest with peak values of 0.11 and 0.06 for normalized horizontal and normalized vertical turbulence intensities, respectively. Froude-scaled turbulent kinetic energy was observed to increase almost linearly from the flume bed up to elevation z/h = 1.0. Both normalized, time-averaged turbulence intensity components and the Froude-scaled time-averaged turbulent kinetic energy results show nearly exponential decay towards the shore. Colour contour plots were used to visualize the evolution of vorticity as flow progressed. Instantaneous vorticity fields were observed to be characterized by patches of counter-rotating eddies. These pairs are generated at the free surface and translated as units in the direction of the flow. Eventually these high vorticity patches are observed to diffuse to the bottom of the flume reaching the flume bed after the crest has passed. While the phase speed is of the order of 150 cm/s, vortex structures were estimated to propagate with a speed of about 6 cm/s. Vorticity of the mean flow revealed a large vortex of peak value of around 150 s−1 developing around the shear boundary layer. The phenomenon of vortex shedding was observed in the evolution of mean flow vorticity where the tail of the initially strong shear layer vortex disintegrates as flow progresses. Finally, a continuous, one-dimensional, complex Morlet wavelet transform was applied to synthesized test signals comprising of three sinusoids of different frequencies. This was done to determine the relationship between the wavelet spatial scales and the period of the signal, from which a one-to-one relationship was obtained. The amount of energy available at each scale of the synthetic signals was then obtained and compared against the expected root-mean-square energy. This was done to calibrate the wavelet algorithm before it was used to extract the turbulent energy of the wave available in the velocity fluctuations. The wavelet energy was compared against the statistically calculated turbulent kinetic energy, and showed very good agreement at each phase, and at each elevation. The wavelet scales in the velocity fluctuations were then subdivided into three bands that are herein referred to as the micro, mid and macro scales. The wavelet energy spectrum of the flow shows that for the early phases of the flow, which correspond to the approach of the crest, up to 80 % of the wave energy is confined in the micro scales. For the remainder of the flow, micro scales contribute almost uniformly, about half of the maximum (40 %) shedding off excess energy which appears in the macro-scales. Mid scales were observed to contain 15 % of the total energy, almost uniformly throughout the wave cycle. Further energy analysis was performed to examine the variation of the total wavelet energy with scale at different phases of the flow. Results indicate that for phases corresponding to the approach of the crest, most of the energy lies in the micro-scales between 5 cm and 15 cm. The average wavelet energy at each phase of the flow was computed over several cycles at each elevation. Results show that for the three elevations considered, the micro-scales contain most of the energy while mid-scales have the least. The available energy of the wave decreases towards the shore.
Experimental investigation of waves breaking over a bar in the surf zone.
(2005) Pillay, Ursula Monica.; Govender, Kessie.
Experiments on regular two dimensional waves breaking over an artificial sand bar were conducted in a glass walled flume to facilitate an investigation of the hydrodynamic processes that evolve in the surf zone. The instantaneous time averaged velocities and the velocity flow fields have been measured using digital video imaging and analysis techniques. The instantaneous velocity flow fields were then processed, resulting in the phase-ensemble-averaged velocities, time averaged velocities and energy and momentum fluxes. The time averaged velocities above the trough level have been found to be much higher than those below. After wave breaking, kinetic energy and momentum flux are shown to increase, reaching a peak value and decreasing thereafter. An estimate of the relative density of the fluid in the wave crests was obtained by comparing the forward and reverse velocity fluxes. Other surf zone physical parameters such as the free surface displacements, wave heights across the flume and potential energies have been derived from the resistive wave gauge measurements. A spectral analysis of the water level time series was conducted and the energy growth in each spectral component was examined. It was noticed that there is a transfer of energy from the primary frequency of the wave maker to higher harmonics. Also, wave breaking has little effect on the amplitudes of the harmonic components. The roller and aeration areas were also computed. Two methods of estimating the roller area have been explored and these are the analysis of the wave gauge measurements of the surface elevation and the analysis of the video images. There was agreement between the results obtained and those published in the literature.
An experimental study of diamond and the nitrogen vacancy centre as a source of single photons.
(2009) Semonyo, Malehlohonolo.; Govender, Kessie.
For applications in Quantum Information and Quantum Key Distribution an on-demand source of single photons is desirable because absolute security is of utmost importance. Photons are quantum systems; hence encoding information onto them offers a secure alternative to classical cryptography as a measurement cannot be performed on photons without altering their properties. The Nitrogen Vacancy (NV) centre in diamond is a good source of such photons. It is photo-stable and its location in diamond offers robustness. It has zero phonon line at 637 nm and its relative short luminescence life-time of about 12 ns makes it suitable for generating single photons. This thesis covers two aspects: Firstly the characterization of defects in diamonds and subsequent selection of diamonds suitable for use in the single photon setup and secondly, the development of the experimental setup for single photon generation. This thesis sets out to describe the development of a laboratory based single photon source using the NV centre in diamond. For this purpose a suite of diamond samples were selected and subjected to various spectroscopic tests in order to characterize and classify the samples, especially the presence of the NV centres and their concentrations. The characterization of the defects was done through the use of the following spectroscopic techniques: Ultraviolet-Visible-Near infrared spectroscopy, Infrared spectroscopy, Electron Spin Resonance and Photoluminescence. These techniques enabled us to understand the types and origins of crystal defects that were present in the diamond samples used in this study and to use this to select diamonds that are most suitable for use in generation of single photons. The experimental setup for single photon generation using the NV centre is based on a confocal microscope arrangement. Single NV centres were identified by measuring the second order autocorrelation function of the fluorescence light emitted by the sample when illuminated with a laser. This measurement was done using a Hanbury-Brown Twiss (HBT) interferometer.
Microcontroller based data acquisition and control of a solar thermal energy system.
(2009) Doho, Goncalves Justino.; Govender, Kessie.
A solar thermal energy system is being rebuilt at University of KwaZulu-Natal School of Physics. A similar system is also being built in the University Eduardo Mondlane – Maputo Mozambique, in a team development work. The system is composed mainly of the following subsystems: (i) An Energy capture subsystem: paraboloidal dish concentrator with a heat receiver, mounted on a dual axis polar mount sun tracking assembly; (ii) An Energy storage subsystem: rock-bed thermal energy storage (TES) system; (iii) An Energy utilization subsystem: any user heat utilization (like a cooking or water boiling appliance); and (iv) A monitoring and control subsystem. The subsystem (iv) for performing a controlled charging of the Thermal Energy Storage from a hot plate simulated solar heat, was formerly developed and it was based on 2 conventional data loggers (HP/Agilent) and programs running on 2 PCs. The present work is aimed at performing the same plus additional monitoring and control tasks, based on a low cost microcontroller design. The monitoring and control subsystem based on the Atmel ATmega 32 MCU has been designed and built, capable of performing data acquisition, data logging and control of relevant system variables such as, hour and declination angles of the tracking concentrator; to cite some of the main variables. Besides a huge work of designing, building, programming and testing the microcontroller system itself, a special focus was given to the monitoring and control of the solar heat concentrator, to perform a dual axis sun tracking, so as to get as much as possible of the available solar radiation. Measurements of various system parameters such as, the sun tracking actual hour and declination angles, the inlet and outlet temperatures of both the heat receiver and the rock bed heat storage, etc., for the system under consideration have been carried out.
Velocity, vorticity and turbulence measurements in the surf zone.
(1999) Govender, Kessie.
The coastal regions of the world play a significant role with regards to social and economic development, and with the ever increasing human activities along the coast, it has become necessary to understand and protect this vast resource. This requires a thorough understanding of the hydrodynamic processes that occur in the surf zone as a result of wave breaking. Laboratory investigations fonn an essential part of unravelling any physical process. It is the aim of this thesis, therefore, to investigate via laboratory measurements, the hydrodynamics of plunging and spilling waves in the surf-zone with the view to obtaining greater physical insight and also to obtain real data for model validation purposes. To achieve the above goal it was also necessary to develop new measuring techniques that were suitable for surf zone application. since conventional water level and fluid velocity measuring devices perfonn poorly in the presence of wave breaker related aeration, a common feature in the surf zone. The experimental investigation of processes occurring in the surf zone was carried out using a plane beach in a 20 m long wave flume. Both spilling and plunging waves are considered. The measurement of the spatial and temporal fluid velocities, water level and aeration area was accomplished using digital video imaging and analysis techniques in conjunction with particle image velocimetry. The diagnostic equipment. employed for the measurement of water levels, consists of a monochrome video camera connected to a video cassette recorder (VCR) and to a video frame grabber residing in a Pc. The video machine is used for mass storage of video data and easy replay of the experiment. The frame grabber is used to digitise and grab a sequence of video frames. The frame grabber can accept a video signal directly from the camera or from the VCR during playback. Fluid velocities were measured using a non-interlaced digital ceo camera connected direct to a frame grabber residing in a pentium Pc. Thus, images were stored direct to the hard disk. Water level measurements were performed with the aim of measuring wave heights as well as the mean water level set-down prior to wave breaking and the subsequent set-up in the surf zone. A time series of wave evolution was obtained by tracking the air-water interface along a vertical line of pixels at a fixed horizontal position in the video image. This method has the advantage of being non intrusive, and is considered more reliable than some of the more traditional techniques employed in the surf zone. Mean water levels were measured every 0.1 m along the measurement section of the flume, which is approximately 8 m in length. The spatial and temporal particle velocities were measured initially by tracking individual neutrally buoyant particles. This technique is referred to as particle image velocimetry (PIV) . More extensive measurements were later accomplished using digital cross correlation techniques, referred to as digital correlation image velocimetry (DCIV). A longitudinal section of the flume was illuminated with a sheet of light and the flume was seeded with neutrally buoyant particles. Velocities are then estimated by tracking individual seeds or a group of seeds in consecutive video frames. The velocity flow field was measured at the break point and in the surf zone. The average and turbulent velocities are estimated through phase ensemble-averaging. The undertow current and shoreward mass flux are derived from the flow field through averaging over the wave phase. A sensitivity analysis on this data is done to ensure statistical convergence. The measured velocity flow fields were analyzed and compared with linear wave theory and measurements made using Laser Doppler anemometry (LDA). The turbulent structures generated in the surf zone are investigated through the computation of the phase ensembleaveraged turbulence intensities and vorticity. The vorticity of the averaged flow field is computed and analyzed. The time averaged Reynolds stress and the turbulent kinetic energy is also computed. The Reynolds stress near the bed is shown to increase almost linearly with distance above the bed. Spectral analysis of the spatial velocities was used to obtain estimates of the scale lengths, and a comparison with the Kolmogorov frequency scaling law in the equilibrium region of the spectrum was also performed. The measurement of the aeration/roller geometry for spilling and plunging waves was accomplished using the video techniques used for water level measurements. The normalised wave roller area, in spilling waves, is shown to be nearly constant through the inner surfzone, increasing slightly in the middle of the surf-zone. The roller slope tends to show a decreasing trend in the surf zone.