Euchaeta antarctica, a prominent calanoid copepod species inhabiting the Antarctic region, plays a pivotal role in the Southern Ocean’s ecosystem. When engaged in cruise swimming, E. antarctica propels itself through metachronal stroking of its swimming legs, which raises many questions regarding its fundamental locomotion mechanics. Further, this species generates noteworthy flow disturbances that mediate ecological interactions by transmitting hydrodynamic signals to both predators and prey. In the present investigation, we employ a high-speed tomographic particle image velocimetry (tomo-PIV) system to visualize and quantify the dynamic 3D velocity field surrounding a freely swimming adult E. antarctica. A comparative analysis of cruising speeds among the adult E. antarctica and smaller congeners, E. rimana and E. elongata, as well as the juvenile stage of E. antarctica (CV), reveals a linear relationship between organism length and cruising speed. Furthermore, we meticulously quantified the fluid velocity, vorticity, dissipation rate, and shear strain rate fields generated by adult E. antarctica during cruising behavior, with a particular focus on its turning maneuvers. The results unveil the presence of peak vorticity and shear strain rate values within close proximity of 1 to 2 mm from the copepod’s body during straight-line motion. During turning motions, we observe significant increases in these parameters along the animal’s tail. In addition, the volumes surrounded by the 0.5 (1/s) iso-surface of shear strain rate, which is a likely threshold to induce an escape response in prey, are around 12 times the volume of the exoskeletal form for the straight cruise motion and 20 to 24 times during the turning maneuver.

The flow field around bioinspired magnetic-responsive soft materials that mimic the symmetry-breaking mechanisms in swimming animals, such as pteropods and manta rays, is studied using the tomographic particle image velocimetry (tomo-PIV) technique. Magnetic-responsive material appendages are actuated by an oscillating external magnetic field. The fluid flow induced by two types of actuation is quantified. First, a single actuation mode involves alternating upward and downward bending motions. Second, an asymmetric multimodal actuation encompasses upward folding and downward bending motions by locating an asymmetric joint at the midpoint of the appendage. The formed vorticity field, vortex structure, and viscous energy dissipation rate in the surrounding fluid are observed to be weaker for the multimodal actuation case. The multimodal appendage moves with reduced flow resistance, leading to faster appendage velocity during the downward power stroke. Furthermore, the study examines the effect of an asymmetric magnetic field cycle on the flow field by extending the time interval of the applied positive voltage (upstroke motion) compared to the duration of the negative applied voltage (downstroke motion). The asymmetric cycle and extended stopping period provide time for greater dissipation of the formed vorticity field. Thus, the peak values of vorticity and viscous dissipation rate decrease to smaller magnitudes compared to the symmetric cycle case. These findings demonstrate that the utilization of symmetry-breaking morphology and an asymmetric cycle enhances stroke performance, offering promising avenues for achieving greater effectiveness in underwater propulsion.

Euchaeta antarctica is a key calanoid copepod species inhabiting the Southern Ocean surrounding Antarctica. In addition to basic propulsion considerations, the flow fields generated by Euchaeta antarctica are significant due to their ecological interactions with other organisms via hydrodynamic signals to predators and prey. In cruise swimming mode, Euchaeta antarctica generates thrust via metachronal stroking of swimming legs located on the dorsal side of their prosome. In the current study, a high-speed tomographic particle image velocimetry (tomo-PIV) system was employed to visualize and quantify the time-resolved 3D velocity field surrounding a free-swimming adult E. antarctica. Comparison of cruising swimming speed among adult E. antarctica and smaller species E. rimana and E. elongata, as well as smaller stage E. antarctica CV, demonstrate a linear dependence on organism length. The fluid velocity, vorticity, dissipation rate, and shear strain rate fields generated by adult E. antarctica during cruise behavior are quantified, with particular attention to turning. The flow fields reveal peak values of vorticity and shear strain rate within a proximity of 1 to 2 mm from the copepod body during straight motion, whereas significant increases in these quantities were observed along the animal tail during turning motion.

The hydrodynamics induced by deformation of magnetic-responsive soft materials are investigated and the resultant flow field is analyzed using tomographic particle image velocimetry (tomo-PIV). Magnetic-responsive composite arms incorporated with a multifunctional joint design are actuated by an external magnetic oscillating field with a period of roughly 1 sec. The tomo-PIV system quantifies the surrounding aqueous flow at 250 frames per second. The effects of asymmetric multimodal actuation (i.e., folding versus bending) on the flow field are compared with the single actuation mode, by creating an asymmetric joint at the mid-point of the magnetic material. In addition, the effect of non-symmetric time interval of positive/negative magnetic field generation is investigated on the flow field around the magnetic material. Analysis of the kinematics of the soft material, as well as the velocity, vorticity and pressure of the flow field surrounding the magnetic material indicate that the kinematics of the non-symmetric time interval of magnetic field provides sufficient time for the flow to become steady between each upward and downward stroke motions. Thus, the peak values of vorticity and pressure decrease to smaller magnitudes compared to the symmetric case. In addition, the asymmetric joint at the mid-point of the magnetic material arm affects the flow in a way that the peak values of both vorticity and pressure are larger in the downward motion (rigid body motion, i.e. folding) compared to the upward motion (elastic deformation, i.e. bending).

To provide insight to the dynamics of weakly non-linear standing internal waves, the density and velocity fields are measured using combined planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) techniques. A laboratory scale apparatus was created to generate standing internal waves in a two-layer stratified system. Experimental results are presented for two configurations with a density jump of 1.1 kg/m3 and 1.5 kg/m3 (separately). The interface location, wave amplitude and period, interface thickness, convection transport terms, fluid velocity, shear strain rate, and vorticity are quantified and analyzed at fixed phases in the wave cycle. The comparison between the internal wave frequency computed from the experimental results and the dispersion relationship resulting from the theoretical third-order Stokes internal-wave solution confirms that the laboratory-generated waves demonstrate non-linear behavior. The interface detected from experimental PLIF images indicated that due to the non-linear effects, a steeper wave with a sharper-looking interface at anti-node locations was formed in comparison with the theoretical linear sinusoidal shape. Further, the magnitude of shear strain rate and vorticity computed from experimental PIV measurements had a sharp, non-linear increase along the interface compared to the one computed from the linear theory. This non-linear trend in shear strain rate and vorticity can lead to the generation of sharper interface and short-period (i.e., higher frequency) non-linear internal waves.

The effect of initial perturbations on the evolution of the inclined Richtmyer-Meshkov turbulent mixing layer before and after reshock initiated by a shock wave with Mach number 1.55 is investigated through three-dimensional (3D) simulations using the flash code. The 3D simulations aim to reproduce both predominantly single-mode and multimode interfaces between light and heavy gases ($N_2\text{−}{\mathrm{CO}}_2$, Atwood number, $A\approx 0.22$; amplitude to wavelength ratio of 0.088) which were created in an inclined shock tube facility to analyze the effects of initial conditions on mixing development in the entire flow field. The two-dimensional center slices of 3D simulations are compared with the experimental results to validate the computational code. Mixing width, mixed mass, mixed-mass thickness, and circulation in addition to concentration fields are shown to be in good agreement with the experimental data. The three-dimensional density and vorticity fields are first presented to qualitatively describe the flow behavior before and after reshock. Several measured density/velocity-related quantities indicate that the growth of the mixing material is strongly dependent on initial conditions. Before reshock and at early times after reshock, flow is clearly maintaining the memory of initial perturbations. However, at late time after reshock, although the large wavelength feature still dominates the flow motion, and the morphology of the two different interfaces indicates several differences, by breakdown of large-scale coherent structures to much finer scales, the memory of small scales of the multimode initial perturbation is not as clear as pre-reshock. Regarding three-dimensionality of the flow, before reshock in the multimode case, the baroclinic vorticity production, circulation, turbulent kinetic energy, and turbulent mass flux suggest that the small-scale roll-up features along the large inclined wavelength quickly evolves in all three dimensions. The coherent vortex tubes break down to smaller wormlike vortex structures, and turbulent fluctuations in the out-of-plane dimension are comparable to the spanwise direction. After reshock, this three-dimensionality of mixing growth was observed in the flow for both initial conditions. The results of this work represent a significant extension of previous computational studies performed on this specific topic. A different code with a different numerical method is validated through comparison with the experimental data. The initial perturbations are directly measured from the experimental results. Moreover, the entire three-dimensional experimental shock tube domain is simulated, and more quantities are investigated to understand the mixing mechanism and instability evolution in all three dimensions.

Internal waves are a fascinating physical phenomenon that play an important role in the mixing and dynamics of both atmospheric and oceanographic flows. This experimental study addresses non-linear internal waves due to their importance in shaping the circulation and distributions of heat and carbon within density stratified systems. We aim to fully understand the dynamics of internal waves by measuring the density and velocity fields using combined PLIF/PIV measurements and comparing the experimental results with the theoretical non-linear wave solution. Non-linear theory is required due to the non-negligible amplitude of the wave compared to the wavelength. A laboratory-scale apparatus was created to replicate the flow characteristics of standing internal waves in a two-layer stratified system. Experimental results are presented for configurations with a density jump of 1.1 and 1.5 σ_t (separately). The interface location, density gradient, wave amplitude and period, velocity and vorticity fields, kinetic energy, and shear strain rate are quantified at several phases in one wave cycle. The experimental results are compared with the corresponding predictions based on third-order Stokes internal-wave theory. The results showed that the 3^rd-order non-linear theory does an outstanding job of describing the internal wave flow.

The objective of this study is to provide insight to the bio-physical interaction and the role of biological versus physical forcing in mediating organism distributions in and near internal waves. A laboratory-scale configuration is presented with a density jump of 1 σ_t. Theoretical analysis of the two-layer system provided guidance to the target forcing frequency needed to generate a standing internal wave with a single dominant frequency of oscillation.  The results show a close match to the target wave parameters.  Marine copepod (mixed population of Acartia tonsa, Temora longicornis, and Eurytemora affinis) behavior assays were conducted for three different physical arrangements: (1) no density stratification (i.e. control), (2) stagnant two-layer density stratification, and (3) two-layer density stratification with internal wave motion.  Digitized trajectories of copepod swimming behavior indicate that in the control (case 1) the animals showed no preferential aggregation.  In the stagnant density jump treatment (case 2) copepods preferentially moved horizontally, parallel to the density interface.  In the internal wave treatment (case 3) copepods demonstrated loopy, orbital trajectories near the density interface.  Noted differences with simulated trajectories and a consideration of the potential hydrodynamic cues indicate that copepod behavior response has a substantial influence on the swimming trajectories in the internal wave region.

This experimental study investigates the effects of Reynolds number ($5000\le \text{Re}\le 20000$, where $\text{Re}=UH/\nu$) and initial release diameter (2.2 mm $\le D\le$ 9.4 mm) on the scalar power spectra, fractal geometry, and turbulent length scales of high-Schmidt-number passive scalar fields resulting from an isokinetic release in a turbulent boundary layer. The turbulence analysis is based on 12 000 scalar fields collected using the planar laser-induced fluorescence technique for each case at six locations downstream. The scalar integral length scale and scalar Taylor microscale are calculated directly from the fields using the autocorrelation function and variance/gradient of the concentration fluctuation fields. With increasing downstream distance, the Taylor microscale decreases and the integral length scale increases, each to an asymptotic value. This indicates a larger range of scales exists as the scalar becomes more mixed, as one would expect. For locations beyond $x/H\ge 10$ (where $H$ is the flow depth), the self-similarity condition is observed by considering the ratio between the scalar integral length scale and scalar Taylor microscale. Local isotropy is approached as measured by computing the ratio of longitudinal to transverse scalar Taylor microscales, and a change in the growth rate is observed for the fractal dimension computed from a planar section of the interfaces in the concentration fluctuation fields. The spectral slope magnitude in the inertial-convective regime decreases near the source ($x/H<10$) due to the large-scale anisotropy. In the self-similar regime ($x/H\ge 10$), the scaling-exponent is found to be dependent on the initial release diameter. The lower wave-number portion of the inertial-convective regime, where the scales are larger than or closer to the scale of the nozzle diameter, scales close to ${}^{}$ scaling in agreement with the cascade-bypass situation, and the spectral slope in the upper wave-number portion of the inertial-convective regime is found to be closer to $-5/3$. The viscous-convective scaling behavior deviated significantly from Batchelor’s -1 scaling law, clearly disputing the generality of Batchelor’s arguments. Intermittency analysis, using computation of the intermittency factor as well as probability density functions of the fluctuating scalar gradient, suggests that the discrepancy between theory and observations for the scaling of the viscous-convective regime can be explained by the high intermittency in the small scales of the scalar fluctuations.

Internal waves are ubiquitous features in coastal marine environments and have been observed to mediate vertical distributions of zooplankton. Internal waves possess fine-scale hydrodynamic cues that copepods and other zooplankton are known to sense, such as fluid density gradients and velocity gradients (quantified as shear strain rate). The role of copepod behavior in response to cues associated with internal waves is largely unknown. The objective is to provide insight to the bio-physical interaction and the role of biological vs. physical forcing in mediating organism distributions. A laboratory-scale internal wave apparatus is designed to facilitate fine-scale observations of copepod behavior in flows that replicate in situ conditions of internal waves in two-layer stratification. An experimental configuration is presented with a density jump of 1 σ_t. Theoretical analysis of the two-layer system provided guidance to the target forcing frequency needed to generate a standing internal wave with a single dominate frequency of oscillation. Flow visualization and signal processing of the interface location were used to quantify the wave characteristics. The results show a close match to the target wave parameters. Marine copepod (mixed population of Acartia tonsa, Temora longicornis, and Eurytemora affinis) behavior assays were conducted for three different physical arrangements: (1) no density stratification (i.e., control), (2) stagnant two-layer density stratification, and (3) two-layer density stratification with internal wave motion. Digitized trajectories of copepod swimming behavior indicate that in the control (case 1) the animals showed no preferential aggregation. In the stagnant density jump treatment (case 2) copepods preferentially moved horizontally, parallel to the density interface. In the internal wave treatment (case 3) copepods demonstrated loopy, orbital trajectories near the density interface. Analysis of advected trajectories in the internal wave, with and without superimposed copepod swimming, reveal distinct differences with the observed copepod trajectories in the internal wave treatment. These differences and a consideration of the potential hydrodynamic cues indicate that copepod behavior response has a substantial influence on the swimming trajectories in the internal wave region.

The purpose of this study is to further our understanding of the role of internal waves as a potential to influence the behavior and patchiness of copepods. A standing internal wave with a single dominate frequency of oscillation is generated using a laboratory-scale internal wave apparatus. One experimental configuration is presented with a density jump of 1 σ_t. The wave characteristics are quantified through flow visualization and signal processing. The analytical analysis of two layer stratification system based on the wave characteristics, that includes nonlinear effects, provided the target forcing frequency needed to generate a standing internal wave with a single dominate frequency of oscillation. Three physical arrangements are investigated to study the behavioral assays of marine copepods: (1) no density stratification, (2) stagnant two-layer density stratification, and (3) two-layer density stratification with internal wave motion. Digitized trajectories of copepod swimming behavior indicate that in the control (case 1) the animals showed no preferential motion in terms of direction. In the stagnant density jump treatment (case 2), copepods preferentially moved horizontally, parallel to the density interface. In the internal wave treatment (case 3), copepods demonstrated orbital trajectories near the density interface. Comparing the analytical back-and-forth oscillation trajectories of passive, neutrally-buoyant particles with the experimental orbital trajectories of copepods near the internal wave flow indicates a strong effect of animal swimming behavior on its trajectory in this region.

In Richtmyer-Meshkov instability (RMI) the initial shape of the perturbed interface, the incident shock strength (Mach number) and the gas combination forming the density interface (which is defined by the Atwood number), can each affect evolution of instability. Although effects of the interface initial conditions and Mach number have been investigated in previous experimental works, there is still little data on Atwood number effects on RMI. We present a systematic study of the Atwood number-dependence of shock-driven variable density flows under incident shock and reshock. The density gradient is provided by the interface of two gases namely Nitrogen (seeded with Acetone) and either Carbon Dioxide or Sulfur Hexafluoride (0.22<A<0.67). The Mach number is maintained at 1.55.

The impact of Mach number on Richtmyer-Meshkov instability evolution is studied experimentally at the Georgia Tech Shock Tube and Advanced Mixing Laboratory by varying Mach numbers in the range 1.2<M<2. A density gradient is formed by the interface of two gases namely Nitrogen (seeded with Acetone) and Carbon Dioxide which results in Atwood number 0.22. Two interface perturbations are studied. The first is a predominantly single-mode long-wavelength interface which is formed by inclining the entire tube to 80° relative to the horizontal yielding an amplitude-to-wavelength ratio, η/λ=0.088. The second is a multi-mode initial condition which contains additional shorter wavelength perturbations due to the imposition of shear and buoyancy on the inclined perturbation. Simultaneous planar diagnostics (PLIF and PIV) are used to capture density and velocity statistics and enable calculation of joint statistics. Several quantities, including mixing width, mixedness, vorticity, circulation, the density self-correlation parameter, turbulent mass flux, and other measurements of turbulence are examined at same non-dimensionalized times before and after reshock. Finally, turbulent mixing transition for different Mach numbers and different initial conditions is investigated by analyzing turbulent length scales and energy spectra.

The inclined shock tube facility at Georgia Tech is used to study the evolution of turbulent quantities for a Richtmyer-Meshkov instability initiated from an inclined interface and a complex interface. The complex interface is formed by perturbing the inclined interface with counter flowing jets, which create shear and buoyancy effects. Experiments are performed using an incident shock wave with strength of Mach 1.55 to impulsively accelerate an interface with 80 degree angle of inclination between N₂-Acetone mixture and CO₂ gas resulting in an Atwood number of 0.23, after gas compression. The evolution of quantities such as turbulent stresses and the cross correlation across the mixing width along with density field are obtained by implementing simultaneous high resolution PLIF and PIV measurement techniques. In the current investigation, the given data are compared between experiments initiated with complex initial interface and experiments initiated with flat inclined initial condition, thus outlining the effect of the initial perturbations on mixing evolution at intermediate and late-time as well as the transition to turbulent regime before and after re-shock.

In the presented work, Delft3D, a coastal hydrodynamic modeling system, capable of simulating hydrodynamic processes due to waves, tides, rivers, winds and coastal currents, is used to predict the nearshore wave energy in Matosinhos, Portugal. DelftDashboard is then used in co-operation to make a precise grid and bathymetry for our wave model. The wind parameters used to stimulate the waves are obtained from the magicseaweed online database. The results obtained from simulating the modeled waves are validated by comparing them with observation data from the aforementioned online database. Finally, wave energy density of three different locations are analyzed and compared.

The inclined shock tube facility in the Georgia Tech Shock Tube and Advanced Mixing Laboratory was used to study a complex inclined interface initial condition for the Richtmyer-Meshkov instability. The inclined interface is essentially a long wavelength, extremely large amplitude perturbation between two gases. In this case, the light gas was chosen to be nitrogen and the heavy gas carbon dioxide, giving an Atwood number of 0.23. The complex interface is formed by perturbing the inclined interface with counter-flowing jets, which create shear and buoyancy effects. The modal content of the initial conditions was determined by taking the Proper Orthogonal Decomposition of a large set of realizations. PLIF images of the shocked flow-field (M∼1.5) were captured with the angle of the shock tube with respect to the horizontal at 80∘. Enhanced mixing in the complex interface was quantified through p.d.f.s, mixed mass, and the density self-correlation. Work that is currently underway will investigate the effect of these initial conditions on intermediate and late-time mixing as well as the transition to turbulence before reshock by implementing simultaneous PLIF and PIV measurements.

This experimental study has investigated effects of Reynolds number (5000≤Re≤20,000) and initial release diameter (2.2mm≤D≤9.4mm) on scalar power spectra and turbulent length scales of high-Schmidt-number passive scalar fields resulting from an iso-kinetic release in a turbulent boundary layer. The turbulence analysis is based on 12,000 scalar fields collected using the PLIF technique for each case at 6 locations downstream. Although the spectral slope at intermediate scales is found to increase to an asymptotic value higher than -5/3 farther downstream, there is an increase in spectral slope from approximately -1.5 for Re=5000 to roughly -1.2 for Re=20,000 while fixing the release diameter at 4.7 mm. A similar trend is observed for the effect of nozzle diameter on spectral slope, as it increases from almost -1.5 to -1.2 when the nozzle diameter changes from 9.4 mm to 2.2 mm while fixing Re=10,000. The scalar integral scale and scalar Taylor microscales are calculated directly from the scalar fields using the correlation function. It was found that the Taylor microscale decreases and the integral scale increases to an asymptotic value respectively, farther downstream. This indicates a larger range of scales exists as flow becomes more turbulent.

This thesis presents results on the effects of initial conditions (single- and multi-mode) and incident shock wave Mach numbers (M) on several mixing characteristics in Richtmyer-Meshkov instability (RMI) evolution. These goals are achieved by performing two different experimental campaigns using a shock strength with an incident Mach number of 1.9 and 1.55. Each campaign follows the interface evolution after interaction with incident shock and reflected shock from the wall (reshock). In addition, two different initial perturbations are imposed to study RMI evolution at each Mach number. The first perturbation is a predominantly single-mode long-wavelength interface which is formed by inclining the entire tube to 80° relative to the horizontal, and thus can be considered as half the wavelength of a triangular wave. The second initial condition is a multi-mode interface, containing additional shorter wavelength perturbations due to the imposition of shear and buoyancy on the inclined perturbation of the first case. In both single- and multi-mode cases at each Mach number, the interface consists of a nitrogen-acetone mixture as the light gas over carbon dioxide as the heavy gas (Atwood number, A~0.22). The evolving density and velocity fields are measured simultaneously using planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) techniques to provide the first detailed turbulence statistics measurements (i.e., Density, velocity, and density-velocity cross-statistics) using ensemble averaging for shock-accelerated variable density flows at M > 1.5 before and after reshock. The evolution of mixing is investigated via the density fields by computing mixed-mass and mixing layer thickness, along with mixing width, mixedness, and the density self-correlation (DSC). It is shown that the amount of mixing is dependent on both the initial conditions and the incident shock Mach number before reshock. Evolution of the density self-correlation is discussed and the relative importance of different DSC terms is shown through fields and spanwise-averaged profiles. The localized distribution of vorticity and the development of roll-up features in the flow is studied through the evolution of interface wrinkling and length of the interface edge, and indicates that the vorticity concentration shows a strong dependence on the Mach number. The contribution of different terms in the Favre-averaged Reynolds stress is shown, and while the mean density-velocity fluctuation correlation term is dominant, a high dependency on the initial condition and reshock is observed for the turbulent mass-flux term. Regarding the effects of initial conditions, density and velocity data show that a distinct memory of the initial conditions is maintained in the flow before interaction with reshock. After reshock, the influence of the long-wavelength inclined perturbation present in both initial conditions is still apparent, but the distinction between the two cases becomes less evident as smaller scales are present even in the single-mode case. Mixing transition is analyzed through two criteria: Reynolds number (Dimotakis, 2000) and time-dependent length scales (Robey et al., 2003). The Reynolds number threshold is surpassed in all cases after reshock. In addition, the Reynolds number is around the threshold range for the multi-mode, high Mach number case (M~1.9) before reshock. However, the time-dependent length-scale threshold is surpassed by all cases only at the latest time after reshock, while all cases at early times after reshock and the high Mach number case at the latest time before reshock fall around the threshold. The scaling analysis of turbulent kinetic energy spectra after reshock at the latest time, at which mixing transition analysis suggests that an inertial range has formed, indicates power scaling of -1.8±0.05 for the low Mach number case and -2.1±0.1 for the higher Mach number case. This is related to the high anisotropy observed in this flow resulting from strong, large-scale, streamwise fluctuations produced by large-scale shear. This work will help develop the capability to accurately predict and model extreme mixing, potentially leading to advances in a number of fields: energy, environment (atmospheric and oceanographic), aerospace engineering, and most pertinently, inertial confinement fusion (ICF).

This thesis is an investigation of wave power resources in the Portuguese western coast, focusing on the spatial distribution of wave power of coastal region exposed to the highest wave power. The main objective of the study is to provide a detailed description of the spatial distribution of wave power to assist the selection of locations for deployment of Wave Energy Converter (WEC) units in these zones. The study methodology employed to achieve this main objective entails an analysis of modeled wave data at nine wave stations distributed along the Portuguese western coast. The analysis provided a general description of wave power at locations for which wave data is calculated. From this analysis the location exposed to the highest wave power and the one with the most frequent waves were discovered. The study objective was achieved by the Simulating Waves Nearshore (SWAN) model embedded in Delft3D software package with the help of Delft-Dashboard software. The ocean wave data were obtained for a 25-day computational period. A simplified simulation procedure was required in order to make the study practically feasible. The accuracy of the modeled output was investigated by directly comparing it to wave data recorded during the overlapping recording period. It was found that the model slightly underestimated the wave power compared to the measured data with a maximum underestimation of 11%; which is sufficiently accurate for the purpose of this study. The results of this investigation alongside the new method discussed in this thesis can be used for the identification of areas with high wave power concentration in order to find the most suitable location to install the WEC units. Further numerical modeling is required for the detailed design of wave farms, especially if potential sites are located in shallow water.

The swimming characteristics achieved by flapping wings, translating motion, and shell pitching are studied from observations of shelled Antarctic pteropods (aquatic snails nicknamed ‘sea butterflies’). These pteropods (Limacina helicina antarctica) swim with a pair of parapodia (or “wings”) via a unique flapping propulsion mechanism that incorporates similar techniques as observed in small flying insects. The geometric scaling of wing span (L), wing chord (c), and minor shell diameter (d) with respect to major shell diameter (D) reveal geometric similitude. Thus, major shell diameter (D) is the only length scale required to describe the size of the pteropods. The motion of swimming pteropods is characterized using flapping, translational, and rotational Reynolds numbers (i.e. Re_f, Re_U, and Re_Ω). A critical value of flapping Reynolds number, Re_f=35, is found for the onset of translating and pitching locomotion. Finally, the relationship is obtained for the Strouhal number (St_A=fA/U) for the pteropods using the geometric scalings and the translational and flapping Reynolds numbers. The Strouhal number is found to be between 0.2 and 0.4, which indicates general agreement with other oscillating organisms moving with high propulsion efficiency.

The effects of incident shock strength on the mixing transition in the Richtmyer–Meshkov instability (RMI) are experimentally investigated using simultaneous density–velocity measurements. This effort uses a shock with an incident Mach number of 1.9, in concert with previous work at Mach 1.55 (Mohaghar et al., J. Fluid Mech., vol. 831, 2017 pp. 779–825) where each case is followed by a reshock wave. Single- and multi-mode interfaces are used to quantify the effect of initial conditions on the evolution of the RMI. The interface between light and heavy gases ( N2/CO2 , Atwood number, A≈0.22 ; amplitude to wavelength ratio of 0.088) is created in an inclined shock tube at 80∘ relative to the horizontal, resulting in a predominantly single-mode perturbation. To investigate the effects of initial perturbations on the mixing transition, a multi-mode inclined interface is also created via shear and buoyancy superposed on the dominant inclined perturbation. The evolution of mixing is investigated via the density fields by computing mixed mass and mixed-mass thickness, along with mixing width, mixedness and the density self-correlation (DSC). It is shown that the amount of mixing is dependent on both initial conditions and incident shock Mach number. Evolution of the density self-correlation is discussed and the relative importance of different DSC terms is shown through fields and spanwise-averaged profiles. The localized distribution of vorticity and the development of roll-up features in the flow are studied through the evolution of interface wrinkling and length of the interface edge, which indicate that the vorticity concentration shows a strong dependence on the Mach number. The contribution of different terms in the Favre-averaged Reynolds stress is shown, and while the mean density-velocity fluctuation correlation term, ?⟨?⟩⟨ui′uj′⟩ , is dominant, a high dependency on the initial condition and reshock is observed for the turbulent mass-flux term. Mixing transition is analysed through two criteria: the Reynolds number (Dimotakis, J. Fluid Mech., vol. 409, 2000, pp. 69–98) for mixing transition and Zhou (Phys. Plasmas, vol. 14 (8), 2007, 082701 for minimum state) and the time-dependent length scales (Robey et al., Phys. Plasmas, vol. 10 (3), 2003, 614622; Zhou et al., Phys. Rev. E, vol. 67 (5), 2003, 056305). The Reynolds number threshold is surpassed in all cases after reshock. In addition, the Reynolds number is around the threshold range for the multi-mode, high Mach number case ( M∼1.9 ) before reshock. However, the time-dependent length-scale threshold is surpassed by all cases only at the latest time after reshock, while all cases at early times after reshock and the high Mach number case at the latest time before reshock fall around the threshold. The scaling analysis of the turbulent kinetic energy spectra after reshock at the latest time, at which mixing transition analysis suggests that an inertial range has formed, indicates power scaling of −1.8±0.05 for the low Mach number case and −2.1±0.1 for the higher Mach number case. This could possibly be related to the high anisotropy observed in this flow resulting from strong, large-scale streamwise fluctuations produced by large-scale shear.

The effect of initial conditions on transition to turbulence is studied in a variable-density shock-driven flow. Richtmyer–Meshkov instability (RMI) evolution of fluid interfaces with two different imposed initial perturbations is observed before and after interaction with a second shock reflected from the end wall of a shock tube (reshock). The first perturbation is a predominantly single-mode long-wavelength interface which is formed by inclining the entire tube to 80 ∘ relative to the horizontal, yielding an amplitude-to-wavelength ratio, ?/?=0.088 , and thus can be considered as half the wavelength of a triangular wave. The second interface is multi-mode, and contains additional shorter-wavelength perturbations due to the imposition of shear and buoyancy on the inclined perturbation of the first case. In both cases, the interface consists of a nitrogen-acetone mixture as the light gas over carbon dioxide as the heavy gas (Atwood number, A∼0.22 ) and the shock Mach number is M≈1.55 . The initial condition was characterized through Proper Orthogonal Decomposition and density energy spectra from a large set of initial condition images. The evolving density and velocity fields are measured simultaneously using planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) techniques. Density, velocity, and density–velocity cross-statistics are calculated using ensemble averaging to investigate the effects of additional modes on the mixing and turbulence quantities. The density and velocity data show that a distinct memory of the initial conditions is maintained in the flow before interaction with reshock. After reshock, the influence of the long-wavelength inclined perturbation present in both initial conditions is still apparent, but the distinction between the two cases becomes less evident as smaller scales are present even in the single-mode case. Several methods are used to calculate the Reynolds number and turbulence length scales, which indicate a transition to a more turbulent state after reshock. Further evidence of transition to turbulence after reshock is observed in the velocity and density fluctuation spectra, where a scaling close to −5/3 is observed for almost one decade, and in the enstrophy fluctuation spectra, where a scaling close to 1/3 is observed for a similar range. Also, based on normalized cross correlation spectra, local isotropy is reached at lower wave numbers in the multi-mode case compared with the single-mode case before reshock. By breakdown of large scales to small scales after reshock, rapid decay can be observed in cross-correlation spectra in both cases.

An experimental study of a twice-accelerated Richtmyer–Meshkov instability, where reshock provides the second acceleration, focusing on the effects of initial conditions and circulation deposition is presented. Experiments were performed using the inclined shock tube facility at the Shock Tube and Advanced Mixing Laboratory. Three experimental cases are presented that have the same Atwood number, inclination angle, and Mach number, but are differentiated by their pre-reshock development time. Both Mie scattering and particle image velocimetry diagnostics were implemented. Velocity statistics were ensemble-averaged over instantaneous realizations for each case before and after reshock. Results show that while the mix width decreases after reshock, the interface length continues to increase because the reshock wave amplifies small-scale perturbations on the pre-reshock interface, resulting in greater mixing. A more developed interface also experiences greater circulation deposition after reshock. After reshock, the sign of the vorticity near the interface reverses due to a second application of baroclinic torque by the reshock wave. Velocity statistics showed that the cross-correlation u’v’ is nonzero over much of the mixing layer, which indicates that shear and anisotropy are present. Turbulent kinetic energy spectra for the most developed case after reshock exhibited a k^-5/3 inertial range.

Using recent advancements in optical diagnostic technologies (both in imaging and illumination), field measurements using simultaneous PIV and PLIF are presented to investigate the instability, evolution and mixing behavior of Richtmyer-Meshkov Instability (RMI). These measurements enable capture of the temporal evolution of the small scale mixing – a detail not possible to study from single shot measurements owing to the stochastic nature of the same. Preliminary data showing excellent agreement with previous high-spatial resolution studies is shown in the current work. The temporal evolution of vorticity, vortex breakdown, associated interface mixing and the preferential presence of mixing activity around the swirling motions is clearly demonstrated. Finally, the ongoing efforts and potential future uses of such a temporally resolved measurement is briefly discussed.

Effects of initial condition and incident shock strength (Mach number) on the Richtmyer-Meshkov instability (RMI) evolution are presented here. The interface between light (N₂) and heavy (CO₂) gases is inclined with respect to shock propagation by 10 degrees (amplitude to wavelength ratio of 0.088), which forms the predominantly single-mode perturbation. A perturbed, multi-mode inclined interface is also created via shear-buoyancy induced roll-ups between the two gases superposed on the dominant inclined mode. These two interface conditions are accelerated by planar shock waves of M≈1.55 and M≈1.9 to investigate the RMI development and mixing transition in this flow. Ensemble-averaged turbulence statistics are computed using simultaneous planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) measurements at two times after incident shock and two times after reshock. The observations from turbulent kinetic energy spectra and mixing Reynolds number indicate that the criteria for mixing transition can be satisfied at late time after the incident shock for the multi-mode case at higher Mach number, and after reshock for both Mach numbers and both initial conditions.

The current study experimentally investigates the influence of the initial Atwood ratio (At) on the evolution of Richtmyer-Meshkov instability at the Georgia Tech Shock Tube and Advanced Mixing Laboratory. Two Atwood numbers (At=0.22 and 0.67) are studied, which correspond to the gas combinations of nitrogen seeded with acetone vapor (light) over carbon dioxide (heavy) and same light gas over sulfur hexafluoride (heavy) respectively. A perturbed, multi-mode, inclined interface (with an amplitude to wavelength ratio of 0.088) is impulsively accelerated by the incident shock traveling vertically from light to heavy gas with a Mach number ≈1.55. The effect of Atwood ratio on turbulent mixing transition after reshock at the same non-dimensional times between the two cases is examined through ensemble-averaged turbulence statistics from simultaneous planar laser induced ?uorescence (PLIF) and particle image velocimetry (PIV) measurements. Preliminary studies over the smaller Atwood number indicates that turbulent mixing transition criteria can be satisfied after reshock.

In the Georgia Tech Shock Tube and Advanced Mixing Laboratory, the evolution of Richtmyer-Meshkov instability (RMI) which arises from two initial conditions, namely, a predominantly single mode, inclined interface between two gases, and a perturbed, multimodal, inclined interface are studied. The gas combination of nitrogen-acetone as light gas and carbon dioxide as heavy gas (Atwood number of 0.23) with an inclination angle of 80 degrees (η/ λ = 0.097) was chosen in this set of experiments. The interface is visualized using planar laser diagnostics (simultaneous PLIF/PIV measurements), once impulsively accelerated by a Mach 1.55. The ensemble-averaged turbulence measurements of the density, velocity and density-velocity cross-statistics are used to investigate the effects of added secondary modes to the interface on the correlation between turbulence and mixing quantities.

Experiments were performed at the inclined shock tube facility at Georgia Institute of Technology to study a Richtmyer-Meshkov unstable complex interface. The complex density stratification was achieved by counter flowing N2 over CO2 in order to create shear and buoyancy effects. The resulting Atwood number is 0.23 with an incident shock strength of Mach 1.55 and an angle of inclination of 80∘. High-resolution, full-field simultaneous Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) was employed to measure density and velocity statistics, respectively. For the first time with the inclined interface, mixing parameters from the BHR (Besnard-Harlow-Rauenzahn) model, including the density self-correlation and turbulent mass flux, are determined from experiments. Secondary modes added to the interface result in markedly greater mixing compared to the simple inclined interface as measured by mixedness and mixed mass.

The Shock Tube and Advanced Mixing Laboratory (STAML) at Georgia Institute of Technology is using a newly established inclined shock tube facility to study an inclined interface perturbation. This facility allows for simultaneous characterization of density and velocity fields by employing high-resolution, full-field Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), respectively. The incident shock strength of Mach 1.55 was used to impulsively accelerate a N₂-Acetone mixture over CO₂ inclined interface with an Atwood number of 0.23 and an 80° angle of inclination. This angle of inclination results in a linear perturbation as defined by the amplitude-to-wavelength ratio (η/ λ = 0.097). The development of the turbulent mixing layer for both pre- and post-reshock is determined by measuring several quantities, including two BHR model parameters: density self-correlation and turbulent mass flux.