Browsing by Author "Martinuzzi, Robert John"
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- ItemOpen AccessAdded Mass and Vortical Impulse: Theory and Experiment(2019-01-10) Limacher, Eric John; Wood, David H.; Morton, Chris R.; Johansen, Craig T.; Martinuzzi, Robert John; Bates, Larry M.; Smits, LexThe classical decomposition of aerodynamic force into added-mass and circulatory components is derived under the assumption of inviscid flow. In the present thesis, this decomposition is shown to be valid for viscous flows. The classical added-mass force, derived using (acyclic) potential flow theory, is superimposable onto the circulatory force regardless of the presence of a vortical wake. This generalized added-mass and circulatory (GAMC) force decomposition is derived from impulse theory using a Helmholtz decomposition of the velocity field, and is valid for rigid bodies of any shape in unbounded incompressible fluid domains. Two supporting theoretical contributions were made in the course of the derivation, and these have been referred to as the conservation of image-vorticity impulse and the invariance of total vortical impulse to infinity-preserving conformal transformations. The practical utility of the GAMC formulation was investigated by applying it to a numerical simulation (generated by Wang and Eldredge (2013)) of the flow around a pitching plate in a steady free-stream flow. The calculated forces show fairly good agreement with the reported forces, although minor discrepancies suggest further work to quantify errors due to discretization. The GAMC formulation was then applied to particle image velocimetry (PIV) data to estimate force on a linearly accelerating cylinder in quiescent fluid. The resulting estimates capture the trend of the measured force well, but consistent underestimation of 10% to 20% was observed. It is speculated that the underestimation could be a failure to resolve the viscous skin friction due to spatial resolution limitations, and this possibility merits further study. In both the numerical and experimental validations, the GAMC formulation was validated alongside a common expression referred to as the standard impulse formulation (SIF). The inclusion of an image-vorticity impulse term in the GAMC formulation, contrary to the SIF, causes it to be less sensitive to random errors in the acquired velocity field and more tolerant to the omission of near-body vorticity data. These features of the GAMC formulation make it an attractive option for application to PIV studies in which near-body data acquisition is challenging.
- ItemOpen AccessEnd effects of nominally two-dimensional thin flat plates(2020-09-24) Braun, Eric; Agrey, Kaden; Martinuzzi, Robert JohnDifferences in the structure and dynamics of nominally two-dimensional turbulent wakes are investigated experimentally for a thin flat plate, normal to a uniform flow, with two different end conditions: with and without end plates. Both cases are characterized by Karman-like vortex shedding with broadband low frequency unsteadiness. Both wakes evidence a low frequency flapping motion in addition to the slowly drifting base flow common to cylinder wakes. For the case without end plates, an interaction between the drift motion and the vortex formation process is associated with a much stronger modulation of the quasiperiodic vortex shedding amplitude when compared to the case with end plates where a flapping motion is more strongly expressed. These dynamics underlie structural differences in the mean wake and Reynolds stress fields. .
- ItemOpen AccessEvolution Surfaces for Spatiotemporal Visualization of Vortex Features(2019-09-10) Ferrari, Simon; Hu, Yaoping; Martinuzzi, Robert JohnTurbulent fluid flow data is often 4-dimensional (4D), spatially and temporally complex, and requires specific techniques for visualization. Common visualization techniques neglect the temporal aspect of the data, limiting the ability to convey feature motion or offering the user a complicated visualization. To remedy this, we present an approach – evolution surfaces – focused on the spatiotemporal rendering of user-selected flow features (i.e., vortices). By abstracting the spatial representation of these features, the approach renders their spatiotemporal behavior with reduced visual complexity. The behavior of vortex features are presented as surfaces, with textures indicating properties of motion and evolution events (e.g., bifurcation and amalgamation) represented by the surface topology. We evaluated the approach on two datasets generated from empirical measurement and computational simulation (Re = 28000 and Re = 1200 respectively). Our approach’s focus on handling evolution events makes it capable of visualizing higher Reynolds number (Re) flows than other surface-based techniques. This approach has been assessed by fluid dynamicists to assert the validity for flow analysis. Evolution surfaces offer a compact visualization of spatiotemporal vortex behaviors, opening potential avenues for exploration and analysis of fluid flows.
- ItemOpen AccessMachine learning strategies applied to the control of a fluidic pinball(AIP Publishing LLC, 2020-01-10) Raibaudo, Cedric; Zhong, Peng; Noack, Bernd R.; Martinuzzi, Robert JohnThe wake stabilization of a triangular cluster of three rotating cylinders is investigated. Experiments are performed at Reynolds number Re ∼ 2200. Flow control is realized using rotating cylinders spanning the wind-tunnel height. The cylinders are individually connected to identical brushless DC motors. Two-component planar particle image velocimetry measurements and constant temperature hot-wire anemometry were used to characterize the flow without and with actuation. Main open-loop configurations are studied and different controlled flow topologies are identified. Machine learning control is then implemented for the optimization of the flow control performance. Linear genetic algorithms are used here as the optimization technique for the open-loop constant speed-actuators. Two different cost functions J are considered targeting either drag reduction or wake symmetrization. The functions are estimated based on the velocity from three hot-wire sensors in the wake. It is shown that the machine learning approach is an effective strategy for controlling the wake characteristics. More significantly, the results show that machine learning strategies can reveal unanticipated solutions or parameter relations, in addition to being a tool for optimizing searches in large parameter spaces.
- ItemOpen AccessMulti-objective optimization using evolutionary algorithms: Application to the control of flow past a circular cylinder(2018-11-22) Bingham, Conrad Cole; Martinuzzi, Robert John; Morton, Chris R.; Hu, Yaoping; Ziadé, Paul; Westwick, David T.; Epstein, Marcelo D.Modifications to the vortex shedding dynamics from a circular cylinder of diameter D are investigated experimentally in a free surface water channel. The vortex shedding is modified via the placement of a control cylinder of diameter \textit{D}/8 in the vicinity of the main cylinder. A methodology is presented to link changes in the wake dynamics and loading on the main cylinder. The analysis combines Fourier Modal Decomposition, Proper Orthogonal Decomposition, and phase averaging. Based on differences in the wake dynamics, the influence of the control cylinder can be classified according to its placement: (i) in the free stream outside the main cylinder shear layer; (ii) within the main cylinder shear layer; and (iii) in the recirculation region. While fluctuating lift is significantly reduced in all cases, the mean and fluctuating drag are affected differently. A generalized model-free method to optimize parameters for open-loop and closed-loop control in fluid mechanics applications is then presented. A multi-objective evolutionary algorithm (MOEA) is employed to minimize the oscillating lift caused by vortex shedding from the main cylinder. The control cylinder is prescribed a position as well as a periodic motion in two dimensions. The MOEA efficiently handles the larger optimization parameter space. The first objective of the algorithm is to minimize the fluctuating force coefficient $C_{L_{RMS}}$, while the second objective is to minimize of the actuation power required to drive the control cylinder. The final solution suppresses $C_{L_{RMS}}$ by over 90\% using near-zero actuation power. Further, the MOEA automatically provides a sensitivity study as to the influence of the different parameters and also in which spatial area the greatest influence is expressed.
- ItemOpen AccessOn the Permeability and Diffusivity of Articular Cartilage(2020-04-01) Hashlamoun, Kotaybah; Federico, Salvatore; Epstein, Marcelo; Herzog, Walter; Wan, Richard; Martinuzzi, Robert John; Quinn, Thomas M.; Federico, SalvatoreArticular cartilage (AC) consists of a fluid-saturated porous proteoglycan matrix, reinforced by collagen fibres. Since AC is avascular, nutrients transport within AC via diffusion, characterized by diffusivity, and via fluid flow, characterised by permeability. When AC degenerates, diffusivity and permeability are altered, impacting nutrient transport. This work investigates AC diffusivity and permeability, their relationship to one another and to the microstructure in healthy tissues, as a first step towards understanding nutrient transport in degenerated tissues. AC properties are anisotropic, due to the collagen fibre organisation, and heterogeneous, due to the zonal variation of AC composition and architecture. Therefore, permeability and diffusivity are treated as tensors, and are studied as a function of tissue depth. Current permeability and diffusivity models in AC are at different levels: while advanced anisotropic large-deformation models of permeability exist, virtually no model exists for diffusivity. The works consist of two parts: Part I. We propose a multi-scale permeability model under large deformations, accounting for the distortion at the fibre level and for fibre reorientation. When applying compressive strain along the tissue’s depth, the model predicts an increase in the ratio of axial to transverse permeability, which is in agreement with the literature. Part II. We study the relationship between diffusivity and permeability in AC. We propose a tensor representation of diffusivity in terms of permeability, based on the effective medium approach, and accounting for the anisotropy induced by the fibres. The model captures the diffusivity magnitude of uncharged spherical molecules of various sizes and the diffusivity anisotropy of linear molecules. We then propose and validate a direct method for quantifying diffusivity tensor from Fluorescence Recovery After Photobleaching experiments. This method is then used to quantify the diffusivity of 3 kDa and 500 kDa dextran molecules across AC depth. Diffusivity measurements are then analysed using the diffusivity-permeability model, through which the role of fibre alignment in diffusivity anisotropy is highlighted. We then describe some attempts at testing AC transverse permeability using microfluidics-based techniques and the associated challenges. Finally, we highlight some limitations of our work and possibilities for future developments.
- ItemOpen AccessOpen-loop and Closed-loop control of a Flow over a Cluster of Three Cylinders with Variable Spin Rate in Equilateral Triangular Arrangement(2020-01-30) Zhong, Xiao Peng; Martinuzzi, Robert John; Morton, Chris R.; Li, Simon; Nowicki, Edwin PeterThe control of the flow over a cluster of three rotating cylinders arranged in a triangular configuration (pinball-configuration) is simulated using 2D URANS. The free stream flow impinges onto the front cylinder at Reynolds number of 2054 based on the diameter. Open-loop and closed-loop controls change the flow behavior because the rotating surfaces of the cylinders directly modify the vorticity flux into the wake. For the open-loop control, the leeward cylinders are counter-rotating at a constant rate. In the open-loop control, the strength of the shear layers from the leeward cylinders and the central gap flow change and even reverse as the rotation rate varies. As a result, different and unique flow states were realized. The fluctuation of the flow reaches a minimum at certain rotation ratio (angular velocity of a cylinder non-dimensionalized by free-stream flow velocity). In the closed-loop control, the fluctuating lift of each cylinder is used as a feedback signal to control an oscillatory rotation rate about the mean that is set to match an open-loop configuration. The physical model describing the rotation-induced-lift by oscillatory rotation is developed and reveals a phase lag between 0 to pi from the steady-state-lift. Based on this mechanism, a closed-loop controller is designed such that the rotation-induced-lift partially cancels the lift from the natural vortex shedding. This results in lift attenuation. The closed-loop control is tested on the single cylinder case, revealing that the shear layers are elongated and the strength of the vorticity flux across each shear layer is reduced. The closed-loop control is then applied to the pinball-configuration. The same elongation of the shear layers and the reduction in the strength of vorticity flux are observed. Additionally, the closed-loop control changes the central gap flow, which slightly shifts the flow characteristics accordingly.
- ItemOpen AccessSimulation of Flow Over Flat Plates for PV Wind Loads Assessment(2019-05-15) Brydges, Jesse; Wood, David H.; Martinuzzi, Robert John; Ziadé, Paul; Nowicki, Edwin Peter; Wood, David H.The ability to accurately predict wind loads on photovoltaic (PV) modules is becoming increasingly desirable in many aspects of engineering as the popularity of renewable energy grows. The decrease in cost of PV units and batteries, as well as the advancement in battery and PV power output are contributors to this surge in popularity, to name a few. These technological advancements, coupled with increases in urbanization and incentives to move towards clean energy have contributed to a rise in popularity of roof-mounted solar panels on buildings in city environments where buildings are exposed to strong winds phenomena, such as downwashing from surrounding buildings. In-situ full-scale measurements of these wind flows can be challenging, and it is common that loading values are desired in the design phase of construction. Historically, the most common method of replicating wind flows in an experimental environment has been with wind tunnels. However, this method can present drawbacks concerning instrumentation, scaling, labour intensity, cost and time consumption. As such, consultants and industry are looking towards computational fluid dynamics (CFD) to perform fast and cost-effective measurements. CFD simulations that involve solar panels and surrounding environments are typically performed using Reynolds Averaged Navier-Stokes (RANS) equations due to the complexity of the geometries involved. In RANS modelling, a number of the turbulence relationships must be modelled. This introduces the possibility of inaccuracy but it reduces the overall time of the simulation and can be applied to more complex geometries. The intent of this study was to examine the behaviour of steady state, incompressible flow around normal and inclined flat plates relative to the oncoming flow at a Reynolds number of 1200 using Reynolds Averaged Navier-Stokes (RANS) equations. The angled plates simulate PV modules in a variety of different environments. Three different turbulence models were used to close the RANS equations; the standard k-epsilon model (SKE), the shear stress transport k-omega model (SST k-ω) and the renormalization group k-epsilon model (RNG k-ϵ). Five Cases were created to validate the numerical setup of the current study, establish a baseline and explore the effects of common environments on the solar modules. The parameters that were collected for the various simulations were the drag (Cd) and lift (Cl) coefficients, the mean recirculation lengths behind the plate (Lw) and the streamwise velocities and turbulence kinetic energy readings along the centrelines of the plates. The simulations were completed using openFOAM, an open source CFD software. Case 1 focused on comparing Cd and Lw on flat plates to experimental data and to direct numerical simulations of the Navier-Stokes equations to ensure accuracy. Case 2 then established a baseline, similar to Case 1 but with angled plates. Cases 3 through 5 then introduced additional geometries for angled plates. The validation process in Case 1 was successful, with the results of the current study coinciding well with existing literature. Cd progressively declined in magnitude from Cases 1 through 4, but generally showed marginal increases in magnitude from Case 4 to 5 when a building step was introduced. Both Cl and Lw generally increased between Cases 2 through 5, while Case 1 showed larger values than Case 2. The SKE turbulence model usually showed larger turbulence kinetic energy values around the plate for all the Cases and the shortest Lw in Cases 4 and 5 when compared to the other turbulence models.
- ItemOpen AccessStructural and dynamic differences in the turbulent wake of cantilevered square and circular cylinders protruding a thin laminar boundary layer(2019-08-16) Kindree, Matthew Gordon; Martinuzzi, Robert John; Wood, David H.; Korobenko, Artem; Hassanzadeh, HassanThis thesis documents a comparative experimental study of the vortical structures and dynamics in the turbulent wake of aspect ratio 4 cantilevered square and circular cylinders at a Reynolds number of 10500 protruding a thin laminar boundary layer. Antisymmetric Kármán-like vortex shedding of half-loop structures is observed in the phase-averaged field of both cylinders. The signature of these shed vortices are dipole structures imprinted on the mean wakes. The half-loop structures, associated spectral signatures, and dipole vortices are concentrated at lower elevations in the circular cylinder wake but span the square cylinder’s entire height and are significantly stronger. A low-frequency instability is observed at high elevations of both cylinder wakes but at different frequencies and is more broadband for the square cylinder. The low frequency signature spans the entire height of the square cylinder and therefore interacts with the vortex shedding. Both low-frequency signatures are shown to be unique to laminar boundary layers. However, these cannot be directly related to instabilities of the laminar horseshoe vortex system. The square cylinder exhibits a significantly more complex mean wake structure with vortices descending from the dipole pair and growing from the ground plate into the far wake. Increased bluffness of the square cylinder geometry leads to stronger rotation of merging structures in the phase-averaged field and formation of these additional mean wake structures. Furthermore, a vortex along the ground plate forms in the mean field of both cylinders which is the signature of a complex interaction between the primary horseshoe vortex legs and the shedding structures. Above the cylinder free-ends, both cylinders exhibit tip vortices planted to their free-end surface that extend into the near wake region. These tip vortices are mostly steady for the circular cylinder but oscillate vertically at the vortex shedding frequency for the square cylinder. Thus, the square cylinder’s free-end flow directly interacts with the vortex shedding in the wake. This work establishes that square and circular cylinder geometries give rise to topologically and dynamically distinct wakes and that the boundary layer state influences the dynamics.
- ItemOpen AccessVisualizing three-dimensional vortex shedding through evolution surface clusters(2019-11-01) Ferrari, Simon; Hu, Yaoping; Morton, Chris R.; Martinuzzi, Robert JohnTurbulent vortex shedding in the wake of a bluff body often contains cycle-to-cycle variations in the shape, trajectory, and intensity of vortices. Existing flow visualization techniques cannot effectively present these variations and, consequently, their influence on the aerodynamics to the user. This paper explores a new flow visualization approach to represent quasi-periodic vortex shedding over multiple shedding cycles concurrently. This approach uses a reduced-dimension representation of spatiotemporal vortex progression (called evolution surfaces) and ensemble visualization techniques (clustering). The resulting visualization can be used to identify topological changes in the behavior and strengths of coherent structures (i.e. vortices) in unsteady flows. This approach is applied in two case studies of bluff body wakes with Reynolds Numbers Re = 1200 (Hemmati et al. 2016c) and Re = 300 (Morton et al. 2018). In prior work, classification of these wakes’ dynamics was based on energy fluctuation and shedding topology. However, these techniques are not well suited for representing characteristic changes between shedding regimes. In the present work, it has been shown that evolution surface clusters help to identify topological changes characterizing cycle-to-cycle variations in vortex behavior, while reducing visual clutter. The results indicate that evolution surface clusters are a promising visualization tool for comparative analysis of unsteady vortex dynamics in turbulent wakes.