Browsing by Subject "Particle Image Velocimetry"

Now showing 1 - 5 of 5
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    Experimental Investigation of Homogeneous Anisotropic Turbulence
    (2019-07) Carter, Douglas
    Motivated by the need to substantiate the existing literature on homogeneous turbulence with experimental data, a novel zero-mean homogeneous turbulence chamber is presented. Despite the anisotropy of the large scale velocity fluctuations, the experimental apparatus is found to well approximate homogeneous, shear-less turbulence over scales larger than the integral lengths of the flow for four separate cases at Reynolds numbers (based on the Taylor microscale) ranging between 154 and 412. This enables a detailed investigation of the turbulence statistics as obtained by 2D particle image velocimetry, which confirms the existence of inertial scaling ranges in both the second-order structure functions and energy spectra. It is found that the anisotropy of the flow persists down to the smallest scales, though its influence decreases with decreasing scale. The coherent structures, identified using a percolation analysis, are however isotropic in their geometry and generally collapse across cases using the Taylor microscale as a normalization length scale. Two types of scale interaction analyses are applied to the turbulent fields and indicate that there exists substantial coupling between scales small and large; challenging the classic assumption that a range of scales might emerge which is independent of the large scales. Employing the generalized Karman-Howarth-Monin equation in scale space, the energy cascade is found to move energy downscale across all cases, which is also confirmed using a filter space technique. The magnitude of the non-linear energy transfer in scale space is however found to be increasingly anisotropic for increasing large-scale RMS velocity ratio u'_1/u'_2. Using a conditional sampling procedure based on the activity of the small-scales, the non-linear energy transfer is found to have a strong dependence on the relative small-scale activity (as well as the presence of coherent structures), which causes enhanced downscale non-linear energy transfer or upscale non-linear energy transfer of moderate magnitude depending on the subset. In addition to showcasing accurate PIV measurements of homogeneous turbulence over a large range of scales, the results point to the complex nature of the energy cascade in the jet-array driven facility, with simultaneous upscale and downscale transfers at each instant as well as spatially concurrent interactions across all scales of the flow.
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    Internal Flow Characterization of Ventilated Partial Cavitation Using Particle Image Velocimetry
    (2020-06) Yoon, Kyungduck
    Understanding the gas leakage mechanisms of a Ventilated Partial Cavitation (VPC), an artificial air pocket formed by injecting air behind a flow separation device in water flow, is important for its potential application to drag reduction during ship operations. While the gas leakage mechanism of VPC involves the coupling of both internal air phase and external water phase flows, the literature lacks characterization of internal flow. Our study provides the first experimental investigation of the internal flows of VPC formed by air injection past a backward-facing step. Planar Particle Image Velocimetry (PIV) with fog particles as tracers is employed to perform flow visualization and time-resolved PIV to provide the internal flow characterization of VPC. Distinctive characteristics are observed in each regime, both in instantaneous and time-averaged senses. Further, the analogy between internal flows of VPC and a single-phased flow over a backward-facing step is suggested to explain the behaviors of VPC including the cavity lengths and the effect of flow parameters on each regime and further on the transition across regimes.
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    Near-wake studies of a utility-scale wind turbine using natural snowfall-based visualization and Particle Image Velocimetry
    (2018-12) Dasari, Teja
    Wind turbine technology has been extensively researched in the past century. A major part of this research effort was to improve the efficiencies of energy extraction and the lifespan of the turbines to make the technology economically viable. One important barrier in this regard is the insufficient knowledge on the wakes of the turbines which directly affect the turbine performance and also cause premature failure of the turbines (Frandsen et al. 2006; Barthelmie et al. 2007). Only when the complex behavior of wakes is understood can they be successfully controlled/modified/mitigated to improve overall wind turbine/farm efficiencies. In order to achieve this objective, a clear need exists for a complete understanding of wind turbine loading, subsequent vortex system, role of ambient turbulence and coherent turbulent structures within the wake (Sørensen 2011). Furthermore, the overall wake development and behavior is strongly dependent on the near-wake (1 – 4D) characteristics. The current research work employs natural snowfall based visualization techniques demonstrated by Toloui et al. 2014 and Hong et al. 2014 to further probe the near-wake behavior of the 2.5 MW EOLOS wind turbine located in Rosemount, MN. The broad objectives of the study are to examine the near-wake (within 0.2 to 1 D) dynamics under different regions of turbine operation, ambient conditions and turbine loading. The specific objectives are to (i) better visualize the wind turbine blade generated coherent structures; (ii) study the characteristic behaviors of these coherent structures and explore correlations with the turbine operation and response characteristics; (iii) provide unique and realistic near-wake visualization data to the wind energy research community; (iv) explore the characteristics of the extreme near-wake in a holistic fashion with unprecedented spatiotemporal resolutions that can provide critical insights on utility-scale wake behavior. Accordingly, visualization data are collected with varying regions of interest ranging from ~ 20 m to ~120 m. The typical whole-wake measurements include visualization and super-large-scale Particle Image Velocimetry (SLPIV) measurements in the near wake of the turbine in a field of view (FOV) of the size of a football field (~ 120 m vertical × 70 m streamwise). The SLPIV measurements provide velocity deficit and turbulent kinetic energy assessments over the entire rotor span. The instantaneous velocity fields from SLPIV indicate the presence of intermittent wake contraction states which are in clear contrast with the expansion states typically associated with wind turbine wakes. These contraction states feature a pronounced upsurge of velocity in the central portion of the wake. The wake velocity ratio R_w, defined as the ratio of the spatially-averaged velocity of the inner wake to that of the outer wake, is introduced to categorize instantaneous near wake into expansion (R_w<1) and contraction states (R_w>1). Based on R_w criterion, the wake contraction occurs 25% of the time during the 30-minute time duration of SLPIV measurements. The contraction states are found to be correlated with the rate of change of blade pitch by examining the distribution and samples of time sequences of wake states with different turbine operation parameters. Moreover, blade pitch change is shown to be strongly correlated to the tower and blade strains measured on the turbine, and the result suggests that the flexing of the turbine tower and the blades could indeed lead to the interaction of rotor with the turbine wake, causing wake contraction. Similar visualization data collected along the wake symmetry plane, along the tower axis, revealed an accelerating flow field behind the nacelle of the turbine. This region is also characterized by relatively higher turbulence characteristics due to the shear production of TKE. This region of TKE with relatively high values (or peak in TKE) is found to waver about the hub elevation which might be an effect of yaw error on the turbine. The smaller field of view studies representing visualization of tip vortex behavior, near the elevation corresponding to the bottom blade tip, over a broad range of turbine operational conditions, demonstrate the presence of the state of consistent vortex formation as well as various types of disturbed vortex states. The histograms corresponding to the consistent and disturbed states are examined over a number of turbine operation/response parameters, including turbine power and tower strain as well as the fluctuation of these quantities, with different conditional sampling restrictions. This analysis establishes a clear statistical correspondence between these turbine parameters and tip vortex behaviours under different turbine operation conditions, which is further substantiated by examining samples of time series of these turbine parameters and tip vortex patterns. This study not only offers benchmark datasets for comparison with the-state-of-the-art numerical simulation, laboratory and field measurements but also sheds light on understanding wake characteristics and its downstream development, turbine performance and regulation, as well as developing novel turbine or wind farm control strategies.
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    Turbulent boundary layers perturbed by an array of cylinders
    (2017-09) Tan, Yan Ming
    Turbulent boundary layers (Reτ = 2500) were perturbed by a spanwise array of cylinders, and the effects on the large-scale flow organization within the logarithmic layer were investigated. Boundary layer and vortex packet recovery trends were quantified downstream of several arrays. Two array spacings, each with two cylinder heights were considered. For S = 0.2δ arrays, cylinder heights of H = 0.2δ (H+ = 500, aspect ratio, AR = 4) and H = δ (H+ = 2500, AR = 20) were investigated. For the S = 0.6δ arrays, cylinder heights were H = 0.2δ and H = 0.05δ (H+ = 125, AR = 1). Stereoscopic and planar PIV measurements were acquired in both fixed and flying configurations at three measurement heights across the logarithmic layer, z+ = 125, 300 and 500. Furthermore, 3-D PTV volumes were acquired downstream of the S= 0.2δ arrays over a depth of 155 < z+ < 465. Results of time-averaged velocity statistics, instantaneous velocity fields and structural analyses of low and high uniform streamwise momentum zones were discussed. In addition, a vortex packet identification algorithm (VPIA) was developed to quantify relaxation trends of individual packet signatures in the flow downstream of the arrays. All of the arrays affected mean and RMS streamwise velocities averaged across the span downstream, due to the blockage posed to the oncoming flow. Undulating wakes due to Karman shedding occurred behind the cylinders, while the average wake structure at the cylinder tips suggested formation of streamwise aligned tip vortices. For the S = 0.2δ array, relaxation trends differed for the two cylinder heights, H = 0.2δ and H = δ. Downstream of the H = δ array, instantaneous PIV and VPIA results showed a bottom-up mechanism for the recovery of the large-scale flow organization. Flow features recovered first closer to the wall (z+ = 125), then later at z+ = 300, while hardly any recovery was seen at z+ = 500 up to 7δ downstream of the array, the furthest measurement location. In contrast, some indications of top-down recovery were observed for the flow perturbed by the shorter H = 0.2δ array. In this case, however, flow features and packets closer to the wall at z+ = 125 remained altered up to 7δ downstream, even though streamwise velocity statistics relaxed substantially to the unperturbed values. The difference in recovery trends between the two cylinder heights was related to weaker and stronger outer-inner interactions respectively, relative to the unperturbed flow. For the S = 0.6δ arrays, perturbations to mean and RMS velocity statistics were weaker than for the S = 0.2δ arrays as blockage was reduced substantially. Nevertheless, the flow downstream of the S = 0.6δ, H = 0.2δ array was profoundly affected, such that the energy contained in the 0.6δ spanwise wavelength was increased throughout the logarithmic layer over a distance of 7δ. The energy increase was related to the array preferentially re-aligning incoming high and low uniform momentum zones to spanwise locations corresponding to cylinder locations and the regions between them respectively. The re-alignment effects were stronger for low and high momentum zones that were longer than 0.5δ. The H = 0.05δ case also showed similar but much weaker trends.
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    Velocity & Vorticity Transport In 3D-Printed Idealized & Realistic Human Airways Using Magnetic Resonance Velocimetry (MRV) & Particle Image Velocimetry (PIV)
    (2019-07) Jalal, Sahar
    Furthering our knowledge of respiratory fluid dynamics is greatly beneficial to understanding lung diseases and improving aerosol drug delivery and mechanical ventilatory techniques. To this end, we develop an in-vitro platform to study detailed flow features in human airways. Idealized and realistic replicas of the bronchial tree are inserted in a flow loop circulating aqueous fluid, and detailed information on the structure-function relationship is collected using Magnetic Resonance Velocimetry (MRV) and refractive-index-matched Particle Image Velocimetry (PIV). By ‘structure’ here we indicate the anatomical and morphological features, while ‘function’ refers to momentum transport and mixing. We extract and analyze velocity and vorticity fields, as well as flow descriptors that characterize the longitudinal and lateral dispersion along the bronchial tree. We consider regimes of steady inhalation, steady exhalation, and oscillatory ventilation for a range of physiologically relevant Reynolds (Re = 100 – 5000) and Womersley (Wo = 1.2 – 12) numbers. Longitudinal dispersion is found to be higher during inhalation, while lateral dispersion is higher during exhalation. Counter-rotating streamwise vortices are observed along the airway tree due to the local curvature of the branches (Dean mechanism) and constitute one of the main transport mechanisms. At the higher Re, however, inertia induces significant non-local effects, and the vortices are transported across successive generation of bronchial branching. Flow reversal, a phenomenon consequential for gas mixing, particle transport and mechano-transduction at the epithelium, is also identified in both idealized and realistic airway geometries during steady and oscillatory regimes. The net flow drift during the ventilation cycle (steady streaming) is experimentally evaluated for the first time, and found to be much smaller than the advective flow, although not insignificant for the realistic airway geometry. The instantaneous flow fields and Reynolds stresses measured in the idealized airway model indicate great sensitivity to the inflow conditions, and show that the flow at the bifurcation is prone to unsteadiness even at regimes sometimes treated as laminar in earlier numerical studies.