Browsing by Author "Limacher, Eric John"

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    Added 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, Lex
    The 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.
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    Experimental Investigation and Modelling of Vortex-Induced Vibrations near Scoured Boundaries
    (2024-06-05) Hardika, Michel Supernikovic; Morton, Chris; Martinuzzi, Robert; Limacher, Eric John; Wong, Ron
    An experimental investigation into the vortex induced vibrations (VIV) of a circular cylinder under scour conditions was conducted. These conditions replicate river crossings of exposed pipelines. The thesis focuses on developing a model to predict the VIV response, and the characterization and flow evolution of the scour VIV response. The development of the model involves the calibration of a coupled two-equation model for a large VIV dataset from a one degree of freedom (1DOF) elastically mounted rigid cylinder in an open free stream that spans across a range of mass, damping, and flow velocities. This experiment was conducted in a water channel where cylinder displacement and force data were collected. The model adequately predicts the lock-in reduced velocity range and oscillation amplitudes. Its empirical coefficients are related to the mass, damping, and flow velocity, improving the prediction capability over previous models. A second data set was collected to address the characterization of VIV response and flow development under scour conditions. In particular, the experiments had one structural configuration with the 1DOF cylinder placed adjacent to different rigid flat and curved boundaries based on the profiles found in past scour studies. The collected data includes displacement, force, and Particle Image Velocity (PIV) measurements across a wide range of flow velocities. The data was processed to produce oscillation amplitude and lift responses, oscillation frequency contour maps, and a Low Order Representation (LOR) of the flow field through a Proper Orthogonal Decomposition (POD) analysis. The results show that the boundary modifies the VIV response in terms of lock-in range, oscillation amplitude, lift, and the appearance of new regimes when compared to the open free stream case. The LOR flow fields reveal the boundary layer coupling with the formation and shedding timing of the cylinder’s vortices, leading to the changes in the response. The results of this investigation aid in understanding the possible oscillation characteristics that submerged structures such as exposed pipelines in riverbeds exhibit. These characteristics inform the design and monitoring of these systems by relating the flow and structural conditions to the expected oscillation response.
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    Multi-fidelity Aerodynamic Analysis of Outboard Horizontal Stabilizers for Small-scale Aircraft
    (2024-12-18) De Alwis, Arjuna; Hinman, William Schuyler; Limacher, Eric John; Benneker, Anne
    The Outboard Horizontal Stabilizer (OHS) configuration is a unique fixed-wing design often used when a traditional tail is impractical and has been proposed as a means to achieve enhanced range and endurance. This study utilized a mixture of numerical approaches to quantify and critically examine the potential performance gains of the OHS concept. A modified non-linear lifting-line approach incorporating wing interactions was used for a fast low-fidelity model, and Reynolds Averaged Navier Stokes simulations were completed using OpenFOAM. A representative small-scale wing-tail combination was arranged into traditional, outboard, split, and extended span configurations and was used as a case study. The results obtained in this work demonstrate a substantial increase in Cl/Cd and Cl^{3/2}/Cd ratios for the OHS when compared to configurations with a similar main wing span, correlating to increased range and endurance, respectively. However, it was observed that simply extending the main wing to the overall span of the OHS configuration yielded higher performance. The importance of performing an aerodynamic analysis while considering static stability and trim is also discussed in detail. Furthermore, the specific conditions that can lead to increased overall lift and reduced induced drag on the stabilizers are highlighted and discussed using different approaches, and a qualitative analysis of the wake flow field was used to explain and confirm conclusions. Finally, a focused parametric study was performed to analyze how stabilizer position and deflection impact aerodynamic performance. It was found that for a fixed tail arm, having the tail just outboard of the main wing would maximize the performance of an OHS configuration. It was further found that the lift-to-drag ratio of an OHS configuration may surpass that of a conventional configuration with the same overall span if the fraction of tail span is reduced while increasing the tail arm, in order to maintain stability. Future studies will focus on a comprehensive optimization analysis to examine when and how OHS configurations may outperform optimal conventional configurations.
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    Sensor-based Temporal Superresolution: Application to turbulent separated flow over a three-dimensional Gaussian hill
    (2023-09-14) Manohar, Kevin Harsh; Martinuzzi, Robert; Morton, Christopher; Limacher, Eric John; Liao, Wenyuan
    The high Reynolds-number turbulent separated flow over a Gaussian speed-bump benchmark geometry presents challenges for predicting smooth-body flow separation. The lack of time-resolved experimental data further hampers the understanding of the three-dimensional unsteady dynamics. This thesis addresses these issues in two parts. First, a data-driven technique using high-rate surface-pressure sensors and long short-term memory (LSTM) neural networks is proposed to estimate aliased velocity dynamics from undersampled particle image velocimetry (PIV) data, revealing low and medium-frequency modes. Second, the three-dimensional unsteady wake dynamics is characterized using additional surface-pressure measurements and two-component PIV. Four dominant frequencies are identified, with a very low-frequency spanwise oscillation of the recirculating zone, two low frequencies associated with the primary separation front motion, and a higher frequency from shear layer vortex shedding. Proper orthogonal decomposition analysis highlights interactions between these modes. The instantaneous vortex topology is conceptualized to infer physical mechanisms that give rise to these frequencies.