 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
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ItemCharacteristics of Reynolds Shear Stress in Adverse Pressure Gradient Turbulent Boundary LayersRomero, S ; Zimmerman, S ; Philip, J ; Klewicki, J (Springer, 2021)The focus of the present work is to characterize the features of the turbulent inertia term (the wallnormal gradient of Reynolds shear stress) through the mean momentum balance and the Reynolds shear stress correlation coefficient (ρuv ). Effects of the Reynolds number and Clauser pressuregradient parameter, β, are discussed. Large eddy simulations of low Reynolds number adverse pressure gradient turbulent boundary layers from Bobke et al. [1], low Reynolds number experimental data from Vila et al. [2] and Volino [3], and newly acquired experimental data at higher Reynolds number from the Flow Physics Facility at The University of New Hampshire are utilized for this analysis. Observations are compared to zero pressure gradient turbulent boundary layer direct numerical simulations of Schlatter and Örlu [4] and Sillero et al. [5], and experimental data from Zimmerman et al. [6] and Zimmerman [7]. These cases show that the correlation coefficient (ρuv ) decreases in magnitude with increasing Reynolds number and β. However, from these initial observations we find that ρuv is more sensitive to changes in the Reynolds number in comparison to the examined range of β. We also find that the location of zerocrossing of the turbulent inertia term seems to scale with δ+ while the minimum of ρuv scales with δ.

ItemA New DataDriven Turbulence Model Framework for Unsteady Flows Applied to WallJet and WallWake FlowsLav, C ; Philip, J ; Sandberg, R (The American Society of Mechanical Engineers, 20191105)The unsteady flow prediction for turbomachinery applications relies heavily on unsteady RANS (URANS). For flows that exhibit vortex shedding, such as the walljet/wake flows considered in this study, URANS is unable to predict the correct momentum mixing with sufficient accuracy. We suggest a novel framework to improve that prediction, whereby the deterministic scales associated with vortex shedding are resolved while the stochastic scales of pure turbulence are modelled. The framework first separates the stochastic from the deterministic length scales and then develops a bespoke turbulence closure for the stochastic scales using a datadriven machinelearning algorithm. The novelty of the method lies in the use of machinelearning to develop closures tailored to URANS calculations. For the walljet/wake flow, three different mass flow ratios (0.86, 1.07 and 1.26) have been considered and a highfidelity dataset of the idealised geometry is utilised for the sake of model development. This study serves as an a priori analysis, where the closures obtained from the machinelearning algorithm are evaluated before their implementation in URANS. The analysis looks at the impact of using all length scales versus the stochastic scales for closure development, and the impact of the extent of the spatial domain for developing the closure. It is found that a twolayer approach, using bespoke trained models for the near wall and the jet/wake regions, produce the best results. Finally, the generalisability of the developed closures is also evaluated by applying a given closure developed using a particular mass flow ratio to the other cases.

ItemReproducing AS/NZS terraintype wind profiles in a shortfetch windtunnelKevin, K ; Philip, J ; Monty, J ; Klewicki, J (AWES, 2018)

ItemExperimental Study of Turbulent NonTurbulent Interface in a Planar Mixing Layer using Kinetic Energy CriteriaBalamurugan, G ; Rodda, A ; Philip, J ; Mandal, AC (FMFP, 2018)Using kinetic energy (KE) criteria, the Turbulent NonTurbulent Interface (TNTI) in a planar mixing layer has been identified and studied in this paper. The mixing layer is generated in a simple way by blocking the top half of the wind tunnel test section with a mesh of suitable solidity. High resolution particle image velocimetry (PIV) is used to acquire the velocity data. The threshold for identifying the interface is obtained using the area algorithm suggested by [2]. The mean and rms profiles of velocity and the spanwise vorticity with respect to the TNTI show finite jump at the interface location. The fractal dimension of the KE interface based on the box counting method is found to be ∼1.2.

ItemImprovement In Unsteady Wake Prediction Through Machine Learning Based Rans Model TrainingLav, C ; Sandberg, R ; Philip, J (ISUAAAT15, 2018)The inability of RANS to correctly capture profile wake dynamics and decay prevents the accurate prediction of unsteady losses and poses additional challenges to the aeromechanic verification of both turbines and compressors. This paper addresses this problem by introducing a novel technique applied to improve wake prediction through a RANS based calculation. In particular, for the first time, a datadriven approach is used to develop bespoke turbulence closures to be used in the context of unsteady RANS (URANS) calculations. The new closure is obtained from an evolutionary machine learning algorithm. The algorithm requires a highfidelity dataset which, in this study, is provided through a DNS for the wake downstream of a normal flat plate with a Reynolds number = 2,000, based on the freestream velocity and plate height. URANS are conducted with the developed closure and the mean flow statistics are compared with the highfidelity data. The results from the developed closure show excellent agreement with the reference data. Furthermore, the generalisability of the developed closure is evaluated by considering other flat plate wake data, which differ in both the aspect ratio and the Reynolds number from the case used in this study to develop the closures. The results once again show remarkable improvement compared with the standard URANS.

ItemDownstream Recovery of Turbulence Kinetic Energy in the Wake of a Turbulent Boundary Layer WingBody Junction FlowZimmerman, S ; Philip, J ; Marino, N ; Klewicki, J (Australasian Fluid Mechanics Society, 2018)A multisensor hotwire probe capable of simultaneously measuring all three components of the velocity vector [Zimmerman et al. 2017] has been deployed in the wake of a turbulent boundary layer wingbody junction flow. The wing shape—a 3:2 semielliptic nose joined to a NACA 0020 airfoil tail—matches that used in a number of existing studies of wingbody junction wake flow (e.g. see the review of Simpson [2001]). Data have been collected in four spanwise/wallnormal measurement planes ranging from 1 to 33 chord lengths behind the trailing edge of the junction. The measurement planes span a domain over which the unperturbed boundary layer would develop from friction Reynolds number Reτ ≈ 8000 –11000. The downstream extent (per chord length) of the present data is the furthest of any experimental effort to date. Despite having a recovery length many times longer than the typical streamwise wavelength of boundary layer ‘superstructure’ motions [Hutchins and Marusic 2007], the turbulence kinetic energy (TKE) profiles at the furthest downstream station still exhibit spanwise inhomogeneity. Data from the measurement planes closer to the junction offer insight into the momentum and turbulence transporting effects of the trailing ‘horseshoe’ vortex, as well as how these effects propagate downstream.

ItemCFD simulations of vertical surface piercing circular cylinders and comparison against experimentsKeough, SJ ; Stephens, DW ; Ooi, A ; Philip, J ; Monty, J (Australian fluid mechanics society, 2018)When predicting the susceptibility of a submarine to above water detection, it is important to consider the impact of the wake generated by the periscope(s). Computational Fluid Dynamics (CFD) tools can be used to predict the physical size and shape of the wake, which can be combined with periscope models for input into detectability prediction models. For this application, it is important that CFD predictions of the wake are accurate not only in the mean calculations, but that the physical characteristics of the wake are captured at instantaneous snapshots in time. In a previous experimental study, Keough et al. [10] presented time resolved measurements of the wake from vertical surface piercing cylinders, utilising an automated method of extracting these measurements as a function of time from video recordings of the experiment. In the present work, CFD simulations have been performed to model this experimental data set. The open source CFD software Caelus was used, with the improved Defence Science and Technology Group version of vofSolver—the multiphase volume of fluid solver. A numerical wave gauge is implemented in order to measure the free surface elevation during the simulation and this data is compared to bow wave data obtained from animations of the CFD results, using the same automated visual tracking technique utilised for the experimental measurements. Analysis of these timeresolved measurements is performed, comparing transient statistics and spectral characteristics of the CFD predictions against the experimental data.

ItemDirect Numerical Simulation of Confined Wall PlumesGeorge, N ; Philip, J ; Ooi, A (Australian fluid mechanics society, 2018)We present results from the direct numerical simulation (DNS) of a wall attached thermal plume in a confined box. The plume originates from a local line heat source of length, L, placed at the bottom left corner of the box. The Reynolds number of the wall plume, based on box height and buoyant velocity scale, is ReH = 14530 and a parametric study is carried out for boxes of two different aspect ratios (ratio of box width to box height) for a particular value of L. In the simulation, the plume develops along the vertical side wall while remaining attached to it before spreading across the top wall to form a buoyant fluid layer and eventually moving downwards and filling the whole box. Further, the original filling box model of Baines and Turner [1] is modified to incorporate the wall shear stress and the results from the DNS are compared against it. A reasonable agreement is observed for the volume and momentum fluxes in the quiescent uniform environment and also for the timedependent buoyancy profile calculated far away from the plume.

ItemTurbulent flow above windgenerated waves: conditional statistics and POD structuresKevin, K ; Philip, J ; Lee, JH ; Bhirawa, T ; Monty, J (Australian Fluid Mechanics Society, 2018)Large fieldofview particle image velocimetry (PIV) measurement is performed to characterise the turbulent boundary layer above evolving wind waves, which are developed over 3.5 m fetch at U∞ = 8.2 m/s. This multicamera experiment captures a streamwise domain of 0.4 m, slightly longer than two dominant wavelength of these wind waves. Instantaneous velocity observations reveal strong flow separations on the leeward side of most dominant waves, and these events are also marked by strong vertical velocity fluctuations. The spatiallyaveraged velocity profile further indicates a large velocity gradient below the wave crest, which occupies a significant proportion of the boundary layer. The conditionallyaveraged flow fields around larger dominant waves show that turbulence stresses are high downwind the wave crest, indicating the highly varying form of the separation events. These events are further elucidated using proper orthogonal decomposition (POD) analysis, where the first few stronger modes reveal several common attributes around the separation events.

ItemVortex Rings in a Stratified FluidElsnab, J ; Zhen, M ; Philip, J ; Klewicki, J (Australian fluid mechanics society, 2018)
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