Mechanical Engineering - Research Publications
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ItemCharacteristics of Reynolds Shear Stress in Adverse Pressure Gradient Turbulent Boundary Layers(Springer, 2021)The focus of the present work is to characterize the features of the turbulent inertia term (the wall-normal 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 pressure-gradient 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 zero-crossing of the turbulent inertia term seems to scale with δ+ while the minimum of ρuv scales with δ.
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ItemStress equation based scaling framework for adverse pressure gradient turbulent boundary layers(Elsevier, 2022-02-01)This paper provides a framework for estimating appropriate turbulent velocity scales in adverse pressure gradient turbulent boundary layers (APG TBLs) via a study of the mean stress balance. We examine the velocity scales of APG TBLs using the relationship between the Reynolds shear stress and pressure stress. It is reasoned that as distance from the wall increases the velocity scaling transitions from one dominated by the wall-shear-stress velocity scale, uτ, to a scaling dominated by the pressure stress. A velocity scale, uhyb, is proposed that varies with distance from the wall and combines the wall-shear-stress velocity with a pressure-stress-based velocity. This investigation uses new high Reynolds number (7000≲Reτ≲7800) experimental measurements, existing lower Reynolds number experimental (600≲Reτ≲2000) and computational (Reτ<700) data sets. The proposed velocity scale realizes similarity in the turbulent stress profiles to a degree that is superior to that achievable via any wall-distance-independent velocity scale when considering the full extent of the flow domain.
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ItemSimultaneous skin friction and velocity measurements in high Reynolds number pipe and boundary layer flows(Cambridge University Press (CUP), 2019-07-25)Streamwise velocity and wall-shear stress are acquired simultaneously with a hot-wire and an array of azimuthal/spanwise-spaced skin friction sensors in large-scale pipe and boundary layer flow facilities at high Reynolds numbers. These allow for a correlation analysis on a per-scale basis between the velocity and reference skin friction signals to reveal which velocity-based turbulent motions are stochastically coherent with turbulent skin friction. In the logarithmic region, the wall-attached structures in both the pipe and boundary layers show evidence of self-similarity, and the range of scales over which the self-similarity is observed decreases with an increasing azimuthal/spanwise offset between the velocity and the reference skin friction signals. The present empirical observations support the existence of a self-similar range of wall-attached turbulence, which in turn are used to extend the model of Baars et al. (J. Fluid Mech., vol. 823, p. R2) to include the azimuthal/spanwise trends. Furthermore, the region where the self-similarity is observed correspond with the wall height where the mean momentum equation formally admits a self-similar invariant form, and simultaneously where the mean and variance profiles of the streamwise velocity exhibit logarithmic dependence. The experimental observations suggest that the self-similar wall-attached structures follow an aspect ratio of in the streamwise, spanwise and wall-normal directions, respectively.
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ItemDownstream Recovery of Turbulence Kinetic Energy in the Wake of a Turbulent Boundary Layer Wing-Body Junction Flow(Australasian Fluid Mechanics Society, 2018)A multi-sensor 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 wing-body junction flow. The wing shape—a 3:2 semi-elliptic nose joined to a NACA 0020 airfoil tail—matches that used in a number of existing studies of wing-body junction wake flow (e.g. see the review of Simpson [2001]). Data have been collected in four spanwise/wall-normal 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.
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ItemVortex Rings in a Stratified Fluid(Australian fluid mechanics society, 2018)