Mechanical Engineering - Research Publications
Now showing items 1-12 of 565
Characteristics of Reynolds Shear Stress in Adverse Pressure Gradient Turbulent Boundary Layers
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. , low Reynolds number experimental data from Vila et al.  and Volino , 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  and Sillero et al. , and experimental data from Zimmerman et al.  and Zimmerman . 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.
Stress equation based scaling framework for adverse pressure gradient turbulent boundary layers
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.
The use of deep learning algorithms to predict mechanical strain from linear acceleration and angular rates of motion recorded from a horse hoof during exercise
(PERGAMON-ELSEVIER SCIENCE LTD, 2022-02-15)
Resolvent analysis-based pressure modeling for trailing edge noise prediction
(American Institute of Aeronautics and Astronautics, 2019-05-20)
This paper presents the first development steps of a hybrid computational aeroacoustics (CAA) method for the prediction of trailing edge (TE) noise, based on a physics-driven prediction of the surface pressure on an airfoil. Starting frommean flowdata for a given configuration, the dominant pressure modes over a foil are modeled with an incompressible formulation of the resolvent method. As a conceptual test of its suitability to predict surface pressures, the framework is used to model the unsteady surface pressure fluctuations generated by instability waves on an infinite flat plate. While a canonical test case, the flat plate is a good starting point for the investigation of airfoil TE noise. Subsequently, the framework is applied to a NACA0012 airfoil at 0° angle of attack. In the flat plate case, hydrodynamic instabilities are excited by a single frequency volume forcing and result in streamwise propagating Tollmien-Schlichting waves. The resolvent captures these instabilities and the resulting surface pressure field with good accuracy. A Mach number dependence is observed for the agreement between resolvent and DNS pressure modes, which may explain the difference in wavelengths between DNS and resolvent results in the NACA0012 airfoil case. Bearing in mind this dependence of the pressure prediction accuracy on the Mach number, the results show promise for resolvent-based TE noise predictions.
Unicortical and bicortical plating in the fixation of comminuted fractures of the clavicle: a biomechanical study
BACKGROUND: Intraoperative neurovascular complications with clavicle fracture fixation are often due to far cortex penetration by drills and screws, but could be avoided using a unicortical construct. The objective of this study was to compare the bending and torsional strength of a unicortical locking screw plate construct and a hybrid (with central locked and outer non-locked long oblique screws) unicortical plate construct for clavicle fracture fixation with that of a conventional bicortical locking screw construct of plate fixation. METHODS: Twenty-four human clavicle specimens were harvested and fractured in a comminuted mid-shaft butterfly configuration. Clavicles were randomly allocated to three surgical fixation groups: unicortical locking screw, bicortical locking screw and hybrid unicortical screw fixation. Clavicles were tested in torsion and cantilever bending. Construct bending and torsional stiffness were measured, as well as ultimate strength in bending. RESULTS: There were no significant differences in bending stiffness or ultimate bending moment between all three plating techniques. The unicortical locked construct had similar torsional stiffness compared with the bicortical locked construct; however, the hybrid technique was found to have significantly lower torsional stiffness to that of the bicortical locking screw construct (mean difference: 87.5 Nmm/degree, P = 0.028). CONCLUSIONS: Unicortical locked screw plate fixation and hybrid unicortical plating fixation with centrally locked screws and outer long, oblique screws may alleviate far cortex penetration, protecting nearby anatomical structures, and may ease implant removal and conversion to bicortical fixation for revision surgery; however, use of long oblique screws may increase the risk of early loosening under torsion.
Liquid film breakup induced by turbulent shear flow
Airblast atomizers of the ‘prefilming’ type are commonly employed for liquid fuel injection in gas turbine engines propelling modern aircraft. Supplied from holes or slits upstream, the liquid fuel forms a thin film over the prefilming surface before being driven to the atomizing edge by a turbulent flow through the turbine. We consider a simplified numerical setup using a volume-of-fluid method to simulate the essential physics of this multiphase problem. A liquid film is ‘sandwiched’ between sheared turbulent gas flows which deform and then rupture the liquid film. The simplified setup allows us to systematically vary parameters such as film thickness and turbulent gas flow Reynolds number to gauge their role in deforming and ultimately rupturing the initially stationary liquid film. This work presents a detailed study of the developing pressure field over the deforming film and related aerodynamic effects, as previously suggested by other authors, in particular the role of the inviscid lift force. Understanding and controlling the route to breakup and atomization of liquid fuels in such systems is of primary practical concern in modern gas turbine engine design.
Introduction and testing of a novel scanning particle tracking velocimetry technique
(LISBON Simposia, 2018)
We present a novel robust three-dimensional particle tracking technique based on particleimagesfrom a scanning laser setup, which can resolve highly seeded flows. Following successful reconstruction of the particle fields via triangulation, a particle tracking routine is applied to calculate Lagrangian particle velocities. The performance of the technique has been assessed using synthetic experiments based on numerical simulations, and on an experimental dataset of a high-Reynolds number turbulent flow. The new approach yields accurate particle 3D location and track measurements both in simulations and in practice.
The temporal boundary layer under the action of decaying free-stream turbulence
The problem of the turbulent boundary layer under decaying free-stream turbulence is numerically investigated using the temporal boundary layer framework. This tool is particularly suited to the problem since the evolution of homogeneous isotropic turbulence is classically described by temporal decay. This study focuses on the interaction between the fully turbulent boundary layer and free-stream turbulence, which has so far received little attention compared to the behaviour of the transitional boundary layer subject to free-stream turbulence. The bulk of our simulations were completed by seeding the free-stream of boundary layers ‘pre-grown’ to a desired thickness with homogeneous isotropic turbulence from a precursor simulation. This strategy allowed us to test different combinations of the turbulence intensity and largeeddy lengthscale of the free-stream turbulence with respect to the velocity- and large-eddy length-scales of the boundary layer; such a parametric investigation would remain otherwise inaccessible given available computing resources. Additionally, the present strategy permits assessment of the direct effect of the locally present free-stream disturbances, as opposed to a ‘downstream’ effect from free-stream disturbances far ‘upstream’. The relative large-eddy turnover timescale between the free-stream turbulence and the boundary layer emerges as an important parameter in predicting if the free-stream turbulence and boundary layer interaction will be ‘strong’ or ‘weak’ before the free-stream turbulence eventually fades away. For a ‘strong’ interaction, the main action of the free-stream turbulence on the boundary layer is to cause increased spreading of the boundary layer away from the wall, which then permits incursions of free-stream fluid deep within it, changing the boundary layer velocity profiles in the outer part of the flow. This has the important effect of increasing the boundary layer thickness _ by flattening the intermittency profile.