Papers by Sharon A. Stephen
The Quarterly Journal of Mechanics and Applied Mathematics, 2006
This study investigates the nonlinear stability of hypersonic viscous flow over a sharp slender c... more This study investigates the nonlinear stability of hypersonic viscous flow over a sharp slender cone. The attached shock and the effects of curvature are taken into account. Asymptotic methods are used for large Reynolds number and large Mach number to examine the viscous modes of instability, which may be described by a triple-deck structure. A weakly nonlinear analysis is carried out allowing an equation for the amplitude of disturbances to be derived. The coefficients of the terms in the amplitude equation are evaluated for axisymmetric and non-axisymmetric disturbances. Thus, the effects of the shock and curvature on the nonlinear stability of the flow may be deduced.
Microconcentric Nebuliser for the Analysis of Small Sample Volumes by Inductively Coupled Plasma Mass Spectrometry
Journal of Analytical Atomic Spectrometry, 1997
ABSTRACT
Heart & Lung, 2008
The purpose of this study was to describe fatigue and the relationships among fatigue intensity, ... more The purpose of this study was to describe fatigue and the relationships among fatigue intensity, self-reported functional status, and quality of life in older adults with stable heart failure. METHODS: A descriptive, correlational design was used to collect quantitative data with reliable and valid instruments. Fifty-three eligible volunteers completed a questionnaire during an interview. Those with recent changes in their medical regimen, other fatigue-inducing illnesses, and isolated diastolic dysfunction were excluded. RESULTS: Fatigue intensity (Profile of Mood States fatigue subscale) was associated with lower quality of life, perceived health, and satisfaction with life. Fatigue was common, and no relationship was found between fatigue intensity and self-reported functional status. Marital status was the only independent predictor of fatigue. CONCLUSIONS: In stable heart failure, fatigue is a persistent symptom. Clinicians need to ask patients about fatigue and assess the impact on quality of life. Self-reported functional status cannot serve as a proxy measure for fatigue.
Atomisierung von angeschl�mmten Lebensmittelproben in der ETA-AAS
Journal of Fluid Mechanics, 2015
In this study, a new centrifugal instability mode, which dominates within the boundary-layer flow... more In this study, a new centrifugal instability mode, which dominates within the boundary-layer flow over a slender rotating cone in still fluid, is used for the first time to model the problem within an enforced oncoming axial flow. The resulting problem necessitates an updated similarity solution to represent the basic flow more accurately than previous studies in the literature. The new mean flow field is subsequently perturbed, leading to disturbance equations that are solved via numerical and short-wavelength asymptotic approaches, yielding favourable comparisons with existing experiments. Essentially, the boundary-layer flow undergoes competition between the streamwise flow component, due to the oncoming flow, and the rotational flow component, due to effect of the spinning cone surface, which can be described mathematically in terms of a control parameter, namely the ratio of streamwise to axial flow. For a slender cone rotating in a sufficiently strong axial flow, the instabili...
Effects of regular and random microstructures on hypersonic boundary layers
6th AIAA Theoretical Fluid Mechanics Conference, 2011
of curvature and of the attached shock are included for axisymmetric and non-axisymmetric disturb... more of curvature and of the attached shock are included for axisymmetric and non-axisymmetric disturbances. The ow in the hypersonic boundary layer is coupled to the ow in the porous layer. The theoretical model of a porous wall developed by Fedorov and his co-workers is used for a random microstructure. The physical parameters chosen correspond to those of their experiments for ow over a felt-metal coating. The resulting transcendental equations are solved numerically. Results for neutrally stable disturbances are presented together with spatial growth rates. These are compared to the case of a regular microstructure. Results are also obtained for a porous wall comprising of layers of regular meshes. The eect of the thickness of the layer is investigated.
Effects of Passive Porous Walls on Hypersonic Boundary Layers
IUTAM Bookseries, 2009
A theoretical linear stability analysis is used to consider the effect of a porous wall on the fi... more A theoretical linear stability analysis is used to consider the effect of a porous wall on the first mode of a hypersonic boundary layer on a sharp slender cone. The effect of curvature and of the attached shock are included. The flow in the hypersonic boundary layer is coupled to the flow in the porous layer. The theoretical model of
Instability of the 3D boundary layer flow over a rotating cone
The flow over a rotating cone is susceptible to crossflow and centrifugal instability modes, depe... more The flow over a rotating cone is susceptible to crossflow and centrifugal instability modes, depending on the sharpness of the cone nose. The boundary layer instability is visualized by the formation of spiral vortices, which wrap around the cone surface in a helical nature. For parameters ranging from propeller nose cones to rotating disks, the instability triggers co-rotating vortices, whereas for sharp spinning missiles, counter-rotating vortices are observed. In the presence of a forced free-stream, the flow is essentially a battle between the oncoming axial flow and the azimuthal shear flow due to the rotating surface. Experiments in the literature have shown that increasing the incident free-stream has a stabilizing effect on these spiral vortices. We derive the mean flow boundary layer equations and investigate the asymptotic stability of the flow to inviscid crossflow modes at large Reynolds number for a cone in still fluid as well as in axial flow. The influence of the cone...
Trace analysis in the food chain
Analytical Proceedings, 1985
Transitionmechanismswithintheboundary-layerflowoverslendervs.broadrotatingcones
40th Fluid Dynamics Conference and Exhibit, 2010
Evaluation of a slurry technique for the determination of lead in spinach by electrothermal atomic-absorption spectrometry
The Analyst, 1985
A simple method for the determination of lead in spinach involving minimum sample preparation is ... more A simple method for the determination of lead in spinach involving minimum sample preparation is described. The powdered spinach is suspended in a thixotropic thickening agent, Viscalex HV30, and the slurry is injected directly into the electrothermal atomiser. Oxygen is introduced during the ashing stage to allow the use of higher ash temperatures and to avoid the build up of carbonaceous residue in the tube. Concentrations of powdered spinach up to 10%m/V can be tolerated in the suspension. Good agreement was achieved between results obtained by a standard additions procedure and by direct calibration with aqueous standards, and also by an alternative wet digestion procedure.
Physics of Fluids, 2009
This investigation is concerned with the linear development of Görtler vortices in the high-Reyno... more This investigation is concerned with the linear development of Görtler vortices in the high-Reynolds-number laminar compressible wake behind a flat plate which is aligned with the centerline of a curved mixing-layer system. The Görtler modes were previously found to exist within curved compressible mixing layers by Owen et al. ͓Phys. Fluids 8, 2506 ͑1997͔͒. This study extends that investigation and demonstrates the effect a wake has on the growth rate and location of such modes. The investigations were made by examining the growth rate and the location of the Görtler modes in the limit of large Görtler number and high wave number within the wake-dominated curved compressible mixing-layer systems based on three wake flow models. An analytic Gaussian wake profile is first used to model the behavior of the basic flow within the mixing layer at the trailing edge of the splitter plate. It is found that the wake has an amplification effect on the growth of the Görtler instability within the concavely curved or "unstably" curved compressible mixing layers. It is also found that within the convexly curved or "stably" curved compressible mixing layers wake modes can occur that behave differently to the "thermal modes," which were previously found within the plain curved compressible system by Owen et al. Another analytic composite model which has some practical applications is then used to predict the behavior of the modes within the systems. Solutions from a numerical wake flow model have been compared with the predictions based on the analytic wake flow models.
Physics of Fluids, 2011
We consider the convective instability of stationary and traveling modes within the boundary laye... more We consider the convective instability of stationary and traveling modes within the boundary layer over a disk rotating in a uniform axial flow. Complementary numerical and high Reynolds number asymptotic analyses are presented. Stationary and traveling modes of type I (crossflow) and type II (streamline curvature) are found to exist within the boundary layer at all axial flow rates considered. For low to moderate axial flows, slowly-traveling type I modes are found to be the most amplified and quickly-traveling type II modes are found to have the lower critical Reynolds numbers. However, nearstationary type I modes are expected to be selected due to a balance being struck between onset and amplification. Axial flow is seen to stabilize the boundary layer by increasing the critical Reynolds numbers and reducing amplification rates of both modes. However, the relative importance of type II modes increases with axial flow and they are therefore expected to dominate for sufficiently high rates. The application to chemical vapour deposition (CVD) reactors is considered.
Journal of Non-Newtonian Fluid Mechanics, 2014
The stability of the flow due to a rotating disk is considered for non-Newtonian fluids, specific... more The stability of the flow due to a rotating disk is considered for non-Newtonian fluids, specifically shearthinning fluids that satisfy the power-law (Ostwald-de Waele) relationship. In this case the basic flow is not an exact solution of the Navier-Stokes equations, however, in the limit of large Reynolds number the flow inside the three-dimensional boundary layer can be determined via a similarity solution. An asymptotic analysis is presented in the limit of large Reynolds number. It is shown that the stationary spiral instabilities observed experimentally in the Newtonian case can be described for shear-thinning fluids by a linear stability analysis. Predictions for the wavenumber and wave angle of the disturbances suggest that shear-thinning fluids may have a stabilising effect on the flow.
Journal of Fluid Mechanics, 1986
The neutral curve for stationary vortex disturbances in rotating-disk flow is computed up to a Re... more The neutral curve for stationary vortex disturbances in rotating-disk flow is computed up to a Reynolds number of 107 using the sixth-order system of linear stability equations which includes the effects of streamline curvature and Coriolis force. It is found that the neutral curve has two minima: one at R = 285.36 (upper branch) and the other at R = 440.88 (lower branch). At large Reynolds numbers, the upper branch tends to Stuart's asymptotic solution while the lower branch tends to a solution that is associated with the wave angle corresponding to the direction of zero mean wall shear.
Journal of Fluid Mechanics, 2012
This study investigates the nonlinear stability of hypersonic viscous flow over a sharp slender c... more This study investigates the nonlinear stability of hypersonic viscous flow over a sharp slender cone with passive porous walls. The attached shock and effect of curvature are taken into account. Asymptotic methods are used for large Reynolds number and large Mach number to examine the viscous modes of instability (first Mack mode), which may be described by a triple-deck structure. A weakly nonlinear stability analysis is carried out allowing an equation for the amplitude of disturbances to be derived. The coefficients of the terms in the amplitude equation are evaluated for axisymmetric and non-axisymmetric disturbances. The stabilizing or destabilizing effect of nonlinearity is found to depend on the cone radius. The presence of porous walls significantly influences the effect of nonlinearity, and results for three types of porous wall (regular, random and mesh microstructure) are compared.
Journal of Fluid Mechanics, 2009
Experimental studies have shown that the boundary-layer flow over a rotating cone is susceptible ... more Experimental studies have shown that the boundary-layer flow over a rotating cone is susceptible to cross-flow and centrifugal instability modes of spiral nature, depending on the cone sharpness. For half-angles (ψ) ranging from propeller nose cones to rotating disks (ψ ≥ 40°), the instability triggers co-rotating vortices, whereas for sharp spinning missiles (ψ < 40°), counter-rotating vortices are observed. In this paper we provide a mathematical description of the onset of co-rotating vortices for a family of cones rotating in quiescent fluid, with a view towards explaining the effect of ψ on the underlying transition of dominant instability. We investigate the stability of inviscid cross-flow modes (type I) as well as modes which arise from a viscous–Coriolis force balance (type II), using numerical and asymptotic methods. The influence of ψ on the number and orientation of the spiral vortices is examined, with comparisons drawn between our two distinct methods as well as wit...
Effect of Postural Changes on Arterial Blood Pressure and Heart Rate in Healthy, Older Adults
Journal of Cardiopulmonary Rehabilitation, 1988
Journal of Algorithms & Computational Technology, 2012
In this study, we provide a mathematical description of the onset of counter-rotating circular vo... more In this study, we provide a mathematical description of the onset of counter-rotating circular vortices observed for a family of slender rotating cones (of half-angles 15° or less) in quiescent fluid. In particular, we apply appropriate scalings in order to simplify the basic-flow profiles, which are subsequently perturbed, accounting for the effects of streamline curvature. A combined large Reynolds number and large vortex wavenumber analysis is used to obtain an estimate for the asymptotic right-hand branch of neutral stability for a slender rotating cone. Our results confirm our earlier predictions pertaining to the existence of the new Görtler mode and capture the effects of the governing centrifugal instability mechanism. Meanwhile, favourable comparisons are drawn with existing numerical neutral stability curve results.
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Papers by Sharon A. Stephen