47 research outputs found
On the global nonlinear instability of the rotating-disk flow over a finite domain
Direct numerical simulations based on the incompressible nonlinear Navier–Stokes equations
of the flow over the surface of a rotating disk have been conducted. An impulsive
disturbance was introduced and its development as it travelled radially outwards
and ultimately transitioned to turbulence has been analysed. Of particular interest was
whether the nonlinear stability is related to the linear stability properties. Specifically
three disk-edge conditions were considered; (i) a sponge region forcing the flow back to
laminar flow, (ii) a disk edge, where the disk was assumed to be infinitely thin, and
(iii) a physically-realistic disk edge of finite thickness. This work expands on the linear
simulations presented by Appelquist et al. (J. Fluid. Mech., vol. 765, 2015, pp. 612-631),
where, for case (i), this configuration was shown to be globally linearly unstable when
the sponge region effectively models the influence of the turbulence on the flow field. In
contrast, case (ii) was mentioned there to be linearly globally stable, and here, where
nonlinearity is included, it is shown that both case (ii) and (iii) are nonlinearly globally
unstable. The simulations show that the flow can be globally linearly stable if the linear
wavepacket has a positive front velocity. However, in the same flow field, a nonlinear
global instability can emerge, which is shown to depend on the outer turbulent region
generating a linear inward-travelling mode that sustains a transition-front within the
domain. The results show that the front position does not approach the critical Reynolds
number for the local absolute instability, R = 507. Instead, the front approaches R = 583
and both the temporal frequency and spatial growth rate correspond to a global mode
originating at this position.Swedish Research Counci
Boundary-layer transition over a rotating broad cone
The Swedish Research Council (VR
A new way to describe the transition characteristics of a rotating-disk boundary-layer flow
A new method of graphically representing the transition stages of a rotating-disk
flow is presented. The probability density function contour map of the fluctuating
azimuthal disturbance velocity is used to show the characteristics of the boundarylayer
flow over the rotating disk as a function of Reynolds numbers. Compared with
the variation of the disturbance amplitude (rms) or spectral distribution, this map more
clearly shows the changing flow characteristics through the laminar, transitional, and
turbulent regions. This method may also be useful to characterize the different stages
in the transition process not only for the rotating-disk flow but also for other flows.Swedish Research Counci
Investigation of the Global Instability of the Rotating-disk Boundary Layer
The development of the flow over a rotating disk is investigated by direct numerical simulations using both the linearized and fully
nonlinear incompressible Navier–Stokes equations. These simulations allow investigation of the transition to turbulence of the
realistic spatially-developing boundary layer. The current research aims to elucidate further the global linear stability properties
of the flow, and relate these to local analysis and discussions in literature. An investigation of the nonlinear upstream (inward)
influence is conducted by simulating a small azimuthal section of the disk (1/68). The simulations are initially perturbed by
an impulse disturbance where, after the initial transient behaviour, both the linear and nonlinear simulations show a temporally
growing upstream mode. This upstream global mode originates in the linear case close to the end of the domain, excited by
an absolute instability at this downstream position. In the nonlinear case, it instead originates where the linear region ends and
nonlinear harmonics enter the flow field, also where an absolute instability can be found. This upstream global mode can be
shown to match a theoretical mode from local linear theory involved in the absolute instability at either the end of the domain
(linear case) or where nonlinear harmonics enter the field (nonlinear case). The linear simulation grows continuously in time
whereas the nonlinear simulation saturates and the transition to turbulence moves slowly upstream towards smaller radial positions
asymptotically approaching a global upstream mode with zero temporal growth rate, which is estimated at a nondimensional radius
of 582.Swedish Research Counci
Turbulence in the rotating-disk boundary layer investigated through direct numerical simulations
Direct numerical simulations (DNS) are reported for the turbulent rotating-disk boundary layer for the
first time. Two turbulent simulations are presented with overlapping small and large Reynolds numbers,
where the largest corresponds to a momentum-loss Reynolds number of almost 2000. Simulation data are
compared with experimental data from the same flow case reported by Imayama et al. (2014), and also a
comparison is made with a numerical simulation of a two-dimensional turbulent boundary layer (2DTBL)
over a flat plate reported by Schlatter and Ă–rlĂĽ (2010). The agreement of the turbulent statistics between
experiments and simulations is in general very good, as well as the findings of a missing wake region and
a lower shape factor compared to the 2DTBL. The simulations also show rms-levels in the inner region
similar to the 2DTBL. The simulations validate Imayama et al.’s results showing that the rotating-disk
turbulent boundary layer in the near-wall region contains shorter streamwise (azimuthal) wavelengths
than the 2DTBL, probably due to the outward inclination of the low-speed streaks. Moreover, all velocity
components are available from the simulations, and hence the local flow angle, Reynolds stresses and
all terms in the turbulent kinetic energy equation are also discussed. However there are in general no
large differences compared to the 2DTBL, hence the three-dimensional effects seem to have only a small
influence on the turbulence.Swedish Research Counci
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Investigation of the structures in the unstable rotating-cone boundary layer
This work reports on the unstable region and the transition process of the boundary-layer flow induced by a rotating cone with a half apex angle of 60 degrees using the probability density function (PDF) contour map of the azimuthal velocity fluctuation, which was first used by Imayamaetal. (Physics of Fluids, vol.24, 2012, 031701) for the similar boundary-layer flow induced by a rotating disk. The PDF shows that the transition behavior of the rotating-cone flow is similar to that on the rotating disk. The effects of roughness elements on the cone surface have been examined. For the cone with roughnesses, we reconstructed the most probable vortex structure within the boundary layer from the hot-wire anemometry time signals. The results show that the PDF clearly describes the overturning process of the high-momentum upwelling of the spiral vortices, which due to vortex meandering cannot be detected in the phase-averaged velocity field reconstructed from the point measurements. At a late stage of the overturning process, our hot-wire measurements captured high-frequency oscillations, which may be related to secondary instability.Swedish Research Council (VR); Japan Society for the Promotion of Science (JSPS)
Transition to turbulence in the rotating-disk boundary-layer flow with stationary vortices
This paper proposes a resolution to the conundrum of the roles of convective and absolute
instability in transition of the rotating-disk boundary layer. It also draws some comparison
with swept-wing
ows. Direct numerical simulations based on the incompressible
Navier{Stokes equations of the
ow over the surface of a rotating disk with modelled
roughness elements are presented. The rotating-disk
ow has been of particular interest
for stability and transition research since the work by Lingwood (J. Fluid. Mech., vol. 299,
1995, pp. 17-33) where an absolute instability was found. Here stationary disturbances
develop from roughness elements on the disk and are followed from the linear stage, growing
to saturation and nally transition to turbulence. Several simulations are presented
with varying disturbance amplitudes. The lowest amplitude corresponds approximately
to the experiment by Imayama et al. (J. Fluid. Mech., vol. 745, 2014, pp. 132-163). For
all cases, the primary instability was found to be convectively unstable, and secondary
modes were found to be triggered spontaneously while the
ow was developing. The
secondary modes further stayed within the domain, and an explanation for this is a
proposed globally unstable secondary instability. For the low-amplitude roughness cases,
the disturbances propagate beyond the threshold for secondary global instability before
becoming turbulent, and for the high-amplitude roughness cases the transition scenario
gives a turbulent
ow directly at the critical Reynolds number for the secondary global
instability. These results correspond to the theory of Pier (J. Eng. Math, vol. 57,
2007, pp. 237-251) predicting a secondary absolute instability. In our simulations, high
temporal frequencies were found to grow with a large ampli cation rate where the secondary
global instability occurred. For smaller radial positions, low-frequency secondary
instabilities were observed, tripped by the global instability.Swedish Research Counci
Insufficient sun exposure has become a real public health problem
This is the final version. Available on open access from MDPI via the DOI in this recordThis article aims to alert the medical community and public health authorities to accumulating evidence on health benefits from sun exposure, which suggests that insufficient sun exposure is a significant public health problem. Studies in the past decade indicate that insufficient sun exposure may be responsible for 340,000 deaths in the United States and 480,000 deaths in Europe per year, and an increased incidence of breast cancer, colorectal cancer, hypertension, cardiovascular disease, metabolic syndrome, multiple sclerosis, Alzheimer’s disease, autism, asthma, type 1 diabetes and myopia. Vitamin D has long been considered the principal mediator of beneficial effects of sun exposure. However, oral vitamin D supplementation has not been convincingly shown to prevent the above conditions; thus, serum 25(OH)D as an indicator of vitamin D status may be a proxy for and not a mediator of beneficial effects of sun exposure. New candidate mechanisms include the release of nitric oxide from the skin and direct effects of ultraviolet radiation (UVR) on peripheral blood cells. Collectively, this evidence indicates it would be wise for people living outside the tropics to ensure they expose their skin sufficiently to the sun. To minimize the harms of excessive sun exposure, great care must be taken to avoid sunburn, and sun exposure during high ambient UVR seasons should be obtained incrementally at not more than 5–30 min a day (depending on skin type and UV index), in season-appropriate clothing and with eyes closed or protected by sunglasses that filter UVR.Sunshine Health Foundation (SHF
Dissecting the Shared Genetic Architecture of Suicide Attempt, Psychiatric Disorders, and Known Risk Factors
BACKGROUND: Suicide is a leading cause of death worldwide, and nonfatal suicide attempts, which occur far more frequently, are a major source of disability and social and economic burden. Both have substantial genetic etiology, which is partially shared and partially distinct from that of related psychiatric disorders. METHODS: We conducted a genome-wide association study (GWAS) of 29,782 suicide attempt (SA) cases and 519,961 controls in the International Suicide Genetics Consortium (ISGC). The GWAS of SA was conditioned on psychiatric disorders using GWAS summary statistics via multitrait-based conditional and joint analysis, to remove genetic effects on SA mediated by psychiatric disorders. We investigated the shared and divergent genetic architectures of SA, psychiatric disorders, and other known risk factors. RESULTS: Two loci reached genome-wide significance for SA: the major histocompatibility complex and an intergenic locus on chromosome 7, the latter of which remained associated with SA after conditioning on psychiatric disorders and replicated in an independent cohort from the Million Veteran Program. This locus has been implicated in risk-taking behavior, smoking, and insomnia. SA showed strong genetic correlation with psychiatric disorders, particularly major depression, and also with smoking, pain, risk-taking behavior, sleep disturbances, lower educational attainment, reproductive traits, lower socioeconomic status, and poorer general health. After conditioning on psychiatric disorders, the genetic correlations between SA and psychiatric disorders decreased, whereas those with nonpsychiatric traits remained largely unchanged. CONCLUSIONS: Our results identify a risk locus that contributes more strongly to SA than other phenotypes and suggest a shared underlying biology between SA and known risk factors that is not mediated by psychiatric disorders