Coupling between cerebrovascular oscillations and CSF flow fluctuations during wakefulness: An fMRI study

It is commonly believed that cerebrospinal fluid (CSF) movement is facilitated by blood vessel wall movements (i.e., hemodynamic oscillations) in the brain. A coherent pattern of low frequency hemodynamic oscillations and CSF movement was recently found during non-rapid eye movement (NREM) sleep via functional MRI. This finding raises other fundamental questions: 1) the explanation of coupling between hemodynamic oscillations and CSF movement from fMRI signals; 2) the existence of the coupling during wakefulness; 3) the direction of CSF movement. In this resting state fMRI study, we proposed a mechanical model to explain the coupling between hemodynamics and CSF movement through the lens of fMRI. Time delays between CSF movement and global hemodynamics were calculated. The observed delays between hemodynamics and CSF movement match those predicted by the model. Moreover, by conducting separate fMRI scans of the brain and neck, we confirmed the low frequency CSF movement at the fourth ventricle is bidirectional. Our finding also demonstrates that CSF movement is facilitated by changes in cerebral blood volume mainly in the low frequency range, even when the individual is awake.

Partially-Covered Fractal Induced Turbulence On Fins Thermal Dissipation

The impacts of partially-covered fractal grids induced turbulence on the forced convective heat transfer across plate-fin heat sink at Reynolds number ReDh=22.0×103 were numerically and experimentally investigated. Results showed that partially covered grids rendered a higher thermal dissipation performance, with partially covered square fractal grid (PCSFG) registering an outstanding increase of 43% in Nusselt number relative to the no grid configuration. The analyzation via an in-house developed single particle tracking velocimetry (SPTV) system displayed the findings of unique “Turbulence Annulus” formation, which provided a small degree of predictivity in the periodic annulus oscillations. Further assessments on PCSFG revealed the preferred inter-fin flow dynamics of (i) high flow velocity, (ii) strong turbulence intensity, (iii) vigorous flow fluctuations, (iv) small turbulence length scale, and (v) heightened decelerated flow events. Furthermore, power spectra density unveiled the powerful vortex shedding effect, with PCSFG achieving fluctuation frequency f=18.5Hz close to an optimal magnitude. Such intricate flow structures pave the way for superior thermal transfer capabilities, benefiting the community in developing for higher efficiency heat transfer systems.

Testing Basic Gradient Turbulent Transport Models for Swirl Burners Using PIV and PLIF

The present paper reports on the combined stereoscopic particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) measurements of turbulent transport for model swirl burners without combustion. Two flow types were considered, namely the mixing of a free jet with surrounding air for different swirl rates of the jet (Re = 5 × 103) and the mixing of a pilot jet (Re = 2 × 104) with a high-swirl co-flow of a generic gas turbine burner (Re = 3 × 104). The measured spatial distributions of the turbulent Reynolds stresses and fluxes were compared with their predictions by gradient turbulent transport models. The local values of the turbulent viscosity and turbulent diffusivity coefficients were evaluated based on Boussinesq’s and gradient diffusion hypotheses. The studied flows with high swirl were characterized by a vortex core breakdown and intensive coherent flow fluctuations associated with large-scale vortex structures. Therefore, the contribution of the coherent flow fluctuations to the turbulent transport was evaluated based on proper orthogonal decomposition (POD). The turbulent viscosity and diffusion coefficients were also evaluated for the stochastic (residual) component of the velocity fluctuations. The high-swirl flows with vortex breakdown for the free jet and for the combustion chamber were characterized by intensive turbulent fluctuations, which contributed substantially to the local turbulent transport of mass and momentum. Moreover, the high-swirl flows were characterized by counter-gradient transport for one Reynolds shear stress component near the jet axis and in the outer region of the mixing layer.

Laser doppler flowmetry assessment of microcirculation in children of 6–7 years old

Introduction. The state of the blood flow within the capillaries and close blood vessels is highly important in practice for the revealing of pathogenetic mechanisms of both systemic and local circulatory disorders. Aim of the study was to define the parameters of microcirculation and the level of blood flow fluctuations (flux) in the distal segments of upper and lower limbs (in fingers of hands and toes of feet) in children of 6–7 years old; and to describe the possible differences in the mechanisms of blood flow modulation in boys and girls. Materials and methods. Skin microcirculation was assessed in middle fingers of hands and great toes of feet in children of 6-7 years old (14 girls and 7 boys in prone position) by means of laser doppler flowmetry. Results. The ranges for parameters of microcirculation (PM) for distal segments of upper and lower limbs in children of mentioned age group were defined, also it was shown that the PM are significantly lower in the lower limbs comparing to those of the upper limbs (both in groups of girls and boys). Asymmetry of PM in the feet was not found; the features of right hand-left hand asymmetry for PM in girls and boys are described. The analysis of modulation of blood flow fluctuations (fluxmotions) of different frequencies showed the profound role of vasomotor (myogenic) rhythm for regulation of microcirculation. Conclusion. Increased neurogenic influences on the modulation of fluxmotions in girls of 6-7 years old may be an evidence of the ongoing development of the mechanisms of blood flow regulation, particularly the association with the growth rate of girls is possible.

Assessment of Dependency of Unsteady Onset Flow and Resultant Tidal Turbine Fatigue Loads on Measurement Position at a Tidal Site

This work determines the variation in the fatigue loading on a tidal turbine at two depth positions and two different locations within a site. Site data were obtained at the European Marine Energy Centre, EMEC, test facility in Scotland, which has been compiled at the University of Edinburgh. The turbine modelled is the 18m Diameter DEEP-gen 1MW horizontal axis turbine. A blade element method is combined with a synthetic turbulence inflow to determine forces along the blade over a period of five tidal cycles. The focus is on establishing the difference between the loads at one tidal site, with an emphasis on the variety of turbulent conditions, with the onset flow fluctuations as great as 17% and the average integral lengthscales varying from 11 to 14 m at hub height. Fatigue loading is assessed using damage equivalent loads, with a 30% variation between turbine positions and 32% between turbine locations within a site, for one design case. When long term loading is assessed, a 41% difference is found for aggregated loads for a near surface turbine and a 28% difference for a near bed turbine.

The Aeroacoustic Response of a Single Square Cylinder in Confined Cross Flow

Unstable flow patterns around arrangements of bluff bodies in different engineering applications can give rise to pressure oscillations, leading to excitation of strong acoustic resonance that can interrupt operation. In certain conditions, flow fluctuations arising from vortex shedding downstream of a circular cylinder are reported to excite severe acoustic resonance. On the other hand, cylinders of a square cross section are known to be particularly susceptible to mechanisms that involve coupling between the flow and a structural mode. It is not documented, however, if such coupling would occur between an acoustic mode and flow fluctuations downstream of a square cylinder. In this work, the possibility of excitation of acoustic resonance due to coupling between unsteady flow downstream of a single square cylinder with an acoustic cross mode of a rectangular duct is experimentally investigated. During the experiments, acoustic resonance was self-excited. Measurements of the acoustic pressure and the flow velocity are carried out for a single square cylinder of an edge length of 25.4 mm. Results show that aeroacoustic response characteristics for this configuration are not completely analogous to the case of a circular cylinder, with a number of features not reported before. A brief summary of the results is presented in this work.