Fluid flows in nature and applications are frequently subject to periodic velocity modulations. Surprisingly, even for the generic case of flow through a straight pipe, there is little consensus regarding the influence of pulsation on the transition threshold to turbulence: while most studies predict a monotonically increasing threshold with pulsation frequency (i.e. Womersley number, α), others observe a decreasing threshold for identical parameters and only observe an increasing threshold at low α. In the present study we apply recent advances in the understanding of transition in steady shear flows to pulsating pipe flow. For moderate pulsation amplitudes we find that the first instability encountered is subcritical (i.e. requiring finite amplitude disturbances) and gives rise to localized patches of turbulence ("puffs") analogous to steady pipe flow. By monitoring the impact of pulsation on the lifetime of turbulence we map the onset of turbulence in parameter space. Transition in pulsatile flow can be separated into three regimes. At small Womersley numbers the dynamics are dominated by the decay turbulence suffers during the slower part of the cycle and hence transition is delayed significantly. As shown in this regime thresholds closely agree with estimates based on a quasi steady flow assumption only taking puff decay rates into account. The transition point predicted in the zero α limit equals to the critical point for steady pipe flow offset by the oscillation Reynolds number (i.e. the dimensionless oscillation amplitude). In the high frequency limit on the other hand puff lifetimes are identical to those in steady pipe flow and hence the transition threshold appears to be unaffected by flow pulsation. In the intermediate frequency regime the transition threshold sharply drops (with increasing α) from the decay dominated (quasi steady) threshold to the steady pipe flow level.

Articles you may be interested inPhotodissociation of (SO2XH) Van der Waals complexes and clusters (XH = C2H2, C2H4, C2H6) excited at 32 040-32090 cm−1 with formation of HSO2 and XThe photochemistry of bromoform is of considerable importance to understanding the impact of short-lived halogen species on bromine chemistry in the atmosphere. In the present work, the products of the ultraviolet photodissociation of bromoform at 234 and 267 nm are determined by time-of-flight mass spectrometry and velocity ion imaging. Both ground Br ( 2 P 3/2 ) and spin-orbit excited Br ( 2 P 1/2 ) atoms are found to be formed via resonance-enhanced multiphoton ionization detection. Radical products are detected via vacuum ultraviolet photoionization at 118 nm. The results indicate that there is a primary molecule bromine elimination channel consisting of CHBr ϩBr 2 . The quantum yields for atomic Br and molecular Br 2 elimination channels are determined from the time-of-flight spectra to be 0.74 and 0.26 at 234 nm, respectively. At 267 nm, they are 0.84 and 0.16, respectively. Energy and angular distributions are deduced from the 2D images of Br, CHBr, and CHBr 2 . The direct studies described in this paper on the photodissociation of bromoform suggest that the current atmospheric photochemical models that do not anticipate the formation of Br 2 need to be reinvestigated to determine their implications for atmospheric bromine chemistry.

Pulsating flows through tubular geometries are laminar provided that velocities are moderate. This in particular is also believed to apply to cardiovascular flows where inertial forces are typically too low to sustain turbulence. On the other hand, flow instabilities and fluctuating shear stresses are held responsible for a variety of cardiovascular diseases. Here we report a nonlinear instability mechanism for pulsating pipe flow that gives rise to bursts of turbulence at low flow rates. Geometrical distortions of small, yet finite, amplitude are found to excite a state consisting of helical vortices during flow deceleration. The resulting flow pattern grows rapidly in magnitude, breaks down into turbulence, and eventually returns to laminar when the flow accelerates. This scenario causes shear stress fluctuations and flow reversal during each pulsation cycle. Such unsteady conditions can adversely affect blood vessels and have been shown to promote inflammation and dysfunction of the shear stress-sensitive endothelial cell layer.

more than 2700 patients have been hospitalized with e-cigarette, or vaping, product use-associated lung injury (EVALI) across the United States. This report describes the outbreak in California, a state with one of the highest case counts and with a legal adult-use (recreational) cannabis market. OBJECTIVE To present clinical characteristics and vaping product exposures of patients with EVALI in California. DESIGN, SETTING, AND PARTICIPANTS Case series describing epidemiologic and laboratory data from 160 hospitalized patients with EVALI reported to the California Department of Public Health by local health departments, who received reports from treating clinicians, from August 7 through November 8, 2019.EXPOSURES Standardized patient interviews were conducted to assess vaping products used, frequency of use, and method of product acquisition. Vaping products provided by a subset of patients were tested for active ingredients and other substances.MAIN OUTCOMES AND MEASURES Demographic and clinical characteristics, level of care, and outcomes of hospitalization were obtained from medical record review.RESULTS Among 160 patients with EVALI, 99 (62%) were male, and the median age was 27 years (range, 14-70 years). Of 156 patients with data available, 71 (46%) were admitted to an intensive care unit, and 46 (29%) required mechanical ventilation. Four in-hospital deaths occurred. Of 86 patients interviewed, 71 (83%) reported vaping tetrahydrocannabinol (THC)-containing products, 36 (43%) cannabidiol (CBD)-containing products, and 39 (47%) nicotine-containing products. Sixty-five of 87 (75%) THC-containing products were reported as obtained from informal sources, such as friends, acquaintances, or unlicensed retailers. Of 87 vaping products tested from 24 patients, 49 (56%) contained THC. Vitamin E or vitamin E acetate was found in 41 (84%) of the THC-containing products and no nicotine products.CONCLUSIONS AND RELEVANCE Patients' clinical outcomes and vaping behaviors, including predominant use of THC-containing products from informal sources, are similar to those reported by other states, despite California's legal recreational cannabis market. While most THC products tested contained vitamin E or vitamin E acetate, other underlying cause(s) of injury remain possible. The California Department of Public Health recommends that individuals refrain from using any vaping or e-cigarette products, particularly THC-containing products from informal sources, while this investigation is ongoing.

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A branching ratio of 1.6 +/- 0.3 for S(3P)/S(1D) is obtained for the dissociation of CS2 with very low fluence 193 nm laser (less than 2 mJ/cm2), in which the S(3P) and S(1D) have been state-selectively ionized using VUV lasers at different wavelengths. The anisotropy parameters betamax(3P) = 0.8 and betamax(1D) = 1.9 indicate that these channels are preferentially populated at different geometries and the lifetime is very short.