We analyse the currently popular vortex identification criteria that are based on pointwise analysis of the velocity gradient tensor. A new measure of spiralling compactness of material orbits in vortices is introduced and using this measure a new local vortex identification criterion and requirements for a vortex core are proposed. The interrelationships between the different criteria are explored analytically and in a few flow examples, using both zero and non-zero thresholds for the identification parameter. These interrelationships provide a new interpretation of the various criteria in terms of the local flow kinematics. A canonical turbulent flow example is studied, and it is observed that all the criteria, given the proposed usage of threshold, result in remarkably similar looking vortical structures. A unified interpretation based on local flow kinematics is offered for when similarity or differences can be expected in the vortical structures educed using the different criteria.

The classical experiments on turbulent friction in rough pipes were performed by Nikuradse in the 1930s. Seventy years later, they continue to defy theory. Here we model Nikuradse's experiments using the phenomenological theory of Kolmogórov, a theory that is widely thought to be applicable only to highly idealized flows. Our results include both the empirical scalings of Blasius and Strickler and are otherwise in minute qualitative agreement with the experiments; they suggest that the phenomenological theory may be relevant to other flows of practical interest; and they unveil the existence of close ties between two milestones of experimental and theoretical turbulence.

It has long been surmised that the mean-velocity profile (MVP) of pipe flows is closely related to the spectrum of turbulent energy. Here we perform a spectral analysis to identify the eddies that dominate the production of shear stress via momentum transfer. This analysis allows us to express the MVP as a functional of the spectrum. Each part of the MVP relates to a specific spectral range: the buffer layer to the dissipative range, the log layer to the inertial range, and the wake to the energetic range. The parameters of the spectrum set the thickness of the viscous layer, the amplitude of the buffer layer, and the amplitude of the wake.

When a hurricane strikes land, the destruction of life and property is largely confined to a narrow coastal area. This is because hurricanes are fueled by the moisture from the ocean, [1][2][3] with the implication that hurricane intensity decays rapidly after striking land. 4,5 In contrast to the effect of a warming climate on hurricane intensification, many aspects of which are fairly well understood, 6-10 little is known of the corresponding effect on hurricane decay.Here we analyze intensity data for North Atlantic landfalling hurricanes 11 over the past 50 years and show that hurricanes decay has slowed, in direct proportion to a contemporaneous rise in the sea-surface temperature. 12 Thus, in the late 1960s, a typical hurricane lost ∼75% of its intensity in the first day past landfall; now, the corresponding decay is only ∼50%. We also show, using computational simulations, that warmer sea surface temperatures induce a slower decay by increasing the stock of moisture which a hurricane carries as it hits land. This 'storm moisture' constitutes a source of heat that is not considered in theoretical models of decay. [13][14][15] Additionally, we show that climate-modulated changes in hurricane tracks 16,17 1 contribute to the increasingly slow decay. Our findings suggest that as the world continues to warm, the destructive power of hurricanes will extend progressively farther inland.Hurricanes thrive on moisture. Moisture from warm tropical oceans fuels the intense winds of a hurricane heat engine. 2,3 In a warming world, the moisture supply is enhanced. Warmer oceans supply more moisture, and warmer air, owing to the Clausius-Clapeyron relation, 18 holds more moisture. As a result, we expect that the maximum intensity a hurricane can achieve over its lifetime increases. 6,9 Indeed, as the world warms, the strongest hurricanes (which, compared with the weaker ones, are less affected by impeding factors, e.g., wind shear) are getting stronger, with the most pronounced intensification seen for the North Atlantic hurricanes. 8 Without moisture, hurricanes wither. A landfall severs 1, 19, 20 a hurricane from the ocean, its moisture source. Consequently, the intensity decays rapidly. (When the intensity drops below 33 m/s, the hurricane, per the Saffir-Simpson scale, 21 is termed a tropical storm; however, for simplicity, we refer to tropical storms also as hurricanes.) How might the hurricane decay rates change in a warming world? In contrast to the extensive studies of hurricanes over ocean, this question has attracted scant attention. Decay timescale, τWe study North Atlantic landfalling hurricanes (Fig. 1a) over 1967-2018 using the best-track database "Atlantic HURDAT2", 11 widely considered the most reliable database amongst all the ocean basins. (See Methods for further discussion on the data.) For each hurricane, we analyze the intensity, V , during the first day past landfall, the period over which the hurricane inflicts most

Two aspects of turbulent flows have been the subject of extensive, split research efforts: macroscopic properties, such as the frictional drag experienced by a flow past a wall, and the turbulent spectrum. The turbulent spectrum may be said to represent the fabric of a turbulent state; in practice it is a power law of exponent \alpha (the "spectral exponent") that gives the revolving velocity of a turbulent fluctuation (or "eddy") of size s as a function of s. The link, if any, between macroscopic properties and the turbulent spectrum remains missing. Might it be found by contrasting the frictional drag in flows with differing types of spectra? Here we perform unprecedented measurements of the frictional drag in soap-film flows, where the spectral exponent \alpha = 3 and compare the results with the frictional drag in pipe flows, where the spectral exponent \alpha = 5/3. For moderate values of the Reynolds number Re (a measure of the strength of the turbulence), we find that in soap-film flows the frictional drag scales as Re^{-1/2}, whereas in pipe flows the frictional drag scales as Re^{-1/4} . Each of these scalings may be predicted from the attendant value of \alpha by using a new theory, in which the frictional drag is explicitly linked to the turbulent spectrum. Our work indicates that in turbulence, as in continuous phase transitions, macroscopic properties are governed by the spectral structure of the fluctuations.Comment: 6 pages, 3 figure