arXiv subjects

Rory T. Cerbus

Publications and source records attributed to Rory T. Cerbus.

7 recordsLinked to original sources

arXiv2024

Completing Moody's friction diagram in the turbulent transitional regime

The Moody diagram, a plot of friction factor versus flow rate, is a well-known engineering tool for estimating head loss in pipe flows. It comprises well-defined relationships between friction factor and flow rate over the majority of parameter space, but there is a gap in the transitional regime between laminar and turbulent flows. It is often left hatched because in this parameter range the friction is deemed indefinite, which Moody remarked could at least partially be due to the different initial conditions used to establish the flow. Here we investigate this issue and seek a systematic dependence for friction in the transitional regime. The novel method we use is to approach the transitional regime from above by reducing the flow speed from a turbulent flow state. We find that in different pipe flow setups, both driven by gravity, a single curve corresponding to a maximum density of the transitional flow structures is found. We test the generality of this result using an alternative method to drive the flow through the pipe, using a mass displacement device. Our investigation of the flow driven by a syringe produces yet a different curve, indicating that the method of driving the flow has a significant impact on both the final states and the paths to them in the transitional regime.

physics.flu-dyn↗

arXiv2024

Air drag controls the runout of small laboratory landslides

Laboratory granular landslides are smaller-scale, simplified, yet well-controlled versions of larger and often tragic natural landslides. Using systematic experiments and scaling analysis, we quantify the influence of grain size, fall height, and landslide volume on runout distance. We also determine the minimum landslide size required to observe this scaling, which we find is set by a combination of air drag, grain size, and fall height.

physics.flu-dyn↗

arXiv2024

A granular scaling approach to landslide runout

A main objective in landslide research is to predict how far they will travel. Landslides are complex, and a complete understanding in principle requires accounting for numerous parameters. Here we engender a simplification by investigating the maximum landslide runout using granular laboratory experiments and a scaling analysis. We find that correctly accounting for the fall height and grain size distribution not only yields an improved correlation of normalized runout, but also quantitatively unites laboratory and field data. In particular, we find that the mobility of landslides increases with the square root of the fall height and with the skewness of the grain size distribution.

physics.flu-dyn↗

arXiv2022

Prandtl-Tietjens intermittency in transitional pipe flows

Pipe flow often traverses a regime where laminar and turbulent flow co-exist. Prandtl and Tietjens explained this intermittency as a feedback between the fluctuations of the internal flow resistance and the constant pressure drop driving the flow. However, because the focus has moved towards studying intermittency without flow fluctuations near the universal critical Reynolds number, their explanation has largely disappeared. Here we refine the mechanism, which has never been put to a quantitative test, to develop a model that agrees with experiments at higher Reynolds numbers, enabling us to demonstrate that Prandtl and Tietjens' mechanism is, in fact, intrinsic to flows where both the pressure gradient and perturbation are constant.

physics.flu-dyn↗

arXiv2017

Kolmogorovian turbulence in transitional pipe flows

As everyone knows who has opened a kitchen faucet, pipe flow is laminar at low flow velocities and turbulent at high flow velocities. At intermediate velocities there is a transition wherein plugs of laminar flow alternate along the pipe with "flashes" of a type of fluctuating, non-laminar flow which remains poorly known. We show experimentally that the fluid friction of flash flow is diagnostic of turbulence. We also show that the statistics of flash flow are in keeping with Kolmogorov's phenomenological theory of turbulence (so that, e.g., the energy spectra of both flash flow and turbulent flow satisfy small-scale universality). We conclude that transitional pipe flows are two- phase flows in which one phase is laminar and the other, carried by flashes, is turbulent in the sense of Kolmogorov.

physics.flu-dyn↗

arXiv2016

Janus spectra in two-dimensional flows

In theory, large-scale atmospheric flows, soap-film flows and other two-dimensional flows may host two distinct types of turbulent energy spectra---in one, $α$, the spectral exponent of velocity fluctuations, equals $3$ and the fluctuations are dissipated at the small scales, and in the other, $α=5/3$ and the fluctuations are dissipated at the large scales---but measurements downstream of obstacles have invariably revealed $α= 3$. Here we report experiments on soap-film flows where downstream of obstacles there exists a sizable interval in which $α$ has transitioned from $3$ to $5/3$ for the streamwise fluctuations but remains equal to $3$ for the transverse fluctuations, as if two mutually independent turbulent fields of disparate dynamics were concurrently active within the flow. This species of turbulent energy spectra, which we term the Janus spectra, has never been observed or predicted theoretically. Our results may open up new vistas in the study of turbulence and geophysical flows.

physics.flu-dyn↗

arXiv2016

Information theory demonstration of the Richardson cascade

Turbulence theory is usually concerned with the statistical moments of the velocity or its fluctuations. One could also analyze the implicit probability distributions. This is the purview of information theory. Here we use information theory, specifically the conditional entropy, to analyze (quasi-)2D turbulence. We recast Richardson's "eddy hypothesis" that large eddies break up into small eddies in time in the language of information theory. In addition to confirming Richardson's idea, we find that self-similarity and turbulent length scales reappear naturally. Not surprisingly, we also find that the direction of information transfer is the same as the direction of the cascade itself. Consequently, intermittency may be considered a necessary companion to all turbulent flows.

nlin.CD↗