Wind fluctuations affect the mean behaviour of naturally ventilated systems

We study the dynamics of a naturally ventilated room in which a point source provides a steady source of buoyancy and which is affected by an opposing unsteady wind. The wind is modelled as a stochastic forcing, which aims at simulating realistic velocity fluctuations as observed in the lower atmosphere. Our main finding is the occurrence of a "noise-induced transition", namely a structural change of the mean behaviour of the system: the warm-cold air interface does not fluctuate around the elevation exhibited when wind is constant, but oscillations occur around a new (significantly lower) interface elevation. We provide the physical explanation for such a counter-intuitive behaviour and show its dependence on (i) wind characteristics (intensity and timescale of fluctuations) and (ii) relative strength of wind over thermal loads. A realistic example case shows that the behaviour highlighted here has potentially major implications in the design and management of naturally ventilated buildings

Noise-driven cooperative dynamics between vegetation and topography in riparian zones

Riparian ecosystems exhibit complex biotic and abiotic dynamics, where the triad vegetation-sediments-stream determines the ecogeomorphological features of the river landscape. Random fluctuations of the water stage are a key trait of this triad, and a number of behaviors of the fluvial environment can be understood only taking into consideration the role of noise. In order to elucidate how randomness shape riparian transects, a stochastic model that takes into account the main links between vegetation, sediments, and the stream is adopted, emphasizing the capability of vegetation to alter the plot topography. A minimalistic approach is pursued, and the probability density function of vegetation biomass is analytically evaluated in any transect plot. This probability density function strongly depends on the vegetation-topography feedback. We demonstrate how the vegetation-induced modifications of the bed topography create more suitable conditions for the survival of vegetation in a stochastically dominated environment

River bedform inception by flow unsteadiness: a modal and nonmodal analysis

River bedforms arise as a result of morphological instabilities of the stream-sediment interface. Dunes and antidunes constitute the most typical patterns, and their occurrence and dynamics are relevant for a number of engineering and environmental applications. Although flow variability is a typical feature of all rivers, the bedform-triggering morphological instabilities have generally been studied under the assumption of a constant flow rate. In order to partially address this shortcoming, we here discuss the influence of (periodic) flow unsteadiness on bedform inception. To this end, our recent one-dimensional validated model coupling Dressler's equations with a refined mechanistic sediment transport formulation is adopted, and both the asymptotic and transient dynamics are investigated by modal and nonmodal analyses

A shallow-water theory of river bedforms in supercritical conditions

A supercritical free-surface turbulent stream flowing over an erodible bottom can generate a characteristic pattern of upstream migrating bedforms known as antidunes. This morphological instability, which is quite common in fluvial environments, has attracted speculative and applicative interests, and has always been modelled in 2D or 3D mathematical frameworks. However, in this work we demonstrate that antidune instability can be described by means of a suitable one-dimensional model that couples the Dressler equations to a mechanistic model of the sediment particle deposition/entrainment. The results of the linear stability analysis match the experimental data very well, both for the instability region and the dominant wavelength. The analytical tractability of the 1D modeling allows us (1) to elucidate the key physical processes which drive antidune instability, (2) to show the secondary role played by sediment inertia, (3) to obtain the dispersion relation in explicit form, and (4) to demonstrate the absolute nature of antidune instabilit

Noise‐driven cooperative dynamics between vegetation and topography in riparian zones

Riparian ecosystems exhibit complex biotic and abiotic dynamics, where the triad vegetation-sediments-stream determines the ecogeomorphological features of the river landscape. Random fluctuations of the water stage are a key trait of this triad, and a number of behaviors of the fluvial environment can be understood only taking into consideration the role of noise. In order to elucidate how randomness shape riparian transects, a stochastic model that takes into account the main links between vegetation, sediments, and the stream is adopted, emphasizing the capability of vegetation to alter the plot topography. A minimalistic approach is pursued, and the probability density function of vegetation biomass is analytically evaluated in any transect plot. This probability density function strongly depends on the vegetation-topography feedback. We demonstrate how the vegetation-induced modifications of the bed topography create more suitable conditions for the survival of vegetation in a stochastically dominated environmen

Effect of sampling time in the laboratory investigation of braided rivers

We focus on the measurement of the bed-elevation of braided networks in flume experiments. In particular, the effect of the survey frequency on the measurement accuracy is studied. To this aim, an innovative measurement system is adopted. It consists of a laser-ultrasonic sensor and can survey the bed-elevation under flowing water. This measurement system was used to profile a flume transect with a frequency of 4 min, without stopping the water discharge. By this technique, the topography of a single transect was continuously acquired during the evolution of a braided river model. Twelve braided rivers generated with different experimental conditions were studied. The main results are (i) there exists a threshold survey frequency (4-8 min in our analysis) which guarantees that the morphological evolution of the braiding channels is fully measured; (ii) if this threshold frequency of survey is exceeded, significant errors occur in the balance of the eroded/deposited sediments and in the evaluation of the bed-elevation dynamics; and (iii) these errors depend on the river stream power

Recovery times of riparian vegetation

Riparian vegetation is a key element in a number of processes that determine the ecogeomorphological features of the river landscape. Depending on the river water stage fluctuations, vegetation biomass randomly switches between growth and degradation phases and exhibits relevant temporal variations. A full understanding of vegetation dynamics is therefore only possible if the hydrological stochastic forcing is considered. In this vein, we focus on the recovery time of vegetation, namely the typical time taken by vegetation to recover a well-developed state starting from a low biomass value (induced, for instance, by an intense flood). The analytical expression of the plot-dependent recovery time is given, the role of hydrological and biological parameters is discussed, and the impact of river-induced randomness is highlighted. Finally, the effect of man-induced hydrological changes (e.g., river damming or climate changes) is explore