1,721,155 research outputs found
A phenomenological model to describe turbulent friction in permeable-wall flows
Full text linkDescribing the canonical properties of turbulent flows over rough-permeable walls such as gravel beds, vegetatedor snow-covered surfaces have, to date, resisted complete theoretical treatment. The major complication in describing such geophysical flows is that the friction factor - Reynolds number relationships significantly deviate from their conventional Nikuradse curves or Moody diagrams derived over impermeable rough boundaries. A novel phenomenological model that describes such anomalous behavior is proposed. It expands the approach in Gioia and Chakraborty (2006) developed for rough-impermeable pipes to include finite velocity effects within the porous wall and canonical length scales governing the momentum exchanges between interstitial and superficial flows. Citation: Manes, C., L. Ridolfi, and G. Katul (2012), A phenomenological model to describe turbulent friction in permeable-wall flow
A multiphase multicomponent flow and transport model for liquid aerosol filtration in coalescing fibrous filters
No full textPore-scale CFD simulations of liquid aerosol filtration in coalescing fibrous filters are currently limited by severe computational constraints. In this paper, we present a new multiphase multicomponent framework for modeling such dynamic filtration process at the Darcy-scale. Moving from the pore- to the Darcy-scale allows for significant reduction in computational efforts with limited repercussion on the representation of the filter geometry. In our model, two phases coexist within the filter, namely the injected oil-mist and the coalesced oil phase. The oil-mist phase consists of a mixture of a gas phase and oil droplets of different sizes. The governing equations are the standard ones of Darcy flow and transport in porous media, where the filtration processes, such as droplet capture and drainage, are accounted for through ad hoc mass exchange terms, which are coherent with the Jump & Channel model by Kampa et al. (2014). The equations are strongly non-linear and are solved through the IMplicit Pressure Explicit Saturation (IMPES) algorithm. Cell-centered finite volumes are used for discretization in space. The model is validated by replicating experiments of oil-mist filtration available in the literature for both wettable and non-wettable media, as well as for combinations of media with different wettability. Simulations are then performed for a variety of operational conditions, demonstrating both the accuracy and robustness of our implementation
State Observers for Nonlinear Systems with Smooth/Bounded input
No full textIt is known that for nonlinear systems the drift-observability property (i.e. observability for zero input) is not sufficient
to guarantee the existence of an asymptotic observer for any input. Many authors studied conditions on systems
structure that ensure uniform observability (i.e. observability for any input). Conditions are available that define
restrict classes of uniformly observable systems. This work considers the problem of state observation with exponential
error rate for smooth nonlinear systems that do not meet conditions of uniform observability: conditions are given
on the input, instead of on the system structure. It is shown that drift-observability, together with a smoothness/
boundedness condition on the input, is sufficient to ensure the existence of an exponential observer. Three types of
observers are presented, that can be constructed under drift-observability assumption. One works well for systems
with maximal relative degree or in the case of input sufficiently small. A second type of observer succeeds for
systems with any relative degree in the case of input sufficiently smooth. The input derivatives up to a certain order
are required for its implementation. Both observers ensure exponential convergence of exponential error to zero. A
third observer suitable in the case of smooth input does not require input derivatives, and ensures exponential decay of
the observation error below a prescribed level. Computer simulations close the paper
On the interplay between pressure and gravitational forces in coalescing filters
No full textThis paper provides insights into processes governing oil-mist filtration in coalescing fillers. In particular, it resolves an apparent inconsistency between different published studies regarding the occurrence (or not) of internal gravity-induced flows. As a result of this, it also clarifies whether in industrially-relevant scenarios such as those pertaining to vertical filter cartridges, non-homogeneous vertical saturation patterns are triggered by these internal flows or are just a result of different oil loading rates. To address these issues, we use Eulerian-Lagrangian CFD simulations, which properly account for the effects of turbulent diffusion of liquid aerosol particles, to replicate an experimental setup available in the literature. In order to interpret results data, we introduce a new dimensionless number, termed S-M, defined as the ratio between pressure gradient and gravitational forces, which provides a bulk characterization of the flow and allows to assess whether internal gravity-induced flows, in a given cartridge-oil system, should be expected or not. We show that S-M explains well the few experimental data available in the literature and identifies specific behaviors associated with limiting S-M values being either very large or close to 1
Delay-independent conditions of exponential ISS for linear time-varying delay differential systems
No full textThis work focuses on linear continuous-time systems with time-varying delays, in the general case of time-varying matrices. Initially, we explore the case of positive systems within this class and introduce two distinct delay-independent conditions of exponential input-to-state stability. Moreover, we provide guaranteed exponential convergence rates for such conditions, explicitly stating the ISS gains. Then, we extend our analysis to systems without sign constraints, adopting a state-bounding approach that takes advantage of the properties of positive systems. Due to the time-varying nature of the systems, all proposed conditions are in terms of an infinite number of inequalities. Hence, implementation issues are discussed and significant special cases in which the conditions can be cast into a linear programming problem of finite dimension are presented
Some Remarks on the Stability of Time-Varying Discrete-Time Positive Delay Systems
No full textThis paper illustrates some remarkable properties of linear time-varying discrete-time positive systems with delays. First, we discuss how a well known property of positive delay systems with time-invariant matrices, namely the equivalence among delay-dependent and delay-independent stability, does not generalize to positive delay systems when the system matrices are time-varying. Then, we illustrate how a stability analysis based on the existence of linear co-positive Lyapunov functions on the zero-delay system and on its dual produces a remarkable dissimilarity to what happens in the time-invariant case: the dual condition is sufficient to prove delay-independent stability, whereas the primal is not even a stability condition. Implications and further results are discussed
An Enhanced Observer for Nonlinear Systems with Time-Varying Measurement Delays
No full textIn this article, we propose an observer for a class of Lipschitz nonlinear systems affected by time-varying and known measurement delays, which is an improvement of the one presented by Cacace et al., 2014. Under the assumption that the delay function is piecewise continuous and differentiable, we prove that exponential convergence to zero of the observation error can be achieved with any desired decay rate, by suitably tuning a gain vector. The delay bound achieved with the observer proposed here is less conservative than the one obtained by Cacace et al., 2014, as confirmed by numerical tests. For the sake of brevity, in this article, only one-step observers are considered. However, a cascade observer can be arranged to cope with arbitrarily long delays
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