Models of Fractal River Basins

Two distinct models for self-similar and self-affine river basins are numerically investigated. They yield fractal aggregation patterns following non-trivial power laws in experimentally relevant distributions. Previous numerical estimates on the critical exponents, when existing, are confirmed and superseded. A physical motivation for both models in the present framework is also discussed.Comment: 16 pages, latex, 9 figures included using uufiles command (for any problem: [email protected]), to be publishes in J. Stat. Phys. (1998

Cellular Models for River Networks

A cellular model introduced for the evolution of the fluvial landscape is revisited using extensive numerical and scaling analyses. The basic network shapes and their recurrence especially in the aggregation structure are then addressed. The roles of boundary and initial conditions are carefully analyzed as well as the key effect of quenched disorder embedded in random pinning of the landscape surface. It is found that the above features strongly affect the scaling behavior of key morphological quantities. In particular, we conclude that randomly pinned regions (whose structural disorder bears much physical meaning mimicking uneven landscape-forming rainfall events, geological diversity or heterogeneity in surficial properties like vegetation, soil cover or type) play a key role for the robust emergence of aggregation patterns bearing much resemblance to real river networks.Comment: 7 pages, revtex style, 14 figure

Electromagnetic Forming of Longitudinal Strengthening Ribs in Roll Formed Automotive Profiles

In the automotive industry, increasing ecological concerns and demands for higher performance have become lightweight construction a key aspect. Due to the gradual introduction of high strength materials on the one side, and greater consideration with regard to continuous manufacturing technologies on the other side, it is possible nowadays to address the demands that structural and complex automotive parts have to face, from the standpoint of lightweight manufacturing. Thickness, shape and impact conditions constitute the main aspects to consider for such parts and shape conditions in particular require from complex, costly and lengthy procedures, especially when discontinuous forming operations such as stamping and hydro forming procedures are selected. However, continuous forming operations like Roll Forming (RF) can prove to be advantageous and suited for scalable parts (e.g in length) and at the same time be economically reasonable. RF lines as well generally incorporate additional installations to perform multiple forming operations destined to imprint strengthening ribs perform punching operations or weld certain parts. It is in this context where the usefulness of the electromagnetic forming technology for completion of auxiliary operations can be proven, given its flexibility and reasonable investment costs. Electromagnetic forming (EMF) is a contact-free technique where large forces can be imparted to a conductive metallic workpiece by a pure electromagnetic interaction. The produced electromagnetic pressure can produce stresses in the workpiece that are several times larger than the material flow stress. Ultimately this can cause the workpiece to deform plastically and to be accelerated achieving high velocities. Once the velocity is imparted to the workpiece, the shape can be developed either by free or die forming. The work described in this paper explores the potential of the EMF process to adequately form shallow longitudinal ribs or stiffeners in components previously formed utilizing an innovative concept called Flexible Roll Forming, developed at The Technical University of Darmstadt in Germany, by means of magnetic pulse forming, maintaining the integrity of the workpiece while trying to meet industry standard tolerances. Profiles exhibiting hat-like cross sections made of AHSS steels were subjected to localized impulses in order to achieve strengthening features in the roll formed part. ZStE340 steel alloy profiles were first roll formed and then inserted in the EM forming installation designed for the occasion. A high strength copper alloy (Cr-Zr-Cu) was used as a conductor for the single turn coil, placed opposite to the sidewall in the moment of the energy delivery. Formed specimens were subsequently measured to account for existing dimensional deviations

Local minimal energy landscapes in river networks

The existence and stability of the universality class associated to local minimal energy landscapes is investigated. Using extensive numerical simulations, we first study the dependence on a parameter $\gamma$ of a partial differential equation which was proposed to describe the evolution of a rugged landscape toward a local minimum of the dissipated energy. We then compare the results with those obtained by an evolution scheme based on a variational principle (the optimal channel networks). It is found that both models yield qualitatively similar river patterns and similar dependence on $\gamma$. The aggregation mechanism is however strongly dependent on the value of $\gamma$. A careful analysis suggests that scaling behaviors may weakly depend both on $\gamma$ and on initial condition, but in all cases it is within observational data predictions. Consequences of our resultsComment: 12 pages, 13 figures, revtex+epsfig style, to appear in Phys. Rev. E (Nov. 2000

A transition from river networks to scale-free networks

A spatial network is constructed on a two dimensional space where the nodes are geometrical points located at randomly distributed positions which are labeled sequentially in increasing order of one of their co-ordinates. Starting with $N$ such points the network is grown by including them one by one according to the serial number into the growing network. The $t$-th point is attached to the $i$-th node of the network using the probability: $\pi_i(t) \sim k_i(t)\ell_{ti}^{\alpha}$ where $k_i(t)$ is the degree of the $i$-th node and $\ell_{ti}$ is the Euclidean distance between the points $t$ and $i$. Here $\alpha$ is a continuously tunable parameter and while for $\alpha=0$ one gets the simple Barab\'asi-Albert network, the case for $\alpha \to -\infty$ corresponds to the spatially continuous version of the well known Scheidegger's river network problem. The modulating parameter $\alpha$ is tuned to study the transition between the two different critical behaviors at a specific value $\alpha_c$ which we numerically estimate to be -2.Comment: 5 pages, 5 figur

Evaluating ecohydrological modelling framework to link atmospheric CO2 and stomatal conductance

The establishment of an accurate stomatal conductance (gs) model in responding to CO2 enrichment under diverse environmental conditions remains an important issue as gs is the key to understand the plant–water–atmosphere interactions. A better representation of gs is important to reduce uncertainties in predicting the climate change impacts on various ecosystem functions. In this study, we evaluated three most commonly used gs formulations for the estimation of the stomatal response to environmental factors using in situ measurements under different environmental conditions. The three gs models were Leuning's modified Ball–Berry model and two specific cases of the optimization models (i.e., Rubisco limitation model and RuBP regeneration limitation model). On the basis of an analysis of 234 data points obtained from experiments under instantaneous, semicontrolled, and the free‐air CO2 experiment conditions, we found that Leuning's modified Ball–Berry model and RuBP‐limited optimization model showed similar performance, and both performed better than Rubisco limitation model. Functional groups (e.g., C3 vs. C4 species) and life form (e.g., annual vs. perennial species) play an important role in determining the gs model performance and thus pose a challenge for gs predictions in mixed vegetation communities

Scale-free Networks from Optimal Design

A large number of complex networks, both natural and artificial, share the presence of highly heterogeneous, scale-free degree distributions. A few mechanisms for the emergence of such patterns have been suggested, optimization not being one of them. In this letter we present the first evidence for the emergence of scaling (and smallworldness) in software architecture graphs from a well-defined local optimization process. Although the rules that define the strategies involved in software engineering should lead to a tree-like structure, the final net is scale-free, perhaps reflecting the presence of conflicting constraints unavoidable in a multidimensional optimization process. The consequences for other complex networks are outlined.Comment: 6 pages, 2 figures. Submitted to Europhysics Letters. Additional material is available at http://complex.upc.es/~sergi/software.ht

Globally and Locally Minimal Weight Spanning Tree Networks

The competition between local and global driving forces is significant in a wide variety of naturally occurring branched networks. We have investigated the impact of a global minimization criterion versus a local one on the structure of spanning trees. To do so, we consider two spanning tree structures - the generalized minimal spanning tree (GMST) defined by Dror et al. [1] and an analogous structure based on the invasion percolation network, which we term the generalized invasive spanning tree or GIST. In general, these two structures represent extremes of global and local optimality, respectively. Structural characteristics are compared between the GMST and GIST for a fixed lattice. In addition, we demonstrate a method for creating a series of structures which enable one to span the range between these two extremes. Two structural characterizations, the occupied edge density (i.e., the fraction of edges in the graph that are included in the tree) and the tortuosity of the arcs in the trees, are shown to correlate well with the degree to which an intermediate structure resembles the GMST or GIST. Both characterizations are straightforward to determine from an image and are potentially useful tools in the analysis of the formation of network structures.Comment: 23 pages, 5 figures, 2 tables, typographical error correcte

Particle-hole symmetry in a sandpile model

In a sandpile model addition of a hole is defined as the removal of a grain from the sandpile. We show that hole avalanches can be defined very similar to particle avalanches. A combined particle-hole sandpile model is then defined where particle avalanches are created with probability $p$ and hole avalanches are created with the probability $1-p$. It is observed that the system is critical with respect to either particle or hole avalanches for all values of $p$ except at the symmetric point of $p_c=1/2$. However at $p_c$ the fluctuating mass density is having non-trivial correlations characterized by $1/f$ type of power spectrum.Comment: Four pages, our figure

The Fractal Properties of Internet

In this paper we show that the Internet web, from a user's perspective, manifests robust scaling properties of the type $P(n)\propto n^{-\tau}$ where n is the size of the basin connected to a given point, $P$ represents the density of probability of finding n points downhill and $\tau=1.9 \pm 0.1$ s a characteristic universal exponent. This scale-free structure is a result of the spontaneous growth of the web, but is not necessarily the optimal one for efficient transport. We introduce an appropriate figure of merit and suggest that a planning of few big links, acting as information highways, may noticeably increase the efficiency of the net without affecting its robustness.Comment: 6 pages,2 figures, epl style, to be published on Europhysics Letter