How Costly is it for Poor Farmers to Lift Themselves out of Subsistence?

The main objective of this paper is to provide estimates of the cost of moving out of subsistence for Madagascar's farmers. The analysis is based on a simple asset-return model of occupational choice. Estimates suggest that the entry (sunk) cost associated with moving out of subsistence can be quite large|somewhere between 124 and 153 percent of a subsistence farmer's annual production. Our results make it possible to identify farm characteristics likely to generatee large gains if moved out of subsistence, yielding useful information for the targeting of trade-adjustment assistance programs

Unstable wake dynamics of rectangular flat-backed bluff bodies with inclination and ground proximity

© 2018 Cambridge University Press. The paper investigates experimentally the global wake dynamics of a simplified three-dimensional ground vehicle at a Reynolds number of . The after-body has a blunt rectangular trailing edge leading to a massive flow separation. Both the inclination (yaw and pitch angles) and the distance to the ground (ground clearance) are accurately adjustable. Two different aspect ratios of the rectangular base are considered; wider than it is tall (minor axis perpendicular to the ground) and taller than it is wide (major axis perpendicular to the ground). Measurements of the spatial distribution of the pressure at the base and velocity fields in the wake are used as topological indicators of the flow. Sensitivity analyses of the base pressure gradient expressed in polar form (modulus and phase) varying ground clearance, yaw and pitch are performed. Above a critical ground clearance and whatever the inclination is, the modulus is always found to be large due to the permanent static symmetry-breaking instability, and slightly smaller when aligned with the minor axis of the base rather than when aligned with the major axis. The instability can be characterized with a unique wake mode, quantified by this modulus (asymmetry strength) and a phase (wake orientation) which is the key ingredient of the global wake dynamics. An additional deep rear cavity that suppresses the static instability allows a basic flow to be characterized. It is shown that both the inclination and the ground clearance constrain the phase dynamics of the unstable wake in such way that the component of the pressure gradient aligned with the minor axis of the rectangular base equals that of the basic flow. Meanwhile, the other component related to the major axis adjusts to preserve the large modulus imposed by the instability. In most cases, the dynamics explores only two possible opposite values of the component along the major axis. Their respective probability depends on the geometrical environment of the wake: base shape, body inclination, ground proximity and body supports. An expression for the lateral force coefficients taking into account the wake instability is proposed

Boat-tail effects on the global wake dynamics of a flat-backed body with rectangular section

© 2019 A three-dimensional body of rectangular section with a blunt trailing edge is studied for various angles of after-body boat-tailing. The Reynolds number of the flow is Re=4.0×105 and the body is in the vicinity of the ground. It is found that the wake dynamics is strongly dependent on the coupling of top and bottom boat-tail angles, triggering either long-time bi-stability between two mirror static asymmetric states or an anti-symmetric periodic mode, both leading to large cross-flow force fluctuations. The transition is shown to depend on the aspect ratio of the vertical base and not on the flow orientations imposed by the coupling of angles. Within the limitation of the aspect ratio for which the static states persist and dominate the wake dynamics, as boat-tails are installed at the after-body, they generate a vertical base pressure gradient component on which the static states adapt according to the mechanism proposed by Bonnavion and Cadot (2018)

A numerical investigation of the asymmetric wake mode of a squareback Ahmed body - effect of a base cavity

Numerical simulations of the turbulent flow over the flat backed Ahmed model at Reynolds number $Re\simeq 4\times 10^{5}$ are conducted using a lattice Boltzmann solver to clarify the mean topology of the static symmetry-breaking mode of the wake. It is shown that the recirculation region is occupied by a skewed low pressure torus, whose part closest to the body is responsible for an extra low pressure imprint on the base. Shedding of one-sided vortex loops is also reported, indicating global quasi-periodic dynamics in conformity with the seminal work of Grandemange et al. (J. Fluid Mech., vol. 722, 2013, pp. 51–84). Despite the limited low frequency resolution of the simulation, power spectra of the lateral velocity fluctuations at different locations corroborate the presence of this quasi-periodic mode at a Strouhal number of $St=0.16\pm 0.03$. A shallow base cavity of $5\,\%$ of the body height reduces the drag coefficient by $3\,\%$ but keeps the recirculating torus and its interaction with the base mostly unchanged. The drag reduction lies in a global constant positive shift of the base pressure distribution. For a deep base cavity of $33\,\%$ of the body height, a drag reduction of $9.5\,\%$ is obtained. It is accompanied by a large elongation of the recirculation inside the cavity that considerably attenuates the low pressure sources therein together with a symmetrization of the low pressure torus. The global quasi-periodic mode is found to be inhibited by the cavity.</jats:p

On the three-dimensional temporal spectrum of stretched vortices

The three-dimensional stability problem of a stretched stationary vortex is addressed in this letter. More specifically, we prove that the discrete part of the temporal spectrum is only associated with two-dimensional perturbations.Comment: 4 pages, RevTeX, submitted to PR

Do food scares explain supplier concentration? An analysis of EU agri-food imports

We investigate how rising sanitary risk of agri-food products affects the geographical concentration of European Union (EU) imports at the product level. We first estimate a product-specific measure of sanitary risk based on the count of food alerts at EU borders. Then we regress the evolution of geographical concentration indices on our measure of product risk and year. We find that product sanitary risk indeed affected the EU import pattern. Overall, the EU diversified its import sources, but with diversification at the extensive margin and concentration at the intensive margin. This pattern is stronger for risky products, leading to a two-tier system

Towards adaptive drag reduction of a flat-back 3D bluff body in variable pitch and crosswind by flow orientation at the base separation

AbstractThe sensitivity of the drag to the rear design of a flat-back body is investigated under different body attitudes defined by the pitch ($$-1.5^\circ , 0^\circ , +1.5^\circ$$ - 1 . 5 ∘ , 0 ∘ , + 1 . 5 ∘ ) and yaw (up to $$15^\circ$$ 15 ∘ ). The rear design consists of taper angles at the top and bottom trailing edge varying from $$0^\circ$$ 0 ∘ (no taper) to $$12.5^\circ$$ 12 . 5 ∘ . Compared to the fixed optimal rear design that minimizes drag at the wind-aligned body attitude, the rear design adaptation to the change of attitude produces a noticeable drag reduction up to 5% depending on the pitch angle within a yaw range smaller than $$2^\circ$$ 2 ∘ . It is shown that this drag reduction is related to the vertical wake steady instability interfering with the rear design. For yaw larger than $$2^\circ$$ 2 ∘ and up to $$12^\circ$$ 12 ∘ , an almost constant drag reduction of 2% is found and shown to be a compromise between a beneficial pressure recovery on the flat base and a detrimental pressure drag on the tapers. At larger yaw angles and whatever the pitch angle is, there is no compromise anymore such that any taper angle different from $$0^\circ$$ 0 ∘ produces a drag increase leading eventually to the squareback rear design as the optimal design. Overall, the study emphasizes the potential of adaptive control of the top and bottom trailing edge tapers to arbitrary body attitude even at small yaw angles when the pitch is varied. </jats:p

Asymmetry and global instability of real minivans' wake

© 2018 Elsevier Ltd Three minivan cars tested in real flow conditions are investigated through base pressure distribution, force balance and velocity measurements. Discontinuous transitions of permanent wake reversals and bistable dynamics between two well-defined states are observed varying ground clearance, pitch and yaw, with open and closed air-intake. These transitions prove that the cars undergo the same global z-instability as the square-back Ahmed body (M. Grandemange, M. Gohlke and O. Cadot, Physics of Fluids, 25, 2013). The contribution of the global instability to the lift coefficient is estimated to 0.012 and 0.024 depending on the wake state. Eventually, the potential of direct passive control of the z-instability is demonstrated by improving from 4.6% to 8.3% the drag reduction obtained by closing the front air-intake of the car