Shell Model for Drag Reduction with Polymer Additive in Homogeneous Turbulence

Recent direct numerical simulations of the FENE-P model of non-Newtonian hydrodynamics revealed that the phenomenon of drag reduction by polymer additives exists (albeit in reduced form) also in homogeneous turbulence. We introduce here a simple shell model for homogeneous viscoelastic flows that recaptures the essential observations of the full simulations. The simplicity of the shell model allows us to offer a transparent explanation of the main observations. It is shown that the mechanism for drag reduction operates mainly on the large scales. Understanding the mechanism allows us to predict how the amount of drag reduction depends of the various parameters in the model. The main conclusion is that drag reduction is not a universal phenomenon, it peaks in a window of parameters like Reynolds number and the relaxation rate of the polymer

A simple model for drag reduction

Direct Numerical Simulations established that the FENE-P model of viscoelastic flows exhibits the phenomenon of turbulent drag reduction which is caused in experiments by dilute polymeric additives. To gain analytic understanding of the phenomenon we introduce in this Letter a simple 1-dimensional model of the FENE-P equations. We demonstrate drag reduction in the simple model, and explain analytically the main observations which include (i) reduction of velocity gradients for fixed throughput and (ii) increase of throughput for fixed dissipation.Comment: submitted to PR

Drag Reduction by Polymers in Turbulent Channel Flows: Energy Redistribution Between Invariant Empirical Modes

We address the phenomenon of drag reduction by dilute polymeric additive to turbulent flows, using Direct Numerical Simulations (DNS) of the FENE-P model of viscoelastic flows. It had been amply demonstrated that these model equations reproduce the phenomenon, but the results of DNS were not analyzed so far with the goal of interpreting the phenomenon. In order to construct a useful framework for the understanding of drag reduction we initiate in this paper an investigation of the most important modes that are sustained in the viscoelastic and Newtonian turbulent flows respectively. The modes are obtained empirically using the Karhunen-Loeve decomposition, allowing us to compare the most energetic modes in the viscoelastic and Newtonian flows. The main finding of the present study is that the spatial profile of the most energetic modes is hardly changed between the two flows. What changes is the energy associated with these modes, and their relative ordering in the decreasing order from the most energetic to the least. Modes that are highly excited in one flow can be strongly suppressed in the other, and vice versa. This dramatic energy redistribution is an important clue to the mechanism of drag reduction as is proposed in this paper. In particular there is an enhancement of the energy containing modes in the viscoelastic flow compared to the Newtonian one; drag reduction is seen in the energy containing modes rather than the dissipative modes as proposed in some previous theories.Comment: 11 pages, 13 figures, included, PRE, submitted, REVTeX

IR varieties and their impact

Project number related to IDRC support could not be determine

A holistic approach to participatory crop improvement in wheat

The term "participatory crop improvement" is used to cover all aspects of crop improvement where farmers are involved in a participatory role. In this paper, we describe the approach and results for participatory crop improvement in wheat, in Luna Wada sub district, Gujarat, India. Nine villages were selected for study, and within each village, farmers were ranked into categories by wealth. An initial baseline survey on farming practices was conducted by semi structured interviews on a sample of farmers. Selected farmers from each wealth category kelp weekly farm calendars of all operations in their wheal fields. Selected fields were termed "intensive data plots." They provided a basis for analyzing Ihe farming system and profitability by wealth category, as well as for identifying constrains. The baseline surveys revealed that upper-category farmers benefitted most from the sale of wheat produce. The lower-category farmers consumed a large part of their produce. Intensive data plots showed that upper-category farmers accrued higher net gains from wheat cultivation than Ihe lower-category farmers. Participatory varietal selection (PVS) offered new varieties to farmers for selection. PVS resulted in significant replacement of the old variety Lok 1, grown in about 90% of the area, by many varieties that increased yield levels and on-farm biodiversity. Resource-poor farmers benefitted as much as the better-off farmers from PVS activities. Participating farmers experimented on a simple, cheap agronomic intervention: seed priming. Most farmers intended to adopt it because of its multiple beneficial effects, including increased yields. This holistic approach to participatory methods was effective in analyzing poverty issues, identifying constraints and new opportunities, and monitoring impact