Coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain‐surface impact process. Experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced, including a diminution of the sub‐saltation layer mean wind speed, as the friction speed increases. In this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free‐stream shear stresses, and the grain mass flux increases with friction speed uf* less rapidly than uf3.

223Boundary conditions are developed for rapid granular flows in which the rheology is dominated by grain-grain collisions. These conditions are v 0 = constdv 0 fdy and u 0 = constdu 0 /dy, where v and u are the thermal (fluctuation) and flow velocities respectively, and the subscript indicates that these quantities and their derivatives are to be evaluated at the wall. These boundary conditions are derived from the nature of individual grain-wall collisions, so that the proportionality constants involve the appropriate coefficient of restitution ew for the thermal velocity equation, and the fraction of diffuse (i.e. non-specular) collisions in the case of the flow-velocity equation. Direct application of these boundary conditions to the problem of Couette-flow shows that as long as the channel width h is very large compared with a grain diameter d it is permissible to set v = 0 at the wall and to adopt the no-slip condition. Exceptions occur where dfh is not very small, when the wall is not rough, and when the grain-wall collisions are very elastic. Similar insight into other flows can be obtained qualitatively by a dimensional analysis treatment of the boundary conditions. Finally, the more difficult problem of self-bounding fluids is discussed qualitatively.

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The relative cross sections for the 2N + TT-2iV reactions were determined where the initial two nucleons are in a relative s state, and coupled either to isospin zero or to one. This information is extracted from two-nucleon coincidence measurements of the (7r + , 2p) and the (7r" f pn) reactions on targets of 3 He and 4 He.PACS numbers: 25.80.+f Pion absorption is usually treated as proceeding through the n +2iV-2iV process. Considerable effort 1 has been devoted to understanding the TT + +d -* pp reaction, in which absorption takes place on a T = 0, S =1 nucleon pair. At bombarding energies in the (3,3) resonance region this reaction is believed to be dominated by the intermediate AN configuration with (J 7r ) AAr =2 + and AAT orbital angular momentum 1,^=0. Much less consideration has been given to the analogous process in which the pion is absorbed by two nucleons in a T =1, S =0 state. Such nucleon pairs are present in all nuclei heavier than the deuteron, and understanding absorption on such pairs is important to the understanding of nuclear pion absorption in general. The inverse of this process certainly does play a role in pion production {N A +N B -N c +N D +TT) and is part of the cross sections designated as a ±1 and a 01 in the traditional parametrization. 2 In pion production, however, several partial waves may contribute, and there is no simple way to select the piece of the cross section in which the (N C N D ) pair is restricted to J ,r = 0 + , T =1.If one assumes A dominance in the absorption process then it follows that the reaction TT + 2N -2N can only go through a T = 1 final state. If only isospin is considered, absorption on an initially T =0 nucleon pair is twice as likely as that on an initially T =1 pair. 3 It should be noted, however, that absorption on an initial J 7r = 0 + , T = 1 pair involves intermediate states that are different from the T = 0 case, as may be seen in Table I. In particular, L^^O, which dominates absorption on a deuteron, is not allowed in the T =1 case. Thus more detailed dynamical con-siderations may very well enter in this ratio.In the present work we report on coincidence measurements of the ir + 2N -* 2N process with targets of 4 He and 3 He. We assume 4 He and 3 He to have simple wave functions with all particles in relative s states, so that a comparison of the (TT + 9 2p) and the (n~ ,pn) reactions will be simply related to the isospin cross sections; the former must start with an initial pair of (np) nucleons that could be in either aT=loraT=0 state, while the latter must start with the initial pair of T =1 protons.The experiment was performed with 165-MeV pion beams from the P 3 channel of the Los Alamos Meson Physics Facility. Cryogenic liquid targets were used, with thicknesses of 343 and 629 mg/ cm 2 for 3 He and 4 He, respectively. 4 Protons were detected with a magnetic spectrometer, 5 with a momentum acceptance Ap/p =± 25% and solid angle of 8 msr. Coincident protons and neutrons were TABLE I. Quantum numbers permitted in the A model for n + (2N) {~*...