No abstract

To gain understanding and insight of entrance-flow phenomena, we solve a fluid dynamics problem involving an oscillating flat velocity profile (with zero mean flow) at the end (entrance) of a semi-infinite, rigid tube. A successive-approximation method is used. Analytic solutions are given for the first-order (linear) approximation. Time averages of products of these first-order solutions for velocity components and their spatial derivatives are used to approximate the nonlinear terms in the second-order equations, which are solved numerically. The time-averaged second-order results demonstrate steady bidirectional streaming in the entrance region. Solutions are given to an illustrative problem with a Reynolds number of 20 and a Womersley unsteadiness parameter of (200)1/2. At this low Reynolds number, the streaming effects are small, with the magnitudes of the calculated streaming velocities less than three percent of the amplitude of the velocity specified at the entrance. The calculated magnitudes of streaming velocities would be expected to increase at higher Reynolds numbers; however, the convergence of the second-order solution would be less certain at higher Reynolds numbers. The steady streaming is related to first-order velocity variations that exist near the entrance of the tube.

The problem of whether the initiation of bacterial protein synthesis involves the obligatory formation of a 30S initiation complex intermediate was examined in a model system with N-acetylphenylalanyl-tRNA as initiator 5RNA nad poly(uridylic acid) as mRNA. The time courses of the formation of the 30S and 70S initiation complex with Escherichia coli ribosomes were measured simultaneously by stopping the reaction with dextran sulfate and differentiating the N-acetylphenylalanyl-tRNA bound to 30S ribosomal subunits from that bound to 70S ribosomes with RNase I, which hydrolyzes N-acetylphenylalanyl-tRNA bound to 30S subunits but not that bound to 70S ribosomes. A maximum in the 30S complex concentration was observed within the first 10-15 sec of the reaction, whereas 70S complex formed formed more slowly with a slight initial time lag. When an analog computer was programmed with rate constants determined separately for the formation of the 30S initiation complex from preformed 30S complex, kinetic curves very similar to the empirical curves were obtained for the entire time course of the reaction. The results show clearly that formation of the 70S complex obeys the kinetic laws for consecutive reactions, and the 30S complex is, therefore, an obligatory intermediate in the initiation of polyphenylalanine synthesis in the model system.