A study is made of the wind-driven circulation of a two-layer ocean within a square basin, with a view to describing the observed separation of western boundary currents. The lower layer is allowed to surface and the line along which the upper-layer depth vanishes is interpreted as the region of the surfacing thermocline. For a representative wind stress the theory predicts the gross features of the Gulf Stream flow, the region adjacent to the surfacing line containing the separated boundary current. By assuming that the effects of friction and inertia are confined to regions of a boundary-layer character, the position of a separated current is shown to depend only on the degree of stratification and certain integral properties of the applied wind stress.

No abstract

The development of the boundary layer along a long thin cylinder aligned with the flow is considered. Numerical solutions are presented and compared with previous asymptotic results. Very near the leading edge the flow is given by the Blasius solution for a flat plate. However, there is soon a significant deviation from Blasius flow, with a thinner boundary layer and higher wall shear stress. Linear normal mode stability of the flow is investigated. It is found that for Reynolds numbers less than a critical value of 1060 the flow is unconditionally stable. Also, axisymmetric modes are only the fourth least stable modes for this problem, with the first three three-dimensional modes all having a lower critical Reynolds number. Further, for Reynolds numbers above the critical value, the flow is unstable only for a finite distance, and returns to stability sufficiently far downstream.

No abstract

Aims: To determine whether intercellular signalling can occur between physically separated populations of Escherichia coli. Methods and Results: Intercellular signalling between physically discrete populations of E. coli BL21 was analysed in bi‐partite Petri dishes. Transfer of a growth‐promoting signal resulted in induction of resistance to the antibiotic ampicillin. Optimal expression of the signal occurred when the signalling population was established as a bacterial lawn for 24 h. This represented an entry into the stationary phase of growth, as indicated by the expression profile of the RNA polymerase subunit σ38 (σS; sigma S). The growth‐promoting effect was also observed when E. coli DH5α (luxS–) was used as the signalling population. Preventing passage of air between the two populations resulted in a complete cessation of the growth‐promoting effect. Conclusions: A growth‐promoting signal occurs between physically separated cultures of E. coli. The exact nature of the signal remains to be determined but does not involve the production of autoinducer‐2 from the luxS gene. Signal transmission is likely to involve airborne transfer of a signal species. Significance and Impact of the Study: Intercellular signalling systems exist in bacteria that enable antibiotic resistance to be conferred between physically separated populations.

A proof is presented for the following theorem: Given all possible three-dimensional (3-D) configurations of n omnidirectional acoustic sensors with vanishingly small separations, and given all possible weighting schemes and steer directions, the directivity index achieves an overall maximum value of n2 when the sensors are configured as a linear array steered to end-fire, it being immaterial whether or not the sensor spacings are equal.

No abstract

No abstract