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To the Editor. DEAR SIR, In the June issue of AIRCRAFT ENGINEERING, Mr V. D. Naylor rightly asserts that, according to one‐dimensional theory, the velocity at the throat of a Laval nozzle is the local sonic velocity, whether friction is present or not. However his proof rests on an expansion law pvn=constant, when n≠y, and the throat velocity which he obtains differs according to the value of n. Both the assumption and the conclusion are false. The confusion which has existed on this point is, therefore, deepened.

Heat transfer from a cylinder placed on the vertical centre‐line of a square enclosure partly filled with a porous medium that is saturated with a fluid has been numerically studied. The cylinder is buried in the porous medium. The horizontal upper surface of the porous medium is separated from the rest of the enclosure by a horizontal impermeable barrier that is assumed to offer negligible resistance to heat transfer. The gap between the barrier and the top of the enclosure is filled with the same fluid as that with which the porous medium is saturated. The surface of the cylinder is at a uniform high temperature. The bottom and sides of the enclosure are assumed to be adiabatic while the horizontal upper surface of the enclosure is assumed to be kept at a uniform low temperature. The natural convective flows that occur in the porous medium and in the fluid layer above the barrier have been assumed to be steady, laminar, two‐dimensional and symmetrical about the vertical centre‐line of the enclosure. Fluid properties have been assumed constant except for the density change with temperature which gives rise to the buoyancy forces. The governing equations have been expressed in dimensionless form and solved using a finite element procedure. Results have been obtained for a Prandtl number of 0.7 for a wide range of the governing parameters. The main aim of the study was to determine how the mean heat transfer rate from the cylinder is affected by the size of the fluid gap at the top of the enclosure. The effect of this gap size has been related to changes in the flow pattern in the porous and fluid regions.

DEAR SIR, It is doubtful whether great practical importance can be attached to the precise location of the sonic section in a de Laval nozzle with friction present, under the assumptions of the one‐dimensional theory of flow. The truth of the matter, however, is that the equations of this approximation do lead to the conclusion that the condition of M 1, where M is the Mach number, prevails in the throat of a convergent‐divergent nozzle only in the limiting case of no friction, contrary to Mr Spalding's assertion in the August issue of AIRCRAFT ENGINEERING. It can occur in a section for which dA/dx 0 only if that section is at the exit of a convergent nozzle.

DEAR SIR. The undersigned has read with considerable interest and pleasure the lively discussion on this matter, which has run through the June 1951 to March 1952 issues of AIRCRAFT ENGINEERING. In spite of the remarkable substance and form of presentation of the papers and letters published, a few additional remarks might perhaps be of interest to the readers.

To the Editor, DEAR SIR, In a letter, which has just come to my notice, published in your October 1951 issue, Mr A. V. Cleaver advocates the use of sub‐orbital refuelling techniques as a means of rendering interplanetary flight an economic proposition, and suggests that the importance of this factor was underestimated in the above paper which was reprinted in your August issue.

To investigate the natural convection in open‐ended parallel, convergent, and divergent channels using a fully elliptic procedure without extending the domain outside the channel for the application of the boundary conditions at the inlet and outlet of the channels.

The model is two‐dimensional and fully elliptic in x and y directions, and the equations are solved only inside the channel by the finite volume method using a co‐located arrangement with a segregated procedure and boundary fitted coordinates. The pressure‐velocity coupling is solved by the PRIME algorithm.

The results are shown in terms of velocity vectors, streamlines, isotherms, and the local and the average Nusselt number for all fluids and configurations investigated. For high values of the Rayleigh number, a recirculation region in the outlet of all investigated configurations and Prandtl numbers was observed. Based on the results, a single correlation is proposed to evaluate the average Nusselt number for all fluids and configurations analyzed.

The shown results are based on the following hypothesis: steady‐state, two‐dimensional, laminar flow, and Boussinesq's aproximation. The results are presented in following range of parameters: 10⁵<(S_max/L)Ra_Smax<10⁸, where S_max denotes the maximum distance between the plates and Ra denotes the Rayleigh number; half angle of convergence or divergence (θ): 5° and 15°; and Prandtl numbers: 0.7, 5.0, and 88.

Local and average Nusselt numbers, for Prandtl numbers varying from 0.70 to 88, and a correlation for the average Nusselt number for all fluids and configurations are presented. The results presented in this paper are useful to engineers and researchers involved in thermal design and numerical methods.

Examines health‐care restructuring activities undertaken across North American hospitals over the past decade related to hospital care by nursing professionals (i.e. hospital nurses versus practical nurses or aides). Identifies fundamental lessons learned and highlights important priority research areas that must be undertaken to ensure that future initiatives achieve the intended effect of improving patient outcomes.

In 1980, 8 per cent of women in the UK were classified as obese (body mass index over 30). By 1998, the prevalence of obesity had increased to 21 per cent and there is no sign that this upward trend is moderating. Many women retrospectively attribute the onset of their obesity to pregnancy, yet research in this area has yielded conflicting evidence. Unlike the USA, the UK does not have clinical guidelines for gestational weight gain. Health professionals such as GPs, midwives and obstetricians have a responsibility to monitor normal pregnancy, yet typically in the UK, women are weighed only once i.e. at the antenatal booking appointment, which is primarily undertaken for the interpretation of screening tests. This paper will review recent evidence relating gestational weight gain and maternal outcomes such as the burden of postpartum obesity and the risk of caesarean section. In addition, some practical advice is given for heath professionals to use when monitoring the weight gain of pregnant women in their care.

PROPELLERS, fans and axial compressors are all airscrews, the only difference being in the use to which they are put. Propeller theory and practice has been put on a firm basis by men like Glauert and the parameters used are now well established. The question therefore arises of whether or not it is advisable to hand over the parameters and results of propeller theory to the theory and practice of axial compressors.

WHEN dealing with questions on gas flow, a certain amount of visual simplification in the resulting equations is produced by using the reservoir conditions as our unitary system.