An experimental investigation of the instability and the acoustic radiation of the low Reynolds number axisymmetric supersonic jet has been performed. Hot-wire measurements in the flow field and microphone measurements in the acoustic field were obtained from different size jets at Mach numbers of about 2. The Reynolds number ranged from 8000 to 107000, which contrasts with a Reynolds number of 1·3 × 106for similar jets exhausting into atmospheric pressure.Hot-wire measurements indicate that the instability process in the perfectly expanded jet consists of numerous discrete frequency modes around a Strouhal number of 0·18. The waves grow almost exponentially and propagate downstream at a supersonic velocity with respect to the surrounding air. Measurements of the wavelength and wave speed of theSt= 0·18 oscillation agree closely with Tam's theoretical predictions.Microphone measurements have shown that the wavelength, wave orientation and frequency of the acoustic radiation generated by the dominant instability agree with the Mach wave concept. The sound pressure levels measured in the low Reynolds number jet extrapolate to values approaching the noise levels measured by other experimenters in high Reynolds number jets. These measurements provide more evidence that the dominant noise generation mechanism in high Reynolds number jets is the large-scale instability.

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A high concentration of certain planktonic animals was found in a frontal region in the English Channel. Temperature, salinity and current measurements and direct visual observations (underwater) describe the nature of the front. It is shown that water depth, season, strong tidal mixing and residence time are important factors leading to the formation and maintenance of a turbulent convergent tidal front. I N T R O D U C T I O NThe hydrography of the English Channel has been studied extensively for nearly a century. It is therefore somewhat surprising that no descriptions of persistent convergent fronts have been recorded. Fishermen have recognized for many years that oceanic fronts are areas where fish tend to congregate (Uda, 1938). Similarly fronts in the English Channel are found to contain concentrations of certain planktonic animals.Hydrographic studies in the open ocean have shown that fronts coincide with sharp temperature discontinuities (Cromwell & Reid, 1956). In the front recorded here (Fig. 1) an abrupt change of temperature of 1 °C was observed in crossing the front and its position also included the 16 °C isotherm. Although reports of fronts are not uncommon (Knauss, 1957;Voorhis & Hersey, 1964;Katz, 1969;Voorhis, 1969) the nature of mixing processes at the boundary remain obscure. In this paper current measurements and associated density structure reveal the nature of the convergence and its associated turbulent structure that results in the concentration of planktonic animals. Fig. 1 shows the position of a persistent front between Guernsey and Jersey. It is of at least 10 nautical miles extent and easily identifiable by the rafts of floating sea-weed, debris and oil lumps held in the boundary. Dimensions of these (rafts) might be typically 20 m long and | m deep but varied according to the wind, usually making up a ragged rather than a strictly linear boundary. Puffins, shearwaters and terns were seen apparently feeding along the line of the front. Indeed the appearance was similar to other oceanic fronts (Beebe, 1926;Uda, 1938;Amos et al., 1972). OBSERVATIONS AND RESULTS Description of a front

The conventional ‘bridge-feedback amplifier’ constant-temperature hot-wire anemometer is analysed to determine its static and dynamic response. The effects of moderate feedback amplifier gain, bridge imbalance, stray bridge reactance, amplifier offset voltage, lack of common mode rejection, amplifier frequency response and departure from constant transconducture are included. The root loci of the system are mapped out and the consequences of the analysis are discussed from the viewpoint of both the operator and the designer.