Application de la méthode de fil chaud à la mesure de la turbulence dans l'eau

The measurement of turbulence in water by the hot wire method

Laboratoire de Mécanique des fluides, Université de Grenoble.

Résumé

PART I : DEVELOPMENT OF THE METHOD The article refers to tests carried out at Grenoble University Fluid Mechanics Laboratory on a rig designed to produce turbulence in a tank for the investigation of specific diffusion problems. The first problem after completion of the rig was to measure this artificial turbulence, for which it was decided to use a hot wire anemometer. A preliminary test showed that immersion of a wire heated electrically to a temperature only 20 °C higher than that of the water without special precautions did not produce any measurable indication. This part of the study was therefore carried out in two stages, as follows : 1. General development of the method (described in this part of the article) ; 2. Results of turbulence measurements in the tank (to be discussed in the second part in the next issue of La Houille Blanche) The factors responsible for difficulties encountered when the method was first tried out were investigated during the first stage and can be summed up as follows : (i) Anemometer : The constant-temperature system is the one to use ; it requires specially designed equipment with very stable reference resistances, low oscillation risk, and counter-reaction amplification. (ii) Water : Various factors cause instrument drift, being mainly connected with the following : Dust : Measurement stability is surprisingly sensitive to dust, and the problem even arises in a confined protected medium. This hazard makes the method difficult to use in unprotected surroundings. Scale formation : Due to the temperature rise above that of the water-i.e. the principle of the method itself- fur or scale forms on the sensitive element if the water contains the right ingredients. Release of dissolved air bubbles : This is also due to the temperature rise. Corrosion of the hot film metal elements by the water : If the water is purified to get rid of its dissolved salts, it becomes very aggressive. water temperature variation, unless controlled to within about 0.1 °C. (iii) Pick-ups. Due to the need to keep the system dust-free, fairly large sensitive elements are required. This practically rules out the use of wires, as their electrical resistance would be inadequate if they were to have adequate mechanical strength. Films score heavily on this point as they keep these two functions apart. Non-insulated sensitive elements of certain shapes are liable to oscillate with the anemometer circuits of a certain make-up. Quartz-insulated pick-ups offer major advantages ; the quartz hardly affects the sensitivity of the method at all. Conical pick-up shapes are preferred because of the dust hazard, but they only give the turbulent velocity in a single direction. Moderate heating seems to be advisable to reduce drift due to dissolved bubbles in the water to a minimum, but this makes effective temperature stabilisation even more important. Special instruments are necessary to measure fluctuating electrical values resulting from low-frequency fluctuations in the water. The Laboratory has designed an integrating voltmeter with digital indication for the purpose. A satisfactory layout for the system has been developed, but the problem of making the method suitable for everyday use in any flow still requires further study. All the results discussed in the two articles are from a turbulent research programme for a "Doctorat ès Sciences" thesis under Professor Craya at Grenoble University Faculty of Science. This thesis is due to be upheld some time during the first quarter of 1967.