[M-IS24] Geophysical fluid dynamics-Transfield approach to geoscience

[MIS24-04] High-frequency measurement of a double helical vortex with a sonic anemometer thermometer for laboratory use

*Toshihisa Itano¹ (1.National Defense Academy)

Keywords:multiple-vortex, sonic anemometer thermometer, momentum transport

The multiple-vortex phenomenon, in which the core of a vortex is destabilized to split into a pairs of secondary vortices intertwined with each other and revolving around the radius of the maximum wind of the parent vortex, was discovered in the middle 1960’s and reproduced in a laboratory experiment in early 1970’s. Afterwards, in 1980’s, a similar kind of tornado simulators, which could reproduce the multiple-vortex phenomenon, were designed in several universities and the wind and pressure distributions, etc. were examined in addition to the condition for the formation. However, due to the restrictions on the instruments available at that time, direct measurement by one- or two-dimensional hot-wire anemometer and/or photometric analysis of stroboscopic photography visualized by smoke wire were simply carried out in most of the case. It should be noted that we can’t obtain the turbulent momentum transport and its cospectra by one- dimensional probe of the hot-wire anemometer. Also, even with the two- dimensional probe, two components of the wind velocity are not obtained necessarily: the measuring surface of the probe has to be turned parallel to the wind direction, and the fluctuations of the wind direction must be restricted within 90 degree. In this respect, to measure the multiple vortex phenomenon, where secondary vortices are embedded within the primary vortex, is quite difficult because the wind direction tends to be reversed frequently (Indeed, Monji (1985) gave up tp measure the core part of the multiple-vortex phenomenon.). Laser Doppler velocimeter (LDV) and Cobra probe, a kind of Pitot tube, were introduced for the measurement of laboratory tornadoes in 1990’s and after 2000, respectively. Especially, the latter is an attractive instrument, which can measure not only three components of wind velocity but also the pressure in the same time. Nevertheless, it could not be used for the multiple-vortex phenomenon since it can’t measure the flow from behind. By considering the above situations, we would like to introduce our attempt to measure the multiple-vortex phenomenon with a sonic anemometer thermometer (SAT) for laboratory use.

We focused on a “double helical vortex” which appears in our tornado simulator at National Defense Academy (1.9m<diameter>×0.95m<height>, aspect ratio a=0.857(=0.3m/0.35m)) when the inflow angle is θ=45 degree (i.e. the swirl ratio a (=tanθ/(2a)=0.58)) and the frequency of the fan is 10Hz (i.e. Reynolds number Re=16,800). We set 72 observation points, every 3cm and 5cm in the radial and vertical directions, respectively, within a vertical cross section of the simulator. The WA-790 <Sonic>, the probe of which has the base line of 3cm between a pair of its transducers, is adopted and high frequency observation is carried out for 10 minutes with 10Hz at each point. Results indicate that in most of the region, the basic swirling flow deviates inward (less than 10 degrees) from the tangential direction and the elevation angle increases with height. Meanwhile, the downward flow exists in the core part, where net wind speed is weak, forming an “eye”. Standard deviation of the wind speed is large at the shear region surrounding the “eye”, which indicates the existence of the secondary vortices. Covariance of the wind fluctuations implies the momentum of the basic swirling flow is transported both upward and outward in most of the region, but inward transported is detected around the radius of the maximum wind of the basic swirling flow.