Measurement of a Vortex with Double-Helical Structure in a Laboratory Tornado Simulator by Sonic Anemometer-Thermometer

Natural and artificial tornadoes take a special form called “multiple vortex structure” where two or more secondary vortices revolve around the eye of their parent vortex. The structure is seen when the Swirl ratio, one of the three dimensionless number governing the system, is larger than 0.3 – 0.4. In the meanwhile, around the lower limit of the Swirl ratio to give the multiple vortex structure, two secondary vortices intertwined with each other emerge to form double helical configuration similar to DNA. We reproduced such a vortex in our laboratory tornado simulator at National Defense Academy (NDA), Japan, and attempt to reveal its structure through a velocity measurement.

For this purpose, we adopt a Sonic anemometer-thermometer (SAT) developed for indoor use (Sonic/Kaijo WA-790) instead of conventional anemometers for laboratory experiments, i.e. Pitot tube, Hot-wire anemometer, Laser Doppler Velocimetry (LDV), etc. This is because wind field in the interior of the vortex is unsettled especially when two or more secondary vortices are embedded within it, and then it is geometrically impossible to measure the wind from the rear side of the probe by Pitot tube, even omnidirectional one, and a Hot-wire anemometer, even two- or three-dimensional one, respectively. Similarly, it is difficult to provide optical axes for LDV in a tornado simulator with complex configuration. In this respect, the indoor SAT is desirable since it can measure three components of wind velocity without any restrictions on wind direction. The probe of WA-790 consists of three set of transduces disposed 3 cm apart with each other. Thus, although the probe is considerably larger than that for Pitot tube or Hot-wire anemometer, it is sufficiently small compared with the size of the vortex spawned in our tornado simulator at NDA, which is about 20 cm in diameter.

We carried out velocity measurement at 87 points on the vertical cross section of 40 cm high and 30 cm wide set in the vortex simulator, which has concentric cylindrical structure with inner diameter of 130 cm and outer diameter of 190 cm, respectively and the height of 95 cm. Each run of velocity measurement is set to be 4 min long and three component of wind speed is obtained with 10 Hz. While the parent vortex is revealed on the average wind fields, the structure of the secondary vortices and their momentum transport are examined statistically on the variance and covariance fields, respectively. In addition, to show their time variations, the spectra and co-spectra of wind speed are also investigated.