Montreal, January 17, 2018 [sic] — With winter upon us in full force, outdoor temperatures are plummeting. But inside an intense tornado, it’s always chilly — no matter the time of year. A new study from Concordia proves why that’s the case.

In an article forthcoming in the Journal of Aircraftof the American Institute of Aeronautics and Astronautics, mechanical engineering professor Georgios Vatistas looks into the case of a violent tornado that touched down in 1955 in Scottsbluff, Nebraska.

During the storm, three broadcasters from the mobile unit of a local radio station were reporting live on the scene and had to take shelter in the basement of a stone building. There, as the tornado’s funnel passed overhead, they reported strange climatic changes. The temperature dropped from a mid-summer average, down to chilly, until the broadcasters were actually cold. They also found it difficult to breathe.

For 61 years, the cause of these phenomena remained unexplained. Enter Vatistas — a leading expert on the topic — who was able to formulate an analytical approach that allowed him to construct a new mathematical model of a turbulent compressible vortex.

To do this, he expanded on his previous theoretical developments on vortices to include the effects of turbulence and density variation.

“Using this new advanced approach, we were able to identify the cause of the temperature drop inside vortices for the first time ever,” says Vatistas, who conducted the study with recent Concordia master’s students Badwal Gurpreet Singh (MASc 14) and Rahul Rampal (MASc 14).

“As air pockets move from the outer periphery of the vortex toward its centre, the pockets expand, thereby bringing the temperature and density down.”

In the case of Scottsbluff, Vatistas and his team found that the temperature inside the tornado would have dropped from a comfortably warm background temperature of 27o C to a chilly 12o C. And at the tornado’s centre, the researchers estimated the air density would have been 20 per cent lower than what’s found at high altitudes.

“That’s what’s known as the ‘death zone,’ at 8,000 metres in altitude, beyond which mountaineers should not climb without breathing assistance gear,” Vatistas explains.

That’s why the reporters in Scottsbluff reported feeling deprived of oxygen. Luckily, the tornado passed quickly, and they avoided asphyxiation.

“It’s my hope that this important finding will help researchers better understand the many mysterious manifestations associated with violent atmospheric vortices like tornados and waterspouts.”

The study will also help engineers improve the operation of refrigeration vortex tubes, which are often used in the cooling of cutting tools during machining, various electronic components, hot melts, gas samples and heat seals.

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Cited study

Thermal Properties of Compressible Line Vortices

Georgios H. Vatistas*Gurpreet S. Badwal*Rahul Rampal† Concordia University, Montreal, Quebec H3G 1M8, Canada

*Professor, Department of Mechanical and Industrial Engineering; . Senior Member AIAA (Corresponding Author). †Graduate Research Assistant, Department of Mechanical and Industrial Engineering.

Publication Date (online): January 11, 2017

A simple mathematical model to simulate the temperature, pressure, and density distributions in intense turbulent compressible line vortices is reported. Based on the previous n=2 member of the n family of vortices, a wider-range model that includes the rudimentary combined features of turbulence and compressibility is built. In antithesis to the previous laminar theoretical approaches, where the converging flow cooled down monotonically with decreasing radius, it is now discovered that, far from the axis of rotation, the gas is first heated up, reaching a static temperature maximum that is higher than the ambient value; then, it chills down to a subambient minimum at the vortex center. The cause of this effect is recognized to be the outcome of competition between two fundamental mechanisms: gas heating because of mechanical dissipation, and cooling due to mainly fluid parcel’s dilation. Thus, the present investigation identifies the cause of the Ranque–Hilsch-like thermal effect observed in unconfined compressible vortices. In comparison to laminar vortices, the center of a turbulent gaseous vortex is found to be cooler, thinner, and under lower-pressure conditions. All thermodynamic properties are shown to be functions of the vortex Mach number and level of turbulence.

Read More: http://arc.aiaa.org/doi/abs/10.2514/1.C034106

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