Inflow characteristics for generating axis-switching phenomenon of heated rectangular jets

Flow characteristics due to the axis-switching phenomenon are numerically investigated for heated rectangular jets with typical inlet conditions and three aspect ratios (AR), namely 1.0, 1.5, and 2.0. To examine various inlets and their relation to the axis-switching flow, an inflow generator is newly devised, wherein jet flows from an orifice plate and a long pipe nozzle can be realized. Depending on the inlet conditions, the 45° and 90° axis-switching phenomena for the rectangular jet are observed to appear differently. As the aspect ratio increases and jet temperature decreases, the axis-switching positions are delayed downstream. It is observed that the strong and weak states of the axis-switching flows can be interpreted by the crossover locations of the jet half-widths and the saddle-shaped velocity profile is not the only main factor for axis-switching flow. As the jet temperature increases, a well-mixed state is quickly obtained, and the mixing performance of the square jet is better than that of the rectangular jet. For both isothermal and heated jets, the flow mixing is improved by the axis-switching flow. Various analyses are attempted to find the specific inlet structure for the axis-switching flow. From the results, the special features for inflow criterion producing axis-switching flow can be explained by factors such as the deformation parameter, interrelations between shear strain and normal rates, turbulence-driven and geometry-driven streamwise vortices, and normal Reynolds stresses.