Helical Instability Wave Excitation in Swirling Jets by a Novel Mechanical Device

The purpose of this investigation is to explore the possibility of using artificial mechanical means for excitation of shear layers with application in swirling jet mixing enhancement. For this purpose, a novel mechanical device for excitation of the helical instabilities of swirling jets is designed, fabricated, and used in these experiments. The device consists of a rotating cylinder with internal lobes of small height to induce small perturbations. The number of lobes and the direction of rotation can be varied to induce helical waves at azimuthal wave numbers of m=+0, ±1, ±2, ±3, and ±4. The m=+0 denotes the plane-wave excitation, m=±1 identifies the first helical mode with one lobe, m=±2 is the second helical wave with two lobes and so on. The positive and negative signs imply helical perturbation waves that spin in the same or opposite direction to the swirl direction, respectively. Swirl is induced in the airflow by a 45° vane-type swirl generator. Time mean axial velocity and turbulence measurements of the swirling jet, with and without excitation, are measured by hot-wire anemometer. The results are compared with the baseline (plane-wave excitation) at various helical modes. The acquired data is presented in 3D mesh plots and 2D contour plots. It is observed that new device is effective in excitation of the helical instability waves and in mixing enhancement of the swirling jet.