Abstract
•Typical base configuration has large coherent structures far downstream, weak entrainment.•Jet boat tail(JBT) flow interacts with shear layer immediately at base surface.•Two primary flow instabilities of the baseline and JBT model identified by spectrum analysis.•JBT creates larger vortex structures enhancing entrainment and energizing base flow.•Energized base flow increases base pressure and reduces drag.
This study conducts an implicit large eddy simulation (ILES) of jet boat tail (JBT) flows to investigate its drag reduction mechanism. The concept of JBT passive flow control is to create a circumferential jet around a bluff body toward the center of the base area. It forms a jet cone to have the similar effect of a solid boat tail. The LES is performed for a baseline bluff body and a JBT model modified from the baseline. The LES predicts that the JBT reduces the averaged drag coefficient by 19.3%, a reasonable agreement with the experimental drag reduction of 22.5%. The reduced averaged wake area is also observed for the JBT model, resulting in a decreased drag. In addition, the unsteady flow structures of the baseline and JBT flow are analyzed to study the flow mixing and entrainment mechanism. For the baseline configuration, the coherent vortex structures occur far downstream of the base surface. It hence does not have strong entrainment and energy transfer from freestream to the base area. For the JBT flow, a pulsative jet is induced by the vortex shedding of the shear layer and interacts immediately with the shear layer near the base surface. It generates the small structures that are substantially larger than those of the baseline configuration. The larger vortex structures of JBT enhance the flow entrainment and transfer more energy from the freestream to the base area. It results in higher static pressure in the base area that substantially reduces the pressure drag. Proper orthogonal decomposition of flow field reveals the periodic jet pulsation pattern in the azimuthal direction.
