Ph.D Thesis

Ph.D StudentRokita Tomer
SubjectExperimental and Numerical Investigation of the Subsonic
Flow Field inside and near a Weapons Bay
DepartmentDepartment of Aerospace Engineering
Supervisors PROFESSOR EMERITUS Jerrold Greenberg
DR. Yuval Levy
Full Thesis textFull thesis text - English Version


Motivated by the development of recent military aircraft that carry weapons internally (for improved stealth and aerodynamic efficiency), this research provides a detailed investigation of the subsonic flow field inside and near a weapons bay. The weapons bay configuration is represented by a plate with a rectangular cutout (cavity) immersed in a fluid moving parallel to the plate. Despite its simple geometry, a cavity generates complicated flow that contains a wide range of physical phenomena: unsteady shear-layer, vortex shedding, recirculation eddies, instabilities and three dimensional effects. Characterization and understanding of these phenomena are challenging tasks, especially in high Reynolds number turbulent regimes.

The research focuses on a three-dimensional, high Reynolds cavity configuration, which has a moderate length-to-depth ratio (6.25), and a width-to-depth ratio of 2. The cavity is exposed to a freestream Mach number of 0.40 and a Reynolds number (based on cavity depth) of 1.6 million, with a thick turbulent incoming boundary layer.

The experimental investigation includes the design and implementation of a designated wind-tunnel model, which allows various measurements of flow properties inside and near the cavity, and, in particular, simultaneous measurements of pressures and velocities. The numerical investigation includes time-accurate, three-dimensional numerical flow simulations, utilizing a hybrid RANS/LES approach, which is extensively validated, thereby allowing us to study three dimensional spatial-temporal patterns of cavity flow in depth.

Special attention is paid to the cavity shear-layer whose characteristics and stability are analyzed by application of Linear-Hydrodynamic-Stability-Theory. The analysis reveals that the behavior of the cavity shear-layer is analogous to a free shear-layer, approximately up to mid-length of the cavity, where further downstream non-linear interactions occur. In addition, a linear relation between shear-layer thickness and shear-layer turbulence intensity was found, and a new empirical correlation is proposed.

The mean and unsteady flow structure is first examined qualitatively by advanced flow visualizations techniques and vortex tracking in time and space and then quantitatively by calculation of spatial velocity cross-correlation fields and application of Proper-Orthogonal-Decomposition in both two-dimensions and three-dimensions. This type of analysis sheds new light on three-dimensional cavity flow by disclosing novel non-intuitive dominant structures and modes. Stretched streamwise vortices are shown to be dominant, at least as the spanwise structures that originate from the classical two-dimensional shear-layer instability.

Furthermore, the effect of the approaching boundary-layer thickness on cavity flow properties is investigated. Thinner boundary-layer is shown to cause a higher impingement velocity on the cavity aft wall.