Large-Eddy Simulation of an over-expanded planar nozzle

Abstract

This fluid dynamics video shows visualizations of a Large-Eddy Simulation (LES) of an over-expanded planar nozzle. This configuration represents the experimental setup of Papamoschou et. al. [1,2,3] which found the position of the internal shock to be unstable. Our LES calculations [4] confirm this instability and offer a vibrant and dynamic view of the underlying flow physics. The interaction between shock and turbulent boundary layer is shown as is the subsequent separation region downstream. Numerical Schlieren provide a glimpse of the low frequency shock motion and suggest potential mechanisms for the instability. Key features include the asymmetry of the shock structure (with large and small lambda shocks), compression and expansion waves downstream of the shock and large scale flow reversal. Full details of the experiment and the calculation can be found in the references.

Characterization of Unsteadiness in an Overexpanded Planar Nozzle

“…2a), with the others being qualitatively similar. We observe that the correlation substantially decays for r z ∕L z < 0.2, although a certain degree of anticorrelation persists, in line with the findings of Olson and Lele [30]. This result was judged as a satisfactory compromise between accuracy and computational cost.…”

“…A coflow velocity was not imposed in the external environment. In the spanwise direction, we follow the reference LES [30] by selecting a spanwise width of the computational domain L z ∕H t 0.9 and applying periodic boundary conditions. The 3-D DDES initial condition is obtained from an extrusion in the spanwise The present internal flowfield is characterized by the nozzle pressure ratio NPR p 0 ∕p a , where p 0 and p a denote the chamber and ambient pressure respectively.…”

Comparison of Aero-Optical Measurements from the Flight Test of Full and Hemispherical Turrets on the Airborne Aero-Optics Laboratory

“…When the shock approaches the sonic line at the moment T5, the shock intensity is decreased and the shock eventually disappears at the moment T6, leaving the flow subsonic Then the proposed scenario repeats itself, creating a recurrent pattern of emerging and disappearing shocks. As a final note, the similar shock-evolving mechanism was observed in the shock-induced separation on the wall of a slightly-overexpended supersonic nozzle [13]. The proposed physical mechanism of the shock dynamics also explains the somewhat surprising relativelyslow shock motion, which is on the order of several characteristic turret times,…”

Aero-Optical Mitigation of Shocks Around Turrets at Transonic Speeds Using Passive Flow Control

“…The shock-induced separation changes the pressure gradient immediately downstream the shock, and for cases where the relatively weak shock was formed over the region with small pressure gradients, such as the weak moving shock and the strong moving shock cases, the interaction between the shock and the shock-induced flow separation leads to unsteady pressure gradients downstream of the shock, Gordeyev at al AIAA-2013-0717 7 American Institute of Aeronautics and Astronautics which results in the unsteady shock motion. A similar shock unsteadiness was observed over the hemisphere-oncylinder turret at incoming transonic speed of 0.6 [14] and in the shock-induced separation on the wall of a slightlyoverexpended supersonic nozzle [16]. When the shock becomes stronger, it is less-sensitive to the small wakerelated pressure gradients downstream of it, thus becoming stationary.…”