Novel tools help uncover new insights into complex flows
By Scott Dawson and David González |December 2020
The Fluid Dynamics Technical Committee focuses on the behaviors of liquids and gases in motion, and how those behaviors can be harnessed in aerospace systems.
Ohio State University and Florida State University in February presented research studying dual jets impinging on a ground plane, providing unique insights into the physics underlying vertical takeoff and landing aircraft in near-ground hover. OSU computed large-eddy simulations; FSU performed accompanying experiments. The research has helped elucidate the acoustic feedback mechanism that destabilizes these jets and results in intense acoustic tones. Data-driven and operator-aware decompositions developed at OSU over the past four years are helping to isolate these acoustic disturbances, which can be debilitating to ground personnel. The flow generated between the jets is highly asymmetric and disrupts the typical feedback dynamics of isolated jets. Understanding these dynamics helps in the development of new resonant tone prediction models, thus laying the foundation for potential control strategies. The physics-driven modal decomposition method developed in this collaboration has also illuminated fundamental aspects of the physics of hypersonic transition. In particular, it has identified a trapped-monopole structure in the acoustic component of flow over adiabatic walls, which rupture upon wall-cooling, resulting in freestream radiation.
Between January and March, researchers at NASA’s Langley Research Center in Virginia conducted Phase 2 of the Juncture Flow experiment, which combined novel experimental measurements with computational fluid dynamics simulations to document the flow physics of a separated wing-body juncture flow. Researchers acquired high-quality flow field measurements of the velocity field and Reynolds stresses very near the corner using innovative onboard laser Doppler velocimetry and particle image velocimetry instruments positioned inside a fuselage. The experiment expanded the range of data collection to include more measurement locations in the corner region of interest, planar fields and an additional angle of attack. Corresponding CFD efforts were tightly integrated with this experimental campaign and have resulted in an improved quadratic constitutive relation for the turbulence model used for simulating this type of flow.
Experiments at North Carolina State University demonstrated in April that energizing the boundary layer is quite ineffective in controlling the open separation by a fin shock. In contrast to flow-induced closed separation units, these open separated flows are dominated by a separation vortex that exhibits significant complexity in its organization, topology and geometry. Instead, the experiments demonstrated a near annihilation of the separated flow by direct interactions between the separation vortex and the vortex-laden boundary layer generated by a single sub-boundary layer vortex generator. The fact that North Carolina State accomplished significant mitigation of the separation scale with a small-footprint device is alluring for transforming these investigations to practical geometries. Mitigating and delaying flow separation is critical to expanding the operational envelope of high-speed platforms.
At the AIAA SciTech Forum in Orlando, Florida, in January, researchers from the Air Force Research Laboratory presented the results of simulations they completed in December 2019 that demonstrated that control of transition over a flat plate is achievable via the use of local dynamic surface modification. They based their direct numerical simulations off an experimental arrangement employing control with piezoelectrically driven actuators to stabilize Tollmien-Schlichting instabilities, which are triggered by separate upstream actuation. The simulations demonstrated that a closed-loop control law could achieve similar results to empirically determined optimal control parameters.
Contributors: Nash’at Ahmad, Datta Gaitonde, Venkat Narayanaswamy, Donald Rizzetta, Unnikrishnan Sasidharan-Nair and Spencer Stahl