Measuring, simulating, predicting and controlling fluid motion
By Eylul Bilgin and Pedro Paredes|December 2024
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.
In March, Purdue University researchers in Indiana completed a test campaign in which they raised the maximum quiet Reynolds number in the Boeing/Air Force Office of Scientific Research Mach 6 Quiet Tunnel to 21×10⁶ m–¹. This is a world record for hypersonic low-disturbance tunnels, up from 15×10⁶ m–¹ in 2022. This breakthrough enables the study of hypersonic turbulence under low-disturbance conditions, which the team demonstrated by observing natural transition on boundary layer transition geometry, with transition Reynolds numbers similar to those seen in the BOLT-II flight test in 2022.
In conventional hypersonic wind tunnels, tunnel noise is dominated by acoustic radiation from turbulent nozzle-wall boundary layers, which can directly influence the boundary layer transition, BLT, over the model in the test section. Relating the noisy tunnel measurements of BLT to that in flight has long been difficult, but direct numerical simulations are now offering new insights into this problem. In January, researchers from the Ohio State University, Sandia National Laboratory in New Mexico, National Institute of Aerospace in Virginia and NASA Langley Research Center in Virginia completed a DNS of Mach 8 flow over a sharp cone in the axisymmetric nozzle of the Sandia HWT-8 facility. The DNS realized all the stages of the transition to turbulence caused by tunnel noise, while capturing the relevant physics. Direct comparison with experiments at the Sandia HWT-8 confirmed the accuracy of the numerical procedure for imposing a “tunnel-noise” model to mimic the disturbance environment in a conventional hypersonic wind tunnel, thereby established the foundation for “virtual’’ testing of the hypersonic BLT in a noisy digital wind tunnel.
During AIAA’s Aviation Forum in late July and early August, researchers from the University of Maryland presented the results of their investigations of the flow physics of droplet impingement at hypersonic speeds. They presented newly developed scaling relationships that allow for the collapse of integral forces generated during high-speed droplet impingement. These scaling laws can be used to establish reduced order models applicable for the assessment of hypersonic vehicles flying through adverse weather conditions. The researchers also presented a new simulation approach that captures the transition between continuum and dispersed phase behavior of water. During impingement, water droplets break into sub-droplets, altering acoustic wave behavior and spray dynamics. This new simulation better captures droplet dynamics during aerobreakup. Other applications of this simulation approach include, for example, high-speed water impact and underwater explosions.
In August, MIT researchers presented a closure model for computation fluid dynamics that improves predictions of high-speed flows for entry, descent and landing vehicles. Unlike previous models, the building-block-flow model uses neural networks and combines fluid physics with machine learning to address a broader range of flow regimes. The model accurately predicted turbulence, flow separation, compressibility and heat transfer in rough surfaces. It outperforms earlier models, as demonstrated in both channel flow tests and realistic entry, descent and landing scenarios.
In January, DARPA selected Aurora Flight Sciences of Virginia to build a full-scale X-plane, designated X-65, to demonstrate the viability of primary flight control via active flow control actuators. Plans call for the X-65 to have two sets of control actuators: “traditional flaps and rudders, as well as AFC [active flow control] effectors embedded across all the lifting surfaces,” according to a DARPA press release. “The plane’s performance with traditional control surfaces will serve as a baseline; successive tests will selectively lock down moving surfaces, using AFC effectors instead.” The unoccupied X-65 will have a 9-meter wingspan and be able to fly at speeds up to Mach 0.7. Flight testing is targeted for 2025.
Contributors: Christoph Brehm, Lian Duan, Joseph S. Jewell, Adrian Lozano-Duran and Martiqua Post
