Boundary layers are regions of flow near surfaces that experience significant friction forces due to viscous effects. They play a significant role in aerospace design, having impacts on net drag, aerodynamic heating, and control surface performance. Generally, three types of boundary layers exist, laminar, transitional, and turbulent. The transitional and turbulent boundary layer states are associated with significant heat transfer rates (several times higher than the laminar case) due to general unsteadiness. Turbulent boundary layers are less susceptible to separating, and hence are desirable in some instances such as in inlets. Turbulent boundary layers are also associated with higher drag, hence it is important to understand the extent of turbulent flow across the vehicle surface. Due to the intended scale of modern hypersonic vehicles, transitional boundary layer states have gained a significant amount of attention in recent years. The transition of boundary layers from laminar to turbulent is a difficult process to model and estimates of boundary layer transition typical involve extensive computational tools.
In a joint collaboration with Texas A&M University, we have measured laminar-to-turbulent boundary layer transition processes unique to hypersonic flows. In particular, the second-mode instability appears in high Mach number conditions and is physically represented by a trapped acoustic wave in the boundary layer. Depending on flight conditions, if the edge Mach number is greater than 4.3 the second-mode instability can be the dominant mechanism leading to laminar-to-turbulent transition of the boundary layer.
Linear Array Focused Laser Differential Interferometry (LA-FLDI), a technique developed at UTSI, was deployed in the Texas A&M Mach 6 Quiet Tunnel. The low-disturbance levels of quiet tunnels is ideal for studying the complicated and sensitive boundary layer transition process. The NASA 91-6 flared cone model was used.
LA-FLDI was aligned so that beams passed through the boundary layer on the model. Note, LA-FLDI measures index of refraction changes, which are related to density changes via the Gladstone-Dale relationship. The advantage of this technique is that it can measure these changes at >1MHz acquisition rates. This is important, as second-mode instabilities have characteristic frequencies on the order of hundreds of kilohertz.
The second-mode instability and higher-order harmonics were measured by each of the six channels. The higher-order harmonics are indicative of nonlinear interactions of second-mode wave packets. Furthermore, this measurement marked the first time LA-FLDI was used for detection of the difficult to measure second-mode instability. Continued measurements of boundary layer transition are important for validating and building reliable tools for the engineering of hypersonic vehicles.
LA-FLDI has also been deployed at the UTSI Mach 4 Ludwieg tube. The spectra below show fluctuation spectra measured in the turbulent floor boundary layer. Velocimetry was also done through two-channel correlation calculations. The velocity measured by FLDI tends to be less than the mean flow velocity based on the expected convective velocity of disturbances.