Advances in Sensing and Actuation for Turbulent Boundary Layer Control
Creators: Gibeau, Bradley J
Publisher: name: University of Alberta Library
Year: 2023

<p>The fluid mechanics literature suggests that the ability to manipulate the very-large-scale motions (VLSMs) that exist within turbulent boundary layers (TBLs) would provide influence over the unwanted drag forces, noise, and vibrations associated with these flows. The ability to suppress these negative effects would then allow for greatly improving the function of various engineering systems. Consequently, the work documented within this thesis focuses on developing sensing and actuation capabilities for the targeted control of VLSMs in physical systems. </p>

<p>Wall-mounted pressure sensors are the most practical sensor type for real-world flow control applications. However, it is not known whether such sensors can be used for the targeted control of VLSMs because the relationship between wall pressure and the VLSMs has not been identified. To remedy this, a pressure measurement system capable of capturing wall-pressure fluctuations in a noisy wind tunnel environment is developed. Simultaneous measurements of wall pressure and velocity are then used to identify the relationship between wall pressure and the VLSMs for the first time. It is found that the wall-normal velocity component of the VLSMs is responsible for producing wall-pressure fluctuations at low frequencies via suction and splatting at the wall. This result suggests that it is possible to use the low-frequency wall-pressure signal to identify VLSMs in real time for the purpose of targeted control.</p>

<p>Targeted actuation of the VLSMs has been explored in previous work using jets, spanwise surface motion, and (numerical) body forces. In contrast, the present work considers actuation using active surface deformations applied locally in the wall-normal direction. An ``active surface&#39;&#39; is developed to produce simple surface deformations over a range of actuation frequencies and amplitudes. An initial evaluation of the device is then conducted using a laminar boundary layer (LBL) to form a baseline understanding of the actuation strategy in a steady flow. This initial evaluation reveals that the active surface is capable of producing both high- and low-speed streamwise velocity fluctuations with similar magnitudes. Actuation at low frequencies is found to be the most promising for flow control because the resulting motions are stronger, more stable, and concentrated along the centreline of the actuator. Moreover, a linear modelling technique is found to adequately describe the input-output dynamics of the actuated flow, thus suggesting that the actuation strategy is amenable to the powerful tools of modern control theory. </p>

<p>The active surface is then deployed beneath a TBL where it is used to actuate at the frequencies associated with the VLSMs. The device is found to be capable of producing high- and low-speed motions that are similar to synthetic VLSMs when considering their dimensions and ability to modulate the surrounding turbulence. Additionally, these motions act as a local actuation because their strength decays rapidly with streamwise distance. These characteristics indicate that the active surface may be well-suited for targeted control of the VLSMs using a feed-forward strategy. Alternatively, the turbulence-modulating properties of the actuated motions may also be appropriate for a control strategy. Most notably, the present results indicate that the high-speed motions produced by downward deformations of the active surface act to reduce turbulence production within the logarithmic layer of the TBL. </p>

<p>Finally, the motions produced by the active surface in the LBL and TBL are compared at low actuation frequencies. Both sets of actuated motions appear similar in form but with a few distinct differences. First, the motions produced in the TBL advect more quickly as a percentage of the freestream velocity than those produced in the LBL. Second, the motions produced in the TBL are weaker as a percentage of the freestream velocity when compared to those produced in the LBL. These differences lead to an attempt to scale the results to obtain a collapse of the data. The resulting normalizations suggest that the shape factor of the boundary layer plays a role in determining the advection velocity of the motions produced by the active surface. Similarly, the shape factor and Reynolds number may be part of what determines the strength of the actuated motions.</p>