Laminar boundary layer forcing with active surface deformations

Gibeau, Bradley; Ghaemi, Sina

doi:10.1103/physrevfluids.7.114101

Cited by 3 publications

(30 citation statements)

References 74 publications

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“…It therefore appears that the low-speed motions form from upward surface deformations and the high-speed motions form from downward surface deformations. This is consistent with the results of past investigations (Kim et al 2003;Gibeau & Ghaemi 2022). It is also evident that the magnitudes of the high-and low-speed motions are similar to one another, and that these magnitudes do not change much as St is increased.…”

Section: Phase Averagessupporting

confidence: 92%

“…All four phase averages in the φ-z plane show that the maximum velocity fluctuations are concentrated along z = 0. This indicates that the active surface is producing type-1 modes for St ≤ 0.2, which is what was observed within the LBL of Gibeau & Ghaemi (2022) at the same St. We also see that the strongest of the streamwise velocity fluctuations are bounded by the dotted lines (z/D = ±0.25), which makes sense because this region experiences the largest surface deformations. These dotted lines represent a width that is roughly equal to the maximum width of the VLSMs.…”

Section: Phase Averagessupporting

confidence: 75%

“…Similarly, the shape of the mean velocity profile should be an important contributor to the advection velocity as there is far more momentum close to the wall in the present TBL compared with its laminar counterpart. Indeed, normalization with U ∞ alone does not collapse the present results with those of Gibeau & Ghaemi (2022), as is clearly shown in figure 7(a). We therefore consider the shape factor H = δ * /θ, which is a measure of this near-wall momentum.…”

Section: Structure and Strengthmentioning

confidence: 50%

“…A key conclusion from the work of Gibeau & Ghaemi (2022) was that the lower actuation frequencies appeared to be more well suited for flow control. Here, 'lower' refers to St ≤ 0.2, where St = f a D/U ∞ is the Strouhal number computed using the actuation frequency, actuator diameter and free stream velocity.…”

Section: Introductionmentioning

confidence: 99%

“…The above conclusion is promising for the present investigation because the VLSMs occupy the lowest frequencies of the turbulence spectrum. However, it is not clear how well the results of Gibeau & Ghaemi (2022) obtained using an LBL will translate to a TBL at a much higher Reynolds number. The primary objective of the present investigation is therefore to evaluate the impact of active surface deformations applied locally beneath a TBL at a sufficient Reynolds number to determine whether this particular actuation strategy remains viable for targeting the VLSMs in future experiments.…”

Section: Introductionmentioning

confidence: 99%

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Manipulation of a turbulent boundary layer using active surface deformations

Gibeau

Ghaemi

2023

J. Fluid Mech.

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We experimentally evaluate whether active wall-normal surface deformations are suitable for the targeted control of very-large-scale motions (VLSMs) in a turbulent boundary layer at a friction Reynolds number of $Re_\tau =2600$ . Circular surface deformations with a diameter $D$ roughly equal to the boundary layer thickness $\delta$ are generated periodically at a constant amplitude of $0.03\delta$ and at actuation frequencies of $St=0.05$ to 0.20, where $St$ is the Strouhal number based on $D$ and the free stream velocity $U_\infty$ . The resulting impact on the flow was captured using high-speed particle image velocimetry and analysed using a triple decomposition. We find that the active surface deformations produce high- and low-speed streamwise velocity fluctuations that are concentrated along the centreline of the actuator. These motions have a negligible impact on the mean velocity profile downstream, i.e. they are truly high and low speed with respect to the unactuated base flow. The motions produced at $St\lesssim 0.1$ are comparable to synthetic VLSMs in terms of their lengths and widths but with a reduced wall-normal extent and rapidly decaying strength. These synthetic motions produce a strong modulation of the turbulence similar to that of the naturally occurring VLSMs. Most notably, we observe that synthetic high-speed motions with magnitudes of the order of $0.05U_\infty$ cause up to a 30 % reduction in turbulence production within the logarithmic layer. The strength and turbulence-modulating characteristics of the synthetic motions appear well suited for targeting the naturally occurring VLSMs locally using a control scheme.

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Section: Phase Averagessupporting

confidence: 92%

Section: Phase Averagessupporting

confidence: 75%

Section: Structure and Strengthmentioning

confidence: 50%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Manipulation of a turbulent boundary layer using active surface deformations

Gibeau

Ghaemi

2023

J. Fluid Mech.

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Transition control of the blasius boundary layer using linear robust control theory

Damaren

2023

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The paper considers control system design for linearized three-dimensional perturbations about a nominal laminar boundary layer over a flat plate (the Blasius profile). The objective is prevention of the laminar to turbulent transition using appropriate inputs, outputs, and feedback controllers. They are synthesized with a view to reducing transient energy growth, a known precursor to important transition scenarios. The linearized Navier–Stokes equations are reduced to the Orr–Sommerfeld and Squire equations with wall-normal velocity actuation entering through the boundary conditions on the wall. The sensor output is taken to be the wall-normal derivative of the wall-normal vorticity measured on the plate. Several multivariable output controllers are examined, including simple constant gain output feedback, loop transfer recovery, and $$H_{\infty }$$ H ∞ loop shaping. Reduced order compensators are developed using balanced truncation and analyzed for robustness using the gap metric between reduced order models and full order models. It is demonstrated that the level of minimum transient energy growth that can be achieved is similar for these diverse controller methodologies but falls short of that which can be achieved using optimal state feedback.

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Reactive control of velocity fluctuations using an active deformable surface and real-time PIV

McCormick,

Gibeau,

Ghaemi

2024

J. Fluid Mech.

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This study demonstrates an experimental realization of turbulence control strategies previously explored by Choi et al. (J. Fluid Mech., vol. 262, 1994, pp. 75–110) through numerical simulations. To conduct the experiments, a deformable surface with a streamwise array of 16 independently controlled actuators was developed. A real-time particle image velocimetry (RT-PIV) system was also created for flow measurements. The objective of the control strategy was to target the sweep and ejection motions of the vortex shedding from a spherical cap placed in a laminar boundary layer. Reactive control strategies consisted of wall-normal surface deformations that opposed or complied with the wall-normal (v) or streamwise (u) velocity fluctuations obtained from the RT-PIV. The results showed two primary outcomes of the control approach. Firstly, it effectively hindered the advancement of sweep motions towards the wall. Secondly, it disrupted the periodic shedding of vortices. The v-control with opposing wall motions and u-control with compliant wall motions exhibited strong inhibition of sweep motions, while the v-control with compliant and u-control with opposing wall motions showed weaker inhibition. All reactive control cases resulted in the disruption of vortex shedding. In some instances, this disruption was accompanied by increased turbulent kinetic energy due to the generation of additional flow motions. However, the v-control with opposing wall motions significantly reduced the vortex-shedding energy while maintaining total turbulent kinetic energy close to or below that of the unforced flow. Overall, the experiments show the effectiveness of reactive control strategies in mitigating sweep motions and disrupting vortical structures, offering insights for developing reactive control strategies.

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Laminar boundary layer forcing with active surface deformations

Cited by 3 publications

References 74 publications

Manipulation of a turbulent boundary layer using active surface deformations

Manipulation of a turbulent boundary layer using active surface deformations

Transition control of the blasius boundary layer using linear robust control theory

Reactive control of velocity fluctuations using an active deformable surface and real-time PIV

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