Session G06: Flow Control: Coherent Structures, Vortices and Turbulence (5:00pm - 5:45pm CST)

The interaction between non-parallel planar starting jets and a steady crossflow

A device typically used in flow control applications is employed to generate starting jets characterized by a substantial initial acceleration that is associated with a large peak in over-pressure inside the jet exit plane. In the absence of a crossflow, thick-cored, almost spherical vortex rings are produced despite the high-aspect ratio outlet geometry, see Steinfurth & Weiss (JFM, 2020). Here, we conduct phase-locked PIV measurements to investigate the influence of a steady crossflow with a zero-pressure-gradient, turbulent boundary layer on these starting jets. Depending on the velocity ratio between jet and crossflow r, two fundamentally different categories of flow structures are observed. At $r<4$, hairpin vortices are produced as the vorticity associated with the upstream part of the starting jet is cancelled by the crossflow boundary layer. At $r>4$, the jets penetrate through the boundary layer, and asymmetric vortex rings are observed. With the current effort, further light is shed upon the flow physics of non-parallel starting jets. This may promote the sophisticated selection of actuation parameters in active mixing and separation control.   

A deterministic analysis of the impact of slip surfaces on laminar-to-turbulent transition and turbulence

The effect of slip surfaces on nonlinear invariant solutions to the Navier-Stokes equations is studied by direct numerical simulation in channel geometry. These solutions are also known as exact coherent states and arise via a saddle-node bifurcation. In general, lower-branch solutions are insightful in the study of transition as they lie along the laminar-turbulent boundary in state-space, while upper-branch solutions provide insight into the mean behavior of a turbulent flow. A deterministic analysis of the effect of slip surfaces on transition and turbulence is made by applying it to both lower- and upper-branch solutions. Two solution families are considered, a core mode and a critical-layer mode. Slip surfaces are found to have distinct effects on the dynamics of the system as it leaves each of these solution states. Slip surfaces cause the system to leave a core mode lower-branch solution earlier with a negligible effect on the instability. However, slip surfaces delay the system in leaving a critical-layer mode lower-branch solution, and the flow eventually laminarizes above a critical slip length. Upper-branch solutions are also observed to behave in distinct manners with the inclusion of slip surfaces. Flow dynamics and structures are further discussed.   

Evolution of Synthetic Jets of Different Orifice Geometries in a Laminar Boundary Layer

The use of synthetic jets for active flow control has been in focus for several decades. The aim of the present experimental study is to investigate the interaction of a finite aspect ratio jet having different orifice geometries with a 2-D laminar boundary layer over a flat plate. Three actuators having the same exit area and the same aspect ratio of 18, but different orifice geometries (rectangular, trapezoidal, and triangular), were studied using Stereoscopic Particle Image Velocimetry. Results show that for all three orifice geometries the near-field is unsteady. The far-field of the rectangular orifice consists of a pair of equal strength counter-rotating vortex pair, travelling parallel to the orifice centerline. The far-fields of the triangular and the trapezoidal orifices consist of a single vortex vectoring away from the centerline, towards the tapered side. The vectoring angle is larger for the triangular orifice at approximately 5.8 deg. The study shows that a trapezoidal or a triangular orifice can be used for vectoring a synthetic jet in a uniform crossflow and might be useful for improving its performance in the presence of spanwise flow.   

Optimal resolvent-based estimation for flow control

Obtaining accurate estimates of the flow state using limited measurements is an essential step for any closed-loop flow control strategy. In this work, we develop an optimal causal estimator formulated in terms of resolvent operators obtained from Navier-Stokes equations. This constitutes an extension of recent work that leveraged resolvent analysis to estimate space-time flow statistics and reconstruct time series from limited, non-causal measurements. In the present approach, causality is optimally enforced using a Wiener-Hopf formalism, ensuring that the current estimate depends only on current and previous measurements, making the method applicable for flow control. When equivalent assumptions are made, the approach reproduces the Kalman filter, but it can be efficiently applied to large systems without the need for prior model reduction. Unlike the Kalman filter, it can easily account for nonlinear terms from Navier Stokes with colored-in-time statistics, which significantly improves the accuracy of the estimates. Moreover, the use of the resolvent framework allows a direct physical interpretation of the mechanisms involved in the estimation procedure in terms of coherent flow structures. Finally, we show how our approach can be incorporated into an optimal control framework. Results are demonstrated using the flow over a backward-facing step.