Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session F28: Flow Instability: Control |
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Chair: Rodolfo Ostilla Monico, University of Houston Room: North 228 AB |
Sunday, November 21, 2021 5:25PM - 5:38PM |
F28.00001: The effect of rotational shear on viscous fingering in miscible fluids. Samar Alqatari, Thomas E Videbaek, Zhaoning Liu, Sidney R Nagel When a lower viscosity fluid invades a more viscous one in a small channel, the fluid flow can become unstable and form fingers: the classic viscous fingering instability. In our experiments, we use miscible fluids in a radial Hele-Shaw geometry and study the effect of applying oscillatory rotational shear to the fluid by rotating one of the Hele-Shaw plates with respect to the other. The application of the shear changes the structure of the interface between the two fluids along the direction spanned by the small gap. We find that the rotational shear suppresses the onset of the instability as well as changes the width of the fingers once they grow. We study the effects of shear frequency, shear amplitude, and the fluid viscosity ratio on the instability onset and growth dynamics. |
Sunday, November 21, 2021 5:38PM - 5:51PM |
F28.00002: Controlling secondary flows in Taylor-Couette flow using stress-free boundary conditions Vignesh Jeganathan, Kamran Alba, Rodolfo O Monico Taylor-Couette (TC) flow, the flow between two independently rotating and co-axial cylinders, is known to have pinned secondary flows known as Taylor rolls. We study the possibility of affecting these secondary structures using one- and two-dimensional patterns of stress-free and no-slip boundary conditions on the inner cylinder. For this, we perform direct numerical simulations of TC flow with pure inner cylinder rotation at three different shear Reynolds numbers up to $Re_s=3 \times 10^4$, fixing the radius ratio to $\eta=0.909$. We find that one-dimensional azimuthal patterns do not have a significant effect on the flow. However, one-dimensional axial patterns disrupt the rolls by interfering with the Reynolds stress that is responsible for secondary structures and decrease the torque substantially. Two-dimensional spiral inhomogeneities lie somewhere between the previous two cases. It affects the torque and imparts axial velocity thus moving the pinned secondary flows. We quantify the roll's movement for various angles and the widths of the spiral pattern, and find that the maximum speed is a non-monotonic function of pattern angle and pattern frequency. Finally, we find that two-dimensional checkerboard patterns do not affect the flow or the torque substantially. |
Sunday, November 21, 2021 5:51PM - 6:04PM |
F28.00003: Effects of Optimized Vortex Generators on Mack Mode-Dominated Boundary Layer Transition Connor W Klauss, Clark C Pederson, Pedro Paredes, Meelan Choudhari, Boris Diskin The primary purpose of this study is to examine the potential for Mack mode stabilization and transition delay of an optimized vortex generator (VG) shape on a circular cone at hypersonic flight conditions. The configuration of interest corresponds to a selected trajectory point from the ascent phase of the HiFIRE-1 flight experiment. Both experimental and numerical studies have shown Mack mode amplification to be the leading mechanism of laminar to turbulent transition in hypersonic boundary layers. Calculations with streamwise streaks generated by VGs have shown that they can reduce the net amplification of Mack modes, effectively delaying boundary-layer transition. The shape of the VGs used in this study is optimized by using SU2's adjoint solver for mean streak amplitude along the wake of the VGs. The optimized VGs are found to yield mean streak amplitude values that are several times larger than those corresponding to the baseline design. A grid refinement study is performed to determine the effect of grid resolution on the optimized shape and streak amplitude evolution. The stability of the VG wake is examined using the plane-marching parabolized stability equations (PSE) to establish the potential of the VGs for transition delay and establish the benefit of the VG optimization. |
Sunday, November 21, 2021 6:04PM - 6:17PM |
F28.00004: Experimental control of Tollmien-Schlichting waves using the Wiener-Hopf formalism Diego B Audiffred, Pedro Paulo C Brito, Eduardo Martini, André Cavalieri Sensor and actuator arrangement may lead to a control law that depends on future sensor readings, and such information is not available in a real-time application. When the non-causal part of the control kernel is substantial, simply truncating the kernel to its causal part implies in suboptimal solutions that can result in a considerable degradation of the control. Optimal causal control (and estimation) laws can be obtained using the Wiener-Hopf formalism by imposing causality in the quadratic cost functional to be minimized. This technique has been used in this work for the experimental control of Tollmien-Schlichting (TS) waves over a NACA 0008 wing profile at zero angle of attack and freestream velocity of 10 m/s. A loudspeaker is used to trigger the TS waves, measurements are performed using microphones and the attenuation of the TS waves is accomplished with a plasma actuator. Experimental results show that the Wiener-Hopf technique can be successfully applied for the control of TS waves. It was also observed that as the actuator moves upstream a significant better performance can be obtained with the Wiener-Hopf technique compared to the truncated solution. |
Sunday, November 21, 2021 6:17PM - 6:30PM |
F28.00005: Control of optimal growth of instabilities in Taylor-Couette flow Harvansh Dandelia, Ravi Kant, Vinod Narayanan It is well understood that sub-critical transition may occur due to short-term algebraic growth in many flows. In this work, we study the optimal growth in standard Taylor-Couette flow and control of the optimal perturbation using wall transpiration. In the non-modal stability framework, we develop a state-space model which incorporates control actuation as periodic suction/blowing of fluid through walls. The study is carried out for different flow configurations such as radii of cylinders and their angular velocities. We explored different wave numbers as well. The Reynolds number is defined based on inner cylinder velocity and the gap between two cylinders. The time evolution of governing equation is written in perturbation velocities in radial (r) and azimuthal (θ) directions. The optimal feedback control is obtained using the linear quadratic regulator (LQR) and feed-backed to the system to reduce the maximum optimal growth of the instabilities in the flow. The perturbation kinetic energy is used as cost function. We used Chebyshev spectral collocation method to discretize the equations and the variational method to calculate the optimal growth. A significant reduction is observed in the growth of perturbation energy. It is observed that a reduction of more than 45% is achieved by this method. More detailed results will be presented at the conference. |
Sunday, November 21, 2021 6:30PM - 6:43PM |
F28.00006: Robust and efficient identification of quasi-optimal mixing perturbations using proxy multiscale measures Colm-Cille P Caulfield, Conor Heffernan Understanding and optimizing passive scalar mixing in a diffusive fluid flow at finite P\'eclet number $Pe=U L/\kappa$ (where $U$ and $L$ are characteristic velocity and length scales, and $\kappa$ is the molecular diffusivisity of the scalar) is a fundamental problem of interest in many environmental and industrial flows. Particularly when $Pe \gg 1$, identifying initial perturbations of given energy which optimally and thoroughly mix fluids of initally different properties can be computationally challenging. To address this challenge, we consider the identification of initial perturbations in an idealized two-dimensional flow on a torus that extremize various measures over finite time horizons. We identify such `optimal' initial perturbations using the `direct-adjoint looping' (DAL) method, thus requiring the evolving flow to satisfy the governing equations and boundary conditions at all points in space and time. We demonstrate that minimising multiscale measures commonly known as `mix-norms' (i.e. Sobolev norms of negative index) over short time horizons is a computationally efficient and robust way to identify initial perturbations that thoroughly mix layered scalar distributions over relatively long time horizons, provided the magnitude of the mix-norm's index is not too large. Minimisation of such (bounded index) mix-norms triggers the development of quasi-coherent vortical flow structures which effectively mix such diffusive passive scalar distributions, with the particular properties of these flow structures depending on $Pe$ and also the time horizon of interest. |
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