Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Z32: Flow Instability: Boundary Layers and Transition II |
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Chair: Saikishan Suryanarayanan, University of Texas at Austin Room: 240 |
Tuesday, November 22, 2022 12:50PM - 1:03PM |
Z32.00001: Infection of subcritical roughness wakes by unstable neighbors Saikishan Suryanarayanan, David Goldstein The detailed mechanisms by which a wake of a subcritical discrete roughness element in a zero-pressure gradient laminar boundary layer is destabilized by background disturbances are analyzed. This knowledge is relevant for extending the understanding and control of boundary transition caused by an isolated discrete roughness element to a splatter involving discrete roughness elements of various sizes. Direct numerical simulations are performed for a range of configurations involving multiple discrete roughness elements and the results are analyzed using stability analysis and vorticity dynamics points of view. It is found that transition behind large discrete roughness elements can destabilize the lifted-up vortical structure downstream of a subcritical roughness wake by causing the top of the vortex lines to clump or by causing the entire structure to sway from side to side. The mechanism depends upon the relative locations of the roughness elements. Broader impacts of these findings on distributed roughness transition, wedge spreading on rough surfaces, and mitigation strategies are discussed. |
Tuesday, November 22, 2022 1:03PM - 1:16PM |
Z32.00002: Ultraspherical Spectral Method for Incompressible Boundary Layer Stability Calculations Vaishnavi Ramaswamy, Claire M Namuroy, Wesley L Harris Linear stability theory constitutes an important approach to predicting the amplification of disturbances in fluid boundary layers and eventual transition to turbulence. In this work, we implement the ultraspherical polynomial spectral method (i.e. US method) developed by Olver & Townsend (siam REVIEW 55, no. 3 (2013), pp. 462-489) to the eigenvalue problems associated with incompressible laminar boundary layer stability. The US method is a spectral coefficient method developed for linear, higher-order differential equations involving the representation of derivatives using ultraspherical (or Gegenbauer) polynomials. The method is favourable as it results in sparse, almost-banded, well-conditioned matrices. First, the feasibility of the numerical scheme is demonstrated by solving for the temporal eigenvalue spectrum of the Orr-Sommerfeld equation for plane channel (Poiseuille) flow at Re=10,000 (classic Orszag (J. Fluid Mech., 50.4 (1971), pp. 689-703) test case). The ease of implementation, accuracy of solution, and computational complexity of the US spectral method are compared against existing schemes for this problem including historic asymptotic formulae for the eigenvalues, finite difference, and Chebyshev collocation and coefficient based methods. The US method is then extended to capture the frequencies and wavelengths of boundary layer disturbances leading to transition, for Blasius and other self-similar incompressible boundary layer profiles. Finally, we highlight the favourable aspects of extending the present numerical method to compressible, and more complex boundary layer flows of interest. |
Tuesday, November 22, 2022 1:16PM - 1:29PM |
Z32.00003: Puff dynamics in the azimuthally constrained pipe flow Nazmi Burak Budanur Transition to turbulence in pipe flow is best understood as a nonequilibrium |
Tuesday, November 22, 2022 1:29PM - 1:42PM |
Z32.00004: Control of laminar-to-turbulent transition via external body forces in channel flow Cesar A Leos, Jae Sung Park The effects of an external force control strategy on the transition to turbulence in minimal channel flow is investigated by direct numerical simulation. In this study, the control method is represented by the spanwise external force, and various velocity perturbations are applied to an initial laminar state. A friction Reynolds number considered is from 112 to 310. The external force appears to modify the transition dynamics very little for high perturbation magnitudes, but quite considerably for low perturbations where results show that the transition time strongly depends on the forcing frequency when compared to no control. Furthermore, the results within the Reynolds numbers studied show that the critical perturbation to trigger the transition decreases, suggesting faster transition by forcing with low frequencies for higher Reynolds numbers. A possible mechanism behind this control method is associated with its unique roller-like vortical structures formed near the wall. This interesting phenomenon appears to prevent interactions between inner and outer regions, eventually modulating the transition dynamics. Finally, it is shown that the perturbation magnitude and forcing frequency are important parameters to control the transition to turbulence. |
Tuesday, November 22, 2022 1:42PM - 1:55PM |
Z32.00005: Excitation and evolution of compressible Görtler vortices induced by free-stream vortical disturbances Dongdong Xu, Pierre Ricco We study the nonlinear development of compressible Görtler vortices (GV) excited by free-stream vortical disturbances (FSVD) in a compressible boundary layer over a concave wall. The free-stream Mach number is assumed to be of O(1) and the FSVD are strong enough so that compressibility and nonlinearity are both relevant. The focus is on low-frequency (long-wavelength) components of FSVD, which excite and drive the GV efficiently. The dynamics of the GV are governed by the compressible nonlinear boundary-region equations, supplemented by appropriate initial and boundary conditions, which characterise the impact of FSVD on the boundary layer. The numerical results are compared with experimental measurements in a hypersonic boundary layer at Mach number 6.5. The predicted mean flow distortion and the wall temperature are in good agreement with the experimental results. As the frequency increases, the nonlinearly generated harmonic component with zero frequency and a spanwise wavenumber double that of the FSVD becomes dominant downstream. As a result, the GV become predominantly steady. |
Tuesday, November 22, 2022 1:55PM - 2:08PM |
Z32.00006: The nature of the transition to turbulence in weakly-curved pipe flow Mukund Vasudevan, Yi Zhuang, Bjoern Hof In wall-bounded shear flows, turbulence arises abruptly, and typically in spite of the linear stability of the laminar base flow. Recent experiments and simulations have established an analogy to absorbing-state phase transitions; more specifically in the universality class of directed percolation (DP). While good agreement with this framework has been demonstrated for Couette type and shear-driven flows, the situation in pressure driven flows is less clear. In particular for pipe flow, measurements of universal characteristics such as critical exponents appear beyond reach, given the extremely large temporal and spatial scales that are relevant close to the critical point. Even conservative estimates suggest that such studies would require pipes more than 1010 diameters in length. We circumvent this problem here by introducing a second parameter: pipe curvature. As shown, even for very weakly curved pipes (radius ratio of around 6 x10-4) the relevant time scales, such as puff decay and splitting times are lower by four orders of magnitude, while the overall nature of the transition appears unchanged. This reduction in time scales is explained by a shift of the transition point. In addition preliminary results on the universal aspects of this transition are presented. |
Tuesday, November 22, 2022 2:08PM - 2:21PM |
Z32.00007: Structured input-output analysis of transitional Blasius boundary layer flows using a descriptor state space model. Aishwarya Rath, Chang Liu, Dennice Gayme We extend the recently proposed structured input-output analysis of channel flows (J. Fluid Mech. vol. 927, A25) to the transitional Blasius boundary layer. This approach enables the incorporation of nonlinear effects into input-output analysis through a feedback interconnection between the linear operator and modeled nonlinearity. The optimal perturbation associated with each (streamwise and spanwise) wavenumber pair can then be quantified by the structured singular value of a modified spatio-temporal frequency response operator. Our implementation employs a descriptor state space model (DSS) model that provides a more direct approach for imposing nontraditional boundary conditions (e.g., stress-free), thereby expanding the applications that can be studied. We verified the DSS model for channel flows with the previously reported results for canonical flows. As with previous observations for channel and plane Couette flow, the imposition of a model that preserves the componentwise structure of the nonlinearity in Blasius boundary layer flows weakens the streamwise elongated flow structures and uncovers the highly amplified streamwise dependent flow structures. The results are compared with nonlinear optimal perturbation and direct numerical simulation in the existing literature. |
Tuesday, November 22, 2022 2:21PM - 2:34PM |
Z32.00008: Modal and non-modal transition scenarios of compression ramp flows at a range of Reynolds numbers Nicolas Cerulus, Ricardo Santos, Helio Quintanilha Jr., Leonardo Alves, Vassilis Theofilis Transition to turbulence of laminar compressible flow over a compression ramp has been addressed using modal and non-modal stability analysis. The importance of highly accurate base states has been identified leading us to analyse steady base states obtained by high-order temporal and spatial discretization schemes. These steady states have been compared with theoretical solutions of the triple-deck equations and excellent agreement has been shown. Modal analysis was employed to obtain the neutral loop pertaining to the leading stationary, three-dimensional global flow eigenmode. Globally stable eigenmodes in the spectrum were found to correspond to Mack mode, also seen in experiments. An impedance well between the wall and separation shock is also presented which has the ability to sustain and trap thermoacoustic waves downstream of separation. Non-modal global stability analysis performed demonstrated that large transient energy growth of linear perturbations can lead to transition substantially earlier than the time needed for modally unstable perturbations to grow (exponentially) to nonlinear levels. |
Tuesday, November 22, 2022 2:34PM - 2:47PM |
Z32.00009: On the interaction between rotation-triggered instabilities and the shear-layer instability over a laminar separation bubble on a rotating airfoil and its relation to transition Thales Coelho Leite Fava, Ardeshir Hanifi, Dan S Henningson Rotating airfoils may experience the appearance of a cross-flow velocity due to centrifugal and Coriolis forces, which can be particularly important in regions with separated flow. In non-rotating airfoils with a laminar separation bubble and low to moderate free-stream turbulence levels, laminar-turbulent transition is often triggered by the break-up of Kelvin-Helmholtz (KH) rolls formed over the separated shear layer. This work shows through direct numerical simulations that the cross-flow velocity generated by rotation is inflectional and generates inviscid instabilities that interact with the normal growth and break-up to turbulence of these KH rolls. Furthermore, this interaction may result in a transition location moving upstream if the pressure gradient is low or downstream if it is high, such as in high angles-of-attack cases. Parabolized stability equations are applied to the problem and show that the most amplified frequencies match those observed in the DNS. This indicates a convective nature of the instabilities observed. Global stability analysis is performed, confirming this character of the flow. However, it is noticed that the flow may become absolutely unstable under certain rotation conditions due to the enhancement of the reverse flow in the separation region. |
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