Abstract
The problem of finding optimal forcing and response for unbounded base flows, exemplified by the Blasius boundary layer, is assessed by means of a locally parallel resolvent analysis. A new analysis of previous results in the literature, which stated that a maximum resolvent gain occurs for spanwise wavenumber \(k_z \approx 0.2\), revealed that this result was not domain converged, and larger domains lead to peak amplification for \(k_z \rightarrow 0\); this result is seen to depend strongly on domain size. It is seen that forcing and response modes for low frequency and wavenumber tend to be extended throughout the computational domain, with substantial support in the free stream. Free-stream modes and their gains are found analytically by considering the resolvent operator for uniform flow, and it is seen that low frequencies and wavenumbers lead to a dominance of such free-stream modes in the resolvent analysis of boundary layers. The lack of domain convergence is explained by the analysis, as gains scale with the square of the domain height. We then propose a new approach to evaluate the resolvent gains for this kind of unbounded flows, by means of a weighting function for the chosen norm that neglects response modes above a cut-off height \(y_p\), typically placed outside the boundary layer thickness; this ensures that relevant responses will only be sought in a region of interest, which here corresponds to the boundary layer. The method proved to solve the problem raised by the presence of free-stream modes, resulting in domain-converged forcing and response modes with the shape of streamwise vortices and streaks, respectively. The results were also shown to be independent of the choice of the filter parameters, leading to converged gains for the whole spectrum.
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References
Bertolotti, F.P.: Response of the blasius boundary layer to free-stream vorticity. Phys. Fluids 9(8), 2286–2299 (1997)
Bertolotti, F.P., Joslin, R.D.: Effect of far-field boundary conditions on boundary-layer transition. Technical report, NASA Langley Research Center (1994)
Boiko, A.V., Demyanko, K.V., Nechepurenko, Y.M.: Asymptotic boundary conditions for the analysis of hydrodynamic stability of flows in regions with open boundaries. Russ. J. Numer. Anal. Math. Model. 34(1), 15–29 (2019)
Brandt, L.: The lift-up effect: the linear mechanism behind transition and turbulence in shear flows. Eur. J. Mech. B/Fluids 47, 80–96 (2014). (Enok Palm Memorial Volume)
Brandt, L., Schlatter, P., Henningson, D.S.: Transition in boundary layers subject to free-stream turbulence. J. Fluid Mech. 517, 167–198 (2004)
Brandt, L., Sipp, D., Pralits, J.O., Marquet, O.: Effect of base-flow variation in noise amplifiers: the flat-plate boundary layer. J. Fluid Mech. 687, 503–528 (2011)
Dergham, G., Sipp, D., Robinet, J.C.: Stochastic dynamics and model reduction of amplifier flows: the backward facing step flow. J. Fluid Mech. 719, 406–430 (2013)
Ellingsen, T., Palm, E.: Stability of linear flow. Phys. Fluids 18(4), 487–488 (1975)
Grosch, C.E., Salwen, H.: The continuous spectrum of the Orr–Sommerfeld equation. Part 1. The spectrum and the eigenfunctions. J. Fluid Mech. 87(01), 33–54 (1978)
Hwang, Y., Cossu, C.: Linear non-normal energy amplification of harmonic and stochastic forcing in the turbulent channel flow. J. Fluid Mech. 664, 51–73 (2010)
Hwang, Y., Cossu, C.: Self-sustained process at large scales in turbulent channel flow. Phys. Rev. Lett. 105(4), 044505 (2010)
Jonáš, P., Mazur, O., Uruba, V.: On the receptivity of the by-pass transition to the length scale of the outer stream turbulence. Eur. J. Mech. B/Fluids 19(5), 707–722 (2000)
Jovanovic, M.R., Bamieh, B.: Componentwise energy amplification in channel flows. J. Fluid Mech. 534, 145–183 (2005)
Matsubara, M., Alfredsson, P.H.: Disturbance growth in boundary layers subjected to free-stream turbulence. J. Fluid Mech. 430, 149–168 (2001)
McKeon, B., Sharma, A.: A critical-layer framework for turbulent pipe flow. J. Fluid Mech. 658, 336–382 (2010)
Salwen, H., Grosch, C.E.: The continuous spectrum of the Orr–Sommerfeld equation. Part 2. Eigenfunction expansions. J. Fluid Mech. 104, 445–465 (1981)
Schmid, P.J.: Nonmodal stability theory. Annu. Rev. Fluid Mech. 39, 129–162 (2007)
Schmid, P.J., Henningson, D.S.: Stability and Transition in Shear Flows. Applied Mathematical Sciences, vol. 142. Springer, New York, NY (2001)
Schmidt, O.T., Towne, A., Rigas, G., Colonius, T., Brès, G.A.: Spectral analysis of jet turbulence. J. Fluid Mech. 855, 953–982 (2018)
Tissot, G., Zhang, M., Lajús, F.C., Cavalieri, A.V., Jordan, P.: Sensitivity of wavepackets in jets to nonlinear effects: the role of the critical layer. J. Fluid Mech. 811, 95–137 (2017)
Trefethen, L.N.: Spectral Methods in MATLAB, vol. 10. Society for Industrial Mathematics, Philadelphia (2000)
Acknowledgements
This work was done with the support from CNPq (Grant 310523/2017-6), from CISB, the Swedish-Brazilian Research and Innovation Centre, and from Saab AB. We would like to thank the anonymous referees for the careful comments on the manuscript, and, in particular, to one of the reviewers who suggested the use of shear-based weights applied in Sect. 6.
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Communicated by Vassilios Theofilis.
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Nogueira, P.A.S., Cavalieri, A.V.G., Hanifi, A. et al. Resolvent analysis in unbounded flows: role of free-stream modes. Theor. Comput. Fluid Dyn. 34, 163–176 (2020). https://doi.org/10.1007/s00162-020-00519-x
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DOI: https://doi.org/10.1007/s00162-020-00519-x