Growth of boundary-layer streaks due to free-stream turbulence

Riccó, Pierre; Walsh, Edmond J.; Brighenti, Flavio; McEligot, Donald M.

doi:10.1016/j.ijheatfluidflow.2016.05.003

Cited by 9 publications

(7 citation statements)

References 55 publications

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“…Recently, Ricco et al (2016) highlighted the strengths of this theory compared to other theoretical approaches found in literature for the analysis of bypass transition, and proved its validity by showing good agreement with the experimental data and with the direct numerical simulation data of Wu & Moin (2009). When streamwise concave curvature is present, Klebanoff modes turn into Görtler vortices as they evolve downstream.…”

Section: Incompressible Görtler Vorticessupporting

confidence: 52%

Compressible unsteady Görtler vortices subject to free-stream vortical disturbances

Viaro

Riccó

2019

J. Fluid Mech.

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The perturbations triggered by free-stream vortical disturbances in compressible boundary layers developing over concave walls are studied numerically and through asymptotic methods. We employ an asymptotic framework based on the limit of high Görtler number, the scaled parameter defining the centrifugal effects; we use an eigenvalue formulation where the free-stream forcing is neglected; and we solve the receptivity problem by integrating the compressible boundary-region equations complemented by appropriate initial and boundary conditions that synthesize the influence of the free-stream vortical flow. Near the leading edge, the boundary-layer perturbations develop as thermal Klebanoff modes and, when centrifugal effects become influential, these modes turn into thermal Görtler vortices, i.e. streamwise rolls characterized by intense velocity and temperature perturbations. The high-Görtler-number asymptotic analysis reveals the condition for which the Görtler vortices start to grow. The Mach number is destabilizing when the spanwise diffusion is negligible and stabilizing when the boundary-layer thickness is comparable with the spanwise wavelength of the vortices. When the Görtler number is large, the theoretical analysis also shows that the vortices move towards the wall as the Mach number increases. These results are confirmed by the receptivity analysis, which additionally clarifies that the temperature perturbations respond to this reversed behaviour further downstream than the velocity perturbations. A matched-asymptotic composite profile, found by combining the inviscid core solution and the near-wall viscous solution, agrees well with the receptivity profile sufficiently downstream and at high Görtler number. The Görtler vortices tend to move towards the boundary-layer core when the flow is more stable, i.e. as the frequency or the Mach number increase, or when the curvature decreases. As a consequence, a region of unperturbed flow is generated near the wall. We also find that the streamwise length scale of the boundary-layer perturbations is always smaller than the free-stream streamwise wavelength. During the initial development of the vortices, only the receptivity calculations are accurate. At streamwise locations where the free-stream disturbances have fully decayed, the growth rate and wavelength are computed with sufficient accuracy by the eigenvalue analysis, although the correct amplitude and evolution of the Görtler vortices can only be determined by the receptivity calculations. It is further proved that the eigenvalue predictions of the growth rate and wavenumber worsen as the Mach number increases as these quantities show a dependence on the wall-normal direction. We conclude by qualitatively comparing our results with the direct numerical simulations available in the literature.

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Section: Incompressible Görtler Vorticessupporting

confidence: 52%

Compressible unsteady Görtler vortices subject to free-stream vortical disturbances

Viaro

Riccó

2019

J. Fluid Mech.

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“…Based on many of these experimental and numerical studies, it was found that the maximum energy growth scales as the square of the so-called roughness Reynolds number Re hh = U BL (h)h ν with h being the height of the roughness element, U BL (h) the would-be value of the Blasius velocity profile at this particular height, and ν the kinematic velocity of the fluid considered. Many authors have studied the transient growth and breakdown of streaks under freestream vortical disturbances or turbulence [26][27][28][29]. However, the presence of the roughness element induces the formation of a separation region, whose influence on the stability of the flow is still open to debate and appears to depend on a large number of external parameters.…”

Section: Introductionmentioning

confidence: 99%

Influence of freestream turbulence on the flow over a wall roughness

et al. 2021

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“…On the other hand, sufficiently high levels of broad-band excitation can induce the growth of streamwise elongated streaks that play an important role in bypass transition [2]. The effect of free-stream turbulence on the growth of boundary layer streaks has been the subject of various experimental [14][15][16], numerical [17,18], and theoretical [19,20] studies. In particular, it has been shown that free-stream disturbances that penetrate into the boundary layer are elongated in the streamwise direction [21].…”

Section: Introductionmentioning

confidence: 99%

Stochastic receptivity analysis of boundary layer flow

et al. 2019

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We utilize the externally forced linearized Navier-Stokes equations to study the receptivity of pre-transitional boundary layers to persistent sources of stochastic excitation. Stochastic forcing is used to model the effect of free-stream turbulence that enters at various wall-normal locations and the fluctuation dynamics are studied via linearized models that arise from locally parallel and global perspectives. In contrast to the widely used resolvent analysis that quantifies the amplification of deterministic disturbances at a given temporal frequency, our approach examines the steady-state response to stochastic excitation that is uncorrelated in time. In addition to stochastic forcing with identity covariance, we utilize the spatial spectrum of homogeneous isotropic turbulence to model the effect of free-stream turbulence. Even though locally parallel analysis does not account for the effect of the spatially evolving base flow, we demonstrate that it captures the essential mechanisms and the prevailing length-scales in stochastically forced boundary layer flows. On the other hand, global analysis, which accounts for the spatially evolving nature of the boundary layer flow, predicts the amplification of a cascade of streamwise scales throughout the streamwise domain. We show that the flow structures that can be extracted from a modal decomposition of the resulting velocity covariance matrix, can be closely captured by conducting locally parallel analysis at various streamwise locations and over different wall-parallel wavenumber pairs. Our approach does not rely on costly stochastic simulations and it provides insight into mechanisms for perturbation growth including the interaction of the slowly varying base flow with streaks and Tollmien-Schlichting waves. *

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Growth of boundary-layer streaks due to free-stream turbulence

Cited by 9 publications

References 55 publications

Compressible unsteady Görtler vortices subject to free-stream vortical disturbances

Compressible unsteady Görtler vortices subject to free-stream vortical disturbances

Influence of freestream turbulence on the flow over a wall roughness

Stochastic receptivity analysis of boundary layer flow

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