TY - JOUR

T1 - Mean flow structure and velocity-bed shear stress maxima phase difference in smooth wall, transitionally turbulent oscillatory boundary layers

T2 - Direct numerical simulations

AU - Fytanidis, Dimitrios K.

AU - García, Marcelo H.

AU - Fischer, Paul F.

N1 - Publisher Copyright:
© The Author(s), 2021.

PY - 2021/12/10

Y1 - 2021/12/10

N2 - Direct numerical simulations of oscillatory boundary-layer flows in the transitional regime were performed to explain discrepancies in the literature regarding the phase difference Δφ between the bed-shear stress and free-stream velocity maxima. Recent experimental observations in smooth bed oscillatory boundary-layer (OBL) flows, showed a significant change in the widely used Δφ diagram (Mier et al., J. Fluid Mech., vol. 922, 2021, A29). However, the limitations of the point-wise measurement technique did not allow us to associate this finding with the turbulent kinetic energy budget and to detect the approach to a 'near-equilibrium' condition, defined in a narrow sense herein. Direct numerical simulation results suggest that a phase lag occurs as the result of a delayed and incomplete transition of OBL flows to a stage that mimics the fully turbulent regime. Data from the literature were also used to support the presence of the phase lag and propose a new Δφ diagram. Simulations performed for Reδ = 671 confirmed the sensitivity in the development of self-sustained turbulence on the background disturbances (Reδ = Uoδ/ν, where δ = [2ν/ω]1/2 is the Stokes' length, Uo is the maximum free stream velocity of the oscillation, ν is the kinematic viscosity and ω = 2π/T is the angular velocity based on the period of the oscillation T). Variations of the mean velocity slope and intersect values for oscillatory flows are also explained in terms of the proximity to near-equilibrium conditions. Relaminarization and transition effects can significantly delay the development of OBL flows, resulting in an incomplete transition. The shape and defect factors are examined as diagnostic parameters for conditions that allow the formation of a logarithmic profile with the universal von Kármán constant and intersect. These findings are of relevance for environmental fluid mechanics and coastal morphodynamics/engineering applications.

AB - Direct numerical simulations of oscillatory boundary-layer flows in the transitional regime were performed to explain discrepancies in the literature regarding the phase difference Δφ between the bed-shear stress and free-stream velocity maxima. Recent experimental observations in smooth bed oscillatory boundary-layer (OBL) flows, showed a significant change in the widely used Δφ diagram (Mier et al., J. Fluid Mech., vol. 922, 2021, A29). However, the limitations of the point-wise measurement technique did not allow us to associate this finding with the turbulent kinetic energy budget and to detect the approach to a 'near-equilibrium' condition, defined in a narrow sense herein. Direct numerical simulation results suggest that a phase lag occurs as the result of a delayed and incomplete transition of OBL flows to a stage that mimics the fully turbulent regime. Data from the literature were also used to support the presence of the phase lag and propose a new Δφ diagram. Simulations performed for Reδ = 671 confirmed the sensitivity in the development of self-sustained turbulence on the background disturbances (Reδ = Uoδ/ν, where δ = [2ν/ω]1/2 is the Stokes' length, Uo is the maximum free stream velocity of the oscillation, ν is the kinematic viscosity and ω = 2π/T is the angular velocity based on the period of the oscillation T). Variations of the mean velocity slope and intersect values for oscillatory flows are also explained in terms of the proximity to near-equilibrium conditions. Relaminarization and transition effects can significantly delay the development of OBL flows, resulting in an incomplete transition. The shape and defect factors are examined as diagnostic parameters for conditions that allow the formation of a logarithmic profile with the universal von Kármán constant and intersect. These findings are of relevance for environmental fluid mechanics and coastal morphodynamics/engineering applications.

KW - Boundary layer structure

KW - Coastal engineering

KW - Turbulent boundary layers

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U2 - 10.1017/jfm.2021.827

DO - 10.1017/jfm.2021.827

M3 - Article

AN - SCOPUS:85119094562

SN - 0022-1120

VL - 928

JO - Journal of Fluid Mechanics

JF - Journal of Fluid Mechanics

M1 - A33

ER -