An experimental and numerical study of turbulent oscillatory flow over an irregular rough wall

Danny Dunbar, Dominic Van der A* (Corresponding Author), Pietro Scandura, Thomas O'Donoghue

*Corresponding author for this work

Research output: Contribution to journal โ€บ Article โ€บ peer-review

Abstract

The hydrodynamics of turbulent oscillatory flow over a gravel-based irregular rough wall are investigated using laser-Doppler anemometry (LDA) measurements of velocities in a large oscillatory flow tunnel and direct numerical simulation (DNS) of the Navier-Stokes equations. The same periodic irregular roughness was used for both experiments and DNS. Four flow shapes are investigated: sinusoidal, skewed, asymmetric, and combined skewed-asymmetric. The experiments were conducted for target Reynolds number (based on the Stokes length and standard deviation of free-stream velocity) of ๐‘…๐›ฟ, ๐œŽ = 800 and ๐‘…๐›ฟ, ๐œŽ = 1549; DNS was conducted for flows with target ๐‘…๐›ฟ, ๐œŽ = 800. Boundary layer thickness, bottom phase lead and friction factor are in good agreement with previous studies. For the first time, evidence of Prandtlโ€™s secondary flows of the second kind in oscillatory flow is presented. Turbulence
structure is visualised using isosurfaces of ๐œ†2 (Jeong & Hussain 1995), revealing densely packed structures that grow stronger and weaker in correspondence with the free-stream velocity. Reynolds and dispersive stresses peak just below the highest roughness crest, with dispersive stress vanishing a short distance above the roughness. Bursts of turbulence kinetic energy and wake kinetic energy are generated each flow half-cycle, with variable behaviour depending on flow shape. Non-Gaussian turbulence statistics are observed that originate
near the wall, becoming increasingly non-Gaussian far from the wall. Probability density functions of turbulence statistics can be closely approximated by a 4th -order Gram-Charlier distribution at most phases and elevations, though when statistics deviate more strongly from Gaussian, streamwise and wall-normal (spanwise) statistics are better described by a Pearson type IV (VII) distribution.
Original languageEnglish
Number of pages47
JournalJournal of Fluid Mechanics
Volume955
Issue numberA33
Early online date18 Jan 2023
DOIs
Publication statusE-pub ahead of print - 18 Jan 2023

Keywords

  • coastal engineering
  • turbulent boundary layers
  • surface gravity waves

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