arXiv subjects

A. Duggleby

Publications and source records attributed to A. Duggleby.

3 recordsLinked to original sources

arXiv2007

Dynamics of propagating turbulent pipe flow structures. Part I: Effect of drag reduction by spanwise wall oscillation

The results of a comparative analysis based upon a Karhunen-Loève expansion of turbulent pipe flow and drag reduced turbulent pipe flow by spanwise wall oscillation are presented. The turbulent flow is generated by a direct numerical simulation at a Reynolds number $Re_τ= 150$. The spanwise wall oscillation is imposed as a velocity boundary condition with an amplitude of $A^+ = 20$ and a period of $T^+ = 50$. The wall oscillation results in a 27% mean velocity increase when the flow is driven by a constant pressure gradient. The peaks of the Reynolds stress and root-mean-squared velocities shift away from the wall and the Karhunen-Loève dimension of the turbulent attractor is reduced from 2453 to 102. The coherent vorticity structures are pushed away from the wall into higher speed flow, causing an increase of their advection speed of 34% as determined by a normal speed locus. This increase in advection speed gives the propagating waves less time to interact with the roll modes. This leads to less energy transfer and a shorter lifespan of the propagating structures, and thus less Reynolds stress production which results in drag reduction.

physics.flu-dyn↗

arXiv2007

Dynamics of propagating turbulent pipe flow structures. Part II: Relaminarization

The dynamical behavior of propagating structures, determined from a Karhunen-Lo`eve decomposition, in turbulent pipe flow undergoing reverse transition to laminar flow is investigated. The turbulent flow data is generated by a direct numerical simulation started at a fully turbulent Reynolds number of Re_τ=150, which is slowly decreased until Re_τ=95. At this low Reynolds number the high frequency modes decay first, leaving only the decaying streamwise vortices. The flow undergoes a chugging phenomena, where it begins to relaminarize and the mean velocity increases. The remaining propagating modes then destabilize the streamwise vortices, rebuild the energy spectra, and eventually the flow regains its turbulent state. Our results capture three chugging cycles before the flow completely relaminarizes. The high frequency modes present in the outer layer decay first, establishing the importance of the outer region in the self-sustaining mechanism of wall bound turbulence.

physics.flu-dyn↗

arXiv2007

Dynamical Eigenfunction Decomposition of Turbulent Pipe Flow

The results of an analysis of turbulent pipe flow based on a Karhunen-Lo`eve decomposition are presented. The turbulent flow is generated by a direct numerical simulation of the Navier-Stokes equations using a spectral element algorithm at a Reynolds number Re_τ=150. This simulation yields a set of basis functions that captures 90% of the energy after 2,453 modes. The eigenfunctions are categorised into two classes and six subclasses based on their wavenumber and coherent vorticity structure. Of the total energy, 81% is in the propagating class, characterised by constant phase speeds; the remaining energy is found in the non propagating subclasses, the shear and roll modes. The four subclasses of the propagating modes are the wall, lift, asymmetric, and ring modes. The wall modes display coherent vorticity structures near the wall, the lift modes display coherent vorticity structures that lift away from the wall, the asymmetric modes break the symmetry about the axis, and the ring modes display rings of coherent vorticity. Together, the propagating modes form a wave packet, as found from a circular normal speed locus. The energy transfer mechanism in the flow is a four step process. The process begins with energy being transferred from mean flow to the shear modes, then to the roll modes. Energy is then transfer ed from the roll modes to the wall modes, and then eventually to the lift modes. The ring and asymmetric modes act as catalysts that aid in this four step energy transfer. Physically, this mechanism shows how the energy in the flow starts at the wall and then propagates into the outer layer.

physics.flu-dyn↗