Date of Award
1-1-2024
Document Type
Thesis
Degree Name
M.S. in Engineering Science
First Advisor
Wen Wu
Second Advisor
Shan Jiang
Third Advisor
Wen Wu
School
University of Mississippi
Relational Format
dissertation/thesis
Abstract
Modulation of roughness effects by pulsating pressure gradients (PG) is explored by DNS of turbulent channel flows. Sinusoidal pulsation of PG is examined for w+ = 0 (steady), 0.02, and 0.04. The pulsation magnitude is ⇠25% of the mean PG, which drives the flow at Ret,o = 1500. Comparisons between flows over the smooth wall and sand grain roughness consisting of randomly distributed ellipsoids are made. The mean roughness height is k/H = 0.04. A quintuple decomposition is proposed to isolate the pulsation- and roughness-induced fluctuations. Roughness appears to be the dominant factor characterizing the flow: Townsend’s outer-layer similarity holds for the mean velocity, and Reynolds stresses beyond a 2k-thick roughness sublayer. The mean wake (dispersive) flow stays the same within the roughness sublayer regardless of the pulsation. However, pulsation creates a pulsating wake within the roughness sublayer, significantly amplifying the Reynolds stresses. The equivalent roughness height increases from k+s = 94 (steady) to 225 (w = 0.04). Without the roughness, Stoke layers are limited below the buffer layer for the frequencies examined. Roughness offsets them by 0.2-0.3k and thickens by a factor of 5, leading to wall-normal variations in pulsation amplitude and phase up to the mid-log-law region. Spatial asymmetries between phases indicate that the higher frequency is biased toward the decelerating phase. Increasing frequency may increase the concentration of velocity field variations within the same roughness length where the shielding effect of the roughness preserves the variation. Such behavior may lead to increased vorticity, where high momentum entrapment by the roughness may amplify roughness effects.
Recommended Citation
Miles, John Palmer, "The Impact of Flow Pulsation on Roughness Effects in Turbulent Channel Flow" (2024). Electronic Theses and Dissertations. 2956.
https://egrove.olemiss.edu/etd/2956