Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session G42: Turbulence: Wall-Bounded II - Roughness |
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Chair: Junlin Yuan, Michigan State University Room: 207A |
Sunday, November 19, 2023 3:00PM - 3:13PM |
G42.00001: The effects of Reynolds and Prandtl numbers on the turbulent transport of passive scalars in rough-wall flows Zvi hantsis, Ugo Piomelli Numerical simulations were used to investigate Reynolds- and Prandtl-number effects on scalar transport in channels with rough walls. The Reynolds number based on the friction velocity and the channel half-width ranged from Re_{τ}=1,050 to 7,500, with the corresponding equivalent sand-grain roughness height k_{s}^{+} between 70 and 485. Prandtl numbers between 0.2 and 2.0 were considered. |
Sunday, November 19, 2023 3:13PM - 3:26PM |
G42.00002: Near-wall modeling for LES of turbulent channel flows with rough walls Brett Bornhoft, Suhas S Jain, Sanjeeb Bose, Parviz Moin The interaction between turbulent boundary layers and surface roughness poses significant challenges in various propulsion and turbomachinery devices, as well as in the presence of ice on wings, impacting their aerodynamic performance and operability. However, the high-fidelity simulations of turbulent flows over rough walls using predictive computational tools remain limited. Advances in the development of roughness functions have shown promising results, revealing the necessity of considering at least two or more geometric roughness parameters to characterize near-wall velocity deficit accurately. In this study, we incorporate these geometric roughness functions to model the near-wall region to capture the effect of roughness using LES. The simulation setup consists of a turbulent flow in a channel with rough walls generated from laser scans of sand-blasted roughness (Busse et al., Comp. & Fluids, 2015). Two different simulation strategies are explored: (1) partially grid-resolved roughness using a classical algebraic wall model, and (2) fully subgrid roughness using a modified wall model that incorporates roughness functions. The focus of this work is to evaluate these modeling strategies by comparing them with the existing direct numerical simulations of Thakker et al. (JFM, 2017). The best practices for wall-modeled LES of turbulent flow over rough walls will be discussed. |
Sunday, November 19, 2023 3:26PM - 3:39PM |
G42.00003: Roughness sublayer in non-equilibrium wall-bounded turbulence Junlin Yuan, Sai Chaitanya Mangavelli, Giles J Brereton In rough-wall turbulent flows, the roughness sublayer (RSL) is the near-wall layer where roughness modifies directly the flow dynamics. Despite the importance of this layer in non-equilibrium flows, such as those with strong pressure gradients, the dynamics inside the RSL are not well understood. Direct and large-eddy simulation data are used to show evidence that the RSL thickness and velocities appear to behave as if in a quasi-equilibrium state, in both accelerating and decelerating flows. For example, in an attached flat-plate boundary layer with strong adverse pressure gradient, the wall friction normalized by the RSL edge velocity stays almost constant, while it does not scale on the boundary layer edge velocity. The same scaling is observed for wall friction and form-induced stresses in transient accelerated turbulent channel flows. This is possibly because, although the mean pressure gradient directly accelerates the mean velocity, it is not present in the transport equation of the form-induced velocity. The observed self-preservation in RSL is encouraging for coarse-grained modeling approaches (e.g. RANS and wall-modeled LES) as it implies that equilibrium or quasi-steady RSL models may be applicable to transient and spatial developing flows, though this hypothesis needs to be tested over a wide range of pressure gradient, Reynolds number and roughness topography. |
Sunday, November 19, 2023 3:39PM - 3:52PM |
G42.00004: Interaction of large-scale structures and injected small-scale structures in a turbulent boundary layer Elizabeth Torres De Jesús, Theresa A Saxton-Fox In this experimental study, synthetic small-scale structures were injected in a turbulent boundary layer at different wall-normal positions over a flat plate via the immersion of a small cylinder with a diameter five percent of the boundary layer thickness. Two-dimensional, planar time-resolved particle image velocimetry (PIV) measurements were taken immediately downstream of the cylinder obtaining a field of 3.7δ-by-2.2δ in the streamwise-wall-normal plane. The turbulent statistics indicate that the level of turbulence suppression downstream of the cylinder is dependent on the height where the cylinder is placed. By performing conditional projection averaging of the data using large-scale structures obtained from spectral proper orthogonal decomposition (SPOD), the evolution of the cylinder wake downstream is observed to be affected both in amplitude and advection by local large-scale turbulent structures. Some of the large-scale structures are observed to be lessened in intensity downstream of the cylinder, depending on the wall-normal location of the cylinder. Additional comparisons between coherent structures for the cases with the immersed cylinder and the undisturbed boundary layer may be discussed, such as comparing SPOD modes and quadrant analysis. |
Sunday, November 19, 2023 3:52PM - 4:05PM |
G42.00005: On the attached eddy hypothesis and the turbulent friction factor of pipe flows with rough wall. Daniel A Cruz, Hamidreza A Anbarlooei, Fabio A Ramos, Cecilia M Santos, Gustavo O Celis The characterization of turbulent friction factors in rough pipe flows holds significant importance in fluid dynamics research. Among the various equations employed by engineers, the classical Colebrook-White (CW) equation stands out as the most popular choice. However, despite its accuracy, the CW equation suffers from certain limitations. Being an implicit formulation, it provides limited insight into the near-wall momentum transfer mechanisms.
In this study, we propose an innovative approach by incorporating the attached eddies hypothesis to develop an alternative equation that describes the turbulent friction factor within rough wall pipe flows. Building upon the work of Anbarlooei et al. (H.R. Anbarlooei, D.O.A. Cruz, F. Ramos "New power-law scaling for the friction factor of extreme Reynolds number pipe flows," The Physics of Fluids 32(9):95121.), our formulation expands to include the effects of vortexes generated by the rough wall elements on the momentum transfer near the wall region. The resulting new equation offers explicit representation and demonstrates accurate reproduction of experimental data, addressing the limitations of the classical CW equation. This advancement opens new avenues for understanding and optimizing fluid flow in rough-walled pipes, which has practical implications across various engineering applications. |
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