Inner-outer interactions in a rough-wall turbulent boundary layer

The primary goal of the present effort is to enhance the current understanding of inner-outer interactions in rough-wall turbulent boundary layers. These interactions were recently established over smooth-wall turbulent boundary layers as modulating interactions, where the outer layer large scales amplitude and frequency modulate the near-wall small scales. Given that the outer layer dynamics responsible for these modulations are identical in most high-Reynolds-number (Re) rough-wall flows as well, similar modulation interactions are are explored to identify the similarities and differences of these interactions and establish a spatio-temporal description of the same. This is particularly important given the engineering significance of the flows over rough walls. This work was performed as two parts. In the first part, high temporal-resolution boundary layer hot-wire measurements were made in a wind tunnel, that fully resolved all dynamical scales temporally at fixed points in the flow. Flows over smooth and rough walls were investigated, with the latter being a complex topography indicative of a realistic roughness commonly encountered in engineering applications. Single- and two-probe measurements provided a dual perspective on the large scales, and enabled analysis of analytical techniques commonly employed. With these measurements, it was found that the nature of amplitude and frequency modulation occur even over this complex topography, and that the structure is very similar to that observed in smooth-wall flow. Further, the simultaneous two-probe measurements enabled the investigation of predictive models, which interestingly suggested a possibly stronger modulation in rough-wall flow compared to the smooth-wall case. A `quasi-steady, quasi-homogenous' theory previously developed for smooth-wall flow showed promising predictions of the calibrations constants even in rough-wall flow, lending additional support to the mechanisms speculating that the small scales respond in a quasi-steady manner, irrespective of the origin, to the large scales. With these inner-outer interactions established, the second part of the current work aimed to develop a spatio-temporal description of the modulating mechanisms using high frame-rate particle-image velocimetry (PIV). The experiments, performed in a refractive-index-matched flow facility, enabled the measurements very close to the surface without being impeded by the near-wall reflections that are common in smooth- and rough-wall PIV experiments. Following a preliminary demonstration of the relevant physics observed via point measurements, a representative large-scale structure was defined using conditional averaging. The associated changes to the small-scale turbulence close to the wall indicated similar modulation interactions, and provided a spatial tool to investigate the same. Further, the large-scale -small-scale interaction structure lended support to the speculations made on the same using hot-wire measurements in the current work and in the recent literature for smooth- and rough-wall flow. These experiments identify and emphasize the significance of inner-outer interactions over rough-wall flows, and the necessity to accurately model them to enhance the fidelity of any high Re simulations over rough walls

Structure of turbulent channel flow perturbed by cylindrical roughness elements

The current study investigates the structural modifications imposed in fully-developed turbulent channel flow by an isolated, wall-mounted circular cylinder. The cylinder height is chosen to specifically extend into the logarithmic layer of the flow in order to study its perturbation of the larger flow scales that embody a significant fraction of the turbulent kinetic energy. Hot-wire measurements were made in the wake of the wall-mounted circular cylinder at multiple wall-normal and streamwise positions. Mean streamwise statistics (mean velocity and Reynolds normal stress) and pre-multiplied spectra of perturbed and unperturbed flow were computed, and influence of the cylinder on these statistics were analyzed. The influence of such perturbations on the inner-outer interactions of the channel flow were also investigated. Besides the mean velocity deficit in the wake of the cylinder, a new peak in streamwise Reynolds normal stress away from the wall was observed, coupled with the suppression of the near-wall peak native to the incoming unperturbed flow. Pre-multiplied spectra elaborated on these energy modifications, specifically the occurrence of an energy peak corresponding to a wavelength (λx) ~ 0.45 times the channel half-height (h), an attenuation of large-scale energy close to the wall, and a tertiary peak at two-third's the cylinder height corresponding to a length scale of λx ~ 10h. Further, amplitude modulation effects of the large-scale motions on small scales close to the wall, representative of inner-outer interactions, was found to be greatly enhanced in the near-wall region. All the perturbations were found to decay with streamwise distance downstream towards the unperturbed flow. A clear persistence of the structures at the aforementioned tertiary peak, similar to the wavelengths of the very large scale motions (VLSMs) in canonical wall turbulence, tends to suggest an environment in turbulent flows preferring structures of such wavelengths. Possible mechanisms for the observed suppression of near-wall cycle and the enhanced inner-outer interactions are suggested. The influence of cylinder aspect ratio on the characteristics of perturbed flow are evaluated, and a distinction in wake structure is identified. The necessity of future studies to further understand these significant attributes of perturbation response and recovery of the turbulent wall bounded flows is highlighted

Numerical Analyses of High Temperature Dense, Granular Flows Coupled to High Temperature Flow Property Measurements for Solar Thermal Energy Storage Dataset

The document ‘HighTemperatureFlowData_CP3060.xlsx’ contains high temperature measurements in separate tabs for 1) Particle size and shape 2) Elastic properties 3) Coefficient of restitution 4) Coefficient of static sliding friction and 5) Coefficient of static rolling friction. The documents. The document ‘liggghtsinputscript.txt’ contains the LIGGGHTS input scripts used for modeling.Supplemental data for Justin D. Yarrington, Malavika V. Bagepalli, Gokul Pathikonda, Andrew J. Schrader, Zhuomin M. Zhang, Devesh Ranjan, Peter G. Loutzenhiser, Numerical analyses of high temperature dense, granular flows coupled to high temperature flow property measurements for solar thermal energy storage, Solar Energy, Volume 213, 2021, Pages 350-360, ISSN 0038-092X, https://doi.org/10.1016/j.solener.2020.10.085.High temperature particle flow properties necessary to predict granular flow behavior for solar thermal energy storage applications were measured and calculated for Carbobead CP 30/60 up to 800 °C. The measured properties included elastic and shear moduli, particle-particle coefficients of static sliding and rolling friction, and particle-particle coefficients of restitution. Poisson’s ratio was calculated with elastic and shear moduli. The flow properties were used as inputs for a numerical model using the discrete element method to examine granular flows along an inclined plane at high temperature. The flow behavior was strongly influenced by the coefficients of static friction, which impacted the particle residence time, shear effects from the side walls, and particle flow mass flux. An 8.7%, 15.6%, and 8.5% increase and 37.9% decrease in steady state mass flow rate was observed for 200 °C, 400 °C, 600 °C, and 800 °C, respectively, when compared to room temperature simulations. A 52%, 59%, and 33% decrease in the time to reach steady state was observed for 200 °C, 400 °C, and 600 °C, respectively, while a 53% increase in time was observed for 800 °C. A significant delay in the flow development at 800 °C was observed due to significantly higher frictional forces.U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under Solar Energy Technologies Office (SETO) Agreement Number EE000837

Georgia Tech | Gen3 CSP Mechanical and Radiative Property Database

This is a database of the mechanical and radiative property characterization results for particulate flows at elevated temperatures, as funded through the Solar Energy Technologies Office, Award: DE-EE0008372.The focus of this work is to systematically characterize the heat transfer and flow properties for particulate (granular) flows at elevated temperatures up to 800 °C. This work is intended to address a serious gap within the field related to the understanding and modeling of particulate flow behavior and the related heat transfer at different temperatures, which directly correspond to the operating points of concentrated solar power applications that use particles for heat storage. These objectives will be accomplished using a combination of fundamental experimental measurements, modeling, and simplified flow experimentations over a range of temperatures. Ceramic sintered bauxite proppants will be used as a baseline for comparison with a range of other particles used for various applications. These results will be made available during the project to the research community to provide updated guidance and inputs to current modeling efforts to improve their results.Department of Energy, Solar Energy Technology Office. Award: DE-EE0008372, Grant: GR1000513