Department of Mechanical and Materials Engineering Faculty Publications

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    Roughness Effects on Scalar Transport
    (American Physical Society, 2020-11-18) Hantsis, Zvi; Piomelli, Ugo
    We have studied the transport of a passive scalar for passive scalar with Prandtl numbers near unity in a plane channel with rough walls. The study was carried out by direct numerical simulations of the Navier-Stokes equations; an Immersed Boundary Method was used to model the roughness. The well-known departure from the Reynolds Analogy, which postulates similarity between the statistics of the scalar and the velocity, is verified. Townsend's similarity, suggesting the smooth-wall and rough-wall statistics collapse away from the wall is confirmed. The role of the form-induced production was a focus of this work. Additional form-induced contributions appear in the Reynolds-stress and scalar-variance budgets due to the roughness; they were quantified and compared between passive scalar and momentum. The form-induced production is more significant for the scalar variance than for the streamwise Reynolds stress, and could be the cause of the reduced damping of scalar fluctuations by the roughness. The sheltering caused by tall roughness elements decreases the mean gradients significantly but, in the case of the scalar, is countered by diffusion, so that the form-induced production is larger for the scalar variance. The implications of this finding are discussed.
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    Accuracy of bed-load transport models in eddy-resolving simulations
    (Elsevier, 2021-04-28) D’Alessandro, Gianmarco; Hantsis, Zvi; Marchioli, Cristian; Piomelli, Ugo
    This work investigates the accuracy of commonly used bed-load transport models when applied in combination with high-resolution Navier-Stokes solvers. Empirical bed-load models predict the transport rate of sediments based on the average bottom shear-stress, while eddy-resolving approaches allow for a space- and time-dependent description of the bottom shear-stress distribution. We discuss the effect that a fine-graining of the stress distribution provided by the flow solver has on the transport model prediction, and we examine the space and time scales at which the averaged values of the transport rate, obtained using the local stress distribution, converge to the transport rate predicted using the average stress. To this aim, we performed Direct Numerical Simulation of a channel flow and used the resulting database to mimic Wall-Resolved and Wall-Modelled Large-Eddy Simulations. We compared the prediction of several bed-load transport models to experimental measurements in order to identify and highlight the limitations that stem from the coupling of these models with eddy-resolving techniques. We find that for small values of the Shields parameter (ratio of viscous and gravitational forces) the fine spatial and temporal resolution of wall-resolved simulations can yield overestimation of the bed-load transport rate; whereas more coarse-grained methods, such as wall-modelled Large Eddy Simulation, result in improved predictions. We also show that a short-time averaging of the force exerted by the fluid on the sediments, which we tested in three different configurations (channel flow with smooth and rough walls and flow over an idealized two-dimensional river dune), improves the accuracy of the bed-load transport predictions, thus providing indications about the flow scales that control the transport process.
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    Natural Convective Heat Transfer From the Horizontal Isothermal Surface of Polygons of Octagonal and Hexagonal Shapes
    (ASME, 2019-10-01) Kalendar, Ahmad; Kalendar, Abdulrahim; Alhendal, Yousuf; Karar, Sayed; Alenzi, Adel; Oosthuizen, Patrick
    Heat transfer often occurs effectively from horizontal elements of relatively complex shapes in natural convective cooling of electronic and electrical devices used in industrial applications. The effect of complex surface shapes on laminar natural convective heat transfer from horizontal isothermal polygons of hexagonal and octagonal flat surfaces facing upward and downward of different aspect ratios has been numerically investigated. The polygons’ surface is embedded in a large surrounding plane adiabatic surface, where the adiabatic surface is in the same plane as the surface of the heated element. For the Boussinesq approach used in this work, the density of the fluid varies with temperature, which causes the buoyancy force, while other fluid properties are assumed constants. The numerical solution of the full three-dimensional form of governing equations is obtained by using the finite volume method-based computational fluid dynamics (CFD) code, FLUENT14.5. The solution parameters include surface shape, dimensionless surface width, different characteristic lengths, the Rayleigh number, and the Prandtl number. These parameters are considered as follows: the Prandtl number is 0.7, the Rayleigh numbers are between 103 and 108, and for various surface shapes the width-to-height ratios are between 0 and 1. The effect of different characteristic lengths has been investigated in defining the Nusselt and Rayleigh numbers for such complex shapes. The effect of these parameters on the mean Nusselt number has been studied, and correlation equations for the mean heat transfer rate have been derived.
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    Correlations for Natural Convective Heat Transfer From Vertical and Inclined Cylinders
    (Taylor and Francis, 2017-01-02) Kalendar, Abdulrahim; Karar, Sayed; Kalendar, Ahmad; Oosthuizen, Patrick
    Natural convective heat transfer from the exposed top surface of an inclined isothermal cylinder, with a circular cross section, mounted on a flat adiabatic base plate, has been numerically investigated. The cylinder is mounted normal to the flat adiabatic base plate. The numerical solution has been obtained by solving the dimensionless governing equations, subject to boundary conditions, using the commercial finite-volume method-based code FLUENT. The flow has been assumed to be symmetrical about the vertical center-plane through the cylinder. Results have only been obtained for Prandtl number of 0.7, which is the value existing in the application that originally motivated this study. The simulations consider Rayleigh numbers between 103 and 107, inclination angles between 0º and 180º, and dimensionless cylinder diameters between 0.25 and 1. The effects of dimensionless diameter, Rayleigh numbers, and inclination angles on the mean Nusselt number for the top exposed surface of the cylinder have been studied. Empirical correlations for the heat transfer rates from the top exposed surface of the cylinder have been derived.
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    Material interface control in multi-material topology optimization using pseudo-cost domain method
    (Wiley, 2020-09-20) Shah, Vishrut; Pamwar, Manish; Sangha, Balbir; Kim, Il Yong
    The recent drive for producing lightweight and high performance designs on reduced timelines has promoted the need for computational design generation tools such as Multi-Material Topology Optimization (MMTO). However, MMTO has drawn some industry skepticism as it assumes different material elements to be perfectly fused together. To address this concern, in this article, a novel pseudo-cost domain (PCD) method is proposed which mathematically determines individual material interfaces in MMTO solutions. The proposed methodology employs a user defined joint cost model to weigh the distinct material interfaces relative to each other. An innovative approach to tailor the MMTO design considering the relative cost of each material interface is presented. The proposed methodology can consider any number of materials and their respective interfaces, and it is defined in such a way that increasing the number of materials has minimal effect on computational time. The methodology is formulated in a smooth and differentiable manner and the sensitivity expressions required by gradient-based optimization solvers are presented. A series of example problems are provided to demonstrate the efficacy of the proposed methodology.