In this work a new method of passive drag reduction on bluff bodies by tessellation is presented. Wind-tunnel measurements on tessellated spheres reveal that the variation of the drag coefficient is similar to other types of surface modifications with rotational symmetry such as dimples, manifested by a sudden decrease in the drag coefficient at a critical Reynolds number followed by a nearly constant drag value in the post-critical regime. However, tessellated spheres can achieve a further 10 Re=1.50×10^5 . The predicted values of drag coefficient agree very well with the experiments and confirm the drag reduction. Analysis of the flow reveals that the tessellated panels introduce a smaller pressure “penalty” compared to dimples at the front part of the body. In addition, transition to turbulence occurs later and near the top of the body. As a result the boundary layer grows thinner and global separation is delayed by approximately 10^∘ .

Immersed boundary methods have seen an enormous increase in popularity over the past two decades, especially for problems involving complex moving/deforming boundaries. In most cases, the boundary conditions on the immersed body are enforced via forcing functions in the momentum equations, which in the case of fractional step methods may be problematic due to: i) creation of slip-errors resulting from the lack of explicitly enforcing boundary conditions on the (pseudo-)pressure on the immersed body; ii) coupling of the solution in the fluid and solid domains via the Poisson equation. Examples of fractional-step formulations that simultaneously enforce velocity and pressure boundary conditions have also been developed, but in most cases the standard Poisson equation is replaced by a more complex system which requires expensive iterative solvers. In this work we propose a new formulation to enforce appropriate boundary conditions on the pseudo-pressure as part of a fractional-step approach. The overall treatment is inspired by the ghost-fluid method typically utilized in two-phase flows. The main advantage of the algorithm is that a standard Poisson equation is solved, with all the modifications needed to enforce the boundary conditions being incorporated within the right-hand side. As a result, fast solvers based on trigonometric transformations can be utilized. We demonstrate the accuracy and robustness of the formulation for a series of problems with increasing complexity.

We report a comparative study of three numerical solvers for the direct numerical simulation of the flow over a sphere at Re = 3700. A high-order spectral-element code (Nek5000), a general purpose, unstructured finite-volume solver (OpenFOAM) and an in-house Cartesian solver using the immersed-boundary method (IBM) are employed for the analysis; results are compared against previous numerical and experimental data. Numerical results show that Nek5000 and the IBM code operate within a similar computational performance range, in terms of cost-vs-accuracy analysis, for both global parameters as well as local flow features. On the other hand, OpenFOAM needed a significantly higher number of degrees of freedom (and, overall, a higher cost) to match some of the basic features of the flow, such as the length of the recirculation bubble forming downstream the sphere. For the finest grid resolutions, the three codes are in good agreement for most of the analyzed flow metrics. Overall, our results suggest that high-order methods and second-order, energy-conserving approaches based on the IBM may be both viable options for high-fidelity scale-resolving simulations of turbulent flows with separation.

The fluid dynamics of owls in flapping flight is studied by coordinated experiments and computations. The great horned owl was selected, which is nocturnal, stealthy, and relatively large sized raptor. On the experimental side, perch-to-perch flight was considered in an open wind tunnel. The owl kinematics was captured with multiple cameras from different view angles. The kinematic extraction was central in driving the computations, which were designed to resolve all significant spatio-temporal scales in the flow with an unprecedented level of resolution. The wing geometry was extracted from the planform image of the owl wing and a three-dimensional model, the reference configuration, was reconstructed. This configuration was then deformed in time to best match the kinematics recorded during flights utilizing an image-registration technique based on the large deformation diffeomorphic metric mapping framework. All simulations were conducted using an eddy-resolving, high-fidelity, solver, where the large displacements/deformations of the flapping owl model were introduced with an immersed boundary formulation. We report detailed information on the spatio-temporal flow dynamics in the near wake including variables that are challenging to measure with sufficient accuracy, such as aerodynamic forces. At the same time, our results indicate that high-fidelity computations over smooth wings may have limitations in capturing the full range of flow phenomena in owl flight. The growth and subsequent separation of the laminar boundary layers developing over the wings in this Reynolds number regime is sensitive to the surface micro-features that are unique to each species.

This Work in Progress aims to address the broadening participation challenge in engineering. This paper will describe a complex adaptive systems approach based project aimed at enhancing entry and persistence in engineering of first-generation students, women, under-represented ethnic minorities, and those with socio-economic need. Sponsored by a national agency, the effort involves a large public comprehensive research university, a county-wide community college system, feeder high-school districts, industry collaborators, and a local foundation partner. Motivation It is well established that attracting and retaining more first-generation students, women, underrepresented ethnic minorities and those with socio-economic need in the engineering workforce will augment innovation, creativity, and global competitiveness. A diverse workforce will result in enhanced scientific and technological products, services, and solutions that will be better designed and represent all users. Fostering diversity driven creativity requires a collective effort with a cross-section of social institutions to open a multiplicity of pathways for students to enter, retain and persist in engineering degree pathways. The project seeks to address the ways in which K-12 school districts, community colleges, and a university can adapt their actions to meet the goal of diversifying engineering. Brief Background For many first-generation students, transition to university settings includes developing the confidence to become successful engineering students and envision future possible selves as engineers. A general framework to characterize the challenges in diversifying engineering is: 1) lack of awareness about engineering and what engineers do; 2) absence of enjoyment or an affective response to engineering; 3) dearth of interest in engineering pathways and careers; 4) paucity of opinion formation about the impact of engineering on society; and 5) poor understanding of engineering and its social value. These are further compounded by affordability and challenges with transition to college, especially for first generation students, women, under-represented ethnic minorities, and those with socio-economic need. Strategies implemented in this project are deliberately designed to foster the many related factors that influence identity development by creating a sense of belonging in students; generating confidence, support, and agency for academic success; advocacy for engineering as socially and personally relevant; and engagement in engineering activities beyond coursework. Mentoring is offered to encourage students’ evolving engineering identities. This project seeks to challenge traditional views of engineering as perceived by students and families. It aims to confront stereotypes about who can become an engineer, and has a focus on making engineering socially and personally relevant to the individual. Methods A pilot collective alliance is designed to address the broadening participation objectives of enhancing entry and persistence of targeted populations. The alliance aims to identify and develop effective mechanisms to impact entry and persistence in engineering at scale to expand the alliance for the region, serving as a model for the state and other universities nationally. The project will directly impact opportunities for entry into engineering of 500 high school students and 100 community college students. Efforts to support persistence in engineering at the university will directly impact 200 students. Measures of whether participants evince interest and awareness in engineering and the degree to which they develop engineering identity will be assessed to understand the collective impact of alliance efforts. External developmental evaluation of the complex adaptive systems approach taken by the collective alliance will help identify the mechanisms that advance entry and persistence of targeted populations in broadening participation in engineering. We anticipate that the purposefully designed collective impact efforts implemented through this project will result in higher numbers of first-generation students pursuing engineering. Anticipated Results It is anticipated that the alliance institutions will develop into adaptive systems that respond to the common goal of broadening participation in engineering for first generation students. The ensuing complex adaptive system and the emerging patterns with leverage markers resulting in broadening participation will be identified for expansion. The collective alliance has the potential to transform institutionally engendered identities to be inclusive of the multiple and mutable engineering identities. The project anticipates the development of a model that will be useful regionally and nationally to broaden participation that can reduce and eliminate barriers to engineering education and career pathways.

It is well established that dimples accelerate the drag crisis on a sphere. The result of the early drag crisis is a reduction of the drag coefficient by more than a factor of two when compared to a smooth sphere at the same Reynolds number. However, when the drag coefficients for smooth and dimpled spheres in the post-critical regime are compared, the latter is higher by a factor of two to three. To understand the origin of this behaviour, we conducted direct numerical simulations of the flow around a dimpled sphere, which is similar to commercially available golf balls, in the post-critical regime. By comparing the results to those for a smooth sphere, it is found that dimples, although effective in accelerating the drag crisis, impose a local drag penalty, which contributes significantly to the overall drag force. This finding challenges the broadly accepted view that dimples only indirectly affect the drag force on a sphere by energizing the near-wall flow and delaying global separation.

The purpose of the current study is to examine the engineering interests held by a diverse sample of high school students, along with a battery of social cognitive factors related to interest – including experience with engineering, knowledge and understanding of engineering as a career field, and identity as an engineer. The study is part of an overarching program of research at Arizona State University’s Ira A. Fulton Schools of Engineering, aimed at testing the efficacy of an out-of-school engineering program, Young Engineers Shape the World embedded in an NSF INCLUDES project. This NSF project, Engineers from Day One, aims to facilitate the engineering identities of female, first-generation, and underrepresented minority students, with the goal of increasing these students’ entry and retention in engineering majors. This paper presents findings from efforts to study the awareness, enjoyment, interest, opinion formation, and understanding that high school students have towards engineering. These high school students were enrolled in a year-round program, Young Engineers Shape the World. A questionnaire was administered to a sample of high school students (N = 334, 53.3% female, 60.6% non-white, 77.1% first-generation) via the online survey platform Qualtrics. In addition to collecting demographic information, the questionnaire collected data on students’ experience with engineering, their understanding of who engineers are and what they do, and their identities as future engineers.

In the present work the effects of different types of roughness elements on flow separation over a circular bump is investigated by means of direct numerical simulations. Two types of roughness elements are considered, dimples and spherical beads. The boundary condition setup and the Reynolds number range were selected to replicate in a qualitative sense the initial development of the boundary layer (growth, transition, separation) on full cylinders or spheres, which is primarily responsible for the trends in the behavior of their drag coefficient. Both roughness elements are very effective in causing transition of the boundary layer at a much lower Reynolds number when compared to a smooth surface. For the spherical beads the drag coefficient exhibits a minimum and quickly rises as the Reynolds number increases. For the dimples the minimum drag coefficient remains constant and independent on the Reynolds number within the range considered in this study. This behaviour agrees with experimental observations in the literature for similar types of roughness elements. The differences are due to the way the boundary layer grows over dimples or spherical beads. For the latter, transition shifts upstream and moves toward the stagnation point on the front of the body as the Reynolds number increases. An earlier transition means the boundary layer starts growing thicker earlier and has less momentum to overcome the adverse pressure gradient. As a result the separation point moves upstream too giving rise to increased drag. In contrast the transition and separation points are weakly dependent on the Reynolds number for the case of the dimples.

In the present work the effects of different types of roughness elements on flow separation over a curved boundary is investigsted by means of direct numerical simulations. The geometry and boundary conditions are such that the basic physics of the flow over bluff bodies are represented. Two types of roughness elements are considered, dimples and spherical beads. The Reynolds number, Reh, based on the freestream velocity and height, varied from 3.000 to 30.000. The results are in good qualitative agreement with results for flow over bluff bodies with surface roughness. In particular, the roughness elements are very effective in causing transition of the boundary layer at a much lower Reynolds numbers when compared to a smooth surface. For the spherical beads the drag coefficient exhibits a minimum and quickly rises as the Reynolds number increases. For the dimples the minimum drag coefficient remains constant and independent of the Reynolds number within the range considered in this study. The reason for this different behavior lies in the way the boundary layer grows between the two different roughness elements. For the spherical beads the transition shifts upstream and moves toward the stagnation point on the front of the bump as the Reynolds number increases. An earlier transition means the boundary layer starts growing thicker earlier and has less momentum to overcome the adverse pressure gradient. As a result the separation point moves upstream too giving rise to increased drag. In contrast the transition and separation points are weakly dependent on the Reynolds number for the case of the dimples.

The transition from laminar to turbulent flow over dimples and grooves has been investigated through a series of direct numerical simulations. Emphasis has been given to the mechanism of transition and the momentum transport in the post-dimple boundary layer. It has been found that the dimple geometry plays an important role in the evolution of the turbulent boundary layer downstream. The mechanism of transition in all cases is that of the reorientation of the spanwise vorticity into streamwise oriented structures resembling hairpin vortices commonly encountered in wall bounded turbulent flows. Although qualitatively the transition mechanism amongst the three different cases is similar, important quantitative differences exist. It was shown that two-dimensional geometries like a groove are more stable than three-dimensional geometries like a dimple. In addition, it was found that the cavity geometry controls the initial thickness of the boundary layer and practically results in a shift of the virtual origin of the turbulent boundary layer. Important differences in the momentum transport downstream of the dimples exist but in all cases the boundary layer grows in a self-similar manner.

We studied left ventricular flow patterns for a range of rotational orientations of a bileaflet mechanical heart valve (MHV) implanted in the mitral position of an elastic model of a beating left ventricle (LV). The valve was rotated through 3 angular positions (0, 45, and 90 degrees) about the LV long axis. Ultrasound scans of the elastic LV were obtained in four apical 2-dimensional (2D) imaging projections, each with 45 degrees of separation. Particle imaging velocimetry was performed during the diastolic period to quantify the in-plane velocity field obtained by computer tracking of diluted microbubbles in the acquired ultrasound projections. The resulting velocity field, vorticity, and shear stresses were statistically significantly altered by angular positioning of the mechanical valve, although the results did not show any specific trend with the valve angular position and were highly dependent on the orientation of the imaging plane with respect to the valve. We conclude that bileaflet MHV orientation influences hemodynamics of LV filling. However, determination of ‘optimal’ valve orientation cannot be made without measurement techniques that account for the highly 3-dimensional (3D) intraventricular flow.

Modeling a dilute suspension of particles in a polykinetic Eulerian framework is described using the conditional quadrature method of moments (CQMOM). The particular regimes of interest are multiphase flows comprised of particles with diameters small compared to the smallest length scale of the turbulent carrier flow and particle material densities much larger than that of the fluid. These regimes correspond to moderate granular Knudsen number and large particle Stokes numbers in which interparticle collisions and/or particle trajectory crossing (PTC) can be significant. The probability density function (PDF) of the particle velocity space is discretized with a two-point quadrature, the minimum resolution required to capture PTC which is common to these flows. Both two-dimensional (2D) test cases (designed to assess numerical procedures) and a three-dimensional (3D) fully developed particle-laden turbulent channel flow were implemented for collisionless particles. The driving gas-phase carrier flow is computed using direct numerical simulation of the incompressible Navier–Stokes (N–S) equations and one-way coupled to the particle phase via the drag force. Visualizations and statistical descriptors demonstrate that CQMOM predicts physical features such as PTC, particle accumulation near the channel walls, and more uniform particle velocity profiles relative to the carrier flow. The improvements in modeling compared to monokinetic representations are highlighted.

A series of direct numerical simulations of the flow past a flat plate with two and eight rows of dimples in a staggered arrangement is carried out. The Reynolds number based on the boundary layer thickness and freestream velocity near the inflow plane is 1000 and the dimples are spherical with a depth to diameter ratio of 0.1. The incoming flow is laminar and the boundary layer thickness before the dimples is half the dimple depth. At this low Reynolds number the flow is expected to remain laminar over a smooth flat plate. The presence of the dimples triggers instabilities that cause significant momentum transport. It is shown that the shear layer that forms as the flow separates over the first two rows of dimple becomes unstable and sheds coherent vortex sheets. The vortex sheets become unstable and are transformed into packets of horseshoe vortices. When these vortices evolve over a flat plate or over a series of dimples the flow dynamics are very different with important changes in momentum transport across the boundary layer.

The specific focus of the current effort is on modeling dilute particle-laden turbulent boundary layers in which the gas-phase carrier flow is populated with a second phase of small, dispersed solid particles possessing material densities much larger than that of the carrier flow. A novel approach known as the conditional quadrature method of moments (CQMOM) developed by Yuan and Fox [1], derived from the quadrature-based method of moments (QMOM) developed originally by McGraw [2], is being implemented to model the dispersed particles as an Eulerian phase. Both enabled and disabled inter-particle collision treatments are included in the model for a dispersed phase coupled to the fluid via a drag force acting on the particles. Simulations are conducted with a Reynolds number of 2800 based on the boundary layer thickness at the inlet to the domain. The full 3-D mesh contains 800×128×98 structured cells with overall dimensions in terms of the inlet boundary layer thickness of 80×6 ×4 in the streamwise, spanwise, and wall-normal directions, respectively. The gas-phase carrier flow is computed using Direct Numerical Simulation of the incompressible Navier-Stokes equations. The boundary layer develops spatially from a turbulent inflow condition and drives the particulate phase via drag and collisions. Comparisons are made against simulations performed using Lagrangian-based discrete particle simulation (DPS) of the dispersed phase and demonstrate the utility of the Eulerian moment method approach. Both instantaneous and time-averaged quantities are presented.

A series of direct numerical simulations (DNS) of the flow past a zero pressure gradient flat plate with rows of dimples is carried out. The Reynolds number based on the boundary layer thickness is 1000 and the dimples have a circular profile with a depth to diameter ratio of 0.1. The incoming flow is laminar and the ratio of the incoming boundary layer thickness to the dimple depth determines the critical Reynolds number, where transition to turbulence occurs downstream. This happens as the shear layer that forms at the dimple edge separates over the first row of dimples and becomes unstable creating coherent vortex sheet. The vortex sheet undergoes a complex spanwise instability transforming themselves into a packet of horseshoe vortices. As a result the boundary layer downstream of the dimples has the same qualitative features encountered in wall bounded turbulent boundary layers.