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Browsing by Subject "turbulence"

Now showing 1 - 7 of 7
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    Drag Reduction by Riblets & Sharkskin Denticles: A Numerical Study
    (2015-07) Boomsma, Aaron
    Riblet films are a passive method of turbulent boundary layer control that can reduce viscous drag. They have been studied with great detail for over 30 years. Although common riblet applications include flows with Adverse Pressure Gradients (APG), nearly all research thus far has been performed in channel flows. Recent research has provided motivation to study riblets in more complicated turbulent flows with claims that riblet drag reduction can double in mild APG common to airfoils at moderate angles of attack. Therefore, in this study, we compare drag reduction by scalloped riblet films between riblets in a zero pressure gradient and those in a mild APG using high-resolution large eddy simulations. In order to gain a fundamental understanding of the relationship between drag reduction and pressure gradient, we simulated several different riblet sizes that encompassed a broad range of s+ (riblet width in wall units), similarly to many experimental studies. We found that there was only a slight improvement in drag reduction for riblets in the mild APG. We also observed that peak values of streamwise turbulence intensity, turbulent kinetic energy, and streamwise vorticity scale with riblet width. Primary Reynolds shear stresses and turbulence kinetic energy production however scale with the ability of the riblet to reduce skin-friction. Another turbulent roughness of similar shape and size to riblets is sharkskin. The hydrodynamic function of sharkskin has been under investigation for the past 30 years. Current literature conflicts on whether sharkskin is able to reduce skin friction similarly to riblets. To contribute insights toward reconciling these conflicting views, Direct Numerical Simulations (DNS) are carried out to obtain detailed flow fields around realistic denticles. A sharp interface immersed boundary method is employed to simulate two arrangements of actual sharkskin denticles (from Isurus oxyrinchus) in a turbulent boundary layer at Re? [approximately equal to] 180. For comparison, turbulent flow over drag-reducing scalloped riblets is also simulated with similar flow conditions and with the same numerical method. Although the denticles resemble riblets, both sharkskin arrangements increase total drag by 44-50%, while the riblets reduce drag by 5%. Analysis of the simulated flow fields shows that the turbulent flow around denticles is highly three-dimensional and separated, with 25% of the total drag being form drag. The complex three-dimensional shape of the denticles gives rise to a mean flow dominated by strong secondary flows in sharp contrast with the mean flow generated by riblets, which is largely two-dimensional. The so resulting three- dimensionality of sharkskin flows leads to an increase in the magnitude of the turbulence statistics near the denticles, which further contributes to increasing the total drag. The simulations also show that, at least for the simulated arrangements, sharkskin, in sharp contrast with drag-reducing riblets, is unable to isolate high shear stress near denticle ridges causing a significant portion of the denticle surface to be exposed to high mean shear. Lastly, it has been theorized that sharkskin might act similarly to vortex generators and prevent separation. In order to test this theory, we have conducted simulations with and without sharkskin upstream of a steady separation bubble. Using large eddy simulation, our study shows that sharkskin worsened the weak separation region and enlarged the separation bubble's boundaries. The cause was shown to originate due to the denticles acting as blockages, rather than vortex generators. In fact, our results showed that separation occurred just after the second row of denticles and that the turbulent flow was unable to recover its lost momentum. Streamwise turbulence intensities were decreased compared to the baseline case. Finally, in the present case, the sharkskin induced reversed flow within the denticles--something that was not observed with sharkskin in channel flow.
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    Flow and clay erosion data taken and processed at SAFL in a grid-turbulence tank Fall 2021-Spring 2022
    (2022-05-23) San Juan, Jorge E; Wei, William G; Yang, Judy Q; jsanjuan@umn.edu; San Juan, Jorge E; Environmental Transport Laboratory at St Anthony Falls Lab
    Here we present a data set containing raw imaging and processed data of clay erosion and near-bed turbulence. This data set supports our study of the impact of salinity and consolidation on the microstructure and erosion threshold of cohesive sediments. The flow and erosion measurements were taken inside a grid-turbulence tank at the St. Anthony Falls Laboratory at the University of Minnesota Twin Cities. We set up the water salinity at a prescribed concentration in part-per-thousands and two clay consolidation conditions (unconsolidated and consolidated after 17 hours) for each experimental case. Here we incorporate detailed near-bed turbulence measurements to quantify the erosive effect of the turbulent water flow. We also include laponite (smectite clay)'s surface erosion from laser-induced fluorescence and image processing.
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    Improvement of the Mellor-Yamada Type Planetary Boundary Layer Scheme for Use in Mesoscale Models
    (2020-08) Keester, Adam
    Atmospheric mesoscale models are highly complex and their performance varies widely depending on the models used. Turbulent transport within the boundary layer is especially difficult to analyze, but has a significant impact on mesoscale model applications. In this study, the Mellor-Yamada-Nakanishi-Niino planetary boundary layer model is improved. A new length scale and turbulent closure constants are calculated from two large eddy simulations. The modified MYNN model maintains the original’s accurate eddy coefficients and drastically improves the prediction of the momentum dissipation rate, length scale and stability functions. A 12-member WRF ensemble is used to validate the new model outside of the database on which it is based. The WRF results show that the new model improves the bias and mean absolute error of temperature and relative humidity. There is a significant change in the TKE and length scale predictions that motivates further study of the modified boundary layer scheme.
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    Laboratory Investigation Of Dispere Multiphase-Turbulent Flows, Dilute & Dense Distributions Of Inertial Particles Settling In Air
    (2020-05) Petersen, Alec
    Turbulent multiphase flows are found throughout our universe, all over Earth and in many man-made systems. Despite surrounding us, their dynamics are still in many ways obscure and require further study. These chaotic systems are however quite complicated to both simulate or explore experimentally. In this thesis, we present our laboratory investigation of particle-laden turbulent flows in air. We first focus on the statistical dynamics of dilute multiphase turbulence. Utilizing a zero-mean-flow air turbulence chamber, we drop size-selected solid particles and study their dynamics with particle imaging and tracking velocimetry at multiple resolutions. The carrier flow is simultaneously measured by particle image velocimetry of suspended tracers, allowing the characterization of the interplay between both the dispersed and continuous phases. The turbulence Reynolds number based on the Taylor microscale ranges from 200 – 500, while the particle Stokes number based on the Kolmogorov scale varies between O(1) and O(10). Clustering is confirmed to be most intense for Stokes ≈ 1 , but it extends over larger scales for heavier particles. Individual clusters form a hierarchy of self-similar, fractal-like objects, preferentially aligned with gravity and sizes that can reach the integral scale of the turbulence. Remarkably, the settling velocity of Stokes ≈ 1 particles can be several times larger than the still-air terminal velocity, and the clusters can fall even faster. This is caused by downward fluid fluctuations preferentially sweeping the particles, and we propose that this mechanism is influenced by both large and small scales of the turbulence. The particle-fluid slip velocities show large variance, and both the instantaneous particle Reynolds number and drag coefficient can greatly differ from their nominal values. Finally, for sufficient loadings, the particles generally augment the small-scale fluid velocity fluctuations, which however may account for a limited fraction of the turbulent kinetic energy. We also investigate denser particle-laden flows, specifically plumes driven by the downward buoyancy of inertial particles. With similar tools, we conduct two experiments: one to capture the particle-phase behavior and another to measure the ambient air velocity. Our first focus is on the assumption of self-similarity, which unlike single-phase plumes is not a trivial assumption. We also characterize the mean plume properties observed: the particle-phase velocity and the plume spread comparing their evolution with axial distance from the plume source. From our measurements of the ambient air flow we calculate the entrainment velocity into the particle-laden plumes and using the time-averaged value we estimate the entrainment coefficient along the plume. We find a relatively stable entrainment rate, as expected in the assumption used to formulate many integral plume models. Lastly we compared our experimental results to single and multiphase plume models with the same initial conditions as our experiments. Our multiphase plume model, inspired by the work of Liu (2003) and Lai et al. (2016), well described our velocity measurements, which single phase models were completely unequipped for.
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    Organization And Scaling Of Coherent Structures In The Outer Region Of High-Reynolds-Number Turbulent Boundary Layers
    (2020-05) Heisel, Michael
    Recent advances in high-Reynolds-number turbulence have suggested there is a general self-organization of coherent structures in the logarithmic and wake regions of boundary layer flows. The organization comprises large-scale velocity structures known as uniform momentum zones (UMZs) separated by thin internal shear layers (ISLs). While the velocity structures have been extensively studied in more specific forms such as momentum streaks, streamwise rolls, and bulges, the shear layers have received less attention outside the context of the hairpin packet paradigm. In the present thesis, the universality of this self-organization is evaluated using a novel field-scale particle image velocimetry (PIV) experiment in the logarithmic region of the atmospheric surface layer. The field measurements are validated using collocated sonic anemometry. The experiment reveals the same organization of UMZs and ISLs occurs for atmospheric flows. The properties of the UMZs and ISLs are then compared using ten PIV experiments and a direct numerical simulation, which together span a wide range of surface roughness and three orders of magnitude in Reynolds number. The UMZs unambiguously scale with the friction velocity and wall-normal distance in the logarithmic region, regardless of Reynolds number and surface roughness. The scaling behavior is in agreement with Prandtl's mixing length theory and Townsend's attached eddy hypothesis. The results show that the hypothetical eddies of the logarithmic law of the wall manifest in the structural organization of the flow. Separate analysis focusing on the smaller structures shows that the ISLs and large vortices are both governed by the friction velocity and Taylor microscale. Preliminary evidence suggests these ISL and vortex scaling behaviors both result from mutual interaction with the local large-scale UMZs, possibly through a stretching mechanism. Additional experiments in three dimensions are required to verify the dynamics. The overall findings support the universality of large-scale structures in the outer region and provide promising clues for better understanding scale interaction and energy transfer mechanisms.
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    Particle image velocimetry measurements of smooth- and rough-wall turbulence from the SAFL Atmospheric Boundary Layer wind tunnel
    (2020-01-10) Heisel, Michael; Guala, Michele; mguala@umn.edu; Guala, Michele; St. Anthony Falls Laboratory, University of Minnesota
    Wall-bounded turbulent flows under smooth- and rough-wall surface conditions were measured using particle image velocimetry (PIV) in the Atmospheric Boundary Layer Wind Tunnel at St. Anthony Falls Laboratory (SAFL), University of Minnesota. In the rough-wall case, the tunnel surface was covered with woven wire mesh. The smooth- and rough-wall conditions were each measured for two free-stream velocities (7 m/s and 10 m/s), totaling four flow cases. The friction Reynolds number in the four cases ranges from 3,800 to 14,000. In each case, the PIV imaging field was oriented in the streamwise–wall-normal plane. To enhance the spatial resolution, the measurement field was positioned in the lowest 10 cm of the boundary layer, capturing the roughness sublayer and logarithmic region in the rough-wall cases. Separate high-frequency hotwire anemometer measurements of the full boundary layer profile were used to estimate the scaling parameters such as the boundary layer thickness. This dataset includes the processed velocity vector fields from the PIV measurements and the key scaling parameters.
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    Transition To Elastic Turbulence In Channel Flows
    (2020-04) Hariharan, Gokul
    Materials processing operations such as extrusion and coating often involve the low- inertia flow of viscoelastic fluids through straight channels. Experimental evidence suggest that such flows can transition from a laminar to a disordered flow-state, resulting in defective end-products. On the other hand, such a transition with low inertia is useful for enhancing transport in microfluidic flows where good mixing is hard to achieve. Therefore a fundamental understanding of such a transition is important. Chapter 2 of this thesis considers external disturbances in the form of small-amplitude localized body forces (impulses). They provide a good approximation of the external disturbances that can be realized relatively easily in laboratory experiments. Localized body forces are used to identify the optimal location in a channel that induces the largest kinetic energy growth. A disturbance in the channel that generates the largest kinetic energy growth has a high potential to trigger a transition to a disordered flow-state. Chapter 3 presents tools to accurately resolve steep stress gradients encountered in frequency response calculations of the linearized equations governing channel flow of a viscoelastic fluid. Recently reported well-conditioned spectral methods in conjunction with a reflection technique enable frequency response computations of channel flows of viscoelastic fluids with large elasticity. Applying the methods developed in Chapter 3 to 2D channel flow of a viscoelastic fluid, it is found that the stress can develop large magnitudes even when the velocity has negligible growth. A stress of large magnitude generated by small-amplitude disturbances may provide a new route to a transition to a disordered flow-state observed in recent experiments. Chapter 4 studies stress amplification and conditions in which they become prominent. A first step to perform direct numerical simulations (DNS) of channel flows of viscoelastic fluids using tools developed in Chapter 3 is to develop an algorithm for DNS of channel flows of Newtonian fluids. Chapter 5 extends tools discussed in Chapter 3 to perform direct numerical simulations of channel flows of a Newtonian fluid. Analyzing transition to turbulence in viscoelastic channel flows is a challenging problem that needs a multi-faceted approach involving linear and nonlinear systems theory, robust numerical methods, and complementary experiments. We believe that this dissertation provides new insights into possible mechanisms that may govern the initial stages of a transition to elastic turbulence using linear systems theory and recent numerical methods. We further hope that the numerical methods studied in this dissertation will open new avenues to simulate and analyze flow transition in complex fluids.