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Browsing by Author "Heisel, Michael"

Now showing 1 - 8 of 8
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    Aerodynamics of highway sign structures: from laboratory tests and field monitoring to structural design guidelines
    (American Society of Civil Engineers, 2020-08-20) Heisel, Michael; Daugherty, Carly; Finley, Nicole; Linderman, Lauren; Schillinger, Dominik; French, Catherine E; Guala, Michele
    Field- and model-scale experiments were conducted to quantitatively assess the effects of wind loading on Rural Intersection Conflict Warning System (RICWS) highway sign structures. A field-scale RICWS was instrumented with acceleration and linear displacement sensors to monitor unsteady loads, dynamics, and displacement of the sign under various wind events classified by cup and vane wind velocity measurements. To complement the field-scale results, tests on a 1:18-scale model were conducted under controlled laboratory conditions in the St. Anthony Falls Laboratory towing tank and wind tunnel facilities. Aerodynamic effects on the sign structure were identified through analysis of the mean and oscillating drag and lift forces. Vortices periodically shed by the structure induced forces at a frequency governed by the Strouhal number. The shedding frequency overlapped with the estimated natural frequency during strong wind events, leading to possible resonance. Amplified oscillations were additionally observed when the wind direction was parallel to the structure, possibly due to an aeroelastic instability. The findings highlight the relevance of aerodynamic effects on roadside signs or similar complex planar geometries under unsteady wind loading.
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    On the mixing length eddies and logarithmic mean velocity profile in wall turbulence
    (Cambridge University Press, 2020-01-21) Heisel, Michael; de Silva, Charitha M; Hutchins, Nicholas; Marusic, Ivan; Guala, Michele
    Since the introduction of the logarithmic law of the wall more than 80 years ago, the equation for the mean velocity profile in turbulent boundary layers has been widely applied to model near-surface processes and parameterize surface drag. Yet the hypothetical turbulent eddies proposed in the original logarithmic law derivation and mixing length theory of Prandtl have never been conclusively linked to physical features in the flow. Here, we present evidence that suggests these eddies correspond to regions of coherent streamwise momentum known as uniform momentum zones (UMZs). The arrangement of UMZs results in a step-like shape for the instantaneous velocity profile, and the smooth mean profile results from the average UMZ properties, which are shown to scale with the friction velocity and wall-normal distance in the logarithmic region. These findings are confirmed across a wide range of Reynolds number and surface roughness conditions from the laboratory scale to the atmospheric surface layer.
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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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    Predictive model for local scour downstream of hydrokinetic turbines in erodible channels
    (American Physical Society, 2018-02-22) Musa, Mirko; Heisel, Michael; Guala, Michele
    A modeling framework is derived to predict the scour induced by marine hydrokinetic turbines installed on fluvial or tidal erodible bed surfaces. Following recent advances in bridge scour formulation, the phenomenological theory of turbulence is applied to describe the flow structures that dictate the equilibrium scour depth condition at the turbine base. Using scaling arguments, we link the turbine operating conditions to the flow structures and scour depth through the drag force exerted by the device on the flow. The resulting theoretical model predicts scour depth using dimensionless parameters and considers two potential scenarios depending on the proximity of the turbine rotor to the erodible bed. The model is validated at the laboratory scale with experimental data comprising the two sediment mobility regimes (clear water and live bed), different turbine configurations, hydraulic settings, bed material compositions, and migrating bedform types. The present work provides future developers of flow energy conversion technologies with a physics-based predictive formula for local scour depth beneficial to feasibility studies and anchoring system design. A potential prototype-scale deployment in a large sandy river is also considered with our model to quantify how the expected scour depth varies as a function of the flow discharge and rotor diameter.
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    The spatial structure of the logarithmic region in very-high-Reynolds-number rough wall turbulent boundary layers
    (Cambridge University Press, 2018-10-26) Heisel, Michael; Dasari, Teja; Liu, Yun; Hong, Jiarong; Coletti, Filippo; Guala, Michele
    Using super-large-scale particle image velocimetry (SLPIV), we investigate the spatial structure of the near-wall region in the fully rough atmospheric surface layer with Reynolds number Reτ∼O(10^6). The field site consists of relatively flat, snow-covered farmland, allowing for the development of a fully rough turbulent boundary layer under near-neutral thermal stability conditions. The imaging field of view extends from 3 m to 19 m above the ground and captures the top of the roughness sublayer and the bottom of an extensive logarithmic region. The SLPIV technique uses natural snowfall as seeding particles for the flow imaging. We demonstrate that SLPIV provides reliable measurements of first- and second-order velocity statistics in the streamwise and wall-normal directions. Our results in the logarithmic region show that the structural features identified in laboratory studies are similarly present in the atmosphere. Using instantaneous vector fields and two-point correlation analysis, we identify vortex structures sharing the signature of hairpin vortex packets. We also evaluate the zonal structure of the boundary layer by tracking uniform momentum zones (UMZs) and the shear interfaces between UMZs in space and time. Statistics of the UMZs and shear interfaces reveal the role of the zonal structure in determining the mean and variance profiles. The velocity difference across the shear interfaces scales with the friction velocity, in agreement with previous studies, and the size of the UMZs scales with wall-normal distance, in agreement with the attached eddy framework.
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    The spectral signature of wind turbine wake meandering: a wind tunnel and field-scale study
    (Wiley, 2018-04-06) Heisel, Michael; Hong, Jiarong; Guala, Michele
    Field-scale and wind tunnel experiments were conducted in the 2D to 6D turbine wake region to investigate the effect of geometric and Reynolds number scaling on wake meandering. Five field deployments took place: 4 in the wake of a single 2.5-MW wind turbine and 1 at a wind farm with numerous 2-MW turbines. The experiments occurred under near-neutral thermal conditions. Ground-based lidar was used to measure wake velocities, and a vertical array of met-mounted sonic anemometers were used to characterize inflow conditions. Laboratory tests were conducted in an atmospheric boundary layer wind tunnel for comparison with the field results. Treatment of the low-resolution lidar measurements is discussed, including an empirical correction to velocity spectra using colocated lidar and sonic anemometer. Spectral analysis on the laboratory- and utility-scale measurements confirms a meandering frequency that scales with the Strouhal number St = fD/U based on the turbine rotor diameter D. The scaling indicates the importance of the rotor-scaled annular shear layer to the dynamics of meandering at the field scale, which is consistent with findings of previous wind tunnel and computational studies. The field and tunnel spectra also reveal a deficit in large-scale turbulent energy, signaling a sheltering effect of the turbine, which blocks or deflects the largest flow scales of the incoming flow. Two different mechanisms for wake meandering—large scales of the incoming flow and shear instabilities at relatively smaller scales—are discussed and inferred to be related to the turbulent kinetic energy excess and deficit observed in the wake velocity spectra.
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    Understanding and Mitigating the Dynamic Behavior of RICWS and DMS Under Wind Loading
    (Minnesota Department of Transportation, 2020-06) Linderman, Lauren; Guala, Michele; French, Catherine; Schillinger, Dominik; Finley, Nicole; Heisel, Michael; Nguyen, Lam; Stoter, Stein; Vievering, Josh; Zhu, Qiming
    Dynamic Messaging Signs (DMS) and Rural Intersection Conflict Warning Signs (RICWS) are roadside signs that feature much larger and heavier signs than are typically placed on their respective support systems. The excess weight and size of these signs, in conjunction with their breakaway support systems, introduces vibration problems not seen in the past. The AASHTO 2015 LRFD Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (SLTS) does not yet address vibration design for these nontraditional roadside signs. DMS and RICWS were instrumented in the field and numerically modeled to explore their wind-induced behavior. A dynamic numerical model was validated with experimental field data and used to evaluate the fatigue life of the DMS support system instrumented in the field. The resulting fatigue life differed significantly from the equivalent static pressure analysis prescribed in the AASHTO specification. The fatigue life of the DMS instrumented in the field was conservatively estimated to be 23.8 years. Based on data collected from a RICWS instrumented in the field and experiments done on a scaled model of the RICWS at the St. Anthony Falls Laboratory, vortex shedding was identified as the predominant wind phenomena acting on the RICWS structure. Three modifications were proposed to reduce the impacts of vortex shedding. The investigation of these newer sign types highlights the importance of considering the impact of dynamic behavior and vortex shedding on the structural design.