An official website of the United States government
This study presents the first 3D two-way coupled fluid structure interaction (FSI) simulation of a hybrid anechoic wind tunnel (HAWT) test section with modeling all important effects, such as turbulence, Kevlar wall porosity and deflection, and reveals for the first time the complete 3D flow structure associated with a lifting model placed into a HAWT. The Kevlar deflections are captured using finite element analysis (FEA) with shell elements operated under a membrane condition. Three-dimensional RANS CFD simulations are used to resolve the flow field. Aerodynamic experimental results are available and are compared against the FSI results. Quantitatively, the pressure coefficients on the airfoil are in good agreement with experimental results. The lift coefficient was slightly underpredicted while the drag was overpredicted by the CFD simulations. The flow structure downstream of the airfoil showed good agreement with the experiments, particularly over the wind tunnel walls where the Kevlar windows interact with the flow field. A discrepancy between previous experimental observations and juncture flow-induced vortices at the ends of the airfoil is found to stem from the limited ability of turbulence models. The qualitative behavior of the flow, including airfoil pressures and cross-sectional flow structure is well captured in the CFD. From the structural side, the behavior of the Kevlar windows and the flow developing over them is closely related to the aerodynamic pressure field induced by the airfoil. The Kevlar displacement and the transpiration velocity across the material is dominated by flow blockage effects, generated aerodynamic lift, and the wake of the airfoil. The airfoil wake increases the Kevlar window displacement, which was previously not resolved by two-dimensional panel-method simulations. The static pressure distribution over the Kevlar windows is symmetrical about the tunnel mid-height, confirming a dominantly two-dimensional flow field.
more »
« less
Drag coefficient of bent-awn plumegrass ( Saccharum contortum ) seeds in wind
We present a combination of laboratory experiments and computational fluid dynamics (CFD) simulations to understand the wind-induced drag force and drag coefficient for Saccharum contortum seeds. Seed drop experiments indicate that the settling fall velocities of hair-equipped seeds are within 1–2 m/s, compared to 2.34 times higher settling fall velocity of the seed without hairs. The experimental data illustrate a power-law relationship between drag coefficient (Cd) and Reynolds number (Re) under the free fall condition: Cd∼Re−1.1. CFD simulations show that both viscous and pressure drag force components are important in contributing to wind drag. The presence of hairs substantially increases pressure drag, and its relative importance depends on hair number and orientation. Seed morphology including hair number and orientation influences the drag coefficient under different flow directions relatively to the seed body. The lower drag coefficient observed with crossflow wind compared to free fall suggests that seeds encounter less air resistance while drifting horizontally in the wind, favoring extended flying time and distance. Based on the varying drag coefficients under different conditions, we propose the incorporation of varying drag coefficients in future wind-driven seed dispersal models.
more »
« less
- Award ID(s):
- 1929514
- PAR ID:
- 10649042
- Publisher / Repository:
- Physics of Fluids
- Date Published:
- Journal Name:
- Physics of Fluids
- Volume:
- 36
- Issue:
- 10
- ISSN:
- 1070-6631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Tropical cyclones (TCs) are often generated from preexisting “seed” vortices. Seeds with higher persistence might have a higher chance to undergo TC genesis. What controls seed persistence remains unclear. This study proposes that planetary Rossby wave drag is a key factor that affects seed persistence. Using recently developed theory for the response of a vortex to the planetary vorticity gradient, a new parameter given by the ratio of the maximum wind speed (Vmax) to the Rhines speed at the radius of maximum wind (Rmax), here termed “vortex structural compactness” (Cυ), is introduced to characterize the vortex weakening by planetary Rossby wave drag. The relationship between vortex compactness and weakening rate is tested via barotropicβ-plane experiments. The vortex’s initialCυis varied by systematically varying their initialVmaxandRmaxin idealized wind profile models. Experiments are also conducted with real-world seed vortices from reanalysis data, which possess natural compactness variability. The weakening rate depends strongly on the vortex’s initialCυacross both idealized and real-world experiments, and the initial axis-asymmetry introduces minor differences. Experiments doubling the size of seed vortices cause them to weaken more rapidly, in line with other experiment sets. The dependence of the weakening rate on initial compactness can be predicted from a simple theory, which is more robust for more compact vortices. Our results suggest that a seed’s structure strongly modulates how long it can persist in the presence of a planetary vorticity gradient. Connections to real seeds on Earth are discussed. Significance StatementThis study explores the evolution of tropical cyclone (TC) seeds, which are preexisting weakly rotating rainstorms, in a simple setting that isolates the dynamical effects of the rotating sphere. It is not clear why some seeds can persist for a longer duration and might have a higher chance to eventually undergo genesis. We proposed that a factor called “planetary Rossby wave drag” plays a crucial role in this process. To investigate this, we introduce a new parameter called “compactness” to describe how the size and intensity of a seed vortex determines how quickly it will weaken due to this drag. We conducted experiments with numerical simulations and real-world TC seeds to test our ideas. Our findings show that the initial compactness of seeds strongly influences how quickly they weaken. We have developed a formula to predict how quickly these seeds weaken based on their compactness, which is especially accurate for more compact seeds. This research helps us understand how planetary Rossby wave drag affects the persistence of a TC seed and, ultimately, how it might impact the frequency of TCs.more » « less
-
We experimentally investigate the settling of millimetre-sized thin disks in quiescent air. The range of physical parameters is chosen to be relevant to plate crystals settling in the atmosphere: the diameter-to-thickness aspect ratio is $$\chi =25\unicode{x2013}60$$ , the Reynolds numbers based on the disk diameter and fall speed are $Re=O(10^2)$ and the inertia ratio is $I^*=O(1)$ . Thousands of trajectories are reconstructed for each disk type by planar high-speed imaging, using the method developed by Baker & Coletti ( J. Fluid Mech. , vol. 943, 2022, A27). Most disks either fall straight vertically with their maximum projected area normal to gravity or tumble while drifting laterally at an angle $$<20^\circ$$ . Two of the three disk sizes considered exhibit bimodal behaviour, with both non-tumbling and tumbling modes occurring with significant probabilities, which stresses the need for a statistical characterization of the process. The smaller disks (1 mm in diameter, $Re=96$ ) have a stronger tendency to tumble than the larger disks (3 mm in diameter, $Re=360$ ), at odds with the diffused notion that $Re=100$ is a threshold below which falling disks remain horizontal. Larger fall speeds (and, thus, smaller drag coefficients) are found with respect to existing correlations based on experiments in liquids, demonstrating the role of the density ratio in setting the vertical velocity. The data supports a simple scaling of the rotational frequency based on the equilibrium between drag and gravity, which remains to be tested in further studies where disk thickness and density ratio are varied.more » « less
-
This study investigates the aerodynamic performance of different flying sensors inspired by dandelion seeds, using COMSOL Multiphysics CFD simulation. Dandelion seeds are well known for their ability to remain suspended in the air for extended periods due to their lightweight structure, higher porosity, high drag, and the formation of a separated vortex ring (SVR) above the seed. Mimicking this behavior, five 2D and one 3D geometry were developed and analyzed first through steady-state simulations to explore how different design geometries influence passive flight performance. The primary aim is to identify an optimized structure that can achieve slower descent when realized from an altitude by drones for remote sensing. Steady-state results showed that although the drag coefficient generally decreased with increase in Reynolds numbers, porosity did not exhibit a constant trend across all designs. In some cases, geometries with lower porosity outperformed more porous ones. This may be due to their structural differences. SVR was observed in all designs. However, the distance between these SVR and geometry’s surface was small. While steady-state results give a fair indication of the aerodynamic behavior and relative performances of the various geometries, there are limitations. To address this, transient drop tests, currently under verification, will give a better understanding of the performances of these designs from which the best will be selected.more » « less
-
In the study, a series of wind tunnel tests were conducted to investigate wind effects acting on dome structures (1/60 scale) induced by straight-line winds at a Reynolds number in the order of 106. Computational Fluid Dynamics (CFD) simulations were performed as well, including a Large Eddy Simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) simulation, and their performances were validated by a comparison with the wind tunnel testing data. It is concluded that wind loads generally increase with upstream wind velocities, and they are reduced over suburban terrain due to ground friction. The maximum positive pressure normally occurs near the base of the dome on the windward side caused by the stagnation area and divergence of streamlines. The minimum suction pressure occurs at the apex of the dome because of the blockage of the dome and convergence of streamlines. Suction force is the most significant among all wind loads, and special attention should be paid to the roof design for proper wind resistance. Numerical simulations also indicate that LES results match better with the wind tunnel testing in terms of the distribution pattern of the mean pressure coefficient on the dome surface and total suction force. The mean and root-mean-square errors of the meridian pressure coefficient associated with the LES are about 60% less than those associated with RANS results, and the error of suction force is about 40–70% less. Moreover, the LES is more accurate in predicting the location of boundary layer separation and reproducing the complex flow field behind the dome, and is superior in simulating vortex structures around the dome to further understand the unsteadiness and dynamics in the flow field.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0231717
