James Buchholz

The wake structure and thrust performance of a rigid low-aspect-ratio pitching panel

Thrust performance and wake structure were investigated for a rigid rectangular panel pitching about its leading edge in a free stream. For Re(C) = O(10(4)), thrust coefficient was found to depend primarily on Strouhal number St and the aspect ratio of the panel AR. Propulsive efficiency was sensitive to aspect ratio only for AR less than 0.83; however, the magnitude of the peak efficiency of a given panel with variation in Strouhal number varied inversely with the amplitude to span ratio A/S, while the Strouhal number of optimum efficiency increased with increasing A/S. Peak efficiencies between 9 % and 21 % were measured. Wake structures corresponding to a subset of the thrust measurements were investigated using dye visualization and digital particle image velocimetry. In general, the wakes divided into two oblique jets; however, when operating at or near peak efficiency, the near wake in many cases represented a Kármán vortex street with the signs of the vortices reversed. The three-dimensional structure of the wakes was investigated in detail for AR = 0.54, A/S = 0.31 and Re(C) = 640. Three distinct wake structures were observed with variation in Strouhal number. For approximately 0.20 < St < 0.25, the main constituent of the wake was a horseshoe vortex shed by the tips and trailing edge of the panel. Streamwise variation in the circulation of the streamwise horseshoe legs was consistent with a spanwise shear layer bridging them. For St > 0.25, a reorganization of some of the spanwise vorticity yielded a bifurcating wake formed by trains of vortex rings connected to the tips of the horseshoes. For St > 0.5, an additional structure formed from a perturbation of the streamwise leg which caused a spanwise expansion. The wake model paradigm established here is robust with variation in Reynolds number and is consistent with structures observed for a wide variety of unsteady flows. Movies are available with the online version of the paper.

On the evolution of the wake structure produced by a low-aspect-ratio pitching panel.

Flow visualization is used to interrogate the wake structure produced by a rigid flat panel of aspect ratio (span/chord) 0.54 pitching in a free stream at a Strouhal number of 0.23. At such a low aspect ratio, the streamwise vorticity generated by the plate tends to dominate the formation of the wake. Nevertheless, the wake has the appearance of a three-dimensional von Kármán vortex street, as observed in a wide range of other experiments, and consists of horseshoe vortices of alternating sign shed twice per flapping cycle. The legs of each horseshoe interact with the two subsequent horseshoes in an opposite-sign, then like-sign interaction in which they become entrained. A detailed vortex skeleton model is proposed for the wake formation.

Influence of Collective Boulder Array on the Surrounding Time-averaged and Turbulent Flow Fields

Arrays of large immobile boulders, which are often encountered in steep mountain streams, affect the timing and magnitude of sediment transport events through their interactions with the approach flow. Despite their importance in the quantification of the bedload rate, the collective influence of a boulder array on the approach time-averaged and turbulent flow field has to date been overlooked. The overarching objective is, thus, to assess the collective effects of a boulder array on the time-averaged and turbulent flow fields surrounding an individual boulder within the array, placing particular emphasis on highlighting the bed shear stress spatial variability. The objective of this study is pursued by resolving and comparing the time-averaged and turbulent flow fields developing around a boulder, with and without an array of isolated boulders being present. The results show that the effects of an individual boulder on the time-averaged streamwise velocity and turbulence intensity were limited to the boulder’s immediate vicinity in the streamwise (x/dc < 2–3) and vertical (z/dc < 1) directions. Outside of the boulder’s immediate vicinity, the time-averaged streamwise velocity was found to be globally decelerated. This global deceleration was attributed to the form drag generated collectively by the boulder array. More importantly, the boulder array reduced the applied shear stress exerted on the individual boulders found within the array, by absorbing a portion of the total applied shear. Furthermore, the array was found to have a “homogenizing” effect on the near-bed turbulence thus significantly reducing the turbulence intensity in the near-bed region. The findings of this study suggest that the collective boulder array bears a portion of the total applied bed shear stress as form drag, hence reducing the available bed shear stress for transporting incoming mobile sediment. Thus, the effects of the boulder array should not be ignored in sediment transport predictions. These effects are encapsulated in this study by Equation (6).

Parameter Variation and the Leading-Edge Vortex of a Rotating Flat Plate

Particle image velocimetry was used to characterize the flowfield on flat plates of aspect ratios 2 and 4 undergoing a starting rotation motion at Reynolds numbers, based on tip velocity, of 4,000, 8,000, and 16,000. The starting motion was performed in a tank of quiescent water. For both aspect ratios, a leading-edge vortex was observed on the suction surface of the plate, and its evolution and circulation were characterized with variations in angle of attack, Reynolds number, and azimuthal and spanwise positions. A region of strong counter-rotating vorticity was also observed between the leading-edge vortex and the plate surface, which becomes entrained by the leading-edge vortex. The circulation of the leading-edge vortex, when rendered dimensionless by the plate chord length and tip speed, was found to be relatively insensitive to Reynolds number and the azimuthal position of the plate within the ranges of the measurements; however, a strong (approximately linear) dependence on angle of attack was obs...

The kinematics of the vortex ring structure generated by a bursting bubble

The vortex structure created by a bursting bubble has been photographically studied. A vortex model has been constructed, which describes the evolution of the structure, and the phenomenon has been simulated numerically using two two-dimensional vortex blob models. The bubble is situated on a shallow pool of fluid that is on the top face of a vertical cylinder. Two upward-moving vortex rings with colinear axes are formed within 15 ms after bubble rupture. The two rings leapfrog to form a structure consisting of a vortex ring that sheds lobes of fluid that are sometimes similar in appearance to hairpin vortex loops. This evolution occurs within 100 ms of the initiation of bubble rupture, when the ring Reynolds number is approximately 170. The computational model produced images with a close resemblance to the physical structure, and was used to support the theory that the trailing lobes are rotational and consist of the vortex ring that was initially leading.

A method for detailed movement pattern analysis of tadpole startle response

Prolonged space flight, specifically microgravity, presents a problem for space exploration. Animal models with altered connections of the vestibular ear, and thus altered gravity sensation, would allow the examination of the effects of microgravity and how various countermeasures can establish normal function. We describe an experimental apparatus to monitor the effects of ear manipulations to generate asymmetric gravity input on the tadpole escape response. To perform the movement pattern analysis, an imaging apparatus was developed that uses a high-speed camera to obtain time-resolved, high-resolution images of tadpole movements. Movements were recorded in a temperature-controlled test chamber following mechanical stimulation with a solenoid actuator, to elicit a C-start response. Temperature within the test cell was controlled with a recirculating water bath. Xenopus laevis embryos were obtained using a standard fertilization technique. Tadpole response to a controlled perturbation was recorded in unprecedented detail and the approach was validated by describing the distinct differences in response between normal and one-eared tadpoles. The experimental apparatus and methods form an important element of a rigorous investigation into the response of the tadpole vestibular system to mechanical and biochemical manipulations, and can ultimately contribute to improved understanding of the effects of altered gravity perception on humans.

Vorticity Generation and Transport on a Plunging Wing

Two-component particle image velocimetry and surface pressure measurements are used to characterize the flow field over a plunging nominally two-dimensional flat-plate airfoil at zero geometric angle of attack, and a finite wing with rectangular planform and a semiaspect-ratio sAR = 2. Phase-averaged horizontal and vertical planes of PIV data are used to reconstruct a three-dimensional volume in which the evolution of the vortex structure is rendered, and vorticity transport is quantified within a chordwise planar control volume bounded by the flat plate surface, and containing the leading-edge vortex. It is shown that, for the two-dimensional airfoil, generation of secondary vorticity of opposite sign to the leading-edge vortex occurs at a rate of approximately half that of the leading-edge shear layer flux, suggesting that entrainment of this vorticity into the leading-edge vortex has a significant impact on the strength of the vortex. Also, spanwise convection of vorticity has a non-negligible impact on control-volume circulation during the second half of the stroke. In the case of the finite wing, the initial development of the leading-edge vortex is qualitatively similar to that of the nominally two-dimensional case; however, through the mid-portion of the stroke, the leading-edge vortex rapidly evolves into an arch structure as it convects along the chord, as seen in previous studies. In contrast to the case of the nominally two-dimensional airfoil, spanwise flow acts to significantly deplete the circulation within the leading-edge vortex. The difference between control-volume circulation and the sum of the integrated convective boundary fluxes suggests that the fraction of the total vorticity flux supplied by the finite wing surface beneath the leading-edge vortex is similar to that of the two-dimensional case.

Vortex Shedding and Wake Structure of a Plunging Wing

The structure and dynamics of leading-edge and trailing-edge vortices (LEV and TEV) are investigated for a plunging flat plate airfoil at a chord Reynolds number of 10,000 while varying plunge amplitude and Strouhal number. Digital particle image velocimetry measurements are used to characterize the shedding patterns and interactions of the LEV and TEV. A classification scheme is established to describe the qualitative structure of the wake, which is shown to be dependent primarily on Strouhal number. However, convection of the LEV in the chordwise direction, and its resulting interaction with the TEV is also strongly influenced by plunge amplitude. The development of the leading-edge and trailingedge vortices is tracked using phase-locked measurements throughout the cycle and the circulations of the LEV and TEV structures are measured. A scaling parameter proposed by Buchholz, Green, and Smits 20 for the circulation shed by a pitching panel reduces the variability of the circulation measurements but is found to be lacking physics relevant to the present problem. A modified parameter, taking into account the effect of free-stream velocity, is proposed, which provides an improved scaling of the circulation data.

Techniques to derive geometries for image-based Eulerian computations

PURPOSE The performance of three frequently used level set-based segmentation methods is examined for the purpose of defining features and boundary conditions for image-based Eulerian fluid and solid mechanics models. The focus of the evaluation is to identify an approach that produces the best geometric representation from a computational fluid/solid modeling point of view. In particular, extraction of geometries from a wide variety of imaging modalities and noise intensities, to supply to an immersed boundary approach, is targeted. DESIGN/METHODOLOGY/APPROACH Two- and three-dimensional images, acquired from optical, X-ray CT, and ultrasound imaging modalities, are segmented with active contours, k-means, and adaptive clustering methods. Segmentation contours are converted to level sets and smoothed as necessary for use in fluid/solid simulations. Results produced by the three approaches are compared visually and with contrast ratio, signal-to-noise ratio, and contrast-to-noise ratio measures. FINDINGS While the active contours method possesses built-in smoothing and regularization and produces continuous contours, the clustering methods (k-means and adaptive clustering) produce discrete (pixelated) contours that require smoothing using speckle-reducing anisotropic diffusion (SRAD). Thus, for images with high contrast and low to moderate noise, active contours are generally preferable. However, adaptive clustering is found to be far superior to the other two methods for images possessing high levels of noise and global intensity variations, due to its more sophisticated use of local pixel/voxel intensity statistics. ORIGINALITY/VALUE It is often difficult to know a priori which segmentation will perform best for a given image type, particularly when geometric modeling is the ultimate goal. This work offers insight to the algorithm selection process, as well as outlining a practical framework for generating useful geometric surfaces in an Eulerian setting.

A New Fatigue Analysis Procedure for Composite Wind Turbine Blades

A fatigue analysis procedure including random wind field simulation, aerodynamic analysis, stress analysis by finite element analysis, and fatigue damage evaluation based on tested fatigue data has been developed for large horizontal axis wind turbine blades. In order to simulate realistic wind loads applied on the blade while maintaining affordable computation time, the sectional surface pressure fields obtained from XFOIL are modified to match the lift, drag, and moment coefficients obtained using NREL’s AeroDyn. Thus the modified pressure distribution includes the effect of the dynamic stall and the wake on the turbine rotor aerodynamics. A high-fidelity finite element blade model, which could easily tailor the design of composite materials in the blade, has been parameterized for the detailed stress analyses. Constant life diagrams based on the tested fatigue data have been constructed for fatigue damage evaluation under multi-axial complex stress states of variable amplitude. Starting from the random wind field simulation, the evaluated fatigue damage is determined by two random variables, 10-minute mean wind speed and 10-minute turbulence intensity factor. Consequently, the effect of mean wind speed and atmospheric turbulence toward blade fatigue can be investigated. The proposed fatigue analysis procedure can facilitate the reliability analysis and reliability-based design optimization of composite wind turbine blades considering wind load uncertainty.