J. D. Bugg

Variable threshold outlier identification in PIV data

This paper describes a variable threshold technique that can be applied to any particle image velocimetry (PIV) post-analysis outlier identification algorithm which uses a threshold such as the local median or the cellular neural network techniques. Although these techniques have been shown to work quite well with constant thresholds, the selection of the threshold is not always clear when working with real data. Moreover, if a small threshold is selected, a very large number of valid vectors can be mistakenly rejected. Although careful monitoring may alleviate this danger in many cases, that is not always practical when large data sets are being analysed and there is significant variability in the properties of the vector fields. The method described in this paper adjusts the threshold by calculating a mean variation between a candidate vector and its eight neighbours. The main benefit is that much smaller thresholds can be used without suffering catastrophic loss of valid vectors. The main challenge in obtaining this threshold field is that it must be based on a filtered field to be representative of the underlying velocity field. In this work, a simple median filter which requires no threshold was used for preliminary rejection. A local threshold was then calculated from the mean difference between each vector and its neighbours. The threshold field was also filtered with a Gaussian kernel before use. The algorithm was tested and compared to the base techniques by generating artificial velocity fields with known numbers of spurious vectors. For these tests, the ability of the algorithms to identify bad vectors and preserve good vectors was monitored. In addition, the technique was tested on real PIV data from the developing region of an axisymmetric jet. The variable threshold versions of these algorithms were found to be much less susceptible to erroneously rejecting good vectors. This is because the variable threshold techniques extract information about the local velocity gradient from the data themselves. The user-adjustable parameters for the variable threshold methods were found to be more universal than the constant threshold methods.

Fluid forces on kayak paddle blades of different design

Three kayak paddle blades of different design (Conventional, Norwegian, Turbo) were tested in a low-speed wind tunnel at a maximum chord Reynolds number of Re = 2.2–2.7 × 105 (corresponding to speed through water of ≈1 m/s). The mean drag force and side force acting on each blade were measured, as the yaw and pitch angles were varied. The results were compared with those recorded for a finite rectangular flat plate of similar area and aspect ratio. For zero pitch angle of the blades, the results indicate that the drag coefficient was mostly independent of the blade design as the yaw angle was varied between ± 20°, with only the Norwegian blade design displaying a marginally higher drag coefficient than either of the other two blades or the flat plate. Increasing the pitch angle to 30°, while maintaining the yaw angle at zero, resulted in a 23% reduction of the drag coefficient for the flat plate, but only a 15% reduction of the drag coefficients for the three blades. For all designs, the drag coefficient reduction followed a simple cosine relationship as the pitch angle or yaw angle was increased. The wind tunnel experiments revealed that the side force coefficients for all three paddle blade designs were entirely independent of the blade design and were indistinguishable from those recorded for a flat plate. In summary, the study showed that the nondimensional force coefficients are largely independent of the paddle blade design.

Quantitative investigation of vortical structures in the near-exit region of an axisymmetric turbulent jet

High-resolution particle image velocimetry (PIV) measurements were made at the exit of an axisymmetric free jet at a Reynolds number of 21 900 with a top-hat, low-turbulence exit velocity profile. The data were analysed using the proper orthogonal decomposition (POD) technique to identify the main energy-containing vortices. The vortices so identified have been further quantified by computing their size, position, circulation and direction of rotation. The technique employed resolved vortices of radius 0.055 jet exit diameters and larger. This quantitative information has also been related to observations from flow visualisation images and measurements of the downstream evolution of the jet. The data clearly show the formation of alternating direction toroidal vortices identified in the fluctuating fields which begin to be resolved at a streamwise location of one-half the jet exit diameter. The vortices grow in number approximately linearly in the downstream direction until about one jet diameter. After this, the number of vortices decreases and their size and circulation increase as they move downstream. This is indicative of the vortex pairing process. The high spread rate of the current jet is attributed to the vigorous production of large-scale vortical structures in this near-exit region due to the very near top-hat exit velocity profile and the resulting strong shear layer near the exit.

The effect of surface roughness on the turbulence structure of a plane wall jet

In this paper, an experimental investigation of the turbulence characteristics of a plane wall jet over smooth and rough surfaces, using laser Doppler anemometry (LDA), is reported. The Reynolds number based on the slot height and exit velocity of the jet was approximately Re = 7500. A 36-grit sheet was used as the rough surface, creating a transitionally rough flow regime (44<ks+<70). Both inner and outer scales were used to analyze the effects of surface roughness on the Reynolds stress profiles. Comparisons between the present results and other LDA and hot-wire anemometry studies for a smooth surface indicate a similar behavior for the Reynolds stress profiles. However, the magnitudes of the peak values of the Reynolds stress were higher than in most previous studies due to the lower slot Reynolds number. The present results indicate that surface roughness does not appear to significantly modify the Reynolds stress profiles in the outer region of the jet except for a reduction in the level. In contrast...

Analysis of Coherent Structures in the Far-Field Region of an Axisymmetric Free Jet Identified Using Particle Image Velocimetry and Proper Orthogonal Decomposition

This paper investigates an isothermal free water jet discharging horizontally from a circular nozzle (9 mm) into a stationary body of water. The jet exit velocity was 2.5 m/s and the exit Reynolds number was 22,500. The large-scale structures in the far field were investigated by performing a proper orthogonal decomposition (POD) analysis of the velocity field obtained using a particle image velocimetry system. The number of modes used for the POD reconstruction of the velocity fields was selected to recover 40% of the turbulent kinetic energy. A vortex identification algorithm was then employed to quantify the size, circulation, and direction of rotation of the exposed vortices. A statistical analysis of the distribution of number, size, and strength of the identified vortices was carried out to explore the characteristics of the coherent structures. The results clearly reveal that a substantial number of vortical structures of both rotational directions exist in the far-field region of the jet. The number of vortices decreases in the axial direction, while their size increases. The mean circulation magnitude is preserved in the axial direction. The results also indicate that the circulation magnitude is directly proportional to the square of the vortex radius and the constant of proportionality is a function of the axial location.

Coherent Structures in Shallow Water Jets

This paper reports an experimental investigation of a round jet discharging horizontally from a vertical wall into an isothermal body of water confined in the vertical direction by a flat wall on the bottom and a free surface on top. Specifically, this paper focuses on the effects of vertical confinement on the characteristics of large vortical structures. The jet exit velocity was 2.5 m/s, and the exit Reynolds number was 22,500. Experiments were performed at water layer depths corresponding to 15, 10, and 5 times the jet exit diameter (9 mm). The large-scale structures were exposed by performing a proper orthogonal decomposition (POD) analysis of the velocity field obtained using a particle image velocimetry system. Measurements were made on vertical and horizontal planes—both containing the axis of the jet. All fields-of-view were positioned at an axial location in the range 10 <xlD <80. The number of modes used for the POD reconstruction of the velocity fields was selected to recover ∼40% of the turbulent kinetic energy. A vortex identification algorithm was then employed to quantify the size, circulation, and direction of rotation of the exposed vortices. A statistical analysis of the distribution of number, size, and strength of the identified vortices was carried out to explore the characteristics of the coherent structures. The results clearly reveal the existence of numerous vortical structures of both rotational senses in the jet flow, and their number generally decreases in the axial direction while their size increases. The size of vortices identified in the vertical plane is restricted by the water depth, while they are allowed to increase in size in the horizontal plane. Moreover, the results show a significant decrease in the number of small vortices for the shallowest case in the horizontal plane, with a corresponding increase in the number of large vortices and a significant increase in their size. This behavior was accompanied with an increase in the vortex circulation in the horizontal plane and a reduction in the circulation in the vertical plane. This is indicative of the dominance of the pairing process due to shallowness. Moreover, the balance between the positive and negative vortices in the vertical plane changed because of the formation of negative (clockwise) vortices near the solid wall at downstream locations.

Incomplete similarity of a plane turbulent wall jet on smooth and transitionally rough surfaces

This study assesses the hypothesis of incomplete similarity for a plane turbulent wall jet on smooth and transitionally rough surfaces. Typically, a wall jet is considered to consist of two regions: an inner layer and an outer layer. The degree to which these two regions reach equilibrium with each other and interact to produce the property of self-similarity remains an open question. In this study, the analysis of the outer and inner regions indicates that each region is characterised by a half-width which exhibits its own distinct dependence on the streamwise distance x from the slot, and a single self-similar structure for both regions does not exist. More specifically, the inner and outer layers of the wall jet exhibit different scaling laws, which results in two self-similar mean velocity profiles, both of which retain a dependence on the slot height H. As such, incomplete similarity of the wall jet on smooth and transitionally rough surfaces is confirmed by this study. In addition, comparison of the experimental results for the transitionally rough surface with the smooth wall case indicates that the surface roughness modifies the development of the mean velocity profile in both the inner and outer regions, although the effect on the outer region is relatively small and close to the experimental uncertainty.

The motion of bubbles in a sinusoidally oscillating liquid in microgravity

The sinusoidal motion of single, spherical bubbles in microgravity was studied experimentally aboard the U.S. Space Shuttle. Tests were performed to determine the effect of frequency, acceleration amplitude, bubble size, and fluid viscosity on bubble motion. Five test cells each containing a single bubble were subjected to rectilinear, sinusoidal oscillations. Three nominal bubble sizes and three liquids were used to cover a range of Stokes numbers from 1.3 to 21 and Reynolds numbers from 0.6 to 75. Bubble motion was recorded by video. The ratio of bubble motion amplitude to container motion amplitude was found to be essentially independent of the actual container motion amplitude. Therefore, this ratio could be plotted against frequency to obtain a frequency response for each case. This ratio was found to rise sharply from zero at zero frequency and then approach an asymptote at high frequencies. The strong effect of the walls in these experiments caused the amplitude of bubble motion to be reduced somewhat from that expected for an infinite fluid.

Effect of Aspect Ratio on the Flow Field Above the Free End of a Finite Circular Cylinder

The flow above the free end of a surface-mounted finite-height circular cylinder was studied in a low-speed wind tunnel using particle image velocimetry (PIV). The cylinder was mounted vertically in the wind tunnel, normal to a ground plane. The approaching flow was in the x-direction and the cylinder axis was aligned in the z-direction. Velocity measurements were made above the free-end surface in several vertical (x-z) planes and several horizontal (x-y) planes, for finite circular cylinders of aspect ratios AR = 9, 7, 5 and 3, at a Reynolds number of Re = 4.2×104. The relative thickness of the boundary layer on the ground plane was δ/D = 1.7. In the vertical symmetry plane, the mean velocity measurements show the prominent separation from the circumferential leading edge, the mean recirculation zone above the free-end surface, the arch vortex inside the recirculation zone, and reattachment of the flow onto the free-end surface. Experimental evidence is found for a leading-edge separation bubble, a flow structure which has been reported in some numerical simulations in the literature. As AR decreases, the reattachment point and the centre of the arch vortex move downstream, the recirculation zone becomes thicker, and the centre of the arch vortex moves higher above the free end. Away from the symmetry plane, the recirculation zone becomes thinner, the arch vortex centre moves upstream and closer to the free-end surface, and the reattachment point moves upstream. In the horizontal planes, measurements made very close to the surface can approximate the mean surface streamline topology, revealing the pair of foci representing the termination points of the arch vortex, the prominent curved reattachment line, reverse flow beneath the mean recirculation zone, and the reattachment and separation saddle points on the free-end centerline.Copyright

Quantitative Investigation of Coherent Structures in a Free Jet Using PIV and POD

This paper investigates an isothermal free water jet discharging horizontally from a circular nozzle (9 mm) into a stationary body of water. The jet exit velocity was 2.5 m/s and the exit Reynolds number was 22,500. Particle image velocimetry (PIV) measurements were made at the jet exit and in the far field of the jet. The large-scale structures in the far field were investigated by performing a proper orthogonal decomposition (POD) analysis on the fluctuating velocity fields. The number of modes used for the POD reconstruction of the velocity fields was selected to recover 40% of the turbulent kinetic energy. A vortex identification algorithm was then employed to quantify the size, circulation, and direction of rotation of the exposed vortices. Then, the identified vortex characteristics were analysed by performing a statistical study of their distributions. The results clearly reveal that the jet contains a substantial number of eddies of both rotation directions. Moreover, the number of vortices decreases in the axial direction while their size increases. The mean circulation magnitude is preserved in the axial direction. The results indicate that larger vortices have a higher value of circulation while smaller values of circulation are associated with smaller eddies.Copyright