Showing papers by "Steven L. Ceccio published in 2019"

Multimodal partial cavity shedding on a two-dimensional hydrofoil and its relation to the presence of bubbly shocks

Abstract: The shedding dynamics of partial cavitation forming on a NACA0015 hydrofoil is known from the previous studies to be multimodal, exhibiting abrupt changes in Strouhal number as the cavitation number is reduced (Arndt et al. in Instability of partial cavitation: a numerical/experimental approach. National Academies Press, Washington, D.C., Retrieved from the University of Minnesota Digital Conservancy. http://hdl.handle.net/11299/49781 , 2000). The present study aims to understand the underlying shedding mechanisms responsible for this abrupt change in dynamics. As was observed in Ganesh et al. (J Fluid Mech 802:37–78, 2016), the shedding process can result from both re-entrant liquid flow and the formation of propagating bubbly shock waves within the cavity. Time-resolved X-ray densitometry and high-speed videography are combined with time synchronous measurements of acoustic noise produced by the cavity to examine the formation and shedding of the partial cavitation on a NACA0015 hydrofoil. From the experiments on the hydrofoil, it was observed that the mean cavity length increased with decreasing cavitation number, as expected. At higher pressures, stable partial cavities formed that shed smaller vapor clouds mainly due to the re-entrant liquid flow in the cavity closure. With a reduction in cavitation number, the partial cavity grew in length, and shedding often occurred when the cavity was pinched-off from its leading edge. Once a large region of vapor was shed, the pressure pulse caused by its subsequent collapse could suppress the growth of the newly forming partial cavity. This feedback led to complex, multistep cavity shedding dynamics. At still lower cavitation numbers, bubbly shocks were found to be responsible for the strongest periodic shedding of vapor clouds. Spectral analysis of the acoustic signal revealed a multimodal nature, which was more pronounced at lower cavitation numbers, and similar dynamic behavior was also found for the time varying local void fraction measurements in the vicinity of cavity closure. The occurrence of strongest multimodal behavior was also characterized by the presence of bubbly shock waves as the dominant mechanism of shedding. From the measurements and analysis, it is concluded that at least four different types of shedding modes occurred over a range of cavitation numbers at a fixed attack angle (10°).

The hydroelastic response of a surface-piercing hydrofoil in multi-phase flows. Part 1. Passive hydroelasticity

Abstract: Compliant lift-generating surfaces have widespread applications as marine propellers, hydrofoils and control surfaces, and the fluid–structure interactions (FSI) of such systems have important effects upon their performance and stability. Multi-phase flows like cavitation and ventilation alter the hydrodynamic and hydroelastic behaviours of lifting surfaces in ways that are not yet completely understood. This paper describes experiments on one rigid and two flexible variants of a vertical surface-piercing hydrofoil in wetted, ventilating and cavitating conditions. Tests were conducted in a towing tank and a free-surface cavitation channel. This work, which is Part 1 of a two-part series, examines the passive, or flow-induced, fluid–structure interactions of the hydrofoils. Four characteristic flow regimes are described: fully wetted, partially ventilated, partially cavitating and fully ventilated. Hydroelastic coupling is shown to increase the hydrodynamic lift and yawing moments across all four flow regimes by augmenting the effective angle of attack. The effective angle of attack, which was derived using a beam model to account for the effect of spanwise twisting deflections, effectively collapses the hydrodynamic load coefficients for the three hydrofoils. A generalized cavitation parameter, using the effective angle of attack, is used to collapse the lift and moment coefficients for all trials at a single immersed aspect ratio, smoothly bridging the four distinct flow regimes. None of the hydrofoils approached the static divergence condition, which occurs when the hydrodynamic stiffness negates the structural stiffness, but theory and experiments both show that ventilation increases the divergence speed by reducing the hydrodynamic twisting moment about the elastic axis. Coherent vortex shedding from the blunt trailing edge of the hydrofoil causes vortex-induced vibration at an approximately constant Strouhal number of 0.275 (based on the trailing edge thickness), and leads to amplified response at lock-in, when the vortex-shedding frequency approaches one of the resonant modal frequencies of the coupled fluid–structure system.

On the unsteady behaviour of cavity flow over a two-dimensional wall-mounted fence

Abstract: The topology and unsteady behaviour of ventilated and natural cavity flows over a two-dimensional (2-D) wall-mounted fence are investigated for fixed length cavities with varying free-stream velocity using high-speed and still imaging, X-ray densitometry and dynamic surface pressure measurement in two experimental facilities Cavities in both ventilated and natural flows were found to have a re-entrant jet closure, but not to exhibit large-scale oscillations, yet the irregular small-scale shedding at the cavity closure Small-scale cavity break-up was associated with a high-frequency broadband peak in the wall pressure spectra, found to be governed by the overlying turbulent boundary layer characteristics, similar to observations from single-phase flow over a forward-facing step A low-frequency peak reflecting the oscillations in size of the re-entrant jet region, analogous to ‘flapping’ motion in single-phase flow, was found to be modulated by gravity effects (ie a Froude number dependence) Likewise, a significant change in cavity behaviour was observed as the flow underwent transition analogous to the transition from sub- to super-critical regime in open-channel flow Differences in wake topology were examined using shadowgraphy and proper orthogonal decomposition, from which it was found that the size and number of shed structures increased with an increase in free-stream velocity for the ventilated case, while remaining nominally constant in naturally cavitating flow due to condensation of vaporous structures

Microbubble disperse flow about a lifting surface

Abstract: The formation, size and concentration of microbubbles generated in the wake of a cavitating hydro-foil were investigated experimentally in a variable pressure water tunnel for several Reynolds and cavitation numbers, with and without freestream nuclei. In the absence of freestream nuclei, interactions between the cavity, the overlying boundary layer and associated interfacial effects were invesigated qualitatively and quantitatively using a combination of still and high speed photography. The influence of these features on the physics of cavity breakup and condensation, and subsequent microbubble formation, were examined. Coherent spatial and temporal features of the sheet cavitation were found to be functions of both Reynolds and cavitation numbers. Long range microscopic shadowgraphy was used to measure the dense bubble populations present in the wake, and additionally implemented as a reference technique in the development of the Mie- Scattering Imaging (MSI) technique described below. For the range of microbubble sizes measured, concentrations are shown to increase with Reynolds number and reduce with decreasing cavitation number. The presence of freestream nuclei markedly alters cavity topology, and their effect on flow features and associated microbubble production was also evaluated. Wake microbubble concentrations were found to in-crease when low concentrations of nuclei were introduced but to then decrease with further increase in nuclei seeding. Regardless of seeding concentration, microbubble populations in the wake in-creased as the cavitation number was reduced. For high cavitation numbers the increase in concentration is primarily in bubbles of smaller size, whereas the increase in wake concentration at lower cavitation numbers occurs over a greater size range. These experiments demonstrate the im-portance of cavitation nuclei measurement in hydrodynamic test facilities. Application of an interferometric technique known as Mie- Scattering Imaging (MSI) for the measurement of sparse nuclei seeding populations in such facilities has been developed. A separate pressure chamber, with similar optical path properties to the tunnel test section, was used to develop the technique. Monodisperse bubbles (with diameters between 30 and 150 ➭m) were generated by a microfluidic ‘T’ junction, and individual bubbles were simultaneously imaged with shadowgraphy and MSI. In develop-ment of the MSI technique, approximations from Lorenz-Mie theory were experimentally validated, and the influence of fringe uniformity and intensity for each polarisation (perpendicular or parallel) on measurement precision was investigated. Parallel polarisation was preferred for its more uniform fringe spacing despite a lower intensity. The inverse relation between fringe wavelength and bubble diameter was demonstrated at a measurement angle of 90°. The wavelength of the scattered fringe pattern is predicted using Lorentz-Mie theory and the calibration constant for fringe spacing was obtained. A practical method for the calibration of a second constant related to the imaging optics has also been developed. Using this approach the measured bubble diameters from the shadowgraphy and MSI compared to within 1 μm. The precise bubble location within the beam was measured with shadowgraphy and with this information a method for determining the size dependent measurement volume for both axisymmetric and arbitrary beam profiles was developed. Once re-fined, the technique was used to characterise the concentrations and range of microbubble sizes produced by a nuclei seeding system for various tunnel conditions. Nuclei concentrations between 0-24 bubbles per mL were measured and the distribution of bubble sizes was found to follow a power law for high nuclei concentrations.

Durable superhydrophobic surfaces

Abstract: Durable superhydrophobic components have a superhydrophobic material disposed (e.g., disposed) thereon that exhibits an apparent advancing dynamic contact angle of ≥ about 150° and a roll-off angle of about ≤ 15° for water after at least 100 abrasion cycles. The superhydrophobic material may comprise a low surface energy material and a polymeric material. The superhydrophobic material may be self-healing and capable of recovering its wettability after damage. In yet other aspects, a component comprises a surface that is superhydrophobic and reduces drag in turbulent flow conditions. The surface has an apparent advancing dynamic contact angle of ≥ about 150° and a roll-off angle of ≤ about 15° for water, and a product of dimensionless roughness (k+) and a higher-pressure contact angle hysteresis of less than or equal to about 5.8. Methods of making such materials are also provided.