Showing papers by "Steven L. Ceccio published in 2016"

Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities

Abstract: Partial cavitation in the separated region forming from the apex of a wedge is examined to reveal the flow mechanism responsible for the transition from stable sheet cavity to periodically shedding cloud cavitation. High-speed visualization and time-resolved X-ray densitometry measurements are used to examine the cavity dynamics, including the time-resolved void-fraction fields within the cavity. The experimentally observed time-averaged void-fraction profiles are compared to an analytical model employing free-streamline theory. From the instantaneous void-fraction flow fields, two distinct shedding mechanisms are identified. The classically described re-entrant flow in the cavity closure is confirmed as a mechanism for vapour entrainment and detachment that leads to intermittent shedding of smaller-scale cavities. But, with a sufficient reduction in cavitation number, large-scale periodic cloud shedding is associated with the formation and propagation of a bubbly shock within the high void-fraction bubbly mixture in the separated cavity flow. When the shock front impinges on flow at the wedge apex, a large cloud is pinched off. For periodic shedding, the speed of the front in the laboratory frame is of the order of half the free-stream speed. The features of the observed condensation shocks are related to the average and dynamic pressure and void fraction using classical one-dimensional jump conditions. The sound speed of the bubbly mixture is estimated to determine the Mach number of the cavity flow. The transition from intermittent to transitional to strongly periodic shedding occurs when the average Mach number of the cavity flow exceeds that required for the generation of strong shocks.

Bioinspired surfaces for turbulent drag reduction.

Abstract: In this review, we discuss how superhydrophobic surfaces (SHSs) can provide friction drag reduction in turbulent flow. Whereas biomimetic SHSs are known to reduce drag in laminar flow, turbulence adds many new challenges. We first provide an overview on designing SHSs, and how these surfaces can cause slip in the laminar regime. We then discuss recent studies evaluating drag on SHSs in turbulent flow, both computationally and experimentally. The effects of streamwise and spanwise slip for canonical, structured surfaces are well characterized by direct numerical simulations, and several experimental studies have validated these results. However, the complex and hierarchical textures of scalable SHSs that can be applied over large areas generate additional complications. Many studies on such surfaces have measured no drag reduction, or even a drag increase in turbulent flow. We discuss how surface wettability, roughness effects and some newly found scaling laws can help explain these varied results. Overall, we discuss how, to effectively reduce drag in turbulent flow, an SHS should have: preferentially streamwise-aligned features to enhance favourable slip, a capillary resistance of the order of megapascals, and a roughness no larger than 0.5, when non-dimensionalized by the viscous length scale.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

Ventilated cavities on a surface-piercing hydrofoil at moderate Froude numbers: cavity formation, elimination and stability

Abstract: The atmospheric ventilation of a surface-piercing hydrofoil is examined in a series of towing-tank experiments, performed on a vertically cantilevered hydrofoil with an immersed free tip. The results of the experiments expand upon previous studies by contributing towards a comprehensive understanding of the topology, formation and elimination of ventilated flows at low-to-moderate Froude and Reynolds numbers. Fully wetted, fully ventilated and partially ventilated flow regimes are identified, and their stability regions are presented in parametric space. The stability of partially and fully ventilated regimes is related to the angle of the re-entrant jet, leading to a set of criteria for identifying flow regimes in a laboratory environment. The stability region of fully wetted flow overlaps those of partially and fully ventilated flows, forming bi-stable regions where hysteresis occurs. Ventilation transition mechanisms are classified as formation and elimination mechanisms, which separate the three steady flow regimes from one another. Ventilation formation requires air ingress into separated flow at sub-atmospheric pressure from a continuously available air source. Ventilation washout is caused by upstream flow of the re-entrant jet. The boundary denoting washout of fully ventilated flow is expressed as a semi-theoretical scaling relation, which captures past and present experimental data well across a wide range of Froude and Reynolds numbers.

Acoustic emission due to partial cavity shedding on a hydrofoil

Abstract: The partial cavity shedding dynamics on a NACA 0015 hydrofoil were examined at angles of attack of 7 and 10 degrees over a range of cavitation numbers. Hydrophone measurements in conjunction with high-speed videos and time resolved X-ray densitometry were used to acquire time synchronous measurements of the void fraction field of the cavity and the emitted noise. Cavitation dynamics changed significantly with the reduction of the inlet cavitation number. At higher cavitation numbers, small stable partial cavities were observed. Upon reduction of cavitation number, the cavities grew in length and pinched off from the rear of the cavity, due to a liquid re-entrant jet. Upon further reduction of inlet cavitation number, the dynamics changed significantly with the cloud collapse of the shed vapor inhibiting the cavity growth. X-ray densitometry measurements revealed the presence of a propagation bubbly shockwave as a mechanism of shedding at the lowest cavitation numbers. This process caused complex, multi-st...