The Evolution of Cavitation Events with Speed and Scale of the Flow

This paper focuses on the different forms that individual cavitating events may take when the cavitation number is below the inception value (but not so low as to produce only attached cavities) and individual nuclei trigger individual cavitation events. It is a sequel to those of Kuhn de Chizelle et al. (1992a, 1992b, 1995) which described a set of cavitation scaling observations on simple Schiebe headforms conducted in the US Navy Large Cavitation Channel (LCC). The most common events observed in those experiments were traveling, hemi-spherical shaped bubbles which grew and collapsed as they were convected through the low pressure region on the headform. Several interesting variations were also observed, including the development of bubble tails and the triggering of patches, or local regions of attached cavitation. In the present paper, the frequency of occurrence of the various types of events is analyzed as well as how those probabilities changed with cavitation number, velocity and headform size. In general, the probabilities of tails and patches increased with decreasing cavitation number, but they also increased with increasing headform size and increasing velocity. A specific parametric dependence on these variables is suggested

Void Fraction Measurment Beneath a Stationary Breaking Wave

Impedance based techniques have been used to quantify air entrainment by a stationary breaking wave at the bow of a ship. The present paper describes an impedance based void fraction meter which was developed to make measurements in this high speed, unsteady, multiphase flow, and details of its calibration are provided. In addition, air entrainment data from an experimental simulation of a bow wave are presented. The local, time averaged void fraction was mapped for flow cross sections beneath the plunging wave jet, revealing the location of the clouds of bubbles formed by that jet impacting the incoming water surface. Size distribution functions for the bubbles within the bubble clouds are also presented. The results are correlated with the wave structure described in Waniewski et al. (1997)

Bubble Measurements Downstream of Hydraulic Jumps

A phase Doppler anemometer (PDA) system was used to measure the velocity profiles and air bubble size distributions downstream of two-dimensional hydraulic jumps for different upstream flow conditions in a 1.92m long laboratory flume. The PDA detected bubbles from 1 to 500 [microns] in diameter, and more were found at the bottom of the downstream flow near the elevation of the upstream free surface. This distribution was more marked for smaller bubbles, those with diameters less that 100 [microns]. The migration of the bubbles is controlled by the effects of buoyancy and by turbulent mixing; the relative magnitude of their effects on bubble distribution depends strongly on bubble size, and also on the energy of the upstream flow

Bow Wave Dynamics

Experimental studies of air entrainment by breaking waves are essential for advancing the understanding of these flows and creating valid models. The present study used experimental simulations of a ship bow wave to examine its dynamics and air entrainment processes. The simulated waves were created by a deflecting plate mounted at an angle in a supercritical free-surface flow in a flume. Measurements of the bow wave geometry at two scales and also for a bow wave created by a wedge in a towing tank are presented. Contact line and bow wave profile measurements from the different experiments are compared and demonstrate the similarity of the flume simulations to the towing tank experiments. The bow wave profile data from the towing tank experiments were used to investigate the scaling of the wave with the flow and the dependence on geometric parameters. In addition, surface disturbances observed on the plunging wave are documented herein because of the role they play in air entrainment. The air entrainment itself is explored in Waniewski et al (2001)

Experimental Simulation of a Bow Wave

In flows around ships, the bow wave can entrain a significant amount of air as it breaks continuously on the free surface. The resulting air bubbles persist in the ship wake affecting its radar cross section as well as acting as cavitation nuclei in the flow entering the ship's propeller. In the present investigation, the formation of a bow wave on a ship was simulated in the laboratory using a deflecting plate in a supercritical free surface flow. The experiments were conducted at two scales. The present paper focuses on how the bow wave changes with the angles and flow parameters, information which is a necessary prerequisite for understanding the air entrainment process. Flow visualization studies were performed and an electronic point gage was used to study the three-dimensional shape of the bow waves and the manner in which they break

Measurements of Air Entrainment by Bow Waves

This paper describes measurements of the air entrained in experiments simulating the breaking bow wave of a ship for Froude numbers between two and three. The experiments and the characteristics of the wave itself are detailed in T. Waniewski, 1999, "Air Entrainment by Bow Waves; PhD. theses, Calif. Inst. of Tech." The primary mechanism for air entrainment is the impact of the plunging wave jet, and it was observed that the air bubbles were entrained in spatially periodic bubble clouds. The void fraction and bubble size distributions were measured in the entrainment zone. There were indications that the surface disturbances described in Waniewski divide the plunging liquid jet sheet into a series of plunging jets, each of which produces a bubble cloud

A PCR Assay for the Detection of Wuchereria bancrofti in Blood

To identify Wuchereria bancrofti DNA sequences that could be used as the basis for a simple and rapid parasite detection assay, a genomic library of W. bancrofti was constructed and screened for highly repeated DNA. The repeat found with the highest copy number was 195 basepairs (bps) long, 77% AT, and 300 copies per haploid genome. This sequence was designated the Ssp I repeat because it has a unique recognition site for that restriction endonuclease in all or most of the repeat copies. The Ssp I repeat DNA family is dispersed, genus-specific, and exists in all of the different geographic isolates of W. bancrofti tested. Based on DNA sequence analysis of this repeat, we have developed an assay to detect very small quantities of W. bancrofti DNA using the polymerase chain reaction (PCR). With this PCR assay, the Ssp I repeat was detected in as little as 1 pg of W. bancrofti genomic DNA (about 1% of the DNA in one microfilaria) added to 100 pA of human blood. The PCR assay also amplified Ssp I repeat DNA from geographic isolates of W. bancrofti from around the world but not from other species of filariae or from human or mosquito DNA. Microfilaria-positive human blood samples collected in Mauke, Cook Islands were shown to be Ssp I PCR-positive, while microfilaria-negative samples were PCR-negative. The specificity and sensitivity of the Ssp I PCR assay indicates that this approach has significant potential for improved screening of large human populations for active W. bancrofti infection

Flow patterns in nappe flow regime down low gradient stepped chutes

Although modern gravity dam spillways include often steep chutes operating in skimming flow regime, succession of free-falling nappes (i.e. nappe flow regime) are more common on low gradient chutes and cascades, and this flow situation received little attention to date. New experiments were conducted in nappe flows without hydraulic jump in two large-size facilities with flat slopes. The flow on the stepped cascade displayed complex, three-dimensional patterns. Detailed air-water flow measurements were performed in the jet, at nappe impact and in the downstream flow region. Key results demonstrated that the flow on each step was rapidly varied (RVF), highly three-dimensional and strongly aerated