Tip vortex cavitation studies were made with a hydrofoil that was elliptical in planform, with an aspect ratio of 3, and having a modified NACA 662-415 profile. LDV measurements of the tangential velocity component in the vortex were used to determine that the minimum pressure in the vortex varies with lift coefficient squared, i.e., that the incipient cavitation number σi should follow a Cl2 relation (σi ≈ Cl2). This is in contradiction to previous observations (Arndt et al. 1991) that the tip vortex cavitation index varied approximately with lift coefficient to the power 1.4. By carefully monitoring the tensile strength of the water, i.e., its susceptibility to cavitation, the discrepancy was traced to the capability of the test water to sustain a tensile stress. Cavitation in “weak” water (no tensile strength) does follow the Cl2 relationship, whereas observations in “strong” water (rupture considerably below vapor pressure) more closely followed the previously observed variation, i.e., σi ≈ Cl1.4. Since the structure of the vortex cannot be affected by changes in the water quality, the discrepancy can be explained only by the amount of tension that can be sustained by the test water before inception occurs. Apparently a relatively larger value of tension can be sustained in the vortex is the strength of the vortex is increased (i.e., increasing Cl). This would explain the observed deviation from the expected Cl2 law for water with measurable tensile strength.

The tensile strength of water increases when solid particles are filtered out, and it becomes greater the smaller the remaining particles are. Natural particles are of random shape, making parametric studies on the relationship between tensile strength and particle characteristics difficult. In this investigation, using degassed tap water from which natural particles larger than about 1 μm had been filtered out, the tensile strength was measured before and after seeding with almost spherical solid balls of diameters from 3 up to 76 μm. The smallest balls, one type being hydrophobic, the other hydrophilic, had no measurable influence on the tensile strength, though they were notably larger than the remaining natural nuclei. Seeding with the larger balls, hydrophilic as well as hydrophobic ones, reduced the tensile strength compared with that measured for unseeded, filtered water, but at most down to 1/3. On this basis it is concluded that a greater tensile strength is connected to the almost spherical solid balls than that due to natural particles of the same size. The critical cavities developed from the larger balls had radii much smaller than those of the balls themselves. This supports the hypothesis that cavitation nuclei are related to the fine scale surface structures observed on the balls, and in dependence of their global radii of curvature, critical cavities are developed. A model of this development is presented.

A new method to predict traveling bubble cavitation inception is devised. The crux of the method consists in combining the enhanced predictive capabilities of large-eddy-simulation (LES) for flow computation with a simple but carefully designed stability criterion for the cavitation nuclei. For LES a second-order accurate finite element model based on the Galerkin/least-squares method with Runge-Kutta time integration is applied. The incoming nucleus’ spectrum is approximated by a Weibull distribution. Moreover, it is shown that under typical conditions the stability of the nuclei can be evaluated with an algebraic criterion emerging from the Rayleigh-Plesset equation. This criterion can be expressed as modified critical Thoma number and fits well into the LES approach. The method was applied to study cavitation inception in a flow past a square cylinder. A good agreement with experimental results was achieved. Furthermore, the principal advantage over statistical (time-averaged) methods could be clearly demonstrated, even though the spatial resolution and application of the LES were restricted by limited computational resources. As the latter keep on growing, a wider range of applications will become accessible methods for cavitation prediction based on algebraic stability criteria combined with LES.

Full-scale vibro-acoustical diagnostic measurements of cavitation in four Francis 6 MW double runner turbines were performed. Two types of sensors were used—a hydrophone sensing waterborne noise at the pressure side of a runner and an accelerometer mounted at various points at the outer turbine casing, facing the runner’s pressure side. The correlation of noise and acceleration intensity with suction-side pressure fluctuations and runner position was checked. A simple but efficient method of spectrum normalization, which rejects the influence of the measurement set characteristics and vibro-acoustical characteristics of a turbine, was developed. The resulting spectra reveal the dependence of cavitation source strength on the turbine power as a function of noise or acceleration frequency.

A PCR assay based on the simultaneous detection of IS1245 and IS1311 was developed and used to determine the host range of these insertion elements. Specific PCR products were observed in Mycobacterium malmoense, Mycobacterium scrofulaceum, and Mycobacterium nonchromogenicum, indicating that IS1245 and IS1311 are not limited to the Mycobacterium avium complex.The 1,414-bp insertion element IS1245 belongs to the Staphylococcus aureus IS256 family of insertion sequences. It is present in up to 27 copies in Mycobacterium avium (8) and was found to be stable during in vivo and in vitro passage (1, 14), making it a popular target for restriction fragment length polymorphism strain typing in recent years (5,6,12,19). The host range was originally demonstrated to be limited to the M. avium group (M. avium and subspecies paratuberculosis and silvaticum) by PCR amplification of a 427-bp target sequence within IS1245, leading to its use as a species-specific target for diagnostic detection (11).Recently, Beggs et al. found IS1245 in strains of Mycobacterium intracellulare, demonstrating that the element is present in further species of the M. avium complex (MAC) (2).A closely related insertion element, IS1311 (85% sequence identity to IS1245 at the DNA level), has likewise been used as a target for restriction fragment length polymorphism strain typing of M. avium (5,15,16).In our laboratory, a PCR assay based on the simultaneous detection of a highly homologous 130-bp portion of IS1245 and IS1311 (91% sequence identity) was developed as a diagnostic tool for detection of MAC in clinical specimens and used to determine the host range of these elements.Mycobacterial strains were received from five collaborating mycobacterial laboratories. Species of nontuberculous mycobacteria (NTM) were identified by 16S ribosomal DNA (rDNA) sequencing (10) and/or 23S rDNA probes (GenoType; Hain Diagnostika, Nehren, Germany). Twelve NTM strains (1 Mycobacterium shimoidei strain, 1 Mycobacterium scrofulaceum strain, 6 M. intracellulare strains, 2 Mycobacterium malmoense strains, 1 Mycobacterium chelonae strain, and 1 Mycobacterium szulgai strain) were identified by a combination of standard biochemical methods (9) and high-performance liquid chromatography (4).Cells were lysed by incubation at 100°C for 5 min in the presence of 2 M NaOH and 4% Triton. Crude lysates or DNA purified using QIAamp spin columns (Qiagen, Hilden, Germany) were used as PCR templates.PCR analysis of NTM strains was performed by two independent laboratories (Lab 1 and Lab 2). Amplification was performed on GeneAmp PCR systems 9600 and 2400 (PerkinElmer, Weiterstadt, Germany). A 100-l reaction mixture contained 1 mM MgCl 2, 320 mol of deoxynucleoside triphosphates, 5 U of AmpliTaq Gold (GeneAmp; Perkin-Elmer), 100 pmol of each primer, and 5 to 10 l of lysate with 0.1 to 1 ng of template DNA in 1ϫ PCR Buffer II (Perkin-Elmer). The sense primer N3 (5Ј ACTTCCTGCGCAACGTGCT 3Ј) recognized positions 885 to 903 of IS1245 (GenBank accession no. L33879) and positions 823 to 8...