Masatsugu Maeda

Possibility of Quantitative Prediction of Cavitation Erosion Without Model Test

A scenario for quantitative prediction of cavitation erosion was proposed. The key value is the impact force/pressure spectrum on a solid surface caused by cavitation bubble collapse. As the first step of prediction, the authors constructed the scenario from an estimation of the cavity generation rate to the prediction of impact force spectrum, including the estimations of collapsing cavity number and impact pressure. The prediction was compared with measurements of impact force spectra on a partially cavitating hydrofoil. A good quantitative agreement was obtained between the prediction and the experiment. However, the present method predicted a larger effect of main flow velocity than that observed. The present scenario is promising as a method of predicting erosion without using a model test.

Reduction of skin friction by microbubbles and its relation with near-wall bubble concentration in a channel

Determination of the flow structure near the wall is essential for a clear insight into the phenomenon of skin friction reduction by microbubbles in a turbulent boundary layer. An important parameter, is the bubble concentration or void fraction in the wall region in drag-reducing conditions. The purpose of this paper is to show drag-reducing effects due to microbubbles in a water channel and, more importantly, to show the dependence of the drag-reduction values on the near-wall void fraction. A two-dimensional channel with an aspect ratio of 10 was specially built for this purpose with provisions for air injection through porous plates. Skin friction was directly measured by a miniature floating element transducer with a 5-mm circular sensing disk mounted flush on the top wall 67 channel-heights downstream of the injector. The wall friction in the presence of air bubbles was found to be reduced under the same bulk velocity when compared with the value without air. Detailed void fraction profiles across the channel were obtained by a sampling probe and a fiber-optic probe. Better collapse of the drag reduction data, independent of different profile shapes, was found when plotted against the near-wall void fraction than against a cross-sectional mean void fraction. While this dependence reconfirms that the phenomena are essentially inner-region dependent, the lack of influence of the bubble distribution patterns away from the wall implies lack of outer region influence.

SF20/IL-25, a Novel Bone Marrow Stroma-Derived Growth Factor That Binds to Mouse Thymic Shared Antigen-1 and Supports Lymphoid Cell Proliferation

Using a forward genetic approach and phenotype-based complementation screening to search for factors that stimulate cell proliferation, we have isolated a novel secreted bone marrow stroma-derived growth factor, which we termed SF20/IL-25. This protein signals cells to proliferate via its receptor, which we have identified as mouse thymic shared Ag-1 (TSA-1). Enforced expression of TSA-1 in IL-3-dependent Ba/F3 cells that do not express endogenous TSA-1 rendered cells to proliferate in a dose-dependent manner when stimulated with SF20/IL-25. FDCP2, a factor-dependent hemopoietic cell line that expresses endogenous TSA-1, could also be stimulated to proliferate with SF20/IL-25. Binding of SF20 to TSA-1 was blocked by anti-TSA-1 Ab and SF20-induced proliferation of TSA-1-expressing cells was inhibited by anti-TSA-1. In vitro assay revealed that SF20/IL-25 has no detectable myelopoietic activity but supports proliferation of cells in the lymphoid lineage.

Laser Holographic Observation of Cavitation Cloud on a Foil Section

Observation of a cavitation cloud was performed using an off-axis laser holography system. The cavitation cloud contains an inverse U-shaped vortex cavitation surrounded by many small cavitation bubbles. The density of bubbles with radius larger than 35 μm is on the order of 103 bubbles/cm3. The bubble number distribution was determined from the observation and by counting individual bubbles in reconstructed holographic images of the cavitation cloud.

Study on Unsymmetrical Propeller

In this paper, Unsymmetrical Propellers are proposed in order to reduce the fluctuating pressure at blade frequency induced by a cavitating propeller. The idea behind the unsymmetrical spacing of blade is to give rise to a suitable phase lag between the fluctuating pressures transmitted from each individual blade. Pressure fluctuations can be roughly estimated in terms of the blade spacing by using Fourier expansions. In order to obtain the suitable blade spacing, an iterative calculation procedure is adopted.Finally five model propellers with non-uniform spacing of blades were designed. Experiments were performed with these five propellers and the one with uniform spacing.Comparing these results, the reduction of fluctuating pressure amplitude due to unsymmetrical propellers were verified.