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When a two-dimensional protuberance is placed in an unstable laminar boundary layer at moderate Reynolds numbers, tonal noise can be generated by an acoustic feedback-loop mechanism, not unlike that in case of the trailing-edge noise of an airfoil. In this paper, the generation of tonal protuberance noise was examined experimentally, focusing on the influence of the weak adverse pressure gradient on the critical protuberance height for the onset of tonal noise. It was shown that the weak adverse pressure gradient could cause the critical protuberance height to decrease, while the frequencies of tonal noise were little changed in the present experimental condition. At the onset condition for the tonal noise, the separation bubble formed just upstream of the protuberance was found to be almost the same size in both the zero- and weak adverse-pressure-gradient boundary layers, despite of the difference in the instability nature in the absence of the protuberance. A linear stability analysis revealed that the separation bubble could amplify disturbances by a factor of one hundred or more within a short distance (several times the boundary-layer thickness) at the onset condition. Such rapid disturbance growth was requisite for the sustenance of the feedback-loop mechanism even for the adverse-pressure-gradient case at least in the present Reynolds number range. [ABSTRACT FROM AUTHOR]

Instability and breakdown of a low-speed streak initially forced by disturbances with a frequency-spectrum similar to that of wall turbulence were examined experimentally. A single low-speed streak was generated using a small piece of screen set normal to a boundary-layer plate, and disturbances made by two loudspeakers were introduced through two small holes. Turbulent velocity fluctuations with various spanwise coherences were used as driving signals to two loudspeakers to see how strongly the streak breakdown depended on the nature of disturbances. Signals were deduced at two spanwise locations with distance Δ z + = 24–149 in the buffer region of a turbulent boundary layer by using a pair of hot-wires. The correlation coefficient C t of the two turbulent signals ranged from −0.11 to 0.56. Both sinuous and varicose modes were excited but the former became dominant downstream in all cases. Development of the disturbance excited was compared to those for anti-symmetric ( C t = − 1 ) and symmetric ( C t = 1 ) excitations with the same turbulent spectrum. The comparison showed that the magnitude of the sinuous instability mode excited was dependent on the initial disturbances, but not strongly for − 1 ≤ C t ≤ 0 . That is, in terms of the rms value of velocity fluctuations, the magnitude of the sinuous instability mode for the initial disturbance with C t ∼ 0 was about 75% of that in the case of anti-symmetric forcing. Even for C t ∼ + 0 . 56 , the magnitude of the sinuous mode excited was as large as 50% of that in the anti-symmetric case. [ABSTRACT FROM AUTHOR]

Abstract: Lifespan extension has been demonstrated in dwarfism mouse models relative to their wild-type. The spontaneous dwarf rat (SDR) was isolated from a closed colony of Sprague–Dawley (SD) rats. Growth hormone deficiencies have been indicated to be responsible for dwarfism in SDR. Survival time, the markers of oxidative stress, antioxidant enzymes, and resistance to hyperoxia were compared between SDR and SD rats, to investigate whether SDR, a dwarfism rat model, also extends lifespan and has an enhanced resistance to oxidative stress. SDRs lived 38% longer than SD rats on average. This is the first report to show that dwarf rats exhibit lifespan extensions similar to Ames and Snell mice. Decreased 8-oxo-2′-deoxyguanosine (8-oxodG) content, a marker of oxidative DNA damage, indicated suppressed oxidative stress in the liver, kidney, and lung of SDRs. Increased glutathione peroxidase enzyme activity was consistent with decreased 8-oxodG content in the same tissues. The heart and brain showed a similar tendency, but this was not significant. However, the catalase and superoxide dismutase enzyme activities of SDRs were not different from those of SD rats in any tissue. This was not what the original null hypothesis predicted. SDRs had potent resistance to the toxicity associated with high O2 (85%) exposure. The mean survival time in SDRs was more than 147% that of SD rats with 168h O2 exposure. These results suggest that the enhanced resistance to oxidative stress of SDRs associated with enhanced hydrogen peroxide elimination may support its potential role in lifespan extension. [Copyright &y& Elsevier]