MEAN FLOW EFFECTS OF THE LOW-WAVENUMBER PRESSURE SPECTRUM ON A FLEXIBLE SURFACE.

The Lighthill theory is extended so that it may be used to determine the flow noise induced by a turbulent boundary layer over a plane homogeneous flexible surface. The influence of the surface properties and the mean flow on the sound generation is brought out explicitly through the use of a Green function. The form of the low-wavenumber wall-pressure spectrum on a rigid surface with an arbitrary mean flow profile is determined. The effect of a coating layer is investigated.

Risk-sensitivity and the mean-variance trade-off: decision making in sensorimotor control.

Numerous psychophysical studies suggest that the sensorimotor system chooses actions that optimize the average cost associated with a movement. Recently, however, violations of this hypothesis have been reported in line with economic theories of decision-making that not only consider the mean payoff, but are also sensitive to risk, that is the variability of the payoff. Here, we examine the hypothesis that risk-sensitivity in sensorimotor control arises as a mean-variance trade-off in movement costs. We designed a motor task in which participants could choose between a sure motor action that resulted in a fixed amount of effort and a risky motor action that resulted in a variable amount of effort that could be either lower or higher than the fixed effort. By changing the mean effort of the risky action while experimentally fixing its variance, we determined indifference points at which participants chose equiprobably between the sure, fixed amount of effort option and the risky, variable effort option. Depending on whether participants accepted a variable effort with a mean that was higher, lower or equal to the fixed effort, they could be classified as risk-seeking, risk-averse or risk-neutral. Most subjects were risk-sensitive in our task consistent with a mean-variance trade-off in effort, thereby, underlining the importance of risk-sensitivity in computational models of sensorimotor control.

The effect of shape on the adhesion of fibrillar surfaces.

Experimental data have demonstrated that mushroom-shaped fibrils adhere much better to smooth substrates than punch-shaped fibrils. We present a model that suggests that detachment processes for such fibrils are controlled by defects in the contact area that are confined to its outer edge. Stress analysis of the adhered fibril, carried out for both punch and mushroom shapes with and without friction, suggests that defects near the edge of the adhesion area are much more damaging to the pull-off strength in the case of the punch than for the mushroom. The simulations show that the punch has a higher driving force for extension of small edge defects compared with the mushroom adhesion. The ratio of the pull-off force for the mushroom to that of the punch can be predicted from these simulations to be much greater than 20 in the friction-free case, similar to the experimental value. In the case of sticking friction, a ratio of 14 can be deduced. Our analysis also offers a possible explanation for the evolution of asymmetric mushroom shapes (spatulae) in the adhesion organ of geckos.