The effects of turbulent eddies on the stability and critical swimming speed of creek chub (Semotilus atromaculatus).

The effect of turbulent eddy diameter, vorticity and orientation on the 2 min critical swimming speed and stability of creek chub (Semotilus atromaculatus) is reported. Turbulent eddies were visualized and their properties were quantified using particle image velocimetry (PIV). Flow fields with an increasing range in eddy diameter were created by inserting cylinder arrays upstream from the swimming test section. Eddy vorticity increased with increasing velocity. Two orientations of eddies, eddies spinning about a vertical axis and eddies spinning about a horizontal (wall-to-wall) axis, were investigated. Stability challenges were not observed until the largest (95th percentile) eddy diameters reached 76% of the fish body total length. Under these conditions fish were observed to spin in an orientation consistent with the rotational axis of the large eddies and translate downstream. These losses in postural control were termed 'spills'. Spills were 230% more frequent and lasted 24% longer in turbulent flow fields dominated by horizontal eddies than by vertical eddies of the same diameter. The onset of spills coincided with a 10% and 22% reduction in critical swimming speed in turbulent flows dominated by large vertical and horizontal eddies, respectively. These observations confirm predictions by Pavlov et al., Cada and Odeh, Lupandin, and Liao that the eddy diameter, vorticity and orientation play an important role in the swimming capacity of fishes.

Turbulence: does vorticity affect the structure and shape of body and fin propulsors?

Over the past century, many ideas have been developed on the relationships between water flow and the structure and shape of the body and fins of fishes, largely during swimming in relatively steady flows. However, both swimming by fishes and the habitats they occupy are associated with vorticity, typically concentrated as eddies characteristic of turbulent flow. Deployment of methods to examine flow in detail suggests that vorticity impacts the lives of fishes. First, vorticity near the body and fins can increase thrust and smooth variations in thrust that are a consequence of using oscillating and undulating propulsors to swim. Second, substantial mechanical energy is dissipated in eddies in the wake and adaptations that minimize these losses would be anticipated. We suggest that such mechanisms may be found in varying the length of the propulsive wave, stiffening propulsive surfaces, and shifting to using median and paired fins when swimming at low speeds. Eddies in the flow encountered by fishes may be beneficial, but when eddy radii are of the order of 0.25 of the fish's total length, negative impacts occur due to greater difficulties in controlling stability. The archetypal streamlined "fish" shape reduces destabilizing forces for fishes swimming into eddies.

Phased-array grating compression for high-energy chirped pulse amplification lasers.

The development of phased-array grating compressor is a crucial issue for high-energy, ultra-short pulse petawatt-class lasers. We present a theoretical and experimental analysis of two-grating phasing in a broadband pulse mosaic compressor. The phase defaults induced by misaligned gratings are studied. Monochromatic grating phasing is experimentally achieved with an interferometric technique and pulse compression is demonstrated with a two-phased-array grating system.

Synthetic aperture compression scheme for a multipetawatt high-energy laser.

High-energy petawatt lasers using the chirped-pulse amplification technique require meter-sized gratings to limit the beam fluence on the surface of the grating. An alternative, studied by many groups, is a mosaic grating consisting of smaller, coherently added gratings. We propose what we believe to be a new compression scheme consisting of beam phasing instead of grating mosaic phasing. This synthetic aperture compression scheme allows us to control the beam thanks to a unique segmented mirror equipped with three degrees of freedom. With this configuration, the beam is divided into small subapertures adapted to the classical grating size. After compression, these subapertures are coherently added before the focusing stage. Therefore the alignment processes are simplified.