33 research outputs found
Instantaneous and time-averaged flow fields of multiple vortices in the tip region of a ducted propulsor
The instantaneous and time-averaged flow fields in the tip region of a ducted marine propulsor are examined. In this flow, a primary tip-leakage vortex interacts with a secondary, co-rotating trailing edge vortex and other co- and counter-rotating vorticity found in the blade wake. Planar particle imaging velocimetry (PIV) is used to examine the flow in a plane approximately perpendicular to the mean axis of the primary vortex. An identification procedure is used to characterize multiple regions of compact vorticity in the flow fields as series of Gaussian vortices. Significant differences are found between the vortex properties from the time-averaged flow fields and the average vortex properties identified in the instantaneous flow fields. Variability in the vortical flow field results from spatial wandering of the vortices, correlated fluctuations of the vortex strength and core size, and both correlated and uncorrelated fluctuations in the relative positions of the vortices. This variability leads to pseudo-turbulent velocity fluctuations. Corrections for some of this variability are performed on the instantaneous flow fields. The resulting processed flow fields reveal a significant increase in flow variability in a region relatively far downstream of the blade trailing edge, a phenomenon that is masked through the process of simple averaging. This increased flow variability is also accompanied by the inception of discrete vortex cavitation bubbles, which is an unexpected result, since the mean flow pressures in the region of inception are much higher than the vapor pressure of the liquid. This suggests that unresolved fine-scale vortex interactions and stretching may be occurring in the region of increased flow variability.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47076/1/348_2005_Article_938.pd
Excitation of airborne acoustic surface modes driven by a turbulent flow
This is the author accepted manuscript. The final version is available from AIAA via the DOI in this recordThis experiment demonstrated the generation of trapped acoustic surface waves
excited by a turbulent flow source through the coupling of pressure fluctuations at
the interface between an acoustic metamaterial and a flow environment. The
turbulent flow, which behaves as a stochastic pressure source, was produced using a
fully developed turbulent wall jet. The plate in the wall jet was perforated with a
single cavity. On the flow-side it was capped by a Kevlar weave to ensure the cavity
did not significantly disturb the flow, whilst on the adjacent side the cavity was open
to the quiescent (static) environment. The through-cavity opening, on the quiescent
side, was flush with an acoustic metasurface waveguide, which, through evanescent
diffractive coupling of the pressure field, produced an acoustic surface mode. This
acoustic mode was trapped at the plate surface, with its mode dispersion determined
by the surface geometry. The results of two different metasurface geometries are
discussed; (1) a slotted cavity array, and (2) a meander connected cavity array, each
demonstrating a different trapped surface wave dispersion behavior. Fourier
transform and correlation analyses of spatially-resolved temporal acoustic signals,
measured close to the metamaterial surface, were used to construct the frequency
and wave vector-dependent acoustic mode dispersion. Results demonstrated the flow
can indeed be used to excite these acoustic modes and that their mode dispersion can
be tailored towards realizing novel control of turbulent flow through acoustic-flow
interactionsDefence Science and Technology Laboratory (DSTL
Pre-Bilaterian Origins of the Hox Cluster and the Hox Code: Evidence from the Sea Anemone, Nematostella vectensis
BACKGROUND: Hox genes were critical to many morphological innovations of bilaterian animals. However, early Hox evolution remains obscure. Phylogenetic, developmental, and genomic analyses on the cnidarian sea anemone Nematostella vectensis challenge recent claims that the Hox code is a bilaterian invention and that no “true” Hox genes exist in the phylum Cnidaria. METHODOLOGY/PRINCIPAL FINDINGS: Phylogenetic analyses of 18 Hox-related genes from Nematostella identify putative Hox1, Hox2, and Hox9+ genes. Statistical comparisons among competing hypotheses bolster these findings, including an explicit consideration of the gene losses implied by alternate topologies. In situ hybridization studies of 20 Hox-related genes reveal that multiple Hox genes are expressed in distinct regions along the primary body axis, supporting the existence of a pre-bilaterian Hox code. Additionally, several Hox genes are expressed in nested domains along the secondary body axis, suggesting a role in “dorsoventral” patterning. CONCLUSIONS/SIGNIFICANCE: A cluster of anterior and posterior Hox genes, as well as ParaHox cluster of genes evolved prior to the cnidarian-bilaterian split. There is evidence to suggest that these clusters were formed from a series of tandem gene duplication events and played a role in patterning both the primary and secondary body axes in a bilaterally symmetrical common ancestor. Cnidarians and bilaterians shared a common ancestor some 570 to 700 million years ago, and as such, are derived from a common body plan. Our work reveals several conserved genetic components that are found in both of these diverse lineages. This finding is consistent with the hypothesis that a set of developmental rules established in the common ancestor of cnidarians and bilaterians is still at work today