Flow structure generated by perpendicular blade-vortex interaction and implications for helicopter noise prediction. Volume 1: Measurements

The perpendicular interaction of a streamwise vortex with an infinite span helicopter blade was modeled experimentally in incompressible flow. Three-component velocity and turbulence measurements were made using a sub-miniature four sensor hot-wire probe. Vortex core parameters (radius, peak tangential velocity, circulation, and centerline axial velocity deficit) were determined as functions of blade-vortex separation, streamwise position, blade angle of attack, vortex strength, and vortex size. The downstream development of the flow shows that the interaction of the vortex with the blade wake is the primary cause of the changes in the core parameters. The blade sheds negative vorticity into its wake as a result of the induced angle of attack generated by the passing vortex. Instability in the vortex core due to its interaction with this negative vorticity region appears to be the catalyst for the magnification of the size and intensity of the turbulent flowfield downstream of the interaction. In general, the core radius increases while peak tangential velocity decreases with the effect being greater for smaller separations. These effects are largely independent of blade angle of attack; and if these parameters are normalized on their undisturbed values, then the effects of the vortex strength appear much weaker. Two theoretical models were developed to aid in extending the results to other flow conditions. An empirical model was developed for core parameter prediction which has some rudimentary physical basis, implying usefulness beyond a simple curve fit. An inviscid flow model was also created to estimate the vorticity shed by the interaction blade, and to predict the early stages of its incorporation into the interacting vortex

The Spectral and Statistical Properties of Turbulence Generated by a Vortex/Blade-Tip Interaction

The perpendicular interaction of a streamwise vortex with the tip of a lifting blade was studied in incompressible flow to provide information useful to the accurate prediction of helicopter rotor noise and the understanding of vortex dominated turbulent flows. The vortex passed 0.3 chord lengths to the suction side of the blade tip, providing a weak interaction. Single and two-point turbulence measurements were made using sub-miniature four sensor hot-wire probes 15 chord lengths downstream of the blade trailing edge; revealing the mean velocity and Reynolds stress tensor distributions of the turbulence, as well as its spanwise length scales as a function of frequency. The single point measurements show the flow downstream of the blade to be dominated by the interaction of the original tip vortex and the vortex shed by the blade. These vortices rotate about each other under their mutual induction, winding up the turbulent wakes of the blades. This interaction between the vortices appears to be the source of new turbulence in their cores and in the region between them. This turbulence appears to be responsible for some decay in the core of the original vortex, not seen when the blade is removed. The region between the vortices is not only a region of comparatively large stresses, but also one of intense turbulence production. Velocity autospectra measured near its center suggests the presence quasi-periodic large eddies with axes roughly parallel to a line joining the vortex cores. Detailed two-point measurements were made on a series of spanwise cuts through the flow so as to reveal the turbulence scales as they would be seen along the span of an intersecting airfoil. The measurements were made over a range of probe separations that enabled them to be analyzed not only in terms of coherence and phase spectra but also in terms of wave-number frequency (kappa-omega) spectra, computed by transforming the measured cross-spectra with respect to the spanwise separation of the probes. These data clearly show the influence of the coherent eddies in the spiral wake and the turbulent region between the cores. These eddies produce distinct peaks in the upwash velocity kappa-omega spectra, and strong anisotropy manifested both in the decay of the kappa-omega spectrum at larger wave-numbers and in differences between the kappa-omega spectra of different components. None of these features are represented in the von Karman spectrum for isotropic turbulence that is often used in broadband noise computations. Wave-number frequency spectra measured in the cores appear to show some evidence that the turbulence outside sets tip core waves, as has previously been hypothesized. These spectra also provide for the first time a truly objective method for distinguishing velocity fluctuations produced by core wandering from other motions

Bio-inspired canopies for the reduction of roughness noise

This work takes inspiration from the structure of the down covering the flight feathers of larger species of owls, which contributes to their ability to fly almost silently at frequencies above 1.6 kHz. Microscope photographs of the down show that it consists of hairs that form a structure similar to that of a forest. The hairs initially rise almost perpendicular to the feather surface but then bend over in the flow direction to form a canopy with an open area ratio of about 70 percent. Experiments have been performed to examine the noise radiated by a large open area ratio canopy suspended above a surface. The canopy is found to dramatically reduce pressure fluctuations on the underlying surface. While the canopy can produce its own sound, particularly at high frequencies, the reduction in surface pressure fluctuations can reduce the noise scattered from an underlying rough surface at lower frequencies. A theoretical model is developed which characterizes the mechanism of surface pressure reduction as a result of the mixing layer instability of flow over forest canopies.Office of Naval Research (Grant IDs: N00014-13-1-0244, N00014-14-1-0242

Wind Tunnel Testing of Directionally Sensitive Meander Metasurface and Sub-Resonant Sensor Arrays

This is the author accepted manuscript. The final version is available from the American Institute of Aeronautics and Astronautics via the DOI in this recordLocating a sound source through the disruptive turbulent boundary layer’s pressure fluctuations is of great difficulty in aero/hydroacoustic applications. Conventional phased microphone arrays have trouble with this task because their diaphragms are exposed to the excess noise presented by these pressure fluctuations. In this paper, a meander-style metasurface was modified to make it flow-compatible. A sub-resonant sensor array was also designed to filter out convective pressure fluctuations and improve signal to noise ratio. These novel array designs were tested alongside a conventional phased array in Virginia Tech’s Stability Wind Tunnel. The metasurface array proved its ability to detect sound sources through a turbulent boundary layer although it was less accurate than the other two arrays. The sub-resonant sensor array displayed similar accuracy to the conventional phased array and provided insight into the design of through-cavities for convective pressure filtering. In combination, these results show promise for the development of novel acoustic array designs which can perform optimally through a turbulent boundary layer.Office of Naval Researc

Three-dimensional instability during vortex merging

4 p.The interaction of two parallel vortices of equal circulation is observed experimentally. For low Reynolds numbers ($Re$), the vortices remain two-dimensional and merge into a single one, when their time-dependent core size exceeds approximately 30\% of the vortex separation distance. At higher $Re$, a three-dimensional instability is discovered, showing the characteristics of an elliptic instability of the vortex cores. The instability rapidly generates small-scale turbulent motion, which initiates merging for smaller core sizes and produces a bigger final vortex than for laminar 2D flow

A role for core planar polarity proteins in cell contact-mediated orientation of planar cell division across the mammalian embryonic skin

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. © The Author(s) 2017. Supplementary information accompanies this paper at doi:10.1038/s41598-017-01971-2.The question of how cell division orientation is determined is fundamentally important for understanding tissue and organ shape in both healthy or disease conditions. Here we provide evidence for cell contact-dependent orientation of planar cell division in the mammalian embryonic skin. We propose a model where the core planar polarity proteins Celsr1 and Frizzled-6 (Fz6) communicate the long axis orientation of interphase basal cells to neighbouring basal mitoses so that they align their horizontal division plane along the same axis. The underlying mechanism requires a direct, cell surface, planar polarised cue, which we posit depends upon variant post-translational forms of Celsr1 protein coupled to Fz6. Our hypothesis has parallels with contact-mediated division orientation in early C. elegans embryos suggesting functional conservation between the adhesion-GPCRs Celsr1 and Latrophilin-1. We propose that linking planar cell division plane with interphase neighbour long axis geometry reinforces axial bias in skin spreading around the mouse embryo body.Peer reviewe

Bioinspired trailing-edge noise control

Strategies for trailing edge noise control have been inspired by the downy canopy that covers the surface of exposed flight feathers of many owl species. Previous wind tunnel measurements demonstrate that canopies of similar characteristics can reduce pressure fluctuations on the underlying surface by as much as 30dB, and significantly attenuate roughness noise generated by that surface. In the present work, surface treatments are designed to replicate the effects of the canopy in a form suitable for application to an airfoil. These treatments are installed directly upstream of the trailing edge to modify the boundary layer turbulence prior to acoustic scattering by the edge. Over 20 variants of these designs have been tested by performing aeroacoustic wind tunnel measurements on a tripped DU96-W180 airfoil at chord Reynolds numbers of up to 3 million. Compared to the unmodified airfoil, the treatments provided up to 10dB of broadband attenuation of trailing edge noise. The effectiveness of the treatment is not highly dependent on a particular geometry, but there appears to be strong potential for optimization. The surface treatments remain effective over an angle of attack range that extends over 9 degrees from zero lift. Aerodynamic impact of the treatment appears minimal.Office of Naval Research (Grant IDs: N00014-13-1-0244, N00014-14-1-0242, N62909-12-1-7116 (NICOP)), College of Engineering at Virginia Tech, AVEC Incorporate

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