4 results
Interactions between a shock and turbulent features in a Mach 2 compressible boundary layer
- R. Baidya, S. Scharnowski, M. Bross, C. J. Kähler
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- Journal:
- Journal of Fluid Mechanics / Volume 893 / 25 June 2020
- Published online by Cambridge University Press:
- 22 April 2020, A15
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Large field-of-view (FOV) particle image velocimetry experiments are conducted in the vicinity of a shock wave boundary layer interaction (SWBLI) at Mach 2. The current FOV covers up to 30 boundary layer thicknesses ($\widehat{\unicode[STIX]{x1D6FF}}_{I}$), comprising of upstream and downstream regions relative to the SWBLI, thereby allowing the turbulent boundary layer and shock to be simultaneously captured. The relationship between the boundary layer features and the instantaneous shock location is directly quantified, with the aim of better understanding the mechanisms responsible for oscillation of the reflected shock. The results show that the reflected shock location is clearly influenced by the instantaneous state of the incoming boundary layer. It is found that passage of low-/high-momentum very-large-scale turbulent features through the SWBLI region causes the reflected shock to move upstream/downstream of the mean location. Moreover, interaction with the shock is found to introduce additional velocity fluctuations across a range of spanwise length scales within the boundary layer. The spanwise scales smaller than one $\widehat{\unicode[STIX]{x1D6FF}}_{I}$ recover within one $\widehat{\unicode[STIX]{x1D6FF}}_{I}$ downstream of the SWBLI region. However, at larger spanwise wavelengths, two persistent modes at approximately one and six $\widehat{\unicode[STIX]{x1D6FF}}_{I}$ are observed, where they remain correlated for a longer streamwise extent ($\gg \widehat{\unicode[STIX]{x1D6FF}}_{I}$) than the other spanwise modes. The wall pressure measurements indicate that the low-frequency fluctuations arising from the oscillating shock foot are due to dampening of high-frequency contents beyond the critical frequency associated with the unstable global mode. Thus, the results suggest that low-frequency pressure oscillations are not necessarily an independent phenomenon from the turbulent features entering the SWBLI region and interacting with the shock. Instead, a large scale separation between their dominant time scales is due to the critical frequency of the unstable global mode occurring at frequencies that are orders of magnitudes slower than the dominant frequency of the very-large-scale features.
Structure of a streamwise-oriented vortex incident upon a wing
- C. McKenna, M. Bross, D. Rockwell
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- Journal:
- Journal of Fluid Mechanics / Volume 816 / 10 April 2017
- Published online by Cambridge University Press:
- 06 March 2017, pp. 306-330
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Impingement of a streamwise-oriented vortex upon a fin, tail, blade or wing represents a fundamental class of flow–structure interaction that extends across a range of applications. It can give rise to unsteady loading known as buffeting and to changes of the lift to drag ratio. These consequences are sensitive to parameters of the incident vortex as well as the location of vortex impingement on the downstream aerodynamic surface, generically designated as a wing. Particle image velocimetry is employed to determine patterns of velocity and vorticity on successive cross-flow planes along the vortex, which lead to volume representations and thereby characterization of the streamwise evolution of the vortex structure as it approaches the downstream wing. This evolution of the incident vortex is affected by the upstream influence of the downstream wing, and is highly dependent on the spanwise location of vortex impingement. Even at spanwise locations of impingement well outboard of the wing tip, a substantial influence on the structure of the incident vortex at locations significantly upstream of the leading edge of the wing was observed. For spanwise locations close to or intersecting the vortex core, the effects of upstream influence of the wing on the vortex are to: decrease the swirl ratio; increase the streamwise velocity deficit; decrease the streamwise vorticity; increase the azimuthal vorticity; increase the upwash; decrease the downwash; and increase the root-mean-square fluctuations of both streamwise velocity and vorticity. The interrelationship between these effects is addressed, including the rapid attenuation of axial vorticity in presence of an enhanced defect of axial velocity in the central region of the vortex. When the incident vortex is aligned with, or inboard of, the tip of the wing, the swirl ratio decreases to values associated with instability of the vortex, thereby giving rise to enhanced values of azimuthal vorticity relative to the streamwise (axial) vorticity, as well as relatively large root-mean-square values of streamwise velocity and vorticity.
Flow structure on a simultaneously pitching and rotating wing
- M. Bross, D. Rockwell
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- Journal:
- Journal of Fluid Mechanics / Volume 756 / 10 October 2014
- Published online by Cambridge University Press:
- 02 September 2014, pp. 354-383
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A technique of particle image velocimetry is employed to characterize the three-dimensional flow structure on a wing subjected to simultaneous pitch-up and rotational motions. Distinctive vortical structures arise, relative to the well-known patterns on a wing undergoing either pure pitch-up or pure rotation. The features associated with these simultaneous motions include: stabilization of the large-scale vortex generated at the leading edge, which, for pure pitch-up motion, rapidly departs from the leading-edge region; preservation of the coherent vortex system involving both the tip vortex and the leading-edge vortex (LEV), which is severely degraded for pure rotational motion; and rapid relaxation of the flow structure upon termination of the pitch-up component, whereby the relaxed flow converges to a similar state irrespective of the pitch rate. Three-dimensional surfaces of iso-$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{Q}$ and helicity are employed in conjunction with sectional representations of spanwise vorticity, velocity and vorticity flux to interpret the flow physics.
Assessment of blood neutrophil oxidative burst activity in dairy cows during the period of parturition
- F. Moreira da Silva, C. Burvenich, A. M. Massart Leën, L. Brossé
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- Journal:
- Animal Science / Volume 67 / Issue 3 / December 1998
- Published online by Cambridge University Press:
- 02 September 2010, pp. 421-426
- Print publication:
- December 1998
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Bovine blood neutrophil oxidative burst activity, the total leukocyte count and its differentiation, as well as progesterone, glucose and cortisol levels were studied on 10 cows from 1 week prior to calving until 8 weeks after parturition.
From day –7 before parturition a decrease in the metabolic oxidative burst activity of neutrophils was observed, reaching the lowest value 3 days after calving (0·28 (s.e. 0·06) nmol/l H2O2 per min 106 neutrophils) (P <0·01). Thereafter, values increased with a maximum of activity 26 days after parturition (0·57 (s.e. 0·05) nmol/H202 per min per 106 neutrophils). At the end of the experiment the neutrophils H2O2-productionwas comparable to values observed on day–7.
The maximum total leukocyte count was observed on the day of parturition (8, 108 (s.e. 370) leukocytes per /A blood) (P <0·01) returning to normal levels about 3 weeks after calving. Lymphocytes and neutrophils showed a trend analogous to that of the total leukocytes. Eosinophils and monocytes did not change during the entire period. With the rapid increase of circulating neutrophils observed on the calving day, a marked left shift was observed with a maximum number of immature neutrophils (48·6%) observed on day +3 (1477 (s.e. 242) immature neutrophils per fjl blood) (P <0·01), reaching normal values about 4 weeks after parturition.
A significant increase (F < 0·01) of plasma glucose was found on the day of parturition (91·4 (s.e. 6·8) mg per 100 ml plasma), while the levels of cortisol reached their maximum 3 days after calving (22·05 (s.e. 8·46) nglml plasma).
Progesterone levels averaged 4·07 (s.e. 1·21) jigll 7 days before calving, declining precipitously to 0·06 (s.e. 0·45) Hg/l 3 days after calving (F < 0·01), remaining very low for a period of about 3 weeks. Minimum and maximum anoestrus post-partum were 22 and 57 days, respectively.
Despite great hormonal variation observed in the peripartum period, no significant correlation was found between measured hormones and the oxidative burst activity of neutrophils.