Dynamic Evolution of a Transient Supersonic Trailing Jet Induced by a Strong Incident Shock Wave

The dynamic evolution of a highly underexpanded transient supersonic jet at the exit of a pulse detonation engine is investigated via high-resolution time-resolved schlieren and numerical simulations. Experimental evidence is provided for the presence of a second triple shock configuration along with a shocklet between the reflected shock and the slipstream, which has no analogue in a steady-state underexpanded jet. A pseudo-steady model is developed, which allows for the determination of the post-shock flow condition for a transient propagating oblique shock. This model is applied to the numerical simulations to reveal the mechanism leading to the formation of the second triple point. Accordingly, the formation of the triple point is initiated by the transient motion of the reflected shock, which is induced by the convection of the vortex ring. While the vortex ring embedded shock move essentially as a translating strong oblique shock, the reflected shock is rotating towards its steady state position. This results in a pressure discontinuity that must be resolved by the formation of a shocklet

Modulation of downstream-propagating waves in aeroacoustic resonance

International audienceThe coherent component of turbulence in shock-containing flows undergoing aeroacous-tic resonance often displays a periodic spatial modulation. This modulation is generally thought to be driven either by the hydrodynamic/acoustic standing wave, or by the shock structures within the jet. In this work, we examine this spatial modulation and seek to determine its cause. Specifically, we consider whether the growth of the Kelvin-Helmholtz wavepackets associated with the resonance cycle is modulated by either of these mechanisms. A combined experimental and theoretical analysis is undertaken. Time-independent velocity snapshots of screeching jets are used to produce a reduced order model for the resonance via a Proper Orthogonal Decomposition. Streamwise Fourier filtering is then applied to isolate the negative and postive wavenumber componetns, which for this flow correspond to upstream and downstream-propagating waves. A global stability analysis on an experimentally derived base flow is conducted, producing remarkably similar results to those obtained via experiment. In both the global stability analysis and the experimental decomposition, three distinct structures are observed in the spatial wavenumber spectrum. One of these is associated with the downstream-propagating KH mode. One is associated with the upstream component of screech as previously identified. The third component has positive phase velocity, but a radial structure quite different to the other two waves. We provide evidence that this downstream-propagating wave is the result of an interaction between the KH wavepackets and the shocks embedded in the jet, much the same as the upstream-propagating acoustic wave, and has a structure very similar to duct-like modes previously identified in round jets. A weakly-non-parallel local analysis supplements the global analysis, and suggests that the growth of the KH wave is essentially un-modulated by the shocks, at least at the frequencies associated with screech

Dynamics of round jet impingement

International audienceThe impingement of a round jet issued from a convergent nozzle is known to produce intense tonal noise. These tones are generated by a feedback process involving a Kelvin-Helmholtz instability wave and an upstream propagating wave. The frequency structure of these tones is experimentally explored by varying the jet Mach number from 0.7 to 1.5 and we observe evidence that neutral-upstream modes of the jet are active in the feedback process. Tone frequency predictions obtained using the intrinsic jet modes are compared to the standard model involving free-stream acoustic waves: the neutral waves model provides a convincing agreement with the experimental data

PD-1 Inhibitory Receptor Downregulates Asparaginyl Endopeptidase and Maintains Foxp3 Transcription Factor Stability in Induced Regulatory T Cells

CD4+ T cell differentiation into multiple T helper (Th) cell lineages is critical for optimal adaptive immune responses. This report identifies an intrinsic mechanism by which programmed death-1 receptor (PD-1) signaling imparted regulatory phenotype to Foxp3+ Th1 cells (denoted as Tbet+iTregPDL1 cells) and inducible regulatory T (iTreg) cells. Tbet+iTregPDL1 cells prevented inflammation in murine models of experimental colitis and experimental graft versus host disease (GvHD). Programmed death ligand-1 (PDL-1) binding to PD-1 imparted regulatory function to Tbet+iTregPDL1 cells and iTreg cells by specifically downregulating endo-lysosomal protease asparaginyl endopeptidase (AEP). AEP regulated Foxp3 stability and blocking AEP imparted regulatory function in Tbet+iTreg cells. Also, Aep−/− iTreg cells significantly inhibited GvHD and maintained Foxp3 expression. PD-1-mediated Foxp3 maintenance in Tbet+ Th1 cells occurred both in tumor infiltrating lymphocytes (TILs) and during chronic viral infection. Collectively, this report has identified an intrinsic function for PD-1 in maintaining Foxp3 through proteolytic pathway.Bio-organic Synthesi

Impacts of habitat heterogeneity on the provision of multiple ecosystem services in a temperate floodplain

The relationships between habitat heterogeneity and the provision of multiple ecosystem services are not well understood. This study investigates the impacts of heterogeneity in surface floodwater inundation on the productive efficiency of ecosystem service provision, and the degree to which the relative provision of these ecosystem services is evenly balanced. We analyse indicators of five services. Field data from 100 floodplain quadrats were first analysed to investigate relationships between ecosystem service indicators and floodplain hydrology. Floodplain mosaics of varying hydrological heterogeneity were then simulated using the empirical data. Simulated floodplains with higher hydrological heterogeneity were generally less efficient in providing the target indicators, because they were adapted to the particular hydrological ranges which best provided the target services. Simulated floodplains that were more heterogeneous generally provided more even levels of the target indicators by segregating provision into different habitat types. Heterogeneity in floodplain hydrology may help to balance provision of multiple ecosystem services. However, management of hydrological heterogeneity to achieve this requires a detailed understanding of the relationships between each service and habitat conditions

Models of classroom assessment for course-based research experiences

Course-based research pedagogy involves positioning students as contributors to authentic research projects as part of an engaging educational experience that promotes their learning and persistence in science. To develop a model for assessing and grading students engaged in this type of learning experience, the assessment aims and practices of a community of experienced course-based research instructors were collected and analyzed. This approach defines four aims of course-based research assessment—(1) Assessing Laboratory Work and Scientific Thinking; (2) Evaluating Mastery of Concepts, Quantitative Thinking and Skills; (3) Appraising Forms of Scientific Communication; and (4) Metacognition of Learning—along with a set of practices for each aim. These aims and practices of assessment were then integrated with previously developed models of course-based research instruction to reveal an assessment program in which instructors provide extensive feedback to support productive student engagement in research while grading those aspects of research that are necessary for the student to succeed. Assessment conducted in this way delicately balances the need to facilitate students’ ongoing research with the requirement of a final grade without undercutting the important aims of a CRE education

Azimuthal decomposition of the radiated noise from supersonic shock-containing jets

International audienceAcoustic measurements of unheated supersonic underexpanded jets with ideally-expanded Mach numbers of 1.14, 1.38 and 1.50 are presented. Of the three components of supersonic jet noise, the focus is on the broadband shock-associated noise (BBSAN) component. Motivated by the modelling of BBSAN using the wavepacket framework, a traversable microphone ring is used to decompose the acoustic pressure into azimuthal Fourier modes. Unlike noise radiated downstream, BBSAN is dominated by azimuthal modes 1 through 3 which are approximately 3-4dB/St stronger than the axisymmetric component. Crucially, the relative contribution of successive modes to BBSAN is sensitive to observer angle and jet operating condition. Four azimuthal modes are necessary to reconstruct the total BBSAN signal to within 1dB/St accuracy for the conditions presented here. The analysis suggests, however, that the number of modes required to maintain this accuracy increases as the peak frequency shifts upward. The results demonstrate the need to carefully consider the azimuthal content of BBSAN when comparing acoustic measurements to predictions made by jet noise models built on instability theory