Experimental study of the laminar-turbulent transition of a concave wall in a parallel flow

The instability of the laminar boundary layer flow along a concave wall was studied. Observations of these three-dimensional boundary layer phenomena were made using the hydrogen-bubble visualization technique. With the application of stereo-photogrammetric methods in the air-water system it was possible to investigate the flow processes qualitatively and quantitatively. In the case of a concave wall of sufficient curvature, a primary instability occurs first in the form of Goertler vortices with wave lengths depending upon the boundary layer thickness and the wall curvature. At the onset the amplification rate is in agreement with the linear theory. Later, during the non-linear amplification stage, periodic spanwise vorticity concentrations develop in the low velocity region between the longitudinal vortices. Then a meandering motion of the longitudinal vortex streets subsequently ensues, leading to turbulence

Large Eddy Simulation of Crossflow Vortices on an Infinite Swept Wing

Large Eddy Simulation (LES) was used to model the formation of a crossflow vortex packet in a 3-dimensional swept-wing boundary layer. The capability of LES to model the fine structures near the wall was investigated. An experiment by Chernoray et al.1 was used as a base case and the solution domain replicated their experimental setup with a C-16 airfoil at sweep 45◦ and Rec = 390, 000. Two sub-grid models were used for the investigation: the standard Smagorinsky and the Wall-Adapting Eddy Viscosity (WALE) model. The WALE model is more suitable as it allows the sub-grid scale viscosity to vanish in laminar regions and in the inner regions of the boundary layer. Stagnation streamlines at airfoil leading and trailing edges were taken from a full C-16 grid and used to define the lower boundary of a smaller solution domain which included the wing upper surface. This allowed, for a given computational resource, additional refinement in the area of interest. Results from the full grid matched well with that of the streamline defined domain. The laminar base flow for two LES grids of size 38 million & 161 million nodes was compared against the experiment and the results agreed well although the LES results slightly over-predicted the boundary layer thickness compared to the experiment. Stationary crossflow vortices were generated by strong continuous suction through a 1mm hole. The LES successfully captured the generation and growth of the crossflow vortex packet as well as the breakdown to turbulence on both grids. The fine grid performed better in modelling the growth of the vortices and the location of onset and growth of a dominant ‘z’ mode secondary instability. It was concluded that with suitable grid resolution LES is capable of successfully capturing the onset and development of crossflow vortices at a lower computational cost compared to DNS

A Novel Modular Antigen Delivery System for Immuno Targeting of Human 6-sulfo LacNAc-Positive Blood Dendritic Cells (SlanDCs)

Previously, we identified a major myeloid-derived proinflammatory subpopulation of human blood dendritic cells which we termed slanDCs (e.g. Schäkel et al. (2006) Immunity 24, 767-777). The slan epitope is an O-linked sugar modification (6-sulfo LacNAc, slan) of P-selectin glycoprotein ligand-1 (PSGL-1). As slanDCs can induce neoantigen-specific CD4+ T cells and tumor-reactive CD8+ cytotoxic T cells, they appear as promising targets for an in vivo delivery of antigens for vaccination. However, tools for delivery of antigens to slanDCs were not available until now. Moreover, it is unknown whether or not antigens delivered via the slan epitope can be taken up, properly processed and presented by slanDCs to T cells.Single chain fragment variables were prepared from presently available decavalent monoclonal anti-slan IgM antibodies but failed to bind to slanDCs. Therefore, a novel multivalent anti-slanDC scaffold was developed which consists of two components: (i) a single chain bispecific recombinant diabody (scBsDb) that is directed on the one hand to the slan epitope and on the other hand to a novel peptide epitope tag, and (ii) modular (antigen-containing) linker peptides that are flanked at both their termini with at least one peptide epitope tag. Delivery of a Tetanus Toxin-derived antigen to slanDCs via such a scBsDb/antigen scaffold allowed us to recall autologous Tetanus-specific memory T cells.In summary our data show that (i) the slan epitope can be used for delivery of antigens to this class of human-specific DCs, and (ii) antigens bound to the slan epitope can be taken up by slanDCs, processed and presented to T cells. Consequently, our novel modular scaffold system may be useful for the development of human vaccines

Grundlegende experimentelle Untersuchungen zur Stabilitaet dreidimensionaler Grenzschichtstroemungen

Zur physikalischen Beschreibung des Stabilitaetsproblems dreidimensionaler Grenzschichtstroemungen wurde ein grundlegendes Experiment durchgefuehrt. Durch ein entsprechend ausglegtes Modell wurde eine Grundstroemung simuliert, die derjenigen begleitender Stabilitaetsrechnungen sehr nahekommt. Vergleiche zwischen Experiment und Theorie ergaben, dass die lineare Stabilitaetstheorie die instabilsten Stoerungen in Frequenz, Wellenlaenge und Ausbreitungsrichtung richtig vorhersagt nicht aber deren raeumliche Entwicklung. Ursache dieser Diskrepanz ist die dreidimensionale Deformation der Grenzschicht durch die stationaeren Querstroemungswirbel. Sie fuehrt zu sekundaeren Instabilitaetsentwicklungen die nur durch nichtlineare Theorien beschrieben werden kann. Vergleiche der experimentellen Beobachtungen mit sekundaerer Stabilitaetstheorie und numerischer Simulation zeigen jedoch, dass eine Uebereinstimmung nur dann erzielt werden kann, wenn diese bezueglich der Anfangsbedingungen ausreichend an das Experiment angepasst werden koennen. Um dies sachgerecht durchzufuehren, muessen die Kenntnisse zum Receptivity Problem erweitert werden

Zur Untersuchung der Wechselwirkung angefachter Stoerungen in instabilen dreidimensionalen Grenzschichtstroemungen, ausgehend vom Experiment

Der bisher eingeschlagene Weg der Theorie, den laminar-turbulenten Uebergang in Grenzschichtstroemungen vorauszusagen, verfolgt die Betrachtung der Stabilitaet gegen kleine Stoerungen. Er fuehrt auf die Loesung partieller Differentialgleichungen. Bisherige Erfolge beider Behandlung des Problems beschraenken sich auf idealisierte Stroemungen im Bereich kleiner Stoerungen. Zur Beschreibung der spaeteren Stadien des Transitionsprozesses, die durch die Wechselwirkung zwischen den Stoerungen gekennzeichnet sind, muessen geeignete mathematische Modelle und Anfangsbedingungen eingefuehrt werden. Daraus ergeben sich eine Reihe von Fragen an das Experiment. Das Problem die gesuchten Antworten zu finden besteht nicht nur in der detaillierten messtechnischen Erfassung der Stroemungsvorgaenge, die zum laminar turbulenten Uebergang fuehren, sondern auch in der Simulation, denn eindeutige Aussagen werden im allgemeinen nur dann erhalten, wenn es gelingt, die Stroemung zu idealisieren, indem alles weggeschnitten wird, was die zu untersuchenden Phaenomene ueberdeckt und verschleiert. Der Beitrag zeigt die Vorgehensweise am Beispiel der dreidimensionalen Grenzschichtstroemung. Auf die zur Verfuegung stehenden messtechnischen Hilfsmittel wird kurz eingegangen. Das Zusammenwirken zwischen Experiment und Theorie bei der Behandlung des Stabilitaetsproblems wird an einigen experimentellen Ergebnissen aufgezeigt