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

Investigation of biological macromolecules using atomic force microscope-based techniques

The atomic force microscope (AFM) provides a powerful instrument for investigating and manipulating biological samples down to the subnanometer scale. In contrast to other microscopy methods, AFM does not require labeling, staining, nor fixation of samples and allows the specimen to be fully hydrated in buffer solution during the experiments. Moreover, AFM clearly compares in resolution to other techniques. In general, the AFM can be operated in an imaging or a force spectroscopy mode. In the present work, advantage was taken of this versatility to investigate single biomolecules and biomolecular assemblies. A novel approach to investigate the visco-elastic behavior of biomolecules under force was established, using dextran as an example. While a molecule tethered between a solid support and the cantilever tip was stretched at a constant velocity, the thermally driven oscillation of the cantilever was recorded. Analysis of the cantilever Brownian noise provided information about the visco-elastic properties of dextran that corresponded well to parameters obtained by alternative methods. However, the approach presented here was easier to implement and less time-consuming than previously used methods. A computer controlled force-clamp system was set up, circumventing the need for custom built analogue electronics. A commercial PicoForce AFM was extended by two computers which hosted data acquisition hardware. While the first computer recorded data, the second computer drove the AFM bypassing the manufacturer's microscope control software. To do so, a software-based proportional-integral-differential (PID) controller was implemented on the second computer. It allowed the force applied to a molecule to be held constant over time. After tuning of the PID controller, response times obtained using that force-clamp setup were comparable to those of the recently reported analogue systems. The performance of the setup was demonstrated by force-clamp unfolding of a pentameric Ig25 construct and the membrane protein NhaA. In the latter case, short-lived unfolding intermediates that were populated for less than 10 ms, could be revealed. Conventional single-molecule dynamic force spectroscopy was used to unfold the serine:threonine antiporter SteT from Bacillus subtilis, an integral membrane protein. Unfolding force patterns revealed the unfolding barriers stabilizing structural segments of SteT. Ligand binding did not induce new unfolding barriers suggesting that weak interactions with multiple structural segments were involved. In contrast, ligand binding caused changes in the energy landscape of all structural segments, thus turning the protein from a brittle, rigid into a more stable, structurally flexible conformation. Functionally, rigidity in the ligand-free state was thought to facilitate specific ligand binding, while flexibility and increased stability were required for conformational changes associated with substrate translocation. These results support the working model for transmembrane transport proteins that provide alternate access of the binding site to either face of the membrane. Finally, high-resolution imaging was exploited to visualize the extracellular surface of Cx26 gap junction hemichannels (connexons). AFM topographs reveal pH-dependent structural changes of the extracellular connexon surface in presence of HEPES, an aminosulfonate compound. At low pH (< 6.5), connexons showed a narrow and shallow channel entrance, which represented the closed pore. Increasing pH values resulted in a gradual opening of the pore, which was reflected by increasing channel entrance widths and depths. At pH > 7.6 the pore was fully opened and the pore diameter and depth did not increase further. Importantly, coinciding with pore gating a slight rotation of the subunits was observed. In the absence of aminosulfonate compounds, such as HEPES, acidification did not affect pore diameters and depths, retaining the open state. Thus, the intracellular concentration of taurine, a naturally abundant aminosulfonate compound, might be used to tune gap junction sensitivity at low pH

Process development for the microbial production of erythritol

In dieser Arbeit wird die fermentative Herstellung des Zuckeralkohols Erythrit mit einer osmophilen Hefe untersucht. Es wurden Informationen über das Herstellverfahren gesammelt, die fermentative Herstellung etabliert und ein industrieller Herstellweg vorgeschlagen. Mit Rücksicht auf die Situation der Zuckerindustrie wurden Substrate verwandt, die bei der Zuckerherstellung anfallen und auf ihre Eignung untersucht. Darauf hin wurde ein Verfahrensablauf vorgeschlagen und eine Ermittlung der Herstellkosten durchgeführt. Untersucht wurden Vor- und Nebenprodukte der Zuckerherstellung als Alternativsubstrate zu Saccharose. Keines dieser Substrate konnte eine ähnliche Leistung wie Saccharose erbringen. Der Einsatz von unterschiedlichen Cosubstraten aus der Landwirtschaft zeigte, dass ein spezieller Hefeextrakt (Merck) sowie Kartoffelrestfruchtwasser als Substrate in Frage kommen. Bei der Verwendung von Hefeextrakt (Merck) konnte eine Verbesserung des Prozesses durch den teilweisen Ersatz des Stickstoffes durch eine anorganische Stickstoffquelle erreicht werden. Um eine Aussage über eine Substrat-/Produkthemmung oder-limitierung treffen zu können wurden Untersuchungen hierzu durchgeführt. Aus den Versuchen wurde die maximal erreichbare Erythritkonzentration ermittelt. Eine Kultivierungsstrategie wurde erarbeitet und umgesetzt. Die Nebenprodukte Glycerin, Ethanol und Ribit wurden auf Hemmwirkung hin untersucht. Glycerin zeigt keinen hinderlichen Einfluss auf die Produktbildung. Ribit ist bereits in geringen Mengen hinderlich für die Produktbildung. Der anfängliche pH-Wert hat Einfluss auf die Produktbildung. Ein konstanter pH-Wert ist für die Produktbildung nicht nötig. Der Einfluss des Sauerstoffpartialdrucks (pO2) wurde untersucht. Zu viel pO2 führt zu reiner Biomassebildung, zu wenig pO2 zur Gärung. Es wurden ein Verfahrensschema und ein Berechnungsmodell geschaffen, mit denen die Herstellkosten unter Variation nahezu aller Parameter berechnet werden können.The fermentative production of the sugaralcohol erythritol by using an osmophilic yeast is examined. Information of the production process was gatherd, the fermentative production established and an industrial production method is proposed. With respect to the particular situation of the sugar industry, substrates are used that occur as by- or waste products in the production of sugar were examined. On this basis, ab process was suggested and the manufacturing costs were determined. Precursors and byproducts of the sugar production were compared to sucrose as substrates. None of these products bore similar results as sucrose. The usage of different cosubstrate of agricultural origin showed one yeast extract (Merck) and potatoe nitrogen concentrate to be suited cosubstrates. Using yeast extract (Merck), a process improvement was achied by substituting organic throug inorganic nitrogen. The inhibitory or limiting effect of different substrate-product-concentrations was examined. The maximum achieveable erythritol concentration was determined. A cultivation strategy was suggested and tested. Byproducts as glycerol, ethnaol and ribitol were examined with regards to their inhibitory effect. Glycerol showed no inhibitory effect. Ribitol is already a low concentration inhibitory to erythritol formation. The initial pH-value has influence on product formation, but is not mandatory to be constan for product formation. The influence of dissolved oxygen was examined. To much dissolved oxygen leads to purely biomass, to less to byproduct formationn. Productio costs were calculated on the basis of a process scheme and a dynamic calculation model that allowed the vatiation of allmost all parameters

Modulation of oligodendrocyte differentiation and maturation by combined biochemical and mechanical cues

Extracellular matrix (ECM) proteins play a key role during oligodendrogenesis. While fibronectin (FN) is involved in the maintenance and proliferation of oligodendrocyte progenitor cells (OPCs), merosin (MN) promotes differentiation into oligodendrocytes (OLs). Mechanical properties of the ECM also seem to affect OL differentiation, hence this study aimed to clarify the impact of combined biophysical and biochemical elements during oligodendrocyte differentiation and maturation using synthetic elastic polymeric ECM-like substrates. CG-4 cells presented OPC- or OL-like morphology in response to brain-compliant substrates functionalised with FN or MN, respectively. The expression of the differentiation and maturation markers myelin basic protein - MBP - and proteolipid protein - PLP - (respectively) by primary rat oligodendrocytes was enhanced in presence of MN, but only on brain-compliant conditions, considering the distribution (MBP) or amount (PLP) of the protein. It was also observed that maturation of OLs was attained earlier (by assessing PLP expression) by cells differentiated on MN-functionalised brain-compliant substrates than on standard culture conditions. Moreover, the combination of MN and substrate compliance enhanced the maturation and morphological complexity of OLs. Considering the distinct degrees of stiffness tested ranging within those of the central nervous system, our results indicate that 6.5 kPa is the most suitable rigidity for oligodendrocyte differentiation

Modulation of oligodendrocyte differentiation and maturation by combined biochemical and mechanical cues

Extracellular matrix (ECM) proteins play a key role during oligodendrogenesis. While fibronectin (FN) is involved in the maintenance and proliferation of oligodendrocyte progenitor cells (OPCs), merosin (MN) promotes differentiation into oligodendrocytes (OLs). Mechanical properties of the ECM also seem to affect OL differentiation, hence this study aimed to clarify the impact of combined biophysical and biochemical elements during oligodendrocyte differentiation and maturation using synthetic elastic polymeric ECM-like substrates. CG-4 cells presented OPC- or OL-like morphology in response to brain-compliant substrates functionalised with FN or MN, respectively. The expression of the differentiation and maturation markers myelin basic protein — MBP — and proteolipid protein — PLP — (respectively) by primary rat oligodendrocytes was enhanced in presence of MN, but only on brain-compliant conditions, considering the distribution (MBP) or amount (PLP) of the protein. It was also observed that maturation of OLs was attained earlier (by assessing PLP expression) by cells differentiated on MN-functionalised brain-compliant substrates than on standard culture conditions. Moreover, the combination of MN and substrate compliance enhanced the maturation and morphological complexity of OLs. Considering the distinct degrees of stiffness tested ranging within those of the central nervous system, our results indicate that 6.5 kPa is the most suitable rigidity for oligodendrocyte differentiation

Numerical Experiments on the Stability of Leading Edge Boundary Layer Flow

A numerical study is performed in order to gain insight to the stability of the infinite swept attachment line boundary layer. The basic flow is taken to be of the Hiemenz class with an added cross-flow giving rise to a constant thickness boundary layer along the attachment line. The full Navier-Stokes equations are solved using an initial value problem approach after two-dimensional perturbations of varying amplitude are introduced into the basic flow. A second-order-accurate finite difference scheme is used in the normal-to-the-wall direction, while a pseudospectral approach is employed in the other directions; temporally, an implicit Crank-Nicolson scheme is used. Extensive use of the efficient fast Fourier transform (FFT) algorithm has been made, resulting in substantial savings in computing cost. Results for the two-dimensional linear regime of perturbations are in very good agreement with past numerical and theoretical investigations, without the need for specific assumptions used by the latter, thus establishing the generality of our method

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