During its intraerythrocytic development, the malaria parasite Plasmodium falciparum exposes variant surface antigens (VSAs) on infected erythrocytes to establish and maintain an infection. One family of small VSAs is the polymorphic STEVOR proteins, which are marked for export to the host cell surface through their PEXEL signal peptide. Interestingly, some STEVORs have also been reported to localize to the parasite plasma membrane and apical organelles, pointing toward a putative function in host cell egress or invasion. Using deep RNA sequencing analysis, we characterized P. falciparum stevor gene expression across the intraerythrocytic development cycle, including free merozoites, in detail and used the resulting stevor expression profiles for hierarchical clustering. We found that most stevor genes show biphasic expression oscillation, with maximum expression during trophozoite stages and a second peak in late schizonts. We selected four STEVOR variants, confirmed the expected export of these proteins to the host cell membrane, and tracked them to a secondary location, either to the parasite plasma membrane or the secretory organelles of merozoites in late schizont stages. We investigated the function of a particular STEVOR that showed rhoptry localization and demonstrated its role at the parasite-host interface during host cell invasion by specific antisera and targeted gene disruption. Experimentally determined membrane topology of this STEVOR revealed a single transmembrane domain exposing the semiconserved as well as variable protein regions to the cell surface. IMPORTANCE Malaria claims about half a million lives each year. Plasmodium falciparum, the causative agent of the most severe form of the disease, uses proteins that are translocated to the surface of infected erythrocytes for immune evasion. To circumvent the detection of these gene products by the immune system, the parasite evolved a complex strategy that includes gene duplications and elaborate sequence polymorphism. STEVORs are one family of these variant surface antigens and are encoded by about 40 genes. Using deep RNA sequencing of blood-stage parasites, including free merozoites, we first established stevor expression of the cultured isolate and compared it with published transcriptomes. We reveal a biphasic expression of most stevor genes and confirm this for individual STEVORs at the protein level. The membrane topology of a rhoptry-associated variant was experimentally elucidated and linked to host cell invasion, underlining the importance of this multifunctional protein family for parasite proliferation.
In recent years, overspray fogging has become a powerful means for power augmentation of industrial gas turbines (GT). Most of the studies concerning this topic focus on the problem from a thermodynamic point of view. Only a few studies, however, were undertaken to investigate the droplet behavior in the flow channel of a compressor. In this paper, results of experimental investigation of a water laden flow through a transonic compressor cascade are presented. A finely dispersed spray was used in the measurements (D10 < 10 μm). Results of the droplet behavior are shown in terms of shadowgraphy images and images of the blade surface film pattern. The angle of attack, the incoming velocity, and the water load were varied. The qualitative observations are related to laser Doppler and phase Doppler anemometer (LDA/PDA) data taken in the flow channel and at the outlet of the cascade. The data represent a base for numerical and mean line models of two-phase compressor flow.
The share of renewable sources in the energy sector is increasing steadily leading to higher requirements towards plant flexibility for conventional plants to compensate for the highly fluctuating power supply. To enhance the flexibility of gas turbine power plants overspray fogging systems can be used. Opposed to this positive effect it was shown that a two-phase flow increases losses and minor turning of the blade row as shown by Ober [1]. The reason for this change in aerodynamic performance, however, is not yet fully understood. As shown by Wurz [2] the velocity profile of the airflow changes in the presence of a water film which is related to an increased roughness due to the waviness of the water film. However, the experimental data base for the effect of a water film on compressor airfoils and its effect on aerodynamics is small and the possibility to capture all relevant effects in numerical codes requires an enormous effort. Therefore the objective of this study is to show the feasibility of modeling the influence of a water film as a region of increased surface roughness to estimate the aerodynamic effects on the compressor flow more easily. In the first part of the study the focus is directed to the water wall film pattern inspected experimentally at the transonic open loop wind tunnel at Helmut-Schmidt-University in Hamburg for two different airfoils. This study reveals the areas which are covered with a continuous water film. In the second part results of CFD calculations are presented. A reverse approach is used. Instead of simulating the water film in all details the focus is put on modeling the influence of a water film present. The regions previously being detected to be covered by water are defined as rough walls. The roughness height is varied to match experimentally measured losses published by Ober [1]. The results show the magnitude of influence of a wetted surface on the blade profile loss.
In recent years overspray fogging has become a powerful means for power augmentation of industrial gas turbines. Despite the positive thermodynamic effect on the cycle droplets entering the compressor increase the risk of water droplet erosion and deposition of water on the blades leading to an increase of required torque and profile loss. Due to this detailed information about the structure and the amount of water on the surface is key for compressor performance. Experiments were conducted with a droplet laden flow in a transonic compressor cascade focusing on the film formed by the deposited water. Two approaches were taken. In the first approach the film thickness on the blade was directly measured using white light interferometry. Due to significant distortion of the flow caused by the measurement system a transfer of the measured film thickness to the undisturbed case is not possible. Therefore, a film model is adapted to describe the film flow in terms of height averaged film parameters. In the second approach experiments were conducted in an undisturbed cascade setup and the water film pattern was measured using a non-intrusive quantitative image processing tool. Utilizing the measured flow pattern in combination with findings from literature the rivulet flow structure is resolved. From continuity of the water flow a film thickness is derived showing good agreement with the previously calculated results. Using both approaches a 3D reconstruction of the water film pattern is created giving first experimental results of the film forming on stationary compressor blades under overspray fogging conditions.
For stationary gas turbines, the injection of water sprays into the compressor inlet is used to rapidly increase the power output. To fully utilize this technique a comprehensive understanding of the underlying phenomena is needed. An in-house CFD program was developed at Institute of Aerospace Thermodynamics (ITLR) Stuttgart which was validated on the transonic wind tunnel at Helmut Schmidt University (HSU) Hamburg. The present paper focuses on the water film on the blades formed by water deposition of impinged droplets. Experimental results using a reflective shadowgraphy method reveal the wall film pattern on the blade without disturbing the flow. A numerical model is presented predicting the position of film breakup based on linear stability analysis. The film breakup position is analyzed for three different water loads showing an almost constant position for the onset of film breakup in the experiments which is well captured by the numerical model.
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