National Aerospace Laboratories

About: National Aerospace Laboratories is a facility organization based out in Bengaluru, India. It is known for research contribution in the topics: Coating & Corrosion. The organization has 1838 authors who have published 2349 publications receiving 36888 citations.

Similarity in VITA-detected events in a nearly neutral atmospheric boundary layer

Abstract: Horizontal velocity data recorded in the atmospheric boundary layer at Boulder, Colorado and Cabauw, the Netherlands under near-neutral conditions, are analysed here by the Variable Interval Time Average (VITA) method. It is found that event frequency shows similarity with vita threshold and averaging time over a significant range of parameter values, enabling us to devise an objective method for (i) deciding on the choice of these hitherto arbitrarily selected parameters, and (ii) determining values for a well-defined ‘characteristic’ frequency. The conditional averages obtained from the events in the similarity region, when suitably normalized, are found to be extraordinarily similar to those obtained from low Reynolds number laboratory flows, indicating the existence of similar coherent events at widely varying Reynolds numbers. The characteristic frequency shows little variation with height beyond about 20 m, and a scaling that is poor with height and friction velocity but not inconsistent with outer variables.

Oxidation of MoSi2 and MoSi2-based materials

Abstract: Oxidation experiments, at 500°C, of MoSi2 and MoSi2-based compounds such as Mo(Al,Si)2 and MoSi2+1 wt% C compacts have been carried out. These compacts were prepared byin situ synthesis and a compaction method, starting from the elemental powders. For comparison, commercial MoSi2 and Mo(Al,Si)2 infiltrated into SiC preform were also studied under similar conditions. It was found that the synthesized high density MoSi2 and Mo(Al,Si)2 infiltrated into SiC preform did not show any oxidation even after 100 h of heating in air. The colour of the polished surfaces of commercial MoSi2, Mo(Al,Si)2 and MoSi2+1 wt% C had changed. The SEM of Mo(Al,Si)2 showed open blisters with rods of MoO3 in them whereas MoSi2+1 wt% C surface had MoO3 rods but no blisters and the oxidation was superficial with no penetration into the compact. It is suggested that in compounds, the presence of small amounts of impurities is not as detrimental to pesting as presence of defects like open pores or cracks. Hence, high density of the compact is essential for the prevention of complete disintegration of the compact.