Jet noise suppression by porous plug nozzles

Jet noise suppression data presented earlier by Maestrello for porous plug nozzles were supplemented by the testing of a family of nozzles having an equivalent throat diameter of 11.77 cm. Two circular reference nozzles and eight plug nozzles having radius ratios of either 0.53 or 0.80 were tested at total pressure ratios of 1.60 to 4.00. Data were taken both with and without a forward motion or coannular flow jet, and some tests were made with a heated jet. Jet thrust was measured. The data were analyzed to show the effects of suppressor geometry on nozzle propulsive efficiency and jet noise. Aerodynamic testing of the nozzles was carried out in order to study the physical features that lead to the noise suppression. The aerodynamic flow phenomena were examined by the use of high speed shadowgraph cinematography, still shadowgraphs, extensive static pressure probe measurements, and two component laser Doppler velocimeter studies. The different measurement techniques correlated well with each other and demonstrated that the porous plug changes the shock cell structure of a standard nozzle into a series of smaller, periodic cell structures without strong shock waves. These structures become smaller in dimension and have reduced pressure variations as either the plug diameter or the porosity is increased, changes that also reduce the jet noise and decrease thrust efficiency

The Polls in 2017

Will Jennings examines the opinion polls in the 2017 election, showing how they tracked movements in voting intentions during the campaign, but also how most of them under-estimated the Labour vote on polling day, suggesting a comfortable Conservative victory when the election ended in producing no overall majority. He also notes YouGov’s successful innovation in using a multilevel regression post-stratification model to make detailed constituency projections of the result, which did predict a hung parliament. He further examines both the campaign trends and the performance of the polls into their historical perspective, finding that it is not true that the polls are getting “worse” at predicting election results, as well as discussing the impact of the pollsters’ turnout corrections on their accuracy in 2017