الفهرس 
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Abstract
The objective of the study is to investigate the capability of computational fluid dynamic CFD flow solvers, to quickly predict complicated flow phenomena through the highly three-dimensional flow is a diffusing S-duct.
Curved S-diffusers are widely used as an aircraft intake duct, they can be found in many of today`s comeerical and military aircraft. Commercial aircraft uses ducts to allow the thrust vector from rear mounted engines to be aligned with the axis of the aircraft. Military aircraft uses diffusing S-duct to allow a reduction in the overall aircraft length.Also to decrease the strength of the radar waves returned in the direction if their origin.
Area increase and center line curvature in diffusing s-duct may cause flow separation , which will cause flow distortion at engine face. This inlet distoration can produce a reduction in stability margine for the compressor or fan of the turbine engine. In addition, inlet distortion can result in high cycle fatigue, which may lead to catastrophic loss of aircraft, loss of aircraft operability, and increased maintenance costs.
The study is carried out using the CDF code FLUENT 6.2.16 and using GAMBIT 2.2.3 for drawing the geometry and mesh generation for all cases.
The objective of the current research work is to study the effect of side walls existence on the flow through a curved duct which will help in deep understanding the effect of curved duct, which will help in deep understanding the effect of curved stream lines on the flow behavior.
The effect of variation center line turning angle 15.22.5and 30 on the flow and performance characteristics of s-shaped circular diffusers also is studied. The effect of changing area ratio of diffusing s-duct 1.1.35.1.51 and 1.9) is also performed.
The results of the investigation indicated that a region of flow separation is detected before the end of diffusing s-duct first bend, center line turning angle hardly affects the static pressure recovery at the s-duct outline plane and the duct with 22.5/22.5 turning angle has the most uniform total pressure distribution at the s-duct outline plate, and duct 30/30 has the highest value of distortion. It is also observed that the duct with area ratio 1.51 has the best values for performance parameters, high pressure recovery, low total pressure losses and not much high value of distortion descriptor compared with the other ducts.