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Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;139(6):061101-061101-13. doi:10.1115/1.4035876.

This paper considers the inherent unsteady behavior of the three-dimensional (3D) separation in the corner region of a subsonic linear compressor cascade equipped of 13 NACA 65-009 profile blades. Detailed experimental measurements were carried out at different sections in spanwise direction achieving, simultaneously, unsteady wall pressure signals on the surface of the blade and velocity fields by time-resolved particle image velocimetry (PIV) measurements. Two configurations of the cascade were investigated with an incidence of 4 deg and 7 deg, both at Re=3.8×105 and Ma = 0.12 at the inlet of the facility. The intermittent switch between two statistical preferred sizes of separation, large, and almost suppressed, is called bimodal behavior. The present PIV measurements provide, for the first time, time-resolved flow visualizations of the separation switch with an extended field of view covering the entire blade section. Random large structures of the incoming boundary layer are found to destabilize the separation boundary. The recirculation region, therefore, enlarges when these high vorticity perturbations blend with larger eddies situated in the aft part of the blade. Such a massive detached region persists until its main constituting vortex suddenly breaks down and the separation almost completely vanishes. The increase of the blockage during the separation growth phase appears to be responsible for this mechanism. Consequently, the proper orthogonal decomposition (POD) analysis is carried out to decompose the flow modes and to contribute to clarify the underlying cause-effect relations, which predominate the dynamics of the present flow scenario.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061102-061102-14. doi:10.1115/1.4035877.

A numerical-based (Reynolds-averaged Navier–Stokes (RANS)) investigation into the role of span and wing angle in determining the performance of an inverted wing in ground effect located forward of a wheel is described, using a generic simplified wheel and NACA 4412 geometry. The complex interactions between the wing and wheel flow structures are investigated to explain either increases or decreases for the downforce and drag produced by the wing and wheel when compared to the equivalent body in isolation. Geometries that allowed the strongest primary wing vortex to pass along the inner face of the wheel resulted in the most significant reductions in lift and drag for the wheel. As a result, the wing span and angle combination that would produce the most downforce, or least drag, in the presence of the wheel does not coincide with what would be assumed if the two bodies were considered only in isolation demonstrating the significance of optimizing these two bodies in unison.

Topics: Vortices , Wheels , Wings
Commentary by Dr. Valentin Fuster

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