Jason A. Bourgeois

Alternating half-loop shedding in the turbulent wake of a finite surface-mounted square cylinder with a thin boundary layer

The configuration and energetics of the large-scale vortex structure are presented for quasi-periodic shedding in the turbulent wake of a finite (h/d = 4) square-cross-section surface-mounted cylinder protruding from a thin boundary layer (δ/h = 0.18). The three-dimensional large-scale structure is educed from phase averaged x-y and x-z planar data measured with particle image velocimetry (PIV). Simultaneous measurements of the surface pressure difference on either side of the obstacle were used to phase-align the PIV planar measurements. The topology of the educed structures resembles alternating half-loops interconnecting close to the base plate. The time averaging of this unsteady structure gives rise to mean streamwise vortices akin to those presented in the literature for similar geometries. This topological analysis offers a contrasting interpretation of the mean streamwise vorticity, which has, otherwise, been presumed to originate from structures generated at the leading edge of the free-end. The ...

Quasi-Periodic Structure of Vortical Flows Produced in the Wake of Finite Bluff Bodies Partially Immersed in a Boundary Layer

The quasi-periodic vortex shedding structure in the wake of finite surface-mounted square- and circular-cross-section cylinders is investigated for several aspect ratios. Complex continuous wavelet transforms (CWT’s) are used to obtain a phase function φ(t) from hot-wire measurements. Mean relative phases and phase averaged particle image velocimetry (PIV) measurements indicate an upstream bending of the initially vertical shed vortex structures for all obstacles investigated. This upstream bending mechanism reorients vorticity streamwise and is described in terms of Biot-Savart induction that occurs at the junction of the tip and side shear layers. This mechanism of vorticity concentration/reorientation is inherently three-dimensional and interacts with the nominally two-dimensional mechanism of alternate vortex sheet roll-up from the opposing obstacle side faces. This mechanism typically acts higher along the height for square-as opposed to circular-cross section cylinders and plays a more dominant role for smaller aspect ratios.Copyright

Turbulent Wake of Surface-Mounted Finite Aspect Ratio Bluff Bodies: Effect of Aspect Ratio and Cross Section Shape

Dynamic shedding characteristics in the wake of vertical, surface mounted square and circular cylinders of aspect ratio (H/D) 0.5, 2, 4 and 8 are investigated using high frame-rate particle image velocimetry and thermal anemometry for Reynolds numbers, based on the obstacle width/diameter ranging between 6 × 103 and 105 . The wavelet transform was used to determine the instantaneous phase relationship for either pressure or velocity fluctuations on opposite sides of the obstacles. Except for the smallest aspect ratio, two shedding modes with different vortex configurations are observed in the formation region: i) Alternate formation of vortices accompanied by high-amplitude fluctuation; ii) Events of simultaneous formation of two vortices resulting in low-amplitude activity and varying phase relationship between opposing sides. These counter-rotating coexisting vortices are generally shed alternately. In the case of aspect ratio 0.5 prism, periodic activity is irregular and the shedding behavior is different from higher aspect ratio bodies.Copyright

Experimental and Numerical Investigation of an Aero-Engine Centrifugal Compressor

The complex flow field in turbomachinery poses numerous challenges for turbulence modeling. Herein, results of Laser Doppler Velocimetry (LDV) measurements of a full-scale aeroengine centrifugal compresser are used to validate typical design simulation results using a mixing plane and the k-e, SST, or RSM-SSG turbulence closure models. Generally good agreement between simulation results and LDV measurements was found. The largest discrepancies were found in the near-wall regions: the predicted boundary layers were thicker and the flow more diffusive than measured. Important differences between the simulation results using different closures are discussed.Copyright