Flow over a circular cylinder with its axis aligned with the free stream was investigated experimentally. Both upstream and downstream faces of the cylinder are sharply truncated. The fineness ratio (length to diameter ratio) was varied and the behavior of the leading-edge separating shear layer and its effect on the wake were studied in water using both flow visualization and PIV techniques. For the moderately large fineness ratio, the shear layer reattaches with subsequent boundary layer growth, whereas over a shorter cylinder the shear layer remains detached. This causes differences in the wake recirculation region and the immediate wake patterns. The shear layer structure was analyzed using the proper orthogonal decomposition (POD). The model in the water channel was sting-mounted and in some cases the effect of model support was detected in the wake measurements. To avoid such disturbance from the model support, an experiment was initiated in air using a magnetic model support and balance system. The drag variation with fineness ratio is presented and discussed in light of the flowfield measurements.
Two rakes of cross-wire probes were used to capture the two-point velocity statistics in a flow through an axisymmetric sudden expansion. The expansion ratio of the facility is 3, and has a constant geometry. Measurements were acquired at a Reynolds number equal to 54 000, based on centreline velocity and inlet pipe diameter. The two-point velocity correlations were obtained along a plane normal to the flow (r, θ), at eleven downstream step-height positions spanning from the recirculating region, through reattachment, and into the redeveloping region of the flow. Measurements were acquired by means of a flying-hot-wire technique to overcome rectification errors near the outer wall of the pipe where flow recirculations were greatest. A mixed application of proper orthogonal (in radius) and Fourier decomposition (in azimuth) was performed at each streamwise location to provide insight into the dynamics of the most energetic modes in all regions of the flow. This multi-point analysis reveals that the flow evolves from the Fourier-azimuthal mode m=2 (containing the largest amount of turbulent kinetic energy) in the recirculating region, to m=1 in the reattachment and redeveloping regions of the flow. An eigenvector reconstruction of the kernel, using the most energetic modes from the decomposition, displays the spatial dependence of the Fourier-azimuthal modes and the characteristics that govern the turbulent shear layer and recirculating regions of the flow.
An extension to classical stochastic estimation techniques is presented, following the formulations of Ewing and Citriniti (1999), whereby spectral based estimation coefficients are derived from the cross spectral relationship between unconditional and conditional events. This is essential where accurate modeling using conditional estimation techniques are considered. The necessity for this approach stems from instances where the conditional estimates are generated from unconditional sources that do not share the same grid subset, or possess different spectral behaviors than the conditional events. In order to filter out incoherent noise from coherent sources, the coherence spectra is employed, and the spectral estimation coefficients are only determined when a threshold value is achieved. A demonstration of the technique is performed using surveys of the dynamic pressure field surrounding a Mach 0.30 and 0.60 axisymmetric jet as the unconditional events, to estimate a combination of turbulent velocity and turbulent pressure signatures as the conditional events. The estimation of the turbulent velocity shows the persistence of compact counter-rotating eddies that grow with quasi-periodic spacing as they convect downstream. These events eventually extend radially past the jet axis where the potential core is known to collapse.
P. Jordan, Laurendeau, E., Guitton, A., Tinney, C. E., and Delville, J., “Interpreting the near pressure ﬁeld of unbounded jets,” 10th Confederation of European Aerospace Societies - Aeroacoustics Specialists’ Committee (CEAS-ASC) Workshop. Trinity College, Dublin, Ireland, 2006.