Joseph W. Hall

Cross-Spectral Analysis of the Pressure in a Mach 0.85 Turbulent Jet

Measurements of the near-field pressure in an unheatedMach 0.85 round jet were performed simultaneously with far-field acoustic pressure measurements at Syracuse University’s Skytop Anechoic Chamber facility to directly quantify the strength and frequency content of the propagating portion of the jet’s near-field pressure. The largest contributions were determined to be at the lower wave numbers commonly associated with hydrodynamic pressure fluctuations. This indicates that large low-frequency hydrodynamic fluctuations are obscuring a significant source of acoustic fluctuations in the jet and dictates that care must be taken when using single point measurements of nearfield pressure spectral decay to differentiate between acoustic (propagating) and hydrodynamic (nonpropagating) fluctuations.

Three-Dimensional Turbulent Wall Jets Issuing from Moderate-Aspect-Ratio Rectangular Channels

The development of three-dimensional turbulent wall jets emanating from long channels with outlet cross-sectional aspect ratios from 1 to 8 was investigated by measuring the mean and turbulent flowfields using hot-wire anemometry. The turbulent velocity profiles indicate that the core of the jet behaves like a two-dimensional wall jet before the interaction of the lateral shear layers. Contours of the full flowfield indicate that the turbulent mechanism that causes the lateral growth of the three-dimensional wall jets is located in the lateral shear layers near the wall. Increasing the outlet aspect ratio separates the lateral shear layers, causing a wider core region of two-dimensional wall-jet development that, in turn, delays the onset of far-field three-dimensional wall-jet development. The development of the different aspect-ratio wall jets collapsed onto a single curve when the streamwise coordinate was normalized by the square root of the channel cross-sectional area and the vertical and lateral jet half-widths were normalized by the height and width of the channel, respectively.

The Spanwise Dependence of Vortex-Shedding From Yawed Circular Cylinders

Simultaneous measurements of the fluctuating wall pressure along the cylinder span were used to examine the spanwise characteristics of the vortex-shedding for yaw angles varying from α = 60 deg to α = 90 deg. The Reynolds number based on the diameter of the cylinder was 56,100. The results indicate that yawing the cylinder to the mean flow direction causes the vortex-shedding in the wake to become more disorderly. This disorder is initiated at the upstream end of the cylinder and results in a rapid decrease in correlation length, from 3.3D for a = 90 deg to 1.1D for a = 60 deg. The commonly used independence principle was shown to predict the vortex-shedding frequency reasonably well along the entire cylinder span for a > 70 deg, but did not work as well for α = 60 deg.

Open-Loop Control of Disk Wakes

Open-loop control in the near wake of a disk in subsonic air and water flows was investigated using particle image velocimetry at a Reynolds number based on the disk diameters of 67,000 and 20,000, respectively. The air model had tabs along the edge that were driven radially by electromagnetic actuators, and the water model had slots connected to a piston and cylinder external to the flow. In both systems, the frequency of actuation could be varied, and in air, the spatial extent of the forcing could be controlled. Symmetrical actuation produced a reduction in the recirculation-region length of 10% for both air and water. Additionally, a helical excitation at the fundamental natural vortex-shedding frequency in air caused a reduction of 15 % and caused the wake to oscillate strongly in phase with the excitation. The most effective symmetrical actuation frequencies correspond to twice the natural frequency of the asymmetric mode in the unforced wake. The effects of actuation on the instantaneous and mean velocity fields are presented and discussed in detail.

Coherent Streamwise Vortex Structures in the Near-Field of the Three-Dimensional Wall Jet

A turbulent three-dimensional wall jet with an exit Reynolds number of 250,000 was investigated using stereoscopic particle image velocimetry (PIV) in the near-field region (x/D = 5). The proper orthogonal decomposition (POD) was applied to all three components of the velocity field to investigate the underlying coherent structures in the flow. A low-dimensional reconstruction of the turbulent velocity field using the first five POD modes showed the presence of coherent streamwise vortex structures formed in the outer shear-layers of the wall jet, not unlike those found in the near-field of free jets. The instantaneous streamwise vorticity reconstructed from the low-dimensional reconstructed velocity field indicates the presence of a persistent vortex pair close to the wall and on either side of the jet centerline that appear similar to the mean streamwise vorticity. These regions do not appear to be directly related to the positioning of the streamwise vortex structures in the outer shear-layer.

Vortex Shedding in a Tandem Circular Cylinder System With a Yawed Downstream Cylinder

This investigation examines the flow produced by a tandem cylinder system with the downstream cylinder yawed to the mean flow direction. The yaw angle was varied from α=90deg (two parallel tandem cylinders) to α=60deg; this has the effect of varying the local spacing ratio between the cylinders. Fluctuating pressure and hot-wire measurements were used to determine the vortex-shedding frequencies and flow regimes produced by this previously uninvestigated flow. The results showed that the frequency and magnitude of the vortex shedding varies along the cylinder span depending on the local spacing ratio between the cylinders. In all cases the vortex-shedding frequency observed on the front cylinder had the same shedding frequency as the rear cylinder. In general, at small local spacing ratios the cylinders behaved as a single large body with the shear layers separating from the upstream cylinder and attaching on the downstream cylinder, this caused a correspondingly large, low frequency wake. At other positions where the local span of the tandem cylinder system was larger, small-scale vortices began to form in the gap between the cylinders, which in turn increased the vortex-shedding frequency. At the largest spacings, classical vortex shedding persisted in the gap formed between the cylinders, and both cylinders shed vortices as separate bodies with shedding frequencies typical of single cylinders. At certain local spacing ratios two distinct vortex-shedding frequencies occurred indicating that there was some overlap in these flow regimes.

MR relaxometry of micro-bubbles in the vertical bubbly flow at a low magnetic field (0.2 T)

Measurements of the vertical bubbly flow were performed at a low magnetic field of 0.2T. The void fraction data were acquired. The susceptibility-induced changes in T2 relaxation time were analyzed using the previously introduced approaches by Sukstanskii et al. and Ziener et al., originally developed for the Magnetic Resonance analysis of randomly distributed and isolated spherical inclusions, and a simple model of a spherical particle, respectively. The CPMG signal decay due to the presence of spherical inclusions was approximated as linear vs. CPMG inter-echo times to extract the average inclusions size information. Two equations were derived for a simplified analysis of gas-liquid systems with basic T2 measurements, and without prior knowledge on the gas-liquid susceptibility or a need for the magnetic gradient setup. They can provide estimates for the void fraction and the average inclusion size, provided the CPMG inter-echo time requirements are met. For the control samples, there was a good agreement with the theory. For the bubbly flows, a good agreement was observed between the Magnetic Resonance and optics-based estimates for the slowest airflow rate. The deviation, however, increased for higher airflow rates. The introduced approach lends itself to the characterization of multi-phase systems such as cavitating media and well-separated bubbly flows.

Flow Around a Leading Edge Slat: Part II — Cove Flow Dynamics via Snapshot POD

The dynamics of flow within the cove of a leading edge slat has been investigated using Proper Orthogonal Decomposition by the method of snapshots. The shear-layer that originates from the slat cusp contains a large proportion of the turbulent kinetic energy within the flow. Reconstructions performed with the first 10 POD eigenmodes illustrates that several smaller regions of vorticity persist within the shear-layer. Depending on the Reynolds number and the angle of attack, the shear-layer can impinge upon the leading edge of the main wing element, roll up within the slat cove, or be ejected through the gap between the slat trailing edge and the leading edge of the wing. This unsteady impingement of both the shear-layer, and the smaller features within the shear-layer could result in a very strong acoustic dipole source.Copyright

Turbulence Measurements in the Corner Wall Jet

The corner wall jet is similar to the standard three-dimensional wall jet with the exception that one half of the surface has been rotated counter-clockwise by 90 degrees. The corner wall jet investigated here is formed using a long round pipe with a Reynolds number of 159,000. Contours of the mean and turbulent flow field were measured using hot-wire anemometry. The results indicate that the ratio of lateral to vertical growth in the corner wall jet is approximately half of that in a standard turbulent three-dimensional wall jet.Copyright

An Experimental Investigation of the Noise Sources Associated With Submerged High Flow Acoustic Measurements

A study of the noise sources on an submerged housing designed to facilitate acoustic measurements in high flow situations was investigated experimentally. A scale-mode of the housing was mounted in a low speed wind-tunnel and mean and unsteady surface pressure measurements were taken all over the surface of the housing. The measurements indicate that flow-separation occurs on several of the model edges and that a protruding instrument housing produces a strong turbulent wake. The associated pressure fluctuations produce large, low frequencies fluctuations that are broadband along with discrete tones that occur at higher frequencies.Copyright