Michael K. Ponton

Suppression of Pressure Loads in Cavity Flows

The need to suppress dynamic-pressure loads in open cavities represents an important problem in many aeronautical applications. Many studies have been conducted using passive and active control techniques at the leading and trailing edges of cavities that have shown some success at reducing the dynamic-pressure levels in simulated weapons bays. In this work a leading-edge fence along with a cylindrical rod, suspended in the approaching boundary layer parallel to the leading edge of the cavity, was examined. The overall pressure levels along with the spectral distribution of the surface pressure in the cavity have been shown to he altered in a favorable manner by both of these devices. Suppressing the dynamic-pressure levels in the cavity was also found to alter the correlation between sensors along the floor cavity. Although it was found that both leading-edge devices lift the shear layer away from the cavity, the manner in which it is lifted appears to play an important role in the level of surface-pressure suppression.

The effects of nozzle exit lip thickness on plume resonance

Abstract The acoustic emission and initial shear layer of a cold jet issuing from an underexpanded sonic nozzle were measured where the lip thickness of the nozzle exit varied from 0·015 to 0·625 nozzle diameters. Because the amplitude of plume resonance (or screech) has been shown to be very sensitive to the initial conditions of the plume, the effects of nozzle lip thickness on this sound component in particular was investigated (although it will be shown that other noise components are also affected). Nearfield acoustic measurements have revealed the amplitude and frequency of certain modes of screech to be dependent on nozzle lip thickness. Fluctuating pressure measurements made on the nozzle exit surface have acoustic amplitudes in excess of near field microphone measurements. The dominant mode of instability that exists in the shear layer is also related to this geometric parameter. Detailed shear layer measurements were performed in an attempt to quantify any associated changes in the momentum thickness caused by altering the nozzle exit.

On the near field pressure of a transonic axisymmetric jet

The near field fluctuating pressure of a cold subsonic jet (Mach 0.85) issuing from a contoured convergent nozzle was studied. Conventional time series analysis (i.e., correlation and spectral analysis) as well as a wavelet analysis were used to characterize time-dependent events thereby revealing features consistent with current understandings of jet noise while also uncovering modal features consistent with previous measurements of the jet flow field itself. Specifically, the measurements reveal extensive sources of multiple frequencies in the streamwise direction. The near field information of the present investigation should help to bridge our understanding of the relationship between the velocity field and the far field acoustic emission associated with subsonic jet noise as well as supply a data set that can be used for verification of computational efforts.

Mach Wave Emission from a High-Temperature Supersonic Jet

Solutions to the compressible Rayleigh equation reveal the existence of several families of supersonically convecting instability waves. These waves directly radiate noise to the jet far field. The predicted noise characteristics are compared to previously acquired experimental data for an axisymmetric Mach 2 fully pressure balanced jet (i.e., P e /P a =1.0) operating over a range of jet total temperatures from ambient to 1370 K. The results of this comparison show that the first-order supersonic instability wave and the Kelvin-Helmholtz first-, second-, and third-order modes have directional radiation characteristics that are in agreement with observed data. The assumption of equal initial amplitudes for all of the waves leads to the conclusion that the flapping mode of instability dominates the noise radiation process of supersonic jets. At a jet temperature of 1370 K, supersonic instability waves are predicted to dominate the noise radiated at high frequency at narrow angles to the jet axis

The Relationship between Resourcefulness and Persistence in Adult Autonomous Learning

The purpose of this study was to investigate the tenability of a proposed path-analytic model relating resourcefulness and persistence in the context of adult autonomous learning. Data collected from a nonprobability sample of 492 American adults using valid and reliable measures for resourcefulness and persistence were analyzed. Results suggest that although adults intend to persist in valued learning activities, they often do not choose to engage in such activities. Methods are discussed that may help educators foster autonomous learning tendencies in their students, thereby supporting their development as lifelong learners.

Acoustic Study of B Helical Mode for Choked Axisymmetric Nozzle

An acoustic near-field study was performed for an axisymmetric conical convergent nozzle operating at a pressure ratio corresponding to a fully expanded Mach number of 1.3. The acoustic measurements were performed in the nozzle exit plane using multiple sensors arranged around the periphery of the nozzle. The acquired data were simultaneously digitized. The acoustic spatial characteristics, describing the large-scale structure associated with the preferred shear layer instability mode, were determined for the dominant B screech mode. The nozzle was fitted with a lip thickening device to determine the effect of this geometric variable on the spatial structure of the jet. For the thin-lipped configuration, the flapping structure of the B screech mode was found to precess in a time- dependent manner about the jet axis. Increasing of the nozzle exit lip thickness altered the spatial characteristics of this mode from a flapping to a time-dependent flapping or spinning. Using the autobicoherence spectrum, frequency dependencies were found to exist in the acoustic data of the thick-lipped configuration. These dependencies may be related to the nonlinearity of the jet and/or the time-dependent nature of the B mode spatial structure.

Model and full scale study of twin supersonic plume resonance

This paper examines the effect of both nozzle geometry and scale on the twin supersonic plume resonance phenomenon associated with aircraft having engine nozzle center-to-center spacings less than two diameters. Exit plane near field dynamic pressures were measured for both single and dual nozzle operation in 4.7 percent model and full scale under static conditions. The frequencies associated with this phenomenon were predicted to within 5 percent for a full scale F-15 aircraft. Amplitude levels associated with this phenomenon were found to dominate the dynamic pressure fluctuations in the inter-nozzle region, and reach a level near the structural design limit for this aircraft. The model scale studies, which involved both axisymmetric and rectangular geometry, indicated that amplitude levels could be expected to be much higher in flight. High amplitude levels would likely occur in the overexpanded region for axisymmetric geometry, and in the underexpanded region for rectangular geometry.

External nozzle flap dynamic load measurements on F-15 S/MTD model

Dynamic pressure loads were obtained on 1/12 scale models of the F-15B production aircraft and the F-15 S/MTD experimental aircraft with rectangular nozzles and canards. Flight Mach numbers from 0.51 to 1.20 were studied for aircraft angles of attack from 0 to 10 deg and nozzle pressure ratios from 1.00 to 5.09. The results show that dynamic levels are lower in the internozzle region of twin rectangular nozzles than are levels found with twin axisymmetric nozzles. At other locations, the levels associated with both geometries are of the same order of magnitude when normalized by aircraft dynamic Q. At Mach number of 0.51, the loads spectrum is dominated by plume shock noise processes for both geometries. Above Mach 0.51, this mechanism is associated with either vortex bursting from a forward location or turbulent boundary layer separation over the nozzle external flaps. At supersonic speeds both geometries show significantly decreased load levels.

The effects of initial jet exit conditions on plume resonance

The acoustic emission and boundary layer of a cold jet issuing from an underexpanded sonic nozzle have been measured for lip thicknesses of the nozzle exit varying from 0.015-0.625 nozzle diameters. Near-field acoustic data demonstrate that the amplitude and frequency of certain modes of screech and the dominant mode of instability which exists in the shear layer are dependent on nozzle lip thickness. Changes in momentum thickness due to variations in the nozzle exit were also quantified.

Student Recruitment Strategies in Professional Physical Therapist Education Programs

Background and Purpose. Many economic factors have forced changes upon the profession of physical therapy, resulting in a decline of qualified applicants. The purpose of this study was to identify recruitment practices utilized by professional (entry‐level) physical therapist education programs. Subjects. The subjects were chairs of professional physical therapist education programs in the United States accredited by the Commission on Accreditation in Physical Therapy Education (CAPTE) for the class entering in 2001 (N=189). Methods. A survey instrument was designed and the 53 recruitment strategies were classified as: (1) Traditional, (2) Electronic, (3) Personnel, or (4) Program Change. Results. A total of 109 programs (58%) returned completed surveys. Traditional recruitment strategies were more often utilized; however, an institutional Web page was cited as the most frequently used recruitment strategy. Faculty was more often used in recruitment efforts than recruiters, current students, or alumni. Use of an early admissions program was the most frequently cited program change strategy followed by flexible admissions standards and a change to the Doctor of Physical Therapy (DPT) degree. Discussion and Conclusions. Of the 10 most frequently utilized strategies, 5 were Traditional, 3 were Electronic, and 2 were Personnel. While Traditional recruitment strategies continue to be utilized frequently, program administrators should scrutinize the types and varieties of Electronic, Personnel, and Program Change strategies. The wide spectrum of recruitment strategies available today, along with the potential interaction of external factors, make determination of the effectiveness of a particular strategy very difficult for college administrators. Further research is necessary to determine effectiveness of any particular strategy.