William W. Durgin

On the phenomenon of vortex street breakdown

A von Karman vortex street generated in the usual way was subjected to a deceleration, thereby changing the ratio of longitudinal to lateral spacing between the vortices. Distortion of the individual vortices followed which resulted in annihilation of concentrated vortex regions and creation of a stationary wake flow. This wake flow was itself dynamically unstable and developed into a new vortex street of a different frequency from the initial one. The breakdown of the initial vortex street is qualitatively explained by considering the convection of a concentrated vortex region due to the motion imposed by all the other vortices.

Lower Mode Response of Circular Cylinders in Cross-Flow

Transverse amplitude responses of a circular cylinder in cross-flow were determined as a function of reduced velocities for a variety of spring constants and damping coefficients. Maxima were found at reduced velocities of 5 and 16, and were of comparable amplitude. The first resonance, designated the “fundamental mode,” was due to normal vortex street excitation of the spring-mass system. The second resonance, designated the “lower mode,” occurred when the natural frequency was approximately one-third of the normal vortex shedding frequency. By assuming that the driving force was sinusoidal, it was possible to evaluate the lift coefficients at resonance. Lift coefficients for the lower mode behaved similarly with amplitude ratio but were an order of magnitude lower than lift coefficients for the fundamental mode. A mechanism was used to oscillate the cylinder transversely at prescribed frequencies and amplitudes. Dominant wake frequencies were determined from a frequency analysis of the hot-wire signal for a range of velocities and a fixed frequency of oscillation. It was found that synchronization of the shedding frequency to the forcing frequency did not take place for the lower mode. The familiar “lock-in” region, or frequency synchronization over finite bandwidth, was observed for the fundamental mode only. Since the frequency associated with normal vortex shedding was not suppressed when oscillations took place in the lower mode, it would seem that a low frequency vortex street had not replaced the normal one. It is likely, then, that the spring-mounted cylinder responded subharmonically to the exciting force resulting from vortex shedding. In this regard, however, it was curious that subharmonic response was not found at a frequency ratio of 0.5 as it was at 0.33. A conceptual model, which incorporated features of both the low frequency vortex street and subharmonic response, was developed which accounted for lower mode response at a frequency ratio of 0.33 as well as the lack of response at 0.5.

ULTRASOUND TECHNIQUE FOR PREDICTION OF STATISTICAL CHARACTERISTICS OF GRID-GENERATED TURBULENCE

Ultrasonic technique can be used to determine some characteristics of turbulent flows. In this work, measurements of the difference in propagation times of ultrasonic waves along two paths are combined with the Kolmogorov (2/3)-power law to study characteristics of grid-generated turbulence. The paper focuses on ultrasonic measurements of turbulent flow using the travel-time technique. The theoretical analysis is based on the well-known Kolmogorov theory and supported by experimental data. The present research is of interest from two points of view. First, it demonstrates a non-invasive method of investigation of turbulence, and second, it furthers understanding of the effect of turbulence on sound propagation.

Interpretation of venous occlusion plethysmography using a nonlinear model

Venous occlusion plethysmography (VOP) is a noninvasive technique widely employed for the detection of deep-vein thrombosis. Previous reports that VOP outflow curves are closely fit by a first-order exponential suggest that venous compliance and resistance are nearly constant. Typically, however, the venous compliance function has a sigmoid shape; in addition, the resistance in a collapsing tube must increase. This paradox was resolved by the surprising finding that for realistic nonlinear compliance and resistance these nonlinearities cancel, producing a quasilinear venous outflow that approximates a simple exponential.

Ultrasonic Method for Aircraft Wake Vortex Detection

This paper describes the experimental proof of concept study for an ultrasonic method of wake vortex detection. This new acoustic method uses travel time of acoustic pulses around a closed path to measure the net circulation within the acoustic path. In this application the closed path encloses the vorticity shed from one side of a Piper PA-28 aircraft wing. Magnitude and sign of circulation detected is comparable to the expected circulation generated by the Piper PA-28 test aircraft. This study demonstrates the validity of the acoustic method in detecting aircraft wake vortices. Further investigations and applications using this technique are discussed.

A Transient Electromagnetic Flowmeter and Calibration Facility

nomical means offabricating the transient flowmeter. A transient flow calibration facility consisting ofafree-falling water column was also designed and built. Results ofthe calibrations are presentedandshowthat theflowmeter can accurately measure transient flows up to the maximum observed acceleration of approximately 1

Ultrasonic method for aircraft wake vortex detection

This paper describes the experimental proof of concept study for an ultrasonic method of wake vortex detection. This new acoustic method uses travel time of acoustic pulses around a closed path to measure the net circulation within the acoustic path. In this application the closed path encloses the vorticity shed from one side of a Piper PA-28 aircraft wing. Magnitude and sign of circulation detected is comparable to the expected circulation generated by the Piper PA-28 test aircraft. This study demonstrates the validity of the acoustic method in detecting aircraft wake vortices. Further investigations and applications using this technique are discussed.

Global Projects Prepare WPI Students for the 21st Century

The WPI project based curriculum, which emphasizes discovery based learning are an alternative to the traditional information transfer process, has proved successful in delivering global engineering education. More than 25% of the learning process of the students is integrated into two formal projects, the Major Qualifying Project (MQP) which is designed as a capstone for professional technical competence and the Interactive Qualifying Project (IQP) which relates science and technology to societal concerns and student needs. Both the MQP and IQP may be completed on- or off-campus. Currently, over 50% of the graduating class will have completed one of the projects at an overseas location under WPI’s Global Perspectives Program. Each year, more than twenty faculty members will be advising and sharing a learning experience with the students at international locations spanning six continents.

EXPERIMENTAL INVESTIGATION OF STATISTICAL MOMENTS OF TRAVEL TIME IN GRID-GENERATED TURBULENCE

An experimental tech nique for investigation of the behavior of acoustic wave propagation through a turbulent medium is discussed. The present study utilizes the ultrasonic travel -time technique to diagnose a grid -generated turbulence. The statistics of the travel -time variat ions of ultrasonic wave propagation along a path are used to determine some metrics of the turbulence. Experimental data obtained using ultrasonic technique confirms numerical and theoretical predictions of nonlinear increase of the first -order travel time variance with propagation distance.

Ultrasonic flowmeters in determination of correlation functions of velocity and ultrasound wave fluctuations in grid-generated turbulence.

Worcester Polytechnic Institute, Worcester, MA, 01609 The paper is devoted to the experimental investigation of the statistical characteristics of the grid-generated turbulence produced in a wind tunnel. Ultrasonic time-of-flight method using dual transducers is utilized to develop a methodology for determination of the correlation functions of turbulent velocity and sound speed fluctuations. The ultrasonic flowmeter equation is considered in the form that includes effects of turbulent velocity and sound speed fluctuations. The influence of temperature inhomogeneities on ultrasonic wave propagation is investigated using a set of experiments with a heated grid. Utilization of high-speed digital data acquisition cards and LabView software for the experiments allows collecting a significant amount of statistical data.