An incompressible potential-flow vortex method has been constructed to analyze the flow field of ... more An incompressible potential-flow vortex method has been constructed to analyze the flow field of a ducted wind turbine following that outlined by Lewis (1991). Attention is paid to balancing the momentum change in the flow to the total longitudinal forces acting on the duct-turbine combination: the pressure force on the actuator disk plus the pressure forces acting on the duct, which typically includes a negative component
Throughout our careers we have worked to encourage women to pursue their interests and capabiliti... more Throughout our careers we have worked to encourage women to pursue their interests and capabilities in science, engineering, and medicine. And we are delighted with the continual increase in the percentage of women in these fields. We have also worked to ensure a welcoming and safe environment in academia for women students, faculty, and staff. We believe that universities have a special responsibility to provide a welcoming and effective environment for women students. We believe that this report focuses on the issues that must be addressed for our communities to take the next step. Preventing and effectively addressing sexual harassment of women in colleges and universities has remained a challenge for decades, but over that time a strong research base has been developed that reveals the true nature of sexual harassment and its impacts on women's careers-and also reveals what can be done to successfully address it. The Committee on Women in Science, Engineering, and Medicine developed the idea for this study on the Impacts of Sexual Harassment in Academia more than 2 years ago, and proposed that a special study committee be appointed to examine the research on sexual harassment to determine what could be done to prevent it in academic settings in science, engineering, and medicine. With this charge, our study committee of distinguished scientists, engineers, and physicians, and experts in sexual harassment research, legal studies, and psychology held a series of workshops and undertook a deep analysis of the literature to gather information for our study and to simultaneously help inform the broader community about the problem of sexual harassment. Over the course of the study, which was launched in late 2016, the topic rose in prominence in the national discourse, most significantly with the rise of the #MeToo movement,
Wave Dynamics and Radio Probing of the Ocean Surface, 1986
The interaction of wind-induced short (capillary) and long (gravity) surface waves of disparate s... more The interaction of wind-induced short (capillary) and long (gravity) surface waves of disparate scales is analyzed with the use of an expansion procedure accounting for the lowest-order effects of finite short-wave amplitude on the growth of a small-amplitude long wave. Interactions in both water and air are considered. In the water there is transfer of momentum from the short to the long waves arising from the modulation of the short-wave growth rate by the long wave; this may be interpreted as the effect of the shortwave surface-stress modulations in phase with the long-wave surface elevation and is consistent with earlier treatments employing energy considerations. The interaction in the air is treated with an in viscid model. This shows that the modulation of the short-wave Reynolds stresses by the long wave can give rise to a phase shift between long-wave pressure and surface slope which may significantly increase the rate of transfer of momentum from the wind to the long wave.
An experimental investigation of a semicircular wing flying very close to a solid boundary is per... more An experimental investigation of a semicircular wing flying very close to a solid boundary is performed to verify recent analytical results. Comparison is made between first order theory and data through plots of lift coefficient versus angle of attack for various clearances. Reasonable agreement is obtained for these cases within the limitations of the theory. Lift/drag ratio plots are also presented which show the potential of such a technique for support vehicles. A brief outline of the theoretical development is also included to give some insight into the type of analysis which was used.
The growth of small-scale-turbulence spots in plane Poiseuille flow (at Reynolds numbers below th... more The growth of small-scale-turbulence spots in plane Poiseuille flow (at Reynolds numbers below the the critical values predicted by linear theory) by three-dimensional secondary instabililty of leading-edge oblique waves is investigated theoretically on the basis of the experimental observations of Carlson et al. (1982). A model involving the cyclic interaction of moving regions of mass-flow blocking, waves arising from steady travelling disturbances, and the three-dimensional secondary instability of these waves is proposed, and the results of model computations are found to be in qualitative agreement with the experimental data. Sample spot visualizations, diagrams, and graphs are provided.
The rotational noise of model helicopter rotors in forward flight was studied in an anechoic wind... more The rotational noise of model helicopter rotors in forward flight was studied in an anechoic wind tunnel. The parameters under study were the rotor thrust (blade loading), blade number and advance ratio. The separate effects of each parameter were identified with the other parameters being held constant. The directivity of the noise was also measured. Twelve sets of data for rotational noise as a function of frequency were compared with the theory of Lowson and Ollerhead. In general, the agreement is reasonably good, except for the cases of (1) low and high disk loadings, (2) the four bladed rotor, and (3) low advance ratios. The theory always under-estimates the rotational noise at high harmonics.
The Journal of the Acoustical Society of America, 1974
Intense periodic pressure fluctuations can occur in the fluid in and above a cavity that is expos... more Intense periodic pressure fluctuations can occur in the fluid in and above a cavity that is exposed to high‐speed external airflow. Water table visualization provides an excellent tool to study the flow pattern in and near a cavity because all nonstationary phenomena are slowed by a factor of several hundred. Direct observation by eye or high‐speed movie camera reveals for an oscillating cavity (1) the periodic deflection of the shear layer above the cavity, (2) the cavity internal upstream propagation of an “acoustic” wave originating at the trailing edge bulkhead, (3) the downstream propagation of a pressure wave in the cavity originating at the leading edge bulkhead, and (4) the propagation of a “shock wave” pattern away from the cavity into the surrounding fluid. The effect of geometric and fluid dynamic changes on these non‐stationary phenomena is most easily visualized. In particular, oscillation‐suppression techniques can be readily implemented and studied, before elaborate wind‐tunnel and flight t...
A theoretical investigation of the instability of a vortex ring to short azimuthal bending waves ... more A theoretical investigation of the instability of a vortex ring to short azimuthal bending waves is presented. The theory considers only the stability of a thin vortex ring with a core of constant vorticity (constant /r) in an ideal fluid. Both the mean flow and the disturbance flow are found as an asymptotic solution in e = a /R, the ratio of core radius to ring radius. Only terms linear in wave amplitude are retained in the stability analysis. The solution to 0 (e 2 ) is presented, although the details of the stability analysis are carried through completely only for a special class of bending waves that are known to be unstable on a line filament in the presence of strain (Tsai & Widnall 1976) and have been identified in the simple model of Widnall, Bliss & Tsai (1974) as a likely mode of instability for the vortex ring: these occur at certain critical wavenumbers for which waves on a line filament of the same vorticity distribution would not rotate (w 0 = 0). The ring is found to be always unstable for at least the lowest two critical wavenumbers ( ka = 2.5 and 4.35). The amplification rate and wavenumber predicted by the theory are found to be in good agreement with available experimental results.
An incompressible potential-flow vortex method has been constructed to analyze the flow field of ... more An incompressible potential-flow vortex method has been constructed to analyze the flow field of a ducted wind turbine following that outlined by Lewis (1991). Attention is paid to balancing the momentum change in the flow to the total longitudinal forces acting on the duct-turbine combination: the pressure force on the actuator disk plus the pressure forces acting on the duct, which typically includes a negative component
Throughout our careers we have worked to encourage women to pursue their interests and capabiliti... more Throughout our careers we have worked to encourage women to pursue their interests and capabilities in science, engineering, and medicine. And we are delighted with the continual increase in the percentage of women in these fields. We have also worked to ensure a welcoming and safe environment in academia for women students, faculty, and staff. We believe that universities have a special responsibility to provide a welcoming and effective environment for women students. We believe that this report focuses on the issues that must be addressed for our communities to take the next step. Preventing and effectively addressing sexual harassment of women in colleges and universities has remained a challenge for decades, but over that time a strong research base has been developed that reveals the true nature of sexual harassment and its impacts on women's careers-and also reveals what can be done to successfully address it. The Committee on Women in Science, Engineering, and Medicine developed the idea for this study on the Impacts of Sexual Harassment in Academia more than 2 years ago, and proposed that a special study committee be appointed to examine the research on sexual harassment to determine what could be done to prevent it in academic settings in science, engineering, and medicine. With this charge, our study committee of distinguished scientists, engineers, and physicians, and experts in sexual harassment research, legal studies, and psychology held a series of workshops and undertook a deep analysis of the literature to gather information for our study and to simultaneously help inform the broader community about the problem of sexual harassment. Over the course of the study, which was launched in late 2016, the topic rose in prominence in the national discourse, most significantly with the rise of the #MeToo movement,
Wave Dynamics and Radio Probing of the Ocean Surface, 1986
The interaction of wind-induced short (capillary) and long (gravity) surface waves of disparate s... more The interaction of wind-induced short (capillary) and long (gravity) surface waves of disparate scales is analyzed with the use of an expansion procedure accounting for the lowest-order effects of finite short-wave amplitude on the growth of a small-amplitude long wave. Interactions in both water and air are considered. In the water there is transfer of momentum from the short to the long waves arising from the modulation of the short-wave growth rate by the long wave; this may be interpreted as the effect of the shortwave surface-stress modulations in phase with the long-wave surface elevation and is consistent with earlier treatments employing energy considerations. The interaction in the air is treated with an in viscid model. This shows that the modulation of the short-wave Reynolds stresses by the long wave can give rise to a phase shift between long-wave pressure and surface slope which may significantly increase the rate of transfer of momentum from the wind to the long wave.
An experimental investigation of a semicircular wing flying very close to a solid boundary is per... more An experimental investigation of a semicircular wing flying very close to a solid boundary is performed to verify recent analytical results. Comparison is made between first order theory and data through plots of lift coefficient versus angle of attack for various clearances. Reasonable agreement is obtained for these cases within the limitations of the theory. Lift/drag ratio plots are also presented which show the potential of such a technique for support vehicles. A brief outline of the theoretical development is also included to give some insight into the type of analysis which was used.
The growth of small-scale-turbulence spots in plane Poiseuille flow (at Reynolds numbers below th... more The growth of small-scale-turbulence spots in plane Poiseuille flow (at Reynolds numbers below the the critical values predicted by linear theory) by three-dimensional secondary instabililty of leading-edge oblique waves is investigated theoretically on the basis of the experimental observations of Carlson et al. (1982). A model involving the cyclic interaction of moving regions of mass-flow blocking, waves arising from steady travelling disturbances, and the three-dimensional secondary instability of these waves is proposed, and the results of model computations are found to be in qualitative agreement with the experimental data. Sample spot visualizations, diagrams, and graphs are provided.
The rotational noise of model helicopter rotors in forward flight was studied in an anechoic wind... more The rotational noise of model helicopter rotors in forward flight was studied in an anechoic wind tunnel. The parameters under study were the rotor thrust (blade loading), blade number and advance ratio. The separate effects of each parameter were identified with the other parameters being held constant. The directivity of the noise was also measured. Twelve sets of data for rotational noise as a function of frequency were compared with the theory of Lowson and Ollerhead. In general, the agreement is reasonably good, except for the cases of (1) low and high disk loadings, (2) the four bladed rotor, and (3) low advance ratios. The theory always under-estimates the rotational noise at high harmonics.
The Journal of the Acoustical Society of America, 1974
Intense periodic pressure fluctuations can occur in the fluid in and above a cavity that is expos... more Intense periodic pressure fluctuations can occur in the fluid in and above a cavity that is exposed to high‐speed external airflow. Water table visualization provides an excellent tool to study the flow pattern in and near a cavity because all nonstationary phenomena are slowed by a factor of several hundred. Direct observation by eye or high‐speed movie camera reveals for an oscillating cavity (1) the periodic deflection of the shear layer above the cavity, (2) the cavity internal upstream propagation of an “acoustic” wave originating at the trailing edge bulkhead, (3) the downstream propagation of a pressure wave in the cavity originating at the leading edge bulkhead, and (4) the propagation of a “shock wave” pattern away from the cavity into the surrounding fluid. The effect of geometric and fluid dynamic changes on these non‐stationary phenomena is most easily visualized. In particular, oscillation‐suppression techniques can be readily implemented and studied, before elaborate wind‐tunnel and flight t...
A theoretical investigation of the instability of a vortex ring to short azimuthal bending waves ... more A theoretical investigation of the instability of a vortex ring to short azimuthal bending waves is presented. The theory considers only the stability of a thin vortex ring with a core of constant vorticity (constant /r) in an ideal fluid. Both the mean flow and the disturbance flow are found as an asymptotic solution in e = a /R, the ratio of core radius to ring radius. Only terms linear in wave amplitude are retained in the stability analysis. The solution to 0 (e 2 ) is presented, although the details of the stability analysis are carried through completely only for a special class of bending waves that are known to be unstable on a line filament in the presence of strain (Tsai & Widnall 1976) and have been identified in the simple model of Widnall, Bliss & Tsai (1974) as a likely mode of instability for the vortex ring: these occur at certain critical wavenumbers for which waves on a line filament of the same vorticity distribution would not rotate (w 0 = 0). The ring is found to be always unstable for at least the lowest two critical wavenumbers ( ka = 2.5 and 4.35). The amplification rate and wavenumber predicted by the theory are found to be in good agreement with available experimental results.
Uploads
Papers by sheila widnall