Department of Civil, Environmental, and Geo-Engineering

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Pressure Fields and Cavitation in Turbulent Shear Flows
(National Academy of Sciences, 1979) Arndt, Roger E.A.; George, William K.
Cavitation in turbulent shear flows is the result of a complex interaction between an unsteady pressure field and a distribution of free stream nuclei. Experimental evidence indicates that cavitation is incited by negative peaks in pressure that are as high as ten times the rms level. This paper reviews the current state of knowledge of turbulent pressure fields and presents new theory on spectra in Lagrangian frame of reference. Cavitation data are analyzed in terms of the available theory on the unsteady pressure field. It is postulated that one heretofore unconsidered factor in cavitation scaling is the highly intermittent pressure fluctuations which contribute to the high frequency end of the pressure spectrum. Because of limitations on the response time of cavitation nuclei, these pressure fluctuations play no role in the inception process in laboratory experiments. However, in large scale prototype flows, cavitation nuclei are relatively mroe responsive to a wider range of the pressure spectrum and this can lead to substantially higher values of the critical cavitatino index. Unfortunately, this issue is coulded by the fact that higher cavitation indices can be found in prototype flows because of gas content effects. Some cavitation noise data are also examined within the ocntext of available theory. The spectrum of cavitation noise in free shear flows has some similarity to the noise data found by Blake et al. (1977) with the exception that there appears to be a greater uncertainty in the scaling of the rate of cavitation events which leads to a substantial spread in available data.
Operational Evidence of Changing Travel Patterns
(Institute of Transportation Engineers, 1994) Levinson, David M; Kumar, Ajay
This paper utilizes a traffic counts database covering a ten year period (1976-1985) to identify travel trends for Montgomery County, a suburb of Washington D.C. Generally, travel behavior is analyzed using person based travel survey data. The use of traffic counts to understand travel behavior is a relatively new approach. Unlike household surveys, which are typically characterized by respondent and sample bias, and require special effort for their collection, traffic counts are routinely collected by Departments of Transportation and provide the best available measure of observed traffic volumes. The study provides fresh evidence to support some of the earlier findings: an increase in lateral commuting as a share of travel, changes in work and non-work trip proportions, and increase in peak spreading. An interesting result in this paper relates to a more pronounced directionality in radial as compared with lateral trips. The relative symmetry of traffic flows along lateral routes compared with radial routes results in better utilization of the suburban road network. Non-work trips emerge as the more elastic trips, shifting to off-peak hours with an increase in congestion.
Viscous effects in tip vortex cavitation and nucleation
(1994) Arndt, Roger E.A.; Maines, Brant H.
This paper is concerned with the physics of cavitation in trailing vortices. The research was aimed at investigating the interrelated effects of the vortex structure and bubble dynamics. The experimental phase utilizes a series of hydrofoils and includes lift and drag measurements, oil flow visualization of the boundary layer flow, and observation of both cavitation inception and disinence in strong and weak water. The complex bubble dynamics inherent in the inception process have been studied using an improved photographic technique. The bubble growth process is strongly dependent on the size and number of nuclei in the free stream and on the strength of the vortex. Numerical simulations indicate that the minimum pressure in the vortex is very close to the tip of the lifting surface, in agreement with the observation that the inception process also occurs very close to the tip under most conditions.
Specifying, Estimating and Validating a New Trip Generation Model: Case Study in Montgomery County, Maryland
(Transportation Research Board, 1994) Kumar, Ajay; Levinson, David M
This paper discusses the development of an afternoon peak period trip generation model for both work and non-work trips. Three data sources are used in model development, a Household Travel Survey, a Census-Update Survey, and a Trip Generation Study. Seven one-direction trip purposes are defined, specifically accounting for stops made on the return trip from work to home. Trips are classified by origin and destination activities rather than by production and attraction, so reframing the conventional schema of home-based and non-home-based trips. Prior to estimating the model, the Household Travel Survey was demographically calibrated against the Census-Update to minimize demographic bias. A model of home-end trip generation is estimated using the Household Travel Survey as a cross-classification of the demographic factors of age and household size in addition to dwelling type. Non-home-end generation uses employment by type and population. The model was validated by comparison with a site based Trip Generation Study, which revealed an under-reporting of the relatively short and less regular shopping trips. Normalization procedures are developed to ensure that all ends of a chained trip were properly accounted for.
The Rational Locator: Why Travel Times Have Remained Stable
(American Planning Association, 1994) Levinson, David M; Kumar, Ajay
This paper evaluates household travel surveys for the Washington metropolitan region conducted in 1968 and 1988, and shows that commuting times remain stable or decline over the twenty year period despite an increase in average travel distance, after controlling for trip purpose and mode of travel. The average automobile work-to-home time of 32.5 minutes in both 1968 and 1988 is, moreover, very consistent with a 1957 survey showing an average time of 33.5 minutes in metropolitan Washington. Average trip speeds increased by more than 20 percent, countering the effect of increased travel distance. This change was observed during a period of rapid suburban growth in the region. With the changing distributional composition of trip origins and destinations, overall travel times have remained relatively constant. The hypothesis that jobs and housing mutually co-locate to optimize travel times is lent further support by these data.
Integrating Feedback into the Transportation Planning Model
(Transportation Research Board, 1994) Levinson, David M
This research develops and applies a new structure for the transportation planning model that includes feedback between demand, assignment, and traffic control. New methods, combined with a renewed interest in transportation planning models prompted by the Clean Air Act of 1990 and the Intermodal Surface Transportation Efficiency Act of 1991, warrant a reconsideration of the traditional "four-step" transportation planning model. This paper presents an algorithm for feedback which results in consistent travel times as input to travel demand and output from route assignment. The model, including six stages of Trip Generation, Destination Choice, Mode Choice, Departure Time Choice, Route Assignment and Intersection Control is briefly outlined. This is followed by an application comparing a base year 1990 application with a forecast year of 2010. The 2010 forecast is solved both with and without feedback for comparison purposes. Incorporation of feedback gives significantly different results than the standard model.
Further Studies of Tip Vortex Cavitation
(1994-04) Arndt, Roger E.A.; Maines, Brant H.
Cavitation in vertical flows is an important problem. In particular, cavitation in the vortices trailing from lifting surfaces such as propellers and hydrofoils has been studied extensively in the past few years. Factors which affect tip vortex cavitation include water quality (which relates to the amount of tension that can be supported before cavitation occurs), the details of vortex roll-up close to the tip, and flow unsteadiness. An experimental and numerical investigation has been conducted to examine these effects. The experimental phase reported herein includes lift and drag measurements, oil flow visualization of the boundary layer flow on lifting surfaces, and observation of cavitation inception in strong and weak water. An improved photographic technique has been developed to study the complex bubble dynamics inherent in the inception process. Preliminary results indicate that the growth process is strongly dependent on the size and number of nuclei in the free stream. [missing text] vortices that occur at the tips of lifting surfaces and at the hubs of propellers and Francis turbines. Intermediate between turbulent eddies and tip and hub vortices are a variety of secondary flow phenomena such as the horseshoe vortices that form around bridge piers, chute blocks and struts, and the secondary vortices that are found in the clearance passages of turbomachinery. The focus of this paper is on tip vortex cavitation. Recent research on tip vortex cavitation is discussed. This problem was first studied in detail by McCormick (1954, 1962). Subsequently, little further attention was given to this topic until recently, when there has been a resurgence of interest in the problem. Our understanding of the physics has been considerably enhanced in the past decade (1983-1993). The emphasis of this paper is on the details of the inception process. Developed cavitation is also briefly described.
An Evolutionary Transportation Planning Model: Structure and Application
(Transportation Research Board, 1995) Levinson, David M
This paper describes an evolutionary transportation planning model wherein the demand in a given year depends on the demand of the previous year. The model redistributes a fraction of the work trips each year due to the relocation of a household or taking a new job, while changes in distribution due to growth (or decline) are considered. This hybrid-evolutionary model is compared with an equilibrium model, wherein supply and demand are solved simultaneously. The reasons for preferring the evolutionary method to the equilibrium approach are several: (a) the ability to more easily use observed data and thereby limit modeling to changes in behavior; (b) additional realism in the concept of the model; (c) the provision of a framework for extension to integration with land use models; and (d) the additional information available to policy makers.
A Multi-modal Trip Distribution Model
(Transportation Research Board, 1995) Levinson, David M; Kumar, Ajay
This paper presents a multimodal trip distribution function estimated and validated for the metropolitan Washington region. In addition, a methodology for measuring accessibility, which is used as a measure of effectiveness for networks, using the impedance curves in the distribution model is described. This methodology is applied at the strategic planning level to alternative HOV alignments to select alignments for further study and Right-of-Way preservation.
Activity, Travel, and the Allocation of Time
(American Planning Association, 1995) Levinson, David M
This paper analyzes 1968 and 1987-88 metropolitan Washington, DC household travel surveys to understand the daily allocation of time among different activities of individuals classified by work status and gender. The increase in female labor force participation rates has produced an increase in overall time spent at work per person. The increase in work trips and the simultaneous increase in nonwork trips has resulted in less time spent at home. People are substituting money for time spent at home, buying household services outside the home. The group of individuals who work at home is analyzed separately to obtain an understanding of this growing segment.
Temporal Variations on the Allocation of Time
(Transportation Research Board, 1995) Levinson, David M; Kumar, Ajay
This study investigates the allocation of time and trip-making across time-of-day, day-of-week, and month-of-year, as well as over the past forty years. Some interesting findings result. People are working much more, shopping somewhat more on weekends, and stay at home less today than forty years ago. Time spent in travel on each weekend day (Saturday or Sunday) exceeds that on any weekday, as it did forty years ago. Time spent shopping on a typical day in the busiest month (December) is more than double that in the least busy month (September). Monthly variations in daily time in travel exceed 10 percent. The time of day patterns of shop and other trips for workers and nonworkers are both rational: nonworkers peak in mid-day away from rush hour while workers peak just after work, indicating trip chaining.
Chained Trips in Montgomery County, Maryland
(Institute of Transportation Engineers, 1995-05) Kumar, Ajay; Levinson, David M
This paper analyzes the 1987-88 Metropolitan Washington Council of Governments home interview survey to understand how work trips are combined into trip chains and to relate trip chaining with demographic and travel characteristics. The focus is on the work trips during the morning and afternoon peak period and the stops made on the way for performing nonwork activities. The work trips during the afternoon period are much more likely to involve trip chaining as compared to the morning period. Women are more likely to link work trips with other activities as compared to men. Stops are closer to home than work.
Home or Office:Technology, Attitude, and At-Home Work
(1996) Levinson, David M
This paper examines the influence of technology and employer attitude on the decision to work at home. Using data from a suburban Washington household travel survey, it is found that both technology and a favorable employer attitude are positively associated with the number of hours and days in a two week period during which a respondent works at home. Other factors, including demographics, socio-economics, commuting time, and employer type were not statistically significant. Log-linear and translog forms were better fits than a simpler linear form.
The proper orthogonal decomposition of pressure fluctuations surrounding a turbulent jet
(Cambridge University Press, 1997) Arndt, Roger E.A.; Long, D.F.; Glauser, M.N
It is shown that the pressure signal measured at the outer edge of a jet mixing layer is entirely hydrodynamic in nature and provides a good measure of the large-scale structure of the turbulent ﬂow. Measurement of the pressure signal provides a unique opportunity to utilize proper orthogonal decomposition (POD) to deduce the streamwise structure. Since pressure is a scalar, a signiﬁcant reduction in the numerical and experimental complexity inherent in the analysis of velocity vector ﬁelds results. The POD streamwise eigenfunctions show that the structure associated with any frequency–azimuthal mode number combination displays the general characteristics of ampliﬁcation–saturation–decay of an instability wave, all within about three wavelengths. High-frequency components saturate early in x and low-frequency components saturate further downstream, indicative of the inhomogeneous character of the ﬂow in the streamwise direction. Application of the POD technique allows the phase velocity to be determined taking into account the inhomogeneity of the ﬂow in the streamwise direction. The phase velocity of each instability wave (POD eigenvector) is constant and equal to 0.58Uj , indicating that the jet structure is non-dispersive. Using the shot-noise decomposition, a characteristic event is constructed. This event is found to contain evidence of both pairings and triplings of vortex structures. The tripling results in a rapid increase in the ﬁrst asymmetric (m = 1) component. On average, pairing occurs once every four Uj/D while tripling occurs once every 13Uj/D.
Density and the Journey to Work
(Center for Business and Economic Research, University of Kentucky. Published by Blackwell Publishers, 1997) Levinson, David M; Kumar, Ajay
This paper evaluates the influence of residential density on commuting behavior across U.S. cities while controlling for available opportunities, the technology of transportation infrastructure, and individual socio-economic and demographic characteristics. The measures of metropolitan and local density are addressed separately. We suggest that metropolitan residential density serves principally as a surrogate for city size. We argue that markets react to high interaction costs found in large cities by raising density rather than density being a cause of those high costs. Local residential density measures relative location (accessibility) within the metropolitan region as well as indexing the level of congestion. We conduct regressions to predict commuting time, speed, and distance by mode of travel on a cross-section of individuals nationally and city by city. The results indicate that residential density in the area around the tripmaker's home is an important factor: the higher the density the lower the speed and the shorter the distance. However, density's effect on travel time is ambiguous, speed and distance are off-setting effects on time. The paper suggests a threshold density at which the decrease in distance is overtaken by the congestion effects, resulting in a residential density between 7,500 and 10,000 persons per square mile (neither the highest nor lowest) with the shortest duration auto commutes.
The Limits to Growth Management
(Pion, 1997) Levinson, David M
This paper reviews and critiques the growth management system in Montgomery County, Maryland with the intent of finding generalizable lessons. An overview of the twenty year old system is followed by an analysis of its consequences and implications. The system fails to provide effective price signals, rather relying on proactive command and control policies from the county government. Moreover the system fails to raise sufficient revenue for new infrastructure. The paper suggests that an alternative, reactive, approach, which links the threads of infrastructure financing and adequate public facilities by replacing quotas with a market based approach of cost-based prices, would be more equitable, efficient, and effective in implementing county goals.
Job and Housing Tenure and the Journey to Work
(Springer-Verlag, 1997) Levinson, David M
Tenure at jobs and houses, along with commuting patterns between home and work were studied for residents of metropolitan Washington. Two alternative potential outcomes were considered: (1) Because moving or switching jobs can be used as an opportunity to reduce commuting duration in an era of rising congestion, those who recently moved or changed jobs should have shorter than average commutes; and (2) Because most new residential construction is at the urban fringe, an area of longer commutes, those who recently moved to new homes should have longer commutes. Evaluation of the effect of commuting duration on job and housing tenure suggests that those who move on average maintain commute duration rather than having a major increase or decrease. This corroborates the idea that there are offsetting factors, where increases in commute lengths due to suburbanizing residences are counteracted by the correlated process of suburbanizing jobs.
A Windowed Transportation Planning Model
(Transportation Research Board, 1997) Levinson, David M; Huang, Yuanlin
This research develops and applies a transportation planning model that integrates regional and local area forecasting approaches. While regional models have the scope to model the interaction of demand and congestion, they lack the spatial detail of a local approach. Local approaches typically do not consider the feedback between new project traffic and existing levels of traffic. Using a window, which retains the regional trip distribution information and the consistency between travel demand and congestion, allows the use of a complete transportation network and block level traffic zones while retaining computational feasibility. By combining the two methods, a number of important policy issues can be addressed, including the implications of traffic calming, changes in flow due to alternative traffic operation schemes, the influence of micro-scale zoning changes on nearby intersections, the impact of TDM on traffic congestion, and the consequences of a suburban light rail line.
The Full Cost of High-Speed Rail: An Engineering Approach.
(Springer-Verlag, 1997) Levinson, David M; Mathieu, Jean-Michel; Kanafani, Adib; Gillen, David M
This paper examines the full costs, defined as the sum of private and social costs, of a high speed rail system proposed for a corridor connecting Los Angeles and San Francisco in California. The full costs include infrastructure, fleet capital and operating expenses, the time users spend on the system, and the social costs of externalities, such as noise, pollution, and accidents. Comparing these full costs to those of other competing modes contributes to the evaluation of the feasibility of high speed rail in the corridor. The paper concludes that high speed rail is significantly more costly than expanding existing air service, and marginally more expensive than auto travel. This suggests that high speed rail is better positioned to serve shorter distance markets where it competes with auto travel than longer distance markets where it substitutes for air.
Accessibility and the Journey to Work
(Pergamon, 1998) Levinson, David M
This study analyzes the effect of accessibility to jobs and houses at both the home and work ends of trips on commuting duration for respondents to a household travel survey in metropolitan Washington, DC. A model is constructed to estimate the effects of demographics and relative location on the journey to work. Analysis finds that residences in job-rich areas and workplaces in housing-rich areas are associated with shorter commutes. An implication of this study is that, by balancing accessibility, the suburbanization of jobs maintains stability in commuting durations despite rising congestion, increasing trip lengths, and increased work and non-work trip making.