Browsing by Subject "shock waves"

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    Development of a Queue Warning System Utilizing ATM Infrastructure System Development and Field-Testing
    (Minnesota Department of Transportation, 2017-06) Hourdos, John; Liu, Zhejun; Dirks, Peter; Liu, Henry X.; Huang, Shihong; Sun, Weili; Xiao, Lin
    MnDOT has already deployed an extensive infrastructure for Active Traffic Management (ATM) on I-35W and I-94 with plans to expand on other segments of the Twin Cities freeway network. The ATM system includes intelligent lane control signals (ILCS) spaced every half mile over every lane to warn motorists of incidents or hazards on the roadway ahead. This project developed two separate systems that can identify lane-specific shockwave or queuing conditions on the freeway and use existing ILCS to warn motorists upstream for rear-end collision prevention. The two systems were field tested at two locations in the ATM equipped network that have a high frequency of rear- end collisions. These locations experience significantly different traffic-flow conditions, allowing for the development and testing of two different approaches to the same problem. The I-94 westbound segment in downtown Minneapolis is known for its high crash rate due to rapidly evolving shockwaves while the I-35W southbound segment north of the TH-62 interchange experiences longstanding queues extending into the freeway mainline. The Minnesota Traffic Observatory developed the I-94 Queue Warning system while the University of Michigan, under contract, developed the I-35W system. Prior to the I-94 installation, based on data collected in 2013, there were 11.9 crashes per VMT and 111.8 near crashes per VMT. In the first three months of the system’s deployment, event frequency reduced to 9.34 crashes per million vehicle miles of travel (MVMT) and 51.8 near crashes per MVMT, a 22% decrease in crashes and a 54% decrease in near crashes. The I-35W system did not undergo a similarly thorough evaluation, but for most of the lane segments involved, it showed that queue warning messages help reduce the speed variance near the queue locations and the speed difference between upstream and downstream locations. This also implicated a satisfactory level of compliance rate from travelers.
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    The microphysics of collisionless shocks.
    (2010-09) Wilson III, Lynn Bruce
    Shock waves in interplanetary (IP) space are of considerable interest due to their potential to damage ground based electronic systems and their ability to energize charged particles. The energization of charged particles at IP shocks has the obvious extrapolation to supernova shock waves, which are thought to be a candidate for generating the most energetic particles in the universe. The observations and theory behind collisionless shock wave evolution suggest that IP shocks should, for the most part, be stable structures which require energy dissipation. In a regular fluid, like our atmosphere, energy dissipation is accomplished through binary particle collisions transferring the loss of bulk flow kinetic energy to heat. Plasmas are mostly collisionless fluids, thus requiring other means by which to dissipate energy. The studies herein were performed using wave and particle data primarily from the Wind spacecraft to investigate the microphysics of IP shock energy dissipation mechanisms. Due to their lower Mach numbers, more simplified geometry, and quasiperpendicular nature, IP shock waves are an excellent laboratory to study wave-particle related dissipation mechanisms. Utilization of multiple data sets from multiple high time resolution instruments on board the Wind spacecraft, we have performed studies on the transition region microphysics of IP shocks. The work began with a statistical study of high frequency (&1 kHz) waveform capture data during 67 IP shocks with Mach numbers ranging from ∼1–6 found ion-acoustic wave amplitudes correlated with the fast mode Mach number and shock strength. The ion-acoustic waves (IAWs) were estimated to produce anomalous resistivities roughly seven orders of magnitude above classical estimates. Another study was an examination of low frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular IP shocks found the wave modes to be consistent with oblique precusor whistler waves at four of the events. The strongest event in that study had low frequency waves consistent with shocklets. The shocklets are seen simultaneously with diffuse ion distributions. Both the shocklets and precursor whistlers are seen simultaneously with anisotropic electron distributions unstable to whistler anisotropy and heat flux instabilities. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. Further investigation of the atypical IP shock found the strong heating to be associated with large amplitude (> 100 mV/m) solitary waves and electron Bernstein waves. The observed heating and waveforms are likely due to instabilities driven by the free energy provided by reflected ions at this supercritical IP shock, not the DC macroscopic fields. The particle heating observed for the event with shocklets was observed to be different from other events with similar shock parameters, suggesting a different dissipation mechanism. The work presented in this thesis has helped increase the understanding of the microphysics of IP shocks in addition to raising new questions regarding the energy dissipation mechanisms dominating in the ramp regions. The initial work focused on a statistical study of high frequency waveforms in IP shock ramps. The study results suggested a re-evaluation of the relative importance of anomalous resistivity due to wave-particle interactions. This assertion was further strengthened by the atypical particle heating observed in the 04/06/2000 event which we claimed clearly showed a dependence on the observed waveforms. Thus, the nearly ubiquitous observations of large amplitude IAWs in the ramp regions of IP shocks raises doubts about ignoring these high frequency fluctuations. In addition to these findings, we also observed a low frequency wave mode which is only supposed to exist upstream of quasi-parallel shocks with small radii of curvatures. All of these findings have increased our knowledge of collisionless shock energy dissipation, but they have raised many questions regarding our current theories. We have raised doubts regarding the use of the solar wind electron distributions as one particle population. We have showed evidence to support the energy dependence of wave-particle interactions between low frequency whistler waves and ≤1 keV electrons. Thus, we conclude that in the analysis of IP shocks the microphysics can no longer be disregarded.