Browsing by Subject "Shock waves"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Investigation of the Impact of the I-94 ATM System on the Safety of the I-94 Commons High Crash Area
    (Minnesota Department of Transportation, 2014-05) Hourdos, John; Zitzow, Stephen
    Active Traffic Management (ATM) strategies are being deployed in major cities worldwide to deal with pervasive system congestion and safety concerns. While such strategies include a diverse array of components, in the Twin Cities metropolitan area the deployment of the Intelligent Lane Control Signs (ILCS) allowed for the implementation of Variable Speed Limits (VSL). The VSL system in the Twin Cities aims to detect congestion and preemptively warn upstream drivers to reduce speed. By reducing the severe change in speed between upstream and downstream traffic, safety and operational benefits are sought. This report presents an investigation of the effect the I-94 VSL system has on the safety of the high frequency crash area located on the westbound lanes of the freeway through downtown Minneapolis (I-94/I-35W commons). This report describes several methodologies that were used to examine the impact of the VSL system within the I-94/I-35W commons high crash area. Numerous data sources were utilized, including video records of crash and near crash events, loop detector traffic measurements, machine vision sensor data, and actuations from the VSL system. A before-after approach was taken to examine the incident rates for crashes and near crashes using visually identified events within video data. Utilizing the unique capabilities of the Minnesota Traffic Observatory’s I-94 Freeway Lab, high resolution traffic measurements, collected by machine vision sensors at the bottleneck location, were used within a new cross-correlation based analysis methodology to measure and visualize shockwave activity before and after the implementation of the VSL system.
  • Thumbnail Image
    Item
    Modal and nonmodal stability analysis of shock-wave/boundary-layer interactions
    (2019-04) Hildebrand, Nathaniel
    This dissertation is about the modal and nonmodal stability of an oblique shock wave impinging on a Mach 5.92 laminar boundary layer at a transitional Reynolds number. The adverse pressure gradient of the oblique shock wave causes the laminar boundary layer to separate from the wall, resulting in the formation of a recirculation bubble. For sufficiently large oblique shock angles, the recirculation bubble is unstable to three-dimensional perturbations, and the flow bifurcates from its original laminar state. We use direct numerical simulation (DNS) and global stability analysis (GSA) to show that this first occurs at a critical oblique shock angle of 12.9 degrees. The least-stable global mode is stationary at bifurcation, and it takes place at a nondimensional spanwise wavenumber of 0.25, in good agreement with the DNS results. Examination of the critical global mode reveals that it originates from an interaction between small spanwise corrugations at the incident shock base, streamwise vortices inside the recirculation bubble, and spanwise modulation of the bubble strength. Furthermore, the global mode drives the formation of long streamwise streaks downstream of the bubble. This stationary three-dimensional instability is similar to other mechanisms observed in laminar recirculation bubbles. We show that centrifugal instability plays no role in the self-sustaining mechanism of the stationary global mode. Further, we employ an adjoint solver to corroborate our physical interpretations by showing that the critical global mode is most sensitive to base flow modifications that are entirely contained inside the recirculation bubble. We also perform a parametric study to determine the effect of freestream Mach number on shock-wave/boundary-layer interaction (SWBLI) instability. Along with DNS and GSA, we investigate the physical mechanisms responsible for transient growth in an SWBLI using a power iteration method. This approach lets perturbations propagate upstream and downstream, which allows us to capture the complex physics associated with the recirculation bubble and understand how it amplifies fluctuations. For a Mach 5.92 boundary layer with no oblique shock wave, we demonstrate that the transient response arises from the inviscid Orr mechanism, the Landahl lift-up effect, and first-mode instability. The optimal transient growth for this spatially-developing boundary layer with a nondimensional streamwise domain length of 235 is G=1.69x10^3 and occurs at a spanwise wavenumber of 0.6. This corresponds to an amplification of 4.11x10^1, which is similar to that seen in a variety of parallel boundary layer flows. We compute the optimal transient growth of an SWBLI at the exact same conditions as the spatially-developing boundary layer. The presence of an oblique shock wave changes the optimal transient response such that G=1.36x10^7 at a spanwise wavenumber of 0.6. Hence, the transient growth in an SWBLI is four orders of magnitude larger than the transient growth in a spatially-developing boundary layer. The nondimensional spanwise wavenumber of the optimal transient response also increases from 0.6 to 2.6. Moreover, the corresponding optimal spanwise wavelength for the SWBLI is on the order of twice the boundary-layer thickness, agreeing with SWBLI experiments. These changes are attributed to the sudden change in the streamline curvature in the upstream region of the flow field. Furthermore, the optimal initial condition for the SWBLI consists of elongated streaks in the upstream boundary layer. As this initial condition evolves to its final state, we observe the formation of streamwise streaks in the recirculation bubble (that are further amplified in the downstream boundary layer) along with a large perturbation that comes off of the bubble apex and convects downstream. Our results demonstrate large transient growth in a Mach 5.92 SWBLI and suggest that inevitable imperfections in a hypersonic wind tunnel would play an important role in the early stages of transition to turbulence.
  • Thumbnail Image
    Item
    A Tool for Designing MnPASS Access Spacing
    (Minnesota Department of Transportation, 2018-03) Zitzow, Stephen; Parikh, Gordon; Hourdos, John
    Dynamically priced High Occupancy Toll (HOT) lanes have been recently added to the traffic operations arsenal in an attempt to preserve infrastructure investment in the future by maintaining a control on demand. This study focuses on the operational and design features of HOT lanes. HOT lanes’ mobility and safety are contingent on the design of zones (“gates”) that drivers use to merge in or out of the facility. Existing methodologies for the design of access zones are limited to engineering judgment or studies that take into consideration undersized amount of observations. This project capitalized on the results of an earlier project that performed an assessment of safety and mobility on the HOT facilities in Minnesota highlighted the issues involved in either designs. The product of this project, the MnPASS Access Design application, provides a tool for traffic managers and planners to examine the conditions within an existing or prospective corridor and the distribution of shockwave lengths which are expected. From the distribution of shockwave lengths, decisions can be made regarding access restriction on the HOT lane to ensure that drivers do not attempt to make lane changes at locations prone to dangerous conditions. This tool provides support for the managers and planners by aggregating the entire behavior of the HOT lane within the corridor into a framework for simplified consideration.