Cloud cavitation vortex shedding inside an injector nozzle

The development and collapse of cloud cavitation and its link to surface erosion within a transparent test single-orifice nozzle operating with a closed Diesel fuel hydraulic circuit, has been characterized using high-speed imaging. Data have been obtained for a range of cavitation and Reynolds numbers under fixed lift positions. Post processing of a large number of images acquired with short exposure time (1 μs) allowed the elucidation of the distinct flow phenomena associated with the highly transient two-phase flow. At the inlet of the flow orifice, the vapour cloud was found to occupy the largest part of the nozzle hole cross-section. Coherent vortical structures of a hairpin shape have been detected to onset at the closure region of this vapour cloud and shed downstream in a fully transient manner. The effect of the operating parameters on the temporal and spatial characteristics with regards to the emergence and collapse of the hairpin vortices has been quantified. It has been established that the cavitation-vortex shedding was taking place in a periodical manner, characterized by a Strouhal number

Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging

The effect of viscoelastic additives on the topology and dynamics of the two-phase flow arising within an axisymmetric orifice with a flow path constriction along its main axis has been investigated employing high-flux synchrotron radiation. X-ray Phase Contrast Imaging (XPCI) has been conducted to visualise the cavitating flow of different types of diesel fuel within the orifice. An additised blend containing Quaternary Ammonium Salt (QAS) additives with a concentration of 500 ppm has been comparatively examined against a pure (base) diesel compound. A high-flux, 12 keV X-ray beam has been utilised to obtain time resolved radiographs depicting the vapour extent within the orifice from two views (side and top) with reference to its main axis. Different test cases have been examined for both fuel types and for a range of flow conditions characterised by Reynolds number of 35500 and cavitation numbers (CN) lying in the range 3.0–7.7. It has been established that the behaviour of viscoelastic micelles in the regions of shear flow is not consistent depending on the cavitation regimes encountered. Namely, viscoelastic effects enhance vortical (string) cavitation, whereas hinder cloud cavitation. Furthermore, the use of additised fuel has been demonstrated to suppress the level of turbulence within the orifice

Flow in the closure region of partial attached cavitation.

The flow near the closure region of partial attached cavitation was examined using qualitative and quantitative flow visualization techniques. The flows associated with closed and open attached cavitation were examined on nominally two- and three-dimensional test objects. The results of two-dimensional free-streamline theory were compared with the experimentally observed cavity flows. The phase-averaged and unsteady flow fields were examined using Particle Image Velocimetry (PIV). The inception and topology of attached cavitation were related to the non-cavitating flow. The cavities formed on three-dimensional geometries had both open and closed portions, whereas those formed on two-dimensional geometries were always open. The closed cavities on the three-dimensional geometries had laminar reattachment. Open cavities had either laminar or turbulent reattachment. The open cavities with laminar reattachment shed vortical cloud cavitation periodically, whereas open cavities with turbulent reattachment had irregular shedding of vortical structures. Cavity flows with laminar reattachment formed a re-entrant flow. A thin boundary layer on the cavity interface remained attached to the cavity interface. The cavity fared into the solid surface, and the flow was largely irrotational. The profiles of the closed cavities were qualitatively similar to those predicted by two-dimensional free-streamline theory with a re-entrant flow closure. In contrast, cavities that exhibited turbulent flow reattachment had recirculation within the cavity. The turbulent shear flow downstream of the open partial cavity had similar characteristics to those of a non-cavitating shear flow behind a rearward facing step. An adverse pressure gradient abruptly truncated the cavity in the case of the two-dimensional planar geometries. This resulted in the creation of a turbulent liquid shear flow in the wake of the cavity. The collapse of cloud cavitation was shown to introduce significant vorticity and turbulence into the liquid flow downstream of the cavity. The wall-bounded shear flow downstream of cavities with laminar reattachment was very thin with thickness on the order of the non-cavitating boundary layer. Open cavities with a turbulent reattachment produced a turbulent wake with a thickness on the order of the cavity thickness. This was substantially thicker than the non-cavitating boundary layer.Ph.D.Applied SciencesMechanical engineeringMechanicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131460/2/9909921.pd

Utility Factor Curves for Plug-in Hybrid Electric Vehicles: Beyond the Standard Assumptions

The utility factor (UF) of a plug-in hybrid electric vehicle (PHEV) refers to the ratio of miles traveled in electric mode to the total miles traveled. Standard UF curves provide a prediction of the expected achievable UF by a PHEV given its all-electric range (AER), but such predictions entail assumptions about both the driving patterns (distance traveled and energy intensity) and charging behavior. Studies have attempted to compare the real-world UF achieved by PHEVs to their standard values, but deviations can stem from deviations in assumptions about: (i) achievable electric range, (ii) travel distance and (iii) charging frequency. In this paper, we derive analytical models for modified utility factor curves as a function of both AER and charging behavior. We show that average charging frequency is insufficient to exactly predict UF but can still estimate bounds. Our generalized model can also provide insights into the efficacy of PHEVs in reducing carbon emissions

A Direct Adaptive Controller for ATM ABR Congestion Control

One of the more challenging and yet unresolved issues which is paramount to the success of ATM networks is that of congestion control for Available Bit Rate (ABR) traffic. Unlike other ATM service categories, ABR provides a feedback mechanism, allowing interior nodes to dictate source rates. Previous work has demonstrated how linear control theory can be utilized to create a stable and efficient control system for the purposes of ATM ABR congestion control. This paper extends our previous contribution that assumed a minimum-phase plant, an assumption that is likely violated in practice. Presented here is a direct adaptive controller that uses a finite impulse response (FIR) filter to approximately invert the FIR plant. This controller is well suited for the ATM ABR non-minimum-phase plant. Other control architectures, which motivate the final proposed controller, are also discussed

A Practical Controller for Explicit Rate Congestion Control

This paper examines congestion control for explicit rate data networks. The Available Bit Rate (ABR) service category of Asynchronous Transfer Mode (ATM) networks serves as an example system, however the results of this paper are applicable to other explicit rate systems as well. After a plant model is established, an adaptive control strategy is presented. Several algorithm enhancements are then introduced. These enhancements reduce convergence time, improve queue depth management, and reduce parameter bias. This work differentiates itself from the other contributions in the area of rate-based congestion control in its balanced approach of retaining enough complexity as to afford attractive performance properties, but not so much complexity as to make implementation prohibitively expensive