The head-to-head impact of diesel-fuel droplets on a polished spherical brass target has been investigated experimentally. High-speed imaging was employed to visualize the impact process for wall surface temperatures and Weber and Reynolds numbers in the ranges of 140-340 °C, 30-850, and 210-1135, respectively. The thermohydrodynamic outcome regimes occurring for the aforementioned ranges of parameters were mapped on a We-T diagram. Seven clearly distinguishable postimpact outcome regimes were identified, which are conventionally called the coating, splash, rebound, breakup-rebound, splash-breakup-coating, breakup-coating, and splash-breakup-rebound regimes. In addition, the effects of the Weber number and surface temperature on the wettability dynamics were examined; the temporal variations of the dynamic contact angle, dimensionless spreading diameter, and liquid film thickness forming on the solid particle were measured and are reported.
A combined experimental and computational investigation of micrometric diesel droplets impacting on a heated aluminium substrate is presented. Dual view high-speed imaging has been employed to visualize the evolution of the impact process at various conditions. The parameters investigated include wall-surface temperature ranging from 140 to 400°C, impact Weber and Reynolds numbers of 19–490 and 141–827, respectively, and ambient pressure of 1 and 2 bar. Six possible post-impact regimes were identified, termed as Stick, Splash, Partial-Rebound, Rebound, Breakup-Rebound and Breakup-Stick , and plotted on the We-T map. Additionally, the temporal variation of the apparent dynamic contact angle and spreading factor have been determined as a function of the impact Weber number and surface temperature. Numerical simulations have also been performed using a two-phase flow model with interface capturing, phase-change and variable physical properties. Increased surface temperature resulted to increased maximum spreading diameter and induced quicker and stronger recoiling behaviour, mostly attributed to the change of liquid viscosity.
The primary objective of the present study was to develop a quantitative schlieren-imaging technique that can be used to study the dynamics of instability waves. The technique was initially validated by optically capturing a controlled acoustic wave generated by a compression driver and excellent agreement was obtained with microphone measurements. An underexpanded jet was considered as an ideal test case due to the complexity and multitude of instability mechanism. Further analysis of the underexpanded jet demonstrated that this technique can be used to capture the very high frequency mode related to the phenomenon of screech.
Conditionally automated driving systems (SAE level 3) are able to process the lateral and longitudinal control of a vehicle and warn the driver of the status of the system and ongoing operations. The driver must monitor the system and resume control if prompted to. Previous research in the realm of automated driving explored how in-vehicle information should be presented to optimise drivers’ trust in the system (Wintersberger et al, 2020). For instance, conveying the status and actions of the system contributes to transparency and support adequate trust in the system (Carsten & Martens, 2019). Yet, little is known on the consequences of failing to provide reliable information on the vehicle status and operations on trust. This is particularly salient in the case of silent failures, whereby the system fails to notify the driver of its limit and incapacity to operate reliably (Louw et al., 2019). This lack of empirical evidence is surprising as automation failures are likely to affect drivers’ trust in the system (Payre et al., 2015; 2017), therefore leading to disuse (e.g. no use), misuse (e.g. unsafe operation as reported by the National Transportation Safety Board Tesla crash report, 2017) or abuse (e.g. take advantage of the limits) of such system (Parasuraman, 1997). Past work has stressed that the subjective level of trust of users should be aligned with the capabilities of the automation to mitigate the undesirable effect of overtrust (i.e. using the automated system despite its unreliability) and distrust (i.e. not using the system although it is reliable; Khastgir et al., 2018). This process has been identified as trust calibration (Lee & See, 2004). Even though a wealth of studies has shown what and how information should be presented to support trust calibration, little research attention has been devoted to understand if, how and when failures affect individuals’ trust in the automated system and subsequent impact on driving performance. Addressing this research gap, the present study combines a cyber security and human factors approach to investigate the effect of the type of failure (silent vs. explicit) and its timing (early vs. late during the journey) on individuals’ trust, attitudes and safety. From the cyber security perspective, a threat analysis of in-vehicle digital displays was conducted. This led to a series of use cases being developed when possible malfunction or intrusion (e.g. hacking) would occur. These use cases were developed in a driver-in-the-loop simulator where participants’ responses (N = 37) with respect to trust in the automation, driving performance, and safety were collected. Results from this experiment are discussed in the context of road safety, attitudes and driver behaviour (e.g. manual handover, acceptance and trust).
Viscous oils flowing in the geometrically-complex hydraulic circuits of earth-moving machines are associated with extensive friction losses, thus reducing the fuel efficiency of the vehicles and increasing emissions. The present investigation examines the performance effectiveness of different hydraulic oils, in terms of secondary-flow suppression and pressure-drop reduction. The flow of two non-Newtonian oil compounds, containing poly(alkylmethacrylate) (PMA) and poly(ethylene-co-propylene) (OCP) polymers, respectively, have been comparatively investigated against a base, monograde liquid through Particle Image Velocimetry. An 180 o curved-tube layout and a check-valve replica have been selected as representative examples of the hydraulic components comprising the hydraulic circuit. The flow conditions prevailing in the experimental cases are characterised by Reynolds-number values in the range 76-1385. Precursor viscosity measurements with shear rate along with a theoretical analysis conducted using the FENE and PTT models have verified the influence of viscoelasticity and/or shearthinning on the liquid flow behaviour. PIV results have demonstrated that viscoelastic effects setting in due to the OCP additives tend to reduce the magnitude of the secondary flow pattern, commonly known as a Dean-vortex system, arising in the curved geometry by as much as 15% on average compared to the base liquid. A similar flow behaviour was also demonstrated in the valve replica layout with reference to the geometry-induced coherent vortical motion in the constriction region, where a vorticity decrease up to 38% was observed for the OCP sample. On the contrary, the flow behaviour of the primarily shearthinning PMA oil was found to be comparable to that of the base oil, hence not presenting significant flow-enhancement characteristics.
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