Algebraically diverging modes upstream of a swept bluff body

International audienceClassical stability theory for swept leading-edge boundary layers predicts eigenmodes in the free stream with algebraic decay far from the leading edge. In this article, we extend the classical base flow solution by Hiemenz to a uniformly valid solution for the flow upstream of a bluff body, which includes a three-dimensional boundary layer, an inviscid stagnation-point flow and an outer parallel flow. This extended, uniformly valid base flow additionally supports modes which diverge algebraically outside the boundary layer. The theory of wave packet pseudomodes is employed to derive analytical results for the growth rates and for the eigenvalue spectra of this type of mode. The complete spectral analysis of the flow, including the algebraically diverging modes, will give a more appropriate basis for receptivity studies and will more accurately describe the interaction of perturbations in the free stream with disturbances in the boundary layer. © Cambridge University Press 2011

Fate of mercury in tree litter during decomposition

We performed a controlled laboratory litter incubation study to assess changes in dry mass, carbon (C) mass and concentration, mercury (Hg) mass and concentration, and stoichiometric relations between elements during decomposition. Twenty-five surface litter samples each, collected from four forest stands, were placed in incubation jars open to the atmosphere, and were harvested sequentially at 0, 3, 6, 12, and 18 months. Using a mass balance approach, we observed significant mass losses of Hg during decomposition (5 to 23 % of initial mass after 18 months), which we attribute to gaseous losses of Hg to the atmosphere through a gas-permeable filter covering incubation jars. Percentage mass losses of Hg generally were less than observed dry mass and C mass losses (48 to 63 % Hg loss per unit dry mass loss), although one litter type showed similar losses. A field control study using the same litter types exposed at the original collection locations for one year showed that field litter samples were enriched in Hg concentrations by 8 to 64 % compared to samples incubated for the same time period in the laboratory, indicating strong additional sorption of Hg in the field likely from atmospheric deposition. Solubility of Hg, assessed by exposure of litter to water upon harvest, was very low (<0.22 ng Hg g<sup>−1</sup> dry mass) and decreased with increasing stage of decomposition for all litter types. Our results indicate potentially large gaseous emissions, or re-emissions, of Hg originally associated with plant litter upon decomposition. Results also suggest that Hg accumulation in litter and surface layers in the field is driven mainly by additional sorption of Hg, with minor contributions from "internal" accumulation due to preferential loss of C over Hg. Litter types showed highly species-specific differences in Hg levels during decomposition suggesting that emissions, retention, and sorption of Hg are dependent on litter type

Toward real-time measurement of atmospheric mercury concentrations using cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) is a direct absorption technique that utilizes path lengths up to multiple kilometers in a compact absorption cell and has a significantly higher sensitivity than conventional absorption spectroscopy. This tool opens new prospects for study of gaseous elemental mercury (Hg<sup>0</sup>) because of its high temporal resolution and reduced sample volume requirements (<0.5 l of sample air). We developed a new sensor based on CRDS for measurement of (Hg<sup>0</sup>) mass concentration. Sensor characteristics include sub-ng m<sup>−3</sup> detection limit and high temporal resolution using a frequency-doubled, tuneable dye laser emitting pulses at ~253.65 nm with a pulse repetition frequency of 50 Hz. The dye laser incorporates a unique piezo element attached to its tuning grating allowing it to tune the laser on and off the Hg<sup>0</sup> absorption line on a pulse-to-pulse basis to facilitate differential absorption measurements. Hg<sup>0</sup> absorption measurements with this CRDS laboratory prototype are highly linearly related to Hg<sup>0</sup> concentrations determined by a Tekran 2537B analyzer over an Hg<sup>0</sup> concentration range from 0.2 ng m<sup>−3</sup> to 573 ng m<sup>−3</sup>, implying excellent linearity of both instruments. The current CRDS instrument has a sensitivity of 0.10 ng Hg<sup>0</sup> m<sup>−3</sup> at 10-s time resolution. Ambient-air tests showed that background Hg<sup>0</sup> levels can be detected at low temporal resolution (i.e., 1 s), but also highlight a need for high-frequency (i.e., pulse-to-pulse) differential on/off-line tuning of the laser wavelength to account for instabilities of the CRDS system and variable background absorption interferences. Future applications may include ambient Hg<sup>0</sup> flux measurements with eddy covariance techniques, which require measurements of Hg<sup>0</sup> concentrations with sub-ng m<sup>−3</sup> sensitivity and sub-second time resolution

I'm sensing in the rain: Spatial incongruity in visual-tactile mid-air stimulation can elicit ownership in VR users

Major virtual reality (VR) companies are trying to enhance the sense of immersion in virtual environments by implementing haptic feedback in their systems (e.g., Oculus Touch). It is known that tactile stimulation adds realism to a virtual environment. In addition, when users are not limited by wearing any attachments (e.g., gloves), it is even possible to create more immersive experiences. Mid-air haptic technology provides contactless haptic feedback and offers the potential for creating such immersive VR experiences. However, one of the limitations of mid-air haptics resides in the need for freehand tracking systems (e.g., Leap Motion) to deliver tactile feedback to the user's hand. These tracking systems are not accurate, limiting designers capability of delivering spatially precise tactile stimulation. Here, we investigated an alternative way to convey incongruent visual-tactile stimulation that can be used to create the illusion of a congruent visual-tactile experience, while participants experience the phenomenon of the rubber hand illusion in VR

I Smell Trouble: Using Multiple Scents To Convey Driving-Relevant Information

Cars provide drivers with task-related information (e.g. "Fill gas") mainly using visual and auditory stimuli. However, those stimuli may distract or overwhelm the driver, causing unnecessary stress. Here, we propose olfactory stimulation as a novel feedback modality to support the perception of visual notifications, reducing the visual demand of the driver. Based on previous research, we explore the application of the scents of lavender, peppermint, and lemon to convey three driving-relevant messages (i.e. "Slow down", "Short inter-vehicle distance", "Lane departure"). Our paper is the first to demonstrate the application of olfactory conditioning in the context of driving and to explore how multiple olfactory notifications change the driving behaviour. Our findings demonstrate that olfactory notifications are perceived as less distracting, more comfortable, and more helpful than visual notifications. Drivers also make less driving mistakes when exposed to olfactory notifications. We discuss how these findings inform the design of future in-car user interfaces

Mid-Air Haptic Rendering of 2D Geometric Shapes with a Dynamic Tactile Pointer

IEEE An important challenge that affects ultrasonic midair haptics, in contrast to physical touch, is that we lose certain exploratory procedures such as contour following. This makes the task of perceiving geometric properties and shape identification more difficult. Meanwhile, the growing interest in mid-air haptics and their application to various new areas requires an improved understanding of how we perceive specific haptic stimuli, such as icons and control dials in mid-air. We address this challenge by investigating static and dynamic methods of displaying 2D geometric shapes in mid-air. We display a circle, a square, and a triangle, in either a static or dynamic condition, using ultrasonic mid-air haptics. In the static condition, the shapes are presented as a full outline in mid-air, while in the dynamic condition, a tactile pointer is moved around the perimeter of the shapes. We measure participants' accuracy and confidence of identifying shapes in two controlled experiments ($n_1 = 34; n_2 = 25$). Results reveal that in the dynamic condition people recognise shapes significantly more accurately, and with higher confidence. We also find that representing polygons as a set of individually drawn haptic strokes, with a short pause at the corners, drastically enhances shape recognition accuracy. Our research supports the design of mid-air haptic user interfaces in application scenarios such as in-car interactions or assistive technology in education

Interaction design for online video and television

This course will teach attendees how to design and evaluate interaction with online video and television. It provides attendees a pragmatic toolset, including techniques and guidelines, which can be directly applied in practice. The different tools will be contextualized based on current developments, giving participants a complete overview of the state of the art and industry

Incoherency in Central American Hydroclimate Proxy Records Spanning the Last Millennium

Continued Global Warming is Expected to Result in Reduced Precipitation and a Drier Climate in Central America. Projections of Future Changes Are Highly Uncertain, However, Due to the Spatial Resolution Limitations of Models and Insufficient Observational Data Coverage Across Space and Time. Paleoclimate Proxy Data Are Therefore Critical for Understanding Regional Climate Responses during Times of Global Climate Reorganization. Here We Present Two Lake-Sediment based Records of Precipitation Variability in Guatemala Along with a Synthesis of Central American Hydroclimate Records Spanning the Last Millennium (800–2000 CE). the Synthesis Reveals that Regional Climate Changes Have Been Strikingly Heterogeneous, even over Relatively Short Distances. Our Analysis Further Suggests that Shifts in the Mean Position of the Intertropical Convergence Zone, Which Have Been Invoked by Numerous Studies to Explain Variability in Central American and Circum-Caribbean Proxy Records, Cannot Alone Explain the Observed Pattern of Hydroclimate Variability. Instead, Interactions between Several Ocean-Atmosphere Processes and their Disparate Influences Across Variable Topography Appear to Have Resulted in Complex Precipitation Responses. These Complexities Highlight the Difficulty of Reconstructing Past Precipitation Changes Across Central America and Point to the Need for Additional Paleo-Record Development and Analysis Before the Relationships between External Forcing and Hydroclimate Change Can Be Robustly Determined. Such Efforts Should Help Anchor Model-Based Predictions of Future Responses to Continued Global Warming

Improving immersive experiences for visitors with sensory impairments to the aquarium of the pacific

This case study describes the development of a mid-air haptic solution to enhance the immersive experience of visitors who are deaf, blind or wheelchair users to the Aquarium of the Pacific's movie theatre. During the project we found that adding a sense of touch, using an innovative ultrasound technology, to an immersive experience can improve the sense of engagement users have with the content, and can help to improve agreement with the topics presented. We present guidelines on the design of haptic sensations. By describing how this project took place within the tight timelines of a commercial deployment, we hope to encourage more organisations to do similar work

CARoma Therapy: Pleasant Scents Promote Safer Driving, Better Mood, and Improved Well-Being in Angry Drivers

Driving is a task that is often affected by emotions. The effect of emotions on driving has been extensively studied. Anger is an emotion that dominates in such investigations. Despite the knowledge on strong links between scents and emotions, few studies have explored the effect of olfactory stimulation in a context of driving. Such an outcome provides HCI practitioners very little knowledge on how to design for emotions using olfactory stimulation in the car. We carried out three studies to select scents of different valence and arousal levels (i.e. rose, peppermint, and civet) and anger eliciting stimuli (i.e. affective pictures and on-road events). We used this knowledge to conduct the fourth user study investigating how the selected scents change the emotional state, well-being, and driving behaviour of drivers in an induced angry state. Our findings enable better decisions on what scents to choose when designing interactions for angry drivers