A Review of Parameterizations for Enthalpy and Momentum Fluxes from Sea Spray in Tropical Cyclones

The intensity of tropical cyclones is sensitive to the air-sea fluxes of enthalpy and momentum. Sea spray plays a critical role in mediating enthalpy and momentum fluxes over the ocean’s surface at high wind speeds, and parameterizing the influence of sea spray is a crucial component of any air-sea interaction scheme used for the high wind regime where sea spray is ubiquitous. Many studies have proposed parameterizations of air-sea flux that incorporate the microphysics of sea spray evaporation and the mechanics of sea spray stress. Unfortunately, there is not yet a consensus on which parameterization best represents air-sea exchange in tropical cyclones, and the different proposed parameterizations can yield substantially different tropical cyclone intensities. This paper seeks to review the developments in parameterizations of the sea spray-mediated enthalpy and momentum fluxes for the high wind speed regime and to synthesize key findings that are common across many investigations.

Multi-Channel Spectral Analysis of Non-Synchronous Vibrations of Bladed Disks Measured by Blade Tip Timing

The paper proposes a method for the processing of BTT data deriving from non-synchronous vibrations measured at constant rotor speed by a set of non-uniformly distributed sensors. The sampled data are interpreted as members of a vector space whose characteristics are determined by the signal itself and by sampling pattern. If the signal contains a single harmonic component, its frequency can be estimated through a method that has been named harmonic matching. On the contrary, when more than one harmonic component is present, due to a multi-modal response, a component separation processing is necessary. To this purpose, it is proposed a technique based on the Independent Component Analysis (ICA). This approach is limited to constant speed regime, but has the benefit of using statistical estimators that enable a strong resistance to noise. The method is illustrated using academic examples and is employed to study a flutter-like non-synchronous vibration of a bladed disk.

Experimental Study on Microfluidic Mixing with Different Zigzag Angles

This paper presents experimental investigations of passive mixing in a microfluidic channel with different zigzag angles. Zigzag channel is commonly used for microfluidic mixing because it does not need an additional control unit and can be easily implemented in a lab-on-a-chip system. In this work, microfluidic channels with six different zigzag angles, from θ = 0° to θ = 75°, are tested under ten different flow rates corresponding to Reynolds number from 0.309 to 309. Two colored liquids are mixed with the zigzag channels and mixing performance is evaluated based on the color of the pixels on the region of interest from captured images. According to the results, we found that the mixing performance is almost independent of the zigzag angle in the low-speed regime where its Reynolds number is less than 4. The mixing became very much depending on the zigzag angle in the high-speed regime where its Reynolds number is greater than 100. Microfluidic mixing is needed for Lab-on-a-chip applications in both low flow speed, such as medium perfusion for cell culture, and high flow speed, such as high-speed sensing on a point-of-care device. This work is aimed to provide practical information on zigzag mixing for chip design and applications.

Dynamics of carving runs in alpine skiing. I.The basic centrifugal pendulum.

We studied perfect carving turns of alpine skiing using the simple model of an inverted pendulum which is subject to the gravity force and the force mimicking the centrifugal force emerging in the turns. Depending on the turn speed the model describes two different regimes. In the subcritical ( low speed) regime, there exist three equilibrium positions of the pendulum where the total torque applied to the pendulum vanishes -- the marginally stable vertical position and two unstable tilted positions on both sides of the vertical. The tilted equilibria correspond to the ski turns executed in perfect balance. The vertical equilibrium corresponds to gliding down the fall line without turns. In the supercritical (high speed) regime, the tilted equilibria disappear. In addition to the equilibria the model allows fall-rise solutions, where the pendulum (skier) hits (rises from) the ground, and oscillations about the vertical. These oscillations correspond to the so-called dynamic skiing where the skier never settles to a balanced position in the turn. Analysis of the available data on FIS WC races shows that elite races ski mostly in the supercritical regime. In its current form the model of centrifugal pendulum has no feedback components associated with the skier control over their runs and therefore describes a riderless mono ski. Hence the theory predicts that such a vehicle can execute carving turns automatically.

Gear rattle reduction in an automotive driveline by the adoption of a flywheel with an innovative torsional vibration damper

An innovative device consisting of a flywheel equipped with a torsional vibration damper, based on the magnetorheological effect in elastomeric spring elements, is proposed in this paper. The feasibility study reports the dynamic behaviour of an automotive driveline equipped with the device aiming to explore the effectiveness of the damper in reducing the torsional oscillations of the flywheel, at low-speed regime, responsible for the vibro-acoustic phenomenon known as “gear rattle”. The spring elements of the device are constituted by magneto-rheological elastomeric samples, interposed between the flywheel and the damper disk, working for shear strains. Their dynamical characteristics can be properly tuned by varying the magnetic field surrounding the springs in order to mitigate the forced vibration causes of gear tooth impacts. The good attitude of the device in mitigating the rattle phenomenon is demonstrated by comparing the results provided by a numerical drive line model, equipped with a “monolithic” flywheel, with those obtained by adopting the present innovative vibration damper. The angular accelerations, resulting from the collisions between the teeth during the operation under “idle” conditions at different angular speeds, are thus compared.

On Extreme Winds at L-Band with the SMAP Synthetic Aperture Radar

In this letter, we discuss some observations of the Soil Moisture Active Passive (SMAP) mission’s high-resolution synthetic aperture radar (SAR) for extreme winds and tropical cyclones. We find that the L-band cross-polarized backscatter is far more sensitive to wind speed at extreme winds than the co-polarized backscatter and it is essential to observations of extreme winds with L-band SAR. We introduce a cyclone wind speed retrieval algorithm and apply it to the limited SMAP SAR dataset of cyclones. We show that the SMAP SAR instrument is capable of measuring extreme winds up to the category 5 (70 m/s) wind speed regime providing unique capabilities as compared to traditional scatterometers with C and Ku-band radars.