Effet Venturi | EPFL Graph Search

vignette|Schématisation de l'effet Venturi. La pression au est plus grande qu'au . Et la vitesse du fluide au est plus grande qu'au . vignette|Écoulement dans un tube Venturi. L’effet Venturi, du nom du physicien italien Giovanni Battista Venturi, est le nom donné à un phénomène de la dynamique des fluides, selon lequel un fluide en écoulement subit une dépression là où la vitesse d'écoulement augmente, où la section d'écoulement se réduit, créant ainsi un courant d'air dans sa partie étroite. L'effet est une manifestation du principe de conservation de l'énergie (formalisé dans le cas des écoulements fluides par le théorème de Bernoulli) et peut s'énoncer de la façon suivante : dans le cas d'un écoulement fluide horizontal, lorsque la vitesse d'écoulement augmente, la pression diminue. On peut également le formuler selon cette variante : dans le cas d'un écoulement horizontal, si la section d'écoulement diminue, la pression dans le fluide diminue également ; dans ce cas, on fait

A New Pulsed Multipoint Injection System to Reduce Natural Gas Vehicle Emissions.

Daniel Favrat

Nowadays, most of the natural gas conversion systems available on the market for bifuel vehicles are based on the venturi principle or on the continuous injection principle for the carburation of the engine. Even if the driveability of these systems is satisfactory, their emission performance does often not reflect the full environmental potential offered by natural gas. This is mainly due to the low response of these systems to transient conditions. During the past years, EPFL has been involved in a co-operative program with the industry to develop a new natural gas injection system intended for multipoint sequenced and phased indirect gaseous fuel injection. Mounted on a four cylinder 1.4-liter engine, the system includes specifically developed injectors and a PC based electronic control unit. Emission results on the engine test vench are shown for typical transient sequences as well as steady state conditions. Results on the chassis dynamometer are given for the New European Driving Cycle (94/12/EC directive). Keeping the standard vehicle components (like catalytic converter and lambda sensor), the current emissions results from our novel injection system are well below the legal emissions limits. Those results are compared with the performance of a state of the art multipoint continuous injection system from the market, measured on the same engine. In order to control accurately the narrower lambda window when working with natural gas, the system, among others, accounts for the sensitivity of the lambda sensor to the composition of the exhaust gases. While maintaining a good driveability, this system has shown significant emissions improvements compared to actual natural gas systems. New engine control strategies currently under evaluations are likely to still further reduce emissions.

1998

Entraining avalanches on slopes: results from experiments using PIV on viscoplastic gravity currents.

Christophe Ancey, Belinda Margaret Bates

In order to conduct experiments simulating entrainment by avalanches on slopes, it was necessary to select a material which could act as both a stationary entrainable layer, mimicking undisturbed mud or snow, and a flowing mass, representing the flowing avalanche. Carbopol Ultrez 10, a viscoplastic “micro-gel” exhibiting a yield stress does just that: it remains plastic on a slope until an avalanche arrives and increases the shear within, reducing the viscosity in some or all of the material which may begin to flow as a fluid. Carbopol is transparent and easily seeded with fluorescent tracking micro-particles, without significantly changing the material rheology. We take advantage of its properties to perform Particle Imaging Velocimetry (PIV) on an idealized avalanche, which flows into an entrainment zone where it interacts with a layer of stationary bed material. The internal velocity field is obtained for the flow as it passes over the loose material, showing the entrainment mechanisms active at different slope angles. At the shallowest slope the avalanche is slower and mobilizes the bed almost entirely, causing it to slip along the base and buckle downstream of the front. At steeper slopes the avalanche shears the bed, yet appears to glide over it more easily with a smaller effect downstream. Increasing the concentration of the Carbopol and thus increasing the apparent yield stress leads to more destruction of the bed layer by the avalanche. Three flow phases are identified, beginning with a “rolling phase” where the avalanche has minimal effect on the bed and seems to roll onto it as the front moves forward, then a “gliding phase” where the deposited fluid is pushed downwards and glides downstream, shearing the bed material. Finally, on the shallower slopes and at higher Carbopol concentration, the avalanche digs down to the rigid base, and completely displaces the bed material downstream, with its front riding atop an entirely mobilized plug-flow layer.

2014

Modal behavior of a reduced scale pump-turbine impeller. Part 1: Experiments

François Avellan, Eduard Egusquiza, Mohamed Farhat

An experimental investigation has been carried out to quantify the effects of surrounding fluid on the modal behavior of a reduced scale pump-turbine impeller. The modal properties of the fluid-structure system have been obtained by Experimental Modal Analysis (EMA) with the impeller suspended in air and inside a water reservoir. The impeller has been excited with an instrumented hammer and the response has been measured by means of miniature accelerometers. The Frequency Response Functions (FRF’s) have been obtained from a large number of impacting positions in order to ensure the identification of the main mode shapes. As a result, the main modes of vibration have been well characterized both in air and in water in terms of natural frequency, damping ratio and mode shape. The first mode is the 2 Nodal Diameter (ND), the second one is the 0ND and the following ones are the 3ND coupled with the 1ND. The visual observation of the animated mode shapes and the level of the Modal Assurance Criterion (MAC) have permitted to correlate the homologous modes of vibration of the fluid-structure system in air and in water. From this comparison the added mass effect on the natural frequencies and the fluid effect on the damping ratios have been quantified for the most significant modes. With the surrounding water, the natural frequencies decrease in average by 10%. On the other hand, the damping ratios increase in average by 0.5%. In any case, the damping ratio appears to decrease with the frequency value of the mode.

International Association For Hydraulic Research2010

Entraining avalanches on slopes: results from experiments using PIV on viscoplastic gravity currents.

Christophe Ancey, Belinda Margaret Bates

2014