Advantages of vertical axis tidal turbines set in close proximity: A comparative CFD investigation in the English Channel

A 2D CFD analysis of the hydrodynamic interactions between three closely spaced Vertical Axis Tidal Turbines is performed for two layouts: side-by-side and triangular. Three mechanisms determine a power increase with respect to isolated turbines: (1) turbine blockage that entails flow acceleration outside of the turbines and inside the aisles between adjacent turbines; (2) more favorable direction of the flow approaching the blade during upwind trajectory; (3) wake contraction, that increases the torque generation during the downwind path. Blockage is responsible for a moderate performance increase exhibited by the triangular layout. Change in direction of the flow approaching the blades and wake contraction only occur for the side-by-side layout and cause a significant increase in efficiency. The six-month energy production of the two arrangements is predicted for three real cases in the English Channel: Alderney Race, St. Catherine and Portland Bill, characterized by rectilinear and non-rectilinear currents. Passive stall is assumed as load strategy control; the effect of the rated speed on energy production is analyzed. The side-by-side layout allows not only a higher power gain but also a wider range of flow directions for which a gain is possible, therefore it appears suitable even for non-rectilinear currents

Numerical Analysis of Methane Direct Injection in a Single-cylinder 250 cm3 Spark Ignition Engine

The paper shows the results of the numerical tasks of a study aimed to evaluate the potential of low-pressure (< 20 bar) direct injection systems for internal combustion engines fed with gaseous fuels. Starting from the geometry of a low-cost commercial injector already available for GDI uses, a 2D axisymmetric CFD analyses is performed to assess the influence of injection pressure and valve and seat-valve profiles on jet characteristics, methane-air mixing, and charge distribution at ignition time. Then 3D simulations for the motorcycle single cylinder test-engine are carried out considering as much as possible combustion chamber details and realistic boundary conditions. Although it is possible identifying which operating and geometrical details of injection system are able to support complete mixture homogeneity, this study shows tremendous difficulties, in case of gaseous fuels, to realise satisfactory stratification charges that would be required to obtain satisfactory performance at partial loads

Fluid dynamic mechanisms of enhanced power generation by closely spaced vertical axis wind turbines

We present a comprehensive set of two-dimensional (2D) unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of flow around a pair of counter-rotating vertical-axis wind turbines (VAWTs). The simulations are performed for two possible configurations of the counter-rotating VAWT pair, with various gaps between the two turbines, tip-speed-ratios and wind directions, in order to identify key flow mechanisms contributing to the enhanced performance of a pair of turbines compared to an isolated turbine. One of the key mechanisms identified, for the case of two turbines arrayed side-by-side with respect to the incoming wind, is the change of lateral velocity in the upwind path of each turbine due to the presence of the neighbouring turbine, making the direction of local flow approaching the turbine blade more favourable to generate lift and torque. The results also show that the total power of a staggered pair of turbines cannot surpass that of a side-by-side pair of turbines. Some implications of the present results for the prediction of the performance of single and multiple rows (or a farm) of VAWTs are also discussed. The local flow mechanisms identified in the present study are expected to be of great importance when the size of the farm is relatively small

Hybrid CFD-source terms modelling of a diffuser-augmented vertical axis wind turbine

In order to reduce the calculation time in the analysis of a diffuser augmented H-Darrieus wind turbine (DAWT), a hybrid CFD-source terms 2D approach, relying on a new experimental-based dynamic stall model, was adopted. This method consists in solving the blades load by means of an analytic model, whereas the overall flow field is calculated through a simplified CFD. The new model was compared with a complete CFD exhibiting a satisfying agreement both for the case of a bare turbine and an unoptimized DAWT. By virtue of the rapidity of the new model, an optimization analysis of the diffuser design was carried out. A great power gain was achieved, thanks to unexpected aerodynamic phenomena, but an even greater increase of the frontal area was necessary. The critical issues of the modelling approach and the proposed DAWT are discussed

CFD investigation on the aerodynamic interferences between medium-solidity Darrieus Vertical Axis Wind Turbines

The present study contributes to understand physical mechanisms involved in an achievable power enhancement by setting vertical axis wind turbines in close proximity. The turbines are straight-bladed Darrieus micro-turbines characterized by medium-high solidity and therefore low tip-speed ratio. Preliminary CFD simulations of the isolated turbine explain the reasons why it has a low power output, namely which are laminar flow and laminar separation bubbles on the blades. This fact is expected also considering the low Reynolds number. Subsequently a campaign of CFD simulations has been performed to analyse the aerodynamic interferences in two-rotor configurations. The behaviour of counter-rotating and co-rotating arrangements is analysed at different distances between rotor axes. The simulations show an increasing of power production of about 10% compared to results for the isolated turbine, independently of the sense of rotation. In order to verify wheter vortex shedding suppression might be the cause of the enhanced performance interactions has been simulated between two closely spaced Magnus spinning cylinders with the same tip-speed ratio of the turbines. These last results don’t show reasonable analogies with VAWT wake structures and interactions. Our main conclusion is that accelerated free-stream flow between the turbines is the principle cause of the power extraction enhancement by means of contraction and re-energisation of the turbine wakes. CFD predictions of a four-rotor configuration confirm our hypothesis, nevertheless the wind direction strongly affects the overall efficacy

Hydrodynamic interactions between three closely-spaced Vertical Axis Tidal Turbines

Abstract A CFD analysis of the hydrodynamic interactions between three vertical axis tidal turbines set in close proximity is performed for two layout: side-by-side and triangular. The following key mechanisms are found to determine a power increasing with respect to isolated turbines: (1) turbine blockage that entails flow acceleration outside of the turbines and inside the aisles between adjacent turbines; (2) more favorable direction of the flow approaching the blade during upwind; (3) wake contraction, that increases torque generation during downwind. Blockage is responsible for a moderate performance increasing exhibited by the triangular layout. Change in the direction of the flow approaching the blades and wake contraction only occur for side-by-side layout, and lead to a significant increasing of efficiency. The side-by-side layout allows a wider range of flow directions that make possible a power gain, and thus it could be adoptable for tidal currents characterized by an incomplete inversion of the current direction. The behaviors of the two arrangements are compared for a real case located in the Strait of Messina

Codice KIVA: mglioramenti nella previsione del particolato diesel

Utilizzando il codice KIVA con il modello di ossidazione del particolato di Nagle Strikland-Constable, da un precedente lavoro sono risultati consistenti scostamenti dei valori calcolati da quelli misurati delle emissioni di particolato carbonioso, con eccessivi livelli calcolati per i carichi alti e viceversa livelli troppo bassi per i carichi ridotti. Per indagare sulle cause ditali scostamenti, vengono esaminate, per differenti carichi, gradi di sovralimentazione e velocità di rotazione del motore, sia la sensitività delle emissioni calcolate alle temperature di parete, sia l’influenza del modello di ossidazione, confrontando il modello di Nagle Strikland-Constable, empirico e puramente cinetico, con il modello di Magnussen, basato sulla velocità di dissipazione dei vortici turbolenti. Né l’influenza delle temperature di parete, né il tipo di modello di ossidazione scelto paiono essere la causa primaria degli errati valori calcolati

Investigation of the wake energy recovery of cross-flow turbines in paired configuration by means of 3d-CFD and analysis of the streamwise momentum budget

Literature suggests that cross-flow turbines (CFTs) could be suitable for off-shore wind farms because of the high power-density achievable by shortening the distance between arrays as allowed by a fast wake energy recovery experimentally detectable. By means of 3d-CFD, we analysed the effect of the rotation verse of CFTs in counter-rotating paired configuration on the wake behaviour. Then, we applied the momentum-budget approach to identify the fluid dynamic mechanisms which are more effective in supporting the streamwise momentum recovery. The following results are found. (A) The counter-rotating vortices occurring in the near-wake as a consequence of the vortex shedding at the blade tip are responsible for the vertical advection that enters high momentum flow inside the wake. (B) The turbulent transport contribution is less important, yet it becomes significant starting from the medium wake. (C) For the inner-downwind layout the wake shape appears similar to that of a single turbine, whereas for the inner-upwind layout it is greatly contracted in horizontal direction and enlarged in vertical direction (D). The momentum recovery appears slightly more delayed than in case of a single turbine, yet the velocity deficit appears less extensive for the inner upwind layout that, especially thanks to the wake lateral contraction, could be the preferable in farms consisting in staggered arrays, allowing to shorten the lateral distance between adjacent pairs

Studio CFD di un motore ad idrogeno, alimentato con iniezione diretta a bassa pressione

La strategia di formazione della miscela è un fattore chiave nei motori a combustione interna alimentati ad idrogeno. La formazione esterna della miscela (iniezione indiretta) consente eccellente omogeneità della miscela e sfruttamento pressoché completo della bombola di stoccaggio del combustibile (bassa pressione residua). Tuttavia l’ingente volume specifico dell’idrogeno comporta bassa densità energetica e quindi modesta potenza specifica. L’uso di idrogeno liquido, grazie alla sua bassa temperatura ed al calore latente di evaporazione, consente sia di incrementare la densità energetica della miscela che di allontanare i rischi di preaccensione e ritorno di fiamma, ma richiede sofisticate e costose tecnologie. La formazione interna della miscela (iniezione diretta) assicura elevata densità energetica della carica ed evita ritorni di fiamma, ma non elimina il rischio di preaccensioni. Questo articolo presenta una possibile soluzione di iniezione diretta di idrogeno, la quale ha il pregio di richiedere pressioni di adduzione, quindi anche residue di stoccaggio, relativamente modeste. Poiché l’iniezione deve terminare sufficientemente presto da consentire alla carica di miscelarsi e, in ogni caso, prima che la pressione nel cilindro raggiunga quella di adduzione dell’idrogeno, sono necessarie elevate portate istantanee. Gli elettroiniettori disponibili in commercio non permettono le portate richieste ed inoltre non garantiscono l’elevata tenuta necessaria a contrastare la forte tendenza a trafilare dell’idrogeno. Si è quindi pensato di immettere l’idrogeno nel cilindro tramite una valvola attuata meccanicamente. A monte della valvola si trova un piccolo serbatoio in cui l’idrogeno viene addotto tramite un normale eletroiniettore (per iniezione indiretta di metano) la cui portata risulta adeguata, giacché esso può rimanere aperto per un ampio intervallo di angolo di manovella (al limite l’intero ciclo). Si riportano le simulazioni CFD che stanno conducendo alla realizzazione di un prototipo monocilindrico, derivato dal motore motociclistico Yamaha YZF 450

Effects of the Reynolds number and the tip losses on the optimal aspect ratio of straight-bladed Vertical Axis Wind Turbines

Aspect Ratio (AR) is one of the main design parameters of straight-bladed vertical axis turbines. This paper will examine whether a high AR, with long blades and low tip losses, or a low AR, with a higher diameter and higher losses, is more suitable to achieve the maximum power output given a fixed crosssectional area. Traditional Double-Multiple Stream-Tube (DMST) approaches are limited by a lack of tip loss formulations specifically conceived for vertical axis turbines. Therefore, a CFD-3D investigation covering a power range from micro-generation to MW has been done. Results show that both Reynolds number and tip losses strongly influence the aerodynamic performance of the rotor. More advantages seem to be achieved by limiting tip losses rather than increasing chord-based Reynolds number (Rec), addressing towards high AR at least for medium and large-size turbines. However, as turbine size and wind speed decrease, this difference narrows considerably. For micro turbines, tip losses are balanced by the effects of Rec, thus a variation of AR does not imply a variation of CP. For all the cases that have been analysed, turbine size and therefore Rec does not appreciably affect the normalized CP distribution along the blade, which only depends on AR