Formulazione e messa a punto di una metodologia per il progetto di ventilatori per gallerie del vento con applicazione al "long-pipe" del laboratorio CICLoPE : analisi attraverso l'impiego di modelli matematici in MATLAB

Nell'ottica di un futuro riprogetto, totale o parziale, del ventilatore della galleria del vento del progetto CICLoPE dell'Università di Bologna, è stato messo a punto, grazie a modelli matematici di letteratura, un algoritmo per la determinazione della geometria delle pale di un fan. La procedura si basa su ipotesi di incompressibilità e assenza di vortici di estremità ed è in grado di fornire la geometria del ventilatore una volta che sono state fissate: le condizioni richieste nella sezione di test, l'efficienza del tunnel e alcune proprietà del ventilatore stesso (ad esempio tipologia di profilo aerodinamico e numero di pale). L'algoritmo è in grado di lavorare solamente con la configurazione ventilatore seguito da profili raddrizzatori, ma è in previsione un'estensione che consentirà di studiare anche la configurazione a fan controrotanti (come quella del CICLoPE). Con questo software sono state progettate numerose soluzioni diverse per studiare il legame tra rendimento e geometria del ventilatore. Inoltre sono stati individuati i parametri che permettono di ottenere una pala con rastremazione e svergolatura trascurabili, con lo scopo di abbassare i costi del manufatto. In particolare è stato dimostrato come le configurazioni con diametro della nacelle grande (superiore al 65\% del diametro della sezione di potenza) siano particolarmente adatte a fornire rendimenti alti con la minima complicatezza della pala. Per quanto riguarda l'efficienza aerodinamica del profilo, i test comparativi indicano che questo parametro influisce relativamente poco sul rendimento del macchinario ma modifica profondamente la geometria della pala. Efficienze elevate tendono, secondo lo studio, a richiedere pale estremamente rastremate e poco svergolate; questo porta a preferire l'adozione di profili mediamente efficienti ma dall'ampio intervallo operativo in termini di angolo di attacco.

Studies of spatial clustering of inertial particles in turbulence

We present studies of the spatial clustering of inertial particles embedded in turbulent flow. A major part of the thesis is experimental, involving the technique of Phase Doppler Interferometry (PDI). The thesis also includes significant amount of simulation studies and some theoretical considerations. We describe the details of PDI and explain why it is suitable for study of particle clustering in turbulent flow with a strong mean velocity. We introduce the concept of the radial distribution function (RDF) as our chosen way of quantifying inertial particle clustering and present some original works on foundational and practical considerations related to it. These include methods of treating finite sampling size, interpretation of the magnitude of RDF and the possibility of isolating RDF signature of inertial clustering from that of large scale mixing. In experimental work, we used the PDI to observe clustering of water droplets in a turbulent wind tunnel. From that we present, in the form of a published paper, evidence of dynamical similarity (Stokes number similarity) of inertial particle clustering together with other results in qualitative agreement with available theoretical prediction and simulation results. We next show detailed quantitative comparisons of results from our experiments, direct-numerical-simulation (DNS) and theory. Very promising agreement was found for like-sized particles (mono-disperse). Theory is found to be incorrect regarding clustering of different-sized particles and we propose a empirical correction based on the DNS and experimental results. Besides this, we also discovered a few interesting characteristics of inertial clustering. Firstly, through observations, we found an intriguing possibility for modeling the RDF arising from inertial clustering that has only one (sensitive) parameter. We also found that clustering becomes saturated at high Reynolds number.

Rapid Tooling via Stereolithography

Approximately three years ago, composite stereolithography (SL) resins were introduced to the marketplace, offering performance features beyond what traditional SL resins could offer. In particular, the high heat deflection temperatures and high stiffness of these highly filled resins have opened the door to several new rapid prototyping (RP) applications, including wind tunnel test modelling and, more recently, rapid tooling.

Tab 1-4 Weight and volume reduction as well as surface subsidence during abrasion experiments

Antarctic land surfaces in South Victoria Land, all without a covering of vegetation, are actively formed by winds which often reach velocities of more than 100 km/h. Consequently, deflation and abrasion are essential factors in the process of slope formation. Water erosion, active only during the very short summer period, is limited to a few localities in South Victoria Land. Experiments in a wind tunnel proved that ventifacts in the Dry Valleys can be formed within a few decades or at the most, a few centuries. Yearly corrasion rates average around a maximum of a few millimeters. Considerable variability is caused by the different exposures of ventifacts within the micro relief end the varying resistance of the rocks. The importance of ice crystals (snow) for abrasion processes should not be overestimated.

The ballast pick-up problem. A theoretical approach and two experimental campaigns

The aim of this contribution is to present a theoretical approach and two experimental campaigns (on wind tunnel and on the track) concerning the research work about the ballast train-induced-wind erosion (BTIWE) phenomenon. When a high speed train overpasses the critical speed, it produces a wind speed close to the track large enough to start the motion of the ballast elements, eventually leading to the rolling of the stones (Kwon and Park, 2006) and, if these stones get enough energy, they can jump and then initiate a saltation-like chain reaction, as found in the saltation processes of soil eolian erosion (Bagnold, 1941). The expelled stones can reach a height which is larger than the lowest parts of the train, striking them (and the track surroundings) producing considerable damage that should be avoided. There is not much published work about this phenomenon, in spite of the great interest that exists due to its relevant applications in increasing the maximum operative train speed. Particularly, the initiation of flight of ballast due to the pass of a high speed train has been studied by Kwon and Park (2006) by performing field and wind tunnel experiments.

Hysteresis phenomena in transverse galloping of triangular cross-section bodies

Transverse galloping is a type of aeroelastic instability characterised by large amplitude, low frequency oscillation of a structure in the direction normal to the mean wind direction. It normally appears in bodies with small stiffness and structural damping, provided the incident flow velocity is high enough. In the simplest approach transverse galloping can be considered as a one-degree-of-freedom oscillator subjected to aerodynamic forces, which in turn can be described by using a quasi-steady description. In this frame it has been demonstrated that hysteresis phenomena in transverse galloping is related to the existence of inflection points in the curve giving the dependence with the angle of attack of the aerodynamic coefficient normal to the incident flow. Aiming at experimentally checking such a relationship between these inflection points and hysteresis, wind tunnel experiments have been conducted. Experiments have been restricted to isosceles triangular cross-section bodies, whose galloping behaviour is well documented. Experimental results show that, according to theoretical predictions, hysteresis takes place at the angles of attack where there are inflection points in the lift coefficient curve, provided that the body is prone to gallop at these angles of attack.

Experimental investigation on box-wing configuration for UAS

The range for airframe configurations available for UAS is as diverse as those used for manned aircraft and more since the commercial risk in trying unorthodox solutions is less for the UAS manufacturer. This is principally because the UAS airframes are usually much smaller than the manned aircraft and operators are less likely to have a bias against unconventional configurations. One of these unconventional configurations is the box-wing, which is an unconventional solution for the design of the new UAS generation. The existence of two wings separated in different planes that are, however, significantly close together, means that the aerodynamic analysis by theoretical or computational methods is a difficult task, due to the considerable interference existing. Considering the fact that the flight of most UAS takes place at low Reynolds numbers, it is necessary to study the aerodynamics of the box wing configuration by testing different models in a wind tunnel to be able to obtain reasonable results. In the present work, the study is enhanced by varying not only the sweepback angles of the two wings, but also their position along the models’ fuselage. Certain models have shown being more efficient than others, pointing out that certain relative positions of wing exists that can improve the aerodynamics efficiency of the box wing configuration.

A parametric, experimental analysis of conical vortices on curved roofs of low-rise buildings

Different methods to reduce the high suction caused by conical vortices have been reported in the literature: vertical parapets, either solid or porous, placed at the roof edges being the most analysed configuration. Another method for alleviating the high suction peaks due to conical vortices is the use of some non-standard parapet configuration like cantilever parapets. In this paper the influence of roof curvature on the conical vortex pattern appearing on a curved roof (Fig. 1) when subject to oblique winds is experimentally analysed by testing the mean pressure distribution on the curved roofs of low-rise building models in a wind tunnel. Also, the efficiency of cantilever parapets to reduce mean suction loads on curved roofs is experimentally checked. Very high suction loads have been measured on curved roofs, the magnitude of these high suction loads being significantly decreased when cantilever parapets are used. Thus, the suitability of these parapets to reduce wind pressure loads on curved roofs is demonstrated.

Fourier analysis of the aerodynamic behavior of cup anemometers

The calibration results (the transfer function) of an anemometer equipped with several cup rotors were analyzed and correlated with the aerodynamic forces measured on the isolated cups in a wind tunnel. The correlation was based on a Fourier analysis of the normal-to-the-cup aerodynamic force. Three different cup shapes were studied: typical conical cups, elliptical cups and porous cups (conical-truncated shape). Results indicated a good correlation between the anemometer factor, K, and the ratio between the first two coefficients in the Fourier series decomposition of the normal-to-the-cup aerodynamic force

Investigación experimental del flujo aerodinámico sobre helipuertos embarcados

La investigación del flujo aerodinámico sobre helipuertos embarcados se encuentra estrechamente relacionada con la operación segura de las aeronaves, pues las condiciones del flujo que tiene lugar en ese entorno pueden exceder los límites para los que están certificadas dichas aeronaves. El ambiente aerodinámico en las inmediaciones de un barco es altamente complejo y se encuentra influenciado por gran número de factores (chimeneas, antenas, mástiles, etc.) relacionados con la configuración específica del propio barco. El flujo objeto de investigación corresponde a la estela que se desarrolla sobre la cubierta de vuelo de una fragata, el cual está fuertemente influenciado por la superestructura de la misma, y que cualitativamente es similar al flujo que tiene lugar entre edificios altos o helipuertos situados en áreas urbanas, pues comprende estructuras tipo caja, con bordes afilados, que generan flujos tridimensionales altamente turbulentos. En esta Tesis se aborda el estudio del problema desde el punto de vista experimental, mediante simulación en túnel aerodinámico y medida de las variables del campo fluido sobre maquetas de fragatas a escala reducida. Las herramientas empleadas para tal cometido, han sido técnicas experimentales, tales como la visualización del flujo, la velocimetría láser por imágenes de partículas, la anemometría láser Doppler y los scanners electrónicos de presión, que han permitido investigar el flujo problema con objeto de obtener información, y adquirir así, un conocimiento más profundo de dicho flujo. La explotación de este conocimiento, ha dado lugar al diseño de una nueva solución, basada en la modificación de geometría básica de la fragata, por medio del cambio de la curvatura del techo del hangar, permitiendo suavizar el escalón descendente que se produce aguas abajo del mismo. Las geometrías modificadas han sido ensayada en túnel mediante la misma metodología empleada para la fragata original, de modo que, ha podido establecerse un análisis comparativo, para valorar la efectividad de la solución propuesta, el cual ha mostrado resultados satisfactorios, retirando el flujo adverso de la zona de operación de helicópteros y desplazándolo hacia el hangar, donde resulta menos peligroso, de modo que se reduce la carga del piloto y los riesgos de accidente durante las operaciones a bordo de embarcaciones. ABSTRACT The investigation of aerodynamic flow above the ship’s heliports is directly related to the aircraft safe operation, because the environment flow conditions may exceed the aircraft certification limits. Aerodynamic ship’s environment is highly complex and it is influenced by a large number of factors (stacks, antennae, masts, …) related to each specific ship configuration. The flow under investigation occurs into the wake produced above the flight deck of a frigate, that is strongly influenced by the superstructure. This flow is similar to one produced around tall buildings or heliports located in urban areas, thus in both of them, the air is flowing around sharp-edges box-like structures, producing three-dimensional and highly turbulent flows. This Thesis studies the problem from an experimental point of view, by means of wind tunnel simulations and measurements of the flow field around reduced scale frigates models. Tools used in this work are the experimental techniques, as flow visualization, particle image velocimetry, laser Doppler anemometry and pressure electronic scanners. These techniques provide information about the flow in order to obtain a more complete insight of this kind of flows. The exploitation of this insight is used for the design of a new flow control concept, based on the modification of the basic frigate geometry. This new design consists in the hangar roof curvature modification that produces a smoothing of the descendent step located downstream the hangar. Modified geometries are tested in wind tunnel by means of the same methodology as the original frigate, thus a comparative analysis is established in order to perform an assessment of effectiveness. This analysis has shown good results in displacing the adverse flow from the helicopter operation path to the nearest hangar region, reducing the pilot load and the accident risks during on board operations.

A Spectral Analysis of LDA Turbulent Flow Measurements in a Ship Air Wake

Helicopters are one of the most important tactical elements in maritime operations. The necessity for an improvement in the conditions in which the landing and take-off operations are carried out leads to the study of the flow that separates from the ship?s superstructure over the flight deck. To investigate this flow a series of wind tunnel experiments have been performed by testing a sub-scale model of a generic frigate. Measurements of the flow?s velocity have been taken by means of Laser Doppler Anemometry (LDA) in five points that simulate the last path of the landing trajectory. The data obtained in these experiments is manipulated in a frequency analysis where the corresponding spectra are calculated. Onboard measurements from an actual full scale frigate are analyzed and compared with the wind tunnel results. Conclusions obtained consist of a series of illustrative values of turbulent energy frequency ranges which can be valuable for any study in this field. The comparison shows a clear similarity between both experiments, reasserting the wind tunnel measurements and its reliability.

Transverse galloping of two-dimensional bodies having a rhombic cross-section

Transverse galloping is a type of aeroelastic instability characterized by oscillations perpendicular to wind direction, large amplitude and low frequency, which appears in some elastic two-dimensional bluff bodies when they are subjected to an incident flow, provided that the flow velocity exceeds a threshold critical value. Understanding the galloping phenomenon of different cross-sectional geometries is important in a number of engineering applications: for energy harvesting applications the interest relies on strongly unstable configurations but in other cases the purpose is to avoid this type of aeroelastic phenomenon. In this paper the aim is to analyze the transverse galloping behavior of rhombic bodies to understand, on the one hand, the dependence of the instability with a geometrical parameter such as the relative thickness and, on the other hand, why this cross-section shape, that is generally unstable, shows a small range of relative thickness values where it is stable. Particularly, the non-galloping rhombus-shaped prism?s behavior is revised through wind tunnel experiments. The bodies are allowed to freely move perpendicularly to the incoming flow and the amplitude of movement and pressure distributions on the surfaces is measured.

On the harmonic analysis of cup anemometer rotation speed: A principle to monitor performance and maintenance status of rotating meteorological sensors

The calibration results of one anemometer equipped with several rotors, varying their size, were analyzed. In each case, the 30-pulses pert turn output signal of the anemometer was studied using Fourier series decomposition and correlated with the anemometer factor (i.e., the anemometer transfer function). Also, a 3-cup analytical model was correlated to the data resulting from the wind tunnel measurements. Results indicate good correlation between the post-processed output signal and the working condition of the cup anemometer. This correlation was also reflected in the results from the proposed analytical model. With the present work the possibility of remotely checking cup anemometer status, indicating the presence of anomalies and, therefore, a decrease on the wind sensor reliability is revealed.

On the effects of windbreaks on the aerodynamic loads over parabolic solar troughs

Parabolic reflectors, also known as parabolic troughs, are widely used in solar thermal power plants. This kind of power plants is usually located on desert climates, where the combined action of wind and dust can be of paramount importance. In some cases it becomes necessary to protect these devices from the joined wind and sand action, which is normally accomplished through solid windbreaks. In this paper the results of a wind tunnel test campaign, of a scale parabolic trough row having different windward windbreaks, are reported. The windbreaks herein considered consist of a solid wall with an upper porous fence. Different geometrical configurations, varying the solid wall height and the separation between the parabolic trough row and the windbreak have been considered. From the measured time series, both the mean and peak values of the aerodynamic loads were determined. As it would be expected, mean aerodynamic drag, as well as peak values, decrease as the distance between the windbreak and the parabolic increases, and after a threshold value, such drag loads increase with the distance.

Analysis of air flow past and through the 2415-3S airfoil for an unmanned aerial vehicle with internal propulsion system

This paper deals with the prediction of velocity fields on the 2415-3S airfoil which will be used for an unmanned aerial vehicle with internal propulsion system and in this way analyze the air flow through an internal duct of the airfoil using computational fluid dynamics. The main objective is to evaluate the effect of the internal air flow past the airfoil and how this affects the aerodynamic performance by means of lift and drag forces. For this purpose, three different designs of the internal duct were studied; starting from the base 2415-3S airfoil developed in previous investigation, basing on the hypothesis of decreasing the flow separation produced when the propulsive airflow merges the external flow, and in this way obtaining the best configuration. For that purpose, an exhaustive study of the mesh sensitivity was performed. It was used a non-structured mesh since the computational domain is three-dimensional and complex. The selected mesh contains approximately 12.5 million elements. Both the computational domain and the numerical solution were made with commercial CAD and CFD software, respectively. Air, incompressible and steady was analyzed. The boundary conditions are in concordance with experimental setup in the AF 6109 wind tunnel. The k-e model is utilized to describe the turbulent flow process as followed in references. Results allowed obtaining velocity contours as well as lift and drag coefficients and also the location of separation and reattachment regions in some cases for zero degrees of angle of attack on the internal and external surfaces of the airfoil. Finally, the selection of the configuration with the best aerodynamic performance was made, selecting the option without curved baffles.