Heat Transfer Mechanism In Particle-Laden Turbulent Shearless Flows

Particle-laden turbulent flows are one of the complex flow regimes involved in a wide range of environmental, industrial, biomedical and aeronautical applications. Recently the interest has included also the interaction between scalars and particles, and the complex scenario which arises from the interaction of particle finite inertia, temperature transport, and momentum and heat feedback of particles on the flow leads to a multi-scale and multi-physics phenomenon which is not yet fully understood. The present work aims to investigate the fluid-particle thermal interaction in turbulent mixing under one-way and two-way coupling regimes. A recent novel numerical framework has been used to investigate the impact of suspended sub-Kolmogorov inertial particles on heat transfer within the mixing layer which develops at the interface of two regions with different temperature in an isotropic turbulent flow. Temperature has been considered a passive scalar, advected by the solenoidal velocity field, and subject to the particle thermal feedback in the two-way regime. A self-similar stage always develops where all single-point statistics of the carrier fluid and the suspended particles collapse when properly re-scaled. We quantify the effect of particle inertial, parametrized through the Stokes and thermal Stokes numbers, on the heat transfer through the Nusselt number, defined as the ratio of the heat transfer to the thermal diffusion. A scale analysis will be presented. We show how the modulation of fluid temperature gradients due to the statistical alignments of the particle velocity and the local carrier flow temperature gradient field, impacts the overall heat transfer in the two-way coupling regime

An experimental study of near wall flow parameters in the blade end-wall corner region

The near wall flow parameters in the blade end-wall corner region is investigated. The blade end-wall corner region was simulated by mounting an airfoil section (NACA 65-015 base profile) symmetric blades on both sides of the flat plate with semi-circular leading edge. The initial 7 cm from the leading edge of the flat plate was roughened by gluing No. 4 floor sanding paper to artificially increase the boundary layer thickness on the flat plate. The initial flow conditions of the boundary layer upstream of the corner region are expected to dictate the behavior of flow inside the corner region. Therefore, an experimental investigation was extended to study the combined effect of initial roughness and increased level of free stream turbulence on the development of a 2-D turbulent boundary layer in the absence of the blade. The measurement techniques employed in the present investigation included, the conventional pitot and pitot-static probes, wall taps, the Preston tube, piezoresistive transducer and the normal sensor hot-wire probe. The pitot and pitot-static probes were used to obtain mean velocity profile measurements within the boundary layer. The measurements of mean surface static pressure were obtained with the surface static tube and the conventional wall tap method. The wall shear vector measurements were made with a specially constructed Preston tube. The flush mounted piezoresistive type pressure transducer were employed to measure the wall pressure fluctuation field. The velocity fluctuation measurements, used in obtaining the wall pressure-velocity correlation data, were made with normal single sensor hot-wire probe. At different streamwise stations, in the blade end-wall corner region, the mean values of surface static pressure varied more on the end-wall surface in the corner region were mainly caused by the changes in the curvature of the streamlines. The magnitude of the wall shear stress in the blade end-wall corner region increased significantly in the close vicinity of the corner line. The maximum value of the wall shear stress and its location from the corner line, on both the surfaces forming the corner region, were observed to change along the corner. These observed changes in the maximum values of the wall shear stress and its location from the corner line could be associated with the stretching and attenuation of the horseshoe vortex. The wall shear stress vectors in the blade end-wall corner region were observed to be more skewed on the end-wall surface as compared to that on the blade surface. The differences in the wall shear stress directions obtained with the Preston tube and flow visualization method were within the range in which the Preston tube was found to be insensitive to the yaw angle

Improving Flat Plate Heat Transfer Using Flexible Rectangular Strips

Many engineering systems involve proper transfer of heat to operate. As such, augmenting the heat transfer rate can lead to performance improvement of systems such as heat exchangers and solar photovoltaics panels. Among the many existing and studied heat transfer enhancement techniques, a well-designed passive turbulence generator is a simple and potent approach to augmenting convective heat transfer. Two of the most recognized passive convective heat enhancers are wings and winglets. Their potency is attributed to the long-lasting induced longitudinal vortices which are effective in scooping and mixing hot and cold fluids. Somewhat less studied are flexible turbulence generators, which could further the heat transfer enhancement compared to their rigid counterpart. In the current study, the flexible rectangular strips are proposed, marrying the long-lasting vortex streets with the periodic oscillation, to maximize heat convection. This study was conducted in a closed-looped wind tunnel with 76 cm square cross-section. The effects of flexible strips on the turbulent flow characteristics and the resulting convective heat transfer enhancement from a heated flat surface are detailed in four papers which are presented as Chapters 3, 4, 5 and 6. In Chapter 3, the effect of the thickness of the strip is detailed. The 12.7 mm wide and 38.1 mm tall rectangular strip was cut from an aluminum sheet with thickness of 0.1, 0.2 and 0.25 mm. The incoming wind velocity was maintained at around 10 m/s, giving a Reynolds number based on the strip width of 8500. It is observed that the thinnest 0.1 mm strip could induce a larger downwash velocity and a stronger Strouhal fluctuation at 3H (strip height) downstream, leading to a better heat transfer enhancement. The peak of the normalized Nusselt number (Nu/Nu0) at 3H downstream of the 0.1 mm strip was around 1.67, approximately 0.1 larger than that of the 0.25 mm strip. In Chapter 4, the height effect of the strip is disclosed. The strip was 12.7 mm wide and 0.1 mm thick, with a height of 25.4 mm, 38.1 mm and 50.8 mm. The Reynolds number in this chapter was also fixed at around 8500, based on the strip width and the freestream velocity. It was found that the shortest, 25.4 mm strip could induce the closest-to-wall swirling vortices, and the largest near-surface downwash velocity toward the heated surface. Thus, the largest heat transfer augmentation was observed. At 9W (strip width) downstream, the 25.4 mm-strip provided the Nu/Nu0 peak of around 1.76, 0.26 larger than that associated with the tallest, 50.8 mm-strip. In Chapter 5, the effect of the transversal space of a pair of strips is expounded. A pair of 0.1 mm thick, 12.7 mm wide, and 25.4 mm tall aluminum rectangular flexible strips was placed side-by-side with a spacing of 1W (strip width), 2W and 3W. The Reynolds number based on the strip width was around 8500. The results showed that the 1W-spaced strip pair induced the strongest vortex-vortex interaction, the largest downwash velocity, and the most intense turbulence fluctuation. These resulted in the most effective heat convection. At Y=0 (middle of the strip pair) and X=9W, the largest Nu/Nu0 value of around 1.50 was identified when using the 1W-spaced strip pair. This was approximately 0.24 and 0.33 larger than that of the 2W- and 3W-spaced strip pairs. Chapter 6 presents the effect of freestream turbulence on the flat plate heat convection enhancement with a 12.7 mm wide, 25.4 mm tall and 0.1 mm thick flexible strip. A 6 mm thick sharp-edged orificed perforated plate (OPP) with holes of 38.1 mm diameter (D) was placed at 10D, 13D and 16D upstream of the strip to generate the desirable levels of freestream turbulence. The corresponding streamwise freestream turbulence intensity at the strip was around 11%, 9% and 7%. The Reynolds number based on the strip width and freestream velocity was approximately 6000. The freestream turbulence was found to diminish the effect of flexible strip in terms of the relative heat transfer enhancement (Nu/Nu0). This is due to the significant increase of Nu0 with the increasing freestream turbulence. In other words, the flexible strip could always improve the heat transfer, and the relative improvement is greatest for the largely laminar freestream case in the absence of the OPP. Chapter 7 summarizes the effect of all the parameters in previous chapters on the convective heat transfer enhancement. The results show that the freestream turbulence intensity (Tu) had the most significant effect in augmenting the averaged Nu/Nu0, and the local Nu/Nu0 correlated best with the local ke. The maximal averaged Nu/Nu0 over 23W downstream, within ±1 and ±4 strip widths cross-stream was found for Tu=7% case and Tu=11% case, respectively. Conclusions are drawn and recommendations are provided in Chapter 8

Wind influence on plants: ecophysiological, biomechanical, and aerodynamic aspects

Wind has a wide range of effects on plants: from wind-induced damage in the form of lodging to an increased rate of photosynthesis. Climate models predict that plants will be exposed to elevated wind conditions in many regions around the world due to global warming. To be able to attenuate the negative impact of changing wind patterns on plants, it is necessary to expand our understanding of wind-plant interaction. In this thesis two different aspects of wind-plant interaction are investigated. In the first part, the acclimation response of a model plant, Arabidopsis thaliana, to mechanical stress in the form of continuous wind of a constant speed is explored. A bespoke wind tunnel, suitable for continuous growth of plants, was developed. For the mechanical characterisation of Arabidopsis stems a new multiple resonant frequency method was devised and validated. As a result of the wind treatment, the plants exhibited a positive anemotropic response. This response was documented for the first time in any plant system. Overall, the wind-induced thigmomorphogenetic changes and alterations in the mechanical properties of the primary inflorescence stem were considered to be adaptive to this type of mechanical stress. The mechanical properties can be related to modification in the anatomical tissue organisation and ion content, providing possible sources of the observed changes. In many experiments reported in the literature, wind is mimicked by brushing treatments. In this project, the validity of this approach was investigated by conducting a comparison of the response of Arabidopsis to uni- and bidirectional brushing treatments with the wind-induced changes. While some of the changes to Arabidopsis morphology can be reproduced by matching the vectorial influence of wind and brushing treatments, the changes in the mechanical properties occurred in opposite directions. The unidirectional brushing treatment also evoked a positive tropic response, which can be considered thigmotropic and has been demonstrated for the first time in Arabidopsis shoot. The second part of this thesis explores the aerodynamics of succulent-inspired cylinders using numerical and experimental techniques. Succulents and cacti are, probably, the most well-known examples of convergent evolution in plants, where two different species have independently developed similar traits and features in similar environments but in different parts of the Earth. Investigations inspired by the Saguaro cactus reported in the literature, showed that the presence of ribs on its trunk, among other functions, helps to reduce the aerodynamic forces and decreases unsteady force fluctuations. In contrast to Saguaro, which can have up to 30 ribs, succulents tend to have a considerably lower number of ribs, with only three and four ribs found in many species. This work investigates whether succulent-inspired bluff bodies with a low number of ribs show similar aerodynamic benefits as cactus-shaped cylinders with many ribs. 2D URANS simulations were carried out for two cylinders with three and four ribs, which resemble the succulents Euphorbia trigona and Euphorbia abyssinica, respectively, at Reynolds number 20,000. The succulent-inspired cylinder with four ribs was also investigated using wind tunnel tests with Reynolds numbers ranging from 50,000 to 150,000. For both studied cylinders, a strong angle of attack dependence of all aerodynamic properties was found. For the four-ribbed cylinder no Reynolds number dependence of these properties was observed within the tested range. Overall, succulent-inspired cylinders with a low number of ribs show some aerodynamic advantages in terms of reduction of the mean drag coefficient and the Strouhal number, albeit over a limited range of angles of attack

Aeronautical engineering: A special bibliography, supplement 29, March 1973

This special bibliography lists 410 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1972

New Advances in Fluid Structure Interaction

Fluid–structure interactions (FSIs) play a crucial role in the design, construction, service and maintenance of many engineering applications, e.g., aircraft, towers, pipes, offshore platforms and long-span bridges. The old Tacoma Narrows Bridge (1940) is probably one of the most infamous examples of serious accidents due to the action of FSIs. Aircraft wings and wind-turbine blades can be broken because of FSI-induced oscillations. To alleviate or eliminate these unfavorable effects, FSIs must be dealt with in ocean, coastal, offshore and marine engineering to design safe and sustainable engineering structures. In addition, the wind effects on plants and the resultant wind-induced motions are examples of FSIs in nature. To meet the objectives of progress and innovation in FSIs in various scenarios of engineering applications and control schemes, this book includes 15 research studies and collects the most recent and cutting-edge developments on these relevant issues. The topics cover different areas associated with FSIs, including wind loads, flow control, energy harvesting, buffeting and flutter, complex flow characteristics, train–bridge interactions and the application of neural networks in related fields. In summary, these complementary contributions in this publication provide a volume of recent knowledge in the growing field of FSIs

Full Scale Servo-Actuated Morphing Aileron for Wind Tunnel Tests

Typically, aircraft roll control is accomplished by simultaneously moving ailerons together and in opposite angular direction. Nevertheless, throughout the flying range, more particularly in cruise conditions, it is highly desirable to increase aircraft aerodynamic performance by a differential control of the lift distribution over the wing span. Recent European design studies concerning morphing devices, such as the Clean Sky multifunctional flap or the SARISTU trailing edge device, have largely proved the potential of novel aircraft structural systems, aiming at adaptively modify the wing structural shape to reduce the induced drag penalty associated with off-design flight conditions. In particular, wing camber variation was achieved through adaptive wing trailing edges because of the highly associated L/D ratio enhancements. Such projects proved also the aileron region to be the one where higher cruise benefits could be achieved by local camber variations. Following the enthusiastic results, achieved with the Adaptive trailing edge device, a new challenge has been faced up. The former configuration did in fact refer to the standard position of the flap, leaving apart the aileron region. There are several reasons to leave that part unchanged. The most relevant may be associated to the fact that the aileron has a critical function in the aircraft flight and its collapse could lead to dramatic failures. The investigated configuration would have lied over an extended region of the aileron instead than a limited part, as in the case of a flap, characterised by a large chord. As a direct consequence, the available volumes are reduced and the installation of integrated actuators could have been a problem. Finally, the aeroelastic response of the device is critical as well and its strong modification should have been deeply studied. On the other hand, the studies on the ATED showed as the region, farer from the root, gave a more significant contribution to the aerodynamic behaviour. So, it was really interesting to investigate the possibility to extend the adaptive trailing edge technology to the aileron region. The occasion was given by a joint Italian/Canadian research activity fostered by the Consortium de Recherche et d’Innovation en Aerospatiale au Quebec (CRIAQ). The activity aimed at realising a full-scale demonstrator of a wing section in the tip region for investigating the capability of wing box and trailing edge morphing device, to ensure a certain level of flow control and aerodynamic performance variations, respectively. The first issue was in charge of the Canadian team (ETS, NRC, Thales Aerospace, Bombardier AS), while the Italian group (University of Naples and CIRA) aimed at realising a device for the aileron camber control. The enlisted problems were all evident at the very first approach. Volume limitation forced the designers to follow a different strategy. Instead of having a couple of actuators acting on each rib, the architectural layout was specialised per each single bay. At the aileron root this possibility was maintained, while the more external two bays were commanded by a single actuator. In other words, the last two segments were made of two slave and a master ribs, driven by a single actuator. Calculation showed as this configuration was able to maintain the specified loads. Aeroelastic studies confirmed the reliability of the device, in sense that the selected architecture was demonstrated to be safe in the design flight conditions. The adaptive aileron finally maintained the original capability while ensuring morphing characteristic. This target was accomplished by realising a device with two separate motor system. The first, acting on the main aileron shaft, to preserve its characteristic dynamic response for flight control. The second, acting on the rib, implemented the searched camber variations to follow the aerodynamic necessities related to fuel consumption. Another relevant point concerns the skin. In order to check the possibility of skipping the need of implementing a compliant solution, a heavy and sophisticated element, the single hinged blocks were properly shaped to slide one into the other like a meniscus. This solution was however strongly correlated to manufacture tolerances and the assembly precision, because small deviation could have had a significant impact on the kinematic performance. As usual, vantages and disadvantages try to compensate each other. The innovative device can be considered as a system with augmented capabilities aimed at working in cruise, by means of symmetric deflection, to obtain a near optimum wing geometry enabling optimal aerodynamic performance. The approach, including underlying concepts and analytical formulations, combines design methodologies and tools required to develop such an innovative control surface. A major difficulty in the development of morphing devices is to reach an adequate compromise between high load-carrying capacity to withstand aerodynamic loads and sufficient flexibility to achieve the target shapes. These targets necessitate the use of innovative structural and actuation solutions. When dealing with adaptive structures for lifting surfaces, the level of complexity naturally increases as a consequence of the augmented functionality of the designed system. In specific, an adaptive structure ensures a controlled and fully reversible transition from a baseline shape to a set of different configurations, each one capable of withstanding the associated external loads. To this aim, a dedicated actuation system shall be designed. In addition, the adopted morphing structural kinematics shall demonstrate complete functionality under operative loads. Such a morphing device wants to augment the former device by adapting local wing camber shape and lift distribution through a quasi-static deflection, its excursion ranging into few unit of degrees, positive and negative. In a morphing aircraft design concept, the actuated system stiffness, load capacity and integral volumetric requirements drive flutter, strength and aerodynamic performance. Design studies concerning aircraft flight speed, manoeuvre load factor and actuator response provide sensitivities in structural weight, aeroelastic performance and actuator flight load distributions. Based on these considerations, actuation mechanism constitutes a very fundamental aspect for adaptive structures design because the main prerequisite is to accomplish variable shapes within the physical constraints established by the appropriate actuation arrangement. This thesis addresses the design of a morphing aileron with a specific focus on the structural actuation system sizing and integration while the structural sizing was under Unina responsibility. Particular focus is given to the numerical validation of the entire aileron integrated with the actuation leverage by means of FE model and experimental tests campaign. The aileron actuation system is driven by load bearing servo-electromechanic rotary actuator in a distributed and un-shafted arrangement which combine load carrying and actuation capacities. The use of electro-mechanical actuators is coherent with a “more electric approach” for next-generation aircraft design. Such an actuation architecture allows the control of the morphing structure by using a reduced mass, volume, force and consumed power with respect to conventional solutions. Benefits are obvious. No hydraulic supply buses (easier to maintain and store without hydraulics leaks), improved torque control, more efficiency without fluid losses and elimination of flammable fluids. In addition, it is potentially possible to move individual ribs either synchronously or independently to different angles (twist) in order to enhance aerodynamic benefits during flight. On the other side, actuators susceptibility to jamming may represent the most important drawback that can be tested and prevented by means of an iron bird facility. Finally, the realised system was assembled onto a wing model and tested in a wind tunnel at the National Research Council (NRC) facilities in Ottawa (CAN). On the same model, the adaptive wing box was also installed. The adaptive aileron device proved its functionality in real flow conditions and the main aerodynamic results are herein presented and widely described. The developed device has a lot of further potentialities, that will be object of further works and publications and that are currently explored by the authors: for instance, by giving it a large bandwidth, it could be used as an additional load alleviation device for the outer wing in order to reduce peak loads for gusts. Moreover it can be tailored for active load control distribution in order to modify spanwise lift distribution obtaining a reduced wing root bending moment; in such a manner a lightweight design can be assessed

Three-dimensional turbulent flow past rectangular bluff bodies

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