The role of particles heavier than the fluid (glass spheres in water) in a turbulent open channel flow over a smooth bed is examined at volume concentration about ), as is the Reynolds stress. These findings can be explained if they are referred to the mechanism of particle entrainment and deposition, which takes place close to the wall. This mechanism is related to particle inertia and to the dynamic of the structure of near-wall turbulence, which connects the buffer and outer regions with the very near-wall region. A significant momentum exchange between the two phases, which is particularly effective in the buffer region, is revealed by the quadrant analysis of the Reynolds stresses.
Abstract In this preliminary work, the feasibility of the combination of a volumetric velocimetry technique such as Defocusing Particle Image Velocimetry and a particle phase-discrimination methodology based on ridge detection algorithm for the analysis of turbulent multiphase flows with non-spherical fiber-like particles is discussed. Experimental results of a dilute suspension of fibers in an open-channel apparatus are provided.
An experimental study was conducted to investigate the effect of ice accretions on the aerodynamic characteristics of a 100 mm-chord NACA 0012 airfoil section. Four different configurations of the airfoil model were considered: clean profile and profile with glaze, rime and mixed ice-accretion. shapes. The considered shapes were derived from measurements performed at the NASA Lewis Icing Research Tunnel (IRT). Tests were performed in the closed test section of an open circuit calibration tunnel: the chord Reynolds number was about 200000. Velocity field was measured by means of PIV technique on the clean and mixed airfoils, whereas aerodynamic coefficients for all the different ice accretion shapes were measured as a function of airfoil incidence, by means of a three component balance. The experimental results show remarkable aerodynamic characteristics decay due to the simulated ice formation: glaze configuration shows worst performances with inversion of the lift-incidence curve and a dramatic increase of the drag coefficient. PIV measurements show large regions of separated flow even at low incidence and for moderate amount of ice (mixed shape): in fact, due to the low chord Reynolds number, no flow reattachment occurs downstream the separation.
This study investigates experimentally a heated U-shaped mini-channel heat sink using Infrared Thermography and Particle Image Velocimetry for a water coolant flow of Reynolds numbers of 280, 650, and 1350 (based on the hydraulic channel diameter). The choice of this cell geometry is based on its role as a simplified unit of a serpentine heat exchanger, which is proved to be one of the most promising for cooling processes. The use of the infrared camera allows the detection of temperature fields on top of the external surface of the cell. Therefore, aiming to derive the temperature distribution on the channel roof, a dedicated transfer function is implemented. Moreover, we employed the lumped capacitance model for thermal analysis on both infrared measurements and thermocouple data. The latter are recorded to capture the cooling process of the aluminium base and water temperature at the end of the outlet tube. As a result, thermal transient rate, cooling magnitude and equilibrium temperature are obtained. These parameters indicate that higher Reynolds numbers correspond to increased thermal transient rates, enhanced cooling effects, and lower equilibrium temperatures. A non-uniform distribution of heat transfer along the channel is reported, with the most efficient cooling area localized close to the first 90-degree corner. These findings are consistent with numerical simulations and previous experimental observations. PIV results reveal the presence of two fluid acceleration zones following both 90-degree corners, which contribute to improve the water cooling ability in their respective regions. Additionally, the formation of two recirculating bubbles is reported at the inner wall from corners vertices, whose intensity is dependent on Reynolds number, pushing the main flow towards the outer channel wall and reducing the local heat transfer. Turbulent kinetic energy distributions are also investigated, pointing out the presence of intense areas that better match with regions of minimum equilibrium temperature than maximum velocity zones. This suggests the presence of local turbulent unsteadiness and three-dimensional phenomena, which contributes significantly to cooling enhancement.
We propose a model of the kinetics of reversible breakdown in metal-insulator-metal structures with afnia based on the growth of fractal patterns of defects when the insulator is subject to an external voltage. The probability that a defect is (or is not) generated and the position where it is generated depend on the electric field distribution. The new defect moves accordingly to fractal rules and attach to another defect in a tree branch. When the two electrodes sandwiching the insulating film are connected a conductive filament is formed and the breakdown takes place. The model is calibrated with experiments inducing metastable soft breakdown events in Pt/HfO 2 /Pt capacitors.
The departure from Kolmogorov (1941) scaling of high-order moments of longitudinal velocity increments in a fully developed turbulent channel flow is investigated using data obtained with hot wire anemometry (HWA), laser Doppler velocimetry (LDV) and direct numerical simulation (DNS). The magnitude of the departure increases towards the wall, reaching a local maximum at a distance of about 20 wall units.
