1,721,014 research outputs found
Oscillatory boundary layer close to a rough wall
No full textThe flow field, generated by an oscillating pressure gradient close to a rough wall, is investigated by means of direct numerical simulations of Navier-Stokes and continuity equations. The wall roughness consists of semi-spheres regularly placed on a plane wall. A comparison of the obtained results with the experimental measurements of Keiller and Sleath [D.C. Keiller, J.F.A. Sleath, Velocity measurements close to a rough plate oscillating in its own plane, J. Fluid Mech. 73 (1976) 673-691] supports the numerical findings. As in Keiller and Sleath [D.C. Keiller, J.F.A. Sleath, Velocity measurements close to a rough plate oscillating in its own plane, J. Fluid Mech. 73 (1976) 673-691], a secondary peak in the streamwise velocity component is observed close to flow reversal and the peak is shown to be generated by the coherent vortex structures which are shed by the roughness elements. The flow is found to be dominated by the shear layers which form at the top of the roughness elements during the accelerating phases of the cycle and by the horse-shoe vortices which form close to the base of the semi-spheres. The dynamics of the shear layers and of the horse-shoe vortices is found to have a relevant influence on the pressure distribution and on the force exerted by the fluid on the roughness elements. The obtained results shed light to the mechanism by which the sediment is picked-up from the bottom by the action of sea waves. © 2008 Elsevier Masson SAS. All rights reserved
Direct numerical simulation of an oscillatory boundary layer close to a rough wall
No full textIn the present contribution, the boundary layer generated close to a rough wall by an oscillatory, uniform pressure gradient is studied. The flow simulates the boundary layer generated at the sea bottom by a monochromatic propagating wave. The problem is tackled by numerical means and detailed information on flow dynamics is obtained. In particular, the evolution of vorticity is considered and the coherent vortex structures which are formed within the boundary layer are identified. The force exerted by the fluid on the bed and on the roughness elements is computed along with its pressure and viscous components
DNS del flusso oscillante su una parete scabra
No full textNel presente lavoro si affronta lo studio del campo di moto di un fluido che oscilla
al di sopra di una parete scabra. Sono state effettuate delle simulazioni numeriche dirette delle
equazioni di Navier-Stokes con l’ausilio della tecnica dei “contorni immersi” (Fadlun et al., 2001)
per imporre le condizioni al contorno sulla parete. Il metodo numerico è stato ampiamente testato
e sono stati effettuati confronti con risultati sperimentali. I risultati delle simulazioni numeriche
effettuate hanno consentito un’indagine approfondita del campo di moto e in particolare della
dinamica delle strutture vorticose coerenti. E’ stata inoltre analizzata la distribuzione spaziale e
temporale delle tensioni scambiate tra il fluido e la parete. Mantenendo fissa la geometria della
parete sono state effettuate simulazioni per diversi valori del numero di Reynolds.In this paper the flow field generated by a longitudinal pressure gradient which
oscillates periodically above a rough wall has been studied. Direct numerical simulations of the
Navier-Stokes equation have been performed for different values of the Reynolds number, keeping
the geometry of the wall fixed. In order to force the boundary condition on the rough wall, the
“immersed boundaries” tecnique (Fadlun et al., 2001) has been adopted. The numerical method
has been tested and the results have been compared with experimental data. The obtained results
concern the flow field and the coherent vortex structures. Moreover the spatial and the temporal
distribution of the stresses on the bed is described
Simulazione numerica del flusso oscillante su una parete scabra
No full textNell’articolo è affrontato lo studio del flusso oscillante su una parete coperta da una scabrezza
regolare. La scabrezza considerata è costituita da semisfere disposte su una parete piana secondo una
matrice esagonale. Le equazioni che reggono il moto del fluido sono state risolte numericamente su
una griglia cartesiana. Le condizioni al contorno sulla parete sono state imposte utilizzando la tecnica
dei contorni immersi (Fadlun et al., 2000). Il metodo numerico è stato validato riproducendo i risultati
sperimentali di Keiller & Sleath, (1976). E’ analizzato l’andamento temporale delle strutture vorticose
e delle forze indotte sulla parete dalle oscillazioni del fluido. Il lavoro numerico condotto contribuisce
a chiarire alcuni aspetti dei lavori sperimentali presenti in letteratura (Keiller & Sleath, 1976; Sleath,
1987; Jensen et al., 1989) e risulta essere un potente strumento di indagine nello studio di questo tipo
di flusso
Simplified prediction model of the discharging time of a shell-and-tube LHTES
No full textWe present a simplified theoretical model able to predict the discharging performance of a shell-and-tube latent heat thermal energy storage. The model is validated against two-dimensional axi-symmetric numerical simulations. Here, the heat exchange area, A, the whole PCM volume, V, and the heat exchange wall temperature have been kept constant. According to these constraints, the shell-and-tube shape depends on just one geometrical parameter. Thus, six values of the ratio between the external and internal radius of the PCM module, re/ri, in the range between 2 and 6, are considered. The simplified model matches the discharging time predicted by the numerical simulations. Details of the sensible and the latent heat contribution to the discharging time is provided with respect to the radius ratio. Hence, the latent heat contribution represents about the 70% of the overall discharging time in the range 2⩽re/ri⩽6. The results reveal a scaling law between the Fourier number related to the complete solidification and the radius ratio, Fo∝(re/ri)-5.5, supported by the numerical simulations. Moreover, the rescaled dimensionless time, Fo/(re/ri)-5.5, leads to the self-similar behaviour of the liquid fraction βl time series for all the geometries here investigated. Thus, it represents a promising prediction tool for the design of latent heat thermal energy storage in shell-and-tube configuration
Multi-objective sensitivity analysis of shell-and-tube LHTES performance
Full text linkIn the present paper a sensitivity analysis has been carried out concerning the charging/discharging time and the stored energy performances of a shell-and-tube LHTES with respect to the number of tubes and the tube internal radius. The aim of this analysis is to investigate how the design variables affect the LHTES performance. this could lead to determine the thermal storage optimal design. Thus, the sensitivity analysis has a key role in the selection of several acceptable solutions. The considered LHTES exhibits a cylindrical shell geometry characterized by constant height and diameter. This aspect has allowed to employ simplified theoretical models able to predict the charging/discharging time and the stored energy performance. These models consider a constant heat exchange wall temperature whereas the heat exchange area and the whole PCM volume vary according to the design variables. This analysis represents the first step to solve the multi-objective optimization of the thermal storage design problem and then to determine the best solutions in both design variables and thermal storage performance domains
Direct numerical simulation of an oscillatory boundary layer close to a rough wall
No full textIn the present contribution, the boundary layer generated close to a rough wall by an oscillatory, uniform pressure gradient is studied. The flow simulates the boundary layer generated at the sea bottom by a monochromatic propagating wave. The problem is tackled by numerical means and detailed information on flow dynamics is obtained. In particular, the evolution of vorticity is considered and the coherent vortex structures which are formed within the boundary layer are identified. The force exerted by the fluid on the bed and on the roughness elements is computed along with its pressure and viscous components
Flow patterns and heat transfer around six in-line circular cylinders at low Reynolds number
No full textThe flow field and the heat transfer around six in-line iso-thermal circular cylinders have been studied by means of numerical simulations. Two values of the center to center spacing "" and 4d, where dis the cylinder diameter) at Reynolds number of 100 and Prandtl number of 0.7 have been investigated. Similarly to the in-line two cylinders configuration, in this range, a transition in the flow and in the heat transfer occurs. Two different flow patterns have been identified: the stable shear layer (SSL) mode and the shear layer secondary vortices (SLSV) mode, at 3.6 and 4 spacing ratios respectively. At "" the flow pattern causes the entrainment of cold fluid on the downstream cylinders enhancing the heat transfer. On the other hand, at "" two stable opposite shear layers prevent the cold fluid entrainment over the downstream cylinders reducing their heat exchange. The overall time average heat transfer of the array is enhanced up to 25% decreasing the spacing ratio from 4 to 3.6. Furthermore, it is found that the increased heat transfer is related to the phase shift between the Nusselt time series of successive cylinders
Flow transitions around in line cylinders
No full textThe flow around aligned cylinders is an archetype for several
industrial devices (rod structure of the nuclear reactors, compact heat
exchangers for electronic components, pin-fins heat exchangers for
micro-devices ) and environmental phenomena (diffusion process close to the
vegetation). Cylinders produce instabilities in the flow structures that are very
sensitive to the control parameters such as the inflow velocity, the spacing
between the cylinders and the fluid viscosity. The instabilities leads the
transport phenomena close to the cylinders and they affect the force, the
thermal balance at their surface and the diffusion process. The strong velocity
gradients in confined spaces make that the experimental analysis is difficult,
while the numerical simulation appears to be a promising tool for this purpose
Simplified theoretical model to predict the melting time of a shell-and-tube LHTES
No full textA theoretical model to predict the complete melting time of a Latent Heat Thermal Energy Storage (LHTES) considering a shell-and-tube configuration is presented and applied to several geometries. The shape of the device has been modified according to the internal radius (r i ), external radius (r e ) and total height (L) retaining constant the volume of the storage and the heat exchange area. The model has been validated by means of multiphase numerical simulations of the charging phase. The numerical simulations have been performed considering four configurations, with a radii ratio, r e /r i , equal to 1.5, 2, 4.375 and 6. The comparison between the model predictions and the numerical simulations confirms the reliability of the theoretical model in terms of melting time within the range investigated. The study reveals that, even considering the same storage volume, heat exchange area and wall temperature, for low values of radii ratio (r e /r i ), the shape of the device is able to reduce the charging time of the LHTES up to 50% for a radii ratio r e /r i =1.5 with respect to r e /r i =4.375. Increasing further the radii ratio from r e /r i =4.375 to 6, the melting time decreases. The unsteady numerical simulations support the prediction of the theoretical model. Thus, in the here studied geometrical configurations the proposed approach represents a simplified and accurate design tool to predict the charging time of a LHTES shell-and-tube device
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