1,721,037 research outputs found
Valutazione di conformità alla normativa EN 13544-1 2007+A1:2009 di una pompa volumetrica
No full textSono state eseguite le misure di portata di aria sulla pompa alternativa a cilindro e pistone da utilizzarsi per test secondo la normativa EN 13544-1 2007+A1:200
Il moto medio e la turbolenza in onde di gravità generate in una galleria del vento
No full textIn questa memoria si riportano e si analizzano alcuni esperimenti di laboratorio sulla generazione di onde di gravità per azione diretta del vento. Tali esperimenti si inquadrano in uno studio più ampio mirato alla quantificazioni degli effetti della turbolenza sull’interfaccia tra un liquido e un gas. Qui sono presentati solo i risultati di alcune misure eseguite in acqua. I profili della velocità media orizzontale sono logaritmici e, per fetch sufficientemente esteso, la turbolenza appare pienamente sviluppata. La velocità di attrito nello strato limite in acqua è di un ordine di grandezza più piccola rispetto a quella dello strato limite in aria ed è circa il doppio di quella normalmente misurata in strati limite turbolenti di parete. La maggiore intensità è attribuirsi al micro-frangimento delle onde
Experimental study on oscillating grid turbulence and free surface fluctuation
No full textThis paper analyses the interaction between the turbulence and free surface. The phenomenon takes place in many natural flows and industrial processes. In the present experiments, turbulence is generated by a vertically oscillating grid moving beneath the free surface. Fluid velocity has been measured through a hot film anemometer, and the free surface elevation has been measured by an ultrasonic sensor. Integral length scales and several turbulence estimators have been computed.
In order to detect the generation of turbulence near the free surface, the correlation between free surface elevation and the underneath flow velocity has been studied, as well as the time lag between turbulence and free surface.
The free surface dynamics has been characterised by a velocity scale and a length scale. The kinetic energy associated with the free surface fluctuations increases with the Reynolds number at a rate depending on the frequency of the grid movement. For
Reynolds number larger than 1000, however, the relationships collapse to a single curve characterized by a lower rate. The present experiments do not achieve the inertial sub-range in the vertical velocity fluctuations, and the estimated spectrum
decays with an exponent smaller than -3, which is the typical value for the twodimensional turbulence in the inertial sub-range.
The macro length scale, estimated by using the Taylor's frozen turbulence hypothesis, experiences a decay away from the grid, which follows reasonably well the profile of Thompson and Turner (1975). The micro length scale reduces immediately beneath the free surface, which can be interpreted by the increase of dissipation rate in the subsurface layer. The classification diagram by Brocchini and Peregrine (2001)
indicates that most tests fall in the weak turbulence domain, but some tests fall in the wavy domain. At certain grid strokes, resonance inside the subsurface water layer occurs. The vertical velocity fluctuations and the free surface level show a significant
correlation with a negative phase lag, i.e., the free surface fluctuations are ahead of the vertical velocity fluctuations
Analytical study of the water surface fluctuations induced by grid-stirred turbulence
No full textThe free surface dynamics excited by grid stirred turbulence is analysed with a Lagrangian approach, with the analytical model calibrated by using experimental results. The Lagrangian model for free surface oscillations has been developed by assuming that coherent structures in the water layer close to the free interface have the shape of blobs and are mobilized by a periodic force. The blobs are assumed to behave like nonlinear oscillators with different spring stiffness in upwelling and downwelling and neglecting viscosity as first approximation. In order to calibrate the model, a set of experiments in a grid stirred tank was used where free surface elevation time series were measured and all the necessary variables were computed.
Through calibration and with some working assumptions, the radius of the blobs is shown to be of the order of the integral vertical length scale of turbulence. The forced oscillations are analysed by the method of Rauscher (1938) to predict the amplitude of the oscillations, possible resonances and discontinuities in the amplitude-frequency space. If the forcing term has multiple harmonics, in some cases a blow up of the oscillator is forecast. Some tests including a small damping, in order to evaluate the effects of viscosity, show that a jump is expected in the amplitude-frequency diagram and a range of unstable frequency. The presence of a small damping should also prevent the blow up for multiple harmonics in the forcing term
Experimental analysis of the coherent structures and turbulence past a hydrofoil in stalling condition beneath a water-air interface
No full textIn this paper, measurements beneath the interface of the flow past a hydrofoil in water
during stalling conditions are used to analyse the mean flow and, in more depth, the large eddies and
turbulence. The properties of the time-averaged flow are analysed. In addition to the classical wake,
with a recirculation area near the low pressure side of the hydrofoil, a breaker develops due to the
limited distance between the hydrofoil and the interface, which causes the generation of further
vorticity. Vorticity develops in the recirculation area and at the trailing edge as well. The fluctuating
streamwise and transverse velocity demonstrate a classical distribution with maxima at the edge of the
wake and beneath the breaker. A quadrant analysis of the Reynolds shear stress indicates the main
fluxes of the momentum, which is always directed towards the axis of the wake to reduce the defect
velocity of the wake. The spatial correlations for the longitudinal and transverse fluctuating velocity
are computed at several points along the axis of the wake at four different locations downstream. The
correlation of the transverse fluctuating velocity shows an evident anisotropy, which is more extensive
in the transverse direction and is limited in the streamwise direction. In the streamwise direction, the
correlation of the transverse fluctuating velocity also assumes negative values. The correlation of the
longitudinal fluctuating velocity is significantly isotropic, and the associated length scale increases
downstream. The shape of this last correlation can be fitted with a limited number of eddies; hence, a
stable and repetitive pattern of eddies is predicted, as shown in the wavenumber longitudinal and
transverse spectra as well
Buoyancy transfer in a two-layer system in transient and steady state. Experiments in a wave flume
Get PDFPresentation in a conferenc
PROPAGATION OF GRAVITY CURRENTS OF NON-NEWTONIAN POWER-LAW FLUIDS IN POROUS MEDIA
Get PDFA comprehensive analytical and experimental framework is presented to
describe gravity-driven motions of rheologically complex fluids through
porous media. These phenomena are relevant in geophysical,
environmental, industrial and biological applications.
The fluid is characterized by an Ostwald-DeWaele constitutive equation with
behaviour index n. The flow is driven by the release of fluid at the origin of
an infinite porous domain. In order to represent several possible spreading
scenarios, we consider: i) different domain geometries: plane, radial, and
channelized, with the channel shape parameterized by ; ii) instantaneous
or continuous injection, depending on the time exponent of the volume of
fluid in the current, ; iii) horizontal or inclined impermeable boundaries.
Systematic heterogeneity along the streamwise and/or transverse direction
is added to the conceptualization upon considering a power-law
permeability variation governed by two additional parameters and .
Scalings for current length and thickness are derived in self similar form
coupling the modified Darcy’s law accounting for the fluid rheology with the
mass balance equation. The speed, thickness, and aspect ratio of the
current are studied as a function of model parameters; several different
critical values of emerge and govern the type of dependency, as well as
the tendency of the current to accelerate or decelerate and become thicker
or thinner at a given point. The asymptotic validity of the solutions is limited
to certain ranges of model parameters.
Experimental validation is performed under constant volume, constant and
variable flux regimes in tanks/channels filled with transparent glass beads
of uniform or variable diameter, using shear-thinning suspensions and
Newtonian mixtures. The experimental results for the length and profile of
the current agree well with the self-similar solutions at intermediate and late
times
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