8,172 research outputs found
Turbulent Mixing in Transverse Jets
Turbulent mixing is studied in liquid-phase transverse jets.
Jet-fluid concentration fields were measured using laser-induced fluorescence and digital-imaging techniques, for jets in the Reynolds number range 1000 <= Re <= 20,000, at a jet-to-freestream velocity ratio of 10. Analysis of the measured scalar fields indicates that turbulent mixing is Reynolds-number dependent, as manifest in the evolving probability density functions of jet-fluid concentration.
Enhanced homogenization is found with increasing Reynolds number. Turbulent mixing is also seen to be flow dependent, based on differences between jets discharging into a crossflow and jets into a quiescent reservoir. A novel technique for whole-field measurement of scalar increments was used to study the distribution of difference (scalar increments) of the scalar field. These scalar increments are found to tend toward exponential-tailed distributions with decreasing separation distance. Finally, the scalar field is found to be anisotropic, particularly at small length scales. This is seen in power spectra, directional scalar microscales, and directional PDFs of scalar increments. The local anisotropy of the scalar field is explained in terms of the global dynamics and large-scale strain field of the transverse jet
Recent studies on flame stabilization of premixed turbulent gases
FLAME stabilization is of importance in the practical design
of ramjets and afterburners. It has been studied
extensively in recent years, particularly with reference to
bluff-body flame-holders. In the present survey we describe
the investigations relating to flame holding by bluff bodies as well as new techniques (e.g.,. flame holding by the use of reverse jets) which may prove to be of practical importance in new engine configurations. In Section II we consider the flow field downstream of a bluff-body flame-holder which includes the recirculation zone behind the body and a region of flame spreading farther downstream.
Explicit reference is made to crucial experiments which illustrate the nature and magnitude of the velocity field, the physical extent, the temperature, and the gas composition of the recirculation zone. Experimental studies and theoretical predictions of the angle of flame spreading, as well as some observations on unstable flow and the onset of blowoff, will be reviewed.
The variation of blowoff velocity with flame-holder design,
pressure, and mixture composition is considered briefly in
Section III both for single and for adjacent bluff bodies. Also included is a summary of results for blowoff velocities obtained with a reverse-jet flame-holder and with wall recesses. Theoretical studies on the mechanism of flame stabilization form the subject of Section IV. We shall indicate the points on which various proposed models agree and disagree with experiment and attempt to formulate a composite description which is consistent with most of the currently available experimental data both for bluff-body and for reverse-jet flameholders
Experiments on a jet in a crossflow in the low-velocity-ratio regime
The hairpin instability of a jet in a crossflow (JICF) for a low
jet-to-crossflow velocity ratio is investigated experimentally for a velocity
ratio range of and crossflow Reynolds numbers
. From spectral analysis we characterize the Strouhal number
and amplitude of the hairpin instability as a function of and . We
demonstrate that the dynamics of the hairpins is well described by the Landau
model, and, hence, that the instability occurs through Hopf bifurcation,
similarly to other hydrodynamical oscillators such as wake behind different
bluff bodies. Using the Landau model, we determine the precise threshold values
of hairpin shedding. We also study the spatial dependence of this
hydrodynamical instability, which shows a global behaviour.Comment: 20 pages, 21 figure
Constricted channel flow with different cross-section shapes
Pressure driven steady flow through a uniform circular channel containing a constricted portion is a common problem considering physiological flows such as underlying human speech sound production. The influence of the constriction’s cross-section shape (circle, ellipse, circular sector) on the flow within and downstream from the constriction is experimentally quantified. An analytical boundary layer flow model is proposed which takes into account the hydraulic diameter of the cross-section shape. Comparison of the model outcome with experimental and three-dimensional numerically simulated flow data shows that the pressure distribution within the constriction can be modeled accurately so that the model is of interest for analytical models of fluid–structure interaction without the assumption of two-dimensional flow
The effect of a density difference on shear-layer instability
Measurements of mass flow rate and mean density have been made in separated laminar boundary layers with large transverse density gradients. Two-dimensional shear layers were formed by exhausting a half-jet of one gas into a reservoir of another gas with a different molecular weight. Two freons with a density ratio of 1-98 and unusual properties which permitted the measurement of the mass flow rate with a single hot wire were used. A n analysis of the mass flow rate fluctuations showed that a negative density gradient (i.e. light gas flowing into heavy) increases the amplification rate of the instability oscillations and reduces the frequency and wave number. Opposite trends were observed when the density gradient was positive. These findings are in agreement with recent theoretical predictions
Statistical analysis of the velocity and scalar fields in reacting turbulent wall-jets
The concept of local isotropy in a chemically reacting turbulent wall-jet
flow is addressed using direct numerical simulation (DNS) data. Different DNS
databases with isothermal and exothermic reactions are examined. The chemical
reaction and heat release effects on the turbulent velocity, passive scalar and
reactive species fields are studied using their probability density functions
(PDF) and higher order moments for velocities and scalar fields, as well as
their gradients. With the aid of the anisotropy invariant maps for the Reynolds
stress tensor the heat release effects on the anisotropy level at different
wall-normal locations are evaluated and found to be most accentuated in the
near-wall region. It is observed that the small-scale anisotropies are
persistent both in the near-wall region and inside the jet flame. Two
exothermic cases with different Damkohler number are examined and the
comparison revealed that the Damkohler number effects are most dominant in the
near-wall region, where the wall cooling effects are influential. In addition,
with the aid of PDFs conditioned on the mixture fraction, the significance of
the reactive scalar characteristics in the reaction zone is illustrated. We
argue that the combined effects of strong intermittency and strong persistency
of anisotropy at the small scales in the entire domain can affect mixing and
ultimately the combustion characteristics of the reacting flow
Droplets formation inside a venturi liquid mixer
The formation of a coarse photographic emulsion by entraining a hot oil phase into a cooler aqueous phase by a Venturi device is considered. The main focus is on understanding the mechanism and site of droplet formation in the device, as well as the time-scale of heat flow, to see if it is feasible to feed this emulsion directly to a homogenizer in a continuous process
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