J. Mi

Mixing characteristics of axisymmetric free jets from a contoured nozzle, an orifice plate and a pipe

The differences in mixing performance between axisymmetric turbulent jets issuing from three common types of nozzle, viz. a contoured (or smooth contraction) nozzle, a sharpedged orifice and a long pipe, are investigated. The investigation is carried out using both qualitative flow visualizations and quantitative measurements of the centerline passive temperature. It is revealed that the jet issuing from an orifice plate provides the greatest rate of mixing with ambient fluid, while the pipe jet has the lowest rate. Physical insight into the differences is explored using a planar imaging technique and measurements of power spectra of the fluctuating velocity

Temperature dissipation in a turbulent round jet

Parallel cold wires were used to measure the temperature derivative, in each of the three spatial directions, in the self-preserving region of a turbulent round jet. The temperature derivative variances were inferred from the correlation method and from the temperature derivative spectra after correcting these for the effect of wire separation. Both methods yielded fully consistent results for the components of the average temperature dissipation: the radial and azimuthal values are nearly equal and only slightly larger than the axial component. The resulting departure from isotropy of the temperature dissipation is small, especially when compared with results in other free shear flows. The high-wavenumber behaviour of the corrected temperature derivative spectra conforms closely with isotropy on the jet axis but small departures occur away from the axis. Conditional averages, based on spatially coherent temperature jumps, indicate that, while the organized motion makes a significant contribution to the temperature variance, its contribution to the temperature-derivative variances is small.

The influence of Reynolds number on a plane jet

The present study systematically investigates through experiments the influence of Reynolds number on a plane jet issuing from a radially contoured, rectangular slot nozzle of large aspect ratio. Detailed velocity measurements were performed for a jet exit Reynolds number spanning the range 1500≤Reh≤16 500, where Reh≡Ubh/υ with Ub as the momentum-averaged exit mean velocity, h as the slot height, and υ as the kinematic viscosity. Additional centerline measurements were also performed for jets from two different nozzles in the same facility to achieve Reh=57 500. All measurements were conducted using single hot-wire anemometry to an axial distance (x) of x≤160h. These measurements revealed a significant dependence of the exit and the downstream flows on Reh despite all exit velocity profiles closely approximating a “top-hat” shape. The effect of Reh on both the mean and turbulent fields is substantial for Reh<10 000 but becomes weaker with increasing Reh. The length of the jet’s potential core, initial pri...

Corrections for velocity and temperature derivatives in turbulent flows

Spectral corrections, which are based on local isotropy, are presented for all the spatial derivatives of velocity and temperature fluctuations which feature in the average dissipations of turbulent energy and temperature. The corrections, which compensate for the spectral attenuation due to the separation between sensors, depend only weakly on the choice of the three-dimensional energy (or temperature) spectrum and therefore on the turbulence Reynolds number. Corrections are also obtained for the variances of velocity and temperature derivatives. The diagonal velocity derivatives require smaller corrections than either the off-diagonal velocity derivatives or the temperature derivatives. Corrections of comparable magnitude are required for the average dissipations of turbulent energy and temperature.

Mixing Characteristics of a Flapping Jet from a Self-Exciting Nozzle

Experimental results of the mixing characteristics of a low-frequency flapping jet from a self-exciting nozzle are presented. The simple fluidic device used to generate the flapping motion is also described. The nozzle contains no external trigger and, unlike the flip-flop nozzle of Viets, contains no external feedback path. Both conventional and conditional averaging schemes are employed to characterise the turbulent mixing characteristics of the jet using data obtained from hot-wire anemometry. Flow-visualisation is used to characterise the flapping motion. It is revealed that the dynamic flapping motion enhances the large-scale mixing of the jet while concurrently suppressing the generation of the fine-scale turbulence. The results also indicate that high turbulence intensities, initiated by the flapping motion, are sustained even in the far-field flow region.

Characterization of turbulent jets from high-aspect-ratio rectangular nozzles

Turbulent free jets issuing from rectangular slots with various high aspect ratios (15–120) are characterized. The centerline mean and rms velocities are measured using hot-wire anemometry over a downstream distance of up to 160 slot heights at a slot-height-based Reynolds number of 10 000. Experimental results suggest that a rectangular jet with sufficiently high aspect ratio (>15) may be distinguished between three flow zones: an initial quasi-plane-jet zone, a transition zone, and a final quasi-axisymmetric-jet zone. In the quasi-plane-jet zone, the turbulent velocity field is statistically similar, but not identical, to those of a plane jet.

Joint statistics between temperature and its dissipation rate components in a round jet

The joint statistics between the temperature fluctuation θ and all three components of the temperature dissipation rate eθ are investigated in the self‐preserving region of a slightly heated turbulent round jet. The main factors which determine the correlation between θ and eθ are the asymmetry of p(θ), the probability density function (PDF) of θ, and the anisotropy of the small‐scale turbulence. The assumption of statistical independence between θ and eθ appears to be more closely approximated in this flow than in a turbulent plane jet. Relatedly, the assumption of local isotropy is also more closely satisfied in the round jet than in the plane jet. When θ is in the range ±2 standard deviations, the expectations of all components of eθ, conditioned on θ, are approximately equal in the fully turbulent part of the flow; the magnitude of the conditional expectation is consistent with the independence assumption.

Effect of small vortex-generators on scalar mixing in the developing region of a turbulent jet

Abstract This paper investigates the effect of small tabs placed at the exit plane of an axisymmetric smooth contraction on passive scalar mixing in a slightly heated turbulent jet. It is shown that the presence of tabs profoundly distorts the jet flow field and consequently modifies the scalar mixing characteristics significantly. Tabs cause the mean temperature to decrease more rapidly with downstream distance, implying an increased mixing rate. Furthermore, it is found that two tabs distort the jet from the axisymmetric state more dramatically than four tabs.

Vorticity characteristics of the turbulent intermediate wake

Measurements of the lateral components ωj(j=2 and 3) of the vorticity fluctuation vector have been made, using a vorticity probe consisting of two X-wires, in the intermediate wake of a circular cylinder. The effect of the spatial resolution of the probe on the measurement of ωj has been studied. As the spatial resolution impairs, the variance and flatness factor of ωj decrease whereas the skewness of ωj increases. Reasonably accurate values of ωj2 can be obtained by applying spectral corrections for the spatial resolution effect.Near the beginning of the intermediate wake, the variance of ω2 is larger than that of ω3 due to the significant contribution from ribs which connect consecutive spanwise roll vortices. This difference decreases with downstream distance. Also, the presence of the rolls is reflected by a local extremum in the skewness of ω3 on each side of the wake centerline. The magnitude of the extremum decreases with downstream distance.

Corrections to Taylor’s hypothesis in a turbulent circular jet

The relation between mean square values of the time derivative θ,t(≡∂θ/∂t) and the spatial derivatives θ,i(≡∂θ/∂xi; i=1, 2, 3) of the temperature fluctuation θ, obtained with the assumptions of homogeneity and independence between small scales and large scales, is experimentally verified in the self‐preserving region of a circular jet where the turbulence intensity levels are relatively high. Local isotropy is approximately satisfied in the fully turbulent region of this flow and Wyngaard and Clifford’s [J. Atmos. Sci. 34, 922 (1977)] simplified relation between θ,t2 and θ,12 is shown to be quite adequate in this region.