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The present study aims to understand the process of turbulent entrainment into a jet, as affected by background turbulence, using scalar statistics. Planar laser-induced fluorescence was employed to capture the orthogonal cross sections of the jet at a fixed downstream station with varying background turbulence intensities and length scales. The conditional scalar profiles revealed that the thickness of the scalar turbulent/turbulent interface is greater than that of the traditional turbulent/non-turbulent interface, and the interfacial thickness is an increasing function of the background turbulence intensity. Although nibbling remains the primary entrainment mechanism in the far field, increased occurrence of concentration "holes" within the interfacial layer in the presence of ambient turbulence suggests a more significant role of large-scale engulfment in the turbulent/turbulent entrainment process (although still below 1% of the total mass flux). Enhanced contribution of the area of detached jet patches (i.e., "islands") to that of the main jet is hypothesized to be evidence of intense detrainment events in the background turbulence. This can potentially contribute to a reduced net entrainment into the jet, which manifests as less negative values of scalar skewness within the jet core. [ABSTRACT FROM AUTHOR]

The impact of turbulent coflows on the dynamics of turbulent twin round jets is investigated experimentally. Parallel twin jets, at three jet spacing values and two Reynolds number/jet-to-coflow velocity ratios, were released into turbulent coflows with two distinct levels of turbulence intensity. Velocity measurements were made using acoustic Doppler velocimetry. An increase in the coflow turbulence intensity leads to an earlier merging and combining of the jets and also accelerates the rate of decay with downstream distance of the mean centerline excess velocity of the jets. The mean velocity on the symmetry line, for different values of jet spacing, ratios of jet exit velocity to coflow mean velocity, and coflow turbulence intensity, is self-similar when scaled by the maximum mean velocity on the symmetry line and the corresponding streamwise distance. Moreover, as the turbulence level of the coflow intensifies, the turbulence intensity along the symmetry line of the jets increases. The longitudinal integral length scale on the symmetry line of the twin jets decreases as the coflow turbulence intensity increases. The energy spectra of the coflowing twin jets show that the turbulence in the coflow transfers the energy contained by the larger scales to the smaller scales at a greater rate than that which occurs for jets in a quiescent background. However, as the jet spacing increases, less energy is transferred to the smaller scales. [ABSTRACT FROM AUTHOR]

The effect of a zero-mean-flow turbulent ambient on the geometry of the turbulent/turbulent interface (TTI) of an axisymmetric jet is investigated and compared with the traditional turbulent/non-turbulent interface (TNTI). The ambient turbulence is generated using a random jet array. Orthogonal cross-sections of the jet subjected to different levels of ambient turbulence intensities and length scales are captured using planar laser-induced fluorescence. The enhanced radial transport of concentration from the jet core towards the interface and the increased scalar fluctuations within the jet caused by the ambient turbulence result in steeper mean and root-mean-square scalar conditional jumps across the TTI layer compared with those of the TNTI, respectively. When compared with the quiescent ambient, the mean effect of background turbulence is to stretch and corrugate the interface surface area as evident from the wider probability density of the interface radial position, the lowered occurrence of zero-curvature surface elements, the greater misalignment between the radial and normal unit vectors of the interface, the increased tortuosity and the increased magnitude of the fractal exponent. [ABSTRACT FROM AUTHOR]

The effect of approximately homogeneous isotropic turbulence on the dynamics of an axisymmetric turbulent jet (Re = 10 600 and 5800) in an ambient with a negligible mean flow is interpreted from the statistics of the passive scalar field. The ambient turbulence is generated by a random jet array and scalar concentrations are measured in orthogonal cross sections of the jet using planar laser-induced fluorescence. Statistics of the scalar field of the jet in a turbulent ambient are compared to those in a quiescent ambient, using classical Eulerian averages and those conditioned on the jet centroid. A two-region model for the jet structure in ambient turbulence is proposed based on the centroidal statistics. Following the developing region of the jet, the ambient turbulence disrupts the jet structure, due to modulation of the jet interface, meandering of the jet by large eddies, and entrainment of the turbulent ambient fluid, resulting in a faster concentration decay and reduced entrainment compared to the quiescent ambient. Further downstream, once the ambient turbulence has destroyed the jet, only molecular and turbulent diffusion modify the scalar concentrations. The regions' relative lengths depend primarily on the relative turbulence intensity (ξ) between the ambient and the jet, as assessed using centroidal analysis, which removes the effect of the relative length scale (l) on the jet behavior in the turbulent ambient. The centroidal scalar statistics reveal self-similarity and self-preservation in mean scalar properties before jet breakup, which occurs abruptly once ξ > 0.5. The smaller scales of ambient turbulence modulate the jet boundary and, when entrained, lead to a wider range of centerline concentrations and root mean square concentrations, which are hypothesized to increase local concentration gradients within the jet and reduce jet mixing. [ABSTRACT FROM AUTHOR]

Axially stretched free-surface vortices occur at low-head hydropower intakes under specific flow and geometric conditions. When they are sufficiently strong, they can harm performance or cause premature failure of mechanical components such as turbine blades or guide vanes. Laboratory-scale experimental models are currently used to assess the risk of vortex formation during the design phase, but uncertainty remains as to how vortex characteristics translate from the laboratory scale to the much larger scale of an actual hydropower plant. This paper proposes a semi-empirical model that roughly predicts how the approach flow and intake geometry determine the key vortex characteristics (the core radius, bulk circulation and the depth of the free surface depression). The model is developed using detailed velocity measurements of the approach flow and the flow inside the vortex in a laboratory-scale physical model, using analytical models and insights drawn from previous work. [ABSTRACT FROM AUTHOR]

To validate the use of acoustic Doppler velocimeters (ADVs) for the measurement of turbulent flows, experiments were conducted in an axisymmetric turbulent jet and in approximately homogenous isotropic turbulence with zero mean flow. The jet experiments show that the horizontal RMS velocities measured by the ADV were overestimated when compared to both flying hot-film anemometry measurements and the accepted values in the literature. However, the vertical component of the RMS velocity agrees well with those of other studies. This was furthermore confirmed by the ADV measurements in the isotropic turbulence with zero mean flow. Given that the overestimated RMS velocities in the horizontal directions may be caused by Doppler noise and spikes, two postprocessing filters and a Doppler noise-reduction method were applied to the jet data in an attempt to correct the data. Although the RMS velocities decreased, they remained erroneously higher than the accepted values. A noise-reduction method for axisymmetric flows that corrects the RMS velocities to within the range of acceptable values is presented. Lastly, because the Doppler noise is thought to depend on the mean velocity, a series of laboratory experiments were undertaken to relate Doppler noise to the mean velocity. The results showed no clear relationship between the Doppler noise and the mean flow. In addition, subtracting the Doppler noise (measured for a given mean velocity) from the measured RMS velocities (at the same mean velocity) did not significantly decrease the RMS velocities. [ABSTRACT FROM AUTHOR]