Effect Of Background Turbulence Research Articles

Turbulent entrainment into a cylinder wake from a turbulent background

Abstract

Effect of external turbulence on the short-wavelength instability of a counter-rotating vortex pair

The effect of background turbulence on the evolution and instability of a counter-rotating vortex pair is investigated via the use of direct numerical simulation at a circulation Reynolds number of 2400. The ambient turbulence is modelled as decaying homogeneous isotropic turbulence with the initial levels of turbulence intensity (Tu) varied from 0.0% to 10.0%. The major effect of the external turbulence during the early-time development is that it reduces the vortex separation distance, leading to the mutual interaction of the two vortices. The onset to transition is also found to occur earlier. For a low-level turbulence intensity (Tu ≤ 6.25%), the vortex pair breaks down to turbulence due to the short-wavelength elliptic instability, whose characteristic is qualitatively similar to that in a quiescent background despite exhibiting a shorter wavelength. Increasing the turbulence intensity results in a decrease in the growth rate of the elliptic mode. The long-wavelength Crow mode dominates the breakdown process of the flow once Tu ≥ 7.5%. The Crow instability gives rise to the vortex disconnection and reconnection, resulting in the transformation of the vortex pair into a vortex ring.

Vortex shedding and vorticity fluxes in the wake of cylinders within a random array

ABSTRACTThis work investigates the impact of the background turbulence generated by randomly placed cylinders on the vortex shedding regime and the mechanisms associated to vorticity fluxes. The goals are achieved by exploring velocity databases acquired with a two-dimensional particle image velocimetry system in two types of turbulent flow experiments: flow around a single infinite cylinder and flow within random array of infinite cylinders. Formation lengths, power spectral density functions and vortex distributions are employed to discuss the vortex shedding regime. The effects of background turbulence and vorticity cancellation, due to opposite sign vorticity, on the vorticity fluxes are discussed. The results show that the background turbulence reduces the formation length and consequently increase the shedding frequency. The stronger decay of longitudinal vorticity flux in denser arrays is not accompanied by an increase of the lateral flux of vorticity. Furthermore, it was concluded that the decay of longitudinal vorticity flux is mainly caused by the vorticity cancellation due to the vorticity of opposite sign of close downstream cylinders.

Effect of background turbulence in lidar investigations of clear air turbulence

The effect of atmospheric turbulence on power fluctuations of backscattered laser radiation at the lidar receiving aperture is estimated for divergent laser beams with the Gaussian intensity distribution in the Rytov approximation. Simple analytic formulas are suggested as estimators of the variance of received power fluctuations. The estimates showed that, for paths about 10 km long and sounding region radii of about 1 m, root mean square values of relative power fluctuations are comparable with the relative variation in the power received when changing from sounding of atmospheric regions with background turbulence to highly turbulent regions (clear air turbulence).

Effect of background turbulence on an axisymmetric turbulent jet

AbstractThe effect of different levels of background turbulence on the dynamics and mixing of an axisymmetric turbulent jet at different Reynolds numbers has been investigated. Approximately homogeneous and isotropic background turbulence was generated by a random jet array and had a negligible mean flow (${\langle {U}_{\alpha } \rangle }/ {u}_{\alpha \mathit{rms}} \ll 1$). Velocity measurements of a jet issuing into two different levels of background turbulence were conducted for three different jet Reynolds numbers. The results showed that the mean axial velocities decay faster with increasing level of background turbulence (compared with a jet in quiescent surroundings), while the mean radial velocities increase, especially close to the edges of the jet. Furthermore, the axial root-mean-square velocities of the jet increased in the presence of background turbulence, as did the jet’s width. However, the mass flow rate of the jet decreased, from which it can be inferred that the entrainment into the jet is reduced in a turbulent background. The effect of background turbulence on the entrainment mechanisms is discussed.

The effect of background turbulence on jet entrainment: an experimental study of a plane jet in a shallow coflow

Many effluents are released into water bodies or into the atmosphere, and dilution is relied on to minimize the toxic effects of the pollutants on the environment. Dilution occurs due to entrainment and subsequent mixing of the “clean” (or cleaner) ambient fluid and the effluent stream. In the near field, dilution occurs due to momentum or buoyancy driven entrainment into the effluent stream, which is proportional to a characteristic velocity of the jet or plume. Whereas in the far field, dilution occurs due to turbulent diffusion at a rate dependent on the level of turbulence in the receiving fluid. In order to predict the expected dilution of an effluent stream, it is necessary to answer the question, at what point does the dilution mechanism change and how does it affect dilution rates. An experimental study examining the velocity and concentration decay of plane jets released into shallow coflows with increasing levels of external turbulence, indicated that levels of external turbulence just large enough to disrupt the jet structure reduced dilution rates significantly. This indicates that models, in which the jet dilution and turbulent diffusion are superimposed, will not always give a conservative estimate of effluent dilution.

A Numerical Model of the Fluid Motion at a Density Front in the Presence of Background Turbulence

Abstract The effects of background turbulence on gravity currents produced by lock exchange are investigated using a numerical model with the aim of understanding the fluid motions associated with coastal fronts. It is shown that, at high turbulence intensities, the mutual intrusion of gravity currents is inhibited and the horizontal mass transport is dominated by the turbulent diffusion. The propagation of the front, the horizontal density flux, and the potential energy anomaly are calculated and are compared with available experimental data. The model is extended to include the effects of background rotation. It is found that, in the presence of background turbulence, the geostrophic equilibrium cannot be achieved, and the cross-frontal velocity persists indefinitely. The effects of rotation on the fluid motions were found to be impaired by the background turbulence.

Confidence interval estimates in PIV measurements of turbulent flows

The confidence or fiducial limits attached to the estimates of mean velocity and turbulence intensity obtained by PIV measurements are discussed. The effect of background turbulence and sample size are considered and illustrated by experiment.

Compliant coatings research: A guide to the experimentalist

The study of compliant coatings has been an area of keen interest for about 30 years. Among the potential useful applications for such coatings are drag reduction, sound absorption, vibration control and noise shielding. There are, however, several important issues with regard to the reliability of the available experimental data. In the present article, some of these issues will be addressed with the objective of guiding newcomers to the field. The effects of background turbulence in a wind or water tunnel, accurate drag measurements, compliant wall motion, noise reduction, and the geometry and properties of the coatings used in the experiments will be among the outstanding issues discussed. Attempts will be made to explain some of the seemingly contradictory results available in the open literature.