Effect of Initial Conditions on the Scalar Decay in Grid Turbulence at Low Rλ

Decaying grid turbulence is considered at low Reynolds number (Rλ ~ 50) for different initial conditions. Three different grid geometries are used. Heat is injected via a mandoline at a distance of 1.5 M from the grid. The amount of heating is such that temperature may be treated as a passive scalar. A small contraction (1.36:1) is added at a distance of 11M downstream of the grid. The power-law exponents for the scalar variance are compared with those for the turbulent kinetic energy. These exponents depend on the grid geometry. For the isotropic dissipation rate 〈χ〉iso, the power-law exponent agrees with that inferred from the temperature variance transport equation. Restricting the range of validity of the decay law affects the magnitudes of the origin and decay exponent. Secondorder temperature structure functions collapse when the normalization is based on the local temperature variance and the Corrsin microscale but the asymptotic form of this collapse depends on the initial conditions

Effet de différents régimes photopériodiques sur la puberté du pigeonneau male Columba livia

Ce travail a pour but l’étude de l’influence du régime photopériodique sur l’activité reproductrice de jeunes pigeons domestiques Columba livia à la puberté : poids corporel, volume testiculaire et certains paramètres biochimiques et endocriniens. Les pigeonneaux ont été répartis en 3 lots exposés à différents régimes photopériodiques : jours courts (8L : 16D), éclairement continu (24L : 00 D), photopériode naturelle de printemps (lot Témoin). Les animaux du lot témoin (photopériode naturelle) présentent en fin d’expérimentation un développement testiculaire témoignant du déclenchement de la spermatogenèse, et une augmentation des taux plasmatiques de testostérone, protéines, cholestérol, triglycérides et thyroxine, contrairement au taux de glucose qui baisse. D’autre part, une régression testiculaire rapide chez les pigeonneaux soumis à des jours courts (8L : 16D) a été observée. Les taux plasmatiques de protéines et de glucose augmentent, mais la testostérone, le cholestérol, les triglycérides et la thyroxine diminuent. Enfin, chez les oiseaux exposés à un éclairement continu (24L : 00 D), les gonades se développent rapidement, les taux de testostérone, protéines et thyroxine augmentent ; les taux de glucose, de cholestérol et de triglycérides diminuent.Mots clés : reproduction; thyroxine; photopériode; pigeon. The aim of the present investigation has been to study the influence of various photoperiodic regimes on the reproductive activity of immature male domestic pigeon Columba livia, namely: body weight, testicular volume and some biochemical and hormonal profiles. Immature birds have been divided into 3 groups held under short days (8L: 16 D), continuous light (24L: 00 D), and natural photoperiod (control group). In the control group we see a significant gonadal development, an increase in plasma levels of testosterone, proteins, cholesterol, triglycerides and thyroxin, and a decrease in plasma glucose concentrations. A rapid involution of testes in individuals exposed to short days (8L: 16D) is recorded. Plasma protein and glucose levels increase, while those of testosterone, cholesterol, triglycerides and thyroxin showed a decline. At last, there was an increase, in testicular volume of birds exposed to continuous light (24L: 00 D), testosterone, proteins and thyroxin levels, while those of glucose, cholesterol and triglycerides showed a considerable decline.Keywords: reproduction; thyroxin; photoperiod; birds

Momentum and heat transport in a three-dimensional transitional wake of a heated square cylinder

The transport of momentum and a passive scalar (temperature) in a three-dimensional transitional wake of a heated square cylinder has been carried out through direct numerical simulations using the lattice Boltzmann method at a Reynolds number Rd = 200 (d is the cylinder diameter) and a Prandlt number of 0.7. The simulations shows that while momentum and heat are transported by vortical structures, heat is in general more effectively transported than momentum. It is argued that the nature of the structural flow is responsible for the longitudinal heat flux uθ being larger than the lateral one vθ in the wake region extending up to 45d. It was shown that a gradient transport model could, to a first-order approximation, be used to model uv but would be less accurate for modelling vθ. Also the Reynolds analogy between momentum and heat transports is not verified in this flow. The fluctuating temperature field presents thermal structures similar to the velocity structures with, however, a different spatial organization. In addition the analogy between fluctuating turbulent kinetic energy and the temperature variance is relatively well satisfied throughout the wake flow

Anisotropy measurements in the boundary layer over a flat plate with suction

Laser Doppler velocity measurements are carried out in a turbulent boundary layer subjected to concentrated wall suction (through a porous strip). The measurements are taken over a longitudinal distance of 9x the incoming boundary layer thickness ahead of the suction strip. The mean and rms velocity profiles are affected substantially by suction. Two-point measurements show that the streamwise and wall-normal autocorrelations of the streamwise velocity are reduced by suction. It is found that suction alters the redistribution of the turbulent kinetic energy k between its components. Relative to the no-suction case, the longitudinal Reynolds stress contributes more to k than the other two normal Reynolds stresses; in the outer region, its contribution is reduced which suggests structural changes in the boundary layer. This is observed in the anisotropy of the Reynolds stresses, which depart from the non-disturbed boundary layer. With suction, the anisotropy level in the near-wall region appears to be stronger than that of the undisturbed layer. It is argued that the mean shear induced by suction on the flow is responsible for the alteration of the anisotropy. The variation of the anisotropy of the layer will make the development of a turbulence model quite difficult for the flow behind suction. In that respect, a turbulence model will need to reproduce well the effects of suction on the boundary layer, if the model is to capture the effect of suction on the anisotropy of the Reynolds stresses

Power-law exponent in the transition period of decay in grid turbulence

Hot-wire measurements are carried out in grid-generated turbulence at moderate to low Taylor microscale Reynolds number <i>Re_λ</i> to assess the appropriateness of the commonly used power-law decay for the mean turbulent kinetic energy (e.g. <i>k</i> ∼ <i>xⁿ</i>, with <i>n</i> ⩽−1). It is found that in the region outside the initial and final periods of decay, which we designate a transition region, a power law with a constant exponent <i>n</i> cannot describe adequately the decay of turbulence from its initial to final stages. One is forced to use a family of power laws of the form <i>xⁿⁱ</i>, where <i>n_i</i> is a different constant over a portion <i>i</i> of the decay time during the decay period. Accordingly, it is currently not possible to determine whether any grid-generated turbulence reported in the literature decays according to Saffman or Batchelor because the reported data fall in the transition period where <i>n</i> differs from its initial and final values. It is suggested that a power law of the form <i>k ∼ x^ninit+m(x)</i>, where <i>m(x)</i> is a continuous function of <i>x</i>, could be used to describe the decay from the initial period to the final stage. The present results, which corroborate the numerical simulations of decaying homogeneous isotropic turbulence of Orlandi & Antonia (<i>J. Turbul.</i>, vol. 5, 2004, doi:10.1088/1468-5248/5/1/009) and Meldi & Sagaut (<i>J. Turbul.</i>,vol. 14, 2013, pp. 24–53), show that the values of <i>n</i> reported in the literature, and which fall in the transition region, have been mistakenly assigned to the initial stage of decay

Simulation of gas flow in microchannels with a sudden expansion or contraction

Two-dimensional simulations based on the isothermal lattice-Boltzmann method have been undertaken on microchannels with a sudden expansion or contraction. The study provides insight into the analysis of flows in complicated microdevices. The flow is pressure driven, and computations are performed for several Knudsen numbers, and area and pressure ratios, allowing the effects of compressibility and rarefaction to be assessed. The pressure drop for both the converging and diverging channels shows a discontinuity in slope at the junction, and is accompanied by a jump in velocity. The pressure drop in each section can be predicted well by the theory for straight channels. The mass flow ratio between converging and diverging channels is close to unity, and the streamlines are attached in both cases. It is deduced that compressibility and rarefaction have opposite effects on the flow. These results suggest that complex channels of the type considered here can be understood in terms of their primary units, and they experience only small secondary losses

Effects of low Reynolds number on decay exponent in grid turbulence

This present work is to investigate on the decay exponent (n) of decay power law (q'²~(t-t_o)ⁿ, q'<sup>2</sup is the total turbulent kinetic energy, t is the decay time, t₀ is the virtual origin) at low Reynolds numbers based on Taylor microscale R_λ(≡ u'λ/v) ≤ 64. Hot wire measurements are carried out in a grid turbulence subjected to a 1.36:1 contraction. The grid consists in large square holes (mesh size 43.75 mm and solidity 43%); small square holes (mesh size 14.15 mm and solidity 43%) and woven mesh grid (mesh size 5 mm and solidity 36%). The decay exponent (n) is determined using three different methods: (i) decay of q'², (ii) transport equation for ɛ, the mean dissipation of the turbulent kinetic energy and (iii) λ method (Taylor microscale λ ≡ √5〈q'²〉/〈ɛ_d〉 angular bracket denotes the ensemble). Preliminary results indicate that the magnitude n increases while R_λ(≡ u'λ/v) decreases, in accordance with the turbulence theory

Effects of initial conditions in decaying turbulence generated by passive grids

The effects of initial conditions on grid turbulence are investigated for low to moderate Reynolds numbers. Four grid geometries are used to yield variations in initial conditions and a secondary contraction is introduced to improve the isotropy of the turbulence. The hot-wire measurements, believed to be the most detailed to date for this flow, indicate that initial conditions have a persistent impact on the large-scale organization of the flow over the length of the tunnel. The power-law coefficients, determined via an improved method, also depend on the initial conditions. For example, the power-law exponent m is affected by the various levels of large-scale organization and anisotropy generated by the different grids and the shape of the energy spectrum at low wavenumbers. However, the results show that these effects are primarily related to deviations between the turbulence produced in the wind tunnel and true decaying homogenous isotropic turbulence (HIT). Indeed, when isotropy is improved and the intensity of the large-scale periodicity, which is primarily associated with round-rod grids, is decreased, the importance of initial conditions on both the character of the turbulence and m is diminished. However, even in the case where the turbulence is nearly perfectly isotropic, m is not equal to −1, nor does it show an asymptotic trend in x towards this value, as suggested by recent analysis. Furthermore, the evolution of the second- and third-order velocity structure functions satisfies equilibrium similarity only approximately

Consequences of self-preservation on the axis of a turbulent round jet

On the basis of a two-point similarity analysis, the well-known power-law variations for the mean kinetic energy dissipation rate ϵ¯ and the longitudinal velocity variance u²¯ on the axis of a round jet are derived. In particular, the prefactor for ϵ¯∝(x−x₀)⁻⁴, where x₀ is a virtual origin, follows immediately from the variation of the mean velocity, the constancy of the local turbulent intensity and the ratio between the axial and transverse velocity variance. Second, the limit at small separations of the two-point budget equation yields an exact relation illustrating the equilibrium between the skewness of the longitudinal velocity derivative S and the destruction coefficient G of enstrophy. By comparing the latter relation with that for homogeneous isotropic decaying turbulence, it is shown that the approach towards the asymptotic state at infinite Reynolds number of S+2G/Rλ in the jet differs from that in purely decaying turbulence, although +2G/R_λ∝R⁻¹_λ in each case. This suggests that, at finite Reynolds numbers, the transport equation for ϵ¯ imposes a fundamental constraint on the balance between S and G that depends on the type of large-scale forcing and may thus differ from flow to flow. This questions the conjecture that S and G follow a universal evolution with R_λ; instead, S and G must be tested separately in each flow. The implication for the constant C_ϵ2 in the k−ϵ¯ model is also discussed