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

Effect of mesh grids on the turbulent mixing layer of an axisymmetric jet

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.This paper focuses on the effect that two different mesh grids have on the structure of the mixing layer of an axisymmetric jet. Detailed measurements of mean velocity and turbulent velocity fluctuations are made with an X hot-wire probe in the range 0.5 ≤ x/d ≤ 10, where x is the longitudinal distance from the nozzle exit plane and d is the nozzle diameter. The grids are introduced just downstream of the nozzle exit plane: one completely covers the nozzle (full mesh or FM), the other covers the central, high speed zone (disk mesh or DM). With reference to the undisturbed jet, FM yields a significant reduction in the turbulence intensity and width of the shear layer whereas DM enhances the turbulence intensity and increases the width of the shear layer. Both grids suppress the formation of the Kelvin-Helmholtz instability in the mixing layer. Results are presented, mainly at x/d = 5, both in the spectral domain and in physical space. In the latter context, second and third-order structure functions associated with u (the longitudinal velocity fluctuation) and v (the lateral or radial velocity fluctuation) are presented. All mesh geometries have a more significant effect on the second-order structure function of u than on that of v. The third-order energy transfer term is affected in such a way that, relative to the undisturbed jet, its peak location is shifted to a smaller scale with FM is used and to a larger scale with DM. This is consistent with our observations that FM reduces the turbulence in the shear layer whilst DM enhances it. It is suggested that the large scale vortices that are formed at the edge of the grids play a significant role in the transfer of energy.mp201

Empirical correlations for grid turbulence

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Homogeneous isotropic turbulence (HIT) is approximately generated by passing a uniform flow through a grid followed by a short contraction. The grid flow is slightly heated so that temperature acts as a passive scalar. In the self-similar region of grid flow downstream of the contraction, there is no mean shear and no turbulence production. The amplitudes of velocity and temperature fluctuations simply decay under the effect of viscosity and thermal diffusion. The decay rate is well represented by a power law; this is supported by present measurements in three different grid flows and by previously published data for grid turbulence obtained over different ranges of streamwise distance and/or Reynolds number. From dimensional analysis and the (empirical) power-law correla- tions, basic flow parameters, namely the Kolmogorov/Taylor/ Corrsin microscales and the Reynolds/Péclet numbers, are established as functions of streamwise distance. From this, it is possible to determine the flow parameters for a required grid geometry or initial condition.dc201

The lattice Boltzmann method and the problem of turbulence

This paper reports a brief review of numerical simulations of homogeneous isotopic turbulence (HIT) using the lattice Boltzmann method (LBM). The LBM results shows that the details of HIT are well captured and in agreement with existing data. This clearly indicates that the LBM is as good as current Navier-Stokes solvers and is very much adequate for investigating the problem of turbulence

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

On the anisotropy of a low-Reynolds-number grid turbulence

The anisotropy of a low-Reynolds-number grid turbulence is investigated through direct numerical simulations based on the lattice Boltzmann method. The focus is on the anisotropy of the Reynolds-stress (b_ij) and Reynolds-stress dissipation-rate (d_ij) tensors and the approach taken is that using the invariant analysis introduced by Lumley & Newman (J. Fluid Mech., vol. 82, 1977, pp. 161-178). The grid is made up of thin square floating elements in an aligned configuration. The anisotropy is initially high behind the grid and decays quickly as the downstream distance increases. The anisotropy invariant map (AIM) analysis shows that the return-to-isotropic trend of both b_ij and d_ij is fast and follows a perfectly axisymmetic expansion, although just behind the grid there is an initial tendency toward a one-component state. It is found that the linear relation d_ij = Ab_ij with A = 0.21 is satisfied during the return-to-isotropy phase of the turbulence decay, although close to the grid a form d_ij = f(b_ij), where f is a nonlinear function of b_ij, is more appropriate. For large downstream distances, d_ij becomes almost independent of b_ij, suggesting that despite the absence of an inertial range, the (dissipative) small scales present a high degree of isotropy. It is argued that (i) the very small values of the mean strain rate and (ii) the weak anisotropy of the large scales are in fact responsible for this result

Momentum and scalar transport in a localised synthetic turbulence in a channel flow with a short contraction

A numerical simulation is undertaken to investigate the transport of momentum and a passive scalar in a localised turbulence in a channel with a contraction. The simulation is carried out using a hybrid method which combines the lattice Boltzmann method (LBM, for the velocity field) and the energy equation (for the temperature field). The localised turbulence is generated through pulsed jets issued in the Poiseuille flow developing in the channel at a Reynolds number of about 1000. The aim of the study is twofold : i) determine effect of the contraction on the localised turbulence, and ii) study how the passive scalar behaves in such contracted localised turbulence. The contraction increase the averaged vorticity in the channel flow, which is accompanied by an increase in the averaged kinetic energy. The contraction also tends to reduce the Reynolds stresses. These results are similar those obtained in turbulent pipe flow with an axisymmetric contraction and in a turbulent boundary layer subjected to a favourable pressure gradient. However, it is found that the heat transport in the normal to the wall direction is more dramatically affected (reduced) than that in the direction of the flo

Simulation of gas flow in microchannels with a single 90 degrees bend

Bends are frequently encountered in microdevices and therefore a fundamental understanding of How through them is important. In this paper, we have used the lattice Boltzmann method to study flow of a gas through a microchannel with a single 90 degrees bend. The computations are two-dimensional, isothermal, and for the Reynolds and Knudsen numbers typically observed. We rind that the pressure drop in segments of equal lengths before and after the bend is different, and a recirculatory motion is seen near the bend even when the flow Reynolds number is less than unity. The velocity and pressure distribution between the bend and straight microchannels is almost identical at large Knudsen numbers, indicating that the flow does not feel the presence of the bend in such cases. This is attributed to rarefaction of the gas which hinders transfer of information across the now. (C) 200

Microfluidic characteristics of a multi-holed baffle plate micro-reactor

As part of a larger project aiming at development of a miniaturized hydrogen generator for small mobile/onboard fuel cell applications, a series of experiments was conducted on a novel micro-reactor to examine the effectiveness of its design in promoting the mixing of reactant agents. The reactor is essentially a tubular vessel fitted with a multi-holed baffle plate mounted on a central tube. The mixing phenomenon within the micro-reactor was studied using the micro-PIV (micro-particle image velocimetry) flow visualization technique. Experiments were conducted on a 1:1 scale replica of the reactor. Results indicate that the application of the multi-holed baffle plate considerably improves the mixing performance of the reactor when compared with a simpler co-axial jet tubular reactor. However, the geometrical characteristics of the baffle plate and central tube are found to have dramatic impacts upon the flow structure and mixing patterns within the reactor. Hence, the optimization of the reactor geometry is required to achieve the desirable mixing performance. For the range of Reynolds numbers studied here, the optimum reactor geometry is achieved when the central tube and baffle holes are of similar diameters and baffle holes are located half way between the stream-wise axis and the reactor wall

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