Terms Of Velocity Fluctuations Research Articles

Flame dynamics in the lean burnout zone of an RQL combustion chamber - response to primary zone velocity fluctuations

This study focuses on elucidating the flame dynamics of the lean burnout zone of an Rich-Quench-Lean (RQL) combustion chamber. With a new experimental approach of spatially separating the rich primary zone from the lean burnout zone, the latter can be investigated independently in terms of velocity fluctuations. Acoustically stiff mixing air ports in the lean burnout zone are ensured to prevent any acoustic interaction of the primary crossflow with the secondary mixing air jets. Therefore, defined boundary conditions at the mixing air inlets are used. Resulting no thermoacoustic interaction and additional flame dynamics are generated. In this specific case the lean burnout zone can be treated as a 2-port system allowing the application of existing evaluation methods e.g. acoustic determination of flame-transfer-functions (FTF) from the Rankine-Hugoniot (RH) equation or quantification of the heat release with chemiluminescence in combination with the Multi-Microphone-Method (MMM). Within this research, FTFs acoustically measured with the RH approach are presented and serve as a baseline for comparison with ones measured via a photomultiplier tube (PMT). It is found that the inverse diffusion flame in the burnout zone only reacts to fluctuations in the low frequency range and a clear low pass behavior is observed. The FTFs, calculated via the PMT match those from RH very well. Amplitude weighted phase images, recorded with a high-speed camera setup, visualize changes during excitation which complement and confirm the findings from the FTF.

Spanwise modulation effects of local body force on downstream turbulence growth around two-dimensional hump

Flow characteristics under flow-separation control by the spanwise-modulated body force of a plasma actuator around a two-dimensional hump are numerically investigated. When the Reynolds number is set to Reh=16000, the incoming flow contains much disturbance and a near-wall streak structure appears around the hump. The local body force induces stable streamwise velocity to form a streak-like profile artificially. Under the forcing, turbulence fluctuations increase downstream, with resulting in early flow reattachment. These changes alter the momentum balance around the hump. Particularly, the wall-normal pressure gradient and velocity fluctuation terms in the momentum equation are modified. This somewhat suggests a relation between increases in wall-normal gradient terms and the flow reattachment profile under the control. In addition, the turbulence growth depends on the spanwise modulation wavelength of the forcing. The most effective wavelength among those tested here is λz=0.08, normalized by the hump height, which is λz+=150 in local viscous wall units. This viscous scale corresponds to that of a streak structure appearing around a two-dimensional hump.

Modification of k-ε Turbulent Model Using Kinetic Energy–Preserving Method

In this article, the kinetic energy–preserving (KEP) scheme and, also, the way of applying this scheme to the k–ε turbulent model are introduced. This study aims to introduce a stable method in which a few artificial dissipation terms are added to the governing equations in a way that the intensity of the solution fluctuations is reduced and, therefore, the problem stability increases. In accord with the importance of the study of turbulent flows, the effects of the fluctuating velocity terms on the calculation of all fluxes in the governing equations are scrutinized as well. Also, the influence from applying the KEP scheme on the k–ε turbulent model is investigated. This article reveals that by using the KEP scheme and, afterwards, improving the discretization method of the velocity fluctuation terms in k–ε equations, the accuracy of the results obtained is enhanced without a need to add artificial dissipation terms (or by minimizing their values).

Separation control based on turbulence transition around a two-dimensional hump at different Reynolds numbers

Separation and reattachment around a two-dimensional hump controlled by a two-dimensional periodic excitations induced by a dielectric barrier discharge plasma actuator are investigated at Reynolds numbers of 4000 and 16,000 based on the freestream velocity and the hump height. A two-dimensional excitation is adopted in the present study for promoting two-dimensional instability in the shear layer and a resulting laminar-turbulent transition. Note that the most effective frequency for reattachment is fh(=f∗u∞∗/h∗)=0.20 among the presently considered cases at both Reynolds numbers, which is close to the reference value previously reported for the flow around a backward-facing step (Hasan, 1992).This frequency is the highest among the frequencies that provide sufficient time for the vortex scale to become the hump height. In addition, the momentum balance around the hump is examined by decomposing the averaged momentum equations. The velocity fluctuation terms are found to be considerable in size: they increase around the separation position under periodic excitation control. These terms are balanced with other terms: the gradient of the velocity fluctuation in the streamwise direction is comparable to the sum of convection terms of the time-averaged velocity and that in the wall-normal direction is comparable to the pressure gradient in the wall-normal direction. The control performance for reattachment is correlated with these velocity fluctuation effects. There is a possibility that the excitation control enforces reattachment by increasing turbulence fluctuation and modifying the momentum transfer balance.

Turbulence modelling of shallow water flows using Kolmogorov approach

This study uses an improved k–ε coupled shallow water equations (SWE) model that equipped with the numerical computation of the velocity fluctuation terms to investigate the turbulence structures of the open channel flows. We adapted the Kolmogorov K41 scaling model into the k–ε equations to calculate the turbulence intensities and Reynolds stresses of the SWE model. The presented model was also numerically improved by a recently proposed surface gradient upwind method (SGUM) to allow better accuracy in simulating the combined source terms from both the SWE and k–ε equations as proven in the recent studies. The proposed model was first tested using the flows induced by multiple obstructions to investigate the utilised k–ε and SGUM approaches in the model. The laboratory experiments were also conducted under the non-uniform flow conditions, where the simulated velocities, total kinetic energies (TKE) and turbulence intensities by the proposed model were used to compare with the measurements under different flow non-uniformity conditions. Lastly, the proposed numerical simulation was compared with a standard Boussinesq model to investigate its capability to simulate the measured Reynolds stress. The comparison outcomes showed that the proposed Kolmogorov k–ε SWE model can capture the flow turbulence characteristics reasonably well in all the investigated flows.

Fully Coupled Simulations of Non-Colloidal Monodisperse Sheared Suspensions

In this work we investigate numerically the dynamics of sheared suspensions in the limit of vanishingly small fluid and particle inertia. The numerical model we used is able to handle the multi-body hydrodynamic interactions between thousands of particles embedded in a linear shear flow. The presence of the particles is modelled by momentum source terms spread out on a spherical envelop forcing the Stokes equations of the creeping flow. Therefore all the velocity perturbations induced by the moving particles are simultaneously accounted for. The statistical properties of the sheared suspensions are related to the velocity fluctuation of the particles. We formed averages for the resulting velocity fluctuation and rotation rate tensors. We found that the latter are highly anisotropic and that all the velocity fluctuation terms grow linearly with particle volume fraction. Only one off-diagonal term is found to be non zero (clearly related to trajectory symmetry breaking induced by the non-hydrodynamic repulsion force). We also found a strong correlation of positive/negative velocities in the shear plane, on a time scale controlled by the shear rate (direct interaction of two particles). The time scale required to restore uncorrelated velocity fluctuations decreases continuously as the concentration increases. We calculated the shear induced self-diffusion coefficients using two different methods and the resulting diffusion tensor appears to be anisotropic too. The microstructure of the suspension is found to be drastically modified by particle interactions. First, the probability density function of velocity fluctuations showed a transition from exponential to Gaussian behaviour as particle concentration varies. Second, the probability of finding close pairs while the particles move under shear flow is strongly enhanced by hydrodynamic interactions when the concentration increases.

Transport in porous packings: Statistical characterization of transport, role of fluctuation and data analysis

AbstractThe study of dynamics of transport in porous packings (in particular chromatographic columns) represents an illuminating step towards the statistical and probabilistic analysis of transport phenomena and can be generalized to other transport conditions of environmental interest. This paper addresses the use of Monte Carlo simulation in the analysis of transport in porous media, the local characterization of porosity fields in disordered models of pore networks, and the analysis of dispersion in terms of velocity fluctuations, and presents a simplified treatment of experimental data arising from transport problems.

Response of the shear layers separating from a circular cylinder to small amplitude rotational oscillations

The responses of the shear layers separating from a circular cylinder in water to small amplitude rotational oscillations of the cylinder were investigated experimentally in the Reynolds number (Re) range 250 to 1200 [J. R. Filler, Ph.D. dissertation, Washington State University (1989)]. Magnitude and phase of the response (in terms of velocity fluctuations in the shear layers) were measured using a hot film anemometer connected to a phase-sensitive detector. The hot film was placed in the shear layer approximately one diameter downstream from the cylinder. Synchronous velocity oscillations were found to be roughly linear in the driven rotational amplitude. At lower Re, cylinder oscillations are shown to excite the common Strouhal (or Karman) mode of vortex shedding. Near the Strouhal frequency there is a rapid 180° phase jump characteristic of a driven oscillator. Above Re≈500, a secondary mode of vortex shedding or instability is also excited associated with the instability of the free shear layers. Frequencies of maximum response in this range agree with frequencies of maximum amplification predicted analytically by others for free shear layers. The response phase varied with frequency as expected for waves convected on a shear layer. [Work supported by ONR.]

Theoretical pressure correlation functions in turbulent boundary layers

A Poisson equation is constructed for the pressure in terms of velocity fluctuations and is solved. The pressure correlation has a second-order velocity fluctuation contribution (usually assumed dominant in past work) and a fourth-order velocity correlation contribution. The velocity correlations are assumed to be those, suitably weighted, found from homogeneous turbulence measurements. Results are computed for two boundary layers: an idealized (canonical) turbulent boundary layer and a deliberately thickened boundary layer. The pressure variances are found to be essentially the same as the measured values. The correlations are positive except the streamwise, x correlations. The canonical layer shows a pressure variance which peaks very close to the wall, near the laminar sublayer. The fourth-order velocity fluctuations may generally be neglected for the thickened boundary layer; close to the wall they are an important part of the variance and correlation for the canonical layer.

Notes in Magnetron Injection Gun

The expression for total current fluctuations at the output plane, in terms of velocity fluctuations at cathode plane and beam current is obtained for magnetron injection gun. This expression fails to explain the reduction of noise with increasing value of magnetic field normal to the cathode plane. A qualitative explanation is given in light of two dimensional theory of noise emission.