Sharif Digital Repository

The effects of low frequency, large amplitude sinusoidal pulsation on grid-generated turbulence (PGGT) were experimentally studied. Two-component hot wire anemometry technique was used. Pulsation did not change homogeneous, isotropic character of grid-generated turbulence. © 2001 by ASME  

The prediction of interfacial turbulence characteristics is one of the still challenging of two-phase stratified flow. The evaluation of some important parameters such as interfacial heat transfer coefficient based on turbulence kinetic energy and turbulence dissipation rate in some models, intensifies the importance of turbulence flow correct simulation. High gradient of velocity and turbulence kinetic energy at the interface of two-phase stratified flow leads to a major overestimation or underestimation of flow characteristics without any special treatment. Consideration of a source function of turbulence eddy frequency at the interface is one of the common solution employed in past... 

Using the Markovian method, we study the stochastic nature of electrical discharge current fluctuations in the Helium plasma. Sinusoidal trends are extracted from the data set by the Fourier-Detrended Fluctuation analysis and consequently cleaned data is retrieved. We determine the Markov time scale of the detrended data set by using likelihood analysis. We also estimate the Kramers-Moyal's coefficients of the discharge current fluctuations and derive the corresponding Fokker-Planck equation. In addition, the obtained Langevin equation enables us to reconstruct discharge time series with similar statistical properties compared with the observed in the experiment. We also provide an exact... 

This article describes the results of experimental observations in pulsating Simple Shear Flows (SSF). A uniform-mean-gradient shear flow was generated within the test section of an open circuit wind tunnel. Transverse arrays of honeycomb channels with differing resistances were used to generate shear flow at low shear rates (less than 20 s-1). A set of rotating vanes pulsated the flow field at 8.5 Hz and 18 Hz. Instantaneous velocity was measured by employing a two-component hot wire anemometry technique. The experimental credibility of the facility was established in stationary SSF. In pulsating flows the pulsation effects on mean shear rate, the kinetic energy of turbulence, Reynolds... 

In this paper, pulsating grid-generated turbulence is studied. A two-component hot wire anemometry technique is used. The pulsation effects on characterizing lenght scales and the statistical description of fluctuations are studied in comparison with their stationary counterparts. No significant change in the character of the turbulent flow with pulsation is observed  

The present study investigates the turbulence characteristic of density current experimentally. The 3D Acoustic-Doppler Velocimeter (ADV) was used to measure the instantaneous velocity and characteristics of the turbulent flow. The courses of experiment were conducted in a three-dimensional channel for different discharge flows, concentrations, and bed slopes. Results are expressed at various distances from the inlet, for all flow rates, slopes and concentrations as the distribution of turbulence energy, Reynolds stress and the turbulent intensity. It was concluded that the maximum turbulence intensity happens in both the interface and near the wall. Also it was observed that turbulence... 

A new procedure for simulating turbulent flow in three-dimensional arbitrary geometries is presented. Finite volume method using physical covariant velocities on a staggered grid arrangement was used in this investigation. This work is an extension of previous successful work to three-dimensional cases. The ability of the new algorithm was tested using a conventional two-equation turbulence model on a highly separated turbulent flow test case. The low Reynolds number k-ω turbulence model of Wilcox was utilized to evaluate its capability in modeling highly curved flows. Turbulent flow over a three-dimensional hill, which is appropriate in assessment of ability of turbulence models in... 

Analysis of flow in fluidized beds, a common chemical reactor, is of much current interest due to its fundamental as well as industrial importance. Experimental data for the successive increments of the pressure fluctuations time series in a fluidized bed are analyzed by computing a multiscale probability density function (PDF) of the increments. The results demonstrate the evolution of the shape of the PDF from the short to long time scales. The deformation of the PDF across time scales may be modeled by the log-normal cascade model. The results are also in contrast to the previously proposed PDFs for the pressure fluctuations that include a Gaussian distribution and a PDF with a power-law... 

We employ the horizontal visibility algorithm to map the velocity and acceleration time series in turbulent flows with different Reynolds numbers, onto complex networks. The universal nature of velocity fluctuations in high Reynolds turbulent Helium flow is found to be inherited in the corresponding network topology. The degree distributions of the acceleration series are shown to have stretched exponential forms with the Reynolds number dependent fitting parameter. Furthermore, for acceleration time series, we find a transitional behavior in terms of the Reynolds number in all network features which is in agreement with recent empirical studies  

The present study investigates the turbulence characteristic of turbidity current experimentally. The three-dimensional Acoustic-Doppler Velocimeter (ADV) was used to measure the instantaneous velocity and characteristics of the turbulent flow. The experiments were conducted in a three-dimensional channel for different discharge flows, concentrations, and bed slopes. Results are expressed at various distances from the inlet, for all flow rates, slopes and concentrations as the distribution of turbulence energy, Reynolds stress and the turbulent intensity. It was concluded that the maximum turbulence intensity happens in both the interface and near the wall. Also, it was observed that the... 

Density current is a dense fluid, which is continuously released from a source and spreads down a sloping surface inside a lighter, motionless fluid. A low-Reynolds number k-ε model (Launder and Sharma, 1974) has been used to simulate the behavior of 3-D density currents. Density current with a uniform velocity and concentration enters the channel via a sluice gate into a lighter ambient fluid and moves forward down-slope. The model has been verified with the experimental data sets. Although the k-ε Launder and Sharma model is applied here to a conservative density current, it seems the analysis is valid in general for turbidity current laden with fine particles. Copyright © 2006 by ASME  

A hybrid finite-element-volume FEV method is extended to simulate turbulent non-premixed propane air preheated flame in a minichannel. We use a detailed kinetics scheme, i.e. GRI mechanism 3.0, and the flamelet model to perform the combustion modeling. The turbulence-chemistry interaction is taken into account in this flamelet modeling using presumed shape probability density functions PDFs. Considering an upwind-biased physics for the current reacting flow, we implement the physical influence upwinding scheme PIS to estimate the cell-face mixture fraction variance in this study. To close the turbulence closure, we employ the two-equation standard κ-ε turbulence model incorporated with... 

In this research, the employment of an innovative but simple and cost effective method for turbulent reduction in a low speed wind tunnel is investigated and the effect of this turbulence reduction on the Power Spectrum and FFT-Based signals that obtained from Hot-wire Anemometry is studied. These terms represents the amount of power contained in the signals. Several situations are studied in this paper. One of these situations is when there are four screens in the settling chamber of the wind tunnel and after that by adding the trip strip in the contraction region of the wind tunnel the tests were repeated. In the next stage, three screens were removed from the settling chamber of the wind... 

Laminar flamelet decomposition (LFD) is a dynamic approach for modelling sub-filter scale turbulence-chemistry interactions in Large-Eddy Simulations using a stretched flamelet library. In this work, the performance of the LFD model – that was previously used only in non-premixed combustion—is investigated a priori for premixed combustion using positively-strained flamelets in the reactant-to-product configuration. For this purpose, a DNS database of methane-air premixed flames is utilized. The flames are propagating in a rectangular box under homogeneous isotropic turbulence conditions over a wide range of Karlovitz numbers. The results show that the LFD model can correctly account for the... 

This paper investigates the impact of the inflow conditions on simulations of a round jet discharging from a wall into a large space. The fluid dynamic characteristics of a round jet are studied numerically. A numerical method based on the control volume approach with collocated grid arrangement is employed. The k-ε model is utilized to approximate turbulent stresses by considering six different inlet conditions. The velocity field is presented, and the rate of decay at the jet centerline is determined. The results showed that inflow conditions had a strong influence on the jet characteristics. This paper also investigates both sharp-edged and contoured nozzles. The effects of velocity,... 

The deposition behavior of fine sediment is an important phenomenon, and yet unclear to engineers concerned about reservoir sedimentation. An elliptic relaxation turbulence model (v2̄ - f model) has been used to simulate the motion of turbid density currents laden whit fine solid particles. During the last few years, the v2̄ - f turbulence model has become increasingly popular due to its ability to account for near-wall damping without use of damping functions. In addition, it has been proved that the v2̄ - f model to be superior to other RANS methods in many fluid flows where complex flow features are present. Due to low Reynolds number turbulence of turbidity current,(its critical Reynolds... 

This article reviews the oil-soluble polymeric drag-reducing agents (DRAs) with an outlook to their large-scale application in petroleum industry. A general discussion on mechanism of the DR phenomenon and mechanical degradation of the drag reducers in turbulent flow is offered with an emphasis on the molecular parameters. Furthermore, low charge density associating polymers, as a new class of oil-soluble flow improvers, are described  

Recently, very large eddy simulation approach has attracted a great deal of attention among researchers. This approach can be thought of as an intermediate approach in flow field filtering view point compared with Direct Numerical Simulation and Reynolds-Averaged Navier-Stokes. One famous method to this approach is Partially Averaged Navier-Stokes. Early studies have demonstrated the capability of this technique in flow prediction; however, this method still needs to be evaluated under more flow conditions to ensure its reliable performance. In this study, the performance of PANS k-ω method in the simulation of turbulent flow around a cylinder with square cross section close to a flat... 

Subsonic wind tunnel experiments were conducted to study the turbulence level in the test section. Measurements were performed by introducing trip strip and/or damping screens on the flow field. The results indicated that the introduction of trip strips not only reduced the turbulence intensity compared to cases without it, but also flattened the variations. Further, the experiments which investigated the impact of the damping screens indicated a similar reduction in turbulence intensity; the pattern, however, remained the same. Furthermore, the results for cases wherein both trip strips as well as damping screens were placed on the contraction and in the settling chamber, respectively,... 

Subsonic wind tunnel experiments were conducted to study the effect of forced transition on the pressure distribution in the concave portion of contraction. Further more, the effect of early transition on the turbulence level in the test section of the wind tunnel is studied. Measurements were performed by installing several trip strips at two different positions in the concave portion of the contraction. The results show that installation of the trip strips, have significant effects on both turbulence intensity and on the pressure distribution. The reduction in the free stream turbulence as well as the wall static pressure distribution varied significantly with the location of the trip...