Laser Velocimeter Measurements in Highly Turbulent Recirculating Flows

1984 ◽  
Vol 106 (2) ◽  
pp. 173-180 ◽  
Author(s):  
W. H. Stevenson ◽  
H. D. Thompson ◽  
R. R. Craig
Keyword(s):  
Turbulent Flows ◽  
Turbulence Model ◽  
Quantitative Description ◽  
Sudden Expansion ◽  
Laser Doppler Velocimeter ◽  
Bias Error ◽  
Mean Velocity ◽  
One Dimensional ◽  
Numerical Predictions ◽  
Velocity Bias

This paper presents the results of an extensive study of subsonic separated flows using a laser Doppler velocimeter. Both a rectangular rearward facing step and cylindrical (axisymmetric) sudden expansion geometry were studied. The basic objectives were to resolve the question of whether a velocity bias error does, in fact, occur in LDV measurements in highly turbulent flows of this type and, if so, how it may be eliminated; map the velocity field (mean velocity, turbulence intensity, Reynolds stress, etc.) including the entire recirculation zone; and compare experimental results with numerical predictions based on the k-ε turbulence model. Measurements were carried out using a one-dimensional LDV operating in forward scatter with signal processing by means of a commercial counter-type processor. Results obtained show that velocity bias does occur in turbulent flows and that it can be overcome by proper data acquisition procedures. The results also indicate that the important mean velocity and turbulence quantities can be obtained with reasonable accuracy using a one-dimensional LDV system. Although the k-ε turbulence model provides a good qualitative picture of the flow field, it does not yield a completely adequate quantitative description. Results obtained here illustrate the discrepancies to be expected and provide a basis for further model development.

Uncertainty Quantification of Turbulence Model Coefficients via Latin Hypercube Sampling Method

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Matthew C. Dunn ◽  
Babak Shotorban ◽  
Abdelkader Frendi
Keyword(s):  
Uncertainty Quantification ◽  
Turbulent Flows ◽  
Turbulence Model ◽  
Sampling Method ◽  
Latin Hypercube Sampling ◽  
Recirculation Region ◽  
Free Shear Layer ◽  
Latin Hypercube ◽  
Reattachment Point ◽  
Mean Velocity

The article is concerned with the propagation of uncertainties in the values of turbulence model coefficients and parameters in turbulent flows. These coefficients and parameters are obtained through experiments performed on elementary flows, and they are subject to uncertainty. In this work, the widely used k-ɛ turbulence model is considered. It consists of model transport equations for the turbulence kinetic energy and the rate of turbulent dissipation. Both equations involve various model coefficients about which adequate knowledge is assumed known in the form of probability density functions. The study is carried out for a flow over a 2D backward-facing step configuration. The Latin Hypercube Sampling method is employed for the uncertainty quantification purposes as it requires a smaller number of samples compared to the conventional Monte Carlo method. The mean values are reported for the flow output parameters of interest along with their associated uncertainties. The results show that model coefficient variability has significant effects on the streamwise mean velocity in the recirculation region near the reattachment point and turbulence intensity along the free shear layer. The reattachment point location, pressure, and wall shear are also significantly influenced by the uncertainties of the coefficients.

On Turbulent Flows Dominated by Curvature Effects

1992 ◽  
Vol 114 (1) ◽  
pp. 52-57 ◽  
Author(s):  
G. C. Cheng ◽  
S. Farokhi
Keyword(s):  
Turbulent Flows ◽  
Turbulence Model ◽  
Dominant Role ◽  
Longitudinal Velocity ◽  
Turbulence Models ◽  
Separated Flows ◽  
Local Curvature ◽  
Streamline Curvature ◽  
Curvature Effects ◽  
Numerical Predictions

A technique for improving the numerical predictions of turbulent flows with the effect of streamline curvature is developed. Separated flows and the flow in a curved duct are examples of flow fields where streamline curvature plays a dominant role. New algebraic formulations for the eddy viscosity μt incorporating the k–ε turbulence model are proposed to account for various effects of streamline curvature. The loci of flow reversal (where axial velocities change signs) of the separated flows over various backward-facing steps are employed to test the capability of the proposed turbulence model in capturing the effect of local curvature. The inclusion of the effect of longitudinal curvature in the proposed turbulence model is validated by predicting the distributions of the longitudinal velocity and the static pressure in an S-bend duct and in 180 deg turn-around ducts. The numerical predictions of different curvature effects by the proposed turbulence models are also reported.

Assessment of DES Multiscale Turbulence Models for Prediction of Flow and Heat Transfer in an Axial-Channel Rod Configuration

2008 ◽  
Author(s):  
Debashis Basu ◽  
Kaushik Das ◽  
Scott L. Painter ◽  
Lane D. Howard ◽  
Steven T. Green
Keyword(s):  
Turbulent Flows ◽  
Turbulence Model ◽  
Computational Study ◽  
Axial Flow ◽  
Turbulence Models ◽  
Detached Eddy Simulation ◽  
Mean Velocity ◽  
Model Coefficient ◽  
Channel Configuration ◽  
Des Model

This paper presents results of a computational study conducted to assess the multiscale resolution capabilities and limitations of different Detached Eddy Simulation (DES) multiscale turbulence models in unsteady flow predictions for internal axial flow in a single rod channel configuration. Two different DES models are compared in the present analysis. The DES models are based on the Spalart-Allmaras (S-A) one-equation model and the two-equation realizable k-ε model. A detailed assessment of the DES turbulence model coefficient for the S-A based DES model is presented. The predicted time-averaged mean velocity and turbulent stresses are compared with the available experimental results. Flow unsteadiness, which is important for determining heat, momentum, and mass transfer in the gap region, is presented through time histories and spectra of flow quantities. The unsteady spectra for the velocity fluctuations are also compared with the experimental observations. The results demonstrate that the DES turbulence model coefficient significantly influence the predicted solution. The realizable k-ε-model-based DES model is found to be numerically more stable than the one-equation S-A-based DES model. Predicted results demonstrate that the modifications need to be incorporated in the current DES model formulations for proper prediction of wall bounded internal turbulent flows.

Influence of a Shroud on Swirler Flow Fields

1992 ◽  
Vol 114 (4) ◽  
pp. 768-775 ◽  
Author(s):  
H. Eroglu ◽  
N. Chigier
Keyword(s):  
Flow Fields ◽  
Laser Doppler Velocimeter ◽  
Shear Stresses ◽  
Cell Frequency ◽  
Mean Velocity ◽  
Forward Scatter ◽  
Benchmark Data ◽  
Numerical Predictions ◽  
Flow Deceleration ◽  
Turbulence Parameters

The flow fields for swirlers with and without a shroud were measured using a twocomponent laser-Doppler velocimeter (LDV) system. The primary goal of this study is to investigate the effect of shrouds on swirler flow fields, in order to provide useful design information for the manufacture of gas turbine fuel nozzles, and to supply benchmark data for comparison with numerical predictions. As a result of the measurements, the radial distributions of three mean velocity components, turbulence intensity, and shear stresses were obtained at five locations (x/d = 0.1, 1, 2, 4, and 8) along the axis of the swirlers. The LDV system was operated in the 20 deg off-axis forward scatter mode with beam expanders and Bragg cell frequency shifting on both components. The flow was seeded by 1 μm mean diameter atomized particles of glycerol and water (50/50) mixture. Comparison of flow with and without the shroud showed that the jet diameter was much smaller, and the flow deceleration in the downstream direction was faster, due to the influence of the shroud, at the same supply pressure (750 mm H2O). As a result of the significant reduction in the swirl number due to the addition of the shroud, the recirculation zone disappeared. In addition to its influence on recirculation, the shroud caused a radially inward shift of the maximum mean and turbulence parameters at all axial locations. The anisotropy of turbulence increased as compared to the values for the swirler without the shroud.

Three component velocity measurements of an isothermal confined swirling flow

1997 ◽  
Vol 211 (2) ◽  
pp. 113-122 ◽  
Author(s):  
S A Ahmed
Keyword(s):  
Swirling Flow ◽  
Sudden Expansion ◽  
Recirculation Region ◽  
Laser Doppler Velocimeter ◽  
Velocity Measurements ◽  
Radial Velocity Component ◽  
Numerical Predictions ◽  
Velocity Gradients ◽  
Component Velocity ◽  
High Turbulence

A two-component fibre-optic laser Doppler velocimeter (LDV) system has been employed to measure the flowfield characteristics of a confined, isothermal strongly swirling flow in a combustor model. The primary objectives are to understand such complex flowfields and to provide complete benchmark data for comparisons with numerical predictions based on practical models for turbulent swirling flows, and thereby guide the development of such models. For this confined strongly swirling flow, the measurements show the radial velocity component (close to the swirler exit) to be of the same order as the axial and swirl components. Comprehensive and detailed data show a large central recirculation region close to the dump plane which extends beyond the last measurement station. High velocity gradients and high turbulence activities are common for this type of flow and the current set of data confirms these previous findings. Generally speaking, most of the mixing takes place in the shear layer between the annular swirling jet and the corner recirculation region due to the sudden expansion.

scholarly journals Influence of a Shroud on Swirler Flow Fields

1990 ◽  
Author(s):  
Hasan Eroglu ◽  
Norman Chigier
Keyword(s):  
Flow Fields ◽  
Laser Doppler Velocimeter ◽  
Shear Stresses ◽  
Cell Frequency ◽  
Mean Velocity ◽  
Forward Scatter ◽  
Benchmark Data ◽  
Numerical Predictions ◽  
Flow Deceleration ◽  
Turbulence Parameters

The flow fields for swirlers with and without a shroud were measured using a two-component Laser Doppler Velocimeter (LDV) system. The primary goal of this study is to investigate the effect of shrouds on swirler flow fields, in order to provide useful design information for the manufacture of gas turbine fuel nozzles, and to supply benchmark data for comparison with numerical predictions. As a result of the measurements, the radial distributions of three mean velocity components, turbulence intensity, and shear stresses were obtained at five locations (x/d = 0.1, 1, 2, 4 and 8) along the axis of the swirlers. The LDV system was operated in the 20° off-axis forward scatter mode with beam expanders and Bragg cell frequency shifting on both components. The flow was seeded by 1 μm mean diameter atomized particles of glycerol and water (50/50) mixture. Comparison of flow with and without the shroud showed that the jet diameter was much smaller, and the flow deceleration in the downstream direction was faster, due to the influence of the shroud, at the same supply pressure (750 mm H2O). As a result of the significant reduction in the swirl number due to the addition of the shroud, the recirculation zone disappeared. In addition to its influence on recirculation, the shroud caused a radially inward shift of the maximum mean and turbulence parameters at all axial locations. The anisotropy of turbulence increased as compared to the values for the swirler without the shroud.

scholarly journals Turbulence Characteristics in an Axisymmetric Sudden Expansion

1997 ◽  
Author(s):  
Saad A. Ahmed ◽  
Kamorudeen B. Abidogun
Keyword(s):  
Experimental Data ◽  
Shear Layer ◽  
Turbulent Flows ◽  
Turbulent Transport ◽  
Sudden Expansion ◽  
Laser Doppler Velocimeter ◽  
Reynolds Stresses ◽  
Central Difference ◽  
Dissipation Term ◽  
Layer Flow

Simultaneous two-component laser Doppler velocimeter measurements were made in an axisymmetric sudden expansion to measure the flow properties of a confined, isothermal flowfield of a research dump combustor. Measurements of mean velocities, Reynolds stresses, and triple products were carried out at axial distances ranging from 0.38H (H = step height) to 18H downstream of the dump plane. Detailed experimental data are provided to help in the understanding of the behavior of turbulent transport characteristics of a confined shear layer. In addition, the data from this study will be available for upgrading / or evaluating advanced numerical codes used to predict complex turbulent flows. The turbulent kinetic energy terms: convection, diffusion, and production terms were computed directly from the experimental data using central difference, while the viscous dissipation term was obtained from balance of energy equation. The results indicate that the shear layer flow created by the sudden expansion enhances the combustor performance by serving as a turbulence generator mechanism.

Turbulent Flows in a Model SDR Combustor

1993 ◽  
Vol 115 (3) ◽  
pp. 468-473 ◽  
Author(s):  
T.-M. Liou ◽  
Y.-Y. Wu
Keyword(s):  
Turbulent Flows ◽  
Computational Models ◽  
Fluid Dynamic ◽  
Flow Characteristics ◽  
Joint Probability ◽  
Spreading Rate ◽  
Point Of View ◽  
Laser Doppler Velocimeter ◽  
Mean Velocity ◽  
Ducted Rocket

An experimental study is reported on the isothermal flow fields in a model solid-propellant ducted rocket combustor with two opposing side inlets. The measurements were made by using a four-beam two-color laser-Doppler velocimeter (LDV). Three values of momentum ratio (Ma/Ms) of the axial- to side-inlet jet—0.025, 0.11, and 1.28—were selected to investigate their effects on the flow characteristics. The Reynolds number, based on the air density, combustor height, and bulk velocity, was 4.56 × 104. The flow field was characterized in terms of the mean-velocity vectors and contours, joint probability functions, mean reattachment lengths, spreading rate of the axial-jet width, and Reynolds stress and turbulence kinetic energy contours. The LDV measured mean reattachment lengths were found to well agree with the corresponding flow-visualization photograph. In addition, the three Ma/Ms values provided three characteristic flows which are useful in testing the computational models. Further, correlations between the present cold-flow and previous reading-flow studies were documented in detail. It was found that from the fluid dynamic point of view Ma/Ms = 0.11 was preferable to the other two values of Ma/Ms.

An isothermal experimental investigation of turbulence transport through an abrupt axisymmetric expansion

1998 ◽  
Vol 212 (1) ◽  
pp. 45-55
Author(s):  
S A Ahmed
Keyword(s):  
Experimental Data ◽  
Turbulent Flows ◽  
Swirling Flow ◽  
Energy Balance Equation ◽  
Sudden Expansion ◽  
Laser Doppler Velocimeter ◽  
Turbulence Energy ◽  
Reynolds Stresses ◽  
Dissipation Term ◽  
Flow Mixing

A non-intrusive, two-component laser Doppler velocimeter was employed to measure the flow properties of a confined, isothermal, swirling flowfield in an axisymmetric sudden expansion research combustor. A constant angle swirler was used to stir the flow at the inlet of the combustor. Measurements of mean velocities, Reynolds stresses and triple products were carried out at axial distances ranging from 0.38 H (step height) to 18 H downstream of the swirler. Detailed experimental data are provided to help in the understanding of the behaviour of swirling, recirculating, axisymmetric and turbulent flows. Also, these detailed experimental data will be available for upgrading advanced numerical codes. The turbulent kinetic energy terms, convection, diffusion and production, were computed directly from the experimental data using central differencing, while the dissipation term was obtained from an energy balance equation. The swirling flow data are compared with the simple dump flow in the same experimental arrangement and it is shown that swirl enhances the production and distribution of turbulence energy in the combustor which, in turn, indicates thorough flow mixing and earlier flow recovery.

scholarly journals Variable-Order Fractional Models for Wall-Bounded Turbulent Flows

Entropy ◽  
2021 ◽  
Vol 23 (6) ◽  
pp. 782
Author(s):  
Fangying Song ◽  
George Em Karniadakis
Keyword(s):  
Fractional Derivative ◽  
Turbulent Flows ◽  
Fractional Order ◽  
Variable Order ◽  
Universal Curve ◽  
Reynolds Stresses ◽  
Continuous Change ◽  
Mean Velocity ◽  
Symmetric Variable ◽  
Fractional Models

Modeling of wall-bounded turbulent flows is still an open problem in classical physics, with relatively slow progress in the last few decades beyond the log law, which only describes the intermediate region in wall-bounded turbulence, i.e., 30–50 y+ to 0.1–0.2 R+ in a pipe of radius R. Here, we propose a fundamentally new approach based on fractional calculus to model the entire mean velocity profile from the wall to the centerline of the pipe. Specifically, we represent the Reynolds stresses with a non-local fractional derivative of variable-order that decays with the distance from the wall. Surprisingly, we find that this variable fractional order has a universal form for all Reynolds numbers and for three different flow types, i.e., channel flow, Couette flow, and pipe flow. We first use existing databases from direct numerical simulations (DNSs) to lean the variable-order function and subsequently we test it against other DNS data and experimental measurements, including the Princeton superpipe experiments. Taken together, our findings reveal the continuous change in rate of turbulent diffusion from the wall as well as the strong nonlocality of turbulent interactions that intensify away from the wall. Moreover, we propose alternative formulations, including a divergence variable fractional (two-sided) model for turbulent flows. The total shear stress is represented by a two-sided symmetric variable fractional derivative. The numerical results show that this formulation can lead to smooth fractional-order profiles in the whole domain. This new model improves the one-sided model, which is considered in the half domain (wall to centerline) only. We use a finite difference method for solving the inverse problem, but we also introduce the fractional physics-informed neural network (fPINN) for solving the inverse and forward problems much more efficiently. In addition to the aforementioned fully-developed flows, we model turbulent boundary layers and discuss how the streamwise variation affects the universal curve.

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