Structure of Turbulence in an Incipient-Separating Axisymmetric Flow

1995 ◽  
Vol 117 (3) ◽  
pp. 433-438 ◽  
Author(s):  
Rakesh K. Singh ◽  
Ram S. Azad
Keyword(s):  
Pipe Flow ◽  
Energy Production ◽  
Reynolds Shear Stress ◽  
Axisymmetric Flow ◽  
Adverse Pressure Gradient ◽  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Wall Layer ◽  
Diffuser Flow ◽  
Conical Diffuser

The relative intensity, skewness, and flatness of fluctuating streamwise velocity along the centerline of an 8 deg included angle conical diffuser show dramatic rapid growth in the final stages of the flow under the increasing influence of growing instantaneous reversals in the wall-layer. Pulsed-wire anemometry was effectively used for the measurement of quantitative instantaneous reversals and the turbulent flow field. In the severe adverse pressure gradient of the diffuser flow, the maxima of the streamwise and transverse fluctuating velocities, Reynolds shear stress, and turbulent energy production coincide and move away from their near-wall position in the pipe, also the velocity triple products show completely opposite nature as compared to the pipe flow. These measurements reveal the strong influence of instantaneous backflow on the structure of turbulence. The present results further corroborate the ability of the “structural” turbulence model of Nagano and Tagawa (1990) to predict velocity triple products in an axisymmetric diffuser flow.

scholarly journals Turbulence in a conical diffuser with fully developed flow at entry

1973 ◽  
Vol 57 (3) ◽  
pp. 603-622 ◽  
Author(s):  
P. A. C. Okwuobi ◽  
R. S. Azad
Keyword(s):  
Energy Production ◽  
Correlation Coefficients ◽  
Reynolds Numbers ◽  
Turbulent Energy ◽  
Wall Layer ◽  
Pressure Effects ◽  
Mean Velocity ◽  
Fully Developed Flow ◽  
Conical Diffuser ◽  
Mean Pressure

An experimental study of the structure of turbulence in a conical diffuser having a total divergence angle of 8° and an area ratio of 4: 1 with fully developed flow at entry is described. Theresearch has been done for pipe entry Reynolds numbers of 152 000 and 293000 of profiles of the mean pressure, mean velocity, turbulence intensities, correlation coefficients and the one-dimensional energy spectra, but owing to similar behaviour for these two Reynolds numbers, data are presented for a Reynolds number of 293 000.The results show that the rate of turbulent energy production approximately reaches a maximum value at the edge of the wall layer extending to the point of maximum u1-fluctuation. It is found that, within the layer,$\overline{u^2_1}$varies linearly with the distance from the wall and the linear range grows with distance in the downstream direction.The turbulent kinetic energy balance indicates that the magnitude of the energy convective diffusion due to kinetic and pressure effects is comparable with that of the energy production.

Measurement of the Intermittency Factor in Diffuser-Flow

1971 ◽  
Vol 49 (23) ◽  
pp. 2917-2930 ◽  
Author(s):  
R. S. Azad ◽  
R. H. Hummel
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Time Derivative ◽  
Streamwise Velocity ◽  
Positive Pressure ◽  
Hot Wire ◽  
Diffuser Flow ◽  
Conical Diffuser ◽  
Intermittency Factor ◽  
Channel Analyzer

The intermittency factor in a thick axisymmetric boundary layer with positive pressure gradient (diffuser-flow) was evaluated with hot-wire anemometers and a multi-channel analyzer. The probability density of the signal S(t) (a space–time derivative of the streamwise velocity) is evaluated and then decomposed into three separate curves representing fluids in the turbulent, the transition, and the non-turbulent states respectively. Different values of the intermittency factor γ are calculated by taking different fractions of fluid in the transition state with the turbulent fluid. A model of flow in a conical diffuser is suggested.

Turbulence structure of a reattaching mixing layer

1981 ◽  
Vol 110 ◽  
pp. 171-194 ◽  
Author(s):  
C. Chandrsuda ◽  
P. Bradshaw
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Flow Region ◽  
Turbulent Energy ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Recirculating Flow

Hot-wire measurements of second- and third-order mean products of velocity fluctuations have been made in the flow behind a backward-facing step with a thin, laminar boundary layer at the top of the step. Measurements extend to a distance of about 12 step heights downstream of the step, and include parts of the recirculating-flow region: approximate limits of validity of hot-wire results are given. The Reynolds number based on step height is about 105, the mixing layer being fully turbulent (fully three-dimensional eddies) well before reattachment, and fairly close to self-preservation in contrast to the results of some previous workers. Rapid changes in turbulence quantities occur in the reattachment region: Reynolds shear stress and triple products decrease spectacularly, mainly because of the confinement of the large eddies by the solid surface. The terms in the turbulent energy and shear stress balances also change rapidly but are still far from the self-preserving boundary-layer state even at the end of the measurement region.

Boundary Layer Flashback Model for Hydrogen Flames in Confined Geometries Including the Effect of Adverse Pressure Gradient

2020 ◽  
Author(s):  
Ólafur H. Björnsson ◽  
Sikke A. Klein ◽  
Joeri Tober
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Flow Separation ◽  
Gradient Flow ◽  
Lewis Number ◽  
Adverse Pressure Gradient ◽  
Future Research ◽  
Diffuser Flow ◽  
Combustion Properties ◽  
Quenching Distance

Abstract The combustion properties of hydrogen make premixed hydrogen-air flames very prone to boundary layer flashback. This paper describes the improvement and extension of a boundary layer flashback model from Hoferichter [1] for flames confined in burner ducts. The original model did not perform well at higher preheat temperatures and overpredicted the backpressure of the flame at flashback by 4–5x. By simplifying the Lewis number dependent flame speed computation and by applying a generalized version of Stratford’s flow separation criterion [2], the prediction accuracy is improved significantly. The effect of adverse pressure gradient flow on the flashback limits in 2° and 4° diffusers is also captured adequately by coupling the model to flow simulations and taking into account the increased flow separation tendency in diffuser flow. Future research will focus on further experimental validation and direct numerical simulations to gain better insight into the role of the quenching distance and turbulence statistics.

scholarly journals Effect of discharge and upstream jam angle on the flow distribution beneath a simulated ice jam

2019 ◽  
Vol 46 (5) ◽  
pp. 413-423 ◽  
Author(s):  
Baafour Nyantekyi-Kwakye ◽  
Tanzim Ahmed ◽  
Shawn P. Clark ◽  
Mark F. Tachie ◽  
Karen Dow
Keyword(s):  
Pressure Gradient ◽  
Power Law ◽  
Reynolds Shear Stress ◽  
Measurement Techniques ◽  
Adverse Pressure Gradient ◽  
Mean Bias Error ◽  
Bias Error ◽  
Particle Image Velocimetry System ◽  
Ice Jam ◽  
Two Parameter

The velocity field beneath simulated rough ice jams under various upstream jam angles and discharge were investigated using a particle image velocimetry system. Three discharges were examined at 2.3 L/s, 3.4 L/s, and 4.0 L/s and two upstream ice jam angles were tested at 4° and 6°. Increasing the discharge resulted in high turbulence production beneath the jam. The adverse pressure gradient exerted on the flow increased the levels of the Reynolds shear stress. The measured velocities beneath the jam were used to assess the performances of three traditional field measurement techniques as well as the validity of the two-parameter power law. The two-point measurement technique performed remarkably well with the least mean bias error of 2.0%. The error associated with the different techniques showed their inability to accurately predict the average velocity under high discharge. The two-parameter power law accurately predicted velocity profiles within the equilibrium section of the jam, but failed within the boundary layers when the flow was subjected to a pressure gradient.

What are we learning from simulating wall turbulence?

2007 ◽  
Vol 365 (1852) ◽  
pp. 715-732 ◽  
Author(s):  
Javier Jiménez ◽  
Robert D Moser
Keyword(s):  
Energy Production ◽  
Wall Layer ◽  
Wall Turbulence ◽  
Buffer Layers ◽  
Upper And Lower Bounds ◽  
Nonlinear Structures ◽  
Outer Flow ◽  
Global Properties ◽  
Dynamic Experiments ◽  
Lower Limits

The study of turbulence near walls has experienced a renaissance in the last decade, largely owing to the availability of high-quality numerical simulations. The viscous and buffer layers over smooth walls are essentially independent of the outer flow, and there is a family of numerically exact nonlinear structures that account for about half of the energy production and dissipation. The rest can be modelled by their unsteady bursting. Many characteristics of the wall layer, such as the dimensions of the dominant structures, are well predicted by those models, which were essentially completed in the 1990s after the increase in computer power made the kinematic simulations of the late 1980s cheap enough to undertake dynamic experiments. Today, we are at the early stages of simulating the logarithmic (or overlap) layer, and a number of details regarding its global properties are becoming clear. For instance, a finite Reynolds number correction to the logarithmic law has been validated in turbulent channels. This has allowed upper and lower limits of the overlap region to be clarified, with both upper and lower bounds occurring at much larger distances from the wall than commonly assumed. A kinematic picture of the various cascades present in this part of the flow is also beginning to emerge. Dynamical understanding can be expected in the next decade.

A two-dimensional-three-component model for spanwise rotating plane Poiseuille flow

2019 ◽  
Vol 880 ◽  
pp. 478-496 ◽  
Author(s):  
Shengqi Zhang ◽  
Zhenhua Xia ◽  
Yipeng Shi ◽  
Shiyi Chen
Keyword(s):  
Reynolds Number ◽  
Poiseuille Flow ◽  
Reynolds Shear Stress ◽  
Streamwise Velocity ◽  
Channel Height ◽  
Neutral Stability ◽  
Two Dimensional ◽  
Plane Poiseuille Flow ◽  
Rotation Numbers ◽  
3C Model

Spanwise rotating plane Poiseuille flow (RPPF) is one of the canonical flow problems to study the effect of system rotation on wall-bounded shear flows and has been studied a lot in the past. In the present work, a two-dimensional-three-component (2D/3C) model for RPPF is introduced and it is shown that the present model is equivalent to a thermal convection problem with unit Prandtl number. For low Reynolds number cases, the model can be used to study the stability behaviour of the roll cells. It is found that the neutral stability curves, critical eigensolutions and critical streamfunctions of RPPF at different rotation numbers ($Ro$) almost collapse with the help of a rescaling with a newly defined Rayleigh number $Ra$ and channel height $H$. Analytic expressions for the critical Reynolds number and critical wavenumber at different $Ro$ can be obtained. For a turbulent state with high Reynolds number, the 2D/3C model for RPPF is self-sustained even without extra excitations. Simulation results also show that the profiles of mean streamwise velocity and Reynolds shear stress from the 2D/3C model share the same linear laws as the fully three-dimensional cases, although differences on the intercepts can be observed. The contours of streamwise velocity fluctuations behave like plumes in the linear law region. We also provide an explanation to the linear mean velocity profiles observed at high rotation numbers.

The Prediction of the Axisymmetric Turbulent Jet Issuing into a Co-Flowing Stream

1974 ◽  
Vol 25 (1) ◽  
pp. 69-80 ◽  
Author(s):  
R A Antonia ◽  
R W Bilger
Keyword(s):  
Energy Production ◽  
Turbulent Energy ◽  
Turbulent Jet ◽  
Stream Velocity ◽  
Turbulence Structure ◽  
Air Stream ◽  
Similar Character ◽  
Flowing Stream ◽  
Two Parameter ◽  
External Stream

SummaryThree analyses are presented for predicting the development of an axisymmetric turbulent jet issuing into a co-flowing external air stream. The first analysis is analogous to a method used by Patel to predict the growth of a two-dimensional jet in an external air stream. The method is found to be inadequate when the excess velocity on the axis of the jet becomes small compared with the external stream velocity. The second analysis assumes that the turbulence structure is similar at different streamwise stations but it breaks down when the advection of turbulent energy becomes comparable with the turbulent energy production. In the third approach, a two-parameter model of turbulence developed by Rodi and Spalding, which uses two differential equations for the turbulent energy and the length scale of the turbulence respectively, is found to predict closely the experimental results of Antonia and Bilger for a ratio of jet to external stream velocity of 3.0. The success of this last method emphasises the non-similar character of turbulence.

scholarly journals Stabilisation and drag reduction of pipe flows by flattening the base profile

2019 ◽  
Vol 863 ◽  
pp. 850-875 ◽  
Author(s):  
Elena Marensi ◽  
Ashley P. Willis ◽  
Rich R. Kerswell
Keyword(s):  
Drag Reduction ◽  
Nonlinear Stability ◽  
Pipe Flow ◽  
Body Force ◽  
Initial Conditions ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
The Body ◽  
Random Disturbance ◽  
Good Proxy

Recent experimental observations (Kühnen et al., Nat. Phys., vol. 14, 2018b, pp. 386–390) have shown that flattening a turbulent streamwise velocity profile in pipe flow destabilises the turbulence so that the flow relaminarises. We show that a similar phenomenon exists for laminar pipe flow profiles in the sense that the nonlinear stability of the laminar state is enhanced as the profile becomes more flattened. The flattening of the laminar base profile is produced by an artificial localised body force designed to mimic an obstacle used in the experiments of Kühnen et al. (Flow Turbul. Combust., vol. 100, 2018a, pp. 919–943) and the nonlinear stability measured by the size of the energy of the initial perturbations needed to trigger transition. Significant drag reduction is also observed for the turbulent flow when triggered by sufficiently large disturbances. In order to make the nonlinear stability computations more efficient, we examine how indicative the minimal seed – the disturbance of smallest energy for transition – is in measuring transition thresholds. We first show that the minimal seed is relatively robust to base profile changes and spectral filtering. We then compare the (unforced) transition behaviour of the minimal seed with several forms of randomised initial conditions in the range of Reynolds numbers $Re=2400$–$10\,000$ and find that the energy of the minimal seed after the Orr and oblique phases of its evolution is close to that of a critical localised random disturbance. In this sense, the minimal seed at the end of the oblique phase can be regarded as a good proxy for typical disturbances (here taken to be the localised random ones) and is thus used as initial condition in the simulations with the body force. The enhanced nonlinear stability and drag reduction predicted in the present study are an encouraging first step in modelling the experiments of Kühnen et al. and should motivate future developments to fully exploit the benefits of this promising direction for flow control.

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