Predictions of a Turbulent Separated Flow Using Commercial CFD Codes

2001 ◽  
Vol 123 (4) ◽  
pp. 819-828 ◽  
Author(s):  
Gianluca Iaccarino
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Separation Bubble ◽  
Separated Flow ◽  
Adverse Pressure Gradient ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Flow Features ◽  
The Mean ◽  
Turbulent Separated Flow

Numerical simulations of the turbulent flow in an asymmetric two-dimensional diffuser are carried out using three commercial CFD codes: CFX, Fluent, and Star-CD. A low-Reynolds number k-ε model with damping functions and the four-equation v′2¯−f model are used; the first one is available as a standard feature in all the codes, the v′2¯−f model was implemented using the User Defined Routines. The flow features a large recirculating zone due to the adverse pressure gradient in the diffuser; the v′2¯−f predictions agree very well with the experiments both for the mean velocity and the turbulent kinetic energy. The length of the separation bubble is also computed within 6 percent of the measured value. The k-ε calculations do not show any recirculation and the agreement with the measurements is very poor. The three codes employed show very similar characteristics in terms of convergence and accuracy; in particular, the results obtained using the v′2¯−f are consistent in all the codes, while appreciable differences are obtained when the k-ε is employed.

Structure of Turbulent Flows Over Forward Facing Steps With Adverse Pressure Gradient

2016 ◽  
Vol 138 (11) ◽  
Author(s):  
Hassan Iftekhar ◽  
Martin Agelin-Chaab
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Turbulent Flows ◽  
Large Scale ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Step Height ◽  
Reattachment Length ◽  
Mean Velocity ◽  
The Mean

This paper reports an experimental study on the effects of adverse pressure gradient (APG) and Reynolds number on turbulent flows over a forward facing step (FFS) by employing three APGs and three Reynolds numbers. A particle image velocimetry (PIV) technique was used to conduct velocity measurements at several locations downstream, and the flow statistics up to 68 step heights are reported. The step height was maintained at 6 mm, and the Reynolds numbers based on the step height and freestream mean velocity were 1600, 3200, and 4800. The mean reattachment length increases with the increase in Reynolds number without the APG whereas the mean reattachment length remains constant for increasing APG. The proper orthogonal decomposition (POD) results confirmed that higher Reynolds numbers caused the large-scale structures to be more defined and organized close to the step surface.

Heat transfer from turbulent separated flows

1967 ◽  
Vol 27 (1) ◽  
pp. 97-109 ◽  
Author(s):  
D. B. Spalding
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Reynolds Number ◽  
Separated Flow ◽  
Separated Flows ◽  
Two Dimensional ◽  
Hydrodynamic Properties ◽  
One Dimensional ◽  
Actual Mechanism ◽  
Turbulent Separated Flow

A power-law relation is derived between the Stanton number and the Reynolds number, expressing the law of heat transfer for a wall adjacent to a region of turbulent separated flow. The derivation is based on Prandtl's (1945) proposal for the laws of dissipation, diffusion and generation of turbulent kinetic energy. The constants appearing in these laws are determined by reference to experimental data for the hydrodynamic properties of the constant-stress and the linear-stress layers.The agreement between the resulting predictions and the experimental data of other workers is sufficiently good to suggest that the actual mechanism of heat transfer from separated flows has much in common with that which is postulated. Closer agreement can be expected only after the present one-dimensional analysis has been superseded by a two-dimensional one.

Reattachment of a Two-Dimensional, Incompressible Jet to an Adjacent Flat Plate

1960 ◽  
Vol 11 (3) ◽  
pp. 201-232 ◽  
Author(s):  
C. Bourque ◽  
B. G. Newman
Keyword(s):  
Reynolds Number ◽  
Flat Plate ◽  
Separation Bubble ◽  
Volume Flow ◽  
Turbulent Jet ◽  
Two Dimensional ◽  
General Investigation ◽  
Mean Pressure ◽  
The Mean ◽  
Coandǎ Effect

SummaryAs part of a general investigation into Coanda effect, a study has been made of the reattachment of a two-dimensional, incompressible, turbulent jet to an adjacent, inclined, flat plate. The jet separates from the boundaries at the slot lips and reattaches to the plate downstream, a phenomenon which is associated with the lowering of the pressure between the jet and the plate accompanying the entrainment of fluid there. It is found that the flow becomes independent of both the length of the plate and the Reynolds number when these parameters are sufficiently large: the flow, scaled with respect to the width of the slot, is then uniquely determined by the plate inclination. Two approximate theories are developed for the mean pressure within the separation bubble, the position of reattachment and the increase in volume flow from the slot: the agreement with experiment is fairly satisfactory. These theories are a development of Dodds's analysis for the reattachment of a jet to a plate offset from, and parallel to, the axis of the slot and, for the purpose of comparison, a limited study is also made of this flow.

Full-coverage film cooling. Part 2. Prediction of the recovery-region hydrodynamics

1980 ◽  
Vol 101 (1) ◽  
pp. 159-178 ◽  
Author(s):  
S. Yavuzkurt ◽  
R. J. Moffat ◽  
W. M. Kays
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Film Cooling ◽  
Turbulence Kinetic Energy ◽  
Turbulent Shear ◽  
Mixing Length ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Full Coverage ◽  
The Mean

Hydrodynamic data are reported in the companion paper (Yavuzkurt, Moffat & Kays 1980) for a full-coverage film-cooling situation, both for the blown and the recovery regions. Values of the mean velocity, the turbulent shear stress, and the turbulence kinetic energy were measured at various locations, both within the blown region and in the recovery region. The present paper is concerned with an analysis of the recovery region only. Examination of the data suggested that the recovery-region hydrodynamics could be modelled by considering that a new boundary layer began to grow immediately after the cessation of blowing. Distributions of the Prandtl mixing length were calculated from the data using the measured values of mean velocity and turbulent shear stresses. The mixing-length distributions were consistent with the notion of a dual boundary-layer structure in the recovery region. The measured distributions of mixing length were described by using a piecewise continuous but heuristic fit, consistent with the concept of two quasi-independent layers suggested by the general appearance of the data. This distribution of mixing length, together with a set of otherwise normal constants for a two-dimensional boundary layer, successfully predicted all of the observed features of the flow. The program used in these predictions contains a one-equation model of turbulence, using turbulence kinetic energy with an algebraic mixing length. The program is a two-dimensional, finite-difference program capable of predicting the mean velocity and turbulence kinetic energy profiles based upon initial values, boundary conditions, and a closure condition.

Behavior of Separation Bubble and Reattached Boundary Layer Around a Circular Leading Edge

1994 ◽  
Author(s):  
B. K. Hazarika ◽  
C. Hirsch
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Separation Bubble ◽  
Low Frequency ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Mean Velocity ◽  
Low Frequency Oscillations ◽  
Lower Reynolds Number ◽  
The Mean

The flow around a circular leading edge airfoil is investigated in an incompressible, low turbulence freestream. Hot-wire measurements are performed through the separation bubble, the reattachment and the recovery region till development of the fully turbulent boundary layer. The results of the experiments in the range of Reynolds numbers 1.7×103 to 11.8×103 are analysed and presented in this paper. A separation bubble is present near the leading edge at all Reynolds numbers. At the lowest Reynolds number investigated, the transition is preceded by strong low frequency oscillations. The correlation given by Mayle for prediction of transition of short separation bubbles is successful at the lower Reynolds number cases. The length of the separation bubble reduces considerably with increasing Reynolds number in the range investigated. The turbulence in the reattached flow persists even when the Reynolds number based on momentum thickness of the reattached boundary layer is small. The recovery length of the reattached layer is relatively short and the mean velocity profile follows logarithmic law within a short distance downstream of the reattachment point and the friction coefficient conforms to Prandtl-Schlichting skin-friction formula for a smooth flat plate at zero incidence.

A mechanism for control of turbulent separated flow in rectangular diffusers

2011 ◽  
Vol 687 ◽  
pp. 584-594 ◽  
Author(s):  
Hayder Schneider ◽  
Dominic A. Von Terzi ◽  
Hans-Jörg Bauer ◽  
Wolfgang Rodi
Keyword(s):  
Kinetic Energy ◽  
Performance Enhancement ◽  
Separation Bubble ◽  
Substantial Reduction ◽  
Three Dimensional ◽  
Separated Flow ◽  
Statistical Modelling ◽  
Mean Flow ◽  
Cross Sectional ◽  
Turbulent Separated Flow

AbstractThe turbulent separated flow through an asymmetric diffuser with and without manipulation of incoming turbulence-driven mean secondary vortices (MSVs) from a rectangular duct is investigated by large-eddy simulations. The simulations carried out for two diffuser geometries reveal that by introducing a small amount of mean-flow kinetic energy via the MSVs into the flow, the complex three-dimensional separation behaviour and pressure recovery can be effectively controlled. Manipulated MSVs were found to enhance cross-sectional transport of high-momentum fluid, which determined the location, shape, and size of the separation bubble. The integral effect was a delay or expedition in the onset of separation. This change strongly affected the conversion of mean-flow kinetic energy to pressure, in particular for the front part of the diffuser. In addition, a substantial reduction in total pressure loss could be achieved. The manipulation of the MSVs is an efficient mechanism for performance enhancement in the cases investigated. The results have important implications for both control and statistical modelling of turbulent separated flow in rectangular diffusers.

Mean Separation and Reattachment in Turbulent Pipe Flow Due to an Orifice Plate

1994 ◽  
Vol 116 (2) ◽  
pp. 373-376 ◽  
Author(s):  
N. K. Agarwal
Keyword(s):  
Reynolds Number ◽  
Separated Flow ◽  
Limited Range ◽  
Turbulent Pipe Flow ◽  
Orifice Plate ◽  
Mean Flow ◽  
Reattachment Point ◽  
The Mean ◽  
Length Of Separation ◽  
Turbulent Separated Flow

The mean flow in a pipe with turbulent separated flow due to an orifice plate is experimentally studied. Measurements of time-mean length of separation and reattachment regions, made using a surface fence gauge are presented for a range of orifice sizes. In a limited range of Reynolds number (based on orifice radial height) 3 × 104 to 7.3 × 104 studied, reattachment point location decreased from 12 to 9 step heights. The lengths of separation and reattachment regions are a function of orifice size and the Reynolds number based on the radial height of the orifice plate.

Wake characteristics of two-dimensional perforated plates normal to an air-stream

1971 ◽  
Vol 46 (3) ◽  
pp. 599-609 ◽  
Author(s):  
I. P. Castro
Keyword(s):  
Reynolds Number ◽  
The Other ◽  
Turbulent Intensity ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Perforated Plates ◽  
Number Range ◽  
Air Stream ◽  
The Mean ◽  
Shedding Frequency

The flow in the wakes behind two-dimensional perforated plates has been investigated in the Reynolds number range 2·5 × 104 to 9·0 × 104.Measurements of drag and shedding frequency were made and a pulsed hotwire anemometer was used to measure the mean velocity and turbulent intensity variations in the highly turbulent regions immediately behind the plates.The results indicate the existence of two distinct types of flows: one appropriate to high and the other to low values of plate porosity.

An experimental study of reverse transition in two-dimensional channel flow

1968 ◽  
Vol 31 (3) ◽  
pp. 609-623 ◽  
Author(s):  
M. A. Badri Narayanan
Keyword(s):  
Reynolds Number ◽  
Reynolds Numbers ◽  
Wave Number ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Lateral Direction ◽  
Reverse Transition ◽  
Lissajous Figures ◽  
The Mean ◽  
Prime 2

An experimental investigation on reverse transition from turbulent to laminar flow in a two-dimensional channel was carried out. The reverse transition occurred when Reynolds number of an initially turbulent flow was reduced below a certain value by widening the duct in the lateral direction. The experiments were conducted at Reynolds numbers of 625, 865, 980 and 1250 based on half the height of the channel and the average of the mean velocity. At all these Reynolds numbers the initially turbulent mean velocity profiles tend to become parabolic. The longitudinal and vertical velocity fluctuations ($\overline{u^{\prime 2}}$and$\overline{v^{\prime 2}}$) averaged over the height of the channel decrease exponentially with distance downstream, but$\overline{u^{\prime}v^{\prime}} $tends to become zero at a reasonably well-defined point. During reverse transition$\overline{u^{\prime}}\overline{v^{\prime}}/\sqrt{\overline{u^{\prime 2}}}\sqrt{\overline{v^{\prime 2}}}$also decreases as the flow moves downstream and Lissajous figures taken withu’ andv’ signals confirm this trend. There is approximate similarly between$\overline{u^{\prime 2}} $profiles if the value of$\overline{u^{\prime 2}_{\max}} $and the distance from the wall at which it occurs are taken as the reference scales. The spectrum of$\overline{u^{\prime 2}} $is almost similar at all stations and the non-dimensional spectrum is exponential in wave-number. All the turbulent quantities, when plotted in appropriate co-ordinates, indicate that there is a definite critical Reynolds number of 1400±50 for reverse transition.

Complete self-preservation on the axis of a turbulent round jet

2016 ◽  
Vol 790 ◽  
pp. 57-70 ◽  
Author(s):  
L. Djenidi ◽  
R. A. Antonia ◽  
N. Lefeuvre ◽  
J. Lemay
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Energy Budget ◽  
Dissipation Rate ◽  
Longitudinal Velocity ◽  
Order Structure ◽  
Mean Velocity ◽  
Round Jet ◽  
The Mean

Self-preservation (SP) solutions on the axis of a turbulent round jet are derived for the transport equation of the second-order structure function of the turbulent kinetic energy ($k$), which may be interpreted as a scale-by-scale (s.b.s.) energy budget. The analysis shows that the mean turbulent energy dissipation rate, $\overline{{\it\epsilon}}$, evolves like $x^{-4}$ ($x$ is the streamwise direction). It is important to stress that this derivation does not use the constancy of the non-dimensional dissipation rate parameter $C_{{\it\epsilon}}=\overline{{\it\epsilon}}u^{\prime 3}/L_{u}$ ($L_{u}$ and $u^{\prime }$ are the integral length scale and root mean square of the longitudinal velocity fluctuation respectively). We show, in fact, that the constancy of $C_{{\it\epsilon}}$ is simply a consequence of complete SP (i.e. SP at all scales of motion). The significance of the analysis relates to the fact that the SP requirements for the mean velocity and mean turbulent kinetic energy (i.e. $U\sim x^{-1}$ and $k\sim x^{-2}$ respectively) are derived without invoking the transport equations for $U$ and $k$. Experimental hot-wire data along the axis of a turbulent round jet show that, after a transient downstream distance which increases with Reynolds number, the turbulence statistics comply with complete SP. For example, the measured $\overline{{\it\epsilon}}$ agrees well with the SP prediction, i.e. $\overline{{\it\epsilon}}\sim x^{-4}$, while the Taylor microscale Reynolds number $Re_{{\it\lambda}}$ remains constant. The analytical expression for the prefactor $A_{{\it\epsilon}}$ for $\overline{{\it\epsilon}}\sim (x-x_{o})^{-4}$ (where $x_{o}$ is a virtual origin), first developed by Thiesset et al. (J. Fluid Mech., vol. 748, 2014, R2) and rederived here solely from the SP analysis of the s.b.s. energy budget, is validated and provides a relatively simple and accurate method for estimating $\overline{{\it\epsilon}}$ along the axis of a turbulent round jet.

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