A numerical investigation of wall effects in three-dimensional, laminar flow over a backward facing step with a constant aspect and expansion ratio

2012 ◽  
Vol 71 (9) ◽  
pp. 1073-1102 ◽  
Author(s):  
N. A. Malamataris
Keyword(s):  
Laminar Flow ◽  
Numerical Investigation ◽  
Three Dimensional ◽  
Expansion Ratio ◽  
Wall Effects ◽  
Backward Facing Step

Bifurcation Characteristics of Flows in Rectangular Sudden Expansion Channels

2005 ◽  
Author(s):  
Francine Battaglia ◽  
George Papadopoulos
Keyword(s):  
Reynolds Number ◽  
Laminar Flow ◽  
Critical Reynolds Number ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Expansion Ratio ◽  
Sudden Expansion ◽  
Two Dimensional ◽  
Effective Expansion ◽  
Three Dimensional Simulations

The effect of three-dimensionality on low Reynolds number flows past a symmetric sudden expansion in a channel was investigated. The geometric expansion ratio of in the current study was 2:1 and the aspect ratio was 6:1. Both experimental velocity measurements and two- and three-dimensional simulations for the flow along the centerplane of the rectangular duct are presented for Reynolds numbers in the range of 150 to 600. Comparison of the two-dimensional simulations with the experiments revealed that the simulations fail to capture completely the total expansion effect on the flow, which couples both geometric and hydrodynamic effects. To properly do so requires the definition of an effective expansion ratio, which is the ratio of the downstream and upstream hydraulic diameters and is therefore a function of both the expansion and aspect ratios. When the two-dimensional geometry was consistent with the effective expansion ratio, the new results agreed well with the three-dimensional simulations and the experiments. Furthermore, in the range of Reynolds numbers investigated, the laminar flow through the expansion underwent a symmetry-breaking bifurcation. The critical Reynolds number evaluated from the experiments and the simulations was compared to other values reported in the literature. Overall, side-wall proximity was found to enhance flow stability, helping to sustain laminar flow symmetry to higher Reynolds numbers in comparison to nominally two-dimensional double-expansion geometries. Lastly, and most importantly, when the logarithm of the critical Reynolds number from all these studies was plotted against the reciprocal of the effective expansion ratio, a linear trend emerged that uniquely captured the bifurcation dynamics of all symmetric double-sided planar expansions.

scholarly journals Numerical Simulation of Mixed Convective Flow Over a Three-Dimensional Horizontal Backward Facing Step

2005 ◽  
Vol 127 (9) ◽  
pp. 1027-1036 ◽  
Author(s):  
J. G. Barbosa Saldana ◽  
N. K. Anand ◽  
V. Sarin
Keyword(s):  
Convective Flow ◽  
Three Dimensional ◽  
Step Length ◽  
Expansion Ratio ◽  
Step Height ◽  
Streamwise Direction ◽  
Backward Facing Step ◽  
Thermal Features ◽  
Mixed Convective Flow ◽  
Thermally Conducting

Laminar mixed convective flow over a three-dimensional horizontal backward-facing step heated from below at a constant temperature was numerically simulated using a finite volume technique and the most relevant hydrodynamic and thermal features for air flowing through the channel are presented in this work. The channel considered in this work has an aspect ratio AR=4, and an expansion ratio ER=2, while the total length in the streamwise direction is 52 times the step height (L=52s) and the step length is equal to 2 times the step height (l=2s). The flow at the duct entrance was considered to be hydrodynamically fully developed and isothermal. The bottom wall of the channel was subjected to a constant high temperature while the other walls were treated to be adiabatic. The step was considered to be a thermally conducting block.

Eddy structures in a transitional backward-facing step flow

2007 ◽  
Vol 588 ◽  
pp. 43-58 ◽  
Author(s):  
H. P. RANI ◽  
TONY W. H. SHEU ◽  
ERIC S. F. TSAI
Keyword(s):  
Three Dimensional ◽  
Flow Simulation ◽  
Simulated Data ◽  
Numerical Data ◽  
Expansion Ratio ◽  
Transitional Flow ◽  
Backward Facing Step ◽  
Longitudinal Vortices ◽  
Eddy Structures ◽  
Spanwise Flow

In the present study, flow simulation has been carried out in a backward-facing step channel defined by an expansion ratio of 2.02 and a spanwise aspect ratio of 8 to provide the physical insight into the longitudinal and spanwise flow motions and to identify the presence of Taylor–Görtler-like vortices. The Reynolds numbers have been taken as 1000 and 2000, which fall in the category of transitional flow. The present simulated results were validated against the experimental and numerical data and the comparison was found to be satisfactory. The simulated results show that the flow becomes unsteady and exhibits a three-dimensional nature with the Kelvin–Helmholtz instability oscillations and Taylor–Görtler-Like longitudinal vortices. The simulated data were analysed to give an in-depth knowledge of the complex interactions among the floor and roof eddies, and the spiralling spanwise flow motion. Destabilization of the present incompressible flow system, with the amplified Reynolds number due to the Kelvin–Helmholtz and Taylor–Görtler instabilities, is also highlighted. A movie is available with the online version of the paper.

Global stability of the two-dimensional flow over a backward-facing step

2011 ◽  
Vol 693 ◽  
pp. 1-27 ◽  
Author(s):  
Daniel Lanzerstorfer ◽  
Hendrik C. Kuhlmann
Keyword(s):  
Three Dimensional ◽  
Physical Nature ◽  
Expansion Ratio ◽  
Two Dimensional ◽  
Backward Facing Step ◽  
Critical Mode ◽  
Large Step ◽  
Large Expansion ◽  
Amplification Process ◽  
Two Dimensional Flow

AbstractThe two-dimensional, incompressible flow over a backward-facing step is considered for a systematic variation of the geometry covering expansion ratios (step to outlet height) from 0.25 to 0.975. A global temporal linear stability analysis shows that the basic flow becomes unstable to different three-dimensional modes depending on the expansion ratio. All critical modes are essentially confined to the region behind the step extending downstream up to the reattachment point of the separated eddy. An energy-transfer analysis is applied to understand the physical nature of the instabilities. If scaled appropriately, the critical Reynolds number approaches a finite asymptotic value for very large step heights. In that case centrifugal forces destabilize the flow with respect to an oscillatory critical mode. For moderately large expansion ratios an elliptical instability mechanism is identified. If the step height is further decreased the critical mode changes from oscillatory to stationary. In addition to the elliptical mechanism, the strong shear in the layer emanating from the sharp corner of the step supports the amplification process of the critical mode. For very small step heights the basic state becomes unstable due to the lift-up mechanism, which feeds back on itself via the recirculating eddy behind the step, resulting in a steady critical mode comprising pronounced slow and fast streaks.

scholarly journals Numerical Simulation for Mixed Convective Flow Over a Three-Dimensional Horizontal Backward Facing Step

2004 ◽  
Author(s):  
J. G. Barbosa Saldana ◽  
N. K. Anand ◽  
V. Sarin
Keyword(s):  
Convective Flow ◽  
Three Dimensional ◽  
Step Length ◽  
Expansion Ratio ◽  
The Other ◽  
Step Height ◽  
Streamwise Direction ◽  
Backward Facing Step ◽  
Thermal Features ◽  
Mixed Convective Flow

Laminar mixed convective flow over a three-dimensional horizontal backward-facing step heated from below at a constant temperature was numerically simulated using a finite volume technique and the most relevant hydrodynamic and thermal features for air flowing through the channel are presented in this work. The channel considered in this work has an aspect ratio AR = 4, and an expansion ratio ER = 2, while the total length in the streamwise direction is 52 times the step height (L = 52s) and the step length is equal to 2 times the step height (l = 2s). The flow at the duct entrance was considered to be hydro-dynamically fully developed and isothermal. The bottom wall of the channel was subjected to a constant high temperature while the other walls were treated to be adiabatic. The step was considered to be a thermal conductive block.

scholarly journals Convective instability and transient growth in flow over a backward-facing step

2008 ◽  
Vol 603 ◽  
pp. 271-304 ◽  
Author(s):  
H. M. BLACKBURN ◽  
D. BARKLEY ◽  
S. J. SHERWIN
Keyword(s):  
Convective Instability ◽  
Three Dimensional ◽  
Nonlinear Effects ◽  
Expansion Ratio ◽  
Base Flow ◽  
Time Interval ◽  
Transient Growth ◽  
Two Dimensional ◽  
Backward Facing Step ◽  
Transient Energy

Transient energy growths of two- and three-dimensional optimal linear perturbations to two-dimensional flow in a rectangular backward-facing-step geometry with expansion ratio two are presented. Reynolds numbers based on the step height and peak inflow speed are considered in the range 0–500, which is below the value for the onset of three-dimensional asymptotic instability. As is well known, the flow has a strong local convective instability, and the maximum linear transient energy growth values computed here are of order 80×103 at Re = 500. The critical Reynolds number below which there is no growth over any time interval is determined to be Re = 57.7 in the two-dimensional case. The centroidal location of the energy distribution for maximum transient growth is typically downstream of all the stagnation/reattachment points of the steady base flow. Sub-optimal transient modes are also computed and discussed. A direct study of weakly nonlinear effects demonstrates that nonlinearity is stablizing at Re = 500. The optimal three-dimensional disturbances have spanwise wavelength of order ten step heights. Though they have slightly larger growths than two-dimensional cases, they are broadly similar in character. When the inflow of the full nonlinear system is perturbed with white noise, narrowband random velocity perturbations are observed in the downstream channel at locations corresponding to maximum linear transient growth. The centre frequency of this response matches that computed from the streamwise wavelength and mean advection speed of the predicted optimal disturbance. Linkage between the response of the driven flow and the optimal disturbance is further demonstrated by a partition of response energy into velocity components.

Three-dimensional numerical investigation of laminar flow in blind-tee pipes

2020 ◽  
Vol 217 ◽  
pp. 107962
Author(s):  
Fenghui Han ◽  
Muk Chen Ong ◽  
Yihan Xing ◽  
Wenhua Li
Keyword(s):  
Laminar Flow ◽  
Numerical Investigation ◽  
Three Dimensional

Numerical Investigation of Forced Convection From Small Rectangular Coiled Pipes

2006 ◽  
Author(s):  
Ibrahima Conte ◽  
Zhen Yang ◽  
Xiaofeng Peng
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Numerical Investigation ◽  
Forced Convection ◽  
Flow Behavior ◽  
Three Dimensional ◽  
Maximum Velocity ◽  
Inlet Velocity ◽  
Small Pitch ◽  
Pitch Distance

Investigations are done to numerically analyze forced convection from rectangular coiled pipes using commercial CFD software Fluent 6.0. The problems considered were three-dimensional laminar flow with water inside and outside the pipe. Calculations are done for two rectangular coiled pipes of different values of pitch (distance between two adjacent coils centers). The shift of location of the water flow maximum velocity was significantly observed in the case of smaller pitch. It is expected that the higher pitch value reduces the effects of the torsion. The shift of the flow maximum velocity area allows the fluid to be mixed well inside the pipe. For the outside flow, the flow behavior and the heat transfer should be improved by increasing the outside inlet velocity for the case of small pitch.

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