Shear-Layer Flow Regimes and Wave Instabilities and Reattachment Lengths Downstream of an Abrupt Circular Channel Expansion

1972 ◽  
Vol 39 (3) ◽  
pp. 677-681 ◽  
Author(s):  
L. H. Back ◽  
E. J. Roschke
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Reverse Flow ◽  
Flow Region ◽  
Flow Regimes ◽  
Circular Channel ◽  
Number Range ◽  
Channel Expansion ◽  
Wave Instabilities ◽  
Layer Flow

An experimental investigation of water flow through an abrupt circular-channel expansion is described over a Reynolds number range between 20 and 4200. The shear layer between the central jet and the reverse flow region along the wall downstream behaved differently in the various flow regimes that were observed. With increasing Reynolds number these regimes changed progressively from a laminar flow to an unstable vortex sheetlike flow and then to a more random fluctuating flow. The distance between the step and the reattachment location downstream correspondingly increased, reached a maximum, and then decreased. Of particular significance are the shear layer wave instabilities observed in the shear flow and their relationship to rettachment which apparently has not received much attention previously. Visual observations aided in understanding the results.

Laminar Mixed Convection Adjacent to Three-Dimensional Backward-Facing Step

2001 ◽  
Vol 124 (1) ◽  
pp. 209-213 ◽  
Author(s):  
A. Li and ◽  
B. F. Armaly
Keyword(s):  
Nusselt Number ◽  
Mixed Convection ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Flow Region ◽  
The Other ◽  
Backward Facing Step ◽  
Number Range ◽  
Grashof Number ◽  
Recirculation Flow

Simulations of three-dimensional laminar buoyancy-assisting mixed convection adjacent to a backward-facing step in a vertical rectangular duct are presented to demonstrate the influence of Grashof number on the distributions of the Nusselt number, and the reverse flow regions that develop adjacent to the duct’s walls. The Reynolds number, and duct’s geometry are kept constant: heat flux at the wall downstream from the step is kept uniform but its magnitude varied to cover a Grashof number range of 0–4000; all the other walls in the duct are kept at adiabatic condition; and the flow, upstream of the step, is treated as fully developed and isothermal. Increasing the Grashof number results in increasing the Nusselt number; the size of the secondary recirculation flow region adjacent to the stepped wall; the size of the reverse flow region adjacent to the sidewall and the flat wall; and the spanwise flow from the sidewall toward the center of the duct. On the other hand, the size of the primary recirculation flow region adjacent to the stepped wall decreases and detaches partially from the heated stepped wall as the Grashof number increases. Details are presented and discussed.

Vortex Detachment and Reverse Flow in Pulsatile Laminar Flow Through Axisymmetric Sudden Expansions

1999 ◽  
Vol 121 (3) ◽  
pp. 574-579 ◽  
Author(s):  
S. Tavoularis ◽  
R. K. Singh
Keyword(s):  
Reynolds Number ◽  
Recirculation Zone ◽  
Reverse Flow ◽  
Flow Region ◽  
Vortex Rings ◽  
Mean Velocity ◽  
Zone Length ◽  
Deceleration Phase ◽  
Pulsatile Flows ◽  
Flow Through

Incompressible, steady and pulsatile flows in axisymmetric sudden expansions with diameter ratios of 1:2.25 and 1:2.00 have been simulated numerically over the ranges of time-averaged bulk Reynolds number 0.1 ≤ Re ≤ 400 and Womersley number 0.1 ≤ W ≤ 50. For steady flow, the calculated recirculation zone length increased linearly with an increase in Re, in good agreement with earlier experiments. For pulsatile flows, particularly at higher values of W, the recirculation zone length correlated strongly with the acceleration of the flow and not with the instantaneous Reynolds number; it increased during the deceleration phase and decreased during the acceleration phase. The computed mean velocity and reattachment length were in general agreement with published experimental data. At relatively low W, the computed near-wall, reverse flow region extended along the full domain over part of the cycle, similarly to that in the experiments. At low values of W, the vortex rings created at the expansion remained attached and oscillated back and forth; for an intermediate range of W, they detached and moved downstream; at relatively high W, these vortices became, once more, attached.

Further Investigation of Discharge Coefficient for PTC 6 Flow Nozzle in High Reynolds Number

2015 ◽  
Author(s):  
Noriyuki Furuichi ◽  
Yoshiya Terao ◽  
Shinichi Nakao ◽  
Keiji Fujita ◽  
Kazuo Shibuya
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
High Reynolds Number ◽  
Discharge Coefficient ◽  
Flow Region ◽  
Number Range ◽  
Discharge Coefficients ◽  
Reynolds Number Range ◽  
High Reynolds

The discharge coefficients of the throat tap flow nozzle based on ASME PTC 6 are measured in wide Reynolds number range from Red=5.8×104 to Red=1.4×107. The nominal discharge coefficient (the discharge coefficient without tap) is determined from the discharge coefficients measured for different tap diameters. The tap effects are correctly obtained by subtracting the nominal discharge coefficient from the discharge coefficient measured. Finally, by combing the nominal discharge coefficient and the tap effect determined in three flow regions, that is, laminar, transitional and turbulent flow region, the new equations of the discharge coefficient are proposed in three flow regions.

Experimental and Numerical Investigation of Convective Heat Transfer in a Gas Turbine Can Combustor

2009 ◽  
Author(s):  
Sunil Patil ◽  
Santosh Abraham ◽  
Danesh Tafti ◽  
Srinath Ekkad ◽  
Yong Kim ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Convective Heat Transfer ◽  
Gas Turbine ◽  
Convective Heat ◽  
Swirl Number ◽  
Number Range ◽  
Heat Transfer Augmentation ◽  
Peak Location

Experiments and numerical computations are performed to investigate the convective heat transfer characteristics of a gas turbine can combustor under cold flow conditions in a Reynolds number range between 50,000 and 500,000 with a characteristic swirl number of 0.7. It is observed that the flow field in the combustor is characterized by an expanding swirling flow which impinges on the liner wall close to the inlet of the combustor. The impinging shear layer is responsible for the peak location of heat transfer augmentation. It is observed that as Reynolds number increases from 50,000 to 500,000, the peak heat transfer augmentation ratio (compared to fully-developed pipe flow) reduces from 10.5 to 2.75. This is attributed to the reduction in normalized turbulent kinetic energy in the impinging shear layer which is strongly dependent on the swirl number that remains constant at 0.7 with Reynolds number. Additionally, the peak location does not change with Reynolds number since the flow structure in the combustor is also a function of the swirl number. The size of the corner recirculation zone near the combustor liner remains the same for all Reynolds numbers and hence the location of shear layer impingement and peak augmentation does not change.

Local Flow and Heat Transfer Behavior in Convex-Louver Fin Arrays

1999 ◽  
Vol 121 (1) ◽  
pp. 136-141 ◽  
Author(s):  
N. C. DeJong ◽  
A. M. Jacobi
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Drop ◽  
Shear Layer ◽  
Local Flow ◽  
Number Range ◽  
Flow And Heat Transfer ◽  
Transfer Behavior ◽  
Strip Geometry

Local and surface-averaged measurements of convection coefficients and core pressure-drop data are provided for an array of convex-louver fins. For a Reynolds number range from 200 to 5400, these data are complemented with a flow visualization study and contrasted with new measurements from a similar offset-strip geometry. The results clarify the effects of boundary layer restarting, shear-layer unsteadiness, spanwise vortices, and separation, reattachment, and recirculation on heat transfer in the convex-louver geometry.

Experimental and Numerical Investigation of Convective Heat Transfer in a Gas Turbine Can Combustor

2010 ◽  
Vol 133 (1) ◽  
Author(s):  
Sunil Patil ◽  
Santosh Abraham ◽  
Danesh Tafti ◽  
Srinath Ekkad ◽  
Yong Kim ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Convective Heat Transfer ◽  
Gas Turbine ◽  
Convective Heat ◽  
Swirl Number ◽  
Number Range ◽  
Heat Transfer Augmentation ◽  
Peak Location

Experiments and numerical computations are performed to investigate the convective heat transfer characteristics of a gas turbine can combustor under cold flow conditions in a Reynolds number range between 50,000 and 500,000 with a characteristic swirl number of 0.7. It is observed that the flow field in the combustor is characterized by an expanding swirling flow, which impinges on the liner wall close to the inlet of the combustor. The impinging shear layer is responsible for the peak location of heat transfer augmentation. It is observed that as Reynolds number increases from 50,000 to 500,000, the peak heat transfer augmentation ratio (compared with fully developed pipe flow) reduces from 10.5 to 2.75. This is attributed to the reduction in normalized turbulent kinetic energy in the impinging shear layer, which is strongly dependent on the swirl number that remains constant at 0.7 with Reynolds number. Additionally, the peak location does not change with Reynolds number since the flow structure in the combustor is also a function of the swirl number. The size of the corner recirculation zone near the combustor liner remains the same for all Reynolds numbers and hence the location of shear layer impingement and peak augmentation does not change.

scholarly journals On vortex shedding from an airfoil in low-Reynolds-number flows

2009 ◽  
Vol 632 ◽  
pp. 245-271 ◽  
Author(s):  
SERHIY YARUSEVYCH ◽  
PIERRE E. SULLIVAN ◽  
JOHN G. KAWALL
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Coherent Structures ◽  
Flow Regimes ◽  
Reynolds Numbers ◽  
Frequency Scaling ◽  
Separated Shear Layer ◽  
Low Reynolds Number Flows ◽  
Low Reynolds

Development of coherent structures in the separated shear layer and wake of an airfoil in low-Reynolds-number flows was studied experimentally for a range of airfoil chord Reynolds numbers, 55 × 103 ≤ Rec ≤ 210 × 103, and three angles of attack, α = 0°, 5° and 10°. To illustrate the effect of separated shear layer development on the characteristics of coherent structures, experiments were conducted for two flow regimes common to airfoil operation at low Reynolds numbers: (i) boundary layer separation without reattachment and (ii) separation bubble formation. The results demonstrate that roll-up vortices form in the separated shear layer due to the amplification of natural disturbances, and these structures play a key role in flow transition to turbulence. The final stage of transition in the separated shear layer, associated with the growth of a sub-harmonic component of fundamental disturbances, is linked to the merging of the roll-up vortices. Turbulent wake vortex shedding is shown to occur for both flow regimes investigated. Each of the two flow regimes produces distinctly different characteristics of the roll-up and wake vortices. The study focuses on frequency scaling of the investigated coherent structures and the effect of flow regime on the frequency scaling. Analysis of the results and available data from previous experiments shows that the fundamental frequency of the shear layer vortices exhibits a power law dependency on the Reynolds number for both flow regimes. In contrast, the wake vortex shedding frequency is shown to vary linearly with the Reynolds number. An alternative frequency scaling is proposed, which results in a good collapse of experimental data across the investigated range of Reynolds numbers.

Is There a Best Fixed-Geometry Valve for Micropumps?

2004 ◽  
Author(s):  
Adrian R. Gamboa ◽  
Fred K. Forster
Keyword(s):  
Reynolds Number ◽  
Weighting Function ◽  
Reverse Flow ◽  
Flow Direction ◽  
Local Maximum ◽  
Optimal Shape ◽  
Pump Design ◽  
Number Range ◽  
Optimal Shapes ◽  
Reference Shape

Two frequently used geometries for fixed-valve micropumps are the nozzle-diffuser and Tesla-type valve. However, little work has been done to investigate the relative merits of optimal shapes for each type of valve. In this study 2D steady-state computational fluid dynamics coupled with a formal optimization procedure and experimental evaluation were performed to address this problem. Non-dimensionalization of the problem allowed a comparison of the two valve types independent of physical size, i.e. shape alone was studied. Optimal shapes were found based on maximizing calculated diodicity as a function of Reynolds number in conjunction with a weighting function used to control forward pressure drop. The optimal shape for each valve was then compared numerically and experimentally to reference valves similar to those reported in the literature. The optimal shape for each valve type was found to be significantly different from the reference shape and exhibited significantly improved performance. Both valve types achieved a maximum diodicity of approximately two in the range of Reynolds number 0 ≤ Re ≤ 2000. The optimal Tesla-type valve was characterized by a large return loop and shallow return loop angle. The optimal nozzle-diffuser was characterized by a very long diffuser section that prevented flow separation in the forward flow direction along with increased wall shear stress in the reverse flow direction. The diodicity vs Reynolds number curve for the Tesla-type valve monotonically increased, while the nozzle-diffuser exhibited a local maximum in the mid-Reynolds number range. These characteristics may play an important role when valve size is determined to maximize resonant behavior of a micropump. Thus they influence numerous pump design criteria such as target flow rate-pressure characteristics and overall pump size.

Density ratio effects on reacting bluff-body flow field characteristics

2012 ◽  
Vol 706 ◽  
pp. 219-250 ◽  
Author(s):  
Benjamin Emerson ◽  
Jacqueline O’Connor ◽  
Matthew Juniper ◽  
Tim Lieuwen
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Shear Layer ◽  
Parametric Excitation ◽  
Bluff Body ◽  
Density Ratio ◽  
Global Mode ◽  
Measured Velocity ◽  
Number Range ◽  
Flow Field Characteristics

AbstractThe wake characteristics of bluff-body-stabilized flames are a strong function of the density ratio across the flame and the relative offset between the flame and shear layer. This paper describes systematic experimental measurements and stability calculations of the dependence of the flow field characteristics and flame sheet dynamics upon flame density ratio,${\rho }_{u} / {\rho }_{b} $, over the Reynolds number range of 1000–3300. We show that two fundamentally different flame/flow behaviours are observed at high and low${\rho }_{u} / {\rho }_{b} $values: a stable, noise-driven fixed point and limit-cycle oscillations, respectively. These results are interpreted as a transition from convective to global instability and are captured well by stability calculations that used the measured velocity and density profiles as inputs. However, in this high-Reynolds-number flow, the measurements show that no abrupt bifurcation in flow/flame behaviour occurs at a given${\rho }_{u} / {\rho }_{b} $value. Rather, the flow field is highly intermittent in a transitional${\rho }_{u} / {\rho }_{b} $range, with the relative fraction of the two different flow/flame behaviours monotonically varying with${\rho }_{u} / {\rho }_{b} $. This intermittent behaviour is a result of parametric excitation of the global mode growth rate in the vicinity of a supercritical Hopf bifurcation. It is shown that this parametric excitation is due to random fluctuations in relative locations of the flame and shear layer.

On the Thermal Characteristics of Two-Phase, Liquid-Liquid, Non-Boiling Droplet Flow in Minichannels

2011 ◽  
Author(s):  
Marc Mac Giolla Eain ◽  
Vanessa Egan ◽  
Jeff Punch
Keyword(s):  
Reynolds Number ◽  
Capillary Number ◽  
Thermal Characteristics ◽  
Flow Regimes ◽  
Two Phase ◽  
Flux Boundary Condition ◽  
Number Range ◽  
Droplet Flow ◽  
Micro Fluidics ◽  
Phase Liquid

Two-phase flow regimes offer numerous advantages over traditional single phase flows, resulting in a wide variety of uses in diverse applications such as electronics cooling, heat exchange systems, pharmacology and biological micro-fluidics. This paper experimentally investigates the enhanced heat transfer rates attainable with two-phase liquid-liquid non-boiling droplet flow. A custom experimental facility was constructed, allowing the flow to be analysed in a minichannel geometry subjected to a constant heat flux boundary condition. Parameters of Reynolds number, Capillary number, droplet length and droplet spacing were varied during the course of the experimentation. The experiments were conducted over the Reynolds number range of 46 ≤ Re ≤ 71.8 and a Capillary number range of 0.00849 ≤ Ca ≤ 0.0102. The flow passed through a capillary of 1.5mm internal diameter and 0.25mm wall thickness. Local Nusselt numbers were obtained at the entrance region through the use of infrared thermography. Enhancements of 144% over fully developed Poiseuille flow were encountered. The findings of this paper highlight the thermal characteristics of two-phase liquid-liquid flow regimes and are of practical relevance to applications in both the thermal management and biological micro-fluidics industries.

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