Entry Lengths of Laminar Pipe and Channel Flows

2018 ◽  
Vol 140 (6) ◽  
Author(s):  
Yash Joshi ◽  
B. R. Vinoth
Keyword(s):  
Reynolds Numbers ◽  
Channel Flows ◽  
Newtonian Flow ◽  
Low Reynolds Numbers ◽  
Entry Length ◽  
Newtonian Flows ◽  
Integral Measure ◽  
Displacement Thickness ◽  
Inlet Conditions ◽  
New Entry

Numerical simulations of laminar pipe and channel flows were carried out: (i) to understand the effect of inlet conditions, viz., flat inlet and streamtube inlet, on entry lengths, and (ii) to investigate the flow development in radial/transverse locations. Results show that hydrodynamic entry lengths from the streamtube inlet simulations are significantly lower compared to the entry lengths from the flat inlet simulations for low Reynolds numbers. Moreover, results from the current study (Newtonian flow with no-slip) as well as the results from the literature (non-Newtonian flow with no-slip) showed that for many flow situations, the slowest development of axial velocity in the transverse location happens to be very near to the wall. For the above cases, the existing entry length criteria (centerline as well as global entry length) are not appropriate to define the entry length. We have proposed a new entry length criterion based on the displacement thickness which is an integral measure of the velocity profile. A new entry length correlation using the displacement thickness criterion is proposed for Newtonian flows in pipe and channel based on simulations with the streamtube inlet condition.

scholarly journals Investigation of Riblet Geometry and Start Locations of Herringbone Riblets on Pressure Losses in a Linear Cascade at Low Reynolds Numbers

2020 ◽  
Vol 142 (10) ◽  
Author(s):  
Qiang Liu ◽  
Shan Zhong ◽  
Lin Li
Keyword(s):  
Reynolds Number ◽  
Reynolds Numbers ◽  
Effective Control ◽  
Slight Reduction ◽  
Turning Angle ◽  
Low Reynolds Numbers ◽  
Pressure Losses ◽  
Lower Reynolds Number ◽  
Displacement Thickness ◽  
Low Reynolds

Abstract In this paper, the effects of an array of herringbone riblets with different riblet geometry (height and spacing) and start locations on the pressure losses in a cascade of diffuser blades are investigated over a range of low Reynolds numbers (0.50 × 105–1.00 × 105). The herringbone riblets with a given geometry are found to produce a profound modification to the wake structure above certain critical Reynolds numbers. It is also found that within the range of parameters tested an increase in riblet height and riblet spacing results in an onset of significant control effect at a lower Reynolds number, which is accompanied by a slight reduction in zone-averaged loss coefficient and flow turning angle. An upstream shift of the start position of the riblet array along the blades enables the riblets to become effective at a lower Reynolds number at the expense of a reduced loss reduction and flow turning angle. A semi-empirical relationship between the ratio of riblet height to local baseline boundary layer displacement thickness and the critical Reynolds number is established using the present experimental data. A preliminary methodology for designing the herringbone riblets to ensure an effective control of 2D flow separations around the mid-span of diffuser blades over a specified range of Reynolds numbers is also proposed.

scholarly journals Experimental Investigations of the Influence of Incoming Wakes on the Losses of a Linear Turbine Cascade

1993 ◽  
Author(s):  
M. Ladwig ◽  
L. Fottner
Keyword(s):  
Suction Side ◽  
Reynolds Numbers ◽  
Experimental Investigations ◽  
Turbine Cascade ◽  
Flow Conditions ◽  
Inlet Flow ◽  
Low Reynolds Numbers ◽  
Blade Passage ◽  
Inlet Conditions ◽  
Hot Film

The objective of this work is to enhance the understanding of the influence of wake induced non-uniform, steady inlet flow conditions on the profile losses of highly-loaded turbines. For different Reynolds numbers wake and profile pressure distribution measurements were carried out on a linear subsonic turbine cascade as well as measurements with a single sensor hot-film probe. The non-uniform inlet flow were simulated with two different cascades of cylindrical bars. The measurements with various circumferential positions of the incoming wakes relative to the turbine cascade show at low Reynolds numbers a decrease of the losses compared to uniform inlet conditions, because no separation of the suction side boundary layer occurs. With increasing Reynolds numbers the non-uniform inlet flow conditions cause an increase in the losses compared to uniform inlet conditions, due to the forward shift of transition. Generally, the smallest influence of the non-uniform incoming flow can be observed when the wakes enter the cascade inlet plane between the pressure-side of the profiles and the middle of the blade passage. Incoming wakes have the highest influence when they enter the blade passage near to the suction side of the profiles.

On the Anisotropic Vorticity in Turbulent Channel Flows

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Helge I. Andersson ◽  
Lihao Zhao ◽  
Evan A. Variano
Keyword(s):  
Channel Flow ◽  
Turbulent Channel Flow ◽  
Reynolds Numbers ◽  
Channel Flows ◽  
Vorticity Field ◽  
Particle Suspensions ◽  
Low Reynolds Numbers ◽  
Turbulent Channel Flows ◽  
Low Reynolds

Revisiting the fluctuating vorticity field in the centerplane of a turbulent channel flow, we show that the vorticity is distinctly anisotropic at low Reynolds numbers (Re). This result is in contrast with some earlier conclusions. The anisotropy is a function of Re, and we have compiled data to show that the anisotropy gradually vanishes with increasing Re. Acknowledging the anisotropy is important for current efforts on simulating turbulent particle suspensions.

Effects of viscoelasticity in the high Reynolds number cylinder wake

2012 ◽  
Vol 693 ◽  
pp. 297-318 ◽  
Author(s):  
David Richter ◽  
Gianluca Iaccarino ◽  
Eric S. G. Shaqfeh
Keyword(s):  
High Reynolds Number ◽  
Reynolds Numbers ◽  
Shear Layers ◽  
Cylinder Wake ◽  
Helmholtz Instability ◽  
Newtonian Flow ◽  
Newtonian Flows ◽  
Free Shear Layers ◽  
High Reynolds ◽  
Far Wake

AbstractAt $\mathit{Re}= 3900$, Newtonian flow past a circular cylinder exhibits a wake and detached shear layers which have transitioned to turbulence. It is the goal of the present study to investigate the effects which viscoelasticity has on this state and to identify the mechanisms responsible for wake stabilization. It is found through numerical simulations (employing the FENE-P rheological model) that viscoelasticity greatly reduces the amount of turbulence in the wake, reverting it back to a state which qualitatively appears similar to the Newtonian mode B instability which occurs at lower $\mathit{Re}$. By focusing on the separated shear layers, it is found that viscoelasticity suppresses the formation of the Kelvin–Helmholtz instability which dominates for Newtonian flows, consistent with previous studies of viscoelastic free shear layers. Through this shear layer stabilization, the viscoelastic far wake is then subject to the same instability mechanisms which dominate for Newtonian flows, but at far lower Reynolds numbers.

Characteristics of Coherent Structures in Channel Flows During Low Reynolds Number Mixed Convection

2014 ◽  
Author(s):  
Ahmed Elatar ◽  
Kamran Siddiqui
Keyword(s):  
Mixed Convection ◽  
Coherent Structures ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Velocity Fields ◽  
Channel Flows ◽  
Heated Wall ◽  
Low Reynolds Numbers ◽  
Square Channel ◽  
Low Reynolds

Characteristics of coherent structures generated in channel flows during low Reynolds numbers mixed convection have been investigated in a square channel. The Gr/Re2 ranged between 21 and 206 which indicates that natural convection was dominant over forced convection. Two-dimensional velocity fields were measured using particle image velocimetry (PIV) technique in different planes to obtain a three-dimensional perspective of the flow field in the channel. The coherent structures were detected from the turbulent velocity fields using an algorithm based on the velocity gradient tensor second invariant (Q). The location of each detected coherent structure was recorded and its turbulent kinetic energy was computed. It was found that the strength of coherent structures increased with an increase in the bottom wall temperature. The results also indicate that the coherent structures present in the region away from the bottom heated wall were more energetic compared to the coherent structures present within the thermal boundary layer.

Oscillatory Flow Phenomena in Diffusers at Low Reynolds Numbers

1973 ◽  
Vol 95 (3) ◽  
pp. 401-407 ◽  
Author(s):  
A. H. Stenning ◽  
A. A. Schachenmann
Keyword(s):  
Boundary Layer ◽  
Oscillatory Flow ◽  
Travelling Waves ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Inlet Velocity ◽  
Wall Boundary Layer ◽  
Displacement Thickness ◽  
Flow Phenomena ◽  
Flow Oscillations

In studies of a diffuser operating with inlet flow oscillations, it has been found that large amplification of the inlet velocity oscillations occurs within the diffuser when the throat Reynolds number lies in the range 103 to 104. This phenomenon is caused by travelling waves in the wall boundary layer which are initiated in the laminar portion of the boundary layer and propagate into the turbulent boundary layer, causing large variations in the displacement thickness.

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