scholarly journals Relaminarization of wall turbulence by high-pressure ramps at low Reynolds numbers

2016 ◽  
Vol 20 (suppl. 1) ◽  
pp. 93-102 ◽  
Author(s):  
Kwonyul Song ◽  
Jovan Jovanovic ◽  
Ahmed Al-Salaymeh ◽  
Cornelia Rauh ◽  
Antonio Delgado
Keyword(s):  
High Pressure ◽  
Pipe Flow ◽  
Silicone Oil ◽  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Working Fluid ◽  
Hot Wire ◽  
Low Reynolds Numbers ◽  
Reverse Transition ◽  
Low Reynolds

Reverse transition from the turbulent towards the laminar flow regime was investigated experimentally by progressively increasing the pressure up to 400 MPa in a fully developed pipe flow operated with silicone oil as the working fluid. Using hot-wire anemometry, it is shown indirectly that at low Reynolds numbers a rapid increase in pressure modifies the turbulence dynamics owing to the processes which induce the effects caused by fluid compressibility in the region very close to the wall. The experimental results confirm that under such circumstances, the traditional mechanism responsible for self-maintenance of turbulence in wall-bounded flows is altered in such a way as to lead towards a state in which turbulence cannot persist any longer.

On transition in a pipe. Part 2. The equilibrium puff

1975 ◽  
Vol 69 (2) ◽  
pp. 283-304 ◽  
Author(s):  
I. Wygnanski ◽  
M. Sokolov ◽  
D. Friedman
Keyword(s):  
Pipe Flow ◽  
Reynolds Numbers ◽  
Turbulent Pipe Flow ◽  
Hot Wire ◽  
Low Reynolds Numbers ◽  
Onset Of Turbulence ◽  
Large Perturbation ◽  
Low Reynolds ◽  
Insight Into ◽  
Type Of Transition

Conditionally sampled hot-wire measurements were taken in a pipe at low Reynolds numbers (2700 > Re > 2000) corresponding to the onset of turbulence as a result of a large perturbation in the flow. This type of transition gives rise to a turbulent puff which maintains itself indefinitely at around Re = 2200. The structure of puffs was investigated in some detail and was found to be very different from the structure of fully developed turbulent pipe flow. Nevertheless, it is independent of the character of the disturbance which created it. The purpose of the study was to gain some insight into the mechanism of transition in a pipe.

scholarly journals Dynamics of viscoelastic pipe flow at low Reynolds numbers in the maximum drag reduction limit

2019 ◽  
Vol 874 ◽  
pp. 699-719 ◽  
Author(s):  
Jose M. Lopez ◽  
George H. Choueiri ◽  
Björn Hof
Keyword(s):  
Drag Reduction ◽  
Turbulent Flows ◽  
Pipe Flow ◽  
Domain Size ◽  
Reynolds Numbers ◽  
Weissenberg Number ◽  
Pipe Length ◽  
Low Reynolds Numbers ◽  
Maximum Drag Reduction ◽  
Low Reynolds

Polymer additives can substantially reduce the drag of turbulent flows and the upper limit, the so-called state of ‘maximum drag reduction’ (MDR), is to a good approximation independent of the type of polymer and solvent used. Until recently, the consensus was that, in this limit, flows are in a marginal state where only a minimal level of turbulence activity persists. Observations in direct numerical simulations at low Reynolds numbers ($Re$) using minimal sized channels appeared to support this view and reported long ‘hibernation’ periods where turbulence is marginalized. In simulations of pipe flow at $Re$ near transition we find that, indeed, with increasing Weissenberg number ($Wi$), turbulence expresses long periods of hibernation if the domain size is small. However, with increasing pipe length, the temporal hibernation continuously alters to spatio-temporal intermittency and here the flow consists of turbulent puffs surrounded by laminar flow. Moreover, upon an increase in $Wi$, the flow fully relaminarizes, in agreement with recent experiments. At even larger $Wi$, a different instability is encountered causing a drag increase towards MDR. Our findings hence link earlier minimal flow unit simulations with recent experiments and confirm that the addition of polymers initially suppresses Newtonian turbulence and leads to a reverse transition. The MDR state on the other hand results at these low$Re$ from a separate instability and the underlying dynamics corresponds to the recently proposed state of elasto-inertial turbulence.

Low Reynolds-Number Experiments in an Axial-Flow Turbomachine

1964 ◽  
Vol 86 (3) ◽  
pp. 257-295 ◽  
Author(s):  
J. Neustein
Keyword(s):  
Reynolds Number ◽  
Guide Vane ◽  
Axial Flow ◽  
Reynolds Numbers ◽  
Low Flow ◽  
Working Fluid ◽  
Low Reynolds Numbers ◽  
Blade Row ◽  
Overall Performance ◽  
Low Reynolds

The performance of a single-stage, axial-flow turbomachine was studied experimentally at low Reynolds numbers. The study was made with a turbomachine modeled from a large jet-engine type of axial-flow compressor. Low Reynolds numbers were obtained by using a mixture of glycerine and water as the working fluid. The overall performance was determined over a range of Reynolds numbers RT (based on rotor-tip speed and rotor chord) from 2000 to 150,000. The flow rate at each Reynolds number was varied from near shutoff to the maximum permitted by the turbomachine-tunnel systems. Blade-row characteristics were studied by means of quantitative flow surveys before and after each blade row, and by means of extensive flow-visualization experiments within each blade row. The investigation established that sudden or critical changes in performance do not occur in the type of machine tested, between RT of 150,000 and 20,000. Below 20,000 the performance deteriorated more rapidly. A relatively sharp change in performance occurred between RT of 20,000 and 10,000. The results clarified many of the viscous flow details in each blade row which are associated with the deterioration of performance. These effects were very pronounced at RT of 4000 and below. Consequently, a considerable part of the paper is concerned with results obtained at these lower Reynolds numbers. From the point of view of a designer, information is presented in regard to overall performance, guide-vane turning, and guide-vane and stator total-pressure losses, all as functions of Reynolds number. These results are expected to be indicative of performance in turbomachines similar to the one tested here. Other details are concerned with problems such as wall boundary layers, flow reversal at low flow coefficients, lip-clearance flow, flow patterns near shutoff, and flow comparisons in stators with rotating and stationary hubs.

An investigation of turbulent plane Couette flow at low Reynolds numbers

1995 ◽  
Vol 286 ◽  
pp. 291-325 ◽  
Author(s):  
Knut H. Bech ◽  
Nils Tillmark ◽  
P. Henrik Alfredsson ◽  
Helge I. Andersson
Keyword(s):  
Couette Flow ◽  
Reynolds Stress ◽  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Turbulence Structure ◽  
Plane Couette Flow ◽  
Mean Flow ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Near Wall

The turbulent structure in plane Couette flow at low Reynolds numbers is studied using data obtained both from numerical simulation and physical experiments. It is shown that the near-wall turbulence structure is quite similar to what has earlier been found in plane Poiseuille flow; however, there are also some large differences especially regarding Reynolds stress production. The commonly held view that the maximum in Reynolds stress close to the wall in Poiseuille and boundary layer flows is due to the turbulence-generating events must be modified as plane Couette flow does not exhibit such a maximum, although the near-wall coherent structures are quite similar. For two-dimensional mean flow, turbulence production occurs only for the streamwise fluctuations, and the present study shows the importance of the pressure—strain redistribution in connection with the near-wall coherent events.

Investigation of Bypass Transition in the Presence of a Separation Bubble With Tomographic PIV

2016 ◽  
Author(s):  
Christoph Lyko ◽  
Dirk Michaelis ◽  
Dieter Peitsch ◽  
Mirko Dittmar
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Bypass Transition ◽  
Hot Wire ◽  
Integral Boundary ◽  
Low Reynolds Numbers ◽  
Tomographic Piv ◽  
Low Reynolds

Low pressure turbines of small and medium sized engines may operate at very low Reynolds numbers. In consequence transition is delayed to an extend where laminar separation, detached transition and reattachment occur. The wakes from upstream blade rows lead to overall high turbulence levels which play a key role in the transition process. Freestream eddies buffeting the laminar boundary layer induce streamwise vortices known as Klebanoff Modes. To investigate this type of flow a flat plate was exposed to a pressure distribution. It is based on the PAK-B suction side and was created by a contoured wall facing the plate. The PAK-B is a Pratt & Whitney design and a Mach number scaled version of a highly aft loaded low pressure turbine airfoil. Due to the latter it suffers from a large separation bubble at low Reynolds numbers. The flow has been intensively investigated by hot-wire anemometry with a very high spatial resolution. This allows obtaining very precise information about the location of characteristic flow areas; for instance the separation and reattachment positions. Based on this information, Tomographic PIV was employed to expose detailed features in specific areas of the flow field. This technique provides the velocity vector information inside a flow volume. It complements hot-wire results, which give a time resolved information but only planar velocity magnitudes. Combining these techniques and comparing their results is therefore an excellent way to raise the physical understanding of the flow behaviour. This has been done using velocity profiles, skin friction coefficients and integral boundary layer parameters. As the 3D-PIV information allows calculation of derived quantities, like the vector field rotation, a picture of the coherent structures can be drawn.

scholarly journals A systematic investigation of roughness height and wavelength in turbulent pipe flow in the transitionally rough regime

2015 ◽  
Vol 771 ◽  
pp. 743-777 ◽  
Author(s):  
L. Chan ◽  
M. MacDonald ◽  
D. Chung ◽  
N. Hutchins ◽  
A. Ooi
Keyword(s):  
Outer Layer ◽  
Pipe Flow ◽  
Roughness Parameter ◽  
Reynolds Numbers ◽  
Turbulent Pipe Flow ◽  
Roughness Height ◽  
Average Height ◽  
Low Reynolds Numbers ◽  
Virtual Origin ◽  
Low Reynolds

Direct numerical simulations (DNS) are conducted for turbulent flow through pipes with three-dimensional sinusoidal roughnesses explicitly represented by body-conforming grids. The same viscous-scaled roughness geometry is first simulated at a range of different Reynolds numbers to investigate the effects of low Reynolds numbers and low $R_{0}/h$, where $R_{0}$ is the pipe radius and $h$ is the roughness height. Results for the present class of surfaces show that the Hama roughness function ${\rm\Delta}U^{+}$ is only marginally affected by low Reynolds numbers (or low $R_{0}/h$), and observations of outer-layer similarity (or lack thereof) show no signs of sensitivity to Reynolds number. Then, building on this, a systematic approach is taken to isolate the effects of roughness height $h^{+}$ and wavelength ${\it\lambda}^{+}$ in a turbulent wall-bounded flow in both transitionally rough and fully rough regimes. Current findings show that while the effective slope $\mathit{ES}$ (which for the present sinusoidal surfaces is proportional to $h^{+}/{\it\lambda}^{+}$) is an important roughness parameter, the roughness function ${\rm\Delta}U^{+}$ must also depend on some measure of the viscous roughness height. A simplistic linear–log fit clearly illustrates the strong correlation between ${\rm\Delta}U^{+}$ and both the roughness average height $k_{a}^{+}$ (which is related to $h^{+}$) and $\mathit{ES}$ for the surfaces simulated here, consistent with published literature. Various definitions of the virtual origin for rough-wall turbulent pipe flow are investigated and, for the surfaces simulated here, the hydraulic radius of the pipe appears to be the most suitable parameter, and indeed is the only virtual origin that can ever lead to collapse in the total stress. First- and second-order statistics are also analysed and collapses in the outer layer are observed for all cases, including those where the largest roughness height is a substantial proportion of the reference radius (low $R_{0}/h$). These results provide evidence that turbulent pipe flow over the present sinusoidal surfaces adheres to Townsend’s notion of outer-layer similarity, which pertains to statistics of relative motion.

scholarly journals On the wake flow behind a sphere in a pipe flow at low Reynolds numbers

2020 ◽  
Vol 32 (10) ◽  
pp. 103605 ◽  
Author(s):  
Guang Yin ◽  
Muk Chen Ong
Keyword(s):  
Pipe Flow ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Low Reynolds Numbers ◽  
Low Reynolds

An experimental study on the behaviours of circular synthetic jets at low Reynolds numbers

2012 ◽  
Vol 226 (11) ◽  
pp. 2686-2700 ◽  
Author(s):  
Qingfeng Xia ◽  
Shan Zhong
Keyword(s):  
Reynolds Number ◽  
Silicone Oil ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Operating Conditions ◽  
Flow Visualisation ◽  
Stroke Length ◽  
Low Reynolds Numbers ◽  
Low Reynolds

In the work presented in this article, the behaviour of circular synthetic jets issuing into quiescent surrounding fluid at low Reynolds numbers is experimentally studied for potential mixing applications of synthetic jets at micro-scales or in highly viscous fluids. Sugar solutions and silicone oil are used as the flow media in order to achieve the required low Reynolds numbers. The conditions for jet instability, vortex rollup and synthetic jet formation are investigated using both flow visualisation techniques and particle image velocimetry, and the typical behaviour of synthetic jets at a Reynolds number around unity is also illustrated. The roles of Reynolds number, dimensionless stroke length and Stokes number in determining the characteristics of synthetic jets are examined and found to be largely consistent with the finding obtained at higher Reynolds numbers. Finally, a parameter map of synthetic jet flow patterns is produced based on the results from this study, which can be used to aid the choice of synthetic jet operating conditions for specific applications or anticipate if a desired vortex structure can be obtained at a given synthetic jet operating condition.

scholarly journals Hot-wire investigation of the wake behind cylinders at low Reynolds numbers

1949 ◽  
Vol 198 (1053) ◽  
pp. 174-190 ◽  
Keyword(s):  
Reynolds Number ◽  
Critical Reynolds Number ◽  
Reynolds Numbers ◽  
Sufficient Information ◽  
Hot Wire ◽  
Mean Velocity ◽  
Phase Measurements ◽  
Low Reynolds Numbers ◽  
Laminar Wake ◽  
Low Reynolds

The hot-wire technique has been used to measure the regular vortex street pattern behind a cylinder at low Reynolds number. Measurements of mean velocity distribution were made both below and above the critical Reynolds number at which the periodic motion appears. Amplitude and phase measurements gave sufficient information for computation of the instantaneous flow pattern of the vortex system. The important points resulting from the investigation are that (i) the critical Reynolds number at which vortices are shed is 40, (ii) in the range of Reynolds numbers investigated the vortices are not shed directly from the cylinder but appear some distance downstream as an instability of the laminar wake.

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