Turbulent Slugs Research Articles

Laminar-to-Turbulent Transition of Pipe Flows Triggered by Wall-Mounted, Ring-Type Obstacles

The literature stresses the inherent stability of laminar pipe flows with parabolic velocity profiles and this paper refers to the relevant publications summarizing this work. To cause such flows to turn into their turbulent state requires laminar pipe flows to be triggered externally. Ring-type, wall-mounted obstacles can be used for this purpose, and investigations in this area are of particular interest to the authors’ work, summarized in this paper. In the investigations presented here, however, a special triggering technique was employed that allowed laminar pipe flows to be exposed to obstacle disturbances for only about 30 ms. Individual puffs and slugs could be produced in this way. Comparisons with fixed wall-mounted obstacles showed that the properties of both types of turbulent slugs were the same. Theoretical derivations are described to provide the required obstacle height as a function of the Reynolds number, to trigger fully developed laminar pipe flows to turn into their turbulent state. Corresponding experimental investigations were also performed as described. Very good agreement between the theoretical and experimental results was obtained. All this demonstrates that a relatively simple ‘ad hoc theory’ can derive the required height of ring-type, wall-inserted obstacles to trigger laminar pipe flows with parabolic velocity profiles to turn turbulent. Other ways to trigger laminar pipe flows to turn turbulent were also investigated by employing blowers and plenum chambers and varying the lengths and diameters of pipes. It is demonstrated, in a somewhat qualitative way, that the maintenance of laminar pipe flows requires all components of a test rig to be matched to each other to maintain pipe flows laminar.

Coherent structures in transitional pipe flow

Transition to turbulence in pipe flow is investigated experimentally using a temporally resolved dual-plane particle image velocimetry approach, at a Reynolds number of 3440. The flow is analyzed using proper orthogonal decomposition and it is shown that the flow can be divided into two regions: a pseudolaminar region governed by the presence of azimuthally steady traveling waves, and turbulent slugs. The evolution of the structures within the slugs is identified by using the temporally resolved data along with the dual-plane velocity field. These structures are shown to be remarkably similar to the large-scale motions found in fully turbulent flows, with a streamwise and spatiotemporal extent about four pipe radii. The transition between structures is characterized by the detachment and decay of an old structure and the initiation of a new structure at the wall

Experimental Investigation of Transitional Flow in Porous Media with Usage of a Pore Doublet Model

The transition from laminar to turbulent flow in porous media is studied with a new method. To mimic interconnected pores, a simplified geometry consisting of a pipe with a relatively large diameter that is split into two parallel pipes with different diameters is studied. This is a pore-doublet setup and the pressure drops over the parallel pipes are recorded by pressure transducers for different flow rates. Results show that the flow in the parallel pipes is redistributed when turbulent slugs pass through one of them. The presence of the slugs is revealed by positive skewness in the pressure signals as well as an increase of the standard deviation of the pressure drops and correlation between the pressure drops of the pipes. A frequency analysis of the pressure drops show that lower band frequency pressure variations in one pipe are communicated to the other pipe while higher band frequencies are filtered out.

Directed percolation describes lifetime and growth of turbulent puffs and slugs

We show that directed percolation (DP) simulations in a pipe geometry in 3+1 dimensions capture the observed complex phenomenology of the transition to turbulence. At low Reynolds numbers (Re), turbulent puffs form and spontaneously relaminarize. At high Re, turbulent slugs expand uniformly into the laminar regions. In a spatiotemporally intermittent state between these two regimes of Re, puffs split and turbulent regions exhibit laminar patches. DP also captures some of the quantitative features of the transition, with a superexponentially diverging characteristic lifetime below the transition. Above the percolation threshold, active (turbulent) clusters expand into the inactive (laminar) phase with a well-defined velocity whose scaling with control parameter (Reynolds number or percolation probability) is consistent with experimental results. Our results provide strong evidence in favor of a conjecture of Pomeau.

Visualization of transitional pipe flow using the photochromic tracer method

Two noninvasive measurement techniques are described for investigating transitional flow in a rigid straight tube. Photochromic trace recordings, representing fluid displacement profiles, were made within the core and at the leading and trailing edges of turbulent slugs generated by a disturbance at the tube inlet. Consistent photochromic trace recordings were made possible through the discovery that Doppler ultrasound can be used to detect the passage of turbulent slugs, thereby allowing precise timing of the photochromic traces. Upstream and downstream Doppler probes enabled the leading and trailing edge velocities of slugs to be measured and also enabled the laminar/turbulent transition regions to be investigated in detail. Trailing edge interface velocities were in excellent agreement with existing data, while leading edge velocities agreed less well. Turbulence generation at the trailing edge appeared to be associated with a localized vortical structure which originated in the near wall region and then grew toward the tube centerline. The generation of this vortex is postulated to be induced by a shear layer instability. Mean velocity profiles obtained from within the core of turbulent slugs were similar to those obtained for fully developed turbulent flow.

The structure of turbulence in pulsatile pipe flows

This paper describes the fundamental feature of pulsatile transitional and fully turbulent pipe flows. First, the effect of pulsation on the behavior of turbulent slugs in the developing region of circular pipe is clarified. Second, the distributions of turbulence intensity and Reynolds shear stress in fully turbulent pulsatile pipe flow are compared with their respective distributions in fully turbulent steady pipe flow. Generation region of turbulence and radial propagation time of the turbulence are determined from these distributions. Finally the turbulence structure in pulsatile pipe flows with and without relaminarization, i. e., reverse transition, is made clear by means of the conditional sampling method based on the four quadrant classification.

Behavior of Turbulent Slugs in a Transient Pipe Flow

Behavior of Turbulent Slugs in a Transient Pipe Flow

Behavior of turbulent slug in transient pipe flow.

The generation and propagation of turbulent slugs in a circular pipe were studied under transient flow conditions. The Reynolds number of the final steady state was chosen above the critical Reynolds number of the experimental apparatus. A turbulent slug initiated bout 10 diameters downstream of the entrance, which was preceded by unstable waves with frequency very similar to that of the Tollmien-Schilchting wave. After the passage of the first slug many slugs appeared in the region of x/D ≒ 14 40, where x is the axial distance and D is the pipe diameter. The axial propagation velocities of the leading and trailing edges of the first slug were determined and compared with those of slugs in steady pipe flows. Velocity distributions inside and outside the slug were discussed.

Turbulent bursts, spots and slugs in a generalized Ginzburg-Landau equation

Abstract The time-dependent generalized Ginzburg-Landau equation is generalized by adding a higher-order nonlinearity and by allowing the coefficients to be a function of space. With the addition of external noise, this equation exhibits turbulent bursts, spots and slugs similar to those observed in many open-flow fluid systems. The external noise plays an important role in the spatio-temporal intermittency that is observed. Ideas presented in this paper provide a conceptual framework in which to better understand behaviour occuring in pipe flow and other open-flow systems. A few experiments to support some of these ideas and a mechanism for slug production in pipe flow are suggested.

The propagation of turbulent slugs in a pulsatile pipe flow.

Hot wire measurements were made on the propagation of turbulent slugs in a pulsatile pipe flow. The pulsatile pipe flow was produced by imposing a sinusoidal oscillation on a steady flow. Two types of slug behavior are observed according to whether the pulsation frequency f is low or high, and this paper presents two typical examples with f=0.300 and 2.082 Hz. In both examples the ap·pear·ance of the turbulent slug is periodic, in contrast to the randomness that is characteristic of transition in steady flows. Such periodicity is also known in transitional pulsatile boundary layers on a flat plate. At f=0.300 Hz a turbulent slug generates near the entrance (x/D≒3) and propagates downstream, where x is the axial coordinate and D is the pipe diameter. The velocities of leading and trailing edges of the slug can be predicted by steady flow data. Meanwhile at f=2.082 Hz numerous slugs are generated in the region downstream of x/D≒25 in the middle stage of the accelerating period. After a short time, a new slug appears at x/D≒3 and propagates downstream. Later its leading edge merges with the preceding slugs at x/D≒40 and then these slugs become a single slug. This slug disappears at almost the same time as in an earlier stage of the following accelerating period, due to the combined effect of viscous dissipation and acceleration. The velocity and tur·bu·lence intensity downstream of x/D≒50 were found not to change axially.

Laminar-turbulent transition in a slowly pulsating pipe flow

Transitional pulsating flow in a pipe is investigated experimentally in the frequency region where its behavior is assumed to be quasisteady. Instantaneous velocity measurements were performed at several radial locations at the exit plane of the pipe. The output of flush-mounted hot wires at two upstream positions was also recorded simultaneously. The results indicate that the quasisteady assumption is valid in general. The flow behavior is the function primarily of the instantaneous Reynolds number, although the laminarization process and retransition to turbulence are qualitatively different. The laminarization at subcritical instantaneous Reynolds numbers in a time-dependent flow is a gradual process, similar to the observed turbulence decay in flow geometries where Re is varied spatially. The retransition to turbulence occurs by means of the generation of turbulent slugs at various locations along the pipe. These slugs are convected downstream and coalesce, eventually resulting in fully developed turbulent flow in the whole pipe at higher instantaneous Reynolds numbers.

Triggered transition in the pipe flow of dilute solutions of random-coiling macromolecules

Laminar-to-turbulent transition was studied experimentally in the flow of dilute, distilled water solutions of two polethyleneoxide polymers through three pipes, each 200 diam long and equipped with a ’’trigger’’ orifice, of i.d. half the pipe i.d., to provide a strong inlet disturbance. Polymer molecular weights, 0.7×106 and 5.0×106, and pipe sizes, 0.292, 0.457, 0.945 cm i.d., were chosen to cause transition from the laminar regime (L) into each of three turbulent regimes associated with drag reduction,1 namely, Newtonian (N), polymeric (P), and maximum drag reduction (M). The corresponding three types of laminar-to-turbulent transitions observed, designated L→N, L→P, L→M, had the following characteristics: (1) L→N. In this case the transition occurred from laminar flow into a turbulent regime wherein the polymer solutions did not cause drag reduction, and the transition process appeared to be the same as in the usual Newtonian case.2,3 (2) L→P. In this case the polymer solutions exhibited drag reduction at the lowest fully turbulent flow rates. The critical Reynolds number, below which turbulent slugs were not observed, was essentially the same as Newtonian. At a given Re in the transitional regime, the intermittency factor at the pipe axis was distinctly greater than Newtonian, the more so with increasing drag reducing ability of the polymer solution. The greater intermittency factor seemed to result mainly from an increased turbulent slug formation frequency relative to Newtonian; turbulent slug lengths had mean values essentially the same as Newtonian, although the dispersion about the mean was somewhat greater in the polymer solutions. (3) L→M. In this case the transition occurred from laminar flow into the turbulent regime of maximum drag reduction. Velocity fluctuations appeared at Re≃1500, appreciably lower than the Newtonian critical Re≃2000, and the fluctuation amplitude increased by an order of magnitude over the transitional range, 1500<Re<4000. Intermittency could not be discerned during this type of transition.

On transition in a pipe. Part 1. The origin of puffs and slugs and the flow in a turbulent slug

Conditionally sampled hot-wire measurements were taken in a pipe at Reynolds numbers corresponding to the onset of turbulence. The pipe was smooth and carefully aligned so that turbulent slugs appeared naturally atRe> 5 × 104. Transition could be initiated at lowerReby introducing disturbances into the inlet. For smooth or only slightly disturbed inlets, transition occurs as a result of instabilities in the boundary layer long before the flow becomes fully developed in the pipe. This type of transition gives rise to turbulent slugs which occupy the entire cross-section of the pipe, and they grow in length as they proceed downstream. The leading and trailing ‘fronts’ of a turbulent slug are clearly defined. A unique relation seems to exist between the velocity of the interface and the velocity of the fluid by which relaminarization of turbulent fluid is prevented. The length of slugs is of the same order of magnitude as the length of the pipe, although the lengths of individual slugs differ at the same flow conditions. The structure of the flow in the interior of a slug is identical to that in a fully developed turbulent pipe flow. Near the interfaces, where the mean motion changes from a laminar to a turbulent state, the velocity profiles develop inflexions. The total turbulent intensity near the interfaces is very high and it may reach 15% of the velocity at the centre of the pipe. A turbulent energy balance was made for the flow near the interfaces. All of the terms contributing to the energy balance must vanish identically somewhere on the interface if that portion of the interface does not entrain non-turbulent fluid. It appears that diffusion which also includes pressure transport is the most likely mechanism by which turbulent energy can be transferred to non-turbulent fluid. The dissipation term at the interface is negligible and increases with increasing turbulent energy towards the interior of the slug.Mixed laminar and turbulent flows were observed far downstream for\[ 2000 < Re < 2700 \]when a large disturbance was introduced into the inlet. The flow in the vicinity of the inlet, however, was turbulent at much lowerRe. The turbulent regions which are convected downstream at a velocity which is slightly smaller than the average velocity in the pipe we shall henceforth call puffs. The leading front of a puff does not have a clearly defined interface and the trailing front is clearly defined only in the vicinity of the centre-line. The length and structure of the puff is independent of the character of the obstruction which created it, provided that the latter is big enough to produce turbulent flow at the inlet. The puff will be discussed in more detail later.

Stability studies in pipe flows unsing water and dilute polymer solutions

AbstractThe response of water and 20 ppm. solutions of Separan AP‐30 and Polyox WSR 301 to the periodic motion of a thin sleeve was measured using a laser Doppler velocimeter. The sleeeve was moved in an axial direction by an external coupling at frequencies from 1/4 to 1.O Hz and at amplitudes of 1/2 to 2 in. The flow filed downstream of the distrbance was found to consist of three regions: 1. a near field with a separation wake from the sleeve moving toward the piep centerline; 2. an intermediate field in which alternate laminar and turbulent slugs passed a point; and 3. a far field with decaying turbulent lfow. Defferences in the behavior of water flows and polymer flows appeared in each region. In the near field, wakes spread faster in water flows. In the intermediate field, the laminar slugs disappeared into turbulence more rapidly in water flows. And in the far field, the turbulence was not as developed in the polymer flows.Detailed statistical evaluations were made on the signals in each region for a Reynolds number of 2000. The response of the dilte polymer polymer solutions appeared to be governed by the initial interaction with the sleeve. Fewer high frequencies were generated than in the pure solvent so the periodic disturbance persisted downstream. In pure water the eddies from the separation wake interacted strongly with the periodic distrurbance leading to rapid decay of the periodic portion. Shear rates were not high enough to attribute the dilute polymer differences to viscoelastic effects.

Dispersion of matter in transitional flow through straight tubes

Dispersion of matter in two tubes of diameter 0.684 and 2.77 cm and length 24 m (80 ft) was studied in three gaseous systems, namely, N 2 :C 2 H 4 , N 2 :A, N 2 :He. The Reynolds number variation was 2 x 10 2 to 10 4 . The results were compared with the theory due to Sir Geoffrey Taylor which was up dated by inclusion of an accurate velocity profile. Particular attention was paid to the transition flow regime in which intermittency parameters were measured. It is shown that the theory is adequate beyond the transition regime, but the interpretation of results cannot be quantitatively satisfactory because of lack of precise data on turbulent transfer of matter. Dispersion in the transition regime is influenced by turbulent slugs but experimental results are surprisingly close to predictions from the turbulent flow theory. The position of the maximum of the dispersion parameter was measured and its dependence on the length to diameter ratio of the tubes, and the Schmidt and Reynolds numbers of the system is explained. The reasons for asymmetric response curves for transitional flow lies in the retention of tracer in the recirculating regions set up between adjacent laminar regions and turbulent slugs.

Turbulent and Transition Pipe Flow of Dilute Aqueous Polymer Solutions

The addition of minute quantities of high molecular weight polymers to water can produce a large reduction in the turbulent pipe flow friction. A laser-Doppler instrument for measuring local velocity is used to study transition and turbulent pipe flow of a 50-WPPM aqueous solution of Polyox WSR 301 (polyethylene oxide) at the center of a 14-mm diam pipe. The addition of the polymer to water does not significantly alter the level of the longitudinal turbulence intensity at the center line over the range of Reynolds numbers studied. A surprising result of polymer addition occurs below a Reynolds number of 2,500 in the region of the hump. The increased relative band widths of the frequency spectra in this region imply an uncertainty in the fluid velocity. This result, together with the observed concurrent decrease in the ratio of center line to bulk velocity relative to pure water, shows that the flow is no longer of the steady Poiseuille type after the addition of Polyox WSR 301. The absence of spikes from the frequency spectra further implies that the flow is not intermittent in the sense that laminar and turbulent slugs are alternately passing the point of measurement. At these Reynolds numbers,more » the flow is probably laminar but unsteady. (18 refs.)« less

Propagation Velocity of Turbulent Slugs and Streaks in Transition Pipe Flow

Taking account of experimental observations on the transition process, an elementary theory is developed for estimation of propagation velocity of the rear end of turbulent slugs and streaks which constitute a major feature of the transition process in pipe flow. The theory gives a lower propagation velocity than observed experimentally within the lower range of transition Reynolds numbers while at higher Reynolds numbers the theory gives a value about 7% higher than obtained experimentally. These deviations are due to approximations in the analysis which cannot be avoided at the present level of information on the structure of turbulent shear.

Laminar-turbulent transition process in pulsatile flow.

A controlled ex-vivo study of a simple, sinusoidally oscillating flow in a rigid, constant-area, smooth tube, has produced significant insight into the laminar-turbulent transition phenomenon. The development of turbulence was studied by analyzing the dynamic characteristics of the transition process; i.e., the velocity, growth rate, and intermittency which describe the generation and propagation of turbulent slugs. A new concept, the relaxation time, has been introduced to interpret the effect of a periodic flow component superposed on a mean flow. Classical stability concepts, such as the point of inflection criterion and the Reynolds number, which have been derived from steady-flow analysis, are shown to require modification when applied to an oscillatory flow. Neither the mean nor the instantaneous Reynolds number is a sufficient criterion for determining the transition of laminar to turbulent flow in a pulsatile system. Other necessary criteria are: (1) a source of disturbances, (2) the relaxation time, and (3) the distance from the fluid under observation to the source of disturbance. The concept of relaxation time indicates that slowly oscillating flows of large amplitude tend to suppress or destroy turbulence downstream from sources of disturbance. Qualitative observations are presented which indicate that systolic acceleration may be laminar regardless of the large value of the instantaneous Reynolds number, while diastolic deceleration probably produces disturbed, but not turbulent or highly dissipative, flow.