Oscillatory Pipe Flow Research Articles

Rapid Transition to Turbulence in Pipe Flows Accelerated From Rest

Rapid transition to turbulence in a pipe flow, initially at rest, was achieved by temporally accelerating the flow and then sharply decelerating it to its final Reynolds number. The acceleration phase was characterized by the growth of a laminar boundary layer close to the wall. The subsequent rapid deceleration resulted in inflectional velocity profiles near the wall, followed immediately by transition to turbulence. The time taken to transition was significantly less than the time to transition in a pipe flow monotonically accelerated to the same Reynolds number. Transition is intrinsically different to that observed in oscillatory pipe flows, but is qualitatively similar to pipe flows decelerated to rest.

A study on the unsteady flow in a helical pipe

A study on the unsteady low-frequency oscillatory flow in a helical circular pipe is carried out based upon the blood flow in vessels, using the method of bi-parameter perturbation. The second order perturbation results were obtained and the characteristics were analyzed at different time of the axial velocity, of the secondary flow, and of the wall shearing stress. Also done the analysis of above-mentioned variables that varied along with time and Womersley number. The results indicate that for a helical pipe, the torsion exerts the main influence on the distribution of secondary flow velocity, especially when the absolute value of axial press gradient is rather small. the severe variation of stream function takes place within a very short period, during which time the stream function develops from positive value to negative value and vice versa, while in most cases in a cycle, the variation is smooth. The wall shearing stress changes severely with theta too.

An oscillating hot-wire technique for resolving the magnitude and direction of velocity measurements using single hot-wire sensors

Thermal anemometry is a classic flow-velocity measurement technique that is known to suffer from the inability to discern the flow direction. The current paper describes an innovative approach whereby an oscillating hot wire is used to extract velocity direction and magnitude information from single hot-wire measurements. It is shown that the new sensor operates in one of two modes depending on the velocity amplitude of the wire oscillation. Furthermore, results from applying the technique to measure the phase-averaged velocity in an oscillating pipe flow experiment are presented. The results from the hot-wire measurements show good agreement with those from laser Doppler velocimetery measurements in the same facility.

Flow measurement on oscillating pipe flow near the entrance using the UVP method

Flow measurement on oscillating pipe flow near the entrance using the UVP method

2401 熱音響音波発生機内の振動流れ : スタックおよび共鳴管内で流体のなす仕事

Oscillatory flow in the thermoacoustic sound wave generator is described. In oscillatory pipe flow, characteristic Richardson effect occurs in the vicinity of the wall. Its flow plays an important role in thermoacoustic phenomena in which direct energy conversion from heat to work (sound waves), and vice versa, is performed with no moving parts. Also, flow in the resonance tube is influenced by its flow on the stack. Fluid work in stack and the resonance tube are discussed.

Development and Application of Flow Rate Meter Based on a Laser Doppler Anemometer.

An LDA flow rate meter which is based on the LDA (Laser Doppler anemometer) technique has been developed. The principle of the LDA flow rate meter applied for circular pipe is following. The center line velocity of pulsating pipe flow was measured by the LDA. The velocity distribution profile in the pipe cross section can be numerically estimated, and then the instantaneous flow rate was calculated by the integration of the profile. Firstly, it was applied and evaluated at the pulsating flow made by an artificial heat-lung system. The system has a double roller pump with a vinyl tube of Ω-shape. Secondly, an intermittent flow rate at high fuel pressure injection system was also measured. Swirl injector generated the oscillating pipe flow under high line pressure of 5 MPa at injection frequencies of 20 and 40 Hz. There was a little difficulty at the measurement of injection flow depending on the zero velocity shifting. Finally, a correction method of zero shifting was proposed and the applicability of the flow rate meter was demonstrated.

301 磁性流体管内振動流の UVP による速度分布計測

301 磁性流体管内振動流の UVP による速度分布計測

Measurement of the temperature rise in non-Newtonian oscillatory pipe flows

An experimental investigation of the temperature increase due to viscous dissipation in an oscillating pipe flow is presented. An oscillating pipe of circular section, acting as an extrusion die, was placed at the last stage of a polymer extrusion process and the increase in the temperature of the fluid, due to the superimposed oscillation, was measured. Thermocouples and a temperature control system were mounted on the walls of the oscillating section. Experimental results were obtained following the conditions suggested by Casulli et al. [J. Polym. Eng. Sci. 30 (1990) 1551]. Commercially available low density polyethylene was chosen as the experimental fluid. Results were obtained for the case when the imposed oscillatory motion was parallel to the axial direction of the flow. The bulk temperature of the fluid at the exit of the oscillating section was found to increase with the oscillating frequency and amplitude. If the dimensionless temperature increase is plotted as a function of the characteristic oscillation speed, the experimental results collapse into a single curve. In order to justify the experimental measurements, a theoretical analysis was performed for two simple non-Newtonian fluid models: linear viscoelastic and power-law liquids. Analytic expressions for the velocity and temperature fields were obtained. These models predicted an increase of the bulk temperature with the speed of oscillation in agreement with the experimental results. The direct comparison of the measurements and the predictions showed good order-of-magnitude agreement. We found that the temperature increase predicted using a linear viscoelastic model agrees well with the experiments for low oscillation speeds. For the case of large oscillation speeds, the prediction based on a power-law model resulted in a better agreement than the viscoelastic model prediction.

The oscillatory pipe flow with arbitrary wall injection

The linearized NavierStokes equations play a central role in describing the unsteady motion of a viscous fluid inside a porous tube. Asymptotic solutions of these equations have been found and here...

Flow measurement on an oscillating pipe flow near the entrance using the UVP method

The authors have carried out a study to investigate and clarify the characteristics of purely oscillating pipe flows over the developing region. The main objective of this study is to establish the method of time-dependent velocity profiles obtained by the ultrasonic velocity profile (UVP) measurement method. First, the relationship between the test fluids and the microparticles, as reflectors of ultrasonic pulses, was investigated. In addition, the relationship between the sound speeds of the test fluids and the wall materials was studied. Second, the UVP was used to obtain the instantaneous velocity profiles in oscillating pipe flows, and the developing characteristics of the flows were analyzed. Finally, the “entrance length” (by analogy with a unidirectional pipe flow) required for oscillating pipe flows was analyzed by examining the amplitude of the harmonic spectral components of the oscillating frequency. A fast Fourier transform (FFT) is proposed as the applicable method to estimate the “entrance length”. From the Fourier transform of the velocity on the centerline, nonlinear oscillation of fluid occurs in the “entrance length” of the oscillating flows, and the viscous dissipation of the higher-order velocity harmoncis determines the entrance region. The “entrance length” can be obtained from the dissipation length of the third-order harmonic. These results prove that the UVP method is highly applicable to carry out the flow measurement in the “entrance length” of oscillating pipe flow.

Oscillatory Entrance Flow in a Circular Pipe with a Bell Mouth Opening Connected to a Large Vessel.

Numerical investigations have been made of the oscillatory entrance flow in a straight pipe connected perpendicularly to the wall of a large reservoir. The pipe opening forms a bell mouth, the roundish corner radius of which is the same as the pipe. Calculations are carried out under the conditions of the Womersley numbers of α=2∼20 and the Reynolds numbers of Re=100∼500. The velocity profiles, the streamlines and the distributions of the wall shear stress are illustrated. The influences of the Womersley number and of the configuration of the pipe opening on the entrance flow phenomena are made clear, as occasion calls, by referring to results for a pipe with a sharp edged opening. The flow near the inlet in the outflow as well as the inflow terms exhibits a complicated behavior depending on the Womersley number. The separation at the bell mouth in the outflow term comes to be suppressed with an increase of α. In place of that, for moderate and high α the vortex flow is formed just in front of the bell mouth at the phase that changes from the outflow to the inflow terms. In consequence of the developing of the axial velocity along the pipe axis, the one-directional or bi-directional steady steaming is induced in the entrance region. The inlet length determined from the amplitude of the axial velocity is appreciably shorter than that for a steady flow and it changes proportionately with Re/α2, except for sufficiently high α.

1105 管内往復振動流場における熱伝達特性

1105 管内往復振動流場における熱伝達特性

Présentation d'une méthode d'étude de la stabilité linéaire d'écoulements instationnaires

We propose a linear stability analysis of unsteady viscous flow. We apply this method to an oscillatory pipe flow with an axisymetric 2D perturbation which has received considerable attention. The numerical results are relevant. To cite this article: M. Siouffi et al., C. R. Mecanique 330 (2002) 641–645.

Velocity Profile Measurement on Oscillating Pipe Flow of a Magnetic Fluid by UVP Method.

Velocity Profile Measurement on Oscillating Pipe Flow of a Magnetic Fluid by UVP Method.

Numerical investigations on the heat transfer in turbulent oscillating pipe flow

The examinations on the heat transfer in developing laminar oscillating pipe flow presented before [1] have been extended to include turbulence as well. A suitable low-Reynolds-number k-ɛ-turbulence model was incorporated in an existing 2D-simulation code for oscillating flow conditions and subsequent examinations focused on the heat transfer associated with turbulent oscillating flow. The calculations cover a wide range of the characteristic parameters and the results are summarised in form of new heat transfer correlations to suit with the operating conditions of regenerative thermal machines.

705 円管内振動流の乱流構造に関する数値解析(オーガナイズドセッション7 流れの可視化応用解析)

705 円管内振動流の乱流構造に関する数値解析(オーガナイズドセッション7 流れの可視化応用解析)

LDA Flow Rate Measurements Applied for Analysis of Transient Injection Characteristics.

In the present study an LDA-based technique was applied to analyze details of a transient pipe-flow induced upstream by a fuel ingector. Due to highly accurate measurements of the instantaneous time series of velocity, fuel flow rate and pressure gradient were achieved. The pipe flow has been driven under an injection pressure of 0.6MPa at high injection frequencies of 25 and 50 Hz. The influence of the outflow orifice diameter, varid in range from 100 to 500 μm, on the transient pipe flow dynamics has also been described. The results show that transient behavior of rapid oscillating pipe flow is remarkably and systematically dependent on injection frequency and duration as well as on the orifice diameter.

Transition of unsteady velocity profiles with reverse flow

This paper deals with the stability and transition to turbulence of wall-bounded unsteady velocity profiles with reverse flow. Such flows occur, for example, during unsteady boundary layer separation and in oscillating pipe flow. The main focus is on results from experiments in time-developing flow in a long pipe, which is decelerated rapidly. The flow is generated by the controlled motion of a piston. We obtain analytical solutions for laminar flow in the pipe and in a two-dimensional channel for arbitrary piston motions. By changing the piston speed and the length of piston travel we cover a range of values of Reynolds number and boundary layer thickness. The velocity profiles during the decay of the flow are unsteady with reverse flow near the wall, and are highly unstable due to their inflectional nature. In the pipe, we observe from flow visualization that the flow becomes unstable with the formation of what appears to be a helical vortex. The wavelength of the instability ≃3δ where δ is the average boundary layer thickness, the average being taken over the time the flow is unstable. The time of formation of the vortices scales with the average convective time scale and is ≃39/(Δū/δ), where Δu=(umax−umin) and umax, umin and δ are the maximum velocity, minimum velocity and boundary layer thickness respectively at each instant of time. The time to transition to turbulence is ≃33/(Δū/δ). Quasi-steady linear stability analysis of the velocity profiles brings out two important results. First that the stability characteristics of velocity profiles with reverse flow near the wall collapse when scaled with the above variables. Second that the wavenumber corresponding to maximum growth does not change much during the instability even though the velocity profile does change substantially. Using the results from the experiments and the stability analysis, we are able to explain many aspects of transition in oscillating pipe flow. We postulate that unsteady boundary layer separation at high Reynolds numbers is probably related to instability of the reverse flow region.

Einfluß von Einlaufeffekten auf den Wärmeübergang bei laminarer oszillierender Rohrströmung

A theoretical analysis was performed to evaluate the influence of developing flow on the heat transfer associated with laminar oscillating pipe flow. Simplified analytical approaches are briefly discussed before an investigation based on the numerical solution of the conservation equations for mass, momentum and energy is presented, assuming constant wall temperature and an incompressible viscous fluid. Focusing on operating conditions as found in heat exchangers of regenerative thermal machines, like Stirling engines or Vuilleumier heat pumps, numerous calculations of the flow field and the heat transfer have been executed covering wide ranges of the characteristic dimensionless groups. The results are presented in tems of correlations of the mean Nusselt number averaged on the pipe length and a distribution function describing the local heat transfer. Furthermore, it is shown that the derived correlations are also applicable to compressible fluid flow, provided that the relative pressure amplitude is within the limits typical of regenerative thermal machines.

Heat transfer in the thermally developing region of a laminar oscillating pipe flow

A theoretical analysis was performed focusing on the heat transfer associated with the laminar oscillating flow in a tube. This situation finds applications in the Stirling engines or regenerative-type refrigerators where the working fluid in the heat exchanger undergoes an oscillatory motion. Such an oscillating flow conceivably entails the thermally developing region, not only because the swept length of working fluid is roughly equal to or longer than the characteristic length of the heat exchanger, but also because the wall temperature changes abruptly along the longitudinal direction. For simulation of the practical heat exchanger composed of cooler and heater, two types of thermal boundary conditions are taken into account; either wall temperature or wall heat flux has a square-wave distribution. It is found that the thermally developing length increases in proportion to the oscillation frequency at slow oscillation but eventually approaches an asymptotic value at high frequency. The local average Nusselt number in the developing region is observed to be inversely proportional to the square root of the distance measured from the thermal discontinuity. Out of the thermally developing region, the local Nusselt number is determined only by the oscillation frequency regardless of axial position.