P. J. Lefebvre

Bio: P. J. Lefebvre is an academic researcher. The author has contributed to research in topics: Static pressure & Flow measurement. The author has an hindex of 1, co-authored 1 publications receiving 42 citations.

Experiments on Transition to Turbulence in a Constant-Acceleration Pipe Flow

Abstract: Experiments were conducted to study transition to turbulence in pipe flows started from rest with a linear increase in mean velocity. The data were taken at the Unsteady Flow Loop Facility at the Naval Underwater System Center, using a 5-cm diameter pipe 30 meters long. Instrumentation included static pressure, wall pressure, and wall shear stress sensors, as well as a laser Doppler velocimeter and a transient flowmeter. A downstream control valve was programmed to produce nearly constant mean flow accelerations, a from 2 to 12 m/s 2

Transition of unsteady velocity profiles with reverse flow

Abstract: 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 [simeq R: similar, equals]3[delta] where [delta] 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 [simeq R: similar, equals]39/([Delta]u/[delta]), where [Delta]u=(umax[minus sign]umin) and umax, umin and [delta] are the maximum velocity, minimum velocity and boundary layer thickness respectively at each instant of time. The time to transition to turbulence is [simeq R: similar, equals]33/([Delta]u/[delta]). 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.

Aquatic suction feeding dynamics: insights from computational modelling

Abstract: Aquatic suction feeding in vertebrates involves extremely unsteady flow, externally as well as internally of the expanding mouth cavity. Consequently, studying the hydrodynamics involved in this process is a challenging research area, where experimental studies and mathematical models gradually aid our understanding of how suction feeding works mechanically. Especially for flow patterns inside the mouth cavity, our current knowledge is almost entirely based on modelling studies. In the present paper, we critically discuss some of the assumptions and limitations of previous analytical models of suction feeding using computational fluid dynamics.

Decay of Pressure and Energy Dissipation in Laminar Transient Flow

Abstract: In the present paper, some peculiar characteristics of transient laminar flow are dis-cussed. After presenting a review of the existing literature, attention is focused on tran-sient energy dissipation phenomena. Specifically, results of both laboratory and numericalexperiments are reported, the latter by considering one-dimensional (1D) along withtwo-dimensional (2D) models. The need of modifying a criterion for simulating unsteadyfriction proposed some years ago by one of the writers, and extensively used for water-hammer calculations, is pointed out. Differences between accelerating and deceleratingflows as well as between transients in metallic and plastic pipes are also highlighted.@DOI: 10.1115/1.1839926#Keywords: Transient, Laminar, Energy, Dissipation, Viscoelasticity

Investigation of turbulence behavior in pipe transient using a k–∊model

Abstract: Existing wall shear stress models and turbulence closure equations for water hammer problems are based on far-reaching assumptions in relation to the turbulence behavior during a transient event. It is often postulated that the turbulence behaves in a quasi-steady, frozen or quasi-laminar manner. Presently, the experimental or numerical data needed to investigate turbulence behavior during a transient, and to assess the often used assumptions regarding the turbulence behavior in water hammer flows, are lacking. The recent development of highly efficient numerical scheme makes it feasible to conduct numerical experiments using a k–∊model suitable for time-dependent flows. As a result, this paper develops a k–∊model for water hammer and applies it to transient pipe flows to investigate the turbulence behavior during a water hammer event and to assess the quasi-steady, frozen or quasi-laminar turbulence hypothesis

Unsteady Laminar Duct Flow With a Given Volume Flow Rate Variation

Abstract: In this paper we give a procedure to obtain analytical solutions for unsteady laminar flow in an infinitely long pipe with circular cross section, and in an infinitely long twodimensional channel, created by an arbitrary but given volume flow rate with time. In the literature, solutions have been reported when the pressure gradient variation with time is prescribed but not when the volume flow rate variation is. We present some examples: (a) the flow rate has a trapezoidal variation with time, (b) impulsively started flow, (c) fully developed flow in a pipe is impulsively blocked, and (d) starting from rest the volume flow rate oscillates sinusoidally. [S0021-8936(00)01702-5]