Unstationary Flow Research Articles

Mobilisation of friction in unstationary flows down a model topography

The frictional behaviour of a series of numerical 2D granular mass flows down a model topography is analysed. Effective friction coefficients estimated from final deposits are compared with data from documented natural geophysical flows, and show a consistent behaviour as far as run-out distances are concerned. The latter is used to estimate effective friction coefficients which capture well the frictional behaviour derived from the computation of micro-mechanical stress tensors near the gravity centre. Distinguishing between the different parts of the mass while spreading, we show that the downstream part of the flow exhibits a much larger friction than the core and the tail. A dependence between friction and flow volume is however observed in each region.

Liquid metal flat jet transformation under influence of transverse magnetic field

The article is devoted to an experimental study of a submerged flat jet flow in a transverse magnetic field. Two different approaches to the experimental study of jet flows are described. Detailed information about the experimental program and measuring methods presented here. The flow of a flat jet 6 mm high in a square channel with a side of 56 mm is considered. The channel is positioned so that the plane of the jet is perpendicular to the magnetic field induction. The results of measuring velocity profiles and waveforms by swivel-type probe with potential sensor are presented. Effects that can be interpreted in different ways are found: strongly unstationary flow regimes, mean flow reorganization, and development of near-wall jets. Additional experiments are prepared to obtain more detailed information about the restructuring and development of the jet. In particular, continuous measurements along the channel will be made in the presence of a slight main flow.

Modification of the aerodynamic profile to reduce the impact of non-stationary

The review of works on an unstationary flow on wing profiles which forms change of vortex structure flow and essentially changes its aerodynamic characteristics is considered. Updating of contours of a profile can be, criterion of reduction of an unstationary flow influence, and also lead to reduction of an unstationary flow influence on aerodynamic characteristics.

A level set approach to simulate magnetohydrodynamic instabilities in aluminum reduction cells

Magnetohydrodynamic instabilities at the metal-bath interface in aluminum reduction cells is an important and not fully understood topic. To simulate the two-fluid three-dimensional unstationary flow subject to a background magnetic field, a level set approach is proposed. It features a formulation in terms of the magnetic vector potential to avoid a numerical growth of the divergence of the magnetic field. The same exact projection scheme (with staggered grids) is used for both the velocity field and the magnetic vector potential. Test simulations show that the overall method behaves well in purely hydrodynamic as well as in fully magnetohydrodynamic regimes, in both cases with a single fluid and with two fluids. We also simulate with our technique the metal pad roll instability and trace the behavior of coupled interfacial modes.

Real-time characterization of unstationary flows using a time-reversal mirror

Violation of time-reversal invariance for acoustic waves is a new method for characterizing a vorticity field with a double acoustic time-reversal mirror (TRM). Experiments show that the double TRM works as a vorticity amplifier and, even in the case of small vorticity field, provides the characteristics of the flow. Moreover, the TRM works in real-time and allows one to measure the vorticity field at a 20-Hz frequency rate. The case of a Von Karman vortex street is studied experimentally.

Wavelength coding laser tomography for flow visualizations

In a recent article, we presented an optical method for the simultaneous visualization of two cross-sections of 3D unstationary flows by means of a polarizing coding. We propose here a complementary technique, with identical objectives allowing to remove the limitations of the polarizing filtering. The selection of different recorded “planes” is obtained by means of colored illuminations in association with interferential filterings: the restriction imposed by the use of non-polarizing tracers disappears, and the method becomes usable as well in hydrodynamics as in aerodynamics.

Economics of Hydro Power Plant Cascade Cooperating with a Great Power System in the Conditions of Synchronous Operation

Hydraulic unstationary processes taking place during synchronous operation of hydro plant cascade are described. Due to unstationary flow the head at the beginning of operation is greater than during stationary flow. Wave processes and other characteristics are modelled by hybrid simulator. The hydro power plant cascade is used as a source of intervention and regulation reserve in a considered power system. A method of evaluation of these effects is proposed. It is illustrated by a numerical example.

Comment on "A Prediction of Particle Behavior via the Basset-Boussinesq-Oseen Equation"

N a recent Note, Catalano1 has given an analytical solution of the Basset-Boussinesq-Oseen (BBO) equation. This solution was applied to the interesting problem of the discrepancy between theory and laser velocimetry applications (LDA) measurements in the case of unstationary flows arising especially in Raleigh-Benard convection or at the onset of turbulence in a circular Couette flow. This discrepancy is observed mainly for high frequencies. In the following we would like to comment on the author's assumptions: Accepting, as Catalano did, the BBO equation in the form given by Tchen2 (for a thorough discussion of this topic, we refer to Reek's recent paper),5 we do not agree that the Basset term can be neglected. For example, we look at the experiments cited by Catalano6'7: Taking into account the properties of liquid helium, it turns out that Hinze's4 conditions for the neglect of the Basset term are not fulfilled for marker particles with a diameter less 10 \im. Also, it should be mentioned that Catalano neglected the mass addition term, which represents the force to accelerate the virtual mass of the particle relative to the fluid. In extending Catalano's analytical investigation, we present a solution of the complete BBO equation. We use a standard method3 for solving the inhomogeneous Volterraintegral equation of second kind. Our solution accounts for the Basset term describing the time behavior of the flow in the neighborhood of the particles. We proceeded in this way because we believe that, in unstationary flows of the type considered, near the onset of turbulence, the Bassset term cannot be neglected. The resultant closed-form solution for the particle motion should be examined for the case of Rayleigh-Benard flow as well. The nomenclature we use is the same as Catalano's. At this opportunity we also want to make some corrections of Eqs. (2), (3), (6), (7), (15), (17), (18), (20), and (21) of Ref. 1. Further, we would like to mention that Catalano did not remark that the lift forces [see Eqs. (4) and (5)] are directed perpendicular to the relative velocity vector up—uF. So the inclusion of the lift force is rather questionable in the obviously one-dimensional case presented.1

Analyse quantitative par moyens optiques des instabilities d'ecoulement en regime supersonique

Previous articles described a method of tomographic visualizations of turbulent mixing zones, stationary structures and instabilities in supersonic flows. This paper shows new improvements of this method which thus becomes quantitative and able to give the nature of such instabilities (unstationary flow) and the parameters of the shock waves oscillations.

Tentative application of the tangent simple system method to the study of the viscoelastic behaviour of blood.

The method of the tangent simple systems is applied to the study of the viscoelastic behaviour of human blood in unstationary flow for rectangular steps and triangular ramps of shear rate. The tangent systems we utilize, Maxwell liquids, enable us to determine, at every point of the rheograms, apparent instantaneous values of retardation or relaxation time, viscosity coefficient and elasticity modulus of the studied blood samples, and to plot the curves of variation of these parameters as a function of flow duration. A qualitative interpretation of the results is proposed from data on the aggregation-disaggregation kinetics of red blood cells. Examples are given for samples of normal and pathological bloods.

Sedimentation in Iron Gates Reservoir on the Danube

Long-term data over the Danube River discharge and sediment concentration have been available; a correlation between both has been established. A forecast study for the reservoir sedimentation over a period of 30 yr was carried out by means of a mathematical model which simulated the unstationary flow on mobile bed of the stream charged with load, in balance with its transport capability. The study showed that half of the entered river load will be retained in reservoir. After 5 yr of dam operation, the measurements have indicated differences from the predicted one and have located the critical sites and the tendencies for future sedimentation.