Temperature spectra from a turbulent thermal plume by ultrasound scattering

This paper reports the results of a study on temperature inhomogeneities conducted on a thermal plume by using ultrasound scattering as a non-intrusive measurement technique. The plume rises from a metallic disk which can be heated up to 800 °C. The working fluid is air at atmospheric pressure. In the measurement technique, an incoming ultrasound wave is emitted towards the thermal plume. The incident wave is scattered because of non-linear couplings with the flow instabilities present in the measurement region. The scattered wave carries information about those flow instabilities. The technique allows for the retrieving of this information. The shape of the obtained spectrum of temperature fluctuations as a function of wave vector modulus is consistent with previous theoretical analysis. Three qualitatively different regions were identified: first, a production region characterized by a q² law; secondly, a region with behavior as per q⁻³ associated with a buoyancy region and; finally, a dissipation region associated with a q⁻⁷ law. These spectral regions characterize the energy transfers mechanisms among the length scales of flow investigated here. A coefficient of anisotropy γ was defined to analyze anisotropic features of the flow.     

Comparison of different shear rheometers with regard to creep and creep recovery measurements

The performance of two shear rheometers with regard to creep and creep recovery measurements is investigated. The first one is a commercially available stress-controlled rheometer that uses an air bearing, and the second one is a magnetic bearing torsional creep apparatus that was built at the Institute of Polymer Materials in Erlangen. The creep and creep recovery measurements were performed in the linear-viscoelastic regime of two polyethylene melts at a temperature of 150°C. The creep compliances of the polyethylenes measured by both rheometers are in excellent agreement. The recoverable compliances of both polyethylene melts, however, have lower values in the case of the commercial rheometer than in the case of the magnetic bearing rheometer. The experimental parameters of creep recovery experiments and the features of the two bearings that are responsible for the different results of the two rheometers are discussed. The influence of the level of the applied shear stress on the short time behavior is investigated as well as the residual torque of the bearings which influences the long time region of the recoverable compliances. Also addressed are the influence of the momentum of inertia of the rotor and the bearing friction which is different for the two rheometers.     

Three-dimensional measurements in non-axisymmetric flow experiment using a one-or two-component laser-Doppler anemometer

 A method to determine the velocity components in the third direction for steady-non-axisymmetric nozzle flow experiments has been developed in addition to the two components that could be directly measured by a one-or two-component laser-Doppler anemometer. With the aid of this method, rotation for either the test rig or the anemometer become unnecessary. Results obtained by this approach were used to compare with those directly measured in the downstream wake of a co-axial nozzle with a central five-lobe forced mixer where the flow is non-axisymmetric. Good agreement could be obtained with maximum discrepancy at about 5%. The present approach can be readily applied to many other non-axisymmetric nozzle flow situations with only some minor modifications. Received: 13 May 1996 / Accepted: 10 October 1996     

Measurement of unsteady surface pressure on a slender wing undergoing a self-induced oscillation

Summary The methodology outlined here was shown to be an effective means by which detailed unsteady surface pressure measurements may be obtained on a wing undergoing a self-induced oscillation in a wind tunnel study. Previous methods have relied on forcing the wing through a sinusoidal motion at a fixed amplitude and frequency. The advantage of the technique outlined here is that pressure measurements are obtained for the exact motion time history, including all transient motions. In fact, almost any self-induced or forced motion may be implemented by the system. The method also has the advantage of a significant reduction in the cost of pressure transducers due to the repeatability of the unsteady flowfield.Funds for the support of this study have been allocated by the NASA-Ames Research Center, Moffett Field, California, under Interchange No. NCA2406, and the University of Notre Dame     

In-cylinder fuel distribution in a port-injected model engine using Rayleigh scattering

The spatial and temporal variation of the fuel concentration (air/fuel ratio) in a model engine was quantified by laser Rayleigh scattering. Fuel was simulated by gaseous Freon-12 injected at various timings and quantities into the intake port. The results showed that the fuel concentration in the engine cylinder was strongly dependent on the injection timing and duration and that the Rayleigh system was able to identify spatial variations of the order of one air/fuel ratio at realistic mixture strengths.     

Modelling of two-phase transient flow and combustion of granular propellants

Gas-permeable solid propellants possess great potential for producing high thrusts during extremely short time intervals. The paper presents a theoretical model describing the important physical phenomena taking place in both gaseous and solid phases in such two-phase, unsteady, reactive-flow systems. The governing equations were derived in the form of coupled, non-linear partial differential equations and the resulting formulation was compared with those used by previous authors. A stable, fast-converging, fully-implicit numerical method, first proposed by Spalding and implemented in a general-purpose computer program, was used to solve the equations, so that effects of importance can be studied. Solutions are presented that predict the pressure wave build-up and accelerating flame front for beds of granulated solid propellants fixed in a rigid enclosure, and in gun barrels with an accelerating projectile. The results show that the flame front accelerates and the rate of pressurization increases substantially in the downstream direction. Some discussion is given of the sensitivity of these predictions to the assumed constitutive relations for interphase heat and momentum transfer and the solid burning-rate law. It is argued that the controversy surrounding the hyperbolicity of the equations, which has delayed progress towards theoretical prediction of two-phase flows, is ill-founded, since the equations are shown to possess unique solutions, as indeed do the physical systems they represent.     

Determining transverse impact force on a composite laminate by signal deconvolution

Dynamic impact forces on a composite structure were recovered by using experimentally generated Green's functions and signal deconvolutions. The signal processing is straightforward. Extra windowing and filtering the recorded signals are unnecessary. The Green's functions account for boundary conditions, material properties and structure geometry. This approach can be applied to linearly elastic structures with different boundary conditions. It is realistic and convenient to use for the recovery of impact force on anisotropic or isotropic solid structures.     

Crack trajectories influenced by mechanical and thermal disturbance in anisotropic material

This work is concerned with the application of the volume energy density criterion for predicting the crack trajectories as influenced by mechanical and thermal disturbance in an anisotropic material. Two-dimensional linear thermoelasticity is employed in conjunction with the well-known complex potentials such that a linear relationship is obtained for the boundary conditions across the crack or line of discontinuity. Boundary collocation is then used to determine the unknown coefficients from which the contours of the volume energy density in the cracked plate can be obtained. The crack path is assumed to coincide with the loci where dilatation would dominate. This corresponds to the locations of relative minimum energy density which can be found by visual inspection. An equal and opposite mechanical stress and thermal gradient are applied on the cracked plate. The former and latter enhance symmetric and asymmetric crack growth, respectively. They would complete depending on the magnitude of the mechanical and thermal load. Numerical results are presented for three (3) different cases of a plate whose principal axes of material symmetry are tilted to the crack plane. The influence of anisotropy on crack path is found to be secondary.     

Cluster statistics in a bidimensional suspension: Comparison with percolation

We study the cluster statistics and the viscosity of a two-dimensional suspension of passive macroscopic spheres undergoing shear. The second moment of the finite cluster statistics exhibits a maximum for a 2-D concentrationΦ _S near 0.67 without measurable anomaly in the viscosity. The results of the cluster statistics are compared to those obtained in percolation.