The Stability of Compressible Mixing Layers in Binary Gases

We present the results of a study of the inviscid two-dimensional spatial stability of a parallel compressible mixing layer in a binary gas. The parameters of this study are the Mach number of the fast stream, the ratio of the velocity of the slow stream to that of the fast stream, the ratio of the temperatures, the composition of the gas in the slow stream and in the fast stream, and the frequency of the disturbance wave. The ratio of the molecular weight of the slow stream to that of the fast stream is found to be an important quantity and is used as an independent variable in presenting the stability characteristics of the flow. It is shown that differing molecular weights have a significant effect on the neutral-mode phase speeds, the phase speeds of the unstable modes, the maximum growth rates, and the unstable frequency range of the disturbances. The molecular weight ratio is a reasonable predictor of the stability trends. We have further demonstrated that the normalized growth rate as a function of the convective Mach number is relatively insensitive (≈25%) to changes in the composition of the mixing layer. Thus, the normalized growth rate is a key element when considering the stability of compressible mixing layers, since once the basic stability characteristics for a particular combination of gases is known at zero Mach number, the decrease in growth rates due to compressibility effects at the larger convective Mach numbers is somewhat predictable. (C) 1996 American Institute of Physics

The stability of compressible mixing layers in binary gases

We present the results of a study of the inviscid two-dimensional spatial stability of a parallel compressible mixing layer in a binary gas. The parameters of this study are the Mach number of the fast stream, the ratio of the velocity of the slow stream to that of the fast stream, the ratio of the temperatures, the composition of the gas in the slow stream and in the fast stream, and the frequency of the disturbance wave. The ratio of the molecular weight of the slow stream to that of the fast stream is found to be an important quantity and is used as an independent variable in presenting the stability characteristics of the flow. It is shown that differing molecular weights have a significant effect on the neutral-mode phase speeds, the phase speeds of the unstable modes, the maximum growth rates and the unstable frequency range of the disturbances. The molecular weight ratio is a reasonable predictor of the stability trends. We have further demonstrated that the normalized growth rate as a function of the convective Mach number is relatively insensitive (Approx. 25%) to changes in the composition of the mixing layer. Thus, the normalized growth rate is a key element when considering the stability of compressible mixing layers, since once the basic stability characteristics for a particular combination of gases is known at zero Mach number, the decrease in growth rates due to compressibility effects at the larger convective Mach numbers is somewhat predictable

Absolute-Convective Instabilities and Their Associated Wave Packets in a Compressible Reacting Mixing Layer

In this paper the transition from convective to absolute instability in a reacting compressible mixing layer with finite rate chemistry is examined. The reaction is assumed to be one step, irreversible, and of Arrhenius type. It is shown that absolute instability can exist for moderate heat release without backflow. The effects of the temperature ratio, heat release parameter, Zeldovich number, equivalence ratio, direction of propagation of the disturbances, and the Mach number on the transition value of the velocity ratio are given. The present results are compared to those obtained from the flame sheet model for the temperature using the Lock similarity solution for the velocity profile. Finally, the structure of the wave packets produced by an impulse in the absolutely unstable flow is examined. © 1993 American Institute of Physics

Induced Mach Wave-Flame Interactions in Laminar Supersonic Fuel Jets

A model problem is proposed to investigate the steady response of a reacting, compressible laminar jet to Mach waves generated by wavy walls in a channel of finite width. The model consists of a two-dimensional jet of fuel emerging into a stream of oxidizer which are allowed to mix and react in the presence of the Mach waves. The governing equations are taken to be the steady parabolized Navier-Stokes equations which are solved numerically. The kinetics is assumed to be a one-step, irreversible reaction of the Arrhenius type. Two important questions on the Mach wave-flame interactions are discussed: (i) how is the flame structure altered by the presence of the Mach waves, and (ii) can the presence of the Mach waves change the efficiency of the combustion processes? © 1993 American Institute of Physics

Rubber friction on wet and dry road surfaces: the sealing effect

Rubber friction on wet rough substrates at low velocities is typically 20-30% smaller than for the corresponding dry surfaces. We show that this cannot be due to hydrodynamics and propose a novel explanation based on a sealing effect exerted by rubber on substrate "pools" filled with water. Water effectively smoothens the substrate, reducing the major friction contribution due to induced viscoelastic deformations of the rubber by surface asperities. The theory is illustrated with applications related to tire-road friction.Comment: Format Revtex 4; 8 pages, 11 figures (no color); Published on Phys. Rev. B (http://link.aps.org/abstract/PRB/v71/e035428); previous work on the same topic: cond-mat/041204

Oscillatory oblique stagnation-point flow toward a plane wall

Two-dimensional oscillatory oblique stagnation-point flow toward a plane wall is investigated. The problem is a eneralisation of the steady oblique stagnation-point flow examined by previous workers. Far from the wall, the flow is composed of an irrotational orthogonal stagnation-point flow with a time-periodic strength, a simple shear flow of constant vorticity, and a time-periodic uniform stream. An exact solution of the Navier-Stokes equations is sought for which the flow streamfunction depends linearly on the coordinate parallel to the wall. The problem formulation reduces to a coupled pair of partial differential equations in time and one spatial variable. The first equation describes the oscillatory orthogonal stagnation-point flow discussed by previous workers. The second equation, which couples to the first, describes the oblique component of the flow. A description of the flow velocity field, the instantaneous streamlines, and the particle paths is sought through numerical solutions of the governing equations and via asymptotic analysis

Which Media Services Do Students Use In Fact? Results Of An International Empirical Survey

The dissemination of online information services into higher education has led to constant changes in students' learning behavior. Nowadays they use services like Google and Wikipedia most often not only during free time but also for studying. At the same time, traditional information media such as the textbook or the printed hand-out from the teacher still form basic pillars in their learning environment. To measure the whole variety of media, that are used for learning, an international long term Media Survey in Higher Education ("MESHED") was set up by the authors. It aims to get detailed knowledge about how students use media for study from an international and a long term perspective. This knowledge shall be used to develop recommendations for university media strategy, make prognoses for future media trends in higher education and to figure out influences of external dimensions on the media usage. Beginning with a first survey carried out at Karlsruhe Institute of Technology, Germany in 2009, currently (October 2013) a total of 30 surveys in ten countries were, or currently are carried out. The survey uses a fully standardized questionnaire that measures the acceptance of 48 media services, such as Google search, library catalogues, printed books, e-books, printed journals, e-journals, e-learning-services, virtual class, Wikipedia, open educational resources, bibliographic software and more. It also measures adjacent areas, such as the learning behavior, study success, media usage during free time, usage of IT hardware, education biography and sociodemographic factors. This paper focusses on the results of a survey that was conducted at the University of Barcelona (UB) between March and June 2012. There, about 1,000 samples were collected. The data showed an intense use of a broad variety of media among UB students. Though, not all media services were accepted equally: while especially some university external services, such as Google web search or Wikipedia were used by almost every student, other media, e.g. virtual learning services were used on a very low level. An exploration of hidden structures of media usage behavior, using factor and cluster analysis revealed that especially text and text related media (books, eBooks, library catalogues) seem to have a positive effect on the learning success. A comparison of the Barcelona sample with the data of other countries showed some commonalities, e.g. a high usage of Google and other external services. But there were also hints to cultural differences, such as an explicit maverick or non-social learning behavior of Spanish students. This general tendency also appears in the media sector where they tend to use information media and, compared to students from other countries, use less social media. An additional survey in Canada/Ontario has been conducted in January-February 2013, and at the moment the third survey at the KIT is running. Especially some of the results from Canada show specific aspects, that might be interesting to be compared to the Spanish and German findings

Rubber friction: role of the flash temperature

When a rubber block is sliding on a hard rough substrate, the substrate asperities will exert time-dependent deformations of the rubber surface resulting in viscoelastic energy dissipation in the rubber, which gives a contribution to the sliding friction. Most surfaces of solids have roughness on many different length scales, and when calculating the friction force it is necessary to include the viscoelastic deformations on all length scales. The energy dissipation will result in local heating of the rubber. Since the viscoelastic properties of rubber-like materials are extremely strongly temperature dependent, it is necessary to include the local temperature increase in the analysis. At very low sliding velocity the temperature increase is negligible because of heat diffusion, but already for velocities of order 0.01 m/s the local heating may be very important. Here I study the influence of the local heating on the rubber friction, and I show that in a typical case the temperature increase results in a decrease in rubber friction with increasing sliding velocity for v > 0.01 m/s. This may result in stick-slip instabilities, and is of crucial importance in many practical applications, e.g., for the tire-road friction, and in particular for ABS-breaking systems.Comment: 22 pages, 27 figure