Nonpositive solutions of one functional differential inequality

We establish efficient conditions guaranteeing that every solution of the problem

On Fractional PI λ Controllers: Some Tuning Rules for Robustness to Plant Uncertainties

The objective of this work is to find out optimum settings for a fractional PI ^λ controller in order to fulfill three different robustness specifications of design for the compensated system, taking advantage of the fractional order, λ. Since this fractional controller has one parameter more than the conventional PI controller, one more specification can be fulfilled, improving the performance of the system and making it more robust to plant uncertainties, such as gain and time constant changes. For the tuning of the controller an iterative optimization method has been used, based on a nonlinear function minimization. Two real examples of application are presented and simulation results are shown to illustrate the effectiveness of this kind of unconventional controllers.     

On moving heat sources

The two-dimensional thermal problem due to relative motion of a medium and a suddenly activated circular heat source is solved for several boundary conditions. The solutions can be interpreted as for a moving heat source in a stationary medium or a medium moving past a stationary heat source. Uniform and non-uniform temperature, and uniform and non-uniform heat flux boundary conditions are considered. The effect of velocity and radial direction on the temperature distribution is examined. Average, steady-state Nusselt numbers are derived. The transient response of a continuous line source is obtained as a limiting case of the prescribed heat flux solution. Received on 24 September 1996     

A comparison of flow visualisation and wall pressure measurements for a jet impinging on a plane surface

Several studies of jets impinging on a plane surface have already been made. This paper suggests a new approach to studying impingements of jets. Comparisons have been made between visualisation results and wall pressure measurements. It is shown that only one of these two techniques is sufficient for characterising the flow nature near the wall. Visualisations can be sufficient for determining the location of wall pressure maxima and minima. Detachments and reattachments of the flow are thus located, and the main characteristics of a jet impinging on a plane wall can be shown by simpler experiments such as the spreading over method. Received: 15 October 1998/Accepted: 27 July 1999     

Strain behavior of pressurized cracked thick-walled cylinders

External circumferential strains were measured on large thick-wall pressure vessels containing internal fatigue cracks, using bonded strain gages. When strains measured over the cracks become compressive they predict impending failure. Normalization by the Lamé strain relates them to the fraction of fatigue life consumed and provides estimates of longevity.     

Thermokapillare Grenzflächenkonvektion an Gasblasen in einem Temperaturgradientenfeld

Kurzfassung Bei freien Grenzflächen spielt die thermokapillare Konvektion eine nicht zu vernachlässigende Rolle auf den Wärmetransport. In zwei Experimenten wird deren Einfluß aufgezeigt. In Silikonölen verschiedener Viskosität mit thermisch stabiler Schichtung werden Luftblasen eingebracht und das durch die thermokapillaren Kräfte gegen den Auftrieb erzeugte Geschwindigkeitsfeld vermessen. In einem zweiten Experiment wird die Erhöhung des Wärmeübergangs durch eine Luftblase an einem beheizten Draht gemessen. Trotz kleiner Berührungsfläche kann der Wärmeübergang gegenüber dem Draht ohne Blase verdoppelt werden, was der zusätzlichen Marangoni-Konvektion zuzuschreiben ist. In Wasser wurde dieser Effekt nicht beobachtet, hier wurde durch die Luftblase der Wärmeübergang um 30% reduziert.

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Thermocapillary convection on gasbubbles caused by temperature gradients

The thermocapillary convection on free surfaces is an important mechanism for heat transfer, it is demonstrated in two experiments. Silicon-oil of various viscosity is used as experimental liquid. In the thermal stable stratified liquid air bubbles were injected. Around the bubbles velocity profiles, induced by the thermocapillary force against the buoyancy force were measured. In the second experiment the enhancement of heat transfer induced by air bubbles touching a heated wire is investigated in various liquids. In spite of the small contact area between the bubble and the wire the increase of heat transfer was more than twice of the wire without bubble which is attributed to the Marangoni convection. Using water this enhancement could not be observed. At the bubble the free convection from the wire was hindered and the heat reduced by 30%.

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Herrn Prof. Dr.-Ing. K. Stephan zum 60. Geburtstag gewidmet     

Air-cooled gas-turbine discs: a review of recent research

The flow between corotating compressor or turbine discs and the flow between a turbine disc and an adjacent stationary casing can be respectively modelled by a rotating cavity and by a rotor-stator system. This paper reviews some of the recent experimental and theoretical work on flow and heat transfer in these two classes of rotating-disc systems. Comparisons between the theoretical and measured distributions of velocity, pressure, and Nusselt numbers are made for the rotating cavity with a superimposed radial flow of cooling air. For the rotor-stator system, some recent work on the fluid dynamics is outlined, and particular mention is made of the so-called “ingress problem” and of the use of pre-swirl air to improve the blade-cooling effectiveness.     

Simultaneous stress separation,smoothing of measured thermoelastic isopachic information and enhanced boundary data

A new thermoelastic hybrid method of stress analysis is demonstrated experimentally which simultaneously smooths the measured isopachic data, enhances the boundary information, and separates the stresses at nonboundary locations using only the isopachics. The technique is illustrated by application to a tensile plate containing a hole.     

On the mechanical dissipation of solutions to the Riemann problem for impact involving a two-phase elastic material

Communicated by C. Dafermos