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- 2018 (10) (entfernen)
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- Material moisture (3)
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- Concrete and screed (2)
- Embedded humidity sensors (2)
- Luftgekoppelter Ultraschall (2)
- Moisture measurements (2)
- Pore volume distribution (2)
- Airfoil pitching (1)
- Airfoil surging (1)
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Eingeladener Vortrag
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Structural Health Monitoring (SHM) has become very important in today's rapidly developing time. High buildings, large bridges and complex technical structures need to be monitored continuously and this over long periods.
Visual monitoring cannot evaluate the internal condition of building structures. Thus, material embedded sensors are needed. Cable connection of these sensors pose the disadvantage of weak spots and water intrusion. For concrete embedded sensors the use of batteries is not convenient, because of limited lifetime, difficult charging, and generation of electrical waste.. Hence, monitoring should be implemented preferentially with firmly embedded passive RFID sensor modules. However, since the concrete structure forms an electromagnetically reflective and absorbing barrier, only limited energy can be transmitted to the module. This project requires a highly energy-saving system, which can record different sensor parameters at critical points.
High-Amplitude Surge of a Pitching Airfoil: Complementary Wind- and Water-Tunnel Measurements
(2018)
RECENT interest in gust response, rotorcraft forward flight and wind energy, among other applications, has focused on streamwise oscillations of nominally two-dimensional airfoils in attached and separated flows. The airfoil may be simultaneously held at constant incidence or execute some maneuver, such as pitch.
The relative freestream is spatially uniform but temporally unsteady, and this can be accomplished in a ground-test facility in two ways. The first method is to vary the output of the prime mover, such as the rotational speed of the impeller or the blower driving a wind tunnel, or (alternatively) to vary the pressure drop in the tunnel, thereby varying the flow speed in the test section, despite nominally constant primemover revolutions per minute. The second method is used to keep the tunnel’s operating speed constant, as well as to move the test article in the streamwise direction, fore and aft (for example, via an electric linear motor), such that the relative freestream speed felt by the test article varies according to some waveform. Typically, the latter approach is chosen in water tunnels, where there is too much tunnel-circuit inertia to vary the flow speed directly but where the usually low tunnel test section flow speeds enable large excursions in the relative freestream by oscillation of the test article. In fact, outright reverse flow is possible by moving the test article in the laboratory frame at a higher speed than the water-tunnel flow speed. In either case, a sinusoidal relative-speed waveform is the most intuitively realizable, and this can be combined with similar oscillations in the airfoil incidence angle or other kinematics. Although the two methods of realizing streamwise oscillations are mechanically distinct, experimental comparisons between an oscillating test article in a water tunnel and a stationary test article in a wind tunnel with a louvermechanismhave demonstrated agreement in themeasured lift and drag histories. Such experimentswere performed by Granlund et al. for a 10% freestream amplitude oscillation and fixed airfoil incidence, comparing a free-surface water tunnel and a closed-circuit wind tunnel. After buoyancy was subtracted from the wind-tunnel data (resulting from the louver pressure drop) and the model inertia subtracted from the water-tunnel data (resulting from acceleration of the test article), the remaining lift and drag histories matched well at the low freestream oscillation amplitude regime. The work of Granlund et al. was later extended to high-advance-ratio streamwise oscillations of 50% amplitude by Greenblatt et al., where the aerodynamic histories of the water-tunnel and wind-tunnel facilities were compared in combined pitch and freestreamoscillations (governed by relative pitch phase), pure pitch oscillations, and purely freestream oscillations. Agreement between the two facilities’ data for fixed-incidence streamwise oscillations was reasonably good, and in fact, better than agreement in just the static lift and drag, evidently owing to differences in blockage and model-support systems. Additionally, Greenblatt et al. determined there was no strong coupling between simultaneous freestream oscillations and pitch oscillations on resultant lift and moment coefficients.
Moisture testing and mapping
(2018)
Feuchtemessverfahren
(2018)
Das Ultraschallprüfverfahren wird im Bauwesen für die zerstörungsfreie Prüfung von Bauwerken eingesetzt, insbesondere für Strukturuntersuchungen. Dadurch sollen Lage und Größe von Bauschäden detektiert werden, um geeignete und kostengünstige Sanierungsmaßnahmen ergreifen zu können. Da in der Praxis meist nur eine einseitige Zugänglichkeit gegeben ist (z. B. Betonverschalung eines Tunnels), wird im Impuls-Echoverfahren gemessen. Dies bedeutet, dass ein Ultraschallsignal erzeugt und dessen Echo aufgenommen wird. Prüfköpfe, welche einen Oberflächenkontakt benötigen, erhöhen die benötigte Messzeit dramatisch. Mittels luftgekoppeltem Ultraschall können viele naupraktische Problem umgangen werden und die Messzeit kann deutlich reduziert werden. Als Möglichkeit zur Anregung eines luftgekoppeltem Ultraschallsignals werden fluidische Bauteile diskutiert
Monitoring of the absolute water content in porous materials based on embedded humidity sensors
(2018)
Moisture transport monitoring may indicate the onset of deterioration in porous building materials Prior to damage occurring. Most moisture measurement systems provide only qualitative values, require extensive calibration, or are destructive. Thus, non-destructive and calibration-free monitoring Systems are required. Our approach of moisture monitoring is to embed sensors that measure the relative humidity.
In our experiment, screed samples are monitored during the Hydration and evaporation process. Every test sample is equipped with 10 embedded sensors which measure the relative humidity across the sample thickness. Based on Hillerborg’s approach, the relative humidity is converted into the corresponding pore saturation. In our study, the free water is computed without knowledge of the Sorption isotherm. The free water in the pore system is predicted and validated. The predicted weight decrease corresponds conclusively to gravimetrically measured weights. The embedded sensors yield the absolute liquid water content and enable an experimental, non-destructive monitoring of liquid water in porous materials.
Experiments indicate that vortices trailing finite wings can be perturbed by periodic wing pitching, leading to rapid dissipation and bursting. To illustrate the perturbation mechanism, Betz vortex rollup relations are combined with the Theodorsen theory for unsteady lift response. A sinusoidal pitch motion on a rigid elliptic planform wing is computed in this study as one example. Pitching modifies the instantaneous lift due to the planform variations, via the reduced frequency, and the rollup relations are applied to low-frequency experimental conditions. The combined Betz–Theodorsen theory shows that relatively large spanwise perturbations of the vortex centers can be achieved and may accelerate the exponential growth associated with the Crow instability. In fact, 84.2% of the displacement of the
unsteady trailing vortices due to unsteady pitching is oriented in the direction of the Crow instability. Furthermore, the axial velocity in the vortex center, calculated based on the Batchelor method, varies on the order of the flight speed.
This forms two types of stagnation points produced by approaching and retreating axial core velocities; in the former case, conservation of mass leads to observations of “bursting.” This observation can be explained adequately on the basis of quasi-steady considerations.
Four cement-based and four calcium-sulphate-based screed types are investigated. The samples have a diameter of 300 mm and a height of 35 or 70 mm. Up to ten humidity sensors are embedded directly during the concreting of the screed samples. Thus, the humidity over the sample height is monitored during hardening, hydration, evaporation, and oven drying. Furthermore, the screed samples are weighted during every measurement to determine the total mass and the corresponding moisture loss.
To define the pore system precisely, mercury intrusion porosimetry as well as gas adsorption is performed. According to the data, the entire pore volume distribution is known. The measured pore diameters range from 0.8 nm to 100 μm and the total porosity of the examined screeds ranges between 11 % and 22 %.
Based on these measurement data, moisture transport, pore saturation as well as sorption isotherms and their hysteresis may be calculated quantitatively as described by Strangfeld et al.
Four cement-based and four calcium-sulphate-based screed types are investigated. The samples have a diameter of 300 mm and a height of 35 or 70 mm. Up to ten humidity sensors are embedded directly during the concreting of the screed samples. Thus, the humidity over the sample height is monitored during hardening, hydration, evaporation, and oven drying. Furthermore, the screed samples are weighed during every measurement to determine the total mass and the corresponding moisture loss.
To define the pore system precisely, mercury intrusion porosimetry as well as gas adsorption is performed. According to the data, the entire pore volume distribution is known. The measured pore diameters range from 0.8 nm to 100 µm and the total porosity of the examined screeds ranges between 11% and 22%.
Based on these measurement data, moisture transport, pore saturation as well as sorption isotherms and their hysteresis may be calculated quantitatively as described in “Monitoring of the absolute water content in porous materials based on embedded humidity sensors” (Strangfeld and Kruschwitz, 1921).