Stickstoff

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Textilindustrie

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Der Einflu\ des Randwinkels auf die Benetzung von porösen Körpern

Zusammenfassung Versuchsergebnisse für kapillare Aufsteigung in porösen Körpern, insbesondere für Wasser in Baustoffen, werden gegeben. Es wird gezeigt, da\ für homogene isotrope Körper ein scharfer Flüssigkeitsspiegel beobachtet werden sollte. Abweichungen werden verursacht durch den Einflu\ des Randwinkels und die InhomogenitÄt des porösen Körpers. Für praktische Systeme ist der Benetzungswinkel: (a) gewöhnlich nicht Null, (b) sehr oft nicht konstant in der Zeit, und (c) zuweilen nicht konstant für verschiedene Teile des porösen Körpers. Au\erdem existiert kein einfacher Zusammenhang zwischen dem Randwinkel und der maximalen kapillaren Steighöhe. VorschlÄge werden gemacht, wie der Benetzungsvorgang von praktischen Systemen zu prüfen ist.

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The influence of the contact angle on the wetting of porous media

Experimental results are reported on capillary rise in porous media, in particular water in building materials. It is stressed that capillary rise is a hydrodynamic transport. For short times, a sharp macroscopic liquid front should be observed for homogeneous isotropic media. It appears that this is in fact so for random sphere packings. Deviations from the ideal behaviour are due to the influence of the contact angle and the inhomogeneity of the porous medium. For practical systems the contact angle is (i) usually different from zero, (ii) very often changing with time, (iii) occasionally varying over different parts of the porous medium. Moreover, the capillary driving force is no simple function of the contact angle. The effect of these factors on the phenomena to be observed are discussed. Suggestions are given how to test the wetting behaviour of practical systems.

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Formelzeichen g Schwerebeschleunigung ¦L/T²¦ - h Steighöhe ¦l¦ - H maximale kapillare Steighöhe ¦l¦ - k DurchlÄssigkeit ¦l/T¦ - 0 spezifische OberflÄche ¦L^–1¦ - r Halbmesser einer Kapillare ¦L¦ - s Feuchtigkeitsgehalt ¦-¦ - t Zeit ¦T¦ - w Strömungsgeschwindigkeit ¦L/T¦ - Neigungswinkel gegen die Lotrechte ¦-¦ - ZwischenraumporösitÄt ¦-¦ - dynamische ZÄhigkeit ¦M/LT¦ - Flüssigkeitsleitzahl ¦M/T²¦ - OberflÄchenspannung ¦M/T²¦ - Randwinkel ¦-¦ - Dichte der Flüssigkeit ¦M/L³¦     

Announcement

Vibration reduction in a harmonically excited 1-DOF beam with one-sided spring is realized by control-ling the system state from a stable large amplitude 1/2 subharmonic response towards a coexisting unstable small amplitude harmonic response using feedback linearization. At the unstable harmonic response no control effort is needed because the unstable harmonic response is a long term solution of the uncontrolled system. To reduce control effort when stabilizing the unstable harmonic response, the stable manifold can be used within the control design because at the stable manifold the system state approaches the unstable harmonic response without control effort. Unfortunately, the calculation of this manifold acquires much off-line computational effort while its usage complicates the on-line control design. Therefore, the stable manifold is approximated by the stable eigenvectors of the monodromy matrix. Due to the local validity of the approximation, a two-stage control approach is used. In the first stage, the system state is controlled towards the unstable harmonic response to reach the region where the stable manifold can be approximated accurately by the stable eigenvectors. In the second stage, the system state is controlled towards the stable eigenvectors and approaches the unstable harmonic response with hardly any control effort.     

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