Die Ausdehnung des Kavitationsgebietes

Zusammenfassung

Bei einer Strömung mit Kavitation hat man im wesentlichen zwischen vier Gebieten zu unterscheiden: (Fig. 1)

• I. Das Gebiet des festen Körpers, der umströmt wird,

• II. das Gebiet der strömenden Flüssigkeit, in welchem nahezu konstante Dichte vorhanden ist und in welchem sich der Druck stetig ändert,

• III das eigentliche Kavitationsgebiet, das durch ein Dampf-Flüssigkeitsgemisch, das u. U. auch Luft- und Gasreste enthält, erfüllt ist von wesentlich geringerer Dichte als die Flüssigkeit und in welchem der Druck konstant ist,

• IV. das Gebiet der Rückbildung der Kavitation, in welchem der Anteil des Dampfes am Gemisch abnimmt unter einer der Druckzunahme entsprechenden Dichteänderung.

Abstract

In cavitating flow, the four regions shown in fig. 1 should be distinguished. Here, I is the solid, II the region in the fluid where the density is practically constant and the pressure changes follow the usual law; III, the actual cavitation region of low density fluid-vapour mixture and almost constant pressure; and IV, the region of recovery from cavitation and hence an increasing pressure region. This fourth region can be regarded as the inverse of the second behind an additional solid placed downstream from the first without any essential change of the cavitation zone III as shown in fig. 2. This substitution has been tested and confirmed experimentally, the cavitation zone being even more clearly defined than with the single solid. This justification directs attention to the use of the potential theory to the calculation of the linear extent of the cavitation zone. Fig. 3 and 4 illustrate different types of potential flow, the latter admitting of central cavitation and the former being considered as a detailed example illustrated in fig. 5. Here the second solid is introduced and the maximum induced velocity is that corresponding to the vapour pressure. The hodograph of the various regional velocities is shown in figs. 6 and 7. The stream velocity is AB and the maximum velocity is BC. Calling the ratio of BC to AB, R, the ratio of the length of the cavitation zone to the width of the solidf is shown to be approximately given by λ = 2/R²−1. The value of λ does not vary much with the shape of the solid and thus the foregoing approximation is of general application. When R equals unity the Kirchhoff solution of λ infinite is obtained and when R approaches infinity, through the stream velocity being very small, λ is zero and the flow is completely streamline.

Refers to the paper on page 294–300 of German text.