The paper presented intends to fill up a gap in surveying the Hotelling Rule by taking a company based, microeconomic approach based on analyses of annual reports. Using selected data three fundamental hyphothesis are tested:
1) growth rate of margins (“net margins” including a capital charge) per unit realized by mining companies must exceed a rate equal to their cost of capital,
2) output shall follow deviations from the Hotelling growth line,
3) margins shall follow a path set by individually defined expected rate of return.
The analysis was based on 5 leading gold producers, responsible for ca 15–20% of global primary production, all of them public and listed on a stock exchange for the entire period of 2004–2019/2020. As margin shall grow at a rate compensating individual risk of a company in consideration, they shall not be homogenous. At 1st step industry WACC was adopted to calculate a normalized capital charge. The calculations revealed no support for Hotelling Rule. There is no evidence that over a period of above 15 years margins follow any path determined by a growing expotential function, following a compound rate. Subsequently it was checked whether output volume is corrected due to development of actual versus expected (resulting from the Hotelling Rule) margin values. Selected companies were near indifferent to this parameter while taking decisions in area of volumes supplied. Neither there is no evidence of relation between changes in output and margins. Finally, it was checked whether differences between expected and actual margins’ growth paths could be described by a linear function, resulting from consequent adoption of a risk rate component. Here neither any evidence was found. In conclusion no support for the Hotelling rule was identified.

The highly dynamic and unsteady characteristics of the cavitating flow cause many negative effects such as erosion, noise and vibration. Also, in the real application, it is inevitable to neglect the dissolved air in the water, although it is usually neglected in the previous works to reduce the complexity. The novelty of the present work is analysing the impact of dissolved air on the average/unsteady characteristics of Venturi flow by conducting sets of experimental tests. For this purpose, two different amounts of dissolved air at five pressure levels (i.e. five different sets of cavitation numbers) were considered in the study of cavitating flow inside a Venturi nozzle. The fast Fourier transform analysis of pressure fluctuations proved that the shedding frequency reduces almost by 50% to 66%, depending on the case, with adding the amount of dissolved air. However, the reduction of 14% to 25% is achieved by the vibration transducers. On the other hand, the cavity enlarges as well as bubbly flow is observed in the test chamber at a higher level of dissolved air. Furthermore, it is observed that the re-entrant jet, as the main reason for the cavity detachment, is more effective for the detachment process in cases with a lower level of dissolved air, where the re-entrant jet front penetrates more toward the leading edge.