Fluid rigid body interaction is commonly occurring phenomenon and this phenomenon is of high importance in many engineering applications. The main objective of the present paper is to analyse vibrations due to fluid-rigid body interactions by inclining the frontal area of flat plate to flow. As understood from the existing research, the main challenge is the understanding of non-stationary fluid body interactions. Interaction analysis, optimization and synthesis tasks include space-time programming methods and approximate analytical methods. This article discusses an approximate analytical method in which the object's interaction with fluid flow is divided into two parts in the fluid body interaction space. The first part is the frontal pressure side that arise as a result of change of momentum in the system that can be conveniently represented in a differential form. The second part includes the idea of describing the interaction behind the plate as a certain thin (vacuum) side was accepted. This thin vacuum side also depends on the frontal area flow interaction. The use and precision of the approximate analytical method was verified by experiments in the wind tunnel. The method was used for good analysing of varying frontal area (until flattening) of flat plate in fluid flow. The main parameters for motion excitation are the changes in plate-flow interaction area, velocity and angle of inclination of the flat square plate to the fluid flow. Experiments were performed at 10 m•s-1 keeping in view the wind speeds that were observed in the past in Riga, Latvia.

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The paper is devoted to vibration of the foundations for rotary screw compressors used for gas compression for thermo power plant installed on skid mounting. To evaluate the compressor vibration according to the industry standard VDI 3836, the user should decide whether the foundation is rigid or resilient. The foundation is rigid, if the vertical natural frequency of the foundation is at least 25 % higher than the excitation frequency. The excitation frequency, compressor running speed in Hz is normally known, while the natural frequency is usually not known. Therefore the goal of the study was to find natural frequencies of the skid using analytical calculations, SolidWorks simulation and "bump testing" on site. In the results section it is shown that vibration in axial direction is usually higher for screw compressors than in radial direction, therefore the mathematical model with compressor shaking horizontally was adopted for this study. Limitations of the bump test were discovered. The examples of distinctive and indistinctive spectrum were presented. It was concluded that for the structure of screw compressor foundation the bump testing on site is rather an ineffective tool to detect natural frequencies, because there are too many frequencies appearing in the spectrum and no natural frequencies could be distinguished from the time waveform. Therefore, testing of the equipment foundation natural frequencies has to be performed in the manufacturing facility before installing the equipment and filling the frame with concrete.

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