CAVITATION ON A GROOVED TWO-DIMENSIONAL HYDROFOIL AT A SMALL ANGLE OF ATTACK
| Jazyk: | ruština |
|---|---|
| Rok vydání: | 2019 |
| Předmět: | |
| Zdroj: | Известия Томского политехнического университета: Инжиниринг георесурсов, Vol 329, Iss 11 (2019) |
| Druh dokumentu: | article |
| ISSN: | 24131830 2500-1019 2413-1830 |
| DOI: | 10.18799/24131830/2018/11/206 |
| Popis: | Abstract. Cavitation is one of the main sources of flow instabilities arising during the operation of hydraulic equipment and it is also a cause of erosion wear of its working parts. In this regard, the elaboration and development of various methods of cavitating flow control is an urgent problem for hampering cavitation evolution and reducing its negative impact. One of these methods is a hydrofoil surface modification. The main aim of the study is to investigate experimentally cavitating flow around a ribbed hydrofoil with a streamwise semi-circular grooves on the surface (GV2) representing a scaled/down model of a guide vane of a Francis turbine in comparison with a model of the original vane (GV1). The methods. In order to analyze the spatial structure and time dynamics of vapor cavities and evaluate their integral parameters, a high-speed imaging was applied. The flow velocity over the hydrofoils and in their wakes was measured by a PIV method, distributions of the mean and turbulent flow characteristics were obtained on the basis of the measured ensembles of instantaneous velocity fields. Results. On the vane with the modified surface (GV2), cavitation originates as single isolated bubbles travelling in the grooves that transform into cavitating streaks, when the cavitation number is decreased. While the streaks are located inside the grooves and do not interact with each other, the flow regime remains stable. However, when their size becomes larger than the groove diameter, they extend beyond these hollows, interact and form an entire cavity which immediately becomes unstable and starts to oscillate. In general, the grooves on the hydrofoil surface are capable to hinder the cavitation development to some extent and delay the transition to unsteady flow regimes. When the flow is unsteady, the cavity dynamics on the original hydrofoil (GV1) and GV2 is very different. For example, unlike GV2, the cavity on GV1 never disappears completely, the cavity on GV1 is longer on the average and pulsates at a higher frequency (St=0,09) compared to GV2 (St=0,06). In addition, the cavity behavior during one period of the oscillations turned out to be quite unusual for both models: first it elongates to its maximum size, then somewhat decreases and again grows to the maximum, after which it returns to its initial state. The reason for such dynamics is still unclear. At the transitional flow regime, when cavitating streaks are formed inside the grooves, the intensity of turbulent fluctuations over the GV2 surface is decreased in comparison with the regime of transient bubble cavitation. This occurs because the isolated cavities inside the grooves are likely to restore the shape of the modified foil, making the geometry of its surface closer to the original one (GV1). Thus, the GV2 profile becomes more streamlined due to local cavitation in the grooves. Besides, the grooves on the GV2 surface cause a local flow turbulization close to the wall for all flow regimes under consideration, which is probably the reason of the delay in cavitation evolution on the grooved hydrofoil. |
| Databáze: | Directory of Open Access Journals |
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