Influence of the Reshaped Elbow on the Unsteady Pressure Field in a Simplified Geometry of the Draft Tube

The paper focuses on the influence of the reshaped elbow geometry of the draft tube on the unsteady pressure field under different operating conditions. The experimental investigations are conducted considering two simplified geometrical configurations of the draft tubes with sharp heel elbow and modified/reshaped elbow, respectively. The discriminated power spectra (rotating and plunging components) of the acquired pressure signals on the wall are determined on five levels for seven operating conditions to quantify the influence of the reshaped elbow. The influence of the reshaped elbow on the fundamental frequencies of both rotating and plunging components and on the amplitude of the rotating component is negligible. In contrast, the equivalent amplitude associated with the root mean square of the plunging power spectrum that propagates along the hydraulic passage is mitigated up to 25% by the reshaped geometry of the elbow. The equivalent amplitude on the narrow band around the fundamental frequency of the plunging component is diminished with 40–50% by reshaping the elbow geometry of the simplified draft tube.

COMPARATIVE ANALYSIS OF PRESSURE PULSATIONS CAUSED BY SPIRAL-VORTEX STRUCTURES IN THE FLOW PART OF THE HYDRAULIC TURBINE MODEL

The paper studies the pressure pulsations in the hydraulic turbines flow section arise as a result of vortex structures in non-optimal hydraulic turbine operation regimes. The authors directly compare the approaches to the extraction of synchronous and asynchronous components in pressure pulsation signals, as well as using the decomposition into azimuthal modes. Pressure pulsations were measured using four acoustic sensors while varying the operating regimes of the hydraulic turbine model. The pressure pulsations were compared for the shallow draft tube and the deep draft tube of the hydraulic turbine. It is shown that the level of pressure pul-sations is the same for two types of draft tubes. There is no methodological difference in the application of the two approaches to the extraction of pressure pulsations in the flow. The results shall be applicable in the de-velopment of new methods for suppressing spiral-vortex structures in non-optimal regimes of operation of hydraulic turbines in order to increase their overall efficiency.

NUMERICAL STUDY OF A SWIRLING TURBULENT FLOW THROUGH A CHANNEL WITH AN ABRUBT EXPANSION

A strongly swirling turbulent flow through an abrupt expansion is studied using the highly resolved DNS, LES, and SAS to shed more light on a stagnation region and spiral vortex destruction, though these methods require high computational expenses. The vortex fracture induced by a sudden expansion resembles the so-called vortex rope that occurs in hydropower draft tubes. It is known that large-scale spiral vortex structures can be captured by regular RANS turbulence models. In this paper, a numerical study of a strongly swirling flow, which abruptly expands, is carried out using the Reynolds stress models SSG / LRR-RSM and EARSM with experimental measurements implemented by Dellenback et al. (1988). Calculations are carried out using the finite volume method. The flow dynamics is studied at the Reynolds number of 3.0 × 104 at almost constant large swirl numbers of 0.6. The time-averaged velocity and pressure fields, as well as the root-mean-square values of the velocity fluctuations are recorded and studied qualitatively. The obtained results are compared with known experimental data. The aim of this work is to test the ability of the models to describe anisotropic turbulence. It is shown that the SSG / LRRRSM model is more appropriate for studying such flows.