This work finds its motivation in heat exchanger design and flow control. Flow-induced vibration is studied numerically for combined vortex-induced vibrations and vortex-induced rotations of a horizontally positioned elliptic cylinder. The aspect ratio is taken as 2, and the value of reduced velocities ([Formula: see text]) considered for the present simulation is between 2 and 12. The body can have to and fro motions in a transverse (y) direction, in-line (x) direction as well as in azimuthal ([Formula: see text]) direction, which provides three degrees of freedom (DOF) to the body. It is found that for one-DOF (y-direction only) and two-DOF (y and x directions) cases, lock-in regions are the same while it is wider for the case of the three-DOF system. With the rotational DOF, y-directional motion is amplified and when it is compared with the one-DOF and two-DOF cases, difference in peak amplitude is about 30%. The rotational response reaches a maximum value within the synchronization regime, and the frequency behavior of rotational and transverse oscillations is showing the same characteristics. The phase difference is plotted to check their synchronization with respective forces and moments. For all DOFs and [Formula: see text], synchronized or desynchronized regions, 2S mode of vortex shedding was observed. For one-DOF and two-DOF cases, the transverse vibrational frequency ratio ([Formula: see text]) becomes equal to unity for the range [Formula: see text]. For three-DOF, [Formula: see text] and rotational frequency ratio ([Formula: see text] become close to the unity for [Formula: see text]. The three-DOF system shows smaller wake width and vortex formation length whereas the vortex strength is maximum.

Cavitating vortex rope at part load (PL) condition at lower values of Thoma number induces severe pressure fluctuation and efficiency reduction in a Francis turbine which ultimately hinders continuous energy production. Installation of fins at draft tube (DT) can mitigate these instabilities and can safeguard the turbine operation with lower maintenance costs. Effect of fins on hydraulic performance and internal flow physics at PL condition with the variation is examined in the present numerical investigation. For the two extreme opposite values of , the flow characteristics are predicted accurately for the turbine with and without fins by conducting transient simulations using ANSYS-CFX. The numerical findings on the structured and unstructured grid points are validated with the experimental results. The turbine's performance remains constant for higher values of Thoma numbers and as the value decreases the performance declines. The cavitation vortex rope formation inside the DT with fins is mitigated significantly at the minimum while at the maximum value, the vortex rope with bubble generation is restricted. Compared to the without fins case, the swirl intensity is minimized remarkably (68%) with the presence of fins at the lowest. The maximum cavitation rate is manifested by the DT without-fins which is about 60% higher than the DT with fins. At minimum, extreme pressure pulsations are induced inside the DT without fins which are reduced by 43% in the finned draft tube. Therefore, stable energy production is maximized with the installation of fins at both Thoma numbers.

Vortex rope formation at part load (PL) with cavitation inception causes pressure fluctuations inside the draft tube (DT) of a Francis turbine which may fail the turbine due to resonance and erosion. The pressure surge can be minimized by using anti-swirl fins which ensure safe turbine operation. The present study examines the effect of fin sizes and locations on the internal flow characteristics of the Francis turbine and predicts its adverse effect on the pressure surge. Three cases are investigated in which internal flow physics are compared among DTs with longer fins, shorter fins and no fins. At the cavitation inception point under PL conditions, the characteristics are thoroughly studied numerically using ANSYS-CFX with structured and unstructured grids. Cavitation and PL conditions are designated by Thoma number 0.266 and guide vane angle . Numerical methodology is then verified by an experiment based on International Standard (IEC 60193). The vortex rope occurrence is suppressed using fins on the DT periphery and longer fins that are extended up to the elbow exhibit the lowest strength of the vortex rope. Maximum pressure recovery inside the DT is achieved using longer fins. The swirl intensity remarkably reduces by about 94% with longer fins. The pressure peak of low frequency is about 60% suppressed using longer fins. The PL and cavitation-induced instabilities and vibrations are significantly mitigated by longer fins while shorter fins also moderately minimize it. Hence energy production is preferable with longer fins because of the safe and stable turbine function.