Heated Circular Cylinder Subjected to Forced Spanwise Oscillations

Abstract

Abstract The influence of spanwise vibrations coupled with various levels of heating, on the lift and drag coefficients, is numerically studied in this work. The flow domain consists of a circular region of 64D surrounding the circular cylinder of diameter D. Transient analysis is conducted to solve URANS using Ansys/Fluent for laminar flow at Reynolds number of 100. Spanwise forced oscillations are carried out using user-defined-functions to mimic the flow induced vibrations. Amplitude of oscillation is kept fixed at 0.1D and frequency of oscillation is varied according to the frequency ratios (f/fn) of 0, 0.5, 1, 1.5, and 2, where 0 means the cylinder is stationary. Four levels of heating is applied, ΔT = 0 for isothermal, and ΔT = 300, 600, 900 K for non-isothermal flow, where ΔT is the temperature difference between the cylinder wall and the oncoming fluid. Air is taken as the fluid and temperature dependent properties of air are considered as the properties change significantly in the given temperature range. Mesh sensitivity is done initially to gain good fidelity of the discretized flow domain and the model is validated using the experimental results from the literature. The non-dimensional natural vortex shedding frequency of the stationary cylinder for isothermal flow is found to be 0.165 marking its Strouhal number. It is observed that heating the cylinder decreases the natural vortex shedding frequency. Increasing ΔT to 300 and 600 K decreased the natural vortex shedding frequency by 14.29% and 28.03%, respectively. It is observed that vortex shedding stops at ΔT of 900 K for stationary cylinder and for forced oscillating cylinder only one peak is seen in Fast Fourier Transform (FFT) corresponding to the forcing frequency. It is observed that the rms of the lift coefficient increases with an increase in the frequency ratio at all values of temperatures. FFT of the lift coefficient revealed only one frequency for frequency ratio of 0 and 1 at the natural frequency of the cylinder whereas for other values of frequency ratio, two peaks are observed, one for the natural frequency and the other for the forcing frequency. Lock-in phenomena is observed at the frequency ratio of 1 for isothermal cylinder where a large increase in the average drag coefficient occurred. For all values of frequency ratio, an increase in the temperature difference results in decrease in the lift and increase in the drag coefficient. Increasing ΔT to 300, 600, and 900 K, increases drag by 7.33%, 11.65%, and 16.52%, respectively, for stationary cylinder and a similar trend in observed for the oscillating cylinder. These results show that heating the cylinder decreases the lift coefficient and the natural vortex shedding frequency of the cylinder, whereas it increases the drag coefficient.