Rafael S. Gioria

An Experimental Investigation of the Flow Around Straked Cylinders

This paper presents experimental results concerning the response of circular cylinders with and without strakes. The longitudinal and transverse fluid forces (drag and lift), amplitude response and wake structures of plain and helically straked cylinders are compared. Six different configurations of straked cylinders with pitches (p) equal to 5D, 10D and 15D and heights (h) equal to 0.1D and 0.2D are investigated. Measurements on the dynamic response oscillations of an isolated plain and straked cylinders and flow visualization employing a PIV system are shown. Fixed cylinder drag measurements are also shown. The models are mounted on an elastic base fitted with flexor blades and instrumented with strain gauges or in an air bearing base. The base is fixed on the test-section of a water channel facility. The flexor blades possess a low-damping and the flexor blades base an the air bearing base are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments ranges from 2000 to 10000, and reduced velocities, based on natural frequency in still water, vary up to 13. The drag coefficient is increased by 20% for the h = 0.1 D cylinder, and 60% for the h = 0.2 D cylinder, comparing both with the plain cylinder. The smaller height strokes (h = 0.1 D) do not prevent vortex formation in the region very close to the body, resulting in a decrease of about 50% of the amplitude response compared with the plain cylinder. Lowest amplitude response was found to the p = 10 D and h = 0.2 D case. The analysis of the vorticity contours shows that the shear layer does not roll close to the body (same result for the other cases with h = 0.2 D).Copyright

Two- and Three-Dimensional Simulations of the Flow Around a Cylinder Fitted With Strakes

Two- and three-dimensional simulations of the flow around straked cylinders are presented. For the two-dimensional simulations we used the Spectral/hp Element Method, and carried out simulations for five different angles of rotation of the cylinder with respect to the free stream. Fixed and elastically-mounted cylinders were tested, and the Reynolds number was kept constant and equal to 150. The results were compared to those obtained from the simulation of the flow around a bare cylinder under the same conditions. We observed that the two-dimensional strakes are not effective in suppressing the vibration of the cylinders, but also noticed that the responses were completely different even with a slight change in the angle of rotation of the body. The three-dimensional results showed that there are two mechanisms of suppression: the main one is the decrease in the vortex shedding correlation along the span, whilst a secondary one is the vortex wake formation farther downstream.Copyright

Three Dimensional Wake Structures of Flow Around an Oscillating Circular Cylinder

Direct numerical simulationsthe three-dimensional flow around an oscillating circular cylinder are carried out. Imposed body oscillations are realized for low amplitude of oscillation, A/D = 0.4 and for high amplitudes, A/D = 1.0 . As the intention is to analyze the amplitude influence in the wake dynamics, the frequency of oscillation is fixed and chosen to be inside the lock-in region, 0.95 fs, where fs is the shedding frequency of fixed cylinder. The three-dimensional wake characteristics of the oscillatory body simulations are compared to the fixed body. Floquet stability analysis of two-dimensional oscillatory flow is carried out to complete the investigation and consistently analyze the three-dimensional flow results. The different unstable modes are identified for each of the cases, and they are found to depend basically on the vortex patterns.Copyright

Evaluating the control of a cylinder wake by the method of sensitivity analysis

Numerical simulations and sensitivity analysis are carried out regarding control vortex shedding from a circular cylinder using small rotating and non-rotating control cylinders. Prediction of the changes in unsteady wake, instability growth rate, and frequency produced by the control devices is presented in the framework of sensitivity analysis to steady field force. In the case of the rotating cylinders, this prediction relies on the observation that the small control cylinders introduce a steady force whose direction is not aligned to the base flow direction. This external force direction depends on the control cylinders rotation rates having an additional parameter to attain flow control. The evaluation of the control cylinders rotation rate within the sensitivity analysis framework is a novel contribution in this work. The results of the sensitivity analysis to a steady force are then compared to direct numerical simulation computations of the flow fields serving as supporting data for discussion.

Amplitude threshold in the wake transition of an oscillating circular cylinder

In this work we investigate the effect of the amplitude of oscillation on the secondary transition of the wake of an oscillating circular cylinder. In order to carry out this investigation, Floquet stability analysis is employed for Reynolds numbers 200 and 300, and the amplitude of oscillation is varied from 0.0d to 0.3d, where dis the cylinder diameter. A threshold amplitude is observed: if the oscillation amplitude is smaller than the threshold the flow behaves in a manner similar to the flow around a fixed circular cylinder. Above this threshold, the oscillation affects the three-dimensionalities of the wake depending on the amplitude of oscillation.

Comparison Between the Koopman Modes for the Flow Around Circular Cylinder and Circular Cylinder Fitted with Helical Strakes

The helical strake is a device commonly employed for vortex-induced vibration attenuation or suppression. It works by changing and affecting the flow patterns around a circular cylinder and, thereby, reducing the interaction of the shear layers formed in this massive separated type of flow. In this paper the wake of vortices generated by a bare circular cylinder is compared to the wake generated by a circular cylinder fitted with helical strake with pitch equal to ten cylinder diameters and height equal to 0.2 of the cylinder diameter. Besides the analysis of the measured velocity field, a Koopman decomposition is made and the most energetic modes obtained for each condition are compared. The Reynolds number for the visualizations is \(\text {Re} = 10{,}000.\)

STABILITY ANALYSIS OF THE FLOW AROUND AN AIRFOIL IN STALLED CONFIGURATION

perturbations, using the Floquet stability theory. The time-periodic base ow is computed with the spectral/hp element method. The results are the modulus of the Floquet multiplier versus the wavelength of the perturbation, showing the characteristic spanwise length of the three-dimensionality. It is possible to use these results in order to perform quasi-three-dimensional simulations of the ow past uniform wings, with great computational time saving and low resolution loss on the physics of the phenomena. Non-linear simulations are performed in order to compare results with linear stability analysis.