Juan Sebastián Delnero

Active Flow Control Upon Cavities At Low Reynolds Numbers

Flow past open cavities is well known to give rise to highly coherent and selfsustained oscillations. The oscillations lead to undesirable aeroacoustic resonance. The self-sustained oscillations arise from a feedback loop which is formed as a result of successive events that take place in sequence: roll up of the separating shear layer that is convectively unstable, formation of vortices, vortices hitting the trailing edge of the cavity, propagation of the generated acoustic wave toward to the leading edge, and the receptivity process at the leading edge which set the initial amplitude and phase of the instability waves in the separating shear layer. In particular, when the process is coupled with an acoustic resonance, an intense aerodynamic noise is radiated from the cavity. This character is common to basically all cavity noises regardless of the Mach number. This type of aeroacoustic noise is referred as the cavity noise. But there are scarce bibliography regarding cavity flow control under free incident turbulent flows. So, our main interest is boundary layer control using cavity flows. On doing that we study the characteristics of the cavity flow when the incident free stream is turbulent and how to achieve some control on its vortex pattern inside and the surroundings, and the developed turbulent boundary layer outside the cavity. We intend to use the cavity flow generated as a active flow control system to manage the turbulent boundary layer outside it, using a suction and/or blowing system inside it. An experimental setup was made in order to study the vortex behavior and its incidence on the turbulent boundary layer configuration. As results we show the vortex generated inside the cavity and the turbulent boundary layer behavior related to them, also we include the vortex flow control system instrumentation and characterization.

Flow Interaction Analysis Near the Upstream Cavity Edge under Turbulent Incident Flow

The flow pattern developed by a cavity inmersed in a turbulent incident flow is the solution of a complex relation between its geometry and the incident flow characteristics. There are cavities of many different shapes and sizes which in some situations generate important adverse effects such as wind confort reduction or severe structural problems due to resonance phenomena. Besides the need of reducing this negative consequences associated to the cavity, it is possible to think about the idea of modifying the inner cavity flow pattern to make indirect active control over the external flow under turbulent incident free stream. This reasons make the study of cavity flow a very interesting topic not only from a scientific but also from practical point of view. We intend to use the cavity flow generated as an active flow control system to manage the turbulent boundary layer outside it, using a blowing and/or suction system inside it. An experimental setup was made in the LaCLyFA’s 1m by 1.4m test section turbulent boundary layer wind tunnel, in order to study the vortex behavior and its incidence on the turbulent boundary layer configuration. According to the proposed objectives a wooden cavity was constructed which has an aspect ratio (L/D) of 1 (wide, height and long 10 cm.), with a glass wall for flow visualization. Also, in each vertical wall 18 spanwise air injection tubes for the flow injection system were located. This study consist on HWA velocity measurements with and without flow injection at different frequencies (from 10Hz to 250Hz) for a flow rate of 4L/min per pipe. In the present work we analyse the flow interaction near the upstream edge of the cavity by means of HWA measurements on three longitudinal stations at the centerline starting from 2mm and up to 24mm downwind.

Experimental Dynamic Stall Study In An Airfoil

This paper presents an experimental analysis of the effect known as “double stall” or “dynamic stall” of an airfoil under incident turbulent flow conditions. In particular, it has been used a Wortmann FX 63-137 airfoil, which is largely used in wind turbine blades. The main objective of the present work is to define the general fluid dynamic configuration around the dynamic stall of the airfoil. At first, a load test was carried out at the LaCLyFA boundary layer wind tunnel in order to aerodynamically characterize the airfoil (through a two axis aerodynamic balance). The characteristic curves of the airfoil for different Reynolds number were obtained from the test, which showed significant parameters such as: Clmax, αstall, etc. After that, a pneumatic system that allows a sudden change in the angle of attack was implemented. Once operable, different flow visualizations tests for different Reynolds numbers and different rates of angle of attack variation were performed. The flow visualization techniques used were: tufts over the airfoil and its wake, a suspension of kerosene and oxide of magnesium and injection of a smoke line in the free stream. According to the visualization results measurements by hot-wire anemometry techniques with 3 simultaneous sensors placed in different locations of the model were made. Data were processed, determining statistical quantities, wind velocity scales, power density spectra. Data analysis showed different flows patterns for the static and dynamic stall.

Experimental study of near and far wake generated by a Gurney mini flap in turbulent flow

This work aims at analyzing the rolling asymmetry of the shear layers downstream of the trailing edge provided with a Gurney flap acting as flow control device to improve airfoil lift in incident turbulent flow. Experimental investigations have been made concerning the vortex flow asymmetry in the near and far wake region, able to distort and change the downwash produced by the airfoil. We conducted experimental studies on the wake at low Reynolds number of an airfoil HQ-17 and a Clark-Y, endowed with Gurney flaps in different configurations regarding the position and the oscillating movement at different frequencies. In order to identify patterns vortices in the airfoil wake a flow behavior is studied until one chord distance downstream. We found experimental evidence that shows two different behaviors in the airfoil wake. Near the trailing edge which shows a typical condition supporting a process of the upper and lower vortices formation quite evident behavior. Larger vortex is generated in the pressure zone and lower vortex in the suction zone. Aspects of the mechanism which connects the different evolution and pattern of these initial vortex structures with increased lift due to the presence of the Gurney flap is presented. Furthermore experimental evidence in the distant wake is found, that downwash increase in the case of the Gurney flap acting as an active flow control element compared to the fixed Gurney. It is also important that changes in the wake turbulence in different conditions are found.

Unsteady Flow Injection in Turbulent Cavity Flow

An experimental study was conducted to analyze the effects of nonstationary flow injection as a mechanism to generate indirect active control over the external cavity flow throught the modification of the inner cavity flow pattern, under turbulent incident flow. This study consist on HWA velocity measurements of the cavity shear layer at freestream velocities of 2m/s and 5m/s. The experiments were carried out in the LaCLyFA’s 1m by 1.4m test section turbulent boundary layer wind tunnel, over a wooden cavity of aspect ratio of 1, which has a modificable spanwise arrange of 18 air injection tubes located on both sidewalls. The specific designed flow injection system allows blowing air inside the cavity at different frequencies (from 10Hz to 250Hz) and flow rates. The unsteady characteristics during the blowing on/off transient were analysed in the present work: 1) the wavelet transform was applied to the temporal signal to identify dynamic structures and 2) wavelet crosscorrelation is used to extract the most essential scales governing the correlation features.