1,733 research outputs found
Bluff Bodies Flow Control using Innovative Piezoelectric Actuators
No full textAn active flow control technique based on “smart-tabs” is proposed to delay flow separation on a circular cylinder and on a simplified bi-dimensional automotive geometry body. The actuators are retractable and orientable multilayer piezoelectric tabs which protrude perpendicularly from the model surface. They are mounted along the spanwise direction with constant spacing. The effectiveness of the control was tested in pre-critical and in post-critical regime by evaluating the effects of several control parameters of the tabs like frequency, amplitude, height, angular position and plate incidence with respect to the local flow. Measurements of the mean static pressure distribution around the cylinder were used to estimate the pressure drag and lift coefficient. To take into account friction and the parasite drag of the actuators wake analysis was performed. The maximum drag reduction achieved in the pre-critical regime was of the order of 30%, whereas in the post-critical regime was about 15%, both for the highest forcing available. In pre-critical condition the active forcing plays an important role on drag and pressure fluctuations reduction. Spectral analysis of the signals from instantaneous pressure transducers (electret microphones) indicated an almost complete suppression of the vortex shedding in active forcing conditions. In post-critical regime, instead, most of the effects are due to passive forcing limiting the active contribution to a maximum of 3%. The results related to the automotive geometry confirm the ones obtained on the circular cylinder, with a maximum attained drag reduction of 13.4% with only small contributions due to active forcing. To evaluate the potentialities of the smart-tabs in real applications power absorption measurements are reported together with energy budget considerations. Finally, the identification of the key non-dimensional control parameters, was performed
Bluff Body Flow Control Through Piezoelectric Actuators
No full textDelay separation from bluff bodies leads to drag reduction. An active flow control technique based on piezoelectric actuators has been arranged on a 2D cilynder in subsonic flow. Experimental investigation in precritical and postcritical conditions of the flow has been conducted varying the configurations of the actuators. Drag reduction up to 10% without a complete optimization of the flow control parameters have been obtaine
Control of the flow in a trapped vortex cell
No full textA trapped vortex cell, (TVC), is a flow control device consisting in a cavity embedded on the upper surface of an airfoil, designed to trap a steady vortex. By the interaction with the vortical cell's ow, the incoming boundary layer becomes more energetic, and it is thus less prone to separation. Wind tunnel tests by De Gregorio et al. [2] and by Lasagna et al. [1] have shown that to make this control effective the cavity flow itself must be the target of a control action. In particular, the latter authors have shown that the airfoil drag can be substantially reduced with respect to the clean airfoil by applying suction into the cavity region and that this technique is superior to a classical boundary layer suction system, for the same suction flow rate. One goal of this work is to further investigate on the drag reduction mechanism of the TVC with suction technique. Furthermore, a second control technique, based on a synthetic jet device (SJ) which injects momentum into the cavity in order to energize the vortical cell's flow, is considered. Interest in this technique is raised since it is far more effcient than steady suctio
Virtual Shaping on NACA 0015 by Means of a High Momentum Coefficient Synthetic Jet
No full textResults concerning flow control on a NACA 0015 airfoil using high power synthetic jets are presented for low incidences and for Reynolds numbers ranging from 132000 to 425000. The forcing was operated through a spanwise slit positioned near the leading edge at x/c=1.25% or at x/c=10% on the upper surface. Static pressure distribution measurements around the airfoil, wake surveys and smoke flow visualizations were performed. Pressure distributions were significantly modified around the injection location, showing an area of intense suction which increased the lift and strongly affected the drag. Flow visualizations highlighted that the intense suction was due to a virtual shaping effect caused by the formation of a recirculation bubble capable of displacing the streamlines. Low momentum deficits in the wake velocity distributions and, in certain conditions, jet-like flow was observed for the forced cases. Finally, a scaling law relating the bubble size to the forcing intensity is propose
Wall-pressure based Multi-Time-Delay Linear StochasticEstimation
No full textLinear Stochastic Estimation (LSE) is a well known, widely used data analysis technique, offering quantitative insight into the dynamics of coherent structures12. In the wall-pressure based classical LSE approach the velocity estimate u(t) is obtained from the wall-pressure p(t) as u(t)_LSE = bo*p(t), i.e. by using a single sam- ple of the wall-pressure time history. By contrast, in the Multi-Time-Delay (MTD) technique more than one past samples is used simultaneously for the estimat
Definition of tests on Riblets performence through a rotating cylinder
No full textThe document describes an experimental facility and provides relevant tests requirements to evaluate the performance of riblets surfaces in terms of skin friction drag reduction. The concerned activity has been planned, in the frame of WP 2.2.6, to validate this very promising passive drag reduction technology, considering both conventional and innovative riblets configurations to be teste
Circular cylinder drag reduction using piezoelectric actuators
No full textAn active flow control technique based on "smart-tabs" is proposed to delay flow separation on a circular cylinder. The actuators are retractable and orientable multilayer piezoelectric tabs which protrude perpendicularly from the model surface. They are mounted along the spanwise direction with constant spacing. The effectiveness of the control was tested in precritical and in post-critical regime by evaluating the effects of several control parameters of the tabs like frequency, amplitude, height, angular position and plate incidence with respect to the local flow. Measurements of the mean static pressure distribution around the cylinder were used to estimate the pressure drag coefficient. The maximum drag reduction achieved in the pre-critical regime was of the order of 30%, whereas in the post-critical regime was about 10%, 3% of which due to active forcing. Furthermore, pressure fluctuation measurements were performed and spectral analysis indicated an almost complete suppression of the vortex shedding in active forcing condition
Streamwise vortices originating from synthetic jet-turbulent boundary layer interaction
No full textThe interaction between a flat plate turbulent boundary layer and a synthetic jet issuing from a rectangular slot slanted with respect to the free stream was studied experimentally using Digital Particle Image Velocimetry (DPIV). Instantaneous flow fields were sampled in a cross-plane downstream of the slot. Results concerning the effects of varying the synthetic jet velocity ratio at fixed stroke length L0 and yaw angle, and the effects of varying the orifice yaw angle at a fixed frequency are presented. The formation of a pair of counter-rotating vortical structures, completely embedded in the boundary layer, was observed in the mean flow field when the slot was aligned with the cross-flow. As the slot yaw angle was increased the leeward vortex intensified while the other became weaker. These vortical structures are the traces of streamwise vortices forming upstream, at the slot exit, during the blowing phases. As the jet velocity ratio and the slot yaw angle were increased the vortices grew in size and intensity. The vortex identification technique showed that these vortical structures are intermittently present in the instantaneous flow fields with a percentage growing with the frequency but not influenced by the yaw angle. Conditional averages showed that while the rotational core of the identified vortices is nearly unaffected, their outer region is greatly modified and grows in size and intensity as the jet velocity ratio and the yaw angle increase
Multi-time delay, multi-point Linear Stochastic Estimation of a cavity shear layer velocity from wall-pressure measurements
No full textMulti-time-delay Linear Stochastic Estimation (MTD-LSE) technique is thoroughly described, focusing on its fundamental properties and potentialities. In the multi-time-delay ap- proach, the estimate of the temporal evolution of the velocity at a given location in the flow field is obtained from multiple past samples of the unconditional sources. The technique is applied to estimate the velocity in a cavity shear layer flow, based on wall-pressure measurements from multiple sensor
- …
