A perturbative model for predicting the high-Reynolds-number behaviour of the streamwise travelling waves technique in turbulent drag reduction

The background of this work is the problem of reducing the aerodynamic turbulent friction drag, which is an important source of energy waste in innumerable technological fields. We develop a theoretical framework aimed at predicting the behaviour of existing drag reduction techniques when used at the large values of Re which are typical of applications. We focus on one recently proposed and very promising technique, which consists in creating at the wall streamwise-travelling waves of spanwise velocity. A perturbation analysis of the Navier-Stokes equations that govern the fluid motion is carried out, for the simplest wall-bounded flow geometry, i.e. the plane channel flow. The streamwise base flow is perturbed by the spanwise time-varying base flow induced by the travelling waves. An asymptotic expansion is then carried out with respect to the velocity amplitude of the travelling wave. The analysis, although based on several assumptions, leads to predictions of drag reduction that agree well with the measurements available in literature and mostly computed through DNS of the full Navier-Stokes equations. New DNS data are produced on purpose in this work to validate our method further. The method is then applied to predict the drag-reducing performance of the streamwise-travelling waves at increasing Re, where comparison data are not available. The current belief, based on a Re-range of about one decade only above the transitional value, that drag reduction obtained at low Re is deemed to decrease as Re is increased is fully confirmed by our results. From a quantitative standpoint, however, our outlook based on several decades of increase in Re is much less pessimistic than other existing estimates, and motivates further, more accurate studies on the present subject

Ionic Wind Measurements on Multi-Tip Plasma Actuators

This work presents an experimental investigation about the effects of triangular tips on the active electrodes of plasma actuators. The tests are performed on two sets of actuators, corona and DBD, parameterized by means of the tip sharpness and the tips number per unit length. A total number of 30 actuators is considered. The devices are evaluated on the basis of the far field ionic wind velocity, that has been chosen as a representative test common to both kinds of actuator. The dataset includes velocity profiles and maps, that can be integrated to give mass ows and electromechanical effciencies. Some results are also presented in the parameter space defined by tip sharpness and tips number per unit length: this gives the chance of defining optimal electrode shapes within each set. In general, the longitudinal velocity of the gas increases downstream of the tips in all the actuators tested, but the velocity field is modified to different extents in the two kinds of actuators, and is more complicated for the DBDs than for the coronas. The tips also increase the effciency of all the actuators, particularly for the corona set, where even the stability is remarkably improved

Determination of density and concentration from fluorescent images of a gas flow

A fluorescent image analysis procedure to determine the distribution of species concentration and density in a gas flow is proposed. The fluorescent emission is due to the excitation of atoms/molecules of a gas that is intercepted by an electron blade. The intensity of the fluorescent light is proportional to the local number density of the gas. When the gas flow is a mixture of different species, this proportionality can be exploited to extract the contribution associated to the species from the spectral superposition acquired by a digital camera. This yields a method that simultaneously reveals species concentrations and mass density of the mixture. The procedure is applied to two under-expanded sonic jets discharged into a different gas ambient - Helium into Argon and Argon into Helium - to measure the concentration and density distribution along the jet axis and across it. A comparison with experimental and numerical results obtained by other authors when observing under-expanded jets at different Mach numbers is made with the density distribution along the axis of the jet. This density distribution appears to be self-similar.Comment: New figures in portable .eps forma

Hypersonic Jets in Astrophysical Conditions: Focus on Spreading and Asymmetric Stability Properties

High Mach number jets emanating from young stars show remarkable collimation, low opening angle and resilience against the growth of instabilities, especially the asymmetric ones. In recent laboratory experiments instances of asymmetric three-dimensional low amplitude long waves aligned with the jet axis were observed by Belan et al (2013 Astron. Astrophys. 554 A99). To explore the collimation, spreading, and asymmetric stability properties of hypersonic jets we carried out laboratory experiments and numerical simulations in two and three spatial dimensions. We find that laboratory hydrodynamic jets with high Mach numbers remain collimated, for hundreds of jet radii in length and maintain low opening angles. These findings are confirmed by 3D numerical simulations carried out after time-dependent, asymmetric perturbations are applied at the jet inlet. Both experimental and perturbed simulated jets show non-axial modes with long wavelengths, whose growth does not disrupt the jet in the domain considered

Plasma-Gas Flow Interaction of a Discharge Normal to a Bluff Body Wake

This paper describes the interaction of a gas discharge with an air flow consisting of the wake of a flat plate normal to the airstream. The plate acts both as a bluff body and discharge electrode; the free discharge in the absence of the airstream and free airstream without discharge are orthogonal to each other. The electric forces involved in the free discharge and inertial forces involved in the free airstream are of the same order. The experiment has been carried out in a suitable duct, and the facilities include devices for discharge current measurements, hot wire anemometer, and visualization system. The analysis of the time histories obtained from the hot wire signals and cathodic current signals reveals the existence of an airstream-discharge coupling phenomenon, appearing as synchronized oscillations of the flow and discharge current

Moto non lineare nelle simulazioni gravitazionali a N corpi

Tesi di Laure

Compared ionic wind measurements on multi-tip corona and DBD plasma actuators

Two sets of actuators with triangular tips on their active electrodes (13 corona and 15 DBD) are studied in the laboratory. Far field ionic wind velocity, mass flow and efficiency are measured for all the actuators. The best electrode shape as function of tip sharpness and tips number/unit length is determined for each of the above measurements. The gas velocity increases downstream of the tips in all the actuators, but the DBD flow has a three-dimensional structure more complicated than for coronas. The tips improve the efficiency of all the actuators, and the stability of coronas is remarkably improved

A Comparison Between Corona and DBD Plasma Actuators for Separation Control on an Airfoil

In this work, an experimental study on the effectiveness of the multi-tip geometry for plasma actuators is presented for a separation control problem. The multi-tip geometry consists in a periodical series of triangular tips on the active electrode edge. Both DC corona and AC Dielectric Barrier Discharge (DBD) actuators are considered and their performances are compared. The plasma actuators are installed on the leading edge of a NACA 0015 airfoil and tested at high angles of attack in the wind tunnel for freestream velocities in the range 5 – 20 m/s. The flow control mechanisms are briefly discussed. Force measurements are performed by means of an aerodynamic balance and the force coefficients are derived. Electrical measurements are also performed, to quantify the consumed power. The actuators effectiveness is evaluated through some suitable performance parameters. In general, a properly chosen multi-tip geometry is found to be more performing than traditional straight edge configurations, both for coronas and DBDs. Furthermore, corona actuators give comparable aerodynamic performance with respect to DBDs, but they require an order of magnitude lower power