The present research focuses on the experimental investigation of the effectiveness of synthetic jet actuation on a pair of counter-rotating vortices generated by an unswept, low aspect ratio, squared-tipped wing in order to preserve the mutual induction during their evolution. The synthetic jet is actuated at Crow and Widnall's instability frequencies (F+=0.071,0.55) and at a fixed momentum coefficient Cμ=0.2% with the goal of reducing the vortex strength and the induced circumferential velocity. The effect of the jet exit section area, and thus the characteristic jet velocity, has been investigated by testing three wing models equipped with a synthetic jet issuing through a rectangular slot of constant length and three different values of the height, equal to 0.01, 0.02, and 0.04 chord lengths, respectively. A phase-locked stereoscopic particle image velocimetry setup has been designed and implemented to carry out a parametric study in the near wake of the wing models at four downstream distances from the wing trailing edge, namely equal to 0.1, 0.5, 1 and 2 chord lengths.The slot with a height to chord ratio of 4%, yields the minimum vorticity level, the maximum vortex diffusion with a diameter up to 3 times greater than the baseline reference value, and lower values of the vortex circulation. This effect, together with the periodic motion along a ±45∘ direction experienced by the vortex after the synthetic jet blowing, is beneficial in terms of an anticipated instability of the tip vortices as well as of the mitigation of the blade-vortex-interaction in propellers.

Wingtip vortices have been proven to be detrimental to both aircraft efficiency and safety due to their adverse effects such as wake hazard, blade vortex interaction noise and induced drag. Despite the extensive literature on the subject, the number of experimental works featuring far field velocity measurements under active control is very limited. The present work deals with the experimental investigation of the effectiveness of synthetic jet actuation on the control of wingtip vortices and their wake hazard. In order to preserve the mutual induction of the counter-rotating vortices during their evolution, an unswept, low aspect ratio, squared-tipped, finite-span wing is employed. The synthetic jet actuation is based on triggering the inherent instabilities of Crow and Widnall at different values of the momentum coefficient Cμ with the goal of reducing the vortex strength and obtaining an anticipated vortex break up. Different exit geometries of the synthetic jets have been tested to analyze the effects of the jet velocity and position on the wingtip vortices. Phase-locked measurements of the velocity field in the far wake at a distance from the wing trailing edge from 26 to 80 chord lengths have been performed via stereoscopic particle image velocimetry. The effects of blowing at high momentum coefficient Cμ=1% are demonstrated to be remarkable on the mitigation of the wingtip vortices. On the other hand, both the time and phase-averaged results suggest that, at relatively low values of Cμ (0.2%), using a larger synthetic jet exit section area allows to greatly affect the wingtip vortices' features causing a striking alleviation of the vorticity distribution up to 90% with respect to the baseline reference case. In fact, due to the actuation at low frequency, the vortex instability is prematurely risen up and amplified, leading to early vortices linking and their consequent dissipation.

Nowadays, several efforts are being made to design more efficient, cleaner, and economically accessible engines. Spray-wall interactions are strongly related with the fuel–air mixture and emission formation. As such, they are considered as the most important physical processes in engine research. In the present study, the infrared thermography coupled with an inverse heat transfer data reduction is applied to evaluate the wall heat transfer of an iso-octane spray generated by a multi-hole gasoline direct injector (Spray G) impinging on a heated thin foil. The experimental apparatus includes an Invar foil (50μm in thickness) heated by Joule effect and the injector located at 66.66 injector nozzle diameter above the surface. Thermal images of the impinging spray are acquired from the dry side of the foil at several time delays from the start of injection at two different injection pressures (10and 20MPa) and two different wall temperatures (373and 473K). The experimental data are reduced in the dimensionless form in terms of the spray cooling efficiency ξ, which represents the ratio between the spray cooling heat flux and the heat transfer capability of the fluid, by taking into account the area of impact of the spray. Results show a substantial increment of the heat flux and the spray cooling efficiency by increasing the wall temperature. Also, the increment of the injection pressure has an increasing effect on the area of impact, the heat flux, and the efficiency of the spray for both wall temperatures investigated in the experimental campaign. The spray cone angle and the plume jet axis angle were also estimated from the wall heat flux distribution.

A novel technique for obtaining quantitative infrared-thermography data on metallic (e.g., aluminum or steel) wind-tunnel models is developed. The methodology incorporates a matte-black, thin, wrap-film (3M (TM) Wrap Film Series 1080), which is both commercially available and relatively inexpensive. Compared to painting the model to increase the surface emissivity, using a self-adhesive thin film makes it easier to obtain uniform thickness and roughness and is easier to apply and remove. The relevant surface properties (i.e., thickness, roughness, and directional emissivity) and thermal properties (i.e., thermal conductivity, specific heat capacity, and thermal diffusivity) of the wrap film have been measured and are reported. The high emissivity (0.95) and low thermal conductivity (0.23 W/(m K)) make it an excellent candidate for quantitative infraredthermography. Using the experimentally measured properties, and the reported data-reduction techniques, stagnation-point heat flux was successfully measured on a blunt 7 degrees half-angle circular cone in nominally Mach-6 flow. Fay & Riddell's theory for calculating heat flux at the stagnation point provided a comparison to verify the effectiveness of the wrap-film technique for quantitative infrared thermography. The heat flux evaluated with the wrap-film technique resulted in excellent agreement with the theoretical solution with a mean error of 4.4%.

The effectiveness of synthetic jet actuation on the wingtip vortices produced by an unswept, low aspect ratio, rectangular wing at a chord Reynolds number of 8.16 x 104 is experimentally investigated. The synthetic jet is operated at different actuation frequencies and amplitudes in order to investigate five different control cases characterized by different momentum coefficients and dimensionless frequencies, for a comprehensive parametric study. In particular, the instability frequencies (known as Crow and Widnall) characterizing the wingtip vortex dissipation, are chosen among the tested synthetic jet actuation frequencies. A phase-locked stereoscopic particle image velocimetry is employed to analyze the development of the wingtip vortices in the near wake at a distance from the wing trailing edge of 3 chord lengths. The time-averaged results suggest that the synthetic jet actuation greatly affects the wingtip vorticity distribution causing an outward diffusion which increases with the actuation frequency, with a maximum reduction of 46% on the peak value. On the other hand, when the synthetic jet is operated at the Crow instability frequency, the wingtip induced velocity shows its maximum decrease equal to 29% with respect to the baseline case. The phase-averaged analysis reveals a clear relation between the synthetic jet blowing and suction phases and the periodic variation of the vortices features. In correspondence to the blowing phase, the wingtip vortices undergo a huge diffusion and they are characterized by a larger diameter, higher circulation, and lower induced velocity. These findings confirm that this synthetic jet control configuration can represent a useful device to promote the vortex dissipation and reduce the wake vortex hazard.

Heat transfer measurements in hypersonic wind tunnels are particularly challenging when dealing with fully three-dimensional geometries and/or flows with high spatial temperature gradients. In the present work, a full and consistent description of a general procedure to perform infrared thermography heat transfer measurements is proposed. Radiometric and optical camera calibration, spectral directional emissivity characterization, model oscillations correction and 3D surface temperature reconstruction problems are theoretically and practically analyzed along with new solutions. A novel hybrid heat transfer problem is proposed. This technique is based on the solution of the direct problem during the wind tunnel unsteady start-up to accurately estimate the initial condition for the inverse problem solution during the quasi-steady test phase. The data reduction procedure was validated through an experimental campaign conducted in the Boeing/AFOSR Mach-6 Quiet Tunnel on a sharp cone at 0 deg angle of attack. The experimental Stanton number obtained by solving a one-dimensional heat transfer problem is in agreement with the theoretical laminar solution for a range of free-stream conditions with an average error between -1.1 divided by 0.64% and a standard deviation of 1.55%. Finally, a two-dimensional heat transfer problem has been solved when the cone is tested at 6 deg angle of attack demonstrating the IR procedure capability to analyze multi-dimensional flows and to take into account tangential conduction effects caused by surface temperature gradients due to the presence of crossflow vortices. (C) 2020 Elsevier Ltd. All rights reserved.

An experimental investigation on the effectiveness of synthetic jet actuation in affecting and accelerating wingtip vortices dissipation of an unswept, low aspect ratio, rectangular wing at a Reynolds number Rec = 8.04× 104 has been carried out and reported herein. Active control parameters, Cμ and F+ have been chosen in order to stimulate vortex instability at its own frequency. The actuation is implemented through synhtetic jets generated passing a sinusoidal signal whose frequency has been set to Crow instability frequency to the loudspeaker. Measurements at z/b = 5, i.e. z/c = 10 considering thatA = 2 for the wing model, where the wake has assumed an oscillatory behaviour, have been carried out to verify the control effectiveness. StereoPIV measurements showed that synthetic jets actuation affects vortex evolution, causing a decrease of the axial velocity deficit and of the tangential velocities and of the region characterised by high values of streamwise vorticity.

The regulations about pollutant emissions imposed by Community’s laws encourage the investigation on the combustion optimization in modern engines and in particular in those adopting the gasoline direct injection (GDI) or direct injection spark-ignited (DISI) configuration. It is known that the piston head and cylinder surface temperatures, coupled with the fuel injection pressure, strongly influence the interaction between droplets of injected fluid and the impinged wall. In the present study, the Infrared (IR) thermography is applied to investigate the thermal footprint of an iso-octane spray generated by a multi-hole GDI injector impinging on a heated thin foil. The experimental apparatus includes an Invar foil (50 μm in thickness) heated by Joule effect, clamped within a rigid frame, and the GDI injector located 11 mm above the surface. Thermal images of the impinging spray are acquired from the dry side of the foil at several time delays after the impact instant at two different injection pressures (10 and 20 MPa). The temperature difference distributions are employed to describe the unsteady dynamics of the impingement.

Abstract The research field of combustion optimization in modern engines adopting the gasoline direct injection (GDI) has been recently improved due to the strict regulations about pollutant emissions imposed by states’ laws. The droplet-wall interaction is significant influenced by the fuel injection pressure and the time-dependent temperature on the piston head whose quantitative knowledge is sparse. In the present study, the thermal analysis of an iso-octane spray footprint generated by a multi-hole GDI injector impinging on a thin Invar foil heated by Joule effect is performed by means of Infrared (IR) thermography. Starting from the fuel impact instant, several thermal images are acquired at different time delays employing a phase-averaged approach. In particular, a study on different injection pressures (10 and 20 MPa) and wall temperatures (366 and 473 K) at fixed nozzle-to-wall distance of 11 mm is presented. The unsteady thermal dynamics at the impingement is described by the two-dimensional temperature difference maps and the phase-averaged time dynamics of the temperature drop in the impact points.