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  • The resonance of thermal lags of solid fuel with heat transfer oscillations in the boundary layer flow is the primary mechanism of exciting low frequency instability (LFI) in hybrid rocket combustion. Recent studies reported that swirl injection seems to stabilize combustion by suppressing boundary layer oscillations. This study focused on the experimental investigation of how the boundary layer is perturbed and it leads to the LFI. Also the appearance of temporal stabilization of combustion between two consecutive LFIs was investigated in terms of boundary layer oscillation. Special attentions were also made to monitor the overall behavior of oscillating boundary layer and the occurrence of the LFI as swirl intensity increased. Fluctuating boundary layer was successfully captured by visualized images and POD (Proper Orthogonal Decomposition) analysis. In the results, oscillating boundary layer became stabilized as the swirl intensity increases. And the coupling strength between high frequency p\', q\' diminished and periodical amplification of RI (Rayleigh Index) with similar frequency band of thermal lag was also decayed. Results confirmed that oscillating axial boundary layer triggered by periodic coupling of high frequency p\', q\' is the primary mechanism to excite thermal resonance with thermal lag characteristics of solid fuel. Numerical simulation also confirmed that transitioning to a positive coupling of high frequency p\', q\' is the mechanism responsible for exciting boundary layer oscillation.
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    • Document
  • Multi-Robot systems for Space exploration are an interesting alternative versus traditional large monolithic rovers in terms of performance and reliability. However, for a community of cooperative autonomous robots to achieve complex exploration goals, a solid infrastructure of communications and location determination (as a supporting resource for navigation through the scenario) must exist in advance, which traditionally is built over radiofrequencybased technologies due to its robustness and suitability for extreme environmental conditions, such as in typical Space exploration scenarios. This contribution discusses on the impact of electronic oscillators phase noise in the error introduced in the relative location and velocity determination based on frequency measurement techniques, such as Frequency of Arrival (FoA) or Frequency Difference of Arrival (FDoA). A noise model is obtained for a hypothetic scenario consisting in a system for position and relative speed determination on a lunar surface exploration mission, where different options for local oscillators are proposed, considering practical aspects such as size, volume and power consumption against phase noise. The impact of the phase noise in global velocity and position error is determined, analysed and discussed for different commercial oscillators and configurations.
    Data Types:
    • Document
  • A small hybrid rocket engine has been developed, which has radial and swirl two hollow injectors, a large quarts glass window, a transparent combustion chamber and a divergent nozzle. Flame behaviors under atmospheric pressure combustion have been taken by high-speed photography with 30,000 FPS. The proper orthogonal decomposition (POD) is applied to the luminous intensity of acquired images. The higher POD modes are found to capture the structures of luminous flame streaks and the peak frequency of mode 2 coefficient is found to have some relation with the oscillation frequency of the boundary layer.
    Data Types:
    • Document
  • The results of experiments on transonic buffet control by means of \"plasma wedge\" actuators basing on spark discharge are discussed. Experiments have been carried out in the range of Mach number 0.730.78 using model of rectangular wing with supercritical airfoil, chord 200 mm and span 599 mm. Schlieren visualization and Pitot measurements in the wake of the wing show that actuators effectively influence on mean flow and characteristics of shock wave oscillations. It was also found that control efficiency depends on frequency of discharge.
    Data Types:
    • Document
  • The development of localized disturbances, generated by three-dimensional vibrating surface in the flat plate boundary layer at Re 1 600, is experimentally investigated. It is shown, that vibration of a three-dimensional surface with a large amplitude leads simultaneously to the formation of two types of perturbations in the boundary layer: longitudinal localized structures and two wave packets. Spatial development of oscillations at the central frequency of the wave packets is consistent with the linear theory of hydrodynamic stability.
    Data Types:
    • Document
  • The flow pattern in over-expanded conical nozzles is numerically investigated by means of threedimensional wall-modeled large-eddy simulations (WM-LES). The objective of the study is to identify the origin of the low-frequency oscillations (LFO) and the side-loads generation observed in separated supersonic flows. The computational results are compared with the available experimental data for validation. The simulations bring clear evidence of the existence of broadband and energetically-significant LFO in the vicinity of the separated shock. The magnitude of the shock excursions was less important in the conical case, compared to the planar nozzle, because of the strong adverse pressure gradient that develops upstream of the separation zone. The generated side-loads were analyzed and a dynamic mode decomposition (DMD) analysis reveals the existence of two types of modes: the non-helical modes which are low-frequency modes based mainly in the streamwise forces and the helical modes which are highfrequencies modes, compared to the first Type. The laters mainly concern the side-loads.
    Data Types:
    • Document
  • An experimental investigation has been conducted of the supercritical flows at transonic speeds over the laminar OALT25 airfoil in order to analyze the impact of laminar flow upon the shock wave dynamics and the existence of a laminar buffet like phenomenon. An ensemble of tests has been carried out at freestream Mach number varying in the range 0.7 to almost 0.8, angles of attack from 0.5 to 4 degrees and with different tripping configuration at the upper surface of the wing, while the boundary layer at the lower surface was forced turbulent at 7% of chord. The (airfoil) chord based Reynolds number is about 3 million. Results obtained from pressure taps and sensors measurements, as well as Schlieren visualizations of the flow, and Particle Image Velocimetry vector fields, reveal the presence of laminar buffet phenomenon in sharp contrast with its turbulent counterpart, as it features a (freestream and chord) based normalized frequency of about unity while turbulent buffet occurs for a frequency close to 0.07. While a low frequency mode, at frequency about 0.05 is also present in the laminar situation, the high frequency mode dominates the flow fluctuations. This mode exhibits strong oscillations of the shock foot and vertical deformations of the shock wave, while the low frequency peak moves the shock back and forth over a small portion of chord. The mean flow past the laminar wing is characterized by a laminar separation bubble under the shock foot, which likely contributes much to the novel dynamics revealed by the present experiments. In particular it is found that the flow instability corresponds to profound variations of the bubble extent and vortex shedding in the separated flow behind the shock wave. Two control strategies of the instability are implemented, one consisting of three-dimensional bumps and one consisting of steady jets blowing transversely to freestream. While bumps show little influence on the shock dynamics, jets successfully nullify flow unsteadiness in both laminar and turbulent conditions.
    Data Types:
    • Document
  • The flame stabilization represents a relevant issue in aero-engine design. In fact, the growing demand of pollutant emissions reduction without significant losses of the combustion efficiency has driven the efforts of the scientific community towards lean flames. Lean fuel mixtures, characterized by low temperature flames, could manifest an unstable behaviour which can easily lead to the flame extinction due to the establishment of the blowout condition. This requires the implementation of control systems to avoid flame instability occurrence. The present work shows an investigation of the impact of dielectric barrier discharge (DBD) plasma actuation on lifted flame stabilization in a methane CH4-air Bunsen burner at ambient conditions. Two different plasma actuator configurations, powered with a high voltage (HV)/high frequency sinusoidal signal, have been investigated. Once the best actuator configuration was selected, the efficiency of the plasma actuation has been evaluated in terms of the flame lift-off distance, length and shape. In particular, in order to change the actuator power dissipation, different peak-to-peak voltages Vpp were tested, while the actuation frequency was kept equal to 20 kHz. The application of plasma discharges to flame stabilization leads to plasma-attached flames or plasma-enhanced lifted flames, depending on the air and fuel flow rates. At air flow rate of 1.54 g/s, plasma actuation allowed to decrease the lift-off height until the fuel jet velocity was below about 0.05 m/s thanks to the extension of the flame region upstream, toward the burner exit section. Beyond this value, it had no significant impact on the flame lift-off height, even though the amplitude of the lift-off height oscillations reduced coupled with a more stable behaviour of the lifting flame. The benefit of the plasma actuation increased by reducing the air flow rate to 1.35 g/s. In this condition, plasma-assisted flame reattachment was evident at each fuel velocity, in combination with an increasing flame height proportionally to the fuel jet velocity.
    Data Types:
    • Document
  • Experimental methods for determining dynamic characteristics of landing gears are considered. The main dynamic characteristics of landing gears are resonant frequencies, damping coefficients and resonant oscillation shapes. * Features of ground vibration testing for landing gears * Methods for determining main normal modes * Processing algorithms * Application of impact method * Influence of hydraulic rams on landing gear dynamic characteristic
    Data Types:
    • Document
  • The origin and evolution of perturbations generated by low frequency localized local vibrations of the wall at flow separation behind a backward-facing step on a flat plate is investigated in a subsonic wind tunnel. The results are obtained using the hot-wire anemometry technique. It is found, that the wall vibrations produce disturbances of the separation region named streaky structures accompanied by wave packets of oscillations. Laminar boundary layer separation induces the growth of wave packet followed by near-wall flow turbulization.
    Data Types:
    • Document