Recent Performance-Based Advances in SparkJet Actuator Design for Supersonic Flow Applications

The paper gives a brief overview about the challenges and the concepts for future aeroengine applications based on microjets. Technical requirement and difficulties of the potential applications of Active Flow Control (AFC) in turbomachines are discussed. The importance of an active stabilized turbocompressor for an efficiency increase is described. Future concepts of advanced turbocompressors with the application of active and passive flow control, partially based on MEMS devices are briefly sketched. Finally, the numerical methods necessary to unveil the physical background of AFC are presented.

We assess the suitability of Reynolds-Averaged Navier–Stokes (RANS) simulation using the Spalart-Allmaras (SA) turbulence model as a closure in analysing the performance of fluidic Active Flow Control (AFC) applications. In particular, we focus on the optimal set of actuation parameters found by Tousi et al. (Appl Math Model, 2021) and Tousi et al. (Aerospace Sci Technol 127:107679, 2022) for a SD7003 airfoil at a Reynolds number Re=6×104 and post-stall angle of attack α=14∘ fitted with a Synthetic Jet Actuator (SJA). The Large Eddy Simulation (LES) presented in that work is taken as the reference to identify the best choice of boundary conditions for the turbulence field ν~ at both domain inlet and jet orifice in two-dimensional SA-RANS computations. Although SA-RANS is far less accurate than LES, our findings show that it can still predict macroscopic aggregates such as lift and drag coefficients quite satisfactorily and at a much lower computational cost, provided that turbulence levels of the actuator jet are set to a realistic value. An adequate value of ν~ is instrumental in capturing the correct flow behaviour of the reattached boundary layers for close-to-optimal actuated cases. This validates the use of RANS-SA as a reliable and cost-effective simulation method for the preliminary optimisation of SJA parameters in AFC applications, provided that thorough sensitivity analysis on turbulence-related boundary conditions is performed. Given the strong sensitivity of flow detachment on the laminar or turbulent nature of boundary layers, our results suggest that such analyses are particularly indispensable for vastly separated flow scenarios in general, notably for bluff bodies at moderate transcritical Reynolds numbers.

Practical application of active flow control is dependent upon the development of robust actuators that are reliable, low cost, and responsive, and that consume little power. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) is investigating a promising device for high-speed flow control called the SparkJet actuator. This actuator, which produces a synthetic jet with high exhaust velocities, holds the promise of manipulating highspeed flows without active mechanical components. Combined computational and experimental techniques are being applied to investigate the operating characteristics of a cavity SparkJet device for control of the flow on and near external surfaces. This paper focuses on the assessment of the actuator using Computational Fluid Dynamics. Time-dependent computations of firstand second-generation SparkJet designs exhausting into quiescent air are used to demonstrate the operating cycle. A numerical parametric study of the second-generation design characterizes the performance attributes (single-pulse) of the device as a function of orifice size, chamber volume, and energy deposited. __________________ ∗ Engineer, Senior Professional Staff, AIAA Associate Fellow † Engineer, Senior Professional Staff, AIAA Member ‡ Assistant Supervisor, Aeronautical Science and Technology Group, AIAA Member § Supervisor, Aeronautical Science and Technology Group, AIAA Associate Fellow Nomenclature Aex = Orifice cross-sectional area Cv = Specific heat capacity at constant volume Dex = Orifice diameter E = Internal energy e = Energy per unit mass M = Mass P = Gas pressure Q = Energy deposited q = Heat added per unit mass R = Universal gas constant S = Control surface t = Time T = Gas temperature Td = Plasma discharge temperature Tw = Wall temperature U = Gas velocity Uex = Gas velocity at orifice exit V = Control volume Vch = Actuator chamber volume Vy = Vertical component of gas velocity γ = Ratio of specific heats ρ = Gas density

The dimension error of an investment casting always begins at the injection of the wax pattern, and the knowledge on this stage will help to decide the proper pattern shrinkage allowances, and finally understand the dimensional variations of the casting. This study investigated the influence of the process parameters on the cavity pressure and dimensional variations of the wax patterns with different thicknesses by both simulative and experimental methods. A commercial simulation code, three-dimensional (3D) laser scanner, and the pressure sensor were used. Simulated and measured cavity pressure and diameters were compared and found to be matched very well. It was found that the holding time and packing pressure affect the thickness stability of the wax pattern greatly. However, changes in injection temperature did not affect the magnitude of cavity pressure and contraction greatly. The cavity pressure is more sensitive to packing pressure. There was no notable effect on the shrinkage of the wax pattern when holding time greater than the gates sealing time. The shrinkage of the thickness increases with the increase of the thickness. The work presented here is significant to model the deformation of wax pattern and understand the effect of the process parameters and geometry variables on the wax pattern shrinkage allowances.

Recent technological advances have resulted in high success rates for implantation of nonthoracotomy defibrillation lead systems. Further decreases in defibrillator size, facilitating pectoral placement, will depend in part on lowering defibrillation energy requirements. The purpose of this study was to determine if endocardial defibrillation energy requirements are influenced by electrode size. Thirteen adult mongrel dogs were studied under general anesthesia. A 9 Fr integrated bipolar pace/sense/defibrillation electrode (cathode) was positioned transvenously at the RV apex. The second defibrillation electrode (anode) was positioned at the junction of the RA and SVC. Two diameters of the proximal electrode, 7 Fr and 11 Fr, were sequentially tested in random order in each animal. The DFT for each electrode was determined using a 50-V up-down method. Energy, leading edge voltage, and current, current distribution, and total resistance were measured. The mean defibrillation voltage threshold with the 11 Fr proximal electrode was significantly less than with the 7 Fr proximal electrode (551.1 +/- 76.5 V vs 588.5 +/- 54.6 V, P < 0.01). Similarly, the mean DFT with the 11 Fr electrode was less than with the 7 Fr electrode (20.7 +/- 5.7 J vs 23.3 +/- 4.4 J, P < 0.01). Lower DFTs were found using the larger electrode in 11 of the 13 animals studied. However, there was no difference in defibrillation lead impedance between the two electrode systems. Endocardial defibrillation energy requirements may be lowered with a larger diameter proximal electrode. The mechanism by which this occurs may be due to a more even distribution of current gradients with the larger electrode. Determination of the optimal electrode size requires evaluation in humans, as this may allow further reduction in defibrillation energy requirements and defibrillator size.

Flow analysis around a VTOL aircraft near stall conditions and application of Active Flow Control...

An experimental investigation was performed on a three-dimensional, non-conformal turret with the goal of gaining a more detailed understanding of the spatial and temporal nature of the turret wake. The turret flow field was investigated using particle image velocimetry and dynamic surface pressure measurements, both with and without active flow control. Active flow control for the current work was applied at the leading edge of the turret aperture using steady suction and was matched to a configuration used for control of aero-optic distortions. The results of these experiments give insight into the complex, three-dimensional wake that develops as a result of the turret geometry as well as the significant changes that can be achieved with the application of active flow control, even when applied only to the aperture region of the cap geometry.

The reduction of both the drag and the download that act on a generic rotorcraft fuselage by the application of active flow control was investigated in a wind tunnel test conducted on a 1/3-scale powered rotorcraft model. The fuselage, the ROBIN-mod7, was equipped with a series of eight slots located on the ramp section and arranged in a U-shaped pattern. This U-shaped pattern was located slightly downstream of the baseline separation line and parallel to it. The flow control excitation took the form of zero-net-mass-flow blowing, also called synthetic jets, and steady blowing. The same set of slots were used for both types of excitation. The effectiveness of the control over a wide range of model parameters (angle of attack, advance ratio, and thrust coefficient) was documented. Both types of control were able to successfully produce a significant drag and download reduction. For synthetic jet control acting at a low advance ratio, the maximum drag and download reduction was 22% and 43%, respectively. For the case of steady blowing applied at high advance ratio, the maximum drag and download reduction was 45% and 58%, respectively.

The effect of active flow control on local flow separation behind the installation location of an ultra-high-bypass-ratio nacelle is investigated in a real-scale experiment at the TsAGI T-101 wind tunnel. The investigated model represents a swept 2.5D section of the pylon-wing junction in landing configuration. A flow control system employing periodic excitation is integrated into the unprotected leading edge inboard of the pylon. Tuft visualization as well as pressure and force measurements are used to investigate the vortex-dominated base flow and the effect of active flow control. It is shown that the flow control effect is governed by the normalized parameters of flow control, the momentum coefficient, and the velocity ratio, and is largely independent of the free-stream Mach and Reynolds number within the investigated range. The influence of a variation of the momentum coefficient on the lift gain is investigated. At the highest Reynolds number of an application of active flow control with fully eliminates local separation and increases total lift by approximately 2% of the respective baseline value across a range of 5 deg in angle of attack.

We consider the application of active flow control to modify the radial pressure distribution of a single-phase wall-normal vortex. The present flow is based on the Burgers vortex model but with a no-slip boundary condition prescribed along its symmetry plane. The wall-normal vortex serves as a model for vortices that emerge upstream of turbomachinery, such as pumps. This study characterizes the baseline vortex unsteadiness through numerical simulation and dynamic mode decomposition. The insights gained from the baseline flow are used to develop an active flow control technique with rotating zero-net-mass blowing and suction with the objective of modifying the core-pressure distribution. The effectiveness of the control strategy is demonstrated by achieving a widened vortex core with increased pressure. This change in the flow field weakens the local strength of the wall-normal vortex core, potentially inhibiting the formation of hollow-core vortices, commonly encountered in liquids.

Application of Active Flow Control for Download Alleviation in Rotorcraft

The experimental investigations presented in this paper deal with an active flow control approach at the rear end of two different three-dimensional generic car configurations. Periodic forcing is used to influence the recirculation area behind these models based on the Ahmed body geometry with slant angles of 25°. and 90°. and hence to achieve a reduction in the total drag. Several forcing parameters (jet angle, forcing frequency and intensity) and their influence on the static pressure distribution at the rear end of the models and the total drag are examined. A model predictive closed-loop actuation approach is presented for one steady blowing excitation configuration. The interaction between the periodic compressed air actuator jet and the shear layer is depicted by means of stereo time-resolved Particle Image Velocimetry. The different actuation concepts lead to a drag reduction of up to 5.7% and demonstrate a good correlation with the static pressure distribution at the rear end of the model.KeywordsParticle Image VelocimetryDrag ReductionModel Predictive ControlStrouhal NumberBluff BodyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The presence of anode spot before current zero limits the performance of practical vacuum interrupters. Using an AMF(axial magnetic field) is one of the methods to prevent anode spot formation and to keep arc plasma in a diffuse mode. A lot of papers have been published concerning the magnitude of an AMF and electrode shapes for diffuse arcs. Big electrode diameter and large area of high AMFs ensure better breaking performance. Therefore, the area where AMF per current is over 4mT/kA, which is called the effective area, has been calculated and used in designing electrodes. However, it is difficult to calculate the effective area and the exact diameter as the area does not change proportionally according to electrode diameter. So a new concept is needed in order to design electrodes irrespective of diameter. In this paper, using a high speed camera, the behavior of vacuum arcs were analyzed depending on arc currents and electrode diameter and compared with the results of magnetic field analysis. A new design criterion for diffuse arcs was established by using concepts such as the effective area and the effective current density when arc currents were assumed to flow only through the effective area.

The performance of copper electrowining process is directly linked to the electrode's hanger bar designs and plate assembly methods. The actual physical condition of the plate — hanger bar assembly and the internal resistance of electrodes are critical factors for electrowining practice. Electrode internal resistances affects parallel anode-cathode pair resistances, and correspondingly, balanced copper deposits, current efficiency, cell voltage and power consumption of electrowining cells. This paper summarizes the design characteristics of commercial stainless steel cathodes and cold-rolled lead-alloy anodes. It includes the evaluation of electrode's performance under typical operating conditions using 3D finite element analysis. The proposed evaluation of electrode's internal resistance comprises non-linear effects of electrical conduction, heat generation and electrodeposition. The analysis of the design robustness of electrodes considers the most important elements that help to maintain high current efficiency and electrical performance in the long term: mechanical strength, inherent straightness, resistance to dissolution of copper coating on hanger bars, galvanic corrosion of weld joints, and hanger bar resistance to annealing. Results use industrial measurements, test work by vendors and tankhouse operating data for benchmarking. The performance evaluation considers internal electrical resistance, contact shape design, contact loading, average contact resistance and lifetime current efficiency. The results highlight the relationship of electrode's design robustness and long term performance.

Enhancing vertical axis wind turbine by dynamic stall control using synthetic jets