Synthetic Jet Research Articles

Formation and scaling of a coherent large-scale vortex from the impingement of a laminar synthetic jet

Formation and scaling of a coherent large-scale vortex from the impingement of a laminar synthetic jet

Deflected Synthetic Jet due to Vortices Induced by a Tri-Electrode Plasma Actuator

A tri-electrode dielectric barrier discharge (DBD) plasma actuator, which consists of a traditional DBD plasma actuator and an additional direct current (dc) electrode, is proposed to achieve a deflected synthetic jet ranging from 0 deg to 180 deg by only changing the polarity and magnitude of the dc component. Depending on the polarity of dc voltage, the tri-electrode plasma actuator can be operating in three discharge modes: traditional DBD discharge, extended discharge, and sliding discharge. The characteristics of three discharge modes are revealed and analyzed. The evolution of vortices induced by the tri-electrode plasma actuator under burst-mode actuation is investigated by particle image velocimetry (PIV) measurement and schlieren visualization. Results show that in DBD discharge mode, the vortices induced by the tri-electrode plasma actuator is symmetrical and a vertical synthetic jet forms. However, a deflected synthetic jet forms due to the asymmetrical vortices induced by extended discharge and sliding discharge. The deflection mechanism is analyzed from the viewpoint of body force induced by the tri-electrode plasma actuator. The deflected angle of the synthetic jet is identified from PIV measurement and schlieren visualization and found to be nearly linear with , which probably will be a promising result for further flow control applications.

Interaction of synthetic jets with a massively separated three-dimensional flow field

The interaction of an array of finite span synthetic jet actuators with a massively separated flow was explored experimentally over a cantilevered, swept, and tapered model having a deflected control surface. The interaction of one synthetic jet with the local flow led to a flow field associated with formation and advection of vortex rings that bent into the flow, resulting in flow reattachment in their trajectory. Under specific conditions, when multiple jets were actuated, their interaction was either constructive or distractive, depending on the separation severity and spanwise flow magnitude. This work improves control of three-dimensional flow separation using distributed actuators.

Enhancing forced-convection heat transfer of a channel surface with synthetic jet impingements

A transient numerical simulation was carried out to investigate the convection heat transfer enhancement of a heated surface in an airflow channel using a synthetic jet. The study was performed with a channel-flow average velocity varying up to 3 m/s, which has the corresponding channel Reynolds number of 3,616. The peak-to-peak amplitude of a jet diaphragm was varied from 0.4 to 1.2 mm to investigate its effects on the heat transfer enhancement. The synthetic jet was operated at three different locations relative to the heated surface in the streamwise direction with an operating frequency increased up to 200 Hz. The heat transfer of the heated surface was characterized by evaluating a phase-averaged convection heat transfer coefficient over ten operational cycles of the synthetic jet after the flow and thermal fields reached a quasi-steady state. In addition, the helicity flow visualization was conducted to observe the dynamic and transport characteristics of vortex packets generated by the synthetic jet and their interaction with the heated surface. It was found that the vortex packets represent high-momentum zones with strong swirling motions in the streamwise direction, and their direct impingement or sweeping on the heated surface was highly responsible for the heat transfer enhancement.

Experimental investigation on flow characteristics and unsteady heat transfer of noncircular impinging synthetic jets

This study investigates the flow characteristics and unsteady heat transfer of noncircular synthetic jets impinging on a heated plate. Thermographic phosphor thermometry and time-resolved particle image velocimetry are used to acquire quantitative wall temperature and flow field information, respectively. At a constant orifice-to-wall distance (H/De = 7.5), five orifice configurations (circular, square, elliptic with aspect ratio (AR) of 3, and rectangular with ARs of 3 and 5) are examined at a wall temperature of 80 °C. The orifice configuration has a significant effect on the heat transfer and vortex behavior of the impinging synthetic jets. The square orifice displays better stagnation cooling performance with 42% increase in the maximum stagnation cooling coefficient relative to the circular case due to the larger impinging velocity and deeper penetration into the wall shear layer. Axis-switching of the vortex rings is detected for both the elliptic and rectangular orifices before they impinge onto the wall. The axis-switching enhances the near-wall mixing and turbulent kinetic energy in the elliptic and rectangular orifices, and thus heat transfer compared with the circular case. Additionally, stronger vortices and deeper penetration into the wall shear layer are observed in the major-axis plane than in the minor-axis plane, leading to enhanced heat transfer in the major-axis direction. In particular, the rectangular orifice with an AR of 5 induces strong secondary structures in the trailing jet following the primary vortex ring, and these are responsible for the double-peak distribution of wall temperature in the major-axis direction. This is the first time a double-peak temperature distribution has been reported for noncircular impinging synthetic jets. The rectangular orifice with an AR of 5 also yields a larger cross-stream velocity in the minor-axis direction, indicating a stronger wall jet. Thus, among the cases considered in this study, the rectangular orifice with AR = 5 achieves the optimal cooling performance and the maximum time-averaged cooling area reaches more than 14 times the jet orifice area.

Deep reinforcement learning based synthetic jet control on disturbed flow over airfoil

This paper applies deep reinforcement learning (DRL) on the synthetic jet control of flows over an NACA (National Advisory Committee for Aeronautics) 0012 airfoil under weak turbulent condition. Based on the proximal policy optimization method, the appropriate strategy for controlling the mass rate of a synthetic jet is successfully obtained at Re=3000. The effectiveness of the DRL based active flow control (AFC) method is first demonstrated by studying the problem with constant inlet velocity, where a remarkable drag reduction of 27.0% and lift enhancement of 27.7% are achieved, accompanied by an elimination of vortex shedding. Then, the complexity of the problem is increased by changing the inlet velocity condition and reward function of the DRL algorithm. In particular, the inlet velocity conditions pulsating at two different frequencies and their combination are further applied, where the airfoil wake becomes more difficult to suppress dynamically and precisely; and the reward function additionally contains the goal of saving the energy consumed by the synergetic jets. After training, the DRL agent still has the ability to find a proper control strategy, where significant drag reduction and lift stabilization are achieved, and the agent with considerable energy saving is able to save the energy consumption of the synergetic jets for 83%. The performance of the DRL based AFC proves the strong ability of DRL to deal with fluid dynamics problems usually showing high nonlinearity and also serves to encourage further investigations on DRL based AFC.

Investigation of vortex motion mechanism of synthetic jet in a cross flow

To investigate the mechanism of vortex motion, a pitched synthetic jet is arranged on the bottom of a cross flow and simulated by incompressible Naiver–Stokes equations with large eddy simulation. The Liutex vector identification method is utilized to quantitatively study the rotation strength and the core size of the vortex structures according to various driving frequencies (St = 0.25, 0.5, 0.75, 1.0) and amplitudes (A0 = 1.0, 1.5, 2.0, 2.5) of the synthetic jet during 21 periods. The results show that both the frequency and the amplitude play a significant role in the vortex generation mechanism of the synthetic jet. However, the amplitude makes a positive contribution to the rotation strength and the vortex core size, while the frequency makes a negative contribution. The vortex motion mechanism of a synthetic jet in a cross flow can be summarized as follows: low frequency and small amplitude favor the primary clockwise vortex, while high frequency and large amplitude motivate the anticlockwise vortex generation.

Economic and Environmental Potential of Large-Scale Renewable Synthetic Jet Fuel Production through Integration into a Biomass CHP Plant in Sweden

The potential of bio-electro-jet fuel (BEJF) production with integration into an existing biomass-based combined heat and power (CHP) facility was investigated. The BEJF is produced via Fischer–Tropsch (F–T) synthesis from biogenic CO2 and H2 obtained by water electrolysis. Techno-economic (TEA)- and life. cycle (LCA)- assessments were performed to evaluate the production cost and environmental impact of the BEJF production route. The BEJF mass fraction reached 40% of the total F–T crude produced. A reduction of 78% in heating demands was achieved through energy integration, leading to an increase in the thermal efficiency by up to 39%, based on the F–T crude. The total production cost of BEJF was in the range of EUR 1.6–2.5/liter (EUR 169–250/MWh). The GWP of the BEJF was estimated to be 19 g CO2-eq per MJ BEJF. The reduction potential in GWP in contrast to the fossil jet baseline fuel varied from 44% to more than 86%. The findings of this study underline the potential of BEJF as a resource-efficient, cost-effective, and environmentally benign alternative for the aviation sector. The outcome is expected to be applicable to different geographical locations or industrial networks when the identified influencing factors are met.

On the topology patterns and symmetry breaking in two planar synthetic jets

This article studies the flow structures and main patterns driving the flow dynamics in one and two planar synthetic jets. We perform numerical simulations at different Reynolds numbers (Re), for a similar forcing frequency, to model the synthetic jet flow and the two planar synthetic jets, which present a movement in-phase (synchronous jets) and out-of-phase (asynchronous jets). We identify two types of flow regimes as function of the Reynolds number: (i) the flow is symmetric and (ii) the symmetry is broken at Re ≈110 and Re ≈140 for the single and the synchronous jets, respectively. On the contrary, the flow is always asymmetric in the two asynchronous jets. We calculate the thrust produced by the several jet configurations, finding that the thrust produced by a single jet is always half of the thrust produced by the two synchronous jets; however, this quantity is much smaller in the asynchronous jets. Finally, we use higher order dynamic mode decomposition to identify the main patterns driving the flow dynamics. The solution is periodic in the single and two synchronous jets, with the forcing frequency (St*) as the dominant mode. The emerging rise in amplitude of a low-frequency mode (St0 = St*/6) that sub-harmonic of the forcing frequency as the Reynolds number increases suggests a connection between this mode and the symmetry breaking. A new mode is identified in the asynchronous jets, breaking the flow periodicity.

Role of phase-difference between two adjacent rectangular synthetic jet actuators in active control of flow over a rounded ramp

In the current study, the role of phase-difference between signals of two adjacent synthetic jet actuators (SJAs) in active control of flow over a rounded ramp geometry has been investigated. In order to accurately predict the separation and reattachment locations, wall-resolved large eddy simulation has been utilized to capture the locations of separation and reattachment. The two adjacent SJAs were placed upstream of the separation point. Six phase-differences between the two SJAs were considered, and two momentum coefficients were applied. First, the role of phase-difference in active flow control of a separation bubble behind a ramp-down region was investigated. Furthermore, the impact of an increased momentum ratio on the size and length of the separation zone was investigated to assess the effectiveness of phase-difference with respect to a higher velocity ratio. The effect of increased momentum ratio on the wall pressure fluctuations was also explored. As the second objective of this study, the flow and turbulent features were discussed to unveil the SJA actuation impact on the downstream flow. The time-averaged velocity and turbulent kinetic energy profiles and the turbulent production were examined and compared to the uncontrolled baseline case. It was found that a higher velocity ratio tremendously increased the turbulent energy before the separation point, while further downstream, the level of turbulent energy was uncoupled from the SJA momentum coefficient. Our study showed that by increasing the momentum ratio, the role of phase-difference in reducing the separation thickness lessened. Nevertheless, applying either a positive or a negative phase-difference of pi/2 still postponed the separation point.

Computational Analysis of Actuation Techniques Impact on the Flow Control around the Ahmed Body

Active flow control with jet devices is a promising approach for vehicle aerodynamics control. In this work an extended computational study is performed comparing three different actuation strategies for active flow control around the square back Ahmed body at Reynolds number 500,000 (based on the vehicle height). Numerical simulations are run using a Large Eddy Simulation (LES) approach, well adapted to calculate the unsteady high Reynolds number flow control using periodic jet devices. computations are validated comparing to in-house experiments for uncontrolled and some controlled cases. The novelty of this investigation is mainly related to the in-depth study of the base flow and actuation approaches by an accurate LES method and their comparison to experiments. Here, several simulations are performed to estimate the effect of active controls on the flow topology and the drag reduction. Beside the continuous blowing jet, three periodic actuation techniques including periodic blowing and suction as well as the zero flux synthetic jet devices are explored. The slots are implemented discontinuously in order to achieve a better control efficiency linked to vortex generation. In this framework, spectral analyses on global aerodynamical quantities, rear pressure/drag coefficient behavior examination as well as wake structure investigations are performed in order to compare these jet actuations. As a result, shear layer variations are observed during the blowing phase, but the main flow topology change occurs with suction and synthetic jets. Rear back pressure is therefore substantially increased.

Phase evolution of flow controlled by synthetic jets over NACA 0025 airfoil

Phase evolution of flow controlled by synthetic jets over NACA 0025 airfoil

Resistance of Geopolymer, Epoxy and Cement Mortar to Hydrocarbon-Based Synthetic Engine Lubricant, Hydraulic Fluid, Jet Fuel and Elevated Temperatures

Due to routine maintenance of aircraft on the concrete pavement at army airbases, a large part of the pavement surface is often found saturated with different hydrocarbon-based oil, fuel, and fluid. In addition, the pavement concrete is subjected to the aircraft’s exhaust temperature during operation. This study examined the resistance ability of 3 different cementitious materials: (i) epoxy, (ii) fly ash (FA) based geopolymer with various alkali to fly ash (AL/FA) ratios and (iii) Portland cement (PC) mortar under a simulated airfield circumstance. The mortar specimens were repetitively exposed to a mixture of synthetic engine oil, hydraulic fluids, jet fuel and elevated temperatures (175 °C) for 5 months simultaneously. During the exposures, geopolymer and PC mortar both suffered saponification. The degree of saponification of geopolymer samples is found to be highly reliant on the AL/FA ratios. On the contrary, the epoxy mortar was found to be resistant to saponification. It was also found that the PC mortar developed numerous thermal cracks but epoxy and geopolymer did not experience any visual thermal cracks under the same conditions.

Effects of single synthetic jet on turbulent boundary layer

The turbulent boundary layer (TBL) is actively controlled by the synthetic jet generated from a circular hole. According to the datasets of velocity fields acquired by a time-resolved particle image velocimetry (TR-PIV) system, the average drag reduction rate of 6.2% in the downstream direction of the hole is obtained with control. The results of phase averaging show that the synthetic jet generates one vortex pair each period and the consequent vortex evolves into hairpin vortex in the environment with free-stream, while the reverse vortex decays rapidly. From the statistical average, it can be found that a low-speed streak is generated downstream. Induced by the two vortex legs, the fluid under them converges to the middle. The drag reduction effect produced by the synthetic jet is local, and it reaches a maximum value at x + = 400, where the drag reduction rate reaches about 12.2%. After the extraction of coherent structure from the spatial two-point correlation analysis, it can be seen that the synthetic jet suppresses the streamwise scale and wall–normal scale of the large scale coherent structure, and slightly weakens the spanwise motion to achieve the effect of drag reduction.

An experimental investigation of the flow-field and thermal characteristics of synthetic jet impingement with different waveforms

Present experimental investigation reports the flow and thermal behavior of electromagnetically actuated circular orifice synthetic jet involving sinusoidal and square waveforms. The effect of Reynolds number (1656–2218), dimensionless axial distance (1–25), and the excitation frequency (100–175 Hz) on the thermal performance is studied here. The flow field is investigated using the hot wire anemometry and particle image velocimetry technique to couple the thermal behavior with fluid dynamics. The surface temperature is measured using an infrared thermal imaging camera. The average Nusselt number for synthetic jet with sinusoidal waveform is found to be 20.86% higher compared to the synthetic jet with square waveform. However, in the stagnation region, the square waveform exhibits 18.94% higher heat transfer compared to the sinusoidal waveform. The flow dynamics reveal the underline flow physics for the variation in the heat transfer rate. The velocity contours show a smooth and wide top hat with the sinusoidal waveform. However, in the case of square waveform, the velocity profile is narrow, which results in a decrease in entrainment of fresh ambient fluid into the jet. The sine wave exhibits 73.68% higher mass flow rate compared to square wave pattern at its diaphragm resonance frequency (150 Hz). The power spectral density of the velocity signal, obtained from the hot wire anemometry, is analyzed to understand the formation and propagation of vortex rings. A unified correlation for average Nusselt number is developed for both sinusoidal and square waveforms. The results obtained from the present experimental investigation provide significant guidance for the efficient design of synthetic jets for space-constrained electronics cooling.

The behavior of two adjacent vortex rings produced by synthetic jets

Synthetic jets are expected as an effective method of active boundary layer control. A synthetic jet device generates jets by vibrating the diaphragm of a speaker or a piezoelectric actuator by using a method of suction and blowing. The actuators are low power, zero-net-mass-flux and compact devices. Mixing enhancement is one of the key technologies in boundary layer control and it is necessary to control the behavior of vortex rings to promote mixing the crossflow and the boundary layer. The purpose of this study is to investigate the effect of vortex ring interaction due to differences in orifice distance (S=1.5D, 1.7D, 2.0D) on the behavior of vortex rings. In this experiment, the smoke flow visualization was used to describe the interaction process by changing the distance between adjacent orifices. It was confirmed that the height where two vortex rings collapse that in the case of S=2.0D was lower that in the case of S=1.5D.

Numerical simulation on the formation of a helical mode in the initial region of a round jets using synthetic jets

Axisymmetric mode (varicose mode) and asymmetric modes (helical modes) are involved in a round jet. Although there are many studies on the symmetric modes related to the formation of vortex rings, it seems to be a lack of studies on the helical mode. It is necessary to investigate the effect of the helical modes on the spatial development process of a jet. It has been found in previous research that helical vortex structures are formed from flow visualization in the initial region of a round jet by driving loudspeakers in sequence with phase differences. In this study, the conditions for the formation of helical modes by driving synthetic jets in sequence with phase differences are found by numerical simulations using OpenFOAM.

Heat Transfer Enhancement of Array Continuous-Jet Impingement with an Integrated Central Synthetic Jet

Heat Transfer Enhancement of Array Continuous-Jet Impingement with an Integrated Central Synthetic Jet

Effect of varying frequency of a synthetic jet on flow separation over an airfoil

An experimental investigation on the effects of the synthetic jet actuator (SJA) was conducted on a National Advisory Committee for Aeronautics 0025 airfoil in a low-speed recirculating wind tunnel at a chord Reynolds number of 100 000 and at an angle of attack 12°. Particle image velocimetry was used to visualize the flow separation for the uncontrolled baseline flow, and the flow attachment for the SJA controlled flows. The location of the SJA was at −1.3% from the separation point, and a blowing ratio of 0.8 was chosen for this study. The blowing ratio proved to be effective in suppressing the separation of the flow. The reduced frequency (Ste) was varied between 1, 2, 14, and 58. The momentum bursts from the SJA based on the reduced frequency determined the effectiveness of the control method. The Reynolds stresses and turbulence production decreased dramatically with increasing frequency up to the shear layer frequency (Ste= 14), but further excitation (Ste= 58) resulted in a regain of turbulence levels. Proper orthogonal decomposition was performed which showed that the low frequency operations globally affect the modes in the shear layer while the high frequency operations are confined to the airfoil surface.

Heat Transfer Characteristics of Steady Jet and Synthetic Jet of Same Reynolds Numbers

Heat Transfer Characteristics of Steady Jet and Synthetic Jet of Same Reynolds Numbers