Flow Control Applications Research Articles

Thermal camouflage device with efficient thermal management

Thermal camouflage devices adjust the temperature distribution around the target to hide it in the background. However, the majority of the current research is aimed at the situation that the target is non heat source or does not consider the heat dissipation of the target, which limits the application of relevant methods. If the heating target is merely isolated from its surroundings, it will suffer from severe thermal instability. In order to solve this problem, we're considering about using a special device with anisotropy to camouflage the target's thermal signature and lowering the target's temperature as much as possible. The special device is designed by a thermal metamaterial design optimization method based on composite theory. The thermal camouflage and thermal management performance of the special device are verified by numerical simulations and experiments under various models and target power. The results show that when the internal target warms up, the special device can conceal itself in the background environment while also having good thermal management performance to achieve effective thermal dissipation to the target. The method has potential applications in heat flow control, thermal energy recovery, and other areas of integrated thermal management.

Flow control using single dielectric barrier discharge plasma actuator for flow over airfoil

The single dielectric barrier discharge (SDBD) plasma actuator has been developed in the present work for high-accuracy, high-performance computing of flow control applications. The present physics-based SDBD model is a significant improvement over the one developed by Bagade et al., [“Frequency-dependent capacitance–based plasma model for direct simulation of Navier–Stokes equation,” AIAA J. 55, 180–194 (2017)], which was used for planar geometry using sequential computation. Based on the physics of SDBD operation, phase-averaged fully developed body force over an ac cycle is computed and stored, which is reused. Thus, the intensive body force computations are bypassed in the new model, and the body force due to the SDBD plasma actuator is incorporated in the compressible Navier–Stokes equation that is solved in a body-fitted curvilinear coordinates. Here, the modified SDBD model enables performing large-scale simulations for the aerodynamic flow control at low speed and transonic flow past airfoils used in unmanned aerial vehicles and executive jets. The flow control by SDBD plasma actuation is finally compared with other forms of flow control strategies.

Numerical analyses of a reference wing for combination of hybrid laminar flow control and variable camber

The objective of the LuFo VI-1 project CATeW (Coupled Aerodynamic Technologies for Aircraft Wings) consists in multifidelity analyses to assess the potential for aerodynamic efficiency increase by combined application of hybrid laminar flow control and variable camber technologies to the wing of a transonic transport aircraft. Individually, both technologies have proven to lead to major aerodynamic drag reductions. An evaluation of the coupled technologies is, therefore, expected to show an even higher potential due to synergy effects. To derive conclusions on system level, low-fidelity (LowFi) overall aircraft design methodologies will be applied for the analysis of a medium haul reference aircraft in the course of the project, while complex aerodynamic phenomena are modelled with high-fidelity (HiFi) computational fluid dynamics methods. The paper at hand presents results of aerodynamic analyses on both fidelity levels for the wing of the turbulent reference configuration CATeW-01, featuring the technology combination as a retrofit. Furthermore, this work encompasses adaptations and implementations performed within both the LowFi and HiFi toolchains. The LowFi toolchain already incorporates several modules for the proposed technology combination. A short presentation of the LowFi-toolchain is given, along with the modeling approach in the HiFi framework using mesh deformations and a mass flux boundary condition. Comparative studies of the turbulent flow field around the wing show good agreement of predicted load distributions in both numerical frameworks, studies based upon the HiFi approach attest the potential for efficiency increase due to the variable camber technology, incorporated by means of Adaptive Dropped Hinge Flap (ADHF) deflections. Considering the coupled application, four different constant suction mass flow rates are examined, where the maximum mass flow causes laminar flow extending over the entire suction panel, thus moving the transition location from the wing’s leading edge to the end of the suction panel. When being coupled with ADHF deflections, again the variable camber technology leads to a reduction of the wing’s pressure drag component with the simultaneous application of boundary layer suction further promoting drag reduction with increasing suction rate. While the combined application shows no mutual inhibition, major reciprocal effects are not directly observable when applying the combination as a retrofit to the reference configuration CATeW-01. This is mainly attributed to the limited extend of laminar flow, thus indicating the necessity for optimization in wing geometry and operating parameters, to achieve extensive areas of laminar flow and to promote the aspired synergy effects.

Insightful Facts on Peristalsis Flow of Water Conveying Multi-Walled Carbon Nanoparticles Through Elliptical Ducts With Ciliated Walls

In this research, a mathematical model is disclosed that elucidates the peristaltic flow of carbon nanotubes in an elliptic duct with ciliated walls. This novel topic of nanofluid flow is addressed for an elliptic domain for the very first time. The practical applications of current analysis include the customization of the mechanical peristaltic pumps, artificial cilia and their role in flow control, drug delivery and prime biological applications etc. The dimensional mathematical problem is transformed into its non-dimensional form by utilizing appropriate transformations and dimensionless parameters. Exact mathematical solutions are computed over the elliptic domain for the partial differential equations appearing in this convection heat transfer problem. A thorough graphical assessment is performed to discuss the prime results. The graphical visualization of the flow in this elliptic duct is obtained by plotting streamlines. The viscous effects are playing a vital role in the heat enhancement as compared to the molecular conduction. Since the incrementing Brinkman number results in a declined conduction due to viscous dissipation that eventually results in an enhanced temperature profile. This research first time elucidates the impacts of nanofluid flow on the peristaltic pumping through an elliptic domain having ciliated walls. Considering water as base fluid with multi-wall Carbon nanotubes for this ciliated elliptic domain having sinusoidal boundaries.

Comparing steady and unsteady rectangular jets issuing into a crossflow

The foundational differences of steady and unsteady jets issued into a laminar boundary layer crossflow are considered. Jets have been used widely for flow control applications, due to their ability to enhance mixing and mitigate separation, but it is unclear what role jet steadiness plays in flow control effectiveness. Here we compare experimentally unsteady (synthetic) and steady rectangular jets issued into a flat-plate laminar boundary layer with varying orifice pitch and skew. The coherent streamwise vortices produced by unsteady jets were shown to be much stronger than those produced by steady jets, despite producing similar flow patterns. These differences are rooted in how vorticity is generated in the orifice, through either a Stokes layer (unsteady) or a Blasius boundary layer (steady). Exploring the time- and phase-averaged vorticity transport equation reveals that the time-varying vorticity term is the reason for the enhanced vortical structure. When considering flow control metrics, we find that the unsteady jet produced greater added momentum in the boundary layer and added vorticity when compared to a momentum-matched steady jet. Both the steady and unsteady jets produced similar jet penetration characteristics.

Near-wall characteristics of wall-normal jets generated by an annular dielectric-barrier-discharge plasma actuator

An annular dielectric barrier discharge (DBD) plasma actuator has potential flow control applications due to having a simple design, no moving parts, and being light weight and fully electronic with low power consumption. We have experimentally investigated the detailed near-wall flow characteristics generated by the annular DBD plasma actuator. The actuator generates a wall-normal jet on top of a recirculation zone, with characteristics dependent on the actuation parameter i.e., burst frequency and amplitude. Our results can provide guidance for design and process optimization of several flow control applications i.e. lift enhancement, drag reduction, and mixing enhancement etc.

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.

Investigation of the Flow Instabilities of a Low Specific Speed Pump Turbine Part 2: Flow Control With Fluid Injection

Abstract This is the second part of a two-part paper focusing on the flow instabilities of low-specific speed pump turbines. In this part, results of the flow control application with fluid injection in the vaneless space for suppressing the flow instabilities of a model pump turbine in turbine mode operation at HSLU (Lucerne University of Applied Sciences) Switzerland are presented. Using the experimental data and CFD results, the onset and development of flow instabilities are explored in the first part of this paper. Based on these analyses, a flow control technology using injection of fluid in the vaneless space of the model pump turbine is implemented in order to suppress the flow instabilities and, thereby, to extend the operating range. Through use of an external energy source, air and water injection are applied with discrete nozzles that are circumferentially distributed in the vaneless space. The S-shaped pump turbine characteristics in turbine operating mode are modified so that the slope at speed no load conditions is no more positive meaning an improvement in the stability behavior. To the best of our knowledge, this is the first successful application of flow control with fluid injection in the vaneless space of pump turbines. Fluid injection is applied at two different guide vane openings. The analysis of the unsteady pressure data indicates that fluid injection in the vaneless space suppresses the flow instability such as rotating stall. Moreover, the water injection turns out to be more effective than the air injection for modifying the slope of the pump turbine characteristics.

Suction and Pulsed Blowing Applied to Local Separation Downstream of Slat Cutout

An experimental study on the application of active flow control (AFC) to a .4 scale model of a swept wing in a landing configuration was conducted. The wing is fitted with an ultrahigh bypass ratio (UHBR) engine nacelle. The highly efficient UHBR engines are characterized by a large diameter that interferes with the flow around the wing, especially with slats deflected, degrading its performance. An innovative AFC device, creating steady suction and pulsed blowing (PB), was installed in the leading-edge region of the wing, above the nacelle, and its performance was experimentally evaluated. The effects of the suction and PB mechanisms were examined individually and simultaneously using relevant normalized parameters to pave the way to a full-scale wind-tunnel test. It was shown that the AFC actuators increased lift by up to 3%, redirected the flow to the desired downstream direction, and reduced the size of the separation zone created due to the implementation of the UHBR nacelle. The next step is validating the small-scale results of this study in a full-scale wind-tunnel tests that would be the last step before the technology is flight test ready.

Novel Operating Mode of a Fluidic Oscillator

Abstract Fluidic oscillators show promise for use in aerodynamic flow control applications, with many studies reporting oscillation frequencies in the 1–10 kHz range. Spyropoulos, “A Sonic Oscillator,” introduced a “sonic” oscillator whose oscillation frequency depends on the inlet flow rate. The purpose of this paper is to demonstrate the existence of a second mode of operation (mode II) for such an oscillator, with a separate physical mechanism to the traditional, flow rate-dependent mode (mode I). Mode II is shown to be a back-pressure-driven oscillation that is largely independent of flow rate once instigated. This is explained by a stationary wave formed along the outlet paths, and occurs when conditions on the degree of back pressure and the weakening of the Coandă attachment strength are met. For a fixed device geometry, the conditions reduce to a minimum flow rate threshold so that the combination of high flow rate and constant oscillation frequency could make mode II an attractive flow control solution in an industrial context where minimizing device size is often critical.

Greedy Non-Intrusive Reduced-Order Model’s application in dynamic blowing and suction flow control to suppress the flow separation

The Greedy Non-Intrusive Reduced-Order Model (GNIROM) is applied in dynamic blowing and suction’s flow control to suppress the GAW1 airfoil’s flow separation. The effects of the uniform blowing and uniform suction on the flow fields and aerodynamic coefficients have been studied firstly. By comparing a uniform suction control with a dynamic blowing and suction control, it has been found that the dynamic blowing and suction can provide a higher lift coefficient and a lower energy loss. Finally, for the sake of achieving an optimal dynamic blowing and suction control, an optimization of the dynamic blowing and suction’s parameters is taken based on the GNIROM, consisting of a Non-Intrusive ROM (NIROM) built by the two-level POD and Radial Basis Function (RBF), and the relative greedy method. A cost function has been proposed by considering suppressing the flow separation, improving the lift-drag characteristics and reducing the energy loss together. The GNIROM has been built for unsteady vorticity fields with the parameters of the dynamic blowing and suction varying. Based on the GNIROM’s approximation, the optimal control parameters’ values have been determined quickly. Compared to the initial dynamic blowing and suction control, the optimal one has reduced the drag coefficient by 43.4 percents and the energy loss by 42 percents.

An Electroactive Filter with Tunable Porosity Based on Glycolated Polythiophene

The porosity of filters is typically fixed; thus, complex purification processes require application of multiple specialized filters. In contrast, smart filters with controllable and tunable properties enable dynamic separation in a single setup. Herein, an electroactive filter with controllable pore size is demonstrated. The electroactive filter is based on a metal mesh coated with a polythiophene polymer with ethylene glycol sidechains (p(g3T2)) that exhibit unprecedented voltage‐driven volume changes. By optimizing the polymer coating on the mesh, controllable porosity during electrochemical addressing is achieved. The pores reversibly open and close, with a dynamic range of more than 95%, corresponding to over 30 μm change of pores’ widths. Furthermore, the pores’ widths could be defined by applied potential with a 10 μm resolution. From among hundreds of pores from different samples, about 90% of the pores could be closed completely, while only less than 1% are inactive. Finally, the electroactive filter is used to control the flow of a dye, highlighting the potential for flow control and smart filtration applications.

Performance recovery of plasma actuators in wet conditions

Plasma actuators have been extensively studied for flow control applications over the past two and a half decades. While these studies have been traditionally focused on characterizing their performances as flow control devices, the performance of plasma actuators under adverse conditions such as dew or light rain remains to be less explored. This paper seeks to study the effects of water adhesion from droplets directly sprayed on to a plasma actuator using thrust recovery as the performance metric. It was found in all tests that wet actuators quickly recover plasma glow, before gradually regaining performance comparable to the dry actuator. The measured thrust for the wet actuator after 5 s of operation recovered by 46% and 42% of the thrust of the dry actuator for 50.0–62.5 g m−2 and 125–150 g m−2 of sprayed water droplets, respectively. At 22.5 kVpp and 14 kHz, the highest thrust recovery was recorded at 84% of that of the dry actuator after 80 s of operation. For 17.5 kVpp and 14 kHz the wet thrust recovered by 79%, while for 22.5 kVpp and 10 kHz the wet thrust recovered by 68% of their dry counterpart in 80 s. For 17.5 kVpp and 14 kHz, the thrust almost fully recovered in comparison to the dry actuator after about 290 s of operation. These results indicate that both applied voltage and operating frequency plays a critical role in the performance recovery while the latter may have a stronger influence. Performance recovery for a wet serpentine shaped plasma actuator is also included for general applicability. The power data in all cases show that wet actuators consume more power which with time gradually approach the dry actuator power data. This is because during the initial stages of operation, the rolling mean current of the wet actuator is higher than the dry actuator even though the ionization spikes of dry actuator is stronger.

A comparative analysis of sweeping jet actuators

The characteristics of spatially oscillating jets emanating from various Sweeping Jet Actuators (SJAs) are investigated in the present study by performing numerical analysis. The four different SJA designs commonly referred in the literature for the flow control applications, namely, the feedback free type, single feedback type, sharp dual feedback type and curved dual feedback type are considered for this investigation. The various characteristics of the oscillating jets such as instantaneous velocity field, jet oscillation frequency, spread angle and the mean centerline velocity variation have been evaluated and compared with the aim of finding an effective design for application in flow control. The present study reveals that the oscillation frequency and jet penetration are more for a feedback free SJA when compared to the other designs for the same conditions whereas the spread angle is high for a curved dual feedback SJA. Based on these findings, it is inferred that the feedback free type and curved dual feedback type are more suitable for flow control applications.

Liquid Jet Blasting Using Ultra-High Frequency Supersonic Pulsed Air Jet

This paper reports an experimental study on a liquid injector assembly integrated with an ultra-high frequency pulsed air jet that operates at 21 kHz. The active air-blasting assembly steadily injects a liquid through four micro-nozzles of 0.4 mm diameters each, positioned around a 1 mm nozzle through which the pulsed actuation jet flows out at supersonic velocity. High-frequency compressible air vortexes and shock waves generated by the injector atomize the liquid stream into finer droplets and distribute them to a larger area to improve mixing with the ambiance. The paper presents the design details and preliminary studies on the flow field characteristics of this novel injection scheme, which is a potential candidate for high-speed flow mixing and control applications.

Review of MSM Actuators: Applications, Challenges, and Potential

Magnetic shape memory alloys (MSMAs) are a new and rapidly developing class of smart materials. These alloys demonstrate significant strain (from 6% to 12%) under an external magnetic field, which combined with their wide actuation frequency range, rapid accelerations, and high energy efficiency has resulted in MSMAs being used in a wide range of applications, for example, for energy harvesting, in measurement devices, flow control and pump applications, and as actuators and dampers. MSM actuators inherit the properties of MSMAs and MSM actuators can thus be extremely useful in many areas such as robotics, mobile machines, aerospace engineering and medicine, to name a few. Although the volume of research related to MSM actuators has increased in recent years, only a few types of these actuators exist, and the number of possible applications is currently rather limited. Nevertheless, MSM actuators clearly have great potential. Against this background, this paper reviews the current state of MSMA technology, examines different classes of MSM actuators and their current applications, and presents opportunities that have not yet been properly explored.

Binary Repetitive Model Predictive Active Flow Control Applied to an Annular Compressor Stator Cascade With Periodic Disturbances

Abstract Novel pressure gain combustion concepts invoke periodic flow disturbances in a gas turbine's last compressor stator row. This contribution presents studies of mitigation efforts on the effects of these periodic disturbances on an annular compressor stator rig. The passages were equipped with pneumatic active flow control (AFC) influencing the stator blade's suction side, and a rotating throttling disk downstream of the passages inducing periodic disturbances. For steady blowing, it is shown that with increasing actuation amplitudes cμ, the extension of a hub corner vortex deteriorating the suction side flow can be reduced, resulting in an increased static pressure rise coefficient Cp of a passage. The effects of the induced periodic disturbances could not be addressed intrinsically, by using steady blowing actuation, Considering a corrected total pressure loss coefficient ζ*, which includes the actuation effort, the stator row's efficiency decreases with higher cμ due to the increasing costs of the actuation mass flow. Therefore, a closed-loop approach is presented to address the effects of the disturbances more specifically, thus lowering the actuation cost, i.e., mass flow. For this, a repetitive model predictive control (RMPC) was applied, taking advantage of the periodic nature of the induced disturbances. The presented RMPC formulation is restricted to a binary control domain to account for the used solenoid valves' switching character. An efficient implementation of the optimization within the RMPC is presented, which ensures real-time capability. As a result, Cp increases in a similar magnitude but with a lower actuation mass flow of up to 66%, resulting in a much lower ζ* for similar values of cμ.

Active control of flow over a backward-facing step at high Reynolds numbers

This work provides new insight into the transient flow effects of forcing a backward-facing step flow at high Reynolds number. The detailed flow structure and surface pressure is investigated over the range 118,000⩽ReH⩽472,000 using time-resolved particle-image velocimetry. Notably, this research considerably extends the Reynolds number ranges of previous studies. The effect of periodic shear-layer forcing on the global mean and fluctuating flow structure is examined over the forcing frequency range 0.036⩽StH⩽1.98. The effect of forcing on the base pressure, which has received relatively little attention, is also detailed. These studies allow us to characterise the relationship between the forcing frequencies, with a particular focus on frequencies near and significantly above the shear-layer instability. Importantly, despite the high Reynolds number, we are able to perturb the shear layer to control both the reattachment length and base pressure. Forcing at frequencies close to the shear-layer instability results in a significant reattachment length reduction, as has been well reported, with a corresponding base pressure reduction of up to 45%. At the highest forcing frequency, an increase in mean base pressure of 9.7% is found, with a corresponding increase in reattachment length of 3.9% over the unforced case. This high-frequency forcing reduced initial growth of the shear-layer instability and stabilised the latter half of the reattachment zone. This enabled more flow entrainment upstream to the step-base, resulting in a mean base-pressure increase. Although, this came at the cost of triple the base-pressure fluctuations, an important insight for practical flow-control applications. The spatial distribution of spectral power for key instabilities in the flow, and the influence of forcing on these distributions, is also examined. To our knowledge, the spatial distribution of these key instabilities, with or without control, has not been previously presented for high Reynolds numbers (ReH>104).

Bistabilities in two parallel Kármán wakes

Bistabilities of two equilibrium states discovered in the coupled side-by-side Kármán wakes are investigated through Floquet analysis and direct numerical simulation (DNS) with different initial conditions over a range of gap-to-diameter ratio ( $g^*= 0.2\text {--}3.5$ ) and Reynolds number ( $Re = 47\text {--}100$ ). Two bistabilities are found in the transitional $g^*-Re$ regions from in-phase (IP) to anti-phase (AP) vortex shedding states. By initialising the flow in DNS with zero initial conditions, the flow in the first bistable region (i.e. bistable IP/FF $_C$ at $g^*= 1.4 \text {--} 2.0$ , where FF $_C$ denotes the conditional flip-flop flow) attains flip-flop (FF) flow, it settles into the IP state by initialising the flow with an IP flow. The second bistability is observed between cylinder-scale IP and AP states at large $g^*$ ( $=$ 2.0–3.5). The transition from the FF $_C$ to IP is dependent on initial conditions and irreversible over the parameter space, meaning that the flow cannot revert back to the FF $_C$ state once it jumps to the IP state irrespective of the direction of $Re$ variations. Its counterpart for the bistable IP/AP state is reversible. We also found that the FF $_C$ flow in the first bistable region is primarily bifurcated from synchronised AP with cluster-scale features, possibly because the cluster-scale AP flow is inherently unstable to FF flow instabilities. It is demonstrated that the irreversible bistability exists in other interacting wakes around multiple cylinders. A good understanding of flow bistabilities is pivotal to flow control applications and the interpretation of desynchronised flow features observed at high $Re$ values.

Coupled Boundary-Layer Suction and Airfoil Optimization for Hybrid Laminar Flow Control

The present work incorporates the effects of boundary-layer suction in designing airfoils for hybrid laminar flow control application. The overall airfoil shape, the suction velocity distribution, and its location are optimized for minimum total drag while sustaining laminar flow over 80% of the airfoil top surface. Nondominated Sorting Genetic Algorithm II is used with the aerodynamic load computations from XFOILSUC and transition prediction from the improved Van Ingen method. The airfoil is designed for short-range subsonic conditions for three design lift-coefficient scenarios: 1) 0.4, 2) in the range of 0.3–0.7, and 3) in the range of 0.3–0.7 with suction onset at midchord. The optimized airfoil for scenario 3, with suction onset at midchord, results in a thicker airfoil with natural laminar flow (NLF) on the upper surface comparable to NLF airfoils. The total drag reduces by one-third due to the effects of suction. On the other hand, without limiting the suction onset (scenario 2), a thinner airfoil with longer NLF on the bottom surface and lower total drag is obtained. Optimum suction onset location is predicted to be at 30% of the chord. Suction is also observed to improve the drag bucket at off-design conditions.