Boundary Layer Flow Control Research Articles

Effect of wall microstructure on conical hypersonic boundary layer flow

As an effective boundary layer flow control method, wall microstructure has an important application prospect in heat and drag reduction of hypersonic. In this paper, based on experimental techniques such as high-frequency pressure fluctuation testing, the effects of different microstructure configurations (V-shaped and rectangular) and distribution modes (streamwise and transverse) on the flow of hypersonic conic boundary layer under the condition of Mach 6 hypersonic incoming flow are studied. The experimental results show that compared with the flow of the smooth wall boundary layer, the wall microstructure has a greater influence on the eigenfrequency of the second-mode wave in the boundary layer, and the streamwise increases by about 20 kHz to the rectangular microstructure and decreases by about 30 kHz to the transverse rectangular microstructure. At the same time, the transverse V-shaped microstructure can increase the amplitude of the second mode in the boundary layer by 5.5 times compared with the smooth wall. For the evolution of boundary layer flow, the microstructure arranged along the flow direction will accelerate the linear development of the disturbance wave during the transition, so that the laminar flow will be transformed into turbulent flow in a shorter distance. The microstructure arranged along the transverse direction prolongs the linear development stage of the disturbance wave during the transition, so that the laminar flow turns into turbulent flow over a longer distance. Downstream of different structures, it is shown that transverse rectangular microstructure has lowest temperatures.

Numerical and statistical analyses of three-dimensional non-axisymmetric Homann's stagnation-point flow of nanofluids over a shrinking surface

Nanofluids, which are known for their superior heat transfer properties, have become indispensable in real-world applications. This underscores the critical importance of comprehensive understanding, which forms the core of this study. The goal of this study is to scrutinize Homann's non-axisymmetric stagnation-point flow over a shrinking surface of various single-phase water-based nanofluids (water-copper, water-alumina, and water-titanium dioxide) in a three-dimensional (3D) system. The model formulations are derived into ordinary differential equations via sophisticated similarity variables that satisfy the continuity equation. A boundary value problem solver (bvp4c) with a finite difference method in MATLAB is adapted to carry out numerical calculations while the response surface methodology (RSM) in MINITAB is used to statistically analyze the solution. The opposing flow created by the shrinking surface within a certain degree of shrinkage leads to the formation of two distinct alternative solutions. The current study offers two alternative solutions for boundary layer flow control modeling. However, only the first solution is tested to be stable. The critical point for boundary layer bifurcation is identified, to efficiently manage the flow. Water-copper nanofluid has been proven to perform better in heat transfer than water-alumina and water-titanium dioxide nanofluids. The 2 % volume fraction of copper provides better heat transfer compared to 1 % volume fraction in the present configuration of the flow system. The response optimizer via RSM also validates that the heat transfer performance of copper nanofluid is maximized when the volume fraction of copper is set at a high level with a low shear-to-strain rate.

Design of an Acoustic Synthetic Jet Actuator for Flow Control

Synthetic jet technology is widely adopted in active flow control. An actuator with an oscillating diaphragm is a commonly used excitation device for synthetic jet generation. However, it has a disadvantage wherein the volume at the cross-section of the cavity varies unevenly when the diaphragm vibrates, which makes it difficult to use multiple jets corresponding to one diaphragm. In this paper, an acoustic synthetic jet actuator that can generate multiple jets with one diaphragm was designed. The diaphragm vibrated in a cylindrical cavity, transferring air to another constant-volume square cavity through pipes. The square cavity was covered with a multiple-orifice plate for the expulsion and suction of the ambient air. Through this means, the implementation of multiple jets corresponding to one diaphragm was achieved. The multiple jets are called distributed synthetic jets in this paper. Governing parameters that determined the performance of the distributed synthetic jets were given by theoretical derivation. It was found that, under specific geometry conditions, the governing parameters were mainly the frequency and voltage of the input signal to the actuator. Then, the velocity characteristics of the distributed synthetic jets were measured by using a constant-temperature anemometer and the parameter space was determined. The results showed that it was practicable to apply the acoustic actuator to turbulent boundary layer flow control.

Mechanism analysis of magnetohydrodynamic control in hypersonic turbulent flow

Under the assumption of the low magnetic Reynolds number, the coupled model is established for the turbulent flow field and the externally applied magnetic field. The AUSMPW+ scheme and LUSGS method are used to solve turbulent magnetohydrodynamics (MHD) flow equations, in which the Spalart-Allmaras one-equation turbulence model is used. A series of numerical simulations over various geometry configurations, namely, a flat plate and a compression corner, is conducted by using an external electromagnetic field. Results show that the performance of MHD boundary layer flow control is determined mainly by the Lorentz force in the streamwise direction. With an external magnetic field used, the low velocity fluid in the boundary layer can decelerate and increase the static temperature locally. Moreover, the counter-flow Lorentz force always brings a negative effect on the turbulent skin friction coefficient, and the location for the MHD zone has a great influence on the control efficiency of the ramp-induced separation. A reasonable magnetic field layout scheme should be configured in practical engineering application.

Aerodynamic performance investigation of sweptback wings with bio-inspired leading-edge tubercles

The aerodynamic behavior of sweptback wing configurations with bio-inspired humpback whale (HW) leading-edge (LE) tubercles has been investigated through computational and experimental techniques. Specifically, the aerodynamic performance of tubercled wings with symmetric (NACA 0015) and cambered (NACA 4415) airfoils is validated against the baseline model at various angles of attack ([Formula: see text]. The [Formula: see text]/[Formula: see text] ratio of the HW flipper is strategically reduced to 0.15 for ascertaining the flow control potential of the bio-inspired wings with sweptback configuration. It is a novel effort to quantify the effect of the leading-edge protuberances on stall delay, flow separation control and distribution of streamline vortices at unique [Formula: see text]/[Formula: see text] ratio outside the thickness range of HW flipper morphology. Four tapered sweptback wing models (Baseline A, Baseline B, HUMP 0015, HUMP 4415) are used with the amplitude-to-wavelength ([Formula: see text] ratio of 0.24 and Reynolds number about [Formula: see text]. The chordwise pressure distributions are recorded at the peak, mid and trough regions of the tubercled wings through a detailed wind tunnel testing and validated with numerical analysis. Additionally, the flow characteristics over the bio-inspired surfaces have been qualitatively analyzed through the laser flow visualization (LFV) technique to reveal the influence of laminar separation bubbles (LSBs). The essential aerodynamic characteristics such as boundary layer trip delay, vortex mixing, stall delay, and flow control at different AoA are addressed through consistent experimental data. As the sweptback configuration is a primary choice for airplane wings, the improved aerodynamic characteristics of the tubercled wings can be effectively utilized for the design of novel lifting surfaces, hydroplanes and wind turbines in the near future.

Aerodynamics with state-of-the-art bioinspired technology: Tubercles of humpback whale

Bioinspired aerodynamics is an emerging subject in the design of advanced flight vehicles with superior performance and minimum fuel consumption. In the present review article, a comprehensive evaluation is focused on previous studies and investigations toward the performance enhancement of aerodynamic surfaces with leading-edge (LE) tubercles. The implementation of tubercles has been biologically imitated from humpback whale (HW) flippers. Particularly, aerodynamicists are much interested in this bioinspired technology because of the exclusive maneuvering and flow control potential of HW flippers. LE protuberances are considered as a passive flow control method to improve the aerodynamic performance in various applications like aviation, marine, and wind energy. The aerodynamic and hydrodynamic performance variations caused by specific tubercles amplitude and wavelength are also compared through numerical and wind tunnel testing. The prospective utilization of tubercles on boundary layer flow control is measured with regard to conventional and swept-back wing designs. Flow control mechanisms of tubercles are outlined with several interesting facts in addition to the outcomes of various bioinspired aerodynamic investigations in the recent years.

Characteristics of array of distributed synthetic jets and effect on turbulent boundary layer

An array of distributed round synthetic jets was used to control a fully developed turbulent boundary layer. The study focused on the related skin friction drag reduction and mechanisms involved. The control effects were analyzed by measuring the streamwise velocities using a hot-wire anemometer downstream of the array. A reduction in the skin friction was observed both in the regions downstream of the orifices and in the regions between two adjacent orifices. A statistical analysis with the variable-interval time-averaging (VITA) technique demonstrated a weakened bursting intensity with synthetic jet in the near-wall region. The streamwise vortices were lifted by the upwash effect caused by synthetic jet and induced less low-speed streaks. The control mechanism acted in a way to suppress the dynamic interaction between the streamwise vortices and low-speed streaks and to attenuate the turbulence production in the near-wall region. The forcing frequency was found to be a more relevant parameter when synthetic jet was applied in turbulent boundary layer flow control. A higher forcing frequency induced a higher reduction in the skin friction. The power spectral density and autocorrelation of the fluctuating velocities showed that the synthetic jets gradually decayed in the streamwise direction, having an effect as far as 34.5 times the displacement thickness that was on the trailing edge of the distributed synthetic jets array.

Numerical Investigation of Active Control of Boundary Layer Flow Around S809 Airfoil with Suction Method

Bu çalışmada, sınır tabaka akışı aktif kontrol yöntemlerinden biri olan, sınır tabakadan kanat içerisine hava emilmesi prensibine dayanan emme tekniği kullanılarak bir rüzgar türbini kanadının aerodinamik performansının arttırılması hedeflenmiştir. Emme işlemi daimi bir jet vasıtasıyla gerçekleştirilmiş, kanat modeli olarak rüzgar türbini uygulamalarında yaygın olarak kullanılan S809 kanat profili tercih edilmiştir. Çalışma parametreleri olarak, üç ayrı jet konumu (Ljet = 0.1c, 0.26c, 0.36c) ve üç ayrı jet oranı (Rjet = 0.1, 0.3, 0.5) seçilmiştir. Emme jeti genişliği sabit olup veter uzunluğunun %2.5’i kadar ve emme jeti açısı (θjet) bölgesel jet yüzeyine 90° olacak şekilde ayarlanmıştır. İki boyutlu türbülanslı akış için sayısal analiz; α = 15° hücum açısında ve Re = 106’da SST k-ω türbülans modeli kullanılarak gerçekleştirilmiştir. İlk olarak emme jeti konumunun etkisi, ardından en iyi sonucu veren emme jeti konumu seçilerek emme jeti oranının etkisi araştırılmıştır. Kanat profili etrafındaki akışa ait simülasyon sonuçları incelendiğinde, en iyi sonuç emme jeti konumu 0.36c (Jet-3) ve emme jeti oranı 0.5 olduğunda alınmıştır. Jet kullanılmadığı duruma göre CL/CD oranı 17.92’den 273.03’e yükselmiştir. Emme jeti ile kontrol yönteminin uygulanması ile kontrolsüz duruma göre Cl değeri yaklaşık olarak 1.211’den 1.8’e yükselmiş, Cd değeri ise 0.068’den 0.0066’ya düşmüştür.

Experimental and Numerical Analysis of the Effect of Vortex Generator Installation Angle on Flow Separation Control

In order to explore the effect of the installation angle of vortex generator (VG) on boundary-layer flow control, the vortex characteristics of plate VG and their effect on the aerodynamic characteristics of an airfoil was studied numerically and using wind tunnel experiments. The effects of five VG installation angles (β) of 10°, 15°, 20°, 25°, and 30° on the characteristics of vortices were studied. The results show that the strength of vortices on the leeward side of VG increases with an increased installation angle until, eventually, the vortex core breaks down. During the downstream development of the VG leading-edge separation vortices, these vortices deviate in the radial direction. The larger the installation angle, the larger this deviation distance in the radial direction becomes. The effects of installation angle on the aerodynamic performance of airfoils were studied in a wind tunnel using the same five VG installation angles. The results show that VG can delay flow separation on the airfoil suction surface, thereby increasing lift and reducing drag. The stall angle of the airfoil with VG was increased by 10°. When the installation angle of the VG was 20°, the maximum lift coefficient of airfoil increased by 48.77%. For an airfoil angle of attack (AoA) of 18°, the drag of the airfoil decreased by 88%, and the lift-drag ratio increased by 1146.04%. Considering the best overall distribution of lift-drag ratio, the positive effect of the VG was found to be when β = 20° and the worst VG effectiveness was observed at β = 30°.

Mechanism of compressor airfoil boundary layer flow control using nanosecond plasma actuation

The flow control effects of nanosecond plasma actuation on the boundary layer flow of a typical compressor controlled diffusion airfoil are investigated using large eddy simulation method. Three types of plasma actuation are designed to control the boundary layer flow, and two mechanisms of compressor airfoil boundary layer flow control using nanosecond plasma actuation have been found. The plasma actuations located within the laminar boundary layer flow can induce a small vortex structure through influencing on the density and pressure of the flow field. As the small vortex structure moves downstream along the blade surface with the main flow, it can suppress the turbulent flow mixing and reduce the total pressure loss. The flow control effect of the small vortex structure is summarized as wall jet effect. Differently, the plasma actuation located within the turbulent boundary layer flow can act on the shear layer flow and induce a large vortex structure. While moving downstream, this large vortex structure can suppress the turbulent flow mixing too.

On the wave-cancelling nature of boundary layer flow control

This work deals with the feedforward active control of Tollmien–Schlichting instability waves over incompressible 2D and 3D boundary layers. Through an extensive numerical study, two strategies are evaluated; the optimal linear–quadratic–Gaussian (LQG) controller, designed using the Eigensystem realization algorithm, is compared to a wave-cancellation scheme, which is obtained using the direct inversion of frequency-domain transfer functions of the system. For the evaluated cases, it is shown that LQG leads to a similar control law and presents a comparable performance to the simpler, wave-cancellation scheme, indicating that the former acts via a destructive interference of the incoming wavepacket downstream of actuation. The results allow further insight into the physics behind flow control of convectively unstable flows permitting, for instance, the optimization of the transverse position for actuation. Using concepts of linear stability theory and the derived transfer function, a more efficient actuation for flow control is chosen, leading to similar attenuation of Tollmien–Schlichting waves with only about 10% of the actuation power in the baseline case.

Quasi-radial wall jets as a new concept in boundary layer flow control

ABSTRACTThis work aims to introduce a novel concept of wall jets wherein the flow is radially injected into a medium through a sector of a cylinder, called quasi-radial (QR) wall jets. The results revealed that fluid dynamics of the QR wall jet flow differs from that of conventional wall jets. Indeed, lateral and normal propagations of a conventional three-dimensional wall jet are via shear stresses. While, lateral propagation of a QR wall jet is due to mean lateral component of the velocity field. Moreover, discharged Arrays of conventional three-dimensional wall jets in quiescent air lead to formation of a combined wall jet at large distant from the nozzles, while QR wall jet immediately spread in lateral direction, meet each other and merge together very quickly in a short distance downstream of the jet nozzles. Furthermore, in discharging the conventional jets into an external flow, there is no strong interaction between them as they are moving parallel. While, in QR wall jets the lateral components of the velocity field strongly interact with boundary layer of the external flow and create strong helical vortices acting as vortex generators.

Heat generation mechanisms of DBD plasma actuators

During the last twenty years DBD plasma actuators have been known by their ability for boundary layer flow control applications. However, their usefulness is not limited to this application field, they also present great utility for applications within the field of heat transfer, such as a way to improve the aerodynamic efficiency of film cooling of gas turbine blades, or de-icing and ice formation prevention. Nevertheless, there is a relative lack of information about DBD’s thermal characteristics and its heat generation mechanisms. This happens due to the extremely high electric fields in the plasma region and consequent impossibility of applying intrusive measurement techniques. Against this background, this work describes the physical mechanisms behind the generation of heat associated to the DBD plasma actuators operation. An experimental technique, based on calorimetric principles, was devised in order to quantify the heat energy generated during the plasma actuators operation. The influence of the dielectric thickness, as well as the dielectric material, were also evaluated during this work. The results were exposed and discussed with the purpose of a better understanding of the heat generation mechanisms behind the operation of DBD plasma actuators.

A-24 揮発性有機化合物の放散・吸脱着等のモデリングとその数値予測に関する研究(その14) : 建材表面気流精密制御 Small Test Chamberの開発と物質伝達率の測定

A-24 揮発性有機化合物の放散・吸脱着等のモデリングとその数値予測に関する研究(その14) : 建材表面気流精密制御 Small Test Chamberの開発と物質伝達率の測定

Control of boundary layer flow and lock-on of wake behind a circular cylinder with a normal slit

A two-dimensional flow simulation around a circular cylinder with a normal slit at a Reynolds number of 200 is conducted using Ansys/Fluent software. The slit ratio (S/D) ranges from 0.03 to 0.3. The scenarios of the characteristics of the slit flow, the evolution of the boundary-layer flow, and the wake flow are investigated and disclosed by modified phase-averaged method. Some critical outcomes are outlined. Induced by the shedding vortex street with shorter formation length, the pressure difference between the two slit openings drives the fluid at each slit opening, in form of periodic blowing/suction with zero net mass flux. Periodic blowing/suction at the slit opening serves as a perturbation that significantly delays the boundary-layer flow separation along the rear surface of the slit cylinder and reduces the vortex formation length. With proper slit ratio, this perturbation synchronizes with the evolution of the boundary-layer flow and the formation of vortex-street, leading to the primary lock-on of the wake. In this study, the optimal slit ratio is S/D=0.18 at Re=200. For all slit ratios studied, the frequency and amplitude responses, as well as the slight phase shift of the wake flow structures clearly demonstrate the primary lock-on of the wake. In this study, the one-sided lock-on frequency range lies only within fa/fso>1 because the primary lock-on is caused by the synchronization between the periodic blowing/suction at the slit opening and the shedding vortex street with higher frequency (or reduced vortex formation length) behind the slit cylinder than that of a base-line (or non-slit) cylinder.

Study on Free Surface and Channel Flow Induced by Low-Temperature Plasma via Lattice Boltzmann Method

Active boundary layer flow control and boundary layer manipulation in the channel flow that was based on low temperature plasma were studied by means of a lattice Boltzmann method. Two plasma actuators were placed in a row to obtain the influence rule of their separation distance on the velocity profile at three locations and maximum velocity in the flow field. Two plasma actuators were placed symmetrically inside a channel to examine the effect of channel height and voltage on the velocity profile and flow rate. It was found that the channel height controls the distribution of flow velocity, which affected the flow rate and its direction. Increasing plasma voltage had a negative effect on the flow rate due to the generation of a larger and stronger flow vortex.

Experimental Study of Synthetic Jets with Cross Flow in Boundary Layer

The synthetic jet actuator is a low power, highly compact fluidic device which has potential application in boundary layer flow control. In this study, we have shown how synthetic jets work without cross flow and how effectively they modify the flow structure in the boundary layer. This paper describes the electrodynamics synthetic jet actuator used in our experiments. The experimental set-up for flow control using this type of actuator is detailed. The flowfield resulting from the interaction of a synthetic jet actuator and a quiescent environment and of synthetic jet actuator and a turbulent, flat-plate, zero pressure gradient boundary layer were examined via Particle Image Velocimetry (PIV) . The synthetic jet actuator was tested and two forcing amplitudes were investigated. Inspection of the phase-averaged velocity revealed that spanwise large-scale vortices are generated downstream of the slot and were persisting farther downstream.

Experimental investigation on the starting vortex induced by symmetrical dielectric barrier discharge plasma actuator

Flow control using plasma actuator is a promising research field of aeronautical applications. Due to its low energy consumption, rapid response and simple construction, this actuator has been investigated in various aerodynamics problems, such as boundary layer flow control, drag reduction, lift enhancement, noise reduction, and flow separation control. In order to understand the controlling mechanism of plasma actuator, many researchers have been carried out some experiments on the plasma actuator characterization in quiescent air and obtained the evolution process of starting vortex induced by plasma actuator. But the plasma actuator always works under flow condition. Therefore, understanding the interaction process between the starting vortex and incoming flow is a key to promote this technology development. In this paper, the starting vortex induced by symmetrical Dielectric Barrier Discharge (DBD) plasma actuator in quiescent air or under flow condition was investigated using Particle Image Velocimetry (PIV). Compared with the asymmetrical DBD plasma actuator, the symmetrical plasma actuator adopted the whole metal plate model as the insulated electrode. Three layers of kapton film as dielectric material covered the testing model and the thickness of each layer was 0.05 mm. The copper foil which was 2 mm in width and 0.05 mm in thickness was mounted on the trailing edge of the plate and oriented along the spanwise direction to induce a wall jet in the streamwise direction. The input AC voltage was 8 kV p-p and the frequency of the power source was 3 kHz. The wind speed was 1 m/s. The results suggested that the symmetrical actuator produced one pair of counter-rotating starting vortexes on each side of upper electrode and the trajectory of the starting vortex core was shown to scale with t0.7 in quiescent air. Compared to the evolution law of starting vortex in still air, the development evolution and life time of starting vortex under flow condition was different due to the interaction influence between incoming flow and starting vortex. The breakdown time of downstream starting vortex was earlier and the location of the starting vortex core scaled with t0.45 under flow condition. Conversely, the life time of upstream starting vortex which was in the opposite direction of incoming flow was delayed. The incoming flow enhanced the upstream starting vortex's capability of promoting mixing and entraining high-momentum fluid into boundary layer, therefore the boundary layer became more energetic and capable of withstanding adverse pressure gradient. The jet effect and mixing function could be achieved by the symmetrical plasma actuator. These investigations laid the groundwork for flow control using DBD plasma actuator at high wind speed or high Reynolds number.

Wind tunnel experiments on flow separation control of an Unmanned Air Vehicle by nanosecond discharge plasma aerodynamic actuation**Project supported by the National Natural Science Foundation of China (Grant Nos. 61503302, 51207169, and 51276197), the China Postdoctoral Science Foundation (Grant No. 2014M562446), and the Natural Science Foundation of Shaanxi Province, China (Grant No. 2015JM1001).

Plasma flow control (PFC) is a new kind of active flow control technology, which can improve the aerodynamic performances of aircrafts remarkably. The flow separation control of an unmanned air vehicle (UAV) by nanosecond discharge plasma aerodynamic actuation (NDPAA) is investigated experimentally in this paper. Experimental results show that the applied voltages for both the nanosecond discharge and the millisecond discharge are nearly the same, but the current for nanosecond discharge (30 A) is much bigger than that for millisecond discharge (0.1 A). The flow field induced by the NDPAA is similar to a shock wave upward, and has a maximal velocity of less than 0.5 m/s. Fast heating effect for nanosecond discharge induces shock waves in the quiescent air. The lasting time of the shock waves is about 80 μs and its spread velocity is nearly 380 m/s. By using the NDPAA, the flow separation on the suction side of the UAV can be totally suppressed and the critical stall angle of attack increases from 20° to 27° with a maximal lift coefficient increment of 11.24%. The flow separation can be suppressed when the discharge voltage is larger than the threshold value, and the optimum operation frequency for the NDPAA is the one which makes the Strouhal number equal one. The NDPAA is more effective than the millisecond discharge plasma aerodynamic actuation (MDPAA) in boundary layer flow control. The main mechanism for nanosecond discharge is shock effect. Shock effect is more effective in flow control than momentum effect in high speed flow control.

Plasma channel flows: Electro-fluid dynamic jets

The present work builds on the success of a dielectric barrier discharge driven plasma channel by exploring an electrode configuration that directly actuates the bulk fluid minimizing jet impingement and viscosity related losses. Influence of several electrical and physical parameters including electrode materials are experimentally investigated. Results indicate significant variation of performance with these parameters and suggest that in comparison to surface dielectric barrier plasma actuator driven flows, at least an order of magnitude improvement in efficiency is possible. The jet produced from this plasma channel configuration allows greater versatility for applications in boundary layer flow control and internal flows.