Sensing and control of flow separation using plasma actuators

Single dielectric barrier discharge plasma actuators have been used to control flow separation in a large number of applications. An often used configuration involves spanwise-oriented asymmetric electrodes that are arranged to induce a tangential wall jet in the mean flow direction. For the best effect, the plasma actuator is placed just upstream of where the flow separation will occur. This approach is generally more effective when the plasma actuator is periodically pulsed at a frequency that scales with the streamwise length of the separation zone and the free-stream velocity. The optimum frequency produces two coherent spanwise vortices within the separation zone. It has been recently shown that this periodic pulsing of the plasma actuator could be sensed by a surface pressure sensor only when the boundary layer was about to separate, and therefore could provide a flow separation indicator that could be used for feedback control. The paper demonstrates this approach on an aerofoil that is slowly increasing its angle of attack, and on a sinusoidally pitching aerofoil undergoing dynamic stall. Short-time spectral analysis of time series from a static pressure sensor on the aerofoil is used to determine the separation state that ranges from attached, to imminent separation, to fully separated. A feedback control approach is then proposed, and demonstrated on the aerofoil with the slow angle of attack motion.     

Vortex motion in the early stages of unsteady flow around a circular cylinder

In this paper, processes in the early stages of vortex motion and the development of flow structure behind an impulsively-started circular cylinder at high Reynolds number are investigated by combining the discrete vortex model with boundary layer theory, considering the separation of incoming flow boundary layer and rear shear layer in the recirculating flow region. The development of flow structure and vortex motion, particularly the formation and development of secondary vortex and a pair of secondary vortices and their effect on the flow field are calculated. The results clearly show that the flow structure and vortices motion went through a series of complicated processes before the symmetric main vortices change into asymmetric: development of main vortices induces secondary vortices; growth of the secondary vortices causes the main vortex sheets to break off and causes the symmetric main vortices to become “free” vortices, while a pair of secondary vortices is formed; then the vortex sheets, after breaking off, gradually extend downstream and the structure of a pair of secondary vortices becomes relaxed. These features of vortex motion look very much like the observed features in some available flow field visualizations. The action of the secondary vortices causes the main vortex sheets to break off and converts the main vortices into free vortices. This should be the immediate cause leading to the instability of the motion of the symmetric main vortices. The flow field structure such as the separation position of boundary layer and rear shear layer, the unsteady pressure distributions and the drag coefficient are calculated. Comparison with other results or experiments is also made. This work was presented at the First Asian Congress of Fluid Mechanics, Bangalore in December 1980.     

Suppressing a Laminar Flow Separation Zone by Spark Discharge at Mach Number M = 1.43

The influence of flow perturbations generated by an electric discharge on the region of interaction between a shock wave and laminar boundary layer in the flow on a flat plate at a Mach number of M = 1.43 has been experimentally studied. The oblique shock wave generated by a wedge mounted above the plate induced separation of the flow, while perturbations in the flow were introduced by a spark discharge on the model plate surface. It is established that the discharge leads to the formation of turbulent and thermal spots. The turbulent spot suppresses the separation zone, while the thermal spot leads to a local increase in the boundary layer thickness in the interaction zone.

     

Rotating-disk-type flow over loose boundaries

Rotating-disk-type flow of a liquid over a loose boundary, such as a layer of sand, is investigated. For this flow the formation of a new large-scale spiral pattern has been discovered. The new pattern is reminiscent of the Type-I spiral-vortex structures which characterize the laminar–turbulent transition region of boundary layers over rigid rotating disks. Flow visualizations reveal that the new pattern and the Type-I spiral vortices co-exist in the loose-boundary flow. The research investigating the origin of the new large-scale pattern is reviewed. Then photographs from flow visualizations are analysed to obtain estimates for the critical Reynolds number for which Type-I spiral vortices first appear for the loose-boundary flow and for the critical Reynolds numbers for the laminar–turbulent transition of the boundary layer. The results suggest that Type-I vortices appear at much lower Reynolds numbers over loose boundaries in comparison with flow over rigid rotating disks and that transition also appears to be advanced to much lower Reynolds numbers. The discussion of the results suggests that advanced transition arises from disturbances introduced into the flow after the loose boundary has been mobilized and not from disturbances associated with the roughness that the surfaces of the granular layer represents to the flow while grains are at rest.     

Behaviour of spiral vortices on a rotating cone in axial flow

Summary The purpose of the present paper is to investigate experimentally in detail the boundary layer transition process and the behaviour of spiral vortices appearing in the transition range of the boundary layer on a 30°-cone, rotating in axial flow. Counterrotating spiral vortices in the transition range are visualized with a white smoke method, and observed the time dependent behaviour of them using a drum camera and a light sheet illumination method with a stroboscope flash light. The light passes a slit in order to illuminate only a thin sheet in the flow. With this method, the time dependent growing up and breaking down process of these spiral vortices is greatly clarified. A hot wire anemometer is also used for measuring in the flow field quantitatively. The results show that the spiral vortices are generated in the thin region of the steep shear velocity gradients near the wall. As the vortices grow up in z-direction, they are strongly distorted by the mean velocity field there, and finally they are teared off.With 7 Figures     

Flow visualization studies of vortices

An actual vortex in the Kármán vortex street downstream of a circular cylinder has a core of finite dimension which increases downstream. The circulation of the vortex is nearly constant. The ratiob/a which is 0.281 according to the theory of Kármán, grows from 0.2 to 0.4 in the near wake. In the flow about a circular cylinder rotating in a uniform flow, a Kármán vortex street, Görtler-type vortices and Taylor vortices are generated at the same time. In the flow about a circular cylinder impulsively started with a constant velocity, the primary twin vortices behind the cylinder induce secondary twin vortices near the separation point. At the beginning of the motion, the separation does not occur even though a reverse flow is observed in the boundary layer. Mutual slip-through of a pair of vortex rings was achieved by increasing the Reynolds number. A vortex ring rebounds from a plane surface due to the separation of the flow on the surface induced by the vortex ring, and the secondary vortex ring is formed from the separated shear layer.     

Laminar-turbulent transition in a supersonic boundary layer during initiation of a pulsed surface discharge

The near-surface layer structure in a supersonic airflow (M = 1.5) behind the plane shock wave has been experimentally studied in a shock tube. The flow structure was visualized using nanosecond pulsed distributed surface discharge. Different structures of plasma glow have been observed for the laminar and turbulent flow regimes in the boundary layer. The position of a region of the laminar-turbulent transition for different flow densities (0.11 and 0.19 kg/m³) has been determined and the critical Reynolds number (Re _k ～ 2.4 × 10⁵) for this transition has been evaluated.     

Pinning of planar vortices in a three-dimensional Josephson medium and the applicability of the Bean model

Calculations are made of a system of planar vortices formed at the boundary of a sample with a monotonically increasing external magnetic field with allowance for pinning caused by the cellular nature of the medium. The spacing between the vortices remains approximately constant and does not decrease near the boundary. This behavior indicates that the magnetic field does not depend on the coordinate in the region where the vortices have penetrated. These observations contradict the conventional Bean model. Pis’ma Zh. Tekh. Fiz. 23, 66–69 (June 12, 1997)     

A probe investigation of the structure of electrode microwave discharge in nitrogen

A probe investigation is performed of the structure of electrode microwave discharge in nitrogen at a pressure from 0.5 to 2.5 torr, incident power of up to 500 W, a power of 10 to 50 W absorbed in the system, and a gas flow rate from zero to 1000 cm³/min under normal conditions. It is demonstrated that, if the discharge diameter exceeds considerably that of antenna, the discharge is a close-to-spherical formation with a clearly defined boundary. The antenna is surrounded by a bright luminous film of plasma. A considerable nonuniformity of the parameters of the electron component of plasma and of constant potentials in plasma is demonstrated. It is possible that the discharge structure is related to the existence of a region of self-maintained discharge (luminous film surrounding the electrode) and a region of semi-self-maintained discharge (spherical region). The existence of considerable constant fields in the plasma of an electrode microwave discharge is found, whose emergence may be due to a nonlinear transformation of the microwave power maintaining the discharge. The discharge is surrounded by an isotropic region with decreasing electron temperature and concentration.     

A New Interpretation of Oscillating Flow Experiments in Superfluid Helium II

We consider some classic experimental data from oscillating superfluid helium II flows and reinterpret them in terms of our recent results on superfluid vortices in helium II boundary layer flows. We find evidence that the lowest critical velocity in these experiments is due to a boundary layer effect. We show that the critical velocities measured in the oscillating disk experiments agree well with the critical velocities for the existence of stable stationary superfluid vortices in the boundary layer flow.     

微型三角楔超声速绕流特性的研究

基于大涡模拟(LES)方法,结合WENO格式与自适应网格加密(AMR)技术及沉浸边界法(IBM),对来流马赫数为Ma =2.5条件下的平板上微型三角楔绕流流场进行了数值模拟。数值模拟表明微型三角楔涡流发生器可以显著改变超声速流体边界层结构。计算结果清晰地显示了三角楔上游分离区的流场结构和下游各涡的流态,同时计算表明,微楔对边界层控制过程中,其下游的流向涡对与涡环结构都起了重要作用,并对其作用过程进行了讨论。数值计算与相关实验结果相符,且提供了流场的重要细节,揭示了微楔的控制机理,可为超声速边界层控制研究提供重要支持。     

Numerical experiments on feedback EMHD control of large scale coherent structures in channel turbulence

Summary The current work focusses on the spatio-temporal evolution of large scale coherent structures in the turbulent boundary layer of a plane channel both with and without microtile-based EMHD control. The heuristic concept behind the microtile designs that we have simulated apparently does not yield a successful drag reduction strategy (for the open-loop case [1]). In this work we investigate the flow response when the Lorentz force is applied with feedback conditioned on the advection of large-scale flow structures (e.g. hairpin vortices). We performed a long-time simulation conditioned on the passage of strong ejection events but obtained no reduction in skin friction. Based on short-time simulations we found that the near-wall flow structures undergo merely a spatial phase-shift when advecting above a single Lorentz force actuator: the structures are simply decelerated or accelerated with little change in their appearance, based on flow visualization. During this interaction of the applied Lorentz force with the flow, the Reynolds stress is unchanged.     

电磁激活板的宽度对圆柱绕流控制的影响

电磁力可以改变流体边界层的结构,是控制流体运动的主动控制方法之一。基于电磁场和流体的基本方程,对置于弱电介质中的圆柱电磁激活板周围产生的Lorentz力及其对圆柱绕流的控制进行了数值模拟,着重讨论了电磁激活板的宽度对其周围的电磁场、产生的Lorentz力、流场的控制和涡量变化的影响。电磁场包覆范围为流体分离点至其后部,当N值较小时,分离点后移,但不能够完全抑制流体的分离,极板越宽对尾涡的抑制效果越好;随着N值的增大,由于极板窄的表面涡量大,所以可以首先达到完全抑制流体分离的控制效果;当N值较大时,无论极板宽窄,都可以达到完全抑制流体分离的效果。     

Boundary layer separation and vortex creation in superflow through small orifices

We report on numerical solutions of the Gross-Pitaevskii equation for twodimensional flow of a super fluid condensate through a small orifice. When the super fluid velocity in the center of the orifice exceeds about 60 % of the speed of sound, the flow in the throat of the orifice becomes unstable to a form of boundary layer separation. Low condensate-density regions bulge away from the walls and form the cores of singly quantized vortices. These detach from the boundary and are convected downstream.     

Direct resonance analysis and modeling for a turbulent boundary layer over a corrugated surface

Summary Effects of surface corrugation on turbulent flow in a boundary layer are studied using a model based on the direct resonance theory. The induced mean flow due to weakly nonlinear waves, superimposed on the mean and fluctuating components of turbulence, is determined. The mean turbulent flow is affected by the surface corrugation throughout the boundary layer. The corrugated surface generates higher harmonics and affects the streamwise vortices generated by the waves superimposed on turbulence whose mean flow includes secondary induced mean flow components due to the corrugation.     

Radial structure of low pressure rf capacitive discharges

This paper studies the glow intensity distribution of the discharge plasma against the tube radius and reports the radial profiles of electron temperature and plasma concentration in the rf capacitive discharge registered with a Langmuir probe. An abrupt increase of electron temperature and glow intensity near the tube wall in the weak-current α-mode of the rf capacitive discharge is revealed, the radial distribution of plasma concentration and ion flow to the electrodes possessing a maximum near the radial sheath boundary. In the γ-mode of the rf capacitive discharge the electron temperature decrease in the total plasma volume leads to an electric field weakening and the peak of the glow intensity near the tube wall vanishes. The radial sheath thickness in the α-mode of the rf capacitive discharge obtained with 2D simulation experiences pulsations during the rf field period, the changing radial electric field heating electrons and increasing the plasma concentration near the boundary of the radial sheath.     

驻波场中圆柱形换热管外声流特性的数值研究

基于非线性Navier-Stokes方程推导了一般性的声流控制方程,并利用有限元软件COMSOL对平面驻波声场中单根圆柱形换热管和双圆柱形换热管外包含二阶流场信息的Navier-Stokes方程进行全域求解,得到了一阶和二阶流场信息。研究了雷诺数Re和斯特劳哈尔数Sr对换热管外声流结构的影响规律。研究发现:随Re和Sr的增大,换热管边界层内的涡流区域变小,直至消失,而边界层外的涡流区域逐渐增大;且单换热管外涡流个数由8个减少到4个,双换热管外涡流个数从12个减少到8个。此外,边界层内和边界层外的涡流结构呈反向旋转;边界层外流场沿着振荡方向远离换热管,而在垂直方向流向换热管。     

Vorticity evolution mechanism in near‐field of a transverse jet

Abstract

Flow structure and vorticity evolution processes in the near field of an elevated jet in a crossflow are experimentally studied in a wind tunnel. The instantaneous and time‐averaged flow field characteristics are observed and measured by using a flow visualization technique and a high‐speed Particle Image Velocimeter (PIV). Time histories of the instantaneous velocity of the vortical flows in the shear‐layer are recorded by a hot‐wire anemometer and a high‐speed data acquisition system in order to analyze the frequency characteristics of the traveling coherent structure in the shear‐layer. Experiments are performed between two different jet‐to‐crossflow momentum flux ratios R = 0.08 and 0.56, which are selected from two regimes with different kinds of flow patterns at a fixed crossflow Reynolds number 2051. The behaviors and mechanisms of the vortical flow structure and the vorticity evolution mechanisms appear to be distinct in different flow regimes. By analyzing the pictures of the smoke flow visualization and the instantaneous vorticity contour maps, two kinds of vorticity evolution mechanisms, “shear‐induced vortices” and “swing‐induced vortices”, can be identified in the shear‐layer evolving from the jet exit. The time‐averaged velocity field and vorticity properties are also discussed in this paper.     

Simulation of a DC Discharge in a Transverse Supersonic Gas Flow

This study is devoted to the investigation of a dc discharge in a transverse gas flow. It is shown that the discharge may exist in several forms depending on the gas flow velocity. The standard stationary discharge similar to a discharge in still gas is realized if the displacement rate of the plasma boundary exceeds the gas flow velocity. The displacement rate of the plasma boundary in a diffusion model is defined by the relation V _f = 2 , where D _a is the ambipolar diffusion coefficient, and _i is the frontal ionization frequency. Otherwise, the discharge assumes the form of two plasma wakes formed by the cathode and anode, respectively. The surface of the plasma wakes is oriented at an angle to the flow velocity C _s (sin = V _f /C _s ). If sin is smaller than the ratio of the discharge sustaining voltage in the stationary regime U _st to the breakdown voltage U _bk , the discharge transforms into the pulse–periodic form, when the formation of a structure of the cathode and anode plasma wakes is interrupted by a new gas breakdown. A numerical simulation of the discharge properties is performed. The numerical simulation results are compared to the experimental ones.     

Coherent helical structures in swirl flows

A nonstationary flow with rotating helical filaments formed in the region of swirl flow breakdown has been studied using the method of phase averaging of the output signal of a laser Doppler velocimeter. Vortices in the measured flow field were separated using the λ₂ technique based on an analysis of the velocity gradient tensor. It is shown for the first time that precession of the central monopolar vortex is accompanied by the formation of a pair of secondary vortices with the opposite directions of circulation, which form a helical dipole structure.