Broadband Disturbances Research Articles

Transition delay in a Mach 6 boundary layer using steady blowing and suction strips

Direct numerical simulations (DNS) were carried out to investigate flow control for transition delay using steady blowing/suction strips at the wall of a flared cone at Mach 6 and zero angle of attack. For the numerical investigations of the transition control strategy, the flared cone geometry and the flow conditions of the experiments in the Boeing/Air Force Office of Scientific Research (AFOSR) Mach 6 Quiet Tunnel (BAM6QT) at Purdue University were chosen. For the DNS, transition was initiated by introducing random disturbances at the inflow of the computational domain, emulating ‘natural’ transition in wind-tunnel experiments caused by free-stream noise. In both wind-tunnel experiments and numerical simulations, streamwise ‘hot’ streaks were found on the surface of the flared cone, which are caused by a nonlinear interaction of an axisymmetric second-mode wave and a pair of oblique waves of the same frequency (‘fundamental resonance’). The objective of the flow control strategy proposed here is to delay the transition onset, and thus mitigate the negative consequences associated with the nonlinear transition stages, i.e. the development of hot streaks and large wall-pressure amplitudes that were observed in experiments and DNS. Our previous so-called ‘controlled’ transition simulations have shown that flow control using steady blowing and suction strips can lead to a significant delay of the hot streak development on the surface of the flared cone. The simulation results presented in this paper show that this flow control strategy remains effective, even in a natural transition scenario characterized by broadband disturbances.

Effect of local wall temperature on hypersonic boundary layer stability and transition

Wall temperature significantly affects stability and receptivity of the boundary layer. Changing the wall temperature locally may therefore be an effective laminar flow control technique. However, the situation is complicated when the wall temperature distribution is nonuniform, and researchers have experimentally found that local wall cooling may delay the onset of transition. We attempt to clarify the physical mechanisms whereby the local wall temperature affects the transition and the stability of a hypersonic boundary layer. A numerical investigation of the disturbance evolution in a Mach-6 sharp cone boundary layer with local wall heating or cooling is conducted. Direct numerical simulation (DNS) is performed for the single-frequency and broadband disturbance evolution caused by random forcing. We vary the local wall temperature and the location of heating/cooling, and then use the e N method to estimate the transition onset. Our results show that local wall cooling amplifies high-frequency unstable waves while stabilizing low-frequency unstable waves, with local heating amplifying all unstable waves locally. The disturbance amplitude and second-mode peak frequency obtained by DNS agree well with the previous experimental results. Local cooling/heating has a dual effect on the stability of the hypersonic boundary layer. For local cooling, while it effectively inhibits the growth of the low-frequency unstable waves that dominate the transition downstream, it also further destabilizes the downstream flow. In addition, while upstream cooling can delay the transition, excessive cooling may promote it; local heating always slightly promotes the transition. Finally, recommendations are given for practical engineering applications based on the present results.

Nonlinear active disturbance rejection control for deformable mirror

The disturbance in the adaptive optical system includes not only the low-frequency broadband disturbance but also the high-frequency narrow-band disturbance caused by the vibration of system components and atmospheric turbulence. A nonlinear active disturbance rejection control (NLADRC) method is proposed to solve the control problem of a deformable mirror under high-frequency narrow-band disturbance. This method does not completely depend on the mathematical model, and the algorithm structure is simple. The differential tracker flattens the abrupt part of the signal, and the extended state observer is used to estimate the disturbance in real time. Finally, the nonlinear control rate is used to compensate the disturbance. The simulation results show that compared with the classical PID control and the classical linear active disturbance rejection control, the NLADRC method can effectively improve the bandwidth of the control system and show good suppression performance against high-frequency narrow-band disturbance, and the system is more robust. Both in time domain and frequency domain, it can accurately track the system state, reduce the system delay and system error, and has better dynamic performance.

Number vs size of electro-mechanical tuneable vibration absorbers for aeronautical applications: a case study

This paper presents a case study on the tuning and flexural vibration control effects generated either by many-small-scale or few-large-scale electromagnetic tuneable vibration absorbers on a model section of an aircraft fuselage structure. The absorbers are formed by a coil-magnet seismic transducer connected to a tuneable resistive-inductive shunt. The control of both tonal and broad-band disturbances is investigated with reference to the number, and thus the size, of the units. The principal aim of the paper is to address the key question on whether, for a fixed total weight of the absorbers, it would be preferable to have a small number of comparatively larger and heavier units or a greater number of comparatively smaller and lighter units. To this end, the first part of the paper presents a comprehensive scaling analysis based on fundamental principles of the physical properties and tuning laws of the electromagnetic tuneable vibration absorbers. Then, the second part of the paper presents a simulation study on the optimal-tuning and the vibration control effects of setups with 4, 8, 16, 32 electromagnetic tuneable vibration absorbers whose total weight is kept equal to 6% of the weight of a model fuselage structure. The study shows that arrangements with many small-scale units produce larger vibration control effects than setups formed by comparatively fewer, larger scale, units. However, the operation frequency range of the EM-TVAs tends to shift to higher frequencies with downsizing. Moreover, the size of the optimal shunt inductance that would guarantee the tuning of the EM-TVAs tends to grow as the unit is downscaled such that an artificially large inductor may be required.

Stabilization effect of acoustic metasurfaces on broadband disturbances in a Mach 6 boundary-layer flow

In this study, acoustic metasurfaces are optimized to suppress broadband frequencies of unstable modes in a Mach 6 boundary-layer flow. The metasurface is cut into five pieces, and geometrical parameters of each piece are optimized to afford a comprehensive admittance response to the local frequencies of the first and second modes. It shows the designed broadband metasurface not only is advantageous for stabilizing disturbances above the metasurface region but also inhibits the fast growth rate of low-frequency disturbances on the downstream smooth wall.

Robust feedback controller combined with the remote microphone method for broadband active noise control in headrest

This paper proposes a design method combining the remote microphone method (RMM) with a robust fixed feedback controller to achieve active noise control in a headrest with physical microphones situated far from the target zone. Design objectives of the controller are based on a mixed H2/H∞ method and the proposed closed-loop gain margin constraint, which consider noise reduction performance, waterbed lift, and robustness against plant perturbations due to slight rotations of the head. Meanwhile, the design objectives are constructed with RMM to create a local zone of quiet far away from the physical monitoring microphone. Particle swarm optimization algorithm is employed for this multi-objective design problem to efficiently generate the feedback controller consisting of cascade second-order sections infinite-impulse-response (IIR) filters. Simulations and real-time experiments were conducted to demonstrate the superior noise reduction performance and robust stability of the designed IIR feedback controller in attenuating broadband disturbances measured in a vehicle.

The effects of nose bluntness on broadband disturbance receptivity in hypersonic flow

While nose bluntness is known to have a large impact on the stability of hypersonic vehicles, its influence on the freestream receptivity process has not been fully characterized for a wide range of conditions. This paper investigates the effects of nose bluntness on the second mode receptivity coefficients and the development of boundary layer disturbances over two 7° half-angle circular blunt cones at Mach 10 after perturbation with broadband freestream pulses of different types. The cones have nose radii of 9.525 mm (case B) and 5.08 mm (case I). Unsteady direct numerical simulation (DNS) and linear stability theory (LST) results compare well and predict stronger second mode growth for case I in all pulse cases. Unsteady DNS also shows variations in extramodal excitation between the cones depending on freestream disturbance type. Spectral receptivity coefficients are generated by decomposing the unsteady DNS data into discrete frequency Fourier modes, which are then corrected with LST N-factors. Fast acoustic disturbances demonstrate minimal variation in receptivity coefficients, while temperature and vorticity disturbances have much higher coefficients in case I. Planar slow acoustic pulses induce stronger disturbances outside of the second mode in case I, resulting in higher peak receptivity coefficients. Results show significant variation in receptivity response based on nose bluntness, pulse geometry, and the type of incident perturbation.

Смещение предела устойчивости течений при наличии случайных флуктуаций скорости вращения

Nonlinear stability of the isothermal three-dimensional flows of viscous incompressible fluid in the rotational spherical layer with the noise presence was studied numerically. Transition between stable non-stationary flow and unstable flow in the form of travelling azimuthal waves is under consideration. Small-amplitude noise inserted to the flow by random broadband disturbances of the inner sphere rotational rate about constant at time averaged value, outer sphere is fixed. An approach is proposed to simplify finding the critical Reynolds number, corresponding to the stability limit in the presence of noise. The results obtained by the proposed and well known methods are compared.

Shifting the flow stability limit in the presence of random fluctuation of the rotational velocity

Nonlinear stability of the isothermal three-dimensional flows of viscous incompressible fluid in the rotational spherical layer with the noise presence was studied numerically. Transition between stable non-stationary flow and unstable flow in the form of travelling azimuthal waves is under consideration. Small-amplitude noise inserted to the flow by random broadband disturbances of the inner sphere rotational rate about constant at time averaged value, outer sphere is fixed. An approach is proposed to simplify finding the critical Reynolds number, corresponding to the stability limit in the presence of noise. The results obtained by the proposed and well known methods are compared. Keywords: noise, rotational flows, spherical Couette flow, instability control.

Laser beam jitter control of the link in free space optical communication systems

The pointing and tracking performance of laser beams in free space optical communication (FSOC) systems and other precision laser systems is severely limited by beam jitter. This research proposes an adaptive control method to suppress beam jitter by adjusting the deflection angle of the beam by controlling the fast steering mirror (FSM) driven by piezoelectric ceramics. In order to verify the effectiveness of the control method, we have established an experimental platform. First, the internal controller of the FSM is adjusted to an approximately linear system, aiming to turn the input and output control signals into a linear relationship. Based on the adaptive filter, a filtered-x variable step-size normalized least mean square (FxVSNLMS) algorithm is proposed. To further improve the robustness of the controller, a Proportion Integral Differential (PID) controller and an adaptive controller which work in parallel are then added to the control loop. Finally, experimental results are provided and compare with the traditional controller and other adaptive control methods. Experimental results show that for more complex narrow-band and broad-band mixed beam disturbances, the control scheme developed in this research is significantly improved. This conclusion has also been verified on the ground experimental platform of FSOC.

Disturbance observer-based repetitive control with application to optoelectronic precision positioning system

Rejection of periodic disturbance and/or tracking of periodic reference is of importance in high-precision control systems. Conventional repetitive control is often used to solve the problem, but it cannot precisely set effective frequency points and adversely amplify the non-periodic component disturbance. Therefore, it is not applicable in actual systems where external disturbances exist in the whole frequency domain. In this paper, we propose an improved discrete-time repetitive control method based on the disturbance observer to correct the undesired deviation at repetitive frequencies and mitigate the amplification of the non-components. Moreover, in the observer structure, an intuitive and flexible Q-filter design is presented to suppress low-frequency broadband and intermediate-frequency narrowband disturbances. The conditions of closed-loop stability, performance analysis, and the implementation of the proposed scheme are provided in detail. Finally, the effectiveness of the method is verified by simulation and experimentation on an optoelectronic precision positioning system under the condition of disturbances, and the disturbance suppression and tracking error attenuation are improved.

Thermal Trigger for Solar Flares I: Fragmentation of the Preflare Current Layer

We consider the effects of the heat balance on the structural stability of a preflare current layer. The problem of small perturbations is solved in the piecewise homogeneous MHD approximation taking into account the viscosity, the electrical and thermal conductivity, and the radiative cooling. Solution of the problem allows the formation of an instability of a thermal nature. There is no external magnetic field inside the current layer in equilibrium state, but it can penetrate inside when the current layer is disturbed. Formation of a magnetic field perturbation inside the layer creates a dedicated frequency in a broadband disturbance subject to thermal instability. In the linear phase, the growth time of the instability is proportional to the characteristic time of radiative cooling of plasma and depends on the logarithmic derivatives of the radiative cooling function with respect to the plasma parameters. The instability results in transverse fragmentation of the current layer with a spatial period of 1-10 Mm along the layer in a wide range of coronal plasma parameters. The role of that instability in the triggering for the primary energy release in solar flares is discussed.

Improved stability performance of a feedback active noise control using a Steiglitz-McBride adaptive notch filter and robust secondary path identification based on variable step size Griffiths LMS algorithm

A stable feedback active noise control (FBANC) system with an improved performance in a broadband disturbance environment is proposed in this article. This is achieved by using a Steiglitz-McBride adaptive notch filter (SM-ANF) and robust secondary path identification (SPI) both based on variable step size Griffiths least mean square (LMS) algorithm. The broadband disturbance severely affects not only FBANC input synthesized but also the SPI.TheSM-ANFestimated signal has narrowband component that is utilized for the FBANC input synthesis. Further, the SM-ANF error has broadband component utilized to get the desired signal for SPI. The use of variable step size Griffiths gradient LMS algorithm for SPI enables the removal of broadband disturbance and non-stationary disturbance from the available desired signal for better SPI. For a narrowband noise field, the proposed FBANC improves the convergence rate significantly (20 times) and the noise reduction from 10 dB to 15 dB (50%improvement) over the conventional FBANC (without SM-ANF and variable step size Griffiths LMS adaptation for SPI).

Selectivity of plasma actuators in a boundary layer transition control applications

The paper describes the results of the study of broadband velocity pulsations, induced by dielectric barrier discharge plasma actuator in the 2D subsonic boundary layer, and their role in a boundary layer excitation. The origin of the velocity pulsations is the stochastic dynamics of microdischarges. The presented data include the parametric study of the disturbances power on the supply voltage characteristics and their structure. Also, the preliminary study of the role of the broadband disturbances in the Tollmien-Schlichting wave excitation was performed.

Comparison of smart panels for tonal and broadband vibration and sound transmission active control

ABSTRACT This paper presents a comprehensive overview of the principal features of smart panels equipped with feed-forward and feedback systems for the control of the flexural response and sound transmission due respectively to tonal and to stochastic broadband disturbances. The smart panels are equipped with two types of actuators: first, distributed piezoelectric actuators formed either by small piezoelectric patches or large piezoelectric films bonded on the panels and second, point actuators formed by proof-mass electromagnetic transducers. Also, the panels encompass three types of sensors: first, small capacitive microphone sensors placed in front of the panels; second, distributed piezoelectric sensors formed by large piezoelectric films bonded on the panels and third point sensors formed by miniaturized accelerometers. The proposed systems implement both single-channel and multi-channel feed-forward and feedback control architectures. The study shows that, the vibration and sound radiation control performance of both feed-forward and feedback systems critically depends on the sensor-actuator configurations.

Active headrests with selective delayless subband adaptive filters in an aircraft cabin

Active noise control (ANC) systems try to attenuate noise by generating local zones of quiet. In this paper local ANC systems in a relevant environment are investigated, usingry conditions derived from environments like cars, planes or trains. The experimental setup consists of a wooden aircraft cabin mock-up and an active headrest system using two actuators (loudspeakers) and two sensors (microphones) placed in a headrest to create individual local zones of quiet at the ears of the occupant.Various concepts for active headrest systems are taken into account, such as multichannel controller concepts, virtual sensing techniques and delayless subband adaptive filters, focusing on the delayless subband adaptive filters. Based on these studies a new active headrest system is proposed, that is capable of generating a 10 dB zone of quiet for different non-stationary broadband disturbances, while compensating head movement. Using a selective delayless subband adaptive filter and off the shelf microphones and loudspeakers, the overall hardware demand is kept to a minimum allowing for a low cost application.

Numerical research on airfoil transition delay by alternative current dielectric barrier discharge actuation

A Dielectric Barrier Discharge (DBD) plasma actuator can create a body force which locally accelerates the base flow leading to an attenuation of broadband disturbance to delay the transition. In this study, numerical simulation on an NLF0416 airfoil is conducted to investigate transition delay and drag reduction by a DBD plasma actuator. To simulate plasma’s effect more accurately, boundary-layer data is acquired from Reynolds Averaged Navier Stocks (RANS) equations instead of laminar boundary layer equations, although RANS equations need a much finer boundary-layer grid, and the linear stability analysis method is used to analyze the boundary layer and get the transition point. In this study, the influences of different actuation intensities and positions are investigated, and results show that if the actuation intensity is stronger and the actuation position is closer to the base transition point, more drag reduction can be obtained. However, the efficiency of plasma transition delay is really low. For example, when the actuation voltage is 16 kV, the actuation frequency is 1 kHz, and the main Mach number is 0.1, the saved power due to drag reduction is about 5.09 W, but the power consumed is about 32.61 W, and the efficiency is just 15.6%.

Modular Vibration Control Unit Formed by an Electromagnetic Proof-Mass Transducer and Sweeping Resistive–Inductive Shunt

Abstract This paper presents a vibration control unit formed by an electromagnetic proof-mass transducer connected to a sweeping resistive–inductive (RL)-shunt, which can be used to control broadband flexural vibrations of thin structures. The shunt is composed of a resistor and an inductor in series, whose values vary harmonically in time. The design and practical implementation of an electromagnetic transducer and harmonically varying shunt is first discussed. The unit is then tested on a thin-walled cylinder exposed to a broadband disturbance, considering two operation modes: fixed and sweeping RL-shunts. The former mode sets the unit to control the resonant response of a target flexural mode of the cylinder. The latter mode sets the unit to control the resonant responses of multiple modes of the cylinder with natural frequencies confined in a target frequency band. The study demonstrates the practical feasibility of a unit, which sweeps the natural frequency and the damping ratio of the transducer between 36 Hz and 187 Hz and between 4% and 60%. Also, it shows the unit generates broadband control of the flexural vibration of a cylinder, with reductions of the peak responses of the natural modes resonating in frequency band of the sweep comprised between 2 and 13 dB.

Multiple model hybrid adaptive vibration control for flexible cantilever beam with varying load

To solve the vibration suppression problem for a piezoelectric flexible cantilever beam with varying load, this study proposes a new type of multiple model hybrid adaptive vibration control method with a new multiple model switching cost index function. It employs a pre-filter consisting of secondary path and positive feedback path model to eliminate the positive feedback between the secondary actuator and reference sensor. By constructing a piezoelectric flexible cantilever beam as the controlled plant, we constructed a real-time control verification experiment platform based on a real-time MATLAB xPC target. We did a real-time control experiment comparison for vibration suppression of single-frequency, narrowband, and broadband disturbances. Experimental results show that the proposed multiple model hybrid adaptive vibration control algorithm is feasible and has good vibration suppression performance with rapid convergence speed.

Radiated EMC Kron's Model of 3-D Multilayer PCB Aggressed by Broadband Disturbance

This paper focuses on novel modeling of radiated electromagnetic compatibility (EMC) coupling onto the multilayer printed circuit board (PCB). Kron's method integrates the electromagnetic (EM) emission, Taylor's, and field-to-interconnect coupling models. The equivalent graph of the field-to-interconnect coupling is established. The modeling methodology consists in defining the primitive sub-network elements (vias, interconnect lines, pads, and ground plane). Kron's graph equivalent to the EMC problem is elaborated. Finally, the coupling voltages are calculated via the tensorial equation translated from the graph. The radiated EMC Kron's model is validated with a four-layer PCB from 0.4 to 1.4 GHz by two scenarios of EM radiation. As proof-of-concept, a prototype of four-layer PCB was designed, fabricated, tested, and simulated in full wave with a commercial three-dimensional EM tool. For the first case, the multilayer PCB was illuminated by plane wave emission propagating in different directions. The numerical computation from Kron's formalism was compared with simulation and measurement. The other case is the field-to-interconnect coupling between a microstrip I-line PCB, as an EM field emitter, and the multilayer PCB, as a receiver, in 1-m distance. For both cases, the simulated and calculated voltage couplings onto the multilayer PCB are in good agreement.