Spinning Mode Research Articles

Numerical study of three-dimensional detonation structure transformations in a narrow square tube: from rectangular and diagonal modes into spinning modes

Three-dimensional (3-D) detonation structure transformations from rectangular and diagonal modes into spinning modes in a narrow square tube are investigated by high-resolution simulation. Numerical simulations are performed with a Riemann solver of the HLLC-type, new cell-based structured adaptive mesh refinement data structure, high-order, parallel adaptive mesh refinement reactive flow code. A simplified one-step kinetic reaction model is used to reveal the 3-D detonation structure. The four different types of initial disturbances applied in the ZND profiles lead to the structures of rectangular in phase, rectangular out of phase, rectangular partial out of phase and diagonal, respectively, during the initial stages of detonation propagation. Eventually, all these detonation structures evolve into the self-sustained spinning detonations. The asymmetric disturbance leads to a stable spinning detonation much faster than the rest. The important features in the formation of spinning detonation are revealed using a 3-D visualization, and a remarkable qualitative agreement with experimental and numerical results is obtained with respect to the transverse wave dynamics and detonation front structures. The transverse wave collisions produce the unburnt gas pockets and the energy to sustain the detonation front propagation and distortion. The periodic pressure oscillation of front plays a complex role as it shifts the reaction zone structure with an accompanying change in the driving energy of transition and the detonation parameters which result in the more distorted front and the unstable detonation. Eventually, the unstable distorted detonation evolves into a spinning detonation.

Acoustic interaction as a primary cause of infrasonic spinning mode generation and propagation from wind turbines

Relatively balanced load and velocity related pressure waves from the rearward facing surface of each rotor blade, are at a frequency of 1 cycle per revolution of the turbine and are phase shifted by 120 degrees from each other. The superposition of these infrasonic waves destructively interfere. This action results in a non-propagating rotor locked mode; however, the shielding (reflecting) effect of the tower as each blade passes, interrupts the balanced destructive interference for a small portion of rotor angle three times per revolution. The momentary un-balance between the destructive interfering waves results in the generation of Tyler-Sofrin spinning mode series, which propagate into the far field. The spinning mode radiation angles, coupled with the low decay rate of infrasound, result in higher far field sound pressure levels than would be predicted for a point source. An analysis approach partially derived from Tyler-Sofrin (1962) is presented. Field microphone data including phase measurements identifying spinning modes are also presented.

Visual Measurement of the Racket Trajectory in Spinning Ball Striking for Table Tennis Player

A high-speed monocular vision system is established to track the racket trajectory for estimating the spinning ball initiated by a human. A new fast and robust method is proposed to extract the feature sequence of a racket's pose during the process of striking. Considering the complexity of the working environment and fast movement of the racket, a novel corner extraction algorithm with good robustness and high efficiency is presented, which mainly consists of two parts, i.e., target segmentation and line detection. Then PnP-based algorithm is adopted to get the rough pose of the racket, which is then optimized by orthogonal iteration algorithm to guarantee the orthogonality of the racket's orientation matrix. In order to classify the spinning mode of the ball with a higher accuracy, the racket's movement is represented by the sequence of normal vector of the racket's surface. Experiments are thoroughly conducted to verify the effectiveness of the proposed method.

Azimuthal instabilities in annular combustors: standing and spinning modes

This theoretical study investigates spinning and standing modes in azimuthally symmetric annular combustion chambers. Both modes are observed in experiments and simulations, and an existing model predicts that spinning modes are the only stable state of the system. We extend this model to take into account the effect that the acoustic azimuthal velocity, u , has on the flames, and propose a phenomenological model based on experiments performed on transversely forced flames. This model contains a parameter, δ , that quantifies the influence that the transversal excitation has on the fluctuating heat release. For small values of δ , spinning modes are the only stable state of the system. In an intermediate range of δ , both spinning and standing modes are stable states. For large values of δ , standing modes are the only stable state. This study shows that a flame’s response to azimuthal velocity fluctuations plays an important role in determining the type of thermoacoustic oscillations found in annular combustors.

Modal dynamics of self-excited azimuthal instabilities in an annular combustion chamber

In this paper we describe the time-varying amplitude and its relation to the global heat release rate of self-excited azimuthal instabilities in a simple annular combustor operating under atmospheric conditions. The combustor was modular in construction consisting of either 12, 15 or 18 equally spaced premixed bluff-body flames around a fixed circumference, enabling the effect of large-scale interactions between adjacent flames to be investigated. High-speed OH∗ chemiluminescence imaged from above the annulus and pressure measurements obtained at multiple locations around the annulus revealed that the limit cycles of the modes are degenerate in so much as they undergo continuous transitions between standing and spinning modes in both clockwise (CW) and anti-clockwise (ACW) directions but with the same resonant frequency. Similar behaviour has been observed in LES simulations which suggests that degenerate modes may be a characteristic feature of self-excited azimuthal instabilities in annular combustion chambers. By modelling the instabilities as two acoustic waves of time-varying amplitude travelling in opposite directions we demonstrate that there is a statistical prevalence for either standing m=1 or spinning m=±1 modes depending on flame spacing, equivalence ratio, and swirl configuration. Phase-averaged OH∗ chemiluminescence revealed a possible mechanism that drives the direction of the spinning modes under limit-cycle conditions for configurations with uniform swirl. By dividing the annulus into inner and outer annular regions it was found that the spin direction coincided with changes in the spatial distribution of the peak heat release rate relative to the direction of the bulk swirl induced along the annular walls. For standing wave modes it is shown that the globally integrated fluctuations in heat release rate vary in magnitude along the acoustic mode shape with negligible contributions at the pressure nodes and maximum contributions at the pressure anti-nodes.

Acoustic imaging of a duct spinning mode by the use of an in-duct circular microphone array

An imaging method of acoustic spinning modes propagating within a circular duct simply with surface pressure information is introduced in this paper. The proposed method is developed in a theoretical way and is demonstrated by a numerical simulation case. Nowadays, the measurements within a duct have to be conducted using in-duct microphone array, which is unable to provide information of complete acoustic solutions across the test section. The proposed method can estimate immeasurable information by forming a so-called observer. The fundamental idea behind the testing method was originally developed in control theory for ordinary differential equations. Spinning mode propagation, however, is formulated in partial differential equations. A finite difference technique is used to reduce the associated partial differential equations to a classical form in control. The observer method can thereafter be applied straightforwardly. The algorithm is recursive and, thus, could be operated in real-time. A numerical simulation for a straight circular duct is conducted. The acoustic solutions on the test section can be reconstructed with good agreement to analytical solutions. The results suggest the potential and applications of the proposed method.

"Twin Spin"

We have built haptic interfaces featuring smartphones and tablets that use magnetometerderived orientation sensing to modulate virtual displays, especially spatial sound, allowing, for instance, each side of a karaoke recording to be separately steered around a periphonic display. Embedding such devices into a spinnable affordance allows a "spinning plate"- style interface, a novel interaction technique. Either static (pointing) or dynamic (spinning) modes can be used to control "whirled" multimodal display, including a rotary motion platform, panoramic movies, and the positions of avatars in virtual environments.

Three-dimensional cellular structure of detonations in suspensions of aluminium particles

Recently, we have used scarce available data on the detonation cell size in suspensions of aluminium particles in air and oxygen to adjust the kinetic parameters of our two-phase model of detonations in these mixtures. The calculated detonation cell width was derived by means of two-dimensional (2D) unsteady simulations using an assumption of cylindrical symmetry of the flow in the tube. However, in reality, the detonation cells are three-dimensional (3D). In this work, we have applied the same detonation model which is based on the continuous mechanics of two-phase flows, for 3D numerical simulations of cellular detonation structures in aluminium particle suspensions in oxygen. Reasonable agreement on the detonation cell width was obtained with the aforementioned 2D results. The range of tube diameters where detonations in $$\text{ Al/O}_2$$ mixture at a given particle size and concentration would propagate in the spinning mode has been estimated (these results make a complement to our previous analysis of spinning detonations in Al/air mixtures). Coupling these results with the dependencies of detonation cell size on the mean particle diameter is of great interest for the understanding of fundamental mechanisms of detonation propagation in solid particle suspensions in gas and can help to better guide the experimental studies of detonations in aluminium suspensions. It is shown that the part of detonation wave energy used for transverse kinetic energy of both gas and particles is quite small, which explains why the propagation velocity of spinning and multi-headed detonations reasonably agrees with the ideal CJ values.

High resolution WENO simulation of 3D detonation waves

In this paper, we develop a three-dimensional parallel solver using the fifth order high-resolution weighted essentially non-oscillatory (WENO) finite difference scheme to perform extensive simulation for three-dimensional gaseous detonations. A careful study is conducted for the propagation modes of three-dimensional gaseous detonation wave-front structures in a long square duct. The numerical results indicate that, the instability of detonation, overdrive factor, transverse dimension and initial perturbation are the main factors that influence the appearance of spinning detonation. For unstable detonation with an overdrive factor f=1.0, with the duct width decreasing, the transverse modes tend to be the lower ones. When the duct width is sufficiently small, detonation propagation is coupled to the lowest transverse mode in order to sustain itself. There is a maximum duct width for the appearance of spinning detonation. When the duct width is smaller than this value, the unstable detonation propagates in a spinning mode. When the duct width is larger than this value, spinning detonation disappears. For highly unstable detonation with an overdrive factor f=1.2, the formation of the spinning detonation does not depend on the initial perturbation and it only depends on the high instability of detonation and channel dimension in the transverse directions. Both initial rectangular modes and initial diagonal modes can eventually evolve into spinning modes. For mildly unstable detonation with an overdrive factor f=1.6, it is easier for the initial diagonal mode to trigger the spinning mode than for the initial rectangular mode. The formation of spinning detonation thus not only depends on the high instability of detonation and channel dimension in the transverse directions, but also depends on the initial perturbation. For the stable detonation with an overdrive factor f=1.1, when the initial perturbation is in the transverse cosine mode, the front of the stable detonation has a rectangular structure and slapping waves appear on the walls. When the initial perturbation is in the symmetrical mode along the diagonals of the detonation front, the diagonal detonation is formed and the slapping waves on the walls disappear.

Three-Dimensional Parallel Simulation of Formation of Spinning Detonation in a Narrow Square Tube

Four different formation processes of spinning detonation in a narrow square tube are investigated by three-dimensional parallel simulations. High-resolution simulations are performed with a Riemann solver of the HLLC-type, high-order, parallel AMR reactive flow code. The simulations clearly show that the detonation structure transforms from rectangular and diagonal modes into spinning mode respectively during the propagation processes. Transverse wave dynamic and periodic oscillation of the front plays a significant role in the spinning detonations' formation.

An investigation of plate-type windborne debris flight using coupled CFD–RBD models. Part II: Free and constrained flight

A coupled Computational Fluid Dynamics–Rigid Body Dynamics (CFD–RBD) model is presented, which is capable of modelling the 3D flight of plate-type windborne debris. The RBD model is based on rotational quaternions, which means that complex spinning modes of flight can be modelled accurately and robustly. For debris that is free to fly in the absence of obstructions, it is shown that, depending on the initial plate orientation, a number of distinct flight modes are observed. When the plate is aligned at right angles to the flow, the flight mode depends critically on the initial angle of attack. The plate may wobble or flutter as it flies, never completing a full rotation, or it may enter an autorotational flight mode. Depending on the direction of rotation, the plate is capable of flying faster than the wind. It does this by converting some of its potential energy into rotational energy. The rotation of the plate causes the forces acting on the plate to further accelerate or decelerate it, depending on the rotation direction. When the plate is not aligned at right angles to the flow, the plate always enters a complex 3D spinning mode, with significant crosswind motion, which is absent when the plate is aligned at right angles to the flow. This spinning mode is more typical of plates flying in real wind storms.In addition, using a porous region to represent a simple double-eaved building, plates are launched from the roof and their trajectory is simulated. The complex flow fields around the building result in more realistic plate trajectories. For this particular scenario, large suctions on the windward slope of the roof produce large amounts of lift in the plates released from that position, causing them to fly well above the building, never entering the wake of the building.

Identification of Spinning Mode in the Unsteady Flow Field of a Low Pressure Turbine

This paper presents an experimental investigation of the vane-blade unsteady interaction in an unshrouded low pressure (LP) turbine research rig with uneven blade/vane count (72 blades and 96 vanes). The rig was designed in cooperation with MTU Aero Engines and considerable efforts were put on the adjustment of all relevant model parameters. In particular blade count ratio, airfoil aspect ratio, reduced mass flow, reduced speed, and Mach and Reynolds numbers were chosen to reproduce the full scale LP turbine at take off condition. Measurements by means of a fast-response pressure probe were performed adopting a phase-locked acquisition technique in order to provide the time resolved flow field downstream of the turbine rotor. The probe has been fully traversed both in circumferential and radial traverses. The rotor exit is characterized by strong perturbations due to the tip leakage vortex and the rotor blade wake. Circumferential nonuniformities due to the upstream vane wake and to the downstream exit guide vane potential effects are also identified. Furthermore, in the present configuration with an uneven blade/vane count the nonuniformities due to the stator and rotor row are misaligned along the whole turbine circumference and create a spinning mode that rotates in direction opposite to the rotor at a high frequency. The aeroacoustic theory is employed to explain such further unsteady pattern. The variations of the exit flow angle within a cycle of such pattern are not negligible and almost comparable to the ones within the blade passing period.

Single-Sensor Identification of Spinning Mode Noise from Aircraft Engine

Nomenclature A, C = constant matrices in linear time-invariant system a, b = any integers B = number of blades C0 = speed of sound, m=s c = acoustic perturbation amplitude f = frequency, Hz i = complex part Jm = mth Bessel function of the first kind K = gain of Kalman filter k = spinning mode frequency, Hz ka = axial wave number, 1=m kr = radial wave number, 1=m Mj = duct-flow Mach number N = number of microphones in an array nx = number of states O = observability matrix P = estimate covariance p = flow pressure, Pa t = time, s u, v, w = velocity, m=s V = the number of stator vanes v = measurement noise with covariance R w = modeling noise with covariance Q wt = @w=@t x, r, = cylindrical coordinates x0, R = axial and radial positions of sensors x = system states y = system outputs K = circumferential position of Kth microphone, rad = flow density, kg=m

Forced spreading behavior of droplets undergoing low frequency vibration

The profile of a droplet sitting on a flat surface is related to the contact angle, when the surface is vibrated the droplet oscillations result in a fluctuation in the contact angle over time. Depending on the amplitude of vibration this can be accompanied with an oscillation of the position of the contact line, this motion occurs when the instantaneous contact angle exceeds the advancing angle or falls below the receding angle. In this work we demonstrate that a net spreading can occur over multiple cycles. The result is that the final contact angle, after the cessation of vibration, is much reduced from the initial value. This has applications in the uniform filling of wells, such as in a microplate. The degree of forced spreading is shown to depend on the mode of oscillation, rather than purely the amplitude of vibration, with cases being demonstrated whereby an increase in amplitude decreases the degree of spreading due to an axisymmetric oscillation changing into a ‘spinning’ mode. Four regions of behavior are identified as a function of vibrational amplitude. In the lowest amplitude state no spreading is observed. The second region exhibits modest spreading resulting from axisymmetric oscillation. Upon further increase in amplitude, a spinning mode occurs in which no spreading results. Finally, the fourth region is characterized by a complex oscillation in which large scale spreading can be achieved.

Three-dimensional vesicles under shear flow: Numerical study of dynamics and phase diagram

The study of vesicles under flow, a model system for red blood cells (RBCs), is an essential step in understanding various intricate dynamics exhibited by RBCs in vivo and in vitro. Quantitative three-dimensional analyses of vesicles under flow are presented. The regions of parameters to produce tumbling (TB), tank-treating, vacillating-breathing (VB), and even kayaking (or spinning) modes are determined. New qualitative features are found: (i) a significant widening of the VB mode region in parameter space upon increasing shear rate γ and (ii) a robustness of normalized period of TB and VB with γ. Analytical support is also provided. We make a comparison with existing experimental results. In particular, we find that the phase diagram of the various dynamics depends on three dimensionless control parameters, while a recent experimental work reported that only two are sufficient.

Azimuthal behaviour of flow-excited diametral modes of internal shallow cavities

The aerodynamic excitation of ducted cavity diametral modes gives rise to complex flow–sound interaction mechanisms, in which the axisymmetric free shear layer interacts with the asymmetric acoustic modes. This results in various azimuthal patterns and behaviours depending on different flow and geometrical parameters. The azimuthal behaviour of this self-excitation mechanism is investigated experimentally. Axisymmetric shallow cavities in a duct have been tested over the range of cavity length to depth ratio from 1 to 6 and at Mach numbers up to 0.4. A set of pressure transducers flush mounted to the cavity floor is used to determine the acoustic mode amplitude and orientation. The excited acoustic modes are classified into spinning, partially spinning, and stationary diametral modes. An analytical representation based on the duct acoustics theory is used to analyse the measurements and provides a physical explanation of the observed behaviour of the diametral modes. Splitter plates are installed inside the cavity to form a geometrical preference. The acoustic response of this geometrically altered case show that pressure oscillations at different azimuthal angles along the cavity circumference can be uncorrelated, or even oscillate at different frequencies, while the diametral modes are still strongly excited. Two hot-wire probes are also used in a separate set of measurements to investigate the azimuthal behaviour of the shear layer oscillation. The results show that the shear layer oscillation has the same azimuthal distribution as that of the excited acoustic modes, indicating that the shear layer oscillation at different azimuthal angles can be not only uncorrelated but also occur at different frequencies.

Auroral imaging from a spinning satellite

For optimizing in situ particle and field measurements, auroral research satellites are best operated in a spinning mode. Simultaneous imaging of the optical aurora from such satellites requires either a stable platform or the derotation of the camera itself. Either of these requirements is complex and expensive. Either of these solutions also suffers from the problem that image blur often occurs due to the misalignments between the actual and the nominal spin axes of the satellite. Here we propose a novel solution in which the camera(s) are mounted solidly on the spacecraft to observe parallel to the spin axis of the satellite while a despinning flat 45° mirror directs the field of view toward the spacecraft nadir. The resultant image will appear to rotate in the frame of reference of the detector in the camera. In our scheme the images are exposed rapidly and a derotation algorithm is applied to the coordinates of each pixel in real time before the images are co-added in memory. The derotation algorithm uses only look up tables and integer additions and can be executed rapidly in hardware so that the system can support relatively fast satellite spin cycles. The system was simulated including a 1.8° misalignment between the nominal satellite spin axis (parallel to the mirror rotation axis) and the actual spin axis. It was shown that the look up table based algorithm can despin the images and correct for the axes misalignment, allowing the observation of the aurora at full resolution and with continuous coverage.

Theoretical and Experimental Identification of Acoustic Spinning Mode in a Cylindrical Combustor

The analytical solution for the eigenmodes of a cylindrical combustor allows for several realizations of the first tangential mode. The limiting cases of standing and spinning first tangential modes are well known. The general solution for a first tangential mode, however, allows for a large variety of realizations. In this study the analytical solution of the acoustic problem and a numerical time-resolved simulation of the acoustic pressure field are compared with experimental data from hot-fire tests. The experimentally observed pressure data are in excellent agreement with the simulation for all possible theoretical solutions. The approach enables detailed insight into the dynamics of the pressure field and thus constitutes a tool for the characterization of acoustic pressure waves in a cylindrical combustor even when the amplitude of the investigated eigenmode is small as compared with the mean pressure fluctuation.

Dynamic coupling of a small rigid probe to viscous ortho-terphenyl

We have measured the dielectric relaxation of 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) as a rotational probe in supercooled ortho-terphenyl (OTP). Due to the significant dipole moment of TEMPO compared with OTP, its contribution to the loss spectra can be identified already at moderate concentrations. For time scales ranging from 10 μs to 1 s, it is found that the tumbling mode of TEMPO is a true replica of the structural relaxation of OTP regarding average time constant, relaxation time dispersion, and the temperature dependence. While the present dielectric results are consistent with a decoupling of a spinning mode (about the nitroxyl dipole axis) of TEMPO from viscosity, they do not agree with the strong decoupling of the tumbling mode derived from electron spin resonance experiments.

Effects of α-Si 3N 4 and AlN addition on formation of α-SiAlON by combustion synthesis

Preparation of Yb α-SiAlON was investigated by self-propagating high-temperature synthesis (SHS) from α-Si 3N 4- and α-Si 3N 4/AlN-diluted powder compacts under nitrogen of 2.17 MPa. For the AlN-free samples, the molar ratio of Si 3N 4/Si varies between 0.22 and 0.5. The starting stoichiometry of the AlN-added samples comprises a constant proportion of Si 3N 4/Si equal to 0.22, but a broad range of AlN/Al from 0.33 to 1.0. The self-sustaining combustion wave propagated in the spinning mode on account of highly diluted samples adopted in this study. The overall reaction exothermicity increases with Si 3N 4/Si ratio for the AlN-free samples, while decreases with AlN/Al ratio for the AlN-added powder compacts. As a result, the amount of unreacted Si left in the final product was significantly reduced and the formation of nearly single-phase Yb α-SiAlON was achieved in the sample with Si 3N 4/Si = 0.5. Moreover, the growth of elongated α-SiAlON grains was enhanced in the samples with high contents of Si 3N 4. In contrast, the nitridation of Si was only improved to a certain extent with the addition of AlN and no further improvement was attained by increasing the AlN content. Due to the lack of sufficient liquid phases during combustion and the weak reaction exothermicity, the samples with high contents of AlN were inclined to produce α-SiAlON grains in a fine equiaxed form.