Robust model identification of actuated vortex wakes

We present a low-order modeling technique for actuated flows based on the regularization of an inverse problem. The inverse problem aims at minimizing the error between the model predictions and some reference simulations. The parameters to be identified are a subset of the coefficients of a polynomial expansion which models the temporal dynamics of a small number of global modes. These global modes are found by Proper Orthogonal Decomposition, which is a method to compute the most representative elements of an existing simulation database in terms of energy. It is shown that low-order control models based on a simple Galerkin projection and usual calibration techniques are not viable. They are either ill-posed or they give a poor approximation of the solution as soon as they are used to predict cases not belonging to the original solution database. In contrast, numerical evidence shows that the method we propose is robust with respect to variations of the control laws applied, thus allowing the actual use of such models for control.     

Attenuation of sound due to vortex shedding from a splitter plate in a mean flow duct

A theoretical analysis is given of an experiment being performed at the University of Southampton [1] as part of a programme to quantify the effectiveness of perforated screens in dissipating sound in the presence of tangential mean flow. In the experiment vorticity is generated at the trailing edge of a splitter plate in a mean flow duct by a plane sound wave incident from upstream, acoustic energy being ceded to the kinetic energy of the vortex field. An expression is derived for the dissipated sound power at arbitrary subsonic mean flow Mach number and frequency. The calculation is performed both by a consideration of the net flux of acoustic energy into the trailing edge region of the splitter plate, and by evaluating the rate of working of the vortex lift forces in the field of the acoustic particle velocity. In particular, it is shown that the absorption is independent of frequency, provided the frequency does not exceed the minimum cut-on frequency of transverse acoustic modes within the duct.     

Electromagnetic radiation from vortex flow in Type-II superconductors

We show that a moving vortex lattice, as it comes to a crystal edge, radiates into a free space the harmonics of the washboard frequency, omega(0)=2pi v/a, up to a superconducting gap, Delta/variant Planck's over 2pi. Here v is the velocity of the vortex lattice and a is the intervortex spacing. We compute radiation power and show that this effect can be used for the generation of terahertz radiation and for characterization of moving vortex lattices.     

A self-learning coupled map lattice for vortex shedding in cable and cylinder wakes

A coupled map lattice (CML) with self-learning features is developed to model flow over freely vibrating cables and stationary cylinders at low Reynolds numbers. Coupled map lattices that combine a series of low-dimensional circle maps with a diffusion model have been used previously to predict qualitative features of these flows. However, the simple nature of these CML models implies that there will be unmodeled wake features if a detailed, quantitative comparison is made with laboratory or simulated wake flows. Motivated by a desire to develop an improved CML model, we incorporate self-learning features into a new CML that is first trained to precisely estimate wake patterns from a target numerical simulation. A new convective-diffusive map that includes additional wake dynamics is developed. The new self-learning CML uses an adaptive estimation scheme (multivariable least-squares algorithm). Studies of this approach are conducted using wake patterns from a Navier-Stokes solution (spectral element-based NEKTAR simulation) of freely vibrating cable wakes at Reynolds numbers Re=100. It is shown that the self-learning model accurately and efficiently estimates the simulated wake patterns. The self-learning scheme is then successfully applied to vortex shedding patterns obtained from experiments on stationary cylinders. This constitutes a first step toward the use of the self-learning CML as a wake model in flow control studies of laboratory wake flows.     

Formation and decay of vortex lattices in Bose-Einstein condensates at finite temperatures

The dynamics of vortex lattices in stirred Bose-Einstein condensates have been studied at finite temperatures. The decay of the vortex lattice was observed nondestructively by monitoring the centrifugal distortions of the rotating condensate. The formation of the vortex lattice could be deduced from the increasing contrast of the vortex cores observed in ballistic expansion. In contrast to the decay, the formation of the vortex lattice is insensitive to temperature change.     

Determination of plate source, detector separation from one signal

We address the problem of locating a transient source, such as an acoustic emission source, in a plate. We apply time-frequency analysis to the signals detected at a receiver. These highly dispersive and complex waveforms are measured for source-receiver separations ranging from 40 to 180 plate thicknesses and at frequencies such that 10 to 20 Rayleigh-Lamb branches are included. Reassigned, smoothed, pseudo-Wigner-Ville distributions are generated that exhibit the expected sharp ridges in the time-frequency plane, lying along the predicted frequency-time-of-arrival relations. The source-receiver separation can be determined from such plots.     

Transient growth of stationary flow perturbations at laminar boundary-layer separation

In a subsonic wind tunnel, laminar boundary-layer separation at a backward-facing step on a flat plate modulated by stationary flow perturbations was examined. The stationary disturbances were introduced into the flow by controlled roughness elements spaced on the model surface close to the separation line. Through a comparison of obtained data with results gained in previous studies of other periodic roughness systems, some trends in the generation of stationary disturbances subject to transient growth in separation region were revealed.     

Regularizing a vortex sheet near a separation point

Current methods for computing vortex sheet separation use a regularization parameter which is discontinuous from the body to the vortex sheet. We propose two methods for reducing the errors associated with the discontinuity and improving convergence with respect to the regularization parameter. The “velocity smoothing” method is the simpler of the two, and removes the discontinuity in regularization from one of the two equations where it occurs. The “tapered smoothing” method removes the discontinuity from both equations. In a model problem, both methods are found to converge much more rapidly (with exponents 3/2 and 2 versus 1/2 for the standard method) as the regularization parameter tends to zero. Unsteady algorithms are proposed for evolving the free sheet using the two methods, and are tested in a benchmark problem. Accuracy is significantly improved for similar computational expense.     

Acoustic-excited vortex shedding and acoustic nonlinearity at a rectangular slit with bias flow

The acoustical response of a slit with a mean bias flow is numerically studied. By means of a potential flow model based on the discrete vortex method and a spanwise-averaged three-dimensional Green?s function, both unsteady vortical flow and slit impedance are obtained in a unified theoretical framework. The numerical simulation focuses on the acoustic-excited vortex structures of the slit flow while neglecting the viscous damping effect. Three representative flow features are demonstrated, which are the destabilized jet flow, the rolling up of vortex sheets and formation of vortex pairs, and the reversal flow with alternating vortex shedding on both sides of the slit. These features are corresponding to low, moderate, and high sound amplitude, respectively. The acoustic behavior of the slit can be divided into linear, transition, and nonlinear regimes. During its evolution through the three regimes, the resistance exhibits a constant value, a slight decrease, and a significant increase with the increasing sound amplitude. Correspondingly, the reactance first remains constant and then shows a modest decrease as the sound amplitude increases. The nonlinear effect also causes the gradual decrease of the mean bias velocity in company with the marked increase of the amplitude of the fluctuating velocity in the slit. The mean bias velocity decreases to about 80 percent of its linear value at the transition point where reversal flow begins to occur, and further decreases to only 10 percent in the highly nonlinear region. The slit impedance is also presented as a function of frequency and for different aspect ratios. And the effects of frequency and slit geometry are discussed.     

Separated flow noise of a flat plate at large attack angles

Flow noise associated with separated flow of a flat plate with large attack angles was studied experimentally to obtain its acoustic characteristics and to understand its generation mechanism. The acoustic features show that the separated flow noise could be attributed to acoustic dipole sources associated with the wall-pressure fluctuations on the plate surface. The time derivative of the fluctuating wall-surface pressure is highly correlated with the associated acoustic pressure. The noise intensity source strength is proportional to the mean-square time derivative of the fluctuating surface pressure and its correlation area, being proportional to the sixth power of the oncoming flow velocity and distributed uniformly over the plate surfaces. The associated acoustic intensity is well predicted by these noise source strength distributions.     

Flow field measurements of leading-edge separation vortex formed on a delta wing with vortex flaps

Wind tunnel tests are carried out using a 70 delta wing model with leading-edge vortex flaps. The structure of the leading-edge separation vortex over the leading-edge vortex flap is measured by use of a 5 holes pitot probe, surface pressure measurement technique and oil flow visualization technique. Separation vortices formed on a plain delta wing, on a vortex flap and inboard the vortex flap hinge line are clearly visualized. Results indicate that the flow around the vortex flaps is classified into several different cross flow patterns. The streamwise flap deflection angle is defined to discuss the vortex flap performance. The optimum lift to drag ratio is attained when the amount of the wing angle of attack is not far different from that of the streamwise flap deflection angle, as long as the vortex flap is deflected modestly.     

Formation of vortex flows in thin nematic cells

The director field reorientation and the formation of steady vortex flows in doubly and obliquely oriented thin liquid-crystal cells under the temperature gradient have been theoretically described within the nonlinear generalization of the classical Ericksen-Leslie theory for describing liquid crystal hydrodynamics. This generalization allowing the consideration of thermomechanical contributions to the shear stress and to the entropy balance equation has made it possible to describe the formation of a steady two-vortex flow in doubly oriented liquid-crystal cells.     

Three-dimensional visualization of stratified turbulent wakes at varying Reynolds number

Journal of Visualization - We present a series of three-dimensional visualizations of numerically simulated turbulent wakes in a stably stratified fluid with a focus on the effects of the wake...     

涡流管能量分离过程的热力学模型

根据工程热力学理论对涡流管工作过程进行了热力学分析,在h—s图上定性地表示出热力过程,获得了过程中相关参数的计算公式。通过对管内能量分离过程热力学分析,借助J．Mischner的热力学熵增理论,获得了涡流管内能量分离效应与冷气流率的函数关系式,建立了完整的涡流管能量分离过程的热力学模型。     

Efficiency of energy separation at compressible gas flow in a planar duct

The method of energy separation in a high-speed flow proposed by A.I. Leontyev is investigated numerically. The adiabatic compressible gas flow (of a helium-xenon mixture) with a low Prandtl number in a planar narrow duct and a flow with heat exchange in a duct partitioned by a heat-conducting wall are analysed. The temperature recovery factor on the adiabatic wall, degree of cooling the low-speed flow part, temperature efficiency, and the adiabatic efficiency in a duct with heat exchange are estimated. The data are obtained for the first time, which make it possible to compare the efficiency of energy separation in a high-speed flow with the efficiency of similar processes in vortex tubes and other setups of gas-dynamic energy separation.