Nonstationary Turbulent Flow Research Articles

Phase proper orthogonal decomposition of non-stationary turbulent flow

A phase proper orthogonal decomposition (phase POD) method is demonstrated utilizing phase averaging for the decomposition of spatiotemporal behavior of statistically non-stationary turbulent flows in an optimized manner. The proposed phase POD method is herein applied to a periodically forced statistically non-stationary lid-driven cavity flow, implemented using the snapshot proper orthogonal decomposition algorithm. Space-phase modes are extracted to describe the dynamics of the chaotic flow, in which four central flow patterns are identified for describing the evolution of the energetic structures as a function of phase. The modal building blocks of the energy transport equation are demonstrated as a function of the phase. The triadic interaction term can here be interpreted as the convective transport of bi-modal interactions. Non-local energy transfer is observed as a result of the non-stationarity of the dynamical processes inducing triadic interactions spanning across a wide range of mode numbers.

Numerical simulation of aerodynamics of a high-efficiency orthogonal balanced blade of a wind generator

The results of verification of numerical modeling of a rotating blade of a wind generator with a nonstationary turbulent flow around in a two-dimensional formulation are given. A design free from the drawbacks of a single-blade orthogonal high-efficiency turbine is considered (US Patent Victor Lyatkher, US 8007235 B1, August 30, 2011, RF Patent 2426911 C1).

Experimental and numerical simulation for swirl flow in a combustor

Results of the experimental and numerical simulation for swirl flow in combustion of a lean methane-air mixture in a model combustor at atmospheric pressure are represented. The panoramic method for the flow velocity measurement and the calculation by a large eddy method were used for the investigation of the nonstationary turbulent flow. The numerical modeling for the breakdown of the vortex core of the flow and the topology of large-scale vortex structures forming in it showed the close fit to the experiment. The analysis of obtained data showed that for the case of the intensive swirl of the flow as well as in the case of the flow without combustion, dynamics of the flow with combustion was determined by the global azimuthal instability mode corresponding to the intensive precession of the vortex core. The flame had the similar characteristics of the stability and compactness in the case of stabilization by the low swirl; however, velocity pulsations in the flow corresponded to the development of only local instability modes. Thus, the other kind of vortex breakdown in the case of the low swirl, for which the central recirculation zone is lacking, is not only favorable in view of the reduction of the NOx emission, but also remains a possibility for the effective use of the active control method for the flow and combustion. In particular, the given result may be used for the elimination of the thermoacoustic resonance in combustors.

Direct Measurement of Effective Magnetic Diffusivity in Turbulent Flow of Liquid Sodium

The first direct measurements of effective magnetic diffusivity in turbulent flow of electroconductive fluids (the so-called β effect) under the magnetic Reynolds number Rm≫1 are reported. The measurements are performed in a nonstationary turbulent flow of liquid sodium, generated in a closed toroidal channel. The peak level of the Reynolds number reached Re≈3×10(6), which corresponds to the magnetic Reynolds number Rm≈30. The magnetic diffusivity of the liquid metal was determined by measuring the phase shift between the induced and the applied magnetic fields. The maximal deviation of magnetic diffusivity from its laminar value reaches about 50%.

Measurements of turbulent magnetic diffusivity in a liquid-gallium flow

Direct measurements of the effective conductivity (magnetic diffusivity) in the turbulent flow of a liquid metal have been performed. An nonstationary turbulent flow of a gallium alloy has been excited in a closed toroidal channel with dielectric walls. The Reynolds number reaches a maximum value of Re ≈ 106, which corresponds to the magnetic Reynolds number Rm ≈ 1. The conductivity of the metal in the channel has been determined from the phase shift of forced harmonic oscillations in a series RLC circuit whose inductance is a toroidal coil wound around the channel. The maximum deviation of the effective conductivity of the turbulent medium from the ohmic conductivity of the metal is about 1%.

Direct measurement of effective electro conductivity of turbulent liquid metal

AbstractDirect measurement of effective electro conductivity are performed in a nonstationary turbulent flow of liquid gallium, generated in a closed toroidal channel. The peak level of the Reynolds number reached Re ≈ 106, what corresponds to the magnetic Reynolds number Rm ≈ 1. The conductivity of the liquid metal was determined measuring the phase shift between sinusoidal voltage and current in the RLC‐circuit, the inductance of which was made by the toroidal coil embracing the channel. The maximal deviation of electrical conductivity from its ohmic value reaches about 1%. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Physical model of a hydrodynamically nonstationary turbulent flow

The influence of nonisothermality and hydrodynamic nonstationarity on turbulence generation and development is analyzed. A physical model of a hydrodynamically nonstationary turbulent flow is proposed, the model is created on the basis of previous experimental investigations on the structure of a nonstationary turbulent gas flow in the channel.

Le Chatelier’s principle and its adaptation for closure of averaged equations of hydrodynamics

A generalized formulation of Le Chatelier’s principle is offered, which allows considering the latter as a general physical law of shift and equilibrium in open systems. Application of the generalized formulation to the “kinematic effect of pressure-free flow” allows deriving a rule to calculate the local effect of counteraction to the integral effect in the relaxation process of transition of a nonstationary turbulent flow to the stationary one. In the regime with stationary motion, the local effect of counteraction to the integral effect reaches its maximum under given and constant external conditions. The condition of local effect maximum is used for closure of the equation of the plan velocity diagram. An example is given that confirms the theoretical consideration.

Turbulent time-frequency spectral structures and the extraction of the desired turbulence component

Turbulence is induced in a constant-volume bomb using the motion of a perforated plate. Statistical ensemble-averaged and the cycle-resolved analyses are performed for these raw velocity data. It is shown that the cycle-resolved analysis is a suitable method of characterizing non-stationary turbulent flow. Based on the cycle-resolved analysis, the velocity data are then processed using wavelet packet and moving time-window techniques to obtain turbulent time-frequency spectral structures. From the comparison of the turbulent time-frequency spectral properties, a method to induce turbulence with different characteristic parameters is developed. To extract the desired turbulence frequency content, a multiple cut-off frequencytechnique is proposed and good agreement is found between the results obtained from wavelet analysis and the multiple cut-off frequency method. The integral time scale of the turbulence component has been studied as well. It is shown that the integral time scale is the key characteristic time for the turbulence and this is a function of the turbulence frequency.

Numerical Simulation of Nonstationary Turbulent Flow around an Automobile Profile Near a Mobile Screen

A numerical investigation of nonstationary turbulent flow of an incompressible viscous fluid around the profile of a Volkswagen automobile near a mobile screen has been carried out on the basis of solution of the Reynolds equations closed using a two-parameter dissipation model of turbulence by the finite-volume method.

Effects of a single protrusion on the boundary layer around a cylinder rotating in an axial flow. Conditional measurement in the intermittent region of a turbulence wedge.

A turbulence wedge which develops downstream of a single protrusion installed on a rotating cylinder in an axial flow is experimentally investigated. Turbulent/non-turbulent discrimination is made for this non-stationary turbulent flow. Conditioned by this discrimination, mean and fluctuating velocities and Reynolds stress are obtained using ensemble average over realizations, each of which is the data for one revolution of the cylinder. The radial profiles of the conditional averaged mean velocities in the axial direction at the center of the wedge differs from those at the interfaces of the wedge. The velocity profiles in the peripheral direction show that turbulent fluids in the turbulence wedge are carried by the streamwise vortices. The mean velocity distribution expressed in the streamline coordinates and the polar diagram show that the streamwise vortices consist mainly of turbulent fluid. The peripheral profiles of the fluctuating velocities show that turbulent average contributes to the peaks of the profiles, and the value of non-turbulent average at the interfaces are larger than in the outer region of the wedge.