Lu Zhiming

Lagrangian Structure Function's Scaling Exponents in Turbulent Channel Flow

The Lagrangian structure functions scaling exponents and intermittency of three-dimensional incompressible turbulent channel flow are investigated by using direct numerical simulation. The intermittency in streamwise velocity increments is found to increase in the near-wall region, which can be attributed to the presence of strong mean shear and organized motions in the near-wall region. It is found that the intermittency of transverse velocity increments is weaker than that of longitudinal ones. The present ESS evaluation of ζL(q) for the structure function of the streamwise velocity component in the channel centre is fairly close to experimental estimates of isotropic turbulence.

A calculation method for fully developed flows in curved rectangular tubes

In this paper, A method, consisted of perturbation method, Garlerkin method and finite-difference method, is designed to calculate fully developed flows in curved tubes of rectangular cross-section. It costs less computation than that of direct solving N-S equations, and prevents from building high-order difference equations and extra dealing with the boundary conditions. Numerical results in the situation of small curvature and low Dean number is in accordance with formers numerical and experimental results in quality, and it shows the feasibility of this papers method.

Doubly Periodic Wave Solutions of Jaulent–Miodek Equations Using Variational Iteration Method Combined with Jacobian-function Method

One of the advantages of the variational iteration method is the free choice of initial guess. In this paper we use the basic idea of the Jacobian-function method to construct a generalized trial function with some unknown parameters. The Jaulent–Miodek equations are used to illustrate effectiveness and convenience of this method, some new explicit exact travelling wave solutions have been obtained, which include bell-type soliton solution, kink-type soliton solutions, solitary wave solutions, and doubly periodic wave solutions.

Solitons and Waves in (2+1)-Dimensional Dispersive Long-Wave Equation

For a higher-dimensional integrable nonlinear dynamical system, there are abundant coherent soliton excitations. With the aid of an improved projective Riccati equation approach, the paper obtains several types of exact solutions to the (2+1)-dimensional dispersive long-wave equation, including multiple-soliton solutions, periodic soliton solutions, and Weierstrass function solutions. From these solutions, apart from several multisoliton excitations, we derive some novel features of wave structures by introducing some types of lower-dimensional patterns.

EXPERIMENTAL STUDY ON TURBULENT FEATURES IN THE NEGATIVE TRANSPORT REGION OF ASYMMETRIC PLANE CHANNEL FLOW

Turbulent features of streamwise and vertical components of velocity in the negative transport region of asymmetric plane channel flow have been studied experimentally in details. Experiments show that turbulent fluctuations in negative transport region are suppressed, and their probability distributions are far from Gaussian. Besides, the skewness factors attain their negative maxima at the position of the maximum mean velocity, whereas the flatness factors attain their positive maxima at the same position.

On the mechanism of turbulent coherent structure (III)—A statistical and dynamical model of coherent structure and its heat transfer mechanism

Following Tsai & Ma[1] and Tsai & Liu[2], a statistical and dynamical near-wall turbulent coherent structural model with separate consideration of two different portions: locally generated and upstream-transported large eddies has been established. With this model, heat transfer in a fully developed open channel in the absence of pressure gradient is numerically simulated. Database of fluctuations of velocity and temperature has also been set. Numerical analysis shows the existence of high-low temperature streak caused by near-wall coherent structure and its swing in the lateral direction. Numerical results are in accordance with the computations and experimental results of other researchers.

Detrended analysis of Reynolds stress in a decaying turbulent flow in a wind tunnel with active grids

Multi-scale properties of Reynolds stress in decaying turbulence in a wind tunnel with high Reynolds number are investigated. Two filtering techniques i.e., the zeroth-order and first-order detrending methods are applied to the two velocity components, where the local mean value (resp. local linear trend) is removed in the former (latter) technique. Some basic statistics for thirty measurements show that the variation is very large at first two locations and relatively small at last two locations. Moderately good power law is found for the mean value of local Reynolds stress at last three measurement locations with scaling exponents approximately being 1.0 and a dual power law exists for the mean value of standard deviation of local Reynolds stress at all four measurement locations with scaling exponents being 0.53 and 0.58 for zeroth- and first-order filtering respectively. Present results about local Reynolds stress are useful to build and evaluate the model of sub-grid Reynolds stress in large eddy simulations.

Longitudinal and transverse structure functions in decaying nearly homogeneous and isotropic turbulence

Streamwise evolution of longitudinal and transverse velocity structure functions in a decaying homogeneous and nearly isotropic turbulence is reported for Reynolds numbers Re;t up to 720. First, two theoretical relations between longitudinal and transverse structure functions are examined in the light of recently derived relations and the results show that the low-order transverse structure functions can be well approximated by longitudinal ones within the sub-inertial range. Reconstruction of fourth-order transverse structure functions with a recently proposed relation by Grauer et al. is comparatively less valid than the relation already proposed by Antonia et al. Secondly, extended self-similarity methods are used to measure the scaling exponents up to order eight and the streamwise evolution of scaling exponents is explored. The scaling exponents of longitudinal structure functions are, at first location, close to Zybin＇s model, and at the fourth location, close to She-Leveque model. No obvious wend is found for the streamwise evolution of longitudinal scaling exponents, whereas, on the contrary, transverse scaling exponents become slightly smaller with the development of a steamwise direction. Finally, the stremwise variation of the order-dependent isotropy ratio indicates the turbulence at the last location is closer to isotropic than the other three locations.

Scaling exponents in fully developed asymmetric channel flow

Wavelet analysis is applied to study the global and local scaling exponents in fully developed asymmetric channel flow. Global exponents are calculated by orthogonal wavelets and Extended Scaling Similarity (ESS). The results show that the flow in an asymmetric channel flow exhibits different characteristics of intermittency from that in a symmetric flow. It is also shown that the intermittency property of the streamwise fluctuations is different from that of vertical fluctuations, and the intermittency does not decay with the increase of the distance from the wall. In addition, the Continuous Wavelet Transform(CWT) method is found to be unreliable to calculate the local scaling components.Finally, it is pointed out that the existence and the significance of negative local scaling components need further study.

Models for the counter-gradient-transport phenomena

The counter gradient transport phenomena on momentum, energy and passive scalar in turbulent flows were studied by use of the single response function for TSDIA. As a result, models that can describe qualitatively the phenomena are obtained. Then the results are simplified by use of the inertial range theory, and the results for lower degrees agree with results of predecessor. Finally the counter gradient-transport phenomena in channel flow and circular wake flow are analyzed.