Mixing in horizontally heterogeneous flows

An experimental study of mixing across density interfaces produced by laterally heterogenous turbulence is presented in this paper. The turbulence is generated by a flow or air bubles rising through a density interface produced by brine and fresh water. The mixing efficiency, , of the process is measured comparing the increase in potential energy with the available kinetic energy. We find that there is a decrease in the global mixing efficiency of the process with the length of the tank, the shape of (Ri) depends also on the air flow producing the turbulence, showing a geometrical limit to the ammount of kinetic energy which may be used for mixing.     

Wall Versus Centerline Polymer Injection in Turbulent Channel Flows

This experimental study compares the mean and turbulence characteristics of turbulent channel flows with polymer injection at the wall and at the centerline to assess the impact of the injection location on drag reduction. It also contrasts the drag reduction performance of a hydrolyzed polymer versus a non-ionic polymer under the same conditions. Wall injection of non-ionic and hydrolized polymers resulted in 23% and 9% larger drag reduction than corresponding centerline injection, respectively. In all cases, the polymer was structured and the presence of macromolecular polymer structures, even when concentrated mostly away from the wall, seemed to be able to affect the turbulence structure in the flow.     

开放剪切流空间全局动力学与空间相位斑图间的关系

开放流动空间动力学可基于两类全局能量关系式进行研究；而空间相位斑图则可通过互谱空间演化加以测定。全局能量关系式以时间Fourier系数的形式建立流场任意两点问速度脉动能量间的关系，籍此可定义全局意义上的线性、非线性和线性一非线性机制。基于轴对称剪切流、变密度轴对称圆射流以及平面对称剪切流的实验发现：轴对称旋涡结构的配对由线性、线性一非线性机制表征，对应有序空间相位斑图；并且能量可通过线性一非线性机制在具有相同相速度的扰动间传递。螺旋结构由线性机制表征，对应有序相位斑图。全局自激励振荡由非线性的能量共振表征，对应无序相位斑图。籍此，有序空间相位斑图对应线性和线性一非线性机制；而混沌相位斑图则对应非线性机制。     

Orienting properties of discotic nematic liquid crystals in Jeffrey-Hamel flows

The orienting properties of incompressible discotic nematic liquid crystals for creeping flows between converging and diverging planar walls (Jeffrey-Hamel) are analyzed using the Leslie-Ericksen theory. The dependence of director orientation on the reactive parameter and the flow kinematics is presented. Closed form stationary solutions for the director orientation are found when elastic effects are neglected. Stationary numerical solutions for the velocity and director fields using the full Leslie-Ericksen theory are presented. The director field in converging flow is characterized by azimuthal (radial) centerline orientation, by being asymmetric with respect to the azimuthal (radial) direction, and by having an allowed orientation range that spans two half-quadrants (full quadrants). In the limiting case of perfectly flat disk ( –) the flow-induced director orientation in converging flow is the azimuthal direction, while in diverging flow the director rotates by a full n radians. By reducing the vertex angle between the walls to vanishingly small values, converging flow solutions properly reduce to those of flow between parallel plates, but diverging flows are expected to lead to a new instability.     

Efficient implicit algorithm for the equations of 2-D viscous compressible flow: application to shock-boundary layer interaction

A fully implicit algorithm has been developed to time integrate the equations of 2-D compressible viscous flow. The algorithm was constructed so as to optimize computational efficiency. The time-consuming block matrix inversions usually associated with implicit algorithms have been reduced to the trivial non-iterative inversion of four sets of scalar bidiagonal matrices. Thus, the algorithm requires virtually no more computer storage than an explicit algorithm. The efficient structure of the implicit algorithm is reflected in comparative timings which slow that it requires only a factor of two more computer time per point per time step than a typical explicit algorithm. Therefore, the algorithm allows more economical solution of given flows than existing explicit methods and also allows more difficult problems to be attempted using available computer resources. Application of the algorithm to the problem of shock-boundary layer interaction produces results consistent with both experimental measurements and other calculations.     

幂硬化可压缩材料Ⅰ型动态裂纹尖端奇异场

研究了平面应变条件下幂硬化可压缩材料中定常扩展的Ⅰ型动态裂纹尖端应力应变奇异场．采用Ｊ２流动理论和场量直角坐标分量，得到了应力应变奇异性不同时的裂纹尖端渐近场，其中场量的角变化规律和理想弹塑性材料的完全相同     

Direct Numerical Simulation of a Mach 2 shock interacting with isotropic turbulence

Direct Numerical Simulation (DNS) and linear analysis of a shock interacting with incompressible and compressible isotropic turbulence is conducted. A dependence of amplification ratios on the degree of compressibility of the incoming flow is found. It can be shown that the enhancement of rms values of turbulent quantities across the shock varies according to the ratio of compressible to incompressible kinetic energy (exact definition see eq. 8). Inflow conditions with high values of display reduced amplification ratios of TKE and thermodynamic quantities while vorticity fluctuations are enhanced more strongly. The different behaviour of the turbulent kinetic energy (TKE) is due to the reduced pressure diffusion term in the TKE-equation. Experiments show qualitatively a similar behaviour as the simulation with incompressible inflow conditions, but they could so far not confirm our findings of reduced amplification rates in the compressible case, one of the reasons being the lack of knowledge of all flow parameters upstream of the shock front and the inability to generate isotropic turbulence in real life experiments. For the DNS we use a third order in space shock-capturing scheme based on the ENO algorithm of Harten [10] together with an approximate Riemann solver. This non-TVD scheme turned out to have many advantages over other common Godunov-type high resolution schemes for the specific problem of a shock interacting with turbulent fields.     

Spectral methods for compressible reactive flows

High order simulations are necessary in order to capture fine details in resolving supersonic reactive flows. However, high Mach number compressible flows feature sharp gradients and discontinuities, which present a challenge to successful simulations using high order methods. Spectral methods have proven a powerful tool in simulation of incompressible turbulent flows, and recent advances allow the application of spectral methods to compressible reactive flows. We review the recent advances in the theory and application of spectral methods which allow stable computations of discontinuous phenomena, and the recovery of high order information via postprocessing, and present applications of high Mach number reactive flows. To cite this article: D. Gottlieb, S. Gottlieb, C. R. Mecanique 333 (2005).     

Limitations of temperature measurements with holographic interferometry in the presence of pressure variations

In recent years oscillatory flows have shown to be a promising strategy to enhance heat transfer. However, the mechanisms underlying oscillatory heat transfer enhancement are not yet completely understood. One problem, when investigating heat transfer in oscillatory flows experimentally, is to resolve the temperature distribution as a function of time. This is one reason that most studies reported in the literature so far were restricted to frequencies of a few hertz. As shown in this paper, an appropriate tool to investigate oscillatory heat transfer phenomena at higher frequencies (1000 Hz) is real time holographic interferometry (HI) combined with high-speed cinematography. In the present paper HI was applied to study acoustically driven flow. To apply HI to such a physical situation it was necessary to expand its applicability to cases where changes in the refractive index are caused not only by temperature changes but also by pressure variations. For this purpose a new evaluation formula that accounts for pressure variations was derived. On the example of the acoustic field, we discuss the impact of the pressure variations on temperature measurements. Additionally, an image processing algorithm was developed that allows the measurement of time dependent temperature distributions. The uncertainties of the temperature measurements introduced by the image processing algorithm were found to be in the range of thermocouple measurements.