Measurements of turbulent flows

Fluid flows in nature and technology normally depart from laminarity and are turbulent in the majority of cases, including flows around bodies such as airplanes, vehicles, ships, and in internal flows such as in ducts, turbomachines, propulsors, and even in blood circulation in the human body. Laminarity is the anomaly and not the standard. As will be shown in this chapter, the parameter which is fundamental to the transition from laminarity to turbulence is the Reynolds number, i.e., the ratio of inertial to viscous forces. In Sect. 10.1 the statistical Eulerian description of turbulent flows will be developed followed by a section on Reynolds decomposition and Reynolds equations. Section 10.1.3 finally surveys scales in turbulent flows. In Sect. 10.2 the optical Lagrangian particle-tracking technique, capable of producing robust, single- and multiparticle Lagrangian measurements, is presented. First the image-processing algorithms used to determine the particle trajectories are discussed and then the implementation of the technique in the laboratory is described. A brief presentation of results focusing on the separation of particle pairs in intense turbulence is also given. In Sect. 10.3 a novel type of random flow in a dilute polymer solution of a flexible high-molecular-weight polymer in two different flow setups that share the same feature of high curvature of the flow lines is discussed. In the first part of this section the hydrodynamic description of dilute polymer solution flows and the nondimensional parameters that follow from these equations to characterize these flows are presented. Variation of one of these control parameter responsible for the elastic properties of a fluid can lead to a new elastic instability in various flows that is distinguished by the presence of curvilinear trajectories. The theoretical criteria for this elastic instability in three different flows together with experimental verification are discussed. To complete the basics, the rheometric properties of the polymer solutions used and their relation to Boger fluids are given. The first observation of elastic turbulence, in the flow between two plates, is described. Then the experimental measuring techniques used to characterize the flow are given, and a complete description of the results of measurements together with a discussion of the results is presented. Finally, the role of elastic stress, a recent theory of elastic turbulence, and comparative studies of elastic versus hydrodynamic turbulence are discussed. The last part of the section deals with the description of the elastic turbulence in a curvilinear channel or Dean flow, where a particularly detailed experiment on mixing due to elastic turbulence was conducted. A summary of the results is given finally. Section 10.4 briefly reviews large-eddy simulations (LES) and the specific data requirements for LES (Sect. 10.4.1) and then describes the experimental methods that have been employed to obtain such data starting with arrays of point-measurement techniques (Sect. 10.4.2) and optical planar velocimetry measurement methods (Sect. 10.4.3). Sample results from the latter applied to studies of LES models are presented in (Sect. 10.4.4). The application of optical volumetric techniques for three-dimensional (3-D) velocity measurements are described in Sect. 10.4.5. Scalar fluctuation measurements using optical techniques and their applications to the study of LES variables of interest to scalar mixing and combustion are reviewed in Sect. 10.4.6.