Professor Pentti Saarenrinne
Project: Flow Process Management and Control
Internal turbulent flows can be found especially in the process industry, where fluids are transported, mixed and processed in pipes, channels, pumps, tanks and all kind of special equipment. Thus the phenomena in these flows are very important and they have been studied by EFD group for decades. Due to the recent development of experimental methods, such as PIV, very new kind of information from flow in the process equipment is available. From the instantaneous 2D velocity fields several details of process can be addressed. Examples include the coherent flow structures, development of instabilities, flow separation and turbulence production and decay.
Dispersed multiphase flows are one of the main research topics of experimental fluid dynamics (EFD) at TUT. Several techniques have been developed by the EFD group for image acquisition, signal processing and data analysis allowing the utilization of DI, PIV and PTV methods in denser multiphase flows. The research concentrates on incompressible, non-reactive, isothermal, dispersed multiphase flows, in which one of the phases is continuous. These flows have a significant importance in industrial processes. Typical examples are bubbly flows, sprays, flows with solid particles, and three-phase flows with micro-bubbles and flocculate particles.
Pentti Saarenrinne, Markus Honkanen, Tero Pärssinen and Hannu Eloranta: Digital Imaging and PIV methods in Multiphase Flows, Report 175
In fluid mechanics polymers additives are primarily applied to reduce skin friction in pipe flows, where friction reductions up to 80 % have been reported. The polymers used for drag reduction are called drag reducing agents (DRA) and the used concentrations are very low (in the order of ppm). In stirred tank reactors, the object of adding DRA is to modify the turbulence properties. For example, the goal can be to slow down the growth of crystals in crystallization process by reducing turbulent mixing.
Fluid-structure interaction (FSI) is an important and very complex physical phenomenon occurring in several branches of engineering. Flows of fluid around bodies ranging from musical instruments to process engineering machinery and airliners to skyscrapers can under certain conditions give a rise to complex vibratory motions. In some cases these vibrations may even be destructive. FSI typically involves several disciplines of physics and may lead to very complicated problems including fluid dynamics, solid mechanics and vibration. One common form of FSI is the vortex-induced vibration (VIV). This is caused by fluctuating fluid forces on the body due to the vortex shedding which in turn induces elastic structures to respond to the excitation.
The efficiency of chemical processes such as dissolution, coagulation, and flocculation strongly depends on the mixing that takes place in the process. The Lagrangian turbulence statistics determines how turbulence disperses the components and how quickly a homogeneous mixture is developed. The EFD group has studied mixing both in static and active mixers.
PIV is an optical technique for measuring instantaneous 2D velocity fields. The method is non-intrusive but it needs seeding particles in the flow and a disturbance free optical access into the investigated flow field. The main advantage of PIV compared to traditional flow measurement techniques (e.g. LDA), is it's capability to measure the entire 2D flow field at once with either two or three velocity components.
PIV data is directly applicable for the computation time-mean statistics, such as mean- and RMS-fields and several derived quantities like Reynolds stresses. A visual examination of the 2D PIV data allows certain deductions regarding the spatial scales and the nature of coherent flow structures to be made. However, a careful analysis of these features is sometimes needed. To this end, wavelet, spectral and 2D space-correlation function estimates can be employed. Also individual coherent flow structures, such as vortices can be detected and their characteristics studied.
Besides applying PIV to several academic research projects and industrial problems, the EFD group has studied the PIV technique and especially the post-processing algorithms for data analysis.
Direct imaging (or digital imaging, DI) or comprises a wide collection of techniques using optics and digital 2D sensors to provide image data. Methods of signal processing and image analysis are used to examine the data. In multi-phase flows one can analyze for example velocities, morphology, deformation and break-up - coalescence processes of bubbles, flocks and other solids in the flow. In problems concerning fluid-structure interactions, high-speed digital imaging can be used to observe the movement, deformation and vibration of solid surfaces under the action of fluid mechanical forces.
The EFD group has concentrated on developping DI techniques especially for multi-phase flows.
Markus Honkanen: Turbulent Multiphase Flow Measurements with Digital Particle Image Velocimetry: Application to Bubbly Flows, MSc-thesis
Laser Doppler anemometer (LDA) is an optical technique for fluid velocity measurement. LDA system can normally measure two velocity components in a point, but systems able to measure all three components are also available. The method is non-intrusive but it need seeding particles in the flow and a disturbance free optical access into the investigated flow field. LDA's main advantages are non-intrusiveness and relative small measurement volume. Statistical analysis of the velocity data is straightforward but time series analysis needs special algorithms. Besides applying LDA technique in several academic and industrial research projects, the EFD group has also studied the LDA technique for time series analysis.
Phase Doppler Anemometer (PDA) is an optical technique to measure the velocity and size of particles passing the measurement spot. PDA is an extension to the Laser Doppler Anemometer (LDA) and uses the same optical configuration and principle for the velocity measurement. PDA needs an optical access into the investigated flow field from two specific directions. Its main advantages are non-intrusiveness and relatively small measurement volume. Statistical analysis of the velocity and size data is straightforward but time series analysis for the velocity needs special algorithms. The method also provides size-velocity correlations.
Hot Wire Anemometry (HWA) is a technique for velocity measurement in turbulent aerodynamic flows. Normal HWA-instruments can measure two velocity components in a point but also three component measurement is possible. The method is intrusive but if the flow has a well-defined convective component, the disturbance can be made very small. The probe needs an access into the investigated flow field. HWA's main advantages are continuous signal and good spatial and temporal resolution. Statistical analysis of the velocity data is straightforward and also time series analysis can be made with ordinary tools.