Turbulent Near-Wall Layers

In this experiment we investigate flows of air with fixed characteristics, e.g. velocity, degree of turbulence. The structures of turbulence elements in the flow, especially close to the wall are visualized. The aim of the experiment is to use and develop techniques for the visualization of flow structures. In literature there are mainly two approaches mentioned.

- The most common method is to seed the flow with particles and afterwards to visualize the particles by scattering of laser light. The scattered light of the particles allows an observation of their movement and therefore indicates the flow velocity distribution. This approach is called Particle Image Velocimetry (PIV) or Particle Tracking Velocimetry (PTV) depending on the number of particles used.

- Another possibility is to use certain species/molecules which are already present in the flow. Laserspectroscopic methods allow a molecule specific visualization of those species. The idea is to generate a structure only consisting of one species and afterwards to visualize this structure. The altering of this structure provides information about the flow. This method is called Flow Tagging.

Figure 1: Experimental setup for diagnostics in the near-wall layer	Figure 2: Flow-tagging measurements based on NO2 photolysis and NO-LIF detection

We have developed a method for the observation of near-wall flow structures using flow tagging based on NO₂ photolysis. An Nd:YAG laser (3rd harmonic at 355 nm) is focussed perpendicularly to the flow direction and perpendicularly to the wall into the investigation area. It photodissociates NO2 to NO (the airflow is seeded with about 500 ppm NO₂) along its path. A dye laser is tuned to one of the electronic transitions of the NO molecule close to 226 nm. The dye laser is focussed to a light sheet and visualizes the produced NO molecules with laser-induced fluorescence (LIF) imaging. Perpendicularly to the dye laser sheet and perpendicularly to the flow direction the fluorescence of the NO molecule is observed by a CCD camera. The results are pictures of the “written” NO pattern, in this case a simple line along the beam path of the photodissociating laser. With increasing delay between the generation and the detection of the NO, the shape of the line is altered by turbulence. One obvious result is the expected lower flow velocities close to the wall of the flow channel (on the left and right sides of the picture). This method offers the possibility to observe flow structures very close to the wall.

References:

[1] J. v. Saldern, S. Doose, C. Orlemann, and C. Schulz, "Investigation of small-scale wall near flow structures using NO-tagging," in Proceedings of: Application of laser techniques to fluid mechanics (Lisbon, 2000).
[2] T. Fuyuto, H. Kronemayer, B. Lewerich, W. Koban, K. Akihama, and C. Schulz, "Laser-based temperature imaging close to surfaces with toluene and NO-LIF," in International Conference on Laser Diagnostics, ICOLAD2005 (London, 2005), 53-61.
[3] C. Orlemann, C. Schulz, and J. Wolfrum, "NO-flow tagging by photodissociation of NO2. A new approach for measuring small-scale flow structures," Chem. Phys. Lett. 307, 15-20 (1999).