New experimental techniques

An important aspect of our research is improving existing experimental methods and developing new techniques for experimental research in fluid mechanics. Both flow visualization and quantitative flow diagnostic are of interest. For example, we recently developed a new technique for three-dimensional flow visualization and surface flow visualization. Until recently, methods for surface visualizations were not yet available for experiments in water, and flow visualization was limited to narrow planar slices. The new technique is a modification a classical hydrogen bubble technique, where flow is visualized by producing small hydrogen bubbles via electrolysis. Illuminating these bubbles within a volume by a laser (figure 8) allows three-dimensional flow visualization. For the surface visualization, a by-product of the electrolysis is taken advantage of. Specifically, insoluble salts are created on the hydrogen bubble wire during electrolysis and are introduced in the flow as small solid particulates. When these particles interact with a solid boundary of an experimental model, they leave behind a white residue, as seen in figure 9, visualizing near-surface flow patterns.

experimental setup for the testing of the new flow visualization technique

Figure 8. Experimental setup for the testing of the new flow visualization technique

Hydrogen bubble surface visualization on a dual-step cylinder for ReD = 2,100, D/d = 2, and L/D = 0.5.

Figure 9. Hydrogen bubble surface visualization on a dual-step cylinder for ReD = 2,100, D/d = 2, and L/D = 0.5.

 
 
Another technique that is being developed and applied in our lab is the use of planar, two-dimensional PIV measurements to study three-dimensional vortex dynamics. By collecting simultaneous measurements in orthogonal planes, conditional averaging can be used to reconstruct three dimensional topologies corresponding to a particular flow events of interest. The results are illustrated in Figure 10, and this technique can be utilized in a wide range of vortex-dominated flows.
 
sequence of three-dimensional reconstructions of the wake development of a dual step cylinder at Red=2100, Spanwise vorticity isosurfaces are colored by the sign of the spanwise vorticity

Figure 10. Sequence of three-dimensional reconstructions of the wake development of a dual step cylinder at ReD = 2100. Spanwise vorticity isosurfaces are colored by the sign of the spanwise vorticity.

The final example is the use of embedded microphone arrays - similar to that seen in figure 11 - to study unsteady flow phenomena. It was used in studies performed on an airfoil model operating at low Reynolds numbers. The investigated flow regime involves boundary layer separation, laminar-to-turbulent transition, and possible flow reattachment – all of which are unsteady phenomena difficult to investigate either experimentally or numerically. The tests demonstrated the system can be used to characterize boundary layer behaviour and flow transition. The new system is of interest since it represents a non-intrusive measurement technique that can be implemented in practical engineering applications where online flow diagnostic is required. Moreover, its multi-point, non-intrusive measurement capabilities are invaluable for investigating laminar to turbulent transition, allowing estimating frequency, growth rate, propagation speed, and other characteristics of dominant flow disturbances governing the transition process.

airfoil model with an embedded microphone array

Figure 11. Airfoil model with an embedded microphone array.