Flow Visualization with Laser-Induced Thermal Tufts

Abstract

The laser-induced thermal tuft is a new flow visualization technique for simulating traditional tufts with a thermal plume. A laser is used to heat a point of interest on a windtunnel model, causing downstream convection of thermal energy from the heated spot. A temperature-sensitive coating is used to visualize the thermal plume. This technique can be used to indicate flow direction, locate regions of separated flow, and detect laminar/turbulent transition. One primary advantage of thermal tufts is that the measurement technique is less intrusive than traditional tufts. In addition, thermal tufts may be generated at any optically-accessible point during a test, whereas string tufts must be applied to specified locations before a test. This enables greater experimental efficiency, which is particularly important in large-scale ground-testing facilities. This work extends and develops the thermal tuft concept by employing temperature-sensitive paint, as well as the previously used thermochromic liquid crystals. The effect of various substrate materials on tuft quality is evaluated. Calibrations of tuft length dependency on Reynolds number and laser power are made. Furthermore, a computational model is developed to simulate the tuft shape and structure. Finally, a new variation of the technique is presented, based on thermal ablation of the substrate material.

Optical Method for Measuring Low Wall Shear Stresses Using Thermal Tufts

“…Their measurements show the length of the thermal tuft at a fixed axial position on a flat plate increasing with increasing velocity. This trend is in the opposite direction of the measurements presented by Gregory and Peterson [3] and Baughn et al [13], in which the tuft length decreased when the velocity increased. The reason for the opposite trend reported by Hunt and Pantoya [12] is unknown.…”

“…An experimental test of the shear stress dependency was performed to validate the theory and help resolve the discrepancy between the results of Gregory and Peterson [3] and Hunt and Pantoya [12]. Gregory and Peterson [3] reported that tuft length decreases with velocity, whereas Hunt and Pantoya [12] reported results of the opposite trend.…”

“…Gregory and Peterson [3] reported that tuft length decreases with velocity, whereas Hunt and Pantoya [12] reported results of the opposite trend. Both sets of results, however, correlated tuft length with velocity, whereas the current work demonstrates conclusively that this is erroneous because the correlation of tuft length is with shear stress.…”

“…A typical experimental example of a thermal tuft is shown in Fig. 1 [3]. The thermal tuft is generated by advection causing the heated or cooled air to change the temperature distribution on the surface downstream of the spot.…”

“…Smith et al [11] also produced a cool spot using evaporative cooling. Gregory and Peterson [3] used a laser to produce the heated spot and employed liquid crystal thermography and temperature-sensitive paint to observe the thermal tuft. They also introduced a new variation based on thermal ablation of the substrate material.…”