Effects Of Elevated Free-Stream Turbulence Research Articles

Effect of Elevated Free-Stream Turbulence on Transitional Flow Heat Transfer Over Dual-Scaled Rough Surfaces

The surface roughness over a serviced turbine airfoil is usually multiscaled with varying features that are difficult to be universally characterized. However, it was previously discovered in low free-stream turbulence conditions that the height of larger roughness produces separation and vortex shedding, which trigger early transition and exert a dominant effect on flow pattern and heat transfer. The geometry of the roughness and smaller roughness scales played secondary roles. This paper extends the previous study to elevated turbulence conditions with free-stream turbulence intensity ranging from 0.2% to 6.0%. A simplified test condition on a flat plate is conducted with two discrete regions having different surface roughness. The leading-edge roughness is comprised of a sandpaper strip or a single cylinder. The downstream surface is either smooth or covered with sandpaper of grit sizes ranging from 100 to 40 Ra=37-119 μm. Hot wire measurements are conducted in the boundary layer to study the flow structure. The results of this study verify that the height of the largest-scale roughness triggers an earlier transition even under elevated turbulence conditions and exerts a more dominant effect on flow and heat transfer than does the geometry of the roughness. Heat transfer enhancements of about 30–40%-over the entire test surface are observed. The vortical motion, generated by the backward facing step at the joint of two roughness regions, is believed to significantly increase momentum transport across the boundary layer and bring the elevated turbulence from the freestream towards the wall. No such long-lasting heat transfer phenomenon is observed in low free-stream turbulence cases even though vortex shedding also exists in the low turbulence cases. The heat transfer enhancement decreases, instead of increases, as the downstream roughness height increases.

Effects of Elevated Free-Stream Turbulence on Actively Controlled Separation Bubble1

The effects of elevated free-stream turbulence (FST) on natural and periodically excited separation bubbles were examined experimentally at low Reynolds numbers. The bubble was formed at the leading edge of a flat plate and the FST level was altered by placing a grid across the flow at different locations upstream of the plate. The mixing across the separated shear-layer increased due to the elevated FST and the two-dimensional periodic excitation, flattening, and shortening the bubble. Periodic excitation at frequencies that were at least an order of magnitude lower than the initial Kelvin–Helmholtz instability of the separated shear-layer were very effective at low FST. The fundamental excitation frequency and its first harmonic were amplified above the bubble. High frequency excitation (F+≈3, based on the length of the natural low FST bubble that served as a reference baseline) affected the flow near the leading edge of the bubble but it rapidly decayed in the reattachment region. Lower frequencies F+≈1 were less effective and they decayed at a slower rate downstream of reattachment. An increase in FST level reduced the net effect of the periodic excitation on mixing enhancement and subsequent reattachment process. This was probably due to a destructive interference between the nominally 2D excitation and the random, in space and in time, FST. High FST is known to reduce the spanwise coherence in free shear layers [e.g., Chandrasuda, C., Mehta, R. D., Weir, A. D., and Bradshaw, P., 1978, “Effects of free-stream turbulence on large structures in turbulent mixing layers,” J. Fluid Mech., 85, pp. 693–704] and therefore also the effectiveness of the current control strategy, this not withstanding 2D periodic excitation accelerated the reattachment process and the recovery rate of the attached boundary layer, reducing its momentum loss.

Free-Stream Turbulence Effects on Film Cooling With Shaped Holes

A comprehensive set of generic experiments has been conducted to investigate the effect of elevated free-stream turbulence on film cooling performance of shaped holes. A row of three cylindrical holes as a reference case, and two rows of holes with expanded exits, a fanshaped (expanded in lateral direction), and a laidback fanshaped hole (expanded in lateral and streamwise direction) have been employed. With an external (hot gas) Mach number of Mam=0.3 operating conditions are varied in terms of free-stream turbulence intensity (up to 11%), integral length scale at constant turbulence intensity (up to 3.5 hole inlet diameters), and blowing ratio. The temperature ratio is fixed at 0.59 leading to an enginelike density ratio of 1.7. The results indicate that shaped and cylindrical holes exhibit very different reactions to elevated free-stream turbulence levels. For cylindrical holes film cooling effectiveness is reduced with increased turbulence level at low blowing ratios whereas a small gain in effectiveness can be observed at high blowing ratios. For shaped holes, increased turbulence intensity is detrimental even for the largest blowing ratio M=2.5. In comparison to the impact of turbulence intensity the effect of varying the integral length scale is found to be of minor importance. Finally, the effect of elevated free-stream turbulence in terms of heat transfer coefficients was found to be much more pronounced for the shaped holes.

Combined effects of elevated free-stream turbulence and streamwise acceleration on flow and thermal structures in transitional boundary layers

The combined effects of elevated free-stream turbulence intensity (FSTI) and streamwise acceleration on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate with elevated FSTI levels ranging from 3 to 7%. The acceleration strength, based on the parameter, K [≡ (ν/ U ∞ 2)d U ∞/dx] , rangedfrom 0.39 × 10 −6 to 4.1 × 10 −6. A three-wire probe was used to measure detailed momentum and thermal boundary layer structures, including the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, Reynolds heat fluxes, eddy viscosity, turbulent thermal diffusivity, and turbulent Prandtl number. The results show that FSTI has a greater effect than streamwise acceleration on flow and thermal structures in transitional boundary layers. However, the effect of streamwise acceleration on increasing the transition length is significant. Furthermore, streamwise acceleration suppresses the magnitude of uv and slows down the uv transition process relative to u′ even when elevated FSTI seems to have a dominant effect. Acceleration is also shown to suppress ϵ H more than ϵ M in the transitional region.

Effects of Elevated Free-Stream Turbulence on Flow and Thermal Structures in Transitional Boundary Layers

The effects of elevated free-stream turbulence on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate. Detailed boundary layer measurements using a three-wire probe and wall heat transfer were made with free-stream turbulence intensities of 0.5, 3.8, 5.5, and 6.4 percent, respectively. The onset of transition, transition length, and the turbulent spot formation rate were determined. The statistical results of the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, and Reynolds heat fluxes were presented. The eddy viscosity, turbulent thermal diffusivity, and the turbulent Prandtl number were calculated and related physical mechanisms are discussed.

Effect of Periodic Passing Wakes on the Leading Edge Film Cooling Effectiveness. Studies on 90-degree-inclination Cooling Holes.

Detailed studies are conducted on film effectiveness of discrete cooling holes around the leading edge of a blunt body that is subjected to periodically incoming wakes as well as free-stream turbulence with various levels of intensity. The cooling holes have a similar configuration to that of typical turbine blades except for the spanwise inclination angle. Secondary air is heated so that the temperature difference between the primary and secondary air is about 20 K. In this case, the air density ratio of the primary and secondary flows becomes less than unity ; therefore the density ratio encountered in an actual aero-engine cannot be simulated in the present study. A spoped-wheel-type wake generator is used in this study. Moreover, three types of turbulence grids are used to elevate the free-stream turbulence intensity. We adopt three blowing ratios of the secondary flow to the primary flow. For each of the blowing ratios, wake-affected adiabatic wall temperature around the test surface is measured with the thermocouples in the test model. The effect of elevated free-stream turbulence on the wake-affected film effectiveness is also investigated. Flow visualization by use of liquid crystal is performed to obtain qualitative information on the film-effectiveness distributions.