Freestream

About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.

Instability and transition on the HIFiRE-5 in a Mach-6 Quiet Tunnel

Abstract: : A 2:1 aspect-ratio elliptic cone with a blunt nosetip was tested in the Boeing/ AFOSR Mach-6 Quiet Tunnel to investigate the effects of freestream noise level, surface roughness, angle of attack, and freestream Reynolds number on laminar-to turbulent boundary layer transition on the windward surface The cone had a minor axis half-angle of 7-deg and a nose radius of 095 mm Temperature-sensitive paint enabled a global measurement of the temperature distribution and detection of the transition front Transition apparently arising from two mechanisms was observed: transition along the centerline suspected to arise from the amplification of second mode waves in the inflected boundary layer, and transition roughly halfway between the centerline and leading edges probably due to the breakdown of cross flow vortices Reducing noise level from conventional (root-mean-square pressure 3% of the mean) to quiet (root-mean-square pressure less than 01% of the mean) substantially delayed transition due to both mechanisms Increasing the angle of attack from 0-deg to 4-deg delayed the cross flow transition mode on the windward side Transition moved forward as free stream unit Reynolds number increased from 26-deg 106 /m to 119-deg 106 /m PCB fast-response pressure transducers installed along the model centerline detected apparent instabilities at frequencies from 50 to 150 kHz prior to transition under noisy flow; quiet-flow results are less clear Mass flux profiles along the centerline were measured with a calibrated hot wire

A Critical Assessment of Reynolds Analogy for Turbine Flows

Abstract: The application of Reynolds analogy (2St/c f ≅1) for turbine flows is critically evaluated using experimental data collected in a low-speed wind tunnel. Independent measurements of St and c f over a wide variety of test conditions permit assessments of the variation of the Reynolds analogy factor (i.e., 2St/c f ) with Reynolds number, freestream pressure gradient, surface roughness, and freestream turbulence. While the factor is fairly independent of Reynolds number, it increases with positive (adverse) pressure gradient and decreases with negative (favorable) pressure gradient. This variation can be traced directly to the governing equations for momentum and energy which dictate a more direct influence of pressure gradient on wall shear than on energy (heat) transfer. Surface roughness introduces a large pressure drag component to the net skin friction measurement without a corresponding mechanism for a comparable increase in heat transfer. Accordingly, the Reynolds analogy factor decreases dramatically with surface roughness. Freestream turbulence has the opposite effect of increasing heat transfer more than skin friction, thus the Reynolds analogy factor increases with turbulence level. Physical mechanisms responsible for the observed variations are offered in each case. Finally, synergies resulting from the combinations of pressure gradient and freestream turbulence with surface roughness are evaluated.

Experimental study of boundary layer transition with elevated freestream turbulence on a heated flat plate

Abstract: A detailed investigation to document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional, and low Reynolds number turbulent boundary layers are presented. Measurements were acquired in a low-speed, closed-loop wind tunnel with a freestream velocity of 100 ft/s and zero pressure gradient over a range of freestream turbulence intensities (TI) from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate and Reynolds shear stress were all consistent with previously published data. Reynolds analogy factors for R(sub theta) is less than 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length. The measured laminar value of Reynolds analogy factor was as much as 53 percent higher than the Pr(sup -2/3). A small dependence of turbulent results on TI was observed. Conditional sampling performed in the transitional boundary layer indicated the existence of a near-wall drop in intermittency, pronounced at certain low intermittencies, which is consistent with the cross-sectional shape of turbulent spots observed by others. Non-turbulent intervals were observed to possess large magnitudes of near-wall unsteadiness and turbulent intervals had peak values as much as 50 percent higher than were measured at fully turbulent stations. Non-turbulent and turbulent profiles in transitional boundary layers cannot be simply treated as Blasius and fully turbulent profiles, respectively. The boundary layer spectra indicate predicted selective amplification of T-S waves for TI is approximately 0.4 percent. However, for TI is approximately 0.8 and 1.1 percent, T-S waves are localized very near the wall and do not play a dominant role in transition process.

Transition of compressible high enthalpy boundary layer flow over a flat plate

Abstract: This paper presents the results of an experimental investigation into the characteristics of boundary layer transition to turbulence in hypervelocity air flows. A series of experiments was conducted using a flat plate model, equipped with static pressure and thin film beat transfer transducers, in a free piston shock tunnel. Transition was observed in the stagnation enthalpy range of 2.35 to 19.2 MJ/kg. The transition Reynolds number correlates well with the unit Reynolds number through a simple empirical relation. The influences of Mach number, pressure and wall cooling are examined. The measured heat transfer rates in laminar and turbulent regions are compared with empirical predictions. Freestream disturbances of the test flow were also measured and analysed.