High Free-stream Turbulence Levels Research Articles

Influence of Free-Stream Turbulence on Turbulent Boundary Layer Heat Transfer and Mean Profile Development, Part I—Experimental Data

An experimental research program was conducted to determine the influence of free-stream turbulence on zero pressure gradient, fully turbulent boundary layer flow. Connective heat transfer coefficients and boundary layer mean velocity and temperature profile data were obtained for a constant free-stream velocity of 30 m/s and free-stream turbulence intensities ranging from approximately 1/4 to 7 percent. Free-stream multicomponent turbulence intensity, longitudinal integral scale, and spectral distributions were obtained for the full range of turbulence levels. The test results with 1/4 percent free-stream turbulence indicate that these data were in excellent agreement with classic two-dimensional, low free-stream turbulence, turbulent boundary layer correlations. For fully turbulent boundary layer flow, both the skin friction and heat transfer were found to be substantially increased (up to ∼ 20 percent) for the higher levels of free-stream turbulence. Detailed results of the experimental study are presented in the present paper (Part I). A comprehensive analysis is provided in a companion paper (Part II).

Effect of free stream turbulence on the flow around bluff bodies

Many bodies of practical interest are bluff, and there is a large volume of literature which deals with various aspects of the very complex flows about them. Most of the work done to data has been experimental, with the majority of it in wind tunnels in flows with low levels of free stream turbulence (FST). By contrast, in actual applications one often encounters fluid streams with high levels of FST. This introduces an additional parameter into an already complex problem, one which has to be understood before wind tunnel data may be properly extrapolated and applied. In this review the basic mechanisms by which FST and the overall mean flow over bluff bodies (at high Reynolds numbers) interact are discussed in some detail. There are three basic mechanisms that may be identified: accelerated transition to turbulence in shear layers, enhanced mixing and entrainment, and distortion of FST itself by the mean flow. The overall effect of FST is often the result of more than one of these basic mechanisms. The nature of the effect, and its magnitude, are illustrated by examples which show that in some cases it depends on both the intensity and the scale of the FST, while in others only intensity is important.

Boundary-layer profile measurements in a combustion driven MHD generator

The velocity, temperature, and electron number density profiles were measured in the electrode wall boundary layer of a combustion driven MHD generator. Both subsonic and supersonic conditions were run. The experimental results are compared with predictions of a two-dimensional turbulent boundary-layer computation. For the subsonic condition, high levels of freestream turbulence were measured, about 10-12 percent. The measured velocity profile was fatter than that predicted, although the temperature and electron number density profiles were in agreement. This difference is tentatively ascribed to the high freestream turbulence levels. There was no measurable MHD effect for the subsonic case. For the supersonic condition, the measured velocity, temperature, and electron number density profiles fell under the predicted profiles. The discrepancy may be due to three-dimensional recirculation effects. There was a small amount of MHD interaction, the degree of which was in agreement with predictions. Electron number density nonequilibrium was not identified, but the degree of nonequilibrium predicted was small. Under the appropriate supersonic conditions, primarily at freestream temperatures below 2400K, ionization nonequilibrium is predicted to occur.

Time interval histogram mode data processing of the alpha rhythm output integration : S. L. Visser (Amsterdam, The Netherlands)

An algebraic model using the intermittency concept is proposed for laminar to turbulent transition in attached and separated boundary layers. The model modifies the production terms of the k–ω RANS turbulence model by Wilcox. For attached flows, the model describes bypass transition by taking into account two main effects, which are damping of high-frequency disturbances by a laminar shear layer and breakdown of a near-wall laminar layer perturbed by streaks. For separated flow, the model describes breakdown of a laminar free shear layer. The proposed model is a modified and extended version of an earlier model by the authors for bypass transition only (Kubacki and Dick, Int. J. Heat and Fluid Flow, 58, 68–83, 2016).The primary tuning of the model has been done with flat plate T3C flows of ERCOFTAC with steady approaching flow, relevant for bypass transition and separation-induced transition. Secondary tuning has been done with two cascades with steady approaching flow at high turbulence level, one with N3-60 (Re = 6 × 105) steam turbine stator vanes and one with V103 (Re = 1.385 × 105) compressor blades. Model validation has been done for steady approaching flows of the same cascades, but including a case with low level of free-stream turbulence, and for a cascade with T106A (Re = 1.6 × 105) gas turbine rotor blades for high and low levels of free-stream turbulence. Further validation has been done for flow perturbed by travelling wakes through a cascade of N3-60 vanes at high and low levels of background turbulence level. The transition model produces good results for all cases, both for bypass transition in attached boundary layer state and for transition in separated boundary layer state.

An Experimental Study of Boundary-Layer Transition

Abstract : The transition from laminar to turbulent flow on a smooth flat plate in a zero pressure gradient was studied in a wind tunnel by using conventional surface-tube and hot-wire techniques. In the 0.10 to 0.5% range of free-stream turbulence, the laminar oscillations predicted by the Tollmien-Schlichting theory of laminar-layer stability appeared to play an important role in transition. In the process of transition which was similar to the development of a turbulent wake from a vortex street, the amplified frequency fed the rest of the spectrum. A study of the effect of free-stream turbulence on transition, in terms of the energy concentrated at the amplified frequency, revealed that the free-stream turbulence hastened the feeding of energy from the amplified frequency to the rest of the spectrum. Laminar oscillations appeared to play no part in transition at much higher levels of free-stream turbulence. A suggested possible control for transition theory proposed by Taylor.