The transition of a boundary layer from a laminar to a turbulent state is known to be affected by several flow and surface conditions, including flow Reynolds number, streamwise pressure gradient, free-stream turbulence, surface roughness, and periodic oscillations in free-stream velocity. Although the effects of these parameters on the transition process have been the subject of numerous investigations over the past decades, the interactions between these conditions, when present simultaneously, have yet to be fully documented. The current research program was undertaken to increase our understanding of the process of boundary layer transition, and to quantify the relative importance of these conditions. The majority of the present study consists of hot-wire anemometer measurements of a two-dimensional boundary layer developing over a flat surface. The ranges of Reynolds number, pressure distribution, free-stream turbulence, periodic¬ unsteadiness, and surface roughness were representative of the suction side of low pressure turbine blades, and residted in both attached-flow and separation-bubble transition processes. Based on the experimental results, an improved transition model to predict these effects is presented. This model is applicable to a broader range of flow and surface conditions than available alternatives. The experiments are complemented by numerical simulation of one of the test cases, by means of large-eddy simulation. The simulation provides an opportunity for detailed study of the unsteady flow phenomena and vortex shedding processes that accompany transition and reattachment of the separation bubble. Yaras, for his enthusiastic participation and continuous support during this research. 1 consider myself very fortunate that many thankless tasks, such as the majority of design and commissioning of the experimental test section, had already been completed before I joined this project. I am also very grateful for the cooperation of Profs. J.
Many gases, including carbon dioxide and argon, have been considered as alternative working fluids to air in a number of design studies for closed and semi-closed gas turbine engines. In many of these studies, it has been assumed that if the gas constant R and specific heat ratio γ are included in the speed and flow parameters, the compressor map or turbine characteristic is applicable to other working fluids. However, similarity arguments show that the isentropic exponent itself is a criterion of similarity and that the turbomachinery characteristics, even when appropriately nondimensionalized, will, in principle, vary as the γ of the working fluid varies. This paper examines the effect of γ on turbomachinery characteristics, mainly in terms of compressors. The performance of a centrifugal compressor stage was measured using air (γ=1.4), CO2(γ=1.29), and argon (γ=1.67). For the same values of the nondimensional speed, the pressure ratio, efficiency, and choking mass flow were found to be significantly different for the three test gases. The experimental results have been found to be consistent with a CFD analysis of the impeller. Finally, it is shown that the changes in performance can be predicted reasonably well with simple arguments based mainly on one-dimensional isentropic flow. These arguments form the basis for correction procedures that can be used to project compressor characteristics measured for one value of γ to those for a gas with a different value.
This paper presents a mathematical model for predicting the rate of turbulent spot production. In this model, attached- and separated-flow transition are treated in a unified manner, and the boundary layer shape factor is identified as the parameter with which the spot production rate correlates. The model is supplemented by several correlations to allow for its practical use in the prediction of the length of the transition zone. Second, the paper presents a model for the prediction of the location of transition inception in separation bubbles. The model improves on the accuracy of existing alternatives, and is the first to account for the effects of surface roughness.
This paper presents experimental results documenting the effects of surface roughness and free-stream turbulence on boundary-layer transition. The experiments were conducted on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of low-pressure turbine blades was imposed. The test matrix consists of five variations in the roughness conditions, at each of three free-stream turbulence intensities (approximately 0.5%, 2.5%, and 4.5%), and two flow Reynolds numbers of 350,000 and 470,000. The ranges of these parameters considered in the study, which are typical of low-pressure turbines, resulted in both attached-flow and separation-bubble transition. The focus of the paper is on separation-bubble transition, but the few attached-flow test cases that occurred under high roughness and free-stream turbulence conditions are also presented for completeness of the test matrix. Based on the experimental results, the effects of surface roughness on the location of transition onset and the rate of transition are quantified, and the sensitivity of these effects to free-stream turbulence is established. The Tollmien–Schlichting instability mechanism is shown to be responsible for transition in each of the test cases presented. The root-mean-square height of the surface roughness elements, their planform size and spacing, and the skewness (bias towards depression or protrusion roughness) of the roughness distribution are shown to be relevant to quantifying the effects of roughness on the transition process.
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