A. M. Savill

Three-dimensional large-eddy motions and fine-scale activity in a plane turbulent wake

A pattern recognition technique has been applied to simultaneously sampled multipoint hot-wire anemometry data obtained in the far wake of a circular cylinder. Data from both the streamwise fluctuating velocity field and the temperature field have been analysed employing a computer code that uses a correlation approach to automatically detect and ensemble average flow patterns and patterns for mean-square fluctuations

Reducing bottlenecks in the CAD-to-mesh-to-solution cycle time to allow CFD to participate in design

As CFD has matured to the point that it is capable of reliable and accurate flow simulation, attention is now firmly fixed on how best to deploy that CFD as part of a process to improve actual products. This process consists of capturing and controlling the geometry of a suitable portion of an aeroengine (e.g., a blade row, or an internal cooling system or a fan-plus-nacelle), building a mesh system, solving the flow and responding to an appropriately visualized flow field by changing or accepting the geometry. This paper looks at that process from the point of view of identifying any bottlenecks and argues that current research should be directed at the CAD-to-mesh-to-solution cycle time rather than, as has been traditional, just looking at the solver itself and in isolation. Work aimed at eliminating some of these bottlenecks is described, with a number of practical examples.

Biobjective Design Optimization for Axial Compressors Using Tabu Search

At present, optimization is an enabling technology in innovation. Multi-objective and multidisciplinary optimization tools are essential in the design process for real-world applications. In turbomachinery design, these approaches give insight into the design space and identify the tradeoffs between the competing performance measures. This paper describes the application of a novel multi-objective variant of the tabu search algorithm to the aerodynamic design optimization of turbomachinery blades. The aim is to improve the performance of a specific stage and eventually of the whole engine. The integrated system developed for this purpose is described. It combines the optimizer with an existing geometry parameterization scheme and a well-established computational fluid dynamics package. Its performance is illustrated through a case study in which the flow characteristics most important to the overall performance of turbomachinery blades are optimized.

Predicting turbulent flow in a staggered tube bundle

Abstract This paper presents the results of calculations performed for the turbulent, incompressible flow around a staggered array of tubes for which carefully obtained experimental results are available as part of an established ERCOFTAC-IAHR test case. The Reynolds-averaged Navier–Stokes equations are solved using a pressure-based finite volume algorithm, using collocated cell vertex store on an unstructured and adaptive mesh of tetrahedra. Turbulence closure is obtained with a truncated form of a low-Reynolds number k – e model developed by Yang and Shih. The computational domain covers all seven rows of tubes used in the experimental study and periodic flow is allowed to develop naturally. The results of the computations are surprisingly good and compare favourably with results obtained by others using a wide range of alternative k – e models for a single cylinder with periodic inflow and outflow boundaries on structured meshes.

Intermittency Transport Modeling of Separated Flow Transition

An intermittency transport model is proposed for modeling separated-flow transition. The model is based on earlier work on prediction of attached flow bypass transition and is applied for the first time to model transition in a separation bubble at various degrees of free-stream turbulence. The model has been developed so that it takes into account the entrainment of the surrounding fluid. Experimental investigations suggest that it is this phenomena which ultimately determines the extent of the separation bubble. Transition onset is determined via a boundary layer correlation based on momentum thickness at the point of separation. The intermittent flow characteristic of the transition process is modeled via an intermittency transport equation. This accounts for both normal and streamwise variation of intermittency and hence models the entrainment of surrounding flow in a more accurate manner than alternative prescribed intermittency models. The model has been validated against the well established T3L semicircular leading edge flat plate test case for three different degrees of free-stream turbulence characteristic of turbomachinery blade applications.

Drag measurements on planar riblet surfaces at high subsonic speeds

This paper presents the results of an investigation of riblet performance at high subsonic Mach numbers, and Reynolds numbers of about 20 000 based on the momentum thickness, in both zero and adverse pressure gradient boundary layers. The maximum length Reynolds number of the ribbed section was 3.4×107 so the results were directly relevant to flight applications on the engine nacelles of civil airliners. Seven different sizes of riblets with heights h (equal to spacing s) ranging from 0.0007″ (0.0178mm) to 0.006″ (0.1524 mm) have been studied, covering a range of h+, s+ from 10 to 106. The maximum percentage skin friction reduction, as deduced from velocity profiles measured at the downstream end of the riblet surfaces, under nominally zero pressure gradient conditions was 5.5±1; rather less than that recorded in low speed studies, but consistent with a recent theoretical analysis of the effect of Reynolds number. The values of h+ required for maximum and zero skin friction reduction agreed closely with other data. In addition subsequent floating element drag balance measurements revealed little effect of yaw angles up to 15°, again in line with other findings, and also suggested that the extent of the initial development length on, and recovery length behind, the riblets was approximately 5δ. The adverse pressure gradient studies indicated that riblet performance was essentially unaffected by mild gradients (β=0.25), but diminished to zero in a more severe gradient (β=0.5).

Tests of drag-reducing polymer coated on a riblet surface

Experiments have been carried out at BMT where the drag reduction due to Hoechst U-groove riblets, a polymer coating, and the two combined were measured in a towing tank on a one-third scale model of the Americas Cup winning yacht, Australia II. The results indicated that the riblet/polymer combination offered an overall improvement in drag reduction characteristics over either riblets or polymer coating alone, with a maximum reduction of 3.5% observed for a non-dimensional S+=8. The “qualitative” behaviour of the drag reduction was similar to that recorded in earlier pipe flow experiments, employing an injection of polymer additive and 3M V-groove riblets, but contrary to that recorded in studies of an axisymmetric body, also coated with 3M riblets, in a drop tank filled with a polymer solution.

Résumé of important results presented at the third turbulent drag reduction working party

A working party on drag reduction in turbulent boundary layers was held at ONERA/Chatillon, France during September 29–30, 1988. Two passive methods were discussed, in particular: internal manipulators, or “riblets”, (surface modification in the form of longitudinal striations) and external manipulators, or “LEBUs” also described as Outer Layer Devices, blades or ribbons, consisting of thin flat plate or aerofoil section devices embedded within the flow. Experimental as well as theoretical research was presented. This paper summarizes the results of this work and thus provides an overview of the current state of such research in Europe.

Evaluating Turbulence Model Predictions of Transition

This paper presents initial results from a collaborative assessment of turbulence model predictions for transition under the influence of free-stream turbulence. Results are presented from a number of test cases and some recommendations are made as to the best present models for use in engineering design codes.

Skin friction measurements following manipulation of a turbulent boundary layer

Results are presented from three experiments in which direct, local measurements of the skin friction reduction due to flat-plate turbulence manipulators for overall viscous drag reduction were obtained. The results suggest that only a very small overall net drag reduction will be possible for such devices at moderate momentum thickness-derived Reynolds number values, since maximum skin friction drag is neither large nor sustained.