Hector Iacovides

Progress in the generalization of wall-function treatments

Abstract This paper describes progress in developing an analytical representation of the variation of the dynamic variables and temperature across the near-wall sublayer of a turbulent flow. The aim is to enable the effective “resistance” of the viscous sublayer to the transport of heat and momentum to be packaged in the form of a “wall function”, thus enabling CFD predictions of convective heat transfer to be made without incurring the cost of the very fine near-wall grid that would otherwise have to be adopted. While the general idea is not new, the detailed strategy contains many new features, which have led to a scheme capable of accounting for the effects of buoyancy, pressure gradient and of variations in molecular transport properties. The scheme is applied to the problem of forced and mixed convection in a vertical pipe and to the opposed wall jet with encouraging results.

Recent progress in the computation of flow and heat transfer in internal cooling passages of turbine blades

Abstract This paper describes our efforts to compute convective heat transfer through gas-turbine blade-cooling passages. The influence on the mean and turbulent flow due to surface rib-roughness is the main focus. A number of turbulence models, of effective viscosity (EVM) and second-moment type, are applied to the computation of flow and heat transfer through ribbed-roughened passages. Computations of flow through a rib-roughened U-bend show that second moment closures are necessary in order to correctly reproduce the regions of flow separation. Heat transfer computations through two-and also three-dimensional ribbed passages reveal that low-Re turbulence models are necessary and that a low-Re differential stress closure yields thermal predictions that are superior to those of the low-Re EVM model. A differential version of the Yap length-scale correction term, independent of the wall distance, is introduced to the dissipation rate equation and is found to improve the heat transfer predictions of the low-Re k−ϵ model.

Marine Cloud Brightening

The idea behind the marine cloud-brightening (MCB) geoengineering technique is that seeding marine stratocumulus clouds with copious quantities of roughly monodisperse sub-micrometre sea water particles might significantly enhance the cloud droplet number concentration, and thereby the cloud albedo and possibly longevity. This would produce a cooling, which general circulation model (GCM) computations suggest could—subject to satisfactory resolution of technical and scientific problems identified herein—have the capacity to balance global warming up to the carbon dioxide-doubling point. We describe herein an account of our recent research on a number of critical issues associated with MCB. This involves (i) GCM studies, which are our primary tools for evaluating globally the effectiveness of MCB, and assessing its climate impacts on rainfall amounts and distribution, and also polar sea-ice cover and thickness; (ii) high-resolution modelling of the effects of seeding on marine stratocumulus, which are required to understand the complex array of interacting processes involved in cloud brightening; (iii) microphysical modelling sensitivity studies, examining the influence of seeding amount, seed-particle salt-mass, air-mass characteristics, updraught speed and other parameters on cloud–albedo change; (iv) sea water spray-production techniques; (v) computational fluid dynamics studies of possible large-scale periodicities in Flettner rotors; and (vi) the planning of a three-stage limited-area field research experiment, with the primary objectives of technology testing and determining to what extent, if any, cloud albedo might be enhanced by seeding marine stratocumulus clouds on a spatial scale of around 100×100 km. We stress that there would be no justification for deployment of MCB unless it was clearly established that no significant adverse consequences would result. There would also need to be an international agreement firmly in favour of such action.

A new wall-function strategy for complex turbulent flows

Wall functions are widely used and offer significant computational savings compared with low-Reynolds-number formulations. However, existing schemes are based on assumed near-wall profiles of velocity, turbulence parameters, and temperature which are inapplicable in complex, nonequilibrium flows. A new wall function has therefore been developed which solves boundary-layer-type transport equations across a locally defined subgrid. This approach has been applied to a plane channel flow, an axisymmetric impinging jet, and flow near a spinning disc using linear and nonlinear k–ϵ turbulence models. Computational costs are an order of magnitude less than low-Reynolds-number calculations, while a clear improvement is shown in reproducing low-Re predictions over standard wall functions.

LDA investigation of the flow development through rotating U-ducts

This paper reports results from the use of laser-Doppler anemometry (LDA) to measure the mean and fluctuating flow field in a U-bend of strong curvature, Rc/D = 0.65, that is either stationary or rotating in orthogonal mode (the axis of rotation being parallel to the axis of curvature). The data acquisition system enables a stationary optical fiber probe to collect flow data from a rotating U-bend sweeping past it. Three cases have been examined, all concerning a flow Reynolds number of 100,000; a stationary case, a case of positive rotation (the pressure side of the duct coincides with the outer side of the U-bend) at a rotational number (ΩD/U m ) of 0.2, and a case of negative rotation at a rotational number of -0.2. Measurements have been obtained along the symmetry plane of the duct and also along a plane near the top wall. The most important influence on the development of the mean and turbulence flow fields is exerted by the streamwise pressure gradients that occur over the entry and exit regions of the U-bend. In the stationary case a three-dimensional separation bubble is formed along the inner wall at the 90 deg location and it extends to about two diameters downstream of the bend, causing the generation of high-turbulence levels. Along the outer side, opposite the separation bubble, turbulence levels are suppressed due to streamwise flow acceleration. For the rotation numbers examined, the Coriolis force also has a significant effect on the flow development. Positive rotation doubles the length of the separation bubble and generally suppresses turbulence levels. Negative rotation causes an extra separation bubble at the bend entry, raises turbulence levels within and downstream of the bend, increases velocity fluctuations in the cross-duct direction within the bend, and generates strong secondary motion after the bend exit. It is hoped that the detailed information produced in this study will assist in the development of turbulence models suitable for the numerical computation of flow and heat transfer inside blade-cooling passages.

Progress in the Use of Non-Linear Two-Equation Models in the Computation of Convective Heat-Transfer in Impinging and Separated Flows

The paper considers the application of the Craft et al. [6]non-linear eddy-viscosity model to separating and impinging flows. The original formulation was found to lead to numerical instabilities when applied to flow separating from a sharp corner. An alternative formulation for the variation of the turbulent viscosity parametercμ with strain rate is proposed which, together with a proposed improvement in the implementation of the non-linear model, removes this weakness. It does, however, lead to worse predictions in an impinging jet, and a further modification in the expression for cμ is proposed, which both retains the stability enhancements and improves the prediction of the stagnating flow. The Yap [24] algebraic length-scale correction term, included in the original model, is replaced with a differential form, developed from that proposed by Iacovides and Raisee [10]. This removes the need to prescribe the wall-distance, and is shown to lead to superior heat-transfer predictions in both an abrupt pipe flow and the axisymmetric impinging jet. One predictive weakness still, however, remains. The proposed model, in common with other near-wall models tested for the abrupt pipe expansion, returns a stronger dependence of Nusselt number on the Reynolds number than that indicated by the experimental data.

The computation of flow development through stationary and rotating U-ducts of strong curvature

Abstract This article presents comparisons between predictions, obtained during the course of this investigation, and recently produced measurements of the flow development through a square cross-sectioned U-bend of strong curvature, Rc/D=0.65, that is either stationary or in orthogonal rotation. For the stationary case, four turbulence models have been tested; a high-Re κ-e model interfaced with the low-Re 1-equation model in the near-wall regions, a high-Re algebraic second-moment (ASM) closure with the low-Re 1-equation model in the near-wall regions, and two versions of a low-Re ASM model. The two low-Re ASM models return noticeably better predictions of the flow development. There is, however further scope for improvement, especially in the downstream section. Two rotating flow cases have been computed both with the axis of rotation parallel to the axis of bend curvature; one at a positive rotation number R o ≡ΩD/W b of 0.2 and one at R o ≡-0.2. In the case of positive rotation, where the Coriolis and curvature forces reinforce each other, the flow predictions of the low-Re ASM are in very close agreement with the data. When the U-bend rotates negatively, the complex flow field generated in the downstream section is not well reproduced by the low-Re ASM model. More refined turbulence models are thus necessary when the curvature and Coriolis forces oppose each other.

Computational fluid dynamics applied to internal gas-turbine blade cooling : a review

Abstract This paper reviews current capabilities for predicting flow in the cooling passages and cavities of jet engines. Partly because of the need to enhance heat transfer coefficients, these flow domains entail complicated passage shapes where the flow is turbulent, strongly three-dimensional (3-D) and where flow separation and impingement, complicated by strong effects of rotation, pose severe challenges for the modeler. This flow complexity means that more elaborate models of turbulent transport are needed than in other areas of turbine flow analysis. The paper attempts to show that progress is being made, particularly in respect to the flow in serpentine blade-cooling passages. The first essential in modeling such flows is to adopt a low Reynolds number model for the sublayer region. The usual industrial practice of using wall functions cannot give a better than qualitative account of effects of rotation and curvature. It is shown that Rayleigh number effects can modify heat transfer coefficients in the cooling passages by at least 50%. The use of second-moment closure in the modeling is shown to be bringing marked improvements in the quality of predictions. Areas where, at present, more computational fluid dynamics (CFD) applications are encouraged are impingement cooling and pin-fin studies.

Computation of flow and heat transfer through rotating ribbed passages

Abstract This study focuses on the computation of periodic flow and heat transfer through stationary and rotating ducts of square cross-section, with rib-roughened walls. Square-sectioned ribs, normal to the flow direction are employed along two opposite walls. Flow comparisons are presented for a duct under stationary and rotating conditions, with ribs in a staggered arrangement. The rib-height-to-diameter ratio is 0.1 and the rib-pitch-to-rib-height ratio is 10. Heat transfer comparisons are shown for a stationary duct with in-line ribs. The rib-height-to-diameter ratio is 0.0675 and the rib-pitch-to-rib-height ratio is 10. Body-fitted grids are employed and two zonal models of turbulence are tested; a k–ϵ with the 1-equation model of k transport across the near-wall regions and a low-Re version of the basic DSM model, in which in the near-wall region the dissipation rate, ϵ, is obtained from the wall distance. The numerical approach adopted leads to the efficient calculation of flows through ribbed ducts. Both models yield satisfactory mean flow predictions and the DSM is also able to reproduce most of the features of the turbulence field, under both stationary and rotating conditions. Though the computations of the coefficient of wall heat transfer are not as close to the data as the flow predictions, the DSM thermal computations are clearly superior to those of the k–ϵ/1-equation.

Experiments on local heat transfer in a rotating square-ended U-bend

The paper reports measurements of local heat-transfer coefficients and the associated velocity field measured in turbulent flow through a square-ended U-bend which may be rotated in orthogonal mode. The flow thus has close generic similarities with that arising in the internal cooling passages of modern gas turbine blades. While for no rotation, the separation ahead of the flat end wall of the U-bend provokes an unstable non-repeating flow pattern, the secondary flow created by the ducts rotation gives rise to a strong stable vortex originating near the upstream trailing side of the duct and terminating on the downstream leading side. Within and immediately downstream of the U-bend the corresponding heat transfer data exhibit a significant effect of the rotation of the heat-transfer levels which can be linked with changes in the associated velocity and turbulence field.