C.J. Atkin

A simplified mathematical model for thrombin generation

A new phenomenological mathematical model based directly on laboratory data for thrombin generation and having a patient-specific character is described. A set of the solved equations for cell-based models of blood coagulation that can reproduce the temporal evolution of thrombin generation is proposed; such equations are appropriate for use in computational fluid dynamic (CFD) simulations. The initial values for the reaction rates are either taken from already existing model or experimental data, or they can obtained from simple reasoning under certain assumptions; it is shown that coefficients can be adjusted in order to fit a range of different thrombin generation curves as derived from thrombin generation assays. The behaviour of the model for different platelet concentration seems to be in good agreement with reported experimental data. It is shown that the reduced set of equations used represents to a good approximation a low-order model of the detailed mechanism and thus it can represent a cost-effective and-case specific mathematical model of coagulation reactions up to thrombin generation.

Laminar Flow Control: Leap or Creep?

The recent meteoric rise in oil prices, coupled wit h the increasing attention being paid to the impact of aviation upon climate change, has pro mpted a renewed interest on both sides of the Atlantic in Laminar Flow Control (LFC) Technology. One of the most common claims among research budget holders in the late 1990s was that the aerodynamics of LFC was ‘mature’. The author reviews the state of the LFC design toolset with reference to his experiences in the UK aircraft industry and in gove rnment service over the past 15 years, most of them directed in some way towards the development and exercising of methods for laminar flow design. The paper concludes with a discussion on whether our aerodynamic knowledge is indeed sufficient to make the ‘leap’ t o an optimised laminar flow transport aircraft, or whether a number of critical design id eas need to be evaluated and refined as we ‘creep’ towards the dream of a low-emission aircraf t.

Performance trade-off studies for a retrofit Hybrid Laminar Flow Control system

Recent work in the UK, studying the possible retrofit of a hybrid laminar flow control (HLFC) system to a medium-sized aircraft, is reviewed. The key feature of the work was the use of robust boundary layer tools to design HLFC systems based on direct control of Nfactors using a discrete suction chamber technique. The improved HLFC designs were applied to a representative aircraft configuration, leading to a significant reduction in predicted suction mass flow rates, and therefore in pump power requirements and suction system weight. The use of PSE methods to assess suction requirements was further found to reduce predicted suction rates by nearly 25%, increasing the net drag benefit by 10%. Modifications to the wing geometry that were advantageous for laminar flow usually introduced unacceptably large wave drag penalties: the most promising direction for future research therefore appears to be increasing the chord-wise extent of the suction control system. Nevertheless, extrapolating the predicted retrofit HLFC system performance to the entire wing upper surface, tailplane and fin would suggest a potential 6½ - 7% reduction of total aircraft drag for the representative aircraft at cruise.

Parametric PSE Studies on Distributed Roughness Laminar Flow Control

Non-linear stability analysis tools are used to investigate the development of Saric’s Distributed Roughness flow control mechanism as Reynolds number and pressure gradient are varied. Amplitude plots and flow visualization provide complementary information about the flow mechanism but secondary stability analyses are required to predict with confidence the effectiveness of a particular roughness distribution. Anticipated sensitivity to the choice of roughness spacing has not been confirmed by the analysis, suggesting that the linear N-factor development may not be that crucial to the selection of the control mode. Work is still required to bring the tool set up to the standard required for design.

The influence of the spatial frequency content of discrete roughness distributions on the development of the crossflow instability

An experimental investigation on the influence of the spatial frequency content of roughness distributions on the development of crossflow instabilities has been carried out. From previous research it is known that micro roughness elements can have a large influence on the crossflow development. When the spanwise spacing is chosen such that it is the most unstable wavelength (following linear stability analysis), stationary crossflow waves are amplified. While in earlier studies the focus was on the height or spanwise spacing of roughness elements, in the present study it is chosen to vary the shape of the elements. Through the modification of the shape the forcing at the critical wavelength is increased, while the forcing at the harmonics of the critical wavelength is damped. Experiments were carried in the low turbulence wind tunnel at City University London (Tu=0.006%) on a swept flat plate in combination with displacement bodies to create a sufficiently strong favourable pressure gradient. Hot wire measurements across the plate tracked the development of stationary and travelling crossflow waves. Initially, stronger crossflow waves were found for the elements with stronger forcing, while further downstream the effect of forcing diminished. Spatial frequency spectra showed that the stronger forcing at the critical wavelength (via the roughness shape) dominates the response of the flow while low forcing at the harmonics has no notable effect. Additionally, high resolution streamwise hot wire scans showed that the onset of secondary instability is not significantly influenced by the spatial frequency content of the roughness distribution.

A method of reducing the drag of transport wings

The paper describes research work that has been carried out to reduce the viscous drag of transport aircraft wings by controlling the turbulent contamination on the attachment line. Laminar leading edges will reduce the boundary layer thickness downstream and thus reduce drag. This scenario was postulated by M. Gaster in a paper presented to the AIAA Seattle conference in 2008. The effect of maintaining a laminar attachment line on the overall flow on a wing has been modeled and drag reductions calculated. These values are compared with wind tunnel measurements of viscous drag of a wing with both laminar and turbulent attachment lines.

Improving Purge Air Cooling Effectiveness by Engineered End-Wall Surface Structures ? Part I: Duct Flow

Motivated by the recent advances in additive manufacturing, this study investigated a new turbine end-wall aerothermal management method by engineered surface structures. The feasibility of enhancing purge air cooling effectiveness through a series of small-scale ribs added onto the turbine end-wall was explored experimentally and numerically in this two-part paper. Part I presents the fundamental working mechanism and cooling performance in a 90 deg turning duct (part I), and part II of this paper validates the concept in a more realistic turbine cascade case. In part I, the turning duct is employed as a simplified model for the turbine passage without introducing the horseshoe vortex. End-wall heat transfer and temperature were measured by the infrared thermography. Computational fluid dynamics (CFD) simulation was also performed using ANSYS fluent to compliment the experimental findings. With the added end-wall rib structures, purge air flow was observed to be more attached to the end-wall and cover a larger wall surface area. Both experimental and numerical results reveal a consistent trend on improved film cooling effectiveness. The practical design optimization strategy is also discussed in this paper.

Improving Purge Air Cooling Effectiveness by Engineered End-Wall Surface Structures—Part II: Turbine Cascade

Motivated by the recent advances in additive manufacturing, a novel turbine end-wall aerothermal management method is presented in this two-part paper. The feasibility of enhancing purge air cooling effectiveness through engineered surface structure was experimentally and numerically investigated. The fundamental working mechanism and improved cooling performance for a 90 deg turning duct are presented in Part I. The second part of this paper demonstrates this novel concept in a low-speed linear cascade environment. The performance in three purge air blowing ratios is presented and enhanced cooling effectiveness and net heat flux reduction (NHFR) were observed from experimental data, especially for higher blow ratios. The Computational fluid dynamics (CFD) analysis indicates that the additional surface features are effective in reducing the passage vortex and providing a larger area of coolant coverage without introducing additional aerodynamic loss.

End-Wall Secondary Flow Control Using Engineered Residual Surface Structure

Residual surface roughness is often introduced in the manufacture process with ball-end or fillet-end milling. Instead of paying extra cost to remove these small-scale residual surface structures, there is a potential usage of them as flow control device. This numerical study therefore explores the ability of engineered surface structure in controlling the endwall secondary flow in turbomachinery. The CFD method is validated against the existing experimental data obtained for a 90 degree turning duct flow with a single rib fence placed on the end-wall. The working principle of the engineered surface structure is revealed through detailed analysis on the flow produced by multiple small fences and grooves mimicking the residual surface. The results consistently show that addition of engineered residual structure on flow surface can effectively reduce the magnitude of stream-wise vorticity associated with secondary flow and alleviate its lift-off motion. In the end, a general working mechanism and design guideline for optimizing the residual structure are summarized.Copyright

On the effect of discrete roughness on crossflow instability in very low turbulence environment

Wind tunnel experiments were conducted in a low-turbulence environment (Tu < 0.006%) on the stability of 3D boundary layers. The effect of two different distributions of discrete roughness elements (DREs) on crossflow vortices disturbances and their growth was evaluated. As previously reported, DREs are found to be an effective tool in modulating the behaviour of crossflow modes. However, the effect of 24µm DREs was found to be weaker than previously thought, possibly due to the low level of environmental disturbances herewith. Preliminary results suggest that together with the height of the DREs and their spanwise spacing, their physical distribution across the surface also intimately affects the stability of 3D boundary layers. Finally, crossflow vortices are tracked along the chord of the model and their merging is captured. This phenomena is accompanied by a change in the critical wavelength of the dominant mode.