Stuart Norris

The flow fields involved in hydrodynamic imaging by blind Mexican cave fish (Astyanax fasciatus). Part I: open water and heading towards a wall.

SUMMARY Blind Mexican cave fish (Astyanax fasciatus) sense the presence of nearby objects by sensing changes in the water flow around their body. The information available to the fish using this hydrodynamic imaging ability depends on the properties of the flow field it generates while gliding and how this flow field is altered by the presence of objects. Here, we used particle image velocimetry to measure the flow fields around gliding blind cave fish as they moved through open water and when heading towards a wall. These measurements, combined with computational fluid dynamics models, were used to estimate the stimulus to the lateral line system of the fish. Our results showed that there was a high-pressure region around the nose of the fish, low-pressure regions corresponding to accelerated flow around the widest part of the body and a thick laminar boundary layer down the body. When approaching a wall head-on, the changes in the stimulus to the lateral line were confined to approximately the first 20% of the body. Assuming that the fish are sensitive to a certain relative change in lateral line stimuli, it was found that swimming at higher Reynolds numbers slightly decreased the distance at which the fish could detect a wall when approaching head-on, which is the opposite to what has previously been expected. However, when the effects of environmental noise are considered, swimming at higher speed may improve the signal to noise ratio of the stimulus to the lateral line.

The flow fields involved in hydrodynamic imaging by blind Mexican cave fish (Astyanax fasciatus). Part II: gliding parallel to a wall.

SUMMARY Blind Mexican cave fish (Astyanax fasciatus) are able to sense detailed information about objects by gliding alongside them and sensing changes in the flow field around their body using their lateral line sensory system. Hence the fish are able to build hydrodynamic images of their surroundings. This study measured the flow fields around blind cave fish using particle image velocimetry (PIV) as they swam parallel to a wall. Computational fluid dynamics models were also used to calculate the flow fields and the stimuli to the lateral line sensory system. Our results showed that characteristic changes in the form of the flow field occurred when the fish were within approximately 0.20 body lengths (BL) of a wall. The magnitude of these changes increased steadily as the distance between the fish and the wall was reduced. When the fish were within 0.02 BL of the wall there was a change in the form of the flow field owing to the merging of the boundary layers on the body of the fish and the wall. The stimuli to the lateral line appears to be sufficient for fish to detect walls when they are 0.10 BL away (the mean distance at which they normally swim from a wall), but insufficient for the fish to detect a wall when 0.25 BL away. This suggests that the nature of the flow fields surrounding the fish are such that hydrodynamic imaging can only be used by fish to detect surfaces at short range.

Hemodynamics in Idealized Stented Coronary Arteries: Important Stent Design Considerations

Stent induced hemodynamic changes in the coronary arteries are associated with higher risk of adverse clinical outcome. The purpose of this study was to evaluate the impact of stent design on wall shear stress (WSS), time average WSS, and WSS gradient (WSSG), in idealized stent geometries using computational fluid dynamics. Strut spacing, thickness, luminal protrusion, and malapposition were systematically investigated and a comparison made between two commercially available stents (Omega and Biomatrix). Narrower strut spacing led to larger areas of adverse low WSS and high WSSG but these effects were mitigated when strut size was reduced, particularly for WSSG. Local hemodynamics worsened with luminal protrusion of the stent and with stent malapposition, adverse high WSS and WSSG were identified around peak flow and throughout the cardiac cycle respectively. For the Biomatrix stent, the adverse effect of thicker struts was mitigated by greater strut spacing, radial cell offset and flow-aligned struts. In conclusion, adverse hemodynamic effects of specific design features (such as strut size and narrow spacing) can be mitigated when combined with other hemodynamically beneficial design features but increased luminal protrusion can worsen the stent’s hemodynamic profile significantly.

Impact of bifurcation angle and other anatomical characteristics on blood flow – A computational study of non-stented and stented coronary arteries

The hemodynamic influence of vessel shape such as bifurcation angle is not fully understood with clinical and quantitative observations being equivocal. The aim of this study is to use computational modeling to study the hemodynamic effect of shape characteristics, in particular bifurcation angle (BA), for non-stented and stented coronary arteries. Nine bifurcations with angles of 40°, 60° and 80°, representative of ±1 SD of 101 asymptomatic computed tomography angiogram cases (average age 54±8 years; 57 females), were generated for (1) a non-stented idealized, (2) stented idealized, and (3) non-stented patient-specific geometry. Only the bifurcation angle was changed while the geometries were constant to eliminate flow effects induced by other vessel shape characteristics. The commercially available Biomatrix stent was used as a template and virtually inserted into each branch, simulating the T-stenting technique. Three patient-specific geometries with additional shape variation and ±2 SD BA variation (33°, 42° and 117°) were also computed. Computational fluid dynamics (CFD) analysis was performed for all 12 geometries to simulate physiological conditions, enabling the quantification of the hemodynamic stress distributions, including a threshold analysis of adversely low and high wall shear stress (WSS), low time-averaged WSS (TAWSS), high spatial WSS gradient (WSSG) and high Oscillatory Shear Index (OSI) area. The bifurcation angle had a minor impact on the areas of adverse hemodynamics in the idealized non-stented geometries, which fully disappeared once stented and was not apparent for patient geometries. High WSS regions were located close to the carina around peak-flow, and WSSG increased significantly after stenting for the idealized bifurcations. Additional shape variations affected the hemodynamic profiles, suggesting that BA alone has little effect on a patient׳s hemodynamic profile. Incoming flow angle, diameter and tortuosity appear to have stronger effects. This suggests that other bifurcation shape characteristics and stent placement/strategy may be more important than bifurcation angle in atherosclerotic disease development, progression, and stent outcome.

Modelling Turbine Loads during an Extreme Coherent Gust using Large Eddy Simulation

A group of wind turbines operating in extreme transient wind conditions has been simulated using LES and an actuator model. An extreme wind event is introduced into the simulation domain using transient boundary conditions. The event is based on the extreme coherent gust (ECG) structure from the International Wind Turbine Design Standard IEC61400-1:2005 which consists of a simultaneous gust and wind direction change. Details of the implementation are discussed with regard to adapting the analytical functions described in the standard. A recently developed actuator sector method is used to represent the wind turbines in the simulation. The actuator method is coupled to the aero-elastic wind turbine simulation code FAST to allow dynamic control of the wind turbines based on the ambient flow conditions. Standard baseline controllers are used to regulate generator torque, actuate blade pitch angle and control yaw direction. A span-wise periodic array of turbines operating in a steady atmospheric boundary layer is simulated before the introduction of the ECG structure. The convection of the wind event is analysed, along with the subsequent response of the wind turbines and loading during the wind event is quantified. The simulations demonstrate a methodology for modelling groups of turbines operating in transient wind conditions that can be used to study turbine loads or test new turbine control strategies.

Modeling Gusts Moving Through Wind Farms

A large-eddy simulation code has been used to model the atmospheric boundary layer flowing through a pair of wind turbines. The wind turbines are modeled as actuator discs, with the disc loadings being calculated by a coupled wind turbine simulation code, FAST. In addition to modeling a steady wind, the case of an extreme gust passing through the farm is simulated. The persistence of the gust structure suggests the possibility of designing turbine controllers based partially on upstream information.

Dynamically scaled phantom phase contrast MRI compared to true‐scale computational modeling of coronary artery flow

To examine the feasibility of combining computational fluid dynamics (CFD) and dynamically scaled phantom phase‐contrast magnetic resonance imaging (PC‐MRI) for coronary flow assessment.

A CFD code comparison of wind turbine wakes

A comparison is made between the EllipSys3D and SnS CFD codes. Both codes are used to perform Large-Eddy Simulations (LES) of single wind turbine wakes, using the actuator disk method. The comparison shows that both LES models predict similar velocity deficits and stream-wise Reynolds-stresses for four test cases. A grid resolution study, performed in EllipSys3D and SnS, shows that a minimal uniform cell spacing of 1/30 of the rotor diameter is necessary to resolve the wind turbine wake. In addition, the LES-predicted velocity deficits are also compared with Reynolds-Averaged Navier Stokes simulations using EllipSys3D for a test case that is based on field measurements. In these simulations, two eddy viscosity turbulence models are employed: the k- model and the k--fp model. Where the k- model fails to predict the velocity deficit, the results of the k--fP model show good agreement with both LES models and measurements.

Experimental Investigation of Leading Edge Hook Structures for Wind Turbine Noise Reduction

The interaction of a turbulent flow with the leading edge of a blade is a main noise source mechanism especially for wind turbines, which are often exposed to intense atmospheric turbulence with a wide range of length scales. The present paper describes an experimental study performed to explore the noise reducing effect of hook-like extensions, which are fixed to the leading edge of a low speed airfoil. The measurements took place in an aeroacoustic wind tunnel using microphone array technique, while simultaneously the aerodynamic performance of the modified airfoils was captured. It was found that the hook structures lead to a noise reduction at low frequencies, while the noise at high frequencies slightly increases. The aerodynamic performance does not change significantly.

Large Eddy Simulation of Dynamically Controlled Wind Turbines using Actuator Discs

Accurate modelling of wind turbine wakes in a wind farm is an important feature of developing wind farm layouts. This paper discusses a new technique for coupling wind and turbine modelling with an aero-elastic simulation to dynamically model turbine wakes and responses in a wind farm. The advantage of this approach is a turbine model in a simulation domain with the ability to actively respond to transient wind events through the inclusion of a controller. The coupled nature of the aero-elastic/ow simulation also allows recording of load and control data allowing analysis of single and multiple turbine systems. An aero-elastic turbine modelling code and a research LES code were chosen for this work. The research focused on developing a computationally ecient simulation with the ability to model a turbine exhibiting standard baseline control operating in a turbulent Atmospheric Boundary Layer (ABL) such as that observed in an o-shore environment. Multiple wind turbine instances were introduced to a transient ow domain to investigate wake structures and interaction eects between turbines. Results show the successful implementation of a coupled simulation. Preliminary results indicate the signicance of the controller and demonstrate signicant turbine interaction eects in simulations with multiple turbines. This work demonstrates promising techniques to increase the delity of transient modelling of turbulence and wake structures in wind farms for better prediction of load uctuations and power decits.