Christina Vanderwel

Aerofoil geometry effects on turbulence interaction noise

Fan broadband is one of the dominant noise sources on an aircraft engine, particularly at approach. The dominant noise generation mechanism is due to turbulent- aerofoil interaction noise (TAI). This thesis investigates the effect of changes in 2D aerofoil geometry on TAI noise. The main focus of this thesis is to attempt to reduce it through the development of innovative leading edge geometries. The first two chapters of the thesis deals with an experimental and numerical investigation into the effect of aerofoil geometry on interaction noise on single aerofoils and on cascades. Consistent with previous work, they show that variations in aerofoil parameters, such as aerofoil thickness, leading edge nose radius and camber, produce only a small changes in broadband interaction noise at approach conditions. Subsequent chapters deal with the development of innovative leading edge serration profiles aimed at reducing interaction noise. Chapter 4 is a detailed study into the limitations of single-wavelength serrations in reducing interaction noise. The optimum profile is identified. Chapters 5, 6 and 7 all deal with the development of innovative profiles that can provide up to 10dB of additional noise reductions compared to single-wavelength serrations. For each of the profiles investigated a simple model is developed to aid the understanding of their interaction mechanism.

Wind resource assessment in heterogeneous terrain

High-resolution particle image velocimetry data obtained in rough-wall boundary layer experiments are re-analysed to examine the influence of surface roughness heterogeneities on wind resource. Two different types of heterogeneities are examined: (i) surfaces with repeating roughness units of the order of the boundary layer thickness (Placidi & Ganapathisubramani. 2015 J. Fluid Mech. 782, 541–566. (doi:10.1017/jfm.2015.552)) and (ii) surfaces with streamwise-aligned elevated strips that mimic adjacent hills and valleys (Vanderwel & Ganapathisubramani. 2015 J. Fluid Mech. 774, 1–12. (doi:10.1017/jfm.2015.228)). For the first case, the data show that the power extraction potential is highly dependent on the surface morphology with a variation of up to 20% in the available wind resource across the different surfaces examined. A strong correlation is shown to exist between the frontal and plan solidities of the rough surfaces and the equivalent wind speed, and hence the wind resource potential. These differences are also found in profiles of and (where U is the streamwise velocity), which act as proxies for thrust and power output. For the second case, the secondary flows that cause low- and high-momentum pathways when the spacing between adjacent hills is beyond a critical value result in significant variations in wind resource availability. Contour maps of and show a large difference in thrust and power potential (over 50%) between hills and valleys (at a fixed vertical height). These variations do not seem to be present when adjacent hills are close to each other (i.e. when the spacing is much less than the boundary layer thickness). The variance in thrust and power also appears to be significant in the presence of secondary flows. Finally, there are substantial differences in the dispersive and turbulent stresses across the terrain, which could lead to variable fatigue life depending on the placement of the turbines within such heterogeneous terrain. Overall, these results indicate the importance of accounting for heterogeneous terrain when siting individual turbines and wind farms. This article is part of the themed issue ‘Wind energy in complex terrains’.

Scalar dispersion by coherent structures in uniformly sheared flow generated in a water tunnel

ABSTRACT In order to investigate the role of coherent structures as mechanisms of scalar dispersion, we studied measurements of a passive scalar plume released in a uniformly sheared turbulent flow generated in a water tunnel. The flow had homogeneous turbulence properties in the measurement domain and contained hairpin vortices similar to those in boundary layers, and so was an ideal test bed to study the effects of coherent structures on turbulent dispersion, free from the effects of inhomogeneities or boundaries. Measurements of the velocity and concentration fields were acquired simultaneously using stereo particle image velocimetry and planar laser-induced fluorescence. We found that dye was preferentially located far away from vortices and was less likely to appear in close proximity to vortices, which is attributed to the high dissipation at the periphery of the vortices. However, we also found that dye was not directly correlated with the uniform momentum zones in the flow, suggesting a more complex relationship exists between these zones, the locations of vortices, and dye transport. Considering scalar flux events rather than simply the presence of dye as our condition of interest, a conditional eddy analysis demonstrated that hairpin vortices are responsible for the large scalar flux events as well as the large Reynolds stress events in the flow. The fact that the Reynolds stress was correlated with the scalar flux further confirmed that coherent structures are dominant mechanisms for scalar transport. Furthermore, we found that the scalar flux vector was preferentially inclined by 155° and −25° with respect to the streamwise direction, and was thus approximately orthogonal to the planes of the legs of the most common upright and inverted hairpin structures in the flow. These findings demonstrate that coherent structures play an important and intricate role in turbulent diffusion.

The Fine Structure of a Slender Scalar Plume in Sheared Turbulence

We present measurements of the fine structure of a passive scalar plume in uniformly sheared turbulence generated in a water tunnel. We report on the mixed velocity-scalar statistics of the plume, including the probability density functions of the velocity, scalar, and scalar derivatives, as well as conditional expectations of the velocity and the scalar derivatives, conditioned upon the scalar fluctuations. Such results are particularly relevant to models that are intended to be used for solving the balance equation of the scalar pdf. Specifically, we address the effect of having a highly intermittent scalar field, in which case the scalar pdf is highly skewed and non-Gaussian and the conditional expectations of the velocity components are distinctly non-linear.

On the robustness of the TNO model for aerofoil self-noise prediction

For attached flow, aerofoil self-noise is dominated by the interaction of a turbulent boundary-layers with a sharp trailing-edge. This present paper presents the robustness of well know TNO model for the prediction of aerofoil self-noise predictions. This paper experimentally investigates the effect of aerofoil thickness on trailing edge self-noise. We show that trailing edge noise exhibits a complex dependence on thickness, where it is observed that, very roughly, the spectral shape shifts towards lower frequencies as thickness is increased. In this paper we attempt to capture this behaviour using the theoretical framework developed by Blake in which trailing-edge noise sources are formulated in terms of the boundary layer quantities. Important parameters in the solution to the Poisson equation are the wall-normal gradient of streamwise velocity, the length scale of the energycontaining turbulent eddies, the normal vertical Reynolds stress, as well as its spectral decomposition. These quantities are obtained from hot-wire measurements in the flow. The robustness of the widely used TNO-blake formulation for predicting aerofoil self-noise has been examined which arises due to uncertainty in the boundary layer profile measurements. Finally, classical flat plate theory is used to predict the far-field radiated noise using surface pressure spectrum predicted from the Blake’s model.

Flow Visualization and PIV Measurements of Flow Around a Sport Utility Vehicle Model

The instantaneous flow structure around a 1:18 scaled model of a square-back sport utility vehicle (Hummer H2) was documented in a low-speed water tunnel. The study comprises both flows with the model fixed on a flat plate and flows with the model’s wheels rolling on an endless belt that moved with the speed of the free stream, thus simulating ground effects. The flow structure was investigated using flow visualization by dye injection as well as particle image velocimetry (PIV) for several Reynolds numbers in the range of 7000 to 27700. The flow along the roof, the sidewalls, and the underbody was observed to separate at the rear edges of the body, creating a recirculation zone at the rear of the vehicle, which is associated with pressure loss and a major contribution to aerodynamic drag. In the vertical plane of symmetry, this recirculation zone appears as two counter-rotating vortices. With a fixed ground, the lower vortex was less energetic than the upper vortex because the boundary layer that developed along the ground upstream of the model reduced the momentum of the flow below the vehicle. This boundary layer was also observed to separate from the ground behind the vehicle, creating a third vortex located further downstream along the ground. This boundary layer separation forced the bottom vortex to remain attached to the base of the vehicle, whereas the upper vortex was advected in the wake. The dimensionless frequency (Strouhal number) of the vortex shedding process from the roof was found to be in the range of 0.1 to 0.9. With a moving ground, the upper vortex behaved similarly to that in the fixed ground configuration; however, in the absence of the boundary layer along the ground, the lower vortex was typically stronger and its location showed some variability. In both configurations, the Reynolds number had little influence on the wake topology, mostly increasing the turbulence intensity without modifying the main flow pattern.Copyright