David Dennis

On the limitations of Taylor's hypothesis in constructing long structures in a turbulent boundary layer

Taylors hypothesis of frozen flow has frequently been used to convert temporal experimental measurements into a spatial domain. This technique has led to the discovery of long meandering structures in the log-region of a turbulent boundary layer. There is some contention over whether Taylors approximation is valid over large distances. This paper presents an experiment that compares velocity fields constructed using Taylors approximation with those obtained from particle image velocimetry (PIV), i.e. spatial data, obtained in the logarithmic region of a turbulent boundary layer.

Experimental measurement of large-scale three-dimensional structures in a turbulent boundary layer. Part 1. Vortex packets

Experimental measurements of the three-dimensional (3D) velocity field in a moderate Reynolds number zero pressure-gradient boundary layer are presented. The measurements are analysed to produce 3D correlations and conditional averaging techniques are used to further elucidate the underlying structure. The results show clear evidence of vortex-packet-type structures and shed new light on the detailed 3D structure of such packets in a real zero pressure-gradient boundary layer.

Experimental measurement of large-scale three-dimensional structures in a turbulent boundary layer. Part 2. Long structures

Three-dimensional (3D) measurements of a turbulent boundary layer have been made using high-speed particle image velocimetry (PIV) coupled with Taylors hypothesis, with the objective of characterising the very long streamwise structures that have been observed previously. The measurements show the 3D character of both low- and high-speed structures over very long volumes. The statistics of these structures are considered, as is their relationship to the important turbulence quantities. In particular, the length of the structures and their wall-normal extent have been considered and their relationship to the other components of the velocity fluctuations and the instantaneous stress.

Forced and Self-Excited Instabilities From Lean Premixed, Liquid-Fuelled Aeroengine Injectors at High Pressures and Temperatures

Measurements of unsteady pressure and chemiluminescence during flow forced operation of aeroengine lean direct injection fuel spray nozzles were made, with a goal to determine the response of the flame, subject to a range of air fuel ratios, fuel flow splits between pilot and mains injectors, and cooling flows. A rotating shutter installed at the downstream choked nozzle provided the excitation for forcing the mass flow rate between 100 to 600 Hz, at normalized intensities of 0.1 to 0.7 relative to the mean velocity at the injector. The experiments were performed at inlet conditions of 800 K and 5.7 bar.Self-excitation created by the coupling between the flame and the combustor cavity was observed, in the form of a broad peak around 275 Hz. Numerical studies indicate that the peak is associated with an entropy spot (a region of non-uniform temperature) travelling from the flame to the choked nozzle, followed by the ensuing expansion wave towards the injector and amplification of the excitation. Investigation of previous studies suggests that similar phenomena may have been present in other studies at high pressure. The main impact of the self-excitation is the significant amplification of the velocity fluctuations from 0.1 of the mean velocity away from the self-excitation frequency to around 0.7 at the peak. The flame response, represented by the ratio of the fractional fluctuations in OH* chemiluminescence to the fractional velocity fluctuations at the injector, can be determined under conditions where the self-excitation heat release contributes only a small portion of the forced heat release, based on the measured background. The flame response shows a significant dependence on both air fuel ratio and fuel splits, with a decreasing gain towards higher frequency.The results show that it is possible to generate high amplitude fluctuations on the flow using this method, and demonstrate the role of entropy spots during normal operation in lean direct injection systems. Finally, the results suggest that there is an interaction between the forcing frequency and the self-excitation, which may behave in a non-linear manner, and which deserves further investigation.Copyright

Controlling vortex breakdown in swirling pipe flows: Experiments and simulations

A laminar, incompressible, viscous pipe flow with a controllable swirl induced by wall rotation has been studied both numerically and experimentally up to an axial Reynolds number (Re) of 30. The pipe consists of two smoothly joined sections that can be rotated independently about the same axis. The circumstances of flow entering a stationary pipe from a rotating pipe (so-called decaying swirl) and flow entering a rotating pipe from a stationary pipe (growing swirl) have been investigated. Flow visualisations show that at a certain swirl ratio the flow undergoes a reversal and vortex breakdown occurs. The variation of this critical swirl ratio with Reynolds number is explored and good agreement is found between the experimental and numerical methods. At high Re the critical swirl ratio tends to a constant value, whereas at low Re the product of the Reynolds number and the square of the swirl ratio tends to a constant value in good agreement with an existing analytical solution. For decaying swirl the vort...

3D Structures from Stereoscopic PIV Measurements in a Turbulent Boundary Layer

Experiments using stereoscopic high-speed particle image velocimetry (PIV) to take measurements in a cross-stream (i.e. wall-normal/spanwise) plane in a turbulent boundary layer have been used to produce full 3D velocity fields. The 3D fields were constructed from planar 3C fields by using Taylor’s hypothesis to create a pseudo-spatial x-dimension from the temporally resolved measurements. This has produced a 3D viewof the elongated regions of high and low streamwise momentum found in the boundary layer, often referred to as ‘long structures’, and provided information on the arrangement, length and characteristic angle of these structures. Long structures are also seen to be associated with regions of high Reynolds stress. Vortical motions are visualised using swirling strength, and indicate that there is a prevalence for vortices to surround the low speed long structures. The vortices are characterised, and are found to resemble hairpin vortices in some respects.