James H. Bell

Development of a two-stream mixing layer from tripped and untripped boundary layers

A two-stream mixing layer with a velocity ratio of 0.6 was generated with two different initial conditions, one with both initial boundary layers laminar and one where both boundary layers were tripped. Some recent measurements have shown that relatively large spanwise variations can occur in the mean flow and turbulence properties of plane mixing layers, especially in the untripped case. Therefore, for the first time, all the data presented here were averaged over several spanwise locations. The results indicate that both the near- and far-field growth rates for the untripped case are significantly higher than for the tripped case. The maximum Reynolds stresses and higher order products for the two cases behave very differently in the near-field, but asymptotic to approximately the same constant levels far downstream. The mean velocity and turbulence profiles in this region also collapse adequately for the two cases when plotted in similarity coordinates. The distance required to achieve self-similarity was found to be distinctly shorter for the tripped case, in contrast to previous observations. The higher growth rate for the untripped case is attributed to the presence of streamwise vortices, which result in additional entrainment by the mixing layer.

Measurements of the streamwise vortical structures in a plane mixing layer

An experimental study has been conducted to investigate the three-dimensional structure of a plane, two-stream mixing layer through direct measurements. A secondary streamwise vortex structure has been shown to ride among the primary spanwise vortices in past flow visualization investigations. The main objective of the present study was to establish quantitatively the presence and role of the streamwise vortex structure in the development of a plane turbulent mixing layer at relatively high Reynolds numbers ( Re δ ∼ 2.9 × 10 4 ). A two-stream mixing layer with a velocity ratio, U 2 / U 1 = 0.6 was generated with the initial boundary layers laminar and nominally two-dimensional. Mean flow and turbulence measurements were made on fine cross-plane grids across the mixing layer at several streamwise locations with a single rotatable cross-wire probe. The results indicate that the instability, leading to the formation of streamwise vortices, is initially amplified just downstream of the first spanwise roll-up. The streamwise vortices first appear in clusters containing vorticity of both signs. Further downstream, the vortices re-align to form counter-rotating pairs, although there is a relatively large variation in the scale and strengths of the individual vortices. The streamwise vortex spacing increases in a step-wise fashion, at least partially through the amalgamation of like-sign vortices. For the flow conditions investigated, the wavelength associated with the streamwise vortices scales with the mixing-layer vorticity thickness, while their mean strength decays as approximately 1/ X 1.5 . In the near field, the streamwise vortices grossly distort the mean velocity and turbulence distributions within the mixing layer. In particular, the streamwise vorticity is found to be strongly correlated in position, strength and scale with the secondary shear stress (

Spanwise averaging of plane mixing layer properties

\overline{u^{\prime}w^{\prime}}

Comparison of experimental and computational techniques for plane mixing layers

). The secondary shear stress data suggest that the streamwise structures persist through to what would normally be considered the self-similar region, although they are very weak by this point and the mixing layer otherwise appears to be two-dimensional.

Three-dimensional structure of a plane mixing layer

An attempt is made to demonstrate the effects of streamwise vorticity on such mixing layer global properties as layer thickness and peak Reynolds stresses. Attention is given to differences between results obtained from spanwise averaging over a number of velocity profiles and those given by the more conventional approach of analyzing a single profile measured along the test section centerline. It is found that the growth rate and streamwise development of peak Reynolds stresses in the near field can be significantly affected by spanwise variations in the mixing layer.

Interaction of a streamwise vortex with a turbulent mixing layer

Some studies of forced and unforced plane turbulent mixing layers have been conducted using two experimental and two computational approaches. The present paper contains an overall comparison and discussion of the measured and computed results. The experimental results include flow‐visualization data using the smoke‐laser technique and mean flow and turbulence measurements obtained with hot X wires and a two‐component laser‐Doppler velocimeter (LDV). The mean flow and turbulence results indicate that the two experimental techniques agree reasonably well for this shear flow. Two‐dimensional computations of the measured mixing layers have also been conducted in a coordinated effort; one method uses the inviscid discrete vortex technique for a spatially developing layer and the other is based on an approximation to the Reynolds‐averaged Navier–Stokes equations. The vortex method was found to give excellent results for the forced mixing layer while the Reynolds‐averaged computations, with a modified turbulenc...

Bridging the Gap between Pressure-Sensitive Paint and Balance Measurements

An experimental study is reported which shows the spanwise structure of a plane mixing layer originating from laminar boundary layers. Quantitative measurements are obtained for the first time which show the origin and evolution of streamwise vortices within the mixing layer. After the first spanwise vortex rollup occurs, streamwise structures are triggered in clusters within the braid region of the mixing layer. Further downstream, they realign to form pairs of counterrotating vortices. The maximum vorticity diffuses rapidly with increasing downstream distance while the vortices grow, scaling with the mixing layer vorticity thickness. The presence of the streamwise vortices leads to significant spanwise distortions in the mean and turbulence properties of the mixing layer. A consistent peak in the secondary shear stress exists for each streamwise vortex.

Effects of streamwise vorticity injection on turbulent mixing layer development

An experimental study has been conducted on the interaction of a single streamwise vortex with a plane turbulent mixing layer. The vortex was generated by a half‐delta wing mounted in the settling chamber of a blower‐driven wind tunnel. The single‐stream mixing layer originated from the top lip of the extended contraction exit, with a tripped initial boundary layer. Initially, the vortex rode below the mixing layer and its effect was to locally distort the mean velocity and Reynolds stress distributions in the lower (high‐speed) side of the mixing layer. Once the vortex became embedded within the mixing layer, the distortion, which took the form of a wrinkle, spread through the whole width of the layer. The induced extra strain rates in the mixing layer resulted in the generation of additional, relatively large, Reynolds stresses in the regions around the vortex. In particular, the secondary shear stress (∼(u’w’)) reached a maximum absolute value equivalent to about 40% of the maximum primary shear stress...

Model Deformation and Optical Angle of Attack Measurement System in the NASA Ames Unitary Plan Wind Tunnel

We consider the question of how to reconcile integrated forces and moments from a Pressure-Sensitive Paint (PSP) measurement with measured forces and moments from a balance. We show that it is possible to compute the smallest change in pressure distribution that would be required to reconcile any differences between the two sets of data. We refer to this as the gap distribution and show that it can be expressed in terms of a set of basis functions that are determined by the geometry of the test article. The use of these gap basis functions allows sensitivity factors to be defined by which individual forces and moments can be expressed in terms of a common unit of measure, namely the magnitude of the implied gap distribution. We apply this gap analysis to data from a wind-tunnel test of the NASA Orion Command Module, for which both PSP data and balance data are available. While it is clear that there are errors in the PSP measurements, the analysis suggests that there are also problems with the balance data, especially for the normal component of force. This finding is consistent with earlier doubts that had been cast on the accuracy of the balance data. The analysis procedure involving a pressure gap distribution should be applicable to the comparison of balance data to integrated forces and moments from Computational Fluid Dynamics calculations and other techniques.

A streamwise vortex embedded in a plane mixing layer

This paper investigates the effect of injected strong streamwise vorticity on the structure and development of a plane mixing layer originating from tripped boundary layers. The experiments were conducted in a mixing layer wind tunnel consisting of two separate legs which were driven individually by centrifugal blowers. It was found that, while the vorticity injection increased the growth rate in the near-field, the asymptotic growth rate was reduced by a factor of about two, together with the peak Reynolds stress levels. The result is attributed to the effect of the relatively strong and short wavelength streamwise vorticity in making the spanwise structures more three-dimensional and slowing down their pairing process, thus reducing entrainment, and hence growth.