Raymond Humble

Three-dimensional instantaneous structure of a shock wave/turbulent boundary layer interaction

An experimental study is carried out to investigate the three-dimensional instantaneous structure of an incident shock wave/turbulent boundary layer interaction at Mach 2.1 using tomographic particle image velocimetry. Large-scale coherent motions within the incoming boundary layer are observed, in the form of three-dimensional streamwise-elongated regions of relatively low- and high-speed fluid, similar to what has been reported in other supersonic boundary layers. Three-dimensional vortical structures are found to be associated with the low-speed regions, in a way that can be explained by the hairpin packet model. The instantaneous reflected shock wave pattern is observed to conform to the low- and high-speed regions as they enter the interaction, and its organization may be qualitatively decomposed into streamwise translation and spanwise rippling patterns, in agreement with what has been observed in direct numerical simulations. The results are used to construct a conceptual model of the three-dimensional unsteady flow organization of the interaction.

Unsteady aspects of an incident shock wave/turbulent boundary layer interaction

An incident shock wave/turbulent boundary layer interaction at Mach 2.1 is investigated using particle image velocimetry in combination with data processing using the proper orthogonal decomposition, to obtain an instantaneous and statistical description of the unsteady flow organization. The global structure of the interaction is observed to vary considerably in time. Although reversed flow is often measured instantaneously, on average no reversed flow is observed. On an instantaneous basis, the interaction exhibits a multi-layered structure, characterized by a relatively highvelocity outer region and low-velocity inner region. Discrete vortical structures are prevalent along their interface, which create an intermittent fluid exchange as they propagate downstream. A statistical analysis suggests that the instantaneous fullness of the incoming boundary layer velocity profile is (weakly) correlated with the size of the separation bubble and position of the reflected shock wave. The eigenmodes show an energetic association between velocity fluctuations within the incoming boundary layer, separated flow region and across the reflected shock wave, and portray subspace features that represent the phenomenology observed within the instantaneous realizations.

Experimental Study of an Incident Shock Wave/Turbulent Boundary Layer Interaction Using PIV

*† ‡ Particle Image Velocimetry is applied to the interaction between an incident shock wave and a flat plate turbulent boundary layer at Mach 2.1. The undisturbed boundary layer is characterized by its mean and turbulence properties. The interaction region is characterized by the mean velocity field, which shows the incident and reflected shock wave pattern, as well as the boundary layer distortion. The unsteady flow properties are inspected by means of instantaneous velocity fields. Patches of reversed-flow are frequently observed at several locations. Although significant reversed-flow is measured instantaneously, on average no reversed-flow is observed. Turbulence properties show the highest turbulence intensity in the region behind the impingement of the incident shock wave. Turbulence anisotropy is found to be present, with the streamwise component dominating. A distinct streamwiseoriented region of relatively large Reynolds shear stress magnitude appears in the redeveloping boundary layer and persists downstream. The recovery of the boundary layer towards its initial equilibrium conditions therefore appears to be a gradual process.

Unsteady flow organization of compressible planar base flows

The unsteady flow features of a series of two-dimensional, planar base flows are examined, within a range of low-supersonic Mach numbers in order to gain a better understanding of the effects of compressibility on the organized global dynamics. Particle image velocimetry is used as the primary diagnostic tool in order to characterize the instantaneous near wake behavior, in combination with data processing using proper orthogonal decomposition. The results show that the mean flowfields are simplified representations of the instantaneous flow organizations. Generally, each test case can be characterized by a predominant global mode, which undergoes an evolution with compressibility, within the Mach number range considered. (The term “global mode” is defined herein as an organized global dynamical behavior of the near wake region, recognizing that the near wake dynamics may be describable in terms of several global modes.) At Mach 1.46, the predominant global mode can be characterized by a sinuous or flapping motion. With increasing compressibility, this flapping mode decreases, and the predominant global mode evolves into a pulsating motion aligned with the wake axis at Mach 2.27. These global modes play an important role in the distributed nature of the turbulence properties. The turbulent mixing processes become increasingly confined to a narrower redeveloping wake with increasing compressibility. Global maximum levels of the streamwise turbulence intensity and the kinematic Reynolds shear stress occur within the vicinity of the mean reattachment location, and show no systematic trend with compressibility. In contrast, the global maximum level of the vertical turbulence intensity moves upstream from the redeveloping wake toward the mean reattachment location. The vertical turbulence intensity decays thereafter more slowly than the other turbulence quantities. Overall, the local maximum levels of the turbulence properties decrease appreciably with increasing compressibility.

Unsteady Flow Organization of a Shock Wave/Turbulent Boundary Layer Interaction

The interaction between an incident shock wave and a turbulent boundary layer at Mach 2.1 is investigated using particle image velocity in combination with data processing using the proper orthogonal decomposition. The interaction is found to instantaneously exhibit a two-layer structure, characterized by a relatively high-velocity outer layer, and a low-velocity inner layer. Discrete vortical structures are prevalent at their interface, which appear to play a role in the interaction between the two layers. Low-order eigenmodes from the analysis show an energetic association between the incoming boundary layer, separated flow region, and reflected shock wave. Higher-order eigenmodes show subspace bifurcations leading to smaller-scale features, which are required to properly represent the flow. The subspace features contained in the eigenmodes appear to represent the phenomenology observed in the instantaneous realizations.

Investigation of the Instantaneous 3D Flow Organization of a SWTBLI Using Tomographic PIV

The instantaneous 3D flow organization of the interaction between an incident shock wave and a turbulent boundary layer is investigated with tomographic particle image velocimetry. Large-scale coherent motions within the incoming boundary layer are observed, in the form of streamwise-elongated regions of relatively lowand high-speed flow, similar to those found within incompressible boundary layers. Vortical structures are shown to be associated with the low-speed regions, in a way which can be reconciled by the hairpin vortex packet model of incompressible boundary layers. The instantaneous reflected shock wave pattern appears to conform to the lowand high-speed regions as they enter the interaction. It would appear that the previously observed low-frequency shock wave motion can be reconciled by a mechanism involving these large-scale regions. The results are used to construct a tentative conceptual model of the 3D unsteady flow organization of the interaction.

Unsteady Planar Base Flow Investigation Using Particle Image Velocimetry and Proper Orthogonal Decomposition

*† ‡ Particle Image Velocimetry is applied to unsteady planar compressible base flows in the transonic and supersonic regime to obtain mean and instantaneous velocity fields. The instantaneous fields are analysed by Proper Orthogonal Decomposition to investigate the unsteady flow organisation, as revealed by the associated eigenmodes. The results indicate that two dominant eigenmodes exist in the transonic case, characterised by streamwisealigned regions of alternating upward and downward velocity fluctuations, corresponding to sinuous wake motion. A single dominant eigenmode exists in the supersonic case, characterised by large axisymmetric streamwise velocity fluctuations, corresponding to a wake pulsating motion. The out-of-plane vorticity spatial distributions associated to each eigenmode further highlight coherent flow features. Structural coherence degeneration in the supersonic case is thought to be associated with the increasing role of three-dimensional effects with compressibility.