Laura Michele Hudy

Wall-pressure-array measurements beneath a separating/reattaching flow region

A database of wall-pressure-array measurements was compiled for studying the space–time character of the surface-pressure field within a separating/reattaching flow region. The experimental setup consisted of a long splitter plate located within the wake of a fence and instrumented with an array of flush-mounted microphones. Data were acquired for a Reynolds number of 7900, based on the fence height above the splitter plate. Two distinctive regions, defined based on their location relative to the position of the mean reattachment point (xr) of the shear layer, emerged from this investigation. Upstream, from the fence to 0.25xr, the surface-pressure signature was dominated by large time scale disturbances and an upstream convection velocity of 0.21U∞. Beyond 0.25xr, turbulent structures with smaller time scales and a downstream convection velocity of 0.57U∞ generated most of the pressure fluctuations. Interestingly, the low-frequency wall-pressure signature typically associated with the flapping of the sep...

Stochastic estimation of a separated-flow field using wall-pressure-array measurements

Concurrent, surface-pressure and planar, particle image velocimetry (PIV) measurements were obtained in the separating/reattaching flow region downstream of an axisymmetric, backward-facing step at a Reynolds number of 8081, based on step height. The surface-pressure and PIV measurements were used to investigate the evolution of coherent structures in the flow field by employing proper orthogonal decomposition (POD) and multipoint, linear, stochastic estimation (mLSE) analysis techniques. POD was used to determine the dominant modes in the pressure signature, while mLSE was used to estimate the dominant flow structures above the wall from the wall-pressure POD modes over a series of time steps. It was found that a large-scale, coherent structure develops in place (i.e., temporally) at approximately half the reattachment distance. Once this structure reaches a height equivalent to the step, it sheds and accelerates downstream. This growth in place, and then shedding, resembles the evolution of the flow str...

Stochastic Estimation and Non-Linear Wall-Pressure Sources in a Separating/Reattaching Flow

Simultaneous wall-pressure and PIV measurements are used to study the conditional flow field associated with surface-pressure generation in a separating/reattaching flow established over a fence-with-splitter-plate geometry. The conditional flow field is captured using linear and quadratic stochastic estimation based on the occurrence of positive and negative pressure events in the vicinity of the mean reattachment location. The results shed light on the dominant flow structures associated with significant wall-pressure generation. Furthermore, analysis based on the individual terms in the stochastic estimation expansion shows that both the linear and non-linear flow sources of the coherent (conditional) velocity field are equally important contributors to the generation of the conditional surface pressure.Copyright

Stationary and propagating low-frequency wall-pressure disturbances in a separating/reattaching flow

Wall-pressure array measurements are used to study the space-time behavior of the lowfrequency component of the surface pressure within the reattachment region of the flow over a fence with a wake splitter plate. It is found that the low-frequency fluctuations are mostly contained in low-wavenumber modes that are the result of superposition of stationary and upstream/downstream propagating disturbances. The former is attributed to quasi-steady modulation of the mean-wall pressure as a result of the low-frequency flapping of the separated shear layer. Examination of instantaneous data records, in light of earlier time-averaged analysis, suggests that the stationary disturbance coexists with the propagating ones, forming two different types of dominant wall-pressure signatures. These signatures, which are referred to as A and Z modes, are described herein and some of their characteristics are examined using a decomposition of the instantaneous data records into standing and propagating wave components.