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

Abstract

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 separated shear layer was found to be composed of standing and downstream/upstream propagating wave components. The latter seemed to originate from a point near the middle of the reattachment zone, suggesting the existence of an absolute instability of the recirculation bubble, which may be the cause of the flapping of the shear layer.

Statistical properties of wall pressure fluctuations over a forward-facing step

“…In this case, it is interesting to note that the −1 and the − 7 3 scalings coexist. Results obtained at different positions upstream ͑not shown here͒ confirm that the − 7 3 decay law becomes predominant as the distance to the step decreases. It is also observed that such a behavior is independent of Re h since the two scalings are equally present for the whole set of velocities.…”

“…Downstream of the step up to about x / h = 3.5, we observe a larger dispersion that is associated with the varying dimensions of the recirculation region, which are affected by Re h . A similar result was obtained, for instance, by Hudy et al 7 and Cherry et al 24 The analysis of the instantaneous velocity fields provided by the time-resolved PIV measurements confirms that the position of the flow reattachment is strongly time dependent. The maximum amplitude of the Cp rms coefficient is found to correspond approximately to the location of the mean reattachment point of the recirculation bubble.…”

“…28 and 29 further suggest that the wall pressure spectra would decay as power laws with an exponent close to −1 due to the effect of the coherent structures present in the outer region of the TBL. As the separation region is approached, a power law decay closer to the exponent of − 7 3 is usually observed. The f −7/3 power law is the one expected in homogeneous and isotropic turbulence, 30 and it has been found also within regions of separated flow, for instance, downstream of rearward steps.…”

Direct numerical simulation of the flow around an aerofoil in ramp-up motion

“…In the shear layer near the leading edge (P01), we observed a peak at lower frequency f 1 = 0.25U ∞ /c, corresponding to approximately half the shedding frequency and related to a low frequency flapping motion of the shear layer. 35,[38][39][40][41] In the same spectrum, a third peak is present at f 4 = 9.75U ∞ /c corresponding to the vortex generation due to the leading edge Kelvin-Helmholtz (KH) shear layer instability. Another smooth peak at frequency f 3 = 2.52U ∞ /c is present in the spectrum corresponding to the midchord location (P02).…”

Investigation of a transonic separating/reattaching shear layer by means of PIV

“…Bitter et al 11 performed measurements at Mach = 0.7 and presented also a value of 3.7 for this quantity. Low speed experiments also showed a decreased length of the reattachment location 12,13 indicating that the round shape of the model reduces this quantity significantly. The flow over a cylindrical forebody elongated by a second cylinder of smaller diameter and finite length was in the focus of several numerical investigations [14][15][16][17] and of experiments presented in Depres, Reijasse, and Dussauge 18 .…”