Velocity and Wall Pressure Correlations Over a Forward Facing Step E. Fiorentini, R. Camussi , F. Pereira, M. Felli 1 , G. Aloisio 1 , A. Di Marco 2 1 INSEAN – Italian Ship Model Basin, Rome, Italy 2 Università Roma Tre, Mechanical and Industrial Engineering Dept.. (DIMI), Rome, Italy ABSTRACT This work describes an experimental study of the flow field and wall pressure fluctuations induced by quasi-two- dimensional incompressible turbulent boundary layers overflowing a forward-facing step (FFS). Pressure fluctuations are measured upstream and downstream an instrumented FFS step model installed inside a large-scale recirculation water tunnel while two dimensional velocity fields are measured close to the step via time-resolved PIV. The overall flow physics is studied in terms of averaged velocity and vorticity fields for different Reynolds number based on the step’s height. Pressure spectra and cross-correlations are measured as well, and the convection velocity characterizing the propagation of acoustic and hydrodynamic perturbations is computed as a function of the distance from the vertical side of the step. The evolution of the overall Sound Pressure Level measured at the wall shows that the most critical flow structure is the reattachment region downstream the step where an unsteady recirculation bubble is formed. 1. INTRODUCTION Steps and geometrical irregularities often present on surfaces underneath turbulent boundary layers (TBL) induce unsteady aerodynamic pressure fields which are responsible of drawbacks of primary importance in many practical applications, including interior noise generation, flow induced panel vibrations, hydroacoustic of underwater vehicles (see e.g. [1]). Despite the fact that most of the studies conducted in the field concerned equilibrium TBL [2], it is well known that the presence of flow separations, recirculations and reattachments lead to the generation of wall pressure fluctuations whose overall level might be significantly larger (up to 30dB) than that observed in equilibrium TBL with no separations (see e.g. [3]). A simplified representation of surface irregularities is often accomplished by considering backward or forward facing steps, henceforth denoted as BFS and FFS respectively. The former is a typical test bench widely studied in literature (see, among many, the review given in [4]) while the second case has been more rarely analyzed. Efimtzov et al. [5] measured the pressure spectra upstream and downstream a FFS showing that the region downstream the step is the most critical one. On the other hand, Leclercq et al. [6] analyzing the acoustic field induced by a FFS-BFS sequence, suggested that the most effective region in terms of noise emission is located just upstream the FFS step. In recent papers, Camussi et al. [7 -8] measured pressure fluctuations at the wall of a shallow cavity representing a BFS-FFS sequence. The authors again showed that the region close to the FFS is the most effective in terms of Sound Pressure Level (SPL) at the wall even though the origin of the observed acoustic field was not clarified. It is evident that, from the aeroacoustic viewpoint, the FFS geometry is more critical than the BFS case even though the results available in literature are limited and sometimes contradictory. A better knowledge of the physical mechanisms underlying the wall pressure statistics in the vicinity of a FFS is therefore needed not only to improve our understanding but also to address proper methodologies aimed at controlling the flow and lower the pressure peaks responsible for the induced vibrations. The primary intent of the present work is to cover the lack of experimental results in this field through extensive measurement campaigns providing. pressure time series and velocity fields in order to better clarify the correlation between the wall pressure properties and the flow aerodynamic. An instrumented FSS model has been designed and installed within a large-scale water tunnel where measurements have been carried out at different Reynolds numbers based on the step’s height. The overall flow physics have been characterized through time-resolved PIV measurements conducted close to the step while the wall pressure statistics have been analyzed through pointwise pressure measurements performed in several positions upstream and downstream the step. Details about the measurements set up and flow conditions are given in the next section while the main results and the final remarks are presented in Sec. 3 and 4 respectively.