Flow-field and Acoustic Interaction behind a Wall Mounted Cylinder

The current manuscript is a subset of a larger database where fluid-acoustic and structural vibrations were obtained behind three bluff bodies (cylinder, ramp and vortex generator). The main goal of this effort was to study the effect of shape, orientation, length scales of the protrusion, Mach number and boundary layer thickness on the flow field behind the bluff body. In the current paper we discuss the correlation between fluctuating wall pressure and flow field dynamics at a low Mach number to determine the fluid-acoustic-structural interaction behind a wall mounted cylinder. The effect of the flow field on the wall pressure fluctuations were studied for various cylinder diameters and heights at different Mach numbers and free stream boundary layer thicknesses. Three different zones were identified in the flow field. These include: reversible flow zone behind the cylinder, slow/stagnation region at the reattachment point and a leading edge vortex at the separated shear layer. The fluctuating pressure level was as high as 145dB at the reattachment point, which was over 22dB higher than the baseline without the cylinder. Increasing the Mach number increased the amplitude of the fluctuations over the entire measurement region. Increasing the cylinder height from (127mm to 254mm) was found to extend the high pressure fluctuation zone in the streamwise direction. The separation length increased with increasing diameter or height of the cylinder. The separation length was unaffected by the Mach number (0.15 to 0.2) or turbulent boundary layer thickness (38mm,107mm) in the current study. I. Introduction Continued improvement of the aircraft structural performance is a focus of the airframe design engineer. The never ending push to reduce the aircraft structural weight to help improve the overall performance of the airplane is the goal of this study. Towards this goal, new and advanced measurement techniques can provide insight into flow regimes once deemed complicated. Over the past forty years, many researchers have reported measurements of cross and auto spectra of wall pressure fluctuations beneath turbulent boundary layers [1-7]. These have been aimed at solving practical engineering problems listed below [7],

[1]  W. Willmarth,et al.  Measurements of the fluctuating pressure at the wall beneath a thick turbulent boundary layer , 1962, Journal of Fluid Mechanics.

[2]  G. Corcos.  Resolution of Pressure in Turbulence , 1963 .

[3]  W. L. Keith,et al.  A Comparison of Turbulent Boundary Layer Wall-Pressure Spectra , 1992 .

[4]  W. Willmarth,et al.  Resolution and structure of the wall pressure field beneath a turbulent boundary layer , 1965, Journal of Fluid Mechanics.

[5]  Markus Raffel,et al.  Particle Image Velocimetry: A Practical Guide , 2002 .

[6]  M. Bull,et al.  The thick turbulent boundary layer on a long fine cylinder in axial flow , 1984, The Aeronautical Journal (1968).

[7]  Eric T. Riggs,et al.  Wall pressure fluctuations induced by separated/reattached channel flow , 2002 .

[8]  W. Willmarth,et al.  Pressure Fluctuations Beneath Turbulent Boundary Layers , 1975 .

[9]  Ahmed Naguib,et al.  Wall-pressure-array measurements beneath a separating/reattaching flow region , 2003 .

[10]  In-Won Lee,et al.  Characteristics of wall pressure fluctuations in separated and reattaching flows over a backward-facing step: Part I. Time-mean statistics and cross-spectral analyses , 2001 .

[11]  Mario J. Casarella,et al.  Spectral features of wall pressure fluctuations beneath turbulent boundary layers , 1991 .

[12]  Hyung Jin Sung,et al.  Characteristics of wall pressure fluctuations in separated flows over a backward-facing step: Part II. Unsteady wavelet analysis , 2001 .

[13]  M. Bull.  WALL-PRESSURE FLUCTUATIONS BENEATH TURBULENT BOUNDARY LAYERS: SOME REFLECTIONS ON FORTY YEARS OF RESEARCH , 1996 .

[14]  E. W. Adams,et al.  Effects of the separating shear layer on the reattachment flow structure part 2: Reattachment length and wall shear stress , 1988 .

[15]  N. J. Cherry,et al.  Unsteady measurements in a separated and reattaching flow , 1984, Journal of Fluid Mechanics.

[16]  W. Willmarth,et al.  Wall-pressure fluctuations beneath turbulent boundary layers on a flat plate and a cylinder , 1970, Journal of Fluid Mechanics.

[17]  Parviz Moin,et al.  On the space‐time characteristics of wall‐pressure fluctuations , 1990 .

[18]  Richard M. Lueptow,et al.  The thick, turbulent boundary layer on a cylinder: Mean and fluctuating velocities , 1985 .

[19]  Richard M. Lueptow,et al.  Wall pressure and coherent structures in a turbulent boundary layer on a cylinder in axial flow , 1995, Journal of Fluid Mechanics.

[20]  T. J. Bogar,et al.  Axially symmetric turbulent boundary layers on cylinders: mean velocity profiles and wall pressure fluctuations , 1975, Journal of Fluid Mechanics.

[21]  Richard M. Lueptow.  Turbulent boundary layer on a cylinder in axial flow , 1988 .