Experiments Fully Resolving the 3D Flow and Pressure Fields in the Inner Part of Rigid and Compliant
Abstract
The flow in the inner part of high Reynolds number rough wall boundary layers and its impact on drag and noise generation have been,a challenge to measure, compute or model owing to the geometric complexity, as well as the broad range of time and length scales inv,olved. In particular, there is no experimental data on the flow, pressure fields, and shear stresses between roughness elements, the,ically rough, implying that the missing knowledge has broad implications about our ability to predict their acoustic signature. Rece,nt advancements in micro-scale measurement techniques, some of them introduced in our laboratory, now enable us to tackle this probl,em at the required spatial and temporal resolutions. Hence, this proposal consists of a series of experimental studies aimed fully r,esolving the three-dimensional unsteady flow around rigid and compliant roughness elements embedded in the inner part of a high Reyn,olds number turbulent boundary layer. The experiments will be performed in a recently-completed refractive index matched water tunne,l designed specifically for unobstructed flow measurements in geometrically complex system. The 3D time-resolved velocity and pressu,re distributions will be measured at different scales and resolutions. Microscopic dual view tomographic holography (MDTH) will be u,sed to fully resolve the flow around the roughness elements, and tomographic PIV (or particle tracking), for characterizing the larg,e scale flows in the outer layer. Both methods can be implemented simultaneously to obtain multiscale data spanning four orders of m,agnitude of scales, from microns to centimeters. MDTH, which has been invented in our labs, involves simultaneous acquisition of two, inclined inline holograms, precision 3D matching of the two views, as well as pairing and truncating of the elongated traces of the, same particle to determine their location with an uncertainty of 0.5 ?m. Particle tracking followed by interpolation onto a regular, grid using a constrained cost minimization method (CCM) that minimizes the measurement errors provides the 3D distributions of velo,city and material acceleration. Data analysis is performed using high speed GPU based codes, which have already been applied for cha,racterizing the 3D flow and wall shear stresses around one m,5 kHz. The unsteady 3D pressure distribution will be determined by spatial integration of the material acceleration, accounting for,the impact of viscous stresses near walls. The pressure will be integrated to obtain the forces on the roughness elements, and the r,adiated dipole noise will be calculated by integration of the wall pressure and its time derivative. Low frequency (sub-convective),wall shear stresses will be determined from the near wall velocity gradients by combining tracks from multiple exposures to achieve,the required near wall particle concentration. The rough wall consists of 0.79 mm diameter, and 0.5 mm height cylindrical elements.,The first series of measurements will involve rigid acrylic elements, and the second, a visco-elastic rough wall with the same shape, made of PDMS with low and high stiffness. For the latter cases, the time-resolved wall deformation will also be measured using Mach, Zehnder Interferometry and digital image correlation. Analysis of the unique database will elucidate and quantify the mechanisms af,fecting the unsteady forces and noise generation by the roughness elements. Included are effects of wall and roughness deformations,, well as other non-linear phenomena resulting from interactions of the roughness elements with the outer layer turbulence.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 07, 2022
- Source ID
- N000142212824
Entities
People
- Joseph Katz
Organizations
- Johns Hopkins University
- Office of Naval Research
- United States Navy