Experimental Characterization of 3D Flow Structure, Turbulence, and Dipole Noise Radiated from Roughness Embedded in the Inner Part of turbulent Boundary Layers
Abstract
The objective of this project is to characterize the flow structure, turbulence, pressure fluctuations, and dipole noise in high Rey"nolds numbers rough wall turbulent boundary layers with and without mean pressure gradients. This goal will be achieved by performin"g high resolution measurements of the time-resolved, three-dimensional, and three-component velocity field around roughness elements" embedded in the inner part of the boundary layer. The velocity field will be used forcalculating the material acceleration and integrating it spatially to obtain the pressure distribution.The radiated dipole noise will be determined by integrating the pressure distribution and its time derivatives using Lighthill~s formula. Questions of interest include: (i) the effect of roughness height" (k) spacing ( ) and non-uniformities on the flow structure around the roughness elements and their impact on the wall friction, mom""entum and energy transport, pressure fluctuations, and noise; and (ii) The effects of favorable and adverse mean pressure gradient,"" on the structure, wall friction, noise, and scaling of the rough wall boundary layer.The measurements will be performed in the JHU"" refractive index matched facility, where the refractive index of the fluid is matched with that of the acrylic rough walls, facilit"ating unobstructed optical sensing. The friction Reynolds numbers of the fully developed channel flow and developing boundary layer" will be varied in the 1000-5000 range for smooth walls and 2,000~10,000 for rough walls. Favorable and adverse mean pressure gradie"nts will be generated by usingcurved walls to establish a converging-diverging flow paths. Specific setups include: (i) a baseline consisting of a single roughness element with height varying between 20-100 wall units (~~~10 m) embedded within the inner part of a smooth wall boundary layer; (ii) a pair of identical elements with varying separation between them ( /k=1-4). They will be used for studying the interaction between turbulent necklace vortices shed from neighboring roughness elements and its effect on the wall-"normal momentum and energy transport, (iii) a pair of roughness elements with different sizes; (iv) rough surfaces with uniform roug"hness long enough to establish fully developed boundary layers or channel flows. The same surfaces will be installed and tested inr"egions of zero, favorable and adverse mean pressure gradients; and (v) rough surfaces with embedded non-uniformities, i.e. distribut"ed elements with a (significantly) different height.The overall boundary layer structure will be measured using tomographic and stereo-PIV at moderate resolution. It will provide the mean velocity and Reynolds stress profiles as well as the energy spectra and mea"n wall stress. The volumetric 3D flow around the roughness elements will be measured using the recently-developed, dual-view, tomogr""aphic digital holographic microscopy (tomographic DHM) system at a resolution of 20-30 m, corresponding to~2-3~~ or2-3% of the roug"hness size. Extensive calibrations and software developments demonstrate that this technique is uniquely capable of achieving the re"quired resolution and accuracy. The required concentration of 1-2 m particles will be established by local seeding, i.e. injection a"t low speed far upstream of the sample volume. Acquisition at 6-10kHz for sample volumes of400~250~200~~ 3 will provide a fully-resolved four-dimensional data of the flow around the roughness. Calculation of the material acceleration and its spatial integration to determine the pressure field will be performed by a GPU-based efficient omni-directional code that facilitate processing of large" databases, while minimizing the effects of experimental errors. Thetomographic-DHM velocity and pressure distributions will be uni""que, shedding light on an important flow domain for which very little is known.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 29, 2017
- Source ID
- N000141712955
Entities
People
- Joseph Katz
Organizations
- Johns Hopkins University
- Office of Naval Research
- United States Navy