Boundary Layers and Pressure Fields over Uniform and Complex Compliant Walls
Abstract
Driven by bio-inspired attempts to reduce skin friction, the interactions between compliant surfaces and boundary layers have been investigated extensively. Despite numerous experimental, computational, and modeling efforts, there is no well-founded proof that compliant surfaces can reduce drag in high Reynolds number boundary layers. In contrast, compliant coatings are effective in suppressing the acoustic signature of submerged vessels by shielding the rigid hull from the pressure and shear fields of the outer boundary layer. However, being deformable, interaction of the compliant surface with the surrounding flow might become by itself an acoustic source. Hence, the goals of this experimental study are to characterize the flow-surface interactions and use the resulting understanding to explore means of suppressing the wall deformation and pressure transmission. The experiments are performed in a new refractive index matched water tunnel. A novel system combining Mach#Zehnder interferometry with tomographic particle tracking simultaneously measures the time-resolved distribution of wall deformation at a submicron (10 nm) resolution and the 3D flow field. The 3D pressure distribution is determined by calculating the material acceleration and integrating it spatially using a GPU-based, parallel line, omni-directional algorithm. Statistical analyses of the pressure field and surface deformations are used for characterizing the flow-deformation interactions. Expanding our scope, this proposal has several objectives: First we propose to complete the analysis of recently acquired comprehensive databases focusing on pressure-deformation interactions at varying Reynolds numbers, such as the observed formation of large pressure events that are phase-locked with the deformation, even when they are very small. This analysis involves conditional sampling, correlations, and calculations of wavenumber-frequency auto- and cross-spectra of deformation and pressure. Lighthill#s formula will be used for evaluating the radiated dipole noise. Second, since compliant coatings are often installed as panels, gaps and steps are expected to develop. Hence, the effects of isolated steps and gaps in the compliant coating on the wall deformation, flow structure, unsteady pressure field, and radiated dipole noise will be investigated. We will examine whether the deformations and interactions with the flow are enhanced when the gaps introduce eddies with size and frequency matching those ofthe resonant response of the coating. Third, so far, our research has focused on the surface shape and the flow in the boundary layer above it. We propose to characterize the transmission, attenuation, and spectral changes to the stain field propagating across the compliant wall. These experiments will be performed by seeding the interior of the transparent compliant coating with micro-particles and measuring their motion at high magnification using high speed stereo imaging and holographic microscopy. Results will be compared to theoretical predictions and to pressure measurements on the rigid wall beneath the coating. Fourth, we will explore the feasibility of suppressing the surface deformations and enhancing the attenuation of the transmitted pressure by spatially varying the compliant material properties. Two proof of concept setups are considered. The first consists of periodically inserting strips with hard compliant material, which do not protrude to the surface, inside a softer wall. The resulting variations in resonant conditionsare expected to modulate and suppress the surface deformation waves. The second configuration consists of two parallel visco-elastic layers, with the outer one hard to reduce the interaction with the flow, and the inner one soft or porous to enhance the attenuation of the transmitted signal. For both cases, the surface waves, interaction with the flow, and the transmission across the wall will be measured and characterized.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 24, 2023
- Source ID
- N000142312681
Entities
People
- Joseph Katz
Organizations
- Johns Hopkins University
- Office of Naval Research
- United States Navy