The Response of Elastic and Viscoelastic Surfaces to a Turbulent Boundary Layer

1986 ◽  
Vol 53 (1) ◽  
pp. 206-212 ◽  
Author(s):  
Mohamed Gad-el-Hak
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Surface Deformation ◽  
Phase Speed ◽  
Instability Waves ◽  
Compliant Surface ◽  
Wave Characteristics ◽  
Highly Nonlinear ◽  
Nonlinear Static ◽  
High Phase

The unstable response of elastic and viscoelastic surfaces to a turbulent boundary layer was experimentally investigated in an 18-m towing tank. The compliant surface deformation was measured using a remote optical technique. The “Laser Displacement Gauge” employs a Reticon camera equipped with a linear array of 256 photodiodes spaced 25 microns apart. The device was used to measure the characteristics of two classes of hydroelastic instability waves that form on elastic or viscoelastic surfaces as a reuslt of the interaction with a turbulent boundary layer. The instability waves developing on an elastic surface are symmetric and have a relatively high phase speed and a small wavelength, as compared to the slow and highly nonlinear “static-divergence” waves observed on the viscoelastic surface. The experimentally determined wave characteristics are compared to existing theories on compliant surface instabilities.

On the interaction of compliant coatings with boundary-layer flows

1984 ◽  
Vol 140 ◽  
pp. 257-280 ◽  
Author(s):  
Mohamed Gad-El-Hak ◽  
Ron F. Blackwelder ◽  
James J. Riley
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Surface Deformation ◽  
Water Channel ◽  
Wave Structure ◽  
Boundary Layer Flows ◽  
Instability Waves ◽  
Compliant Surface ◽  
Onset Velocity ◽  
Low Speed Streaks

The interactions of compliant coatings with laminar, transitional and turbulent boundary layers are investigated. A 2 m long flat plate is towed in the range of speeds of 20–140 cm/s in an 18 m water channel using a carriage riding on an oil film. Isotropic and anistropic compliant coatings are used to cover about 20% of the working Plexiglas surface. The compliant material used is a viscoelastic plastisol gel produced by heating a mixture of polyvinyl chloride resin, a plasticizer and a stabilizer, and allowing them to gel. The shear modulus of rigidity of the plastisol was varied by changing the percentage of PVC in the mix. Anisotropy is introduced by placing the plastisol on a rubber surface having longitudinal grooves scaled with the low-speed streaks in the turbulent boundary layer. The most pronounced effect of the surface compliance in a turbulent boundary layer is a hydroelastic instability in the form of a spanwise wave structure on the compliant surface. The compliant-surface deformation was measured using a novel remote optical technique. The onset speed of the hydroelastic instability waves depends on the thickness and the modulus of rigidity of the plastisol. Their wavelength, wave speed and amplitude are found to depend on these plastisol parameters as well as on the towing speed. In a laminar boundary layer with freestream speeds of over twice the corresponding onset velocity for the turbulent case, no similar instability is observed.

scholarly journals Planetary wave characteristics of gravity wave modulation from 30–130 km

2012 ◽  
Vol 10 ◽  
pp. 271-277 ◽  
Author(s):  
P. Hoffmann ◽  
Ch. Jacobi
Keyword(s):  
Planetary Wave ◽  
Middle Atmosphere ◽  
Model Simulation ◽  
Phase Speed ◽  
Circulation Model ◽  
Wave Characteristics ◽  
F Region ◽  
Zonal Wavenumber ◽  
Momentum Fluxes ◽  
High Phase

Abstract. Fast gravity waves (GW) have an important impact on the momentum transfer between the middle and upper atmosphere. Experiments with a circulation model indicate a penetration of high phase speed GW into the thermosphere as well as an indirect propagation of planetary waves by the modulation GW of momentum fluxes into the thermosphere. Planetary wave characteristics derived from middle atmosphere SABER temperatures, GW potential energy and ionospheric GPS-TEC data at midlatitudes reveal a possible correspondence of PW signatures in the middle atmosphere and ionosphere in winter around solar maximum (2002–2005). In the case of the westward propagating 16-day wave with zonal wavenumber 1 a possible connection could be found in data analysis (November–December 2003) and model simulation. Accordingly, GW with high phase speeds might play an essential role in the transfer of PW and other meteorological disturbances up to the ionospheric F-region.

Characteristics of Weakly Nonlinear Interaction of the Instability Waves in a Supersonic Boundary Layer

2008 ◽  
Vol 3 (3) ◽  
pp. 3-13
Author(s):  
Yuri G. Yermolaev ◽  
Aleksandr D. Kosinov ◽  
Nikolay V. Semionov
Keyword(s):  
Boundary Layer ◽  
Nonlinear Interaction ◽  
Stationary Wave ◽  
Supersonic Boundary Layer ◽  
Weakly Nonlinear ◽  
Instability Waves ◽  
Wave Characteristics ◽  
Initial Stage ◽  
Hydrodynamical Stability

The results of an experimental study of weakly nonlinear interaction mechanisms of the instability waves in a supersonic boundary layer on flat plate at Mach number М = 2 are presented in the paper. The downstream evolution of artificial disturbances of small amplitude was studied experimentally. The wave characteristics of traveling disturbances were determined. Obtained, that disturbances evolution at basic frequency was happen in according to the linear theory of hydrodynamical stability. Confirmed, that subharmonical resonance on asymmetrical wave triplet was the reason of amplification of the high inclined subharmonic pulsations. The role of high-frequency disturbances was not significant in the region of weakly nonlinear interactions. The initial stage of a parametrical resonance was characterized by appearance of a stationary wave, jumps of a phase on 180° on frequency of a subharmonic in a cross-section direction, and also not symmetry in amplitude β-spectra.

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