Influence of freestream turbulence on the flow over a wall roughness

AbstractCavitation is a phenomenon that occurs easily during rotation of fluid machinery and can decrease the performance of a pump, thereby resulting in damage to flow passage components. To study the influence of wall roughness on the cavitation performance of a centrifugal pump, a three-dimensional model of internal flow field of a centrifugal pump was constructed and a numerical simulation of cavitation in the flow field was conducted with ANSYS CFX software based on the Reynolds normalization group k-epsilon turbulence model and Zwart cavitation model. The cavitation can be further divided into four stages: cavitation inception, cavitation development, critical cavitation, and fracture cavitation. Influencing laws of wall roughness of the blade surface on the cavitation performance of a centrifugal pump were analyzed. Research results demonstrate that in the design process of centrifugal pumps, decreasing the wall roughness appropriately during the cavitation development and critical cavitation is important to effectively improve the cavitation performance of pumps. Moreover, a number of nucleation sites on the blade surface increase with the increase in wall roughness, thereby expanding the low-pressure area of the blade. Research conclusions can provide theoretical references to improve cavitation performance and optimize the structural design of the pump.

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An instability mechanism for channel flows in the presence of wall roughness

Journal of Fluid Mechanics ◽

10.1017/jfm.2020.493 ◽

2020 ◽

Vol 899 ◽

Cited By ~ 1

Author(s):

Philip Hall

Keyword(s):

Channel Flows ◽

Wall Roughness ◽

Instability Mechanism ◽

Image Position

Abstract

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Investigation of Side Wall Roughness Effect on Optical Losses in a Multimode Si3N4 Waveguide Formed on a Quartz Substrate

Photonics ◽

10.3390/photonics7040104 ◽

2020 ◽

Vol 7 (4) ◽

pp. 104

Author(s):

Anastasia Yakuhina ◽

Alexey Kadochkin ◽

Vyacheslav Svetukhin ◽

Dmitry Gorelov ◽

Sergey Generalov ◽

...

Keyword(s):

Optical Waveguides ◽

Quartz Substrate ◽

Side Wall ◽

Wall Roughness ◽

Roughness Parameters ◽

Optical Losses ◽

Waveguide Width ◽

Integration Density ◽

Frequency Domain Reflectometry ◽

Multimode Operation

This article presents the results of the study of the influence of the most significant parameters of the side wall roughness of an ultra-thin silicon nitride lightguide layer of multimode integrated optical waveguides with widths of 3 and 8 microns. The choice of the waveguide width was made due to the need to provide multimode operation for telecommunication wavelengths, which is necessary to ensure high integration density. Scattering in waveguide structures was measured by optical frequency domain reflectometry (OFDR) of a backscattering reflectometer. The finite difference time domain method (FDTD) was used to study the effect of roughness parameters on optical losses in fabricated waveguides, the roughness parameters that most strongly affect optical scattering were determined, and methods of its significant reduction were specified. The prospects for implementing such structures on a quartz substrate are justified.

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The Effect of Freestream Turbulence on Film Cooling Adiabatic Effectiveness

Volume 3: Turbo Expo 2002, Parts A and B ◽

10.1115/gt2002-30172 ◽

2002 ◽

Cited By ~ 1

Author(s):

James E. Mayhew ◽

James W. Baughn ◽

Aaron R. Byerley

Keyword(s):

Flat Plate ◽

Film Cooling ◽

Density Ratio ◽

Main Flow ◽

Freestream Turbulence ◽

Coverage Area ◽

Liquid Crystal Thermography ◽

Blowing Ratio ◽

Adiabatic Effectiveness ◽

Cooling Air

The film-cooling performance of a flat plate in the presence of low and high freestream turbulence is investigated using liquid crystal thermography. High-resolution distributions of the adiabatic effectiveness are determined over the film-cooled surface of the flat plate using the hue method and image processing. Three blowing rates are investigated for a model with three straight holes spaced three diameters apart, with density ratio near unity. High freestream turbulence is shown to increase the area-averaged effectiveness at high blowing rates, but decrease it at low blowing rates. At low blowing ratio, freestream turbulence clearly reduces the coverage area of the cooling air due to increased mixing with the main flow. However, at high blowing ratio, when much of the jet has lifted off in the low turbulence case, high freestream turbulence turns its increased mixing into an asset, entraining some of the coolant that penetrates into the main flow and mixing it with the air near the surface.

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