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Boundary layers, wakes and other shear layers
Published in Bernard S. Massey, John Ward-Smith, Mechanics of Fluids, 2018
Bernard S. Massey, John Ward-Smith
The flow of a real fluid (except at extremely low pressures) has two fundamental characteristics. One is that there is no discontinuity of velocity; the second is that, at a solid surface, the velocity of the fluid relative to the surface is zero, the so-called no-slip condition. As a result there is, close to the surface, a region in which the velocity increases rapidly from zero and approaches the velocity of the main stream. This region is known as the boundary layer. It is usually very thin, but may sometimes be observed with the naked eye: close to the sides of a ship, for example, is a narrow band of water with a velocity relative to the ship clearly less than that of water further away.
Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
The effects of viscosity and the no-slip condition are also important concepts for understanding aerodynamics. When a fluid moves over the surface of a body, it actually “sticks” to the surface of the body so that there is no relative velocity between the fluid and the surface. This is called the no-slip condition and is caused by intermolecular forces and molecular-scale surface roughness. Due to the effects of viscosity, a boundary layer is formed near the surface where the velocity increases from zero at the surface to the freestream value, U∞, far away (see Figure 193.1). Viscous effects are important only in the boundary layer for most streamlined bodies. Since viscous effects are important in the boundary layer, Bernoulli’s equation cannot be applied there. In fact, in the absence of strong curvature, the pressure at the wall where the velocity is zero is equal to the static pressure in the outer flow and not the total pressure as predicted by Bernoulli’s equation.
Fluid Mechanics
Published in Raj P. Chhabra, CRC Handbook of Thermal Engineering Second Edition, 2017
Stanley A. Berger, Stuart W. Churchill, J. Paul Tullis, Blake Paul Tullis, Frank M. White, John C. Leylegian, John C. Chen, Anoop K. Gupta, Raj P. Chhabra, Thomas F. Irvine, Massimo Capobianchi
1 At a solid surface:V and Τ are continuous. Contained in this boundary condition is the “no-slip” condition, namely, that the tangential velocity of the fluid in contact with the boundary of the solid is equal to that of the boundary. For an inviscid fluid, the no-slip condition does not apply, and only the normal component of velocity is continuous. If the wall is permeable, the tangential velocity is continuous and the normal velocity is arbitrary; the temperature boundary condition for this case depends on the nature of the injection or suction at the wall.
CFD modeling and performance evaluation of multipass solar air heaters
Published in Numerical Heat Transfer, Part A: Applications, 2019
Moustafa Al-Damook, Zain Alabdeen Hussein Obaid, Mansour Al Qubeissi, Darron Dixon-Hardy, Joshua Cottom, Peter J. Heggs
The solution domain of the 2 D SAH (models A, B, and C) is a rectangular duct on the x-y plane, bounded by the inlet, outlet, and wall boundaries (as illustrated in Figure 1). The properties of the air, absorber plate material (aluminum), and copper absorber plates are temperature dependent based on features built into the Comsol CFD software. No-slip condition is assumed for the flow velocity at solid surfaces. The top wall boundary condition of the glass is subjected to U.V and I.R radiation, assuming that the glass was ultra-clear and had no absorption or emission, and the insolation on the upper surface of collector is distributed uniformly across the surface [47]. The mean inlet velocity, inlet air temperature, and insolation values, in comparison between model C (2 D CFD model) and model C-I (experimental data), are detailed in Table 3. Uniform air velocity is introduced at the inlet assuming a fully developed flow. At the exit, a pressure outlet boundary condition is specified at a fixed pressure of 101325 Pa.
Two-dimensional simulation of emptying manoeuvres in water pipelines with admitted air
Published in Urban Water Journal, 2023
Duban A. Paternina-Verona, Luis C. Flórez-Acero, Oscar E. Coronado-Hernández, Héctor G. Espinoza-Román, Vicente S. Fuertes-Miquel, Helena M. Ramos
To solve the governing equations of the two-dimensional CFD model, the pressure implicit with splitting of operators (PISO) methodology was used, coupling the pressure and velocity terms. The quality control of the mesh represents the spatial discretization. Numerical schemes based on Gaussian integration methods are used to define the main variables, such as the velocity of the two fluids, the absolute pressures inside the pipe, and the temperature. The Gaussian integration methods used in this model include Gauss Upwind, Gauss Linear, and Gauss VanLeer. To represent transient, first-order implicit, and bounded processes, Backward-Euler’s method is used. The initial and boundary conditions were based on the experimental conditions. The hydraulic system starts at rest. The air pocket pressure is at atmospheric conditions (101,325 Pa) and the water velocity is null. The temperature of all experiments was 20°C (293 K). For the pipe walls, a no-slip condition was applied for the velocity component and fixed flux pressure for the pressure variable. At the inlet and outlet, the velocity condition was defined by a Pressure inlet-outlet velocity function, and a total pressure function was used for the pressure components to represent the exposure of these boundaries to atmospheric pressure. A special boundary condition called empty was defined on the front and back faces to ensure a suitable two-dimensional analysis in the geometry definition (Greenshields and Weller 2022). During each simulation, the air inflow orifice and water discharge duct were exposed to atmospheric pressure, while the pipe walls were subjected to the pressure generated by fluid displacement and gravitational forces.
Numerical analysis for free flow through side rectangular orifice in an open channel
Published in ISH Journal of Hydraulic Engineering, 2021
Almost every computational fluid dynamics problem is defined under the limits of boundary conditions. Boundary conditions play a very important role in imitating flow conditions of physical systems. Boundary conditions are selected so as to be coherent with existing flow conditions in physical simulation. At the inlet boundary conditions, a mass flow rate is defined by the uniform flow boundary condition. It is assumed that there is no transverse and vertical component of velocity, only a single longitudinal component of velocity exists at the inlet. In the ANSYS ICEM-CFD, water is chosen as flowing media from inlet boundary. For the operating pressure conditions, the hydrostatic pressure distribution was chosen as one of the input parameters. After assigning the inlet boundary condition, the next step was the assignment of the outlet boundary conditions. The orifice is an opening discharging at the atmospheric pressure condition. The outlet is a pressure outlet with average static pressure. The top water surface of the channel flow is an entailment opening at atmospheric pressure and considered as the symmetric boundary conditions. Solid boundaries were entitled to ‘WALL’ boundary conditions, which acclaims that there must be no across the flow through it or normal gradient to the wall is always zero and this condition is known to be ‘no-slip’ condition. The channel bottom was provided with proper friction conditions with specific roughness properties. The fluid domain for the simulation was considered as monophasic and incompressible. In the present study, Re is greater than 1 and Weber number is also greater than 1. Therefore, the surface tension effect has been neglected Mohsin and Kaushal 2016.