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Fundamentals of turbulence and external turbulent flow
Published in S. Mostafa Ghiaasiaan, Convective Heat and Mass Transfer, 2018
For flow parallel to a flat plate, the laminar–turbulent transition takes place over the range Rex = 3 × 105–2.8 × 106, depending on a number of parameters including the surface roughness, level of turbulence in the ambient flow, and the nature of other flow disturbances. The higher limit represents a smooth surface with low main flow turbulence intensity. The following parameters cause the transition to take place at a lower Reynolds number: adverse pressure gradient, free stream turbulence, and wall roughness.
Supersonic Diffusers
Published in George Emanuel, Analytical Fluid Dynamics, 2017
Unless the diffuser can be made adjustable, as a scramjet inlet it is a point design meant for cruise operation. As such, it would operate at a high altitude. Because of the low density a characteristic Reynolds number is relatively small with a laminar boundary layer. As is well known, laminar boundary layers readily separate when exposed to a small adverse pressure gradient. Aside from separation, a laminar boundary layer, in an adverse pressure gradient, starts to transition into turbulence (White, 1974, p. 442).
Viscous fluid flow – boundary layer
Published in Amithirigala Widhanelage Jayawardena, Fluid Mechanics, Hydraulics, Hydrology and Water Resources for Civil Engineers, 2021
Amithirigala Widhanelage Jayawardena
A positive pressure gradient is called an adverse pressure gradient and occurs in the flow around curved boundaries and towards a stagnation point. Increase in pressure is of course accompanied by a reduction of velocity, which at some point along the length may decrease to zero. When this happens, the flow separates from the boundary at a point of inflexion of the velocity profile and is called the boundary-layer separation. The separation is followed by a region called the wake where eddies dissipate energy.
Towards a full-scale CFD guideline for simulating a ship advancing in open water
Published in Ship Technology Research, 2023
Luofeng Huang, Blanca Pena, Giles Thomas
The solution of the fluid domain was obtained by solving the Reynolds-averaged Navier-Stokes (RANS) equations for an incompressible Newtonian fluid: where is the time-averaged velocity, is the velocity fluctuation, ρ is the fluid density, denotes the time-averaged pressure, = µ[∇v + (∇v)T] is the viscous stress term, µ is the dynamic viscosity and g is gravitational acceleration set at 9.81 m/s2. Since the RANS equations have been adopted to account for the turbulent effects, a turbulence model needs to be applied to close the equations, for which, the Shear Stress Transport (SST) k − ω model (Menter 1993) was adopted. The logarithmic law wall function is applied to resolve the boundary layer (Peric 2019). The SST k − ω model has been demonstrated to be a robust RANS turbulence modelling strategy for ships due to its capability to model adverse pressure gradients and flow separation (Paterson et al. 2003). An adverse pressure gradient means the pressure increases in the direction of the flow, which can happen when a water flow encounters a hull, especially around the stern region (ITTC 2014a). The review of Pena and Huang (2021) shows the SST k − ω model is a comprehensive RANS scheme for ship hydrodynamic simulations in both model and full scales.
Studying subcritical opposing channel flows
Published in Journal of Applied Water Engineering and Research, 2020
Abhishek Kumar Pandey, Pranab Kumar Mohapatra, Vikrant Jain, Udit Bhatia
Estimation of Cc is important to know the effective width downstream of the junction. An empirical equation is developed to estimate Cc as a function of Qr, Fr3, and Wr [Equation (10)]. Separation zone occurs due to the adverse pressure gradient. Flows with higher momentum may overcome the adverse pressure gradient. As Fr3 increases, momentum in Channel 3 increases and thus, the spatial extension of the separation zone is restricted. In the present study, decreasing width ratio results in less momentum for the incoming flows in Channel 3, and the incoming flows become less effective in restricting the spatial extension of flow attached to the opposite wall in Channel 3. This results in a larger value of Ws. Study of Frizzel et al. (2006) also shows that Cc increases with Wr. As the discharges in both the inlets tend to equal, Ws starts reducing. This is because the incoming flows from both the inlets can restrict the spatial extension of flow attached to the opposite wall, more effectively. There is no bearing of ζ on Cc.