Enrique Sanmiguel-Rojas

Global mode analysis of axisymmetric bluff-body wakes: Stabilization by base bleed

The flow around a slender body with a blunt trailing edge is unstable in most situations of interest. Usually the flow instabilities are generated within the wake behind the bluff body, inducing fluctuating forces and introducing the possibility of resonance mechanisms with modes of the structure. Base bleed is a simple and well-known means of stabilizing the wake. In the present research, we investigate the global instability properties of the laminar-incompressible flow that develops behind a cylinder with sharp edges and axis aligned with the free stream using a spectral domain decomposition method. In particular, we describe the flow instability characteristics as a function of the Reynolds number, Re=ρW∞D/μ, and the bleed coefficient, defined as the bleed-to-free-stream velocity ratio, Cb=Wb/W∞, where D is the diameter of the body and ρ and μ the density and viscosity of the free stream, respectively. For a truncated cylinder of aspect ratio L/D=5, where L is the length of the body, our calculations ...

Nonlinear waves in the pressure driven flow in a finite rotating pipe

To investigate the nature of nonlinear waves appearing in an axially rotating pipe, we have performed a series of time-depending, three-dimensional numerical simulations of the incompressible Navier–Stokes equations in a rotating long pipe. As a difference with some previous works on the subject, which look for several given types of traveling wave solutions in pipes of infinite length, we leave the flow to evolve freely after a pressure difference is set between two points, one on each end of the finite rotating pipe. We use a recently developed numerical method that allows us to simulate numerically the three-dimensional flow produced in a pipe when Dirichlet boundary condition for the pressure is given on part of the inlet and outlet sections of the pipe. This technique is further improved here so that the pressure is only fixed at just one point on each one of the open boundaries of the pipe. Thus, no restrictions on the flow properties are given in these sections, allowing the free entrance and exit ...

Three-dimensional structure of confined swirling jets at moderately large Reynolds numbers

We have performed a series of three-dimensional (3D) numerical simulations of the incompressible flow discharging from a rotating pipe into a coaxial static cylindrical container through a sudden expansion. We have considered several values of the Reynolds number based on the pipe flow rate ReQ between 50 and 300, and an expansion diameter ratio of 8, and have analyzed the emerging 3D flow structures in the swirling jet exiting from the rotating pipe as the swirl parameter S is increased. The results are compared to axisymmetric numerical simulations of the same problem. Three-dimensional, nonlinear instabilities are found in the swirling jet when ReQ≳98 above a critical value of S, which depends on ReQ, that obviously do not appear in the axisymmetric simulations. These nonlinear instabilities are initially triggered by the linear instabilities inside the rotating pipe, which are already present in the pipe from a much lower value of S, and are transformed in the jet. As S increases further, there exists...

Vortex flow structures and interactions for the optimum thrust efficiency of a heaving airfoil at different mean angles of attack

The thrust efficiency of a two-dimensional heaving airfoil is studied computationally for a low Reynolds number using a vortex force decomposition. The auxiliary potentials that separate the total vortex force into lift and drag (or thrust) are obtained analytically by using an elliptic airfoil. With these auxiliary potentials, the added-mass components of the lift and drag (or thrust) coefficients are also obtained analytically for any heaving motion of the airfoil and for any value of the mean angle of attack α. The contributions of the leading- and trailing-edge vortices to the thrust during their down- and up-stroke evolutions are computed quantitatively with this formulation for different dimensionless frequencies and heave amplitudes (Stc and Sta) and for several values of α. Very different types of flows, periodic, quasi-periodic, and chaotic described as Stc, Sta, and α, are varied. The optimum values of these parameters for maximum thrust efficiency are obtained and explained in terms of the inte...