A minimal model of self propelled locomotion

Fish locomotion is a complicated problem in the context of fluid–structure interaction and it is still not understood what is linked to biology and what is linked to mechanics. Measurements performed on natural fish and artificial systems reveal that swimming at high Reynolds number is found in a narrow range of Strouhal numbers - a dimensionless combination of the swimming velocity, tail beat amplitude and frequency. With a minimal model of aquatic locomotion, we investigate how this number depends on the numerous parameters at play. We show a strong correlation with the drag coefficient, while the effect of the other parameters can be neglected at the first-order approximation.     

Influence of power-law index on transitional Reynolds numbers for flow over a semi-circular cylinder

In this work, the governing partial differential equations (continuity and Cauchy’s momentum equations) describing the flow of power-law type non-Newtonian fluids across a semi-circular cylinder (oriented with its curved surface in the upstream direction) have been solved numerically. In particular, consideration has been given to the delineation of the critical Reynolds numbers denoting the onset of flow separation from the surface of the cylinder and the onset of the laminar vortex shedding regime. This information is germane to establish the scaling of the macroscopic characteristics like drag coefficient and Strouhal number on the governing parameters, namely, Reynolds number and power-law index. The present results clearly suggest that the transitional Reynolds numbers show a strong dependence on the type (shear-thinning and shear-thickening) of fluid behavior as well as on the severity of the shear-dependence of the viscosity. With reference to the behavior seen in Newtonian fluids, the flow remains not only attached to the surface up to higher Reynolds numbers, but shear-thinning behavior also delays the onset of the laminar vortex shedding regime. As expected, shear-thickening fluids, of course, display the opposite characteristics.     

Acoustic streaming on spheres

The phenomenon of secondary motion (acoustic streaming) created by the oscillation of a fluid past a sphere is investigated through numerically solving the full Navier-Stokes equations. The main parameters that affect the structure of acoustic streaming are Reynolds number and Strouhal number. The results are presented in the form of steady streaming patterns as well as the relation between Reynolds and Strouhal numbers and the dimensions of the streaming region.     

Unsteady film cooling with imposed nonuniform pulsations of the main flow

Effects of the main flow pulsations on the unsteady adiabatic film cooling efficiency were investigated. The possibility of using the critical value of the modified Strouhal number for the single-row perforation to identify the quasi-steady flow in the double-row perforation was proved. The penetration of disturbances into the perforation channels due to “plunger” effect was observed. The influence of the imposed pulsations on the adiabatic film cooling efficiency was shown to be weaker for the double-row perforation as compared to the single-row perforation.     

Features of the turbulent flow around symmetric elongated bluff bodies

The flow fields around three elongated bluff bodies with the same chord-to-thickness ratios but distinct leading and trailing edge details were measured at a Reynolds number of 3×10⁴. These models each represent a case where: leading edge shedding dominates, trailing edge shedding dominates and a case where there is a balance between the two. The results show that the vortex street parameters vary between the models, and in particular, the shedding frequencies are significantly altered by the geometry. However, contrary to the current understanding for shorter bluff bodies, the scale of the recirculation region is found to be similar for each model, even though the shedding frequency changes within the range from 0.15 to 0.24. Also, the base pressure does not follow trends with shedding frequency expected from shorter bluff bodies. A force balance of the recirculation region shows that the near wake of each body is significantly affected by the Reynolds shear stress distribution and the resultant force due to the pressure field in the mean recirculation region. These differences infer that the distinct vortex formation characteristics depend on the state of the trailing edge shear layers. The boundary layers at the trailing edge have been quantified, as have the leading edge separation bubbles, and the marked differences in the wake details are shown to depend on the leading edge separation.     

The Flow and Heat Transfer at Start-Up of an Arbitrarily Shaped Body in a Viscous Heat-Conducting Gas

The flow corresponding to the start-up of an arbitrarily shaped body in a viscous heat-conducting gas is analyzed. The established fact of fluid incompressibility at short times is used. In the first approximation, in the neighborhood of each point on the body surface the flow and heat transfer are proved to be the same as for an infinite plate coinciding with the tangential plane at this point. The corrections for the curvature of the body surface are found. For determining the flows near a cylinder of arbitrary shape and near a spherical bluntness, the start-up problems for a circular cylinder and a sphere are considered. The possibility of extending the results to the case of reacting gases is discussed.     

Investigation of turbulent flow past a flagged short finite circular cylinder

This study was conducted to investigate the flow structures of turbulent flow passing over a short finite cylinder in which a rigid flag is attached to the rear of the cylinder, in wake region. The length-to-diameter ratio of the cylinder was chosen to be L/D = 2, whereas the rigid flag had a width-to-diameter ratio of W/D = 1.5. Wall-adapted large-eddy simulation (LES-WALE) was used to resolve unsteady turbulent flow structures. The far field Reynolds number based on cylinder diameter was chosen to be 20,000. The results were compared with the regular case wherein no flag was attached to the cylinder. Results revealed that the flow pattern behind the cylinder with flag was totally different in comparison with the regular case one. However, top free end of the cylinder was not influenced by the flag in contrast with the wake region. At far downstream from the cylinder, most of the flow structures in both cases appeared the same. The horseshoe vortices in both cases appeared to be an unsteady phenomenon, with slightly different patterns. Moreover, in the case of flag attachment, the pressure coefficient was smaller than that of with no flag. Finally, it was shown that the main and secondary Strouhal numbers locations were different in both cases.     

Aerodynamic force evaluation for ice shedding phenomenon using vortex in cell scheme,penalisation and level set approaches

In this work, we propose a formulation to evaluate aerodynamic forces for flow solutions based on Cartesian grids, penalisation and level set functions. The formulation enables the evaluation of forces on closed bodies moving at different velocities. The use of Cartesian grids bypasses the meshing issues, and penalisation is an efficient alternative to explicitly impose boundary conditions so that the body fitted meshes can be avoided. Penalisation enables ice shedding simulations that take into account ice piece effects on the flow. Level set functions describe the geometry in a non-parametric way so that geometrical and topological changes resulting from physics, and particularly shed ice pieces, are straightforward to follow. The results obtained with the present force formulation are validated against other numerical formulations for circular and square cylinder in laminar flow. The capabilities of the proposed formulation are demonstrated on ice trajectory calculations for highly separated flow behind a bluff body, representative of inflight aircraft ice shedding.