Experimental investigation of the mean and fluctuating forces of wavy (varicose) cylinders in a cross-flow

\n Part II describes the experimental assessment and the application of the ideas in Part I concerning the mechanisms that determine the role of blade surface waviness on laminar-turbulent transition and their consequent effect on civil aircraft fan performance. A natural transition wind tunnel was designed and constructed to characterize the impact of surface waviness on transition, using both hotwire anemometry and infrared thermography. The experimental results support the new hypothesis presented in Part I, concerning the way in which blade surface waviness affects fan performance through motion of the transition onset location due to interaction between surface waviness and Tollmien-Schlichting (TS) boundary layer instability. In particular, the theoretical amplification of the TS waves, and the corresponding transition onset location movement due to surface waviness, was borne out over a range of variations in Reynolds number, non-dimensional surface wavelength, non-dimensional surface wave height, and location of surface wave initiation, relevant to composite fan blade parameters. Further, the increase of receptivity coefficient, and thus the initial amplitude of disturbances due to geometric resonance between surface wavelength and TS wavelength, was also confirmed by the experiments. Surface waviness was estimated, in some cases, to result in a nearly 1% decrease in fan efficiency compared to a non-wavy blade. Suggestions are given for mitigation of the effects of waviness, including the idea of blade curvature rescheduling as a method to delay transition and thus decrease loss.

Effects of Surface Waviness on Fan Blade Boundary Layer Transition and Profile Loss — Part II: Experimental Assessments and Applications

A critical adjustment was established by the attachment of the lightweight smart motor (LWSM) to an open circuit wind tunnel in order to determine lift and drag forces. LWSM rotated efficiently and the system can assemble and dissemble various cylinders with complete control, in comparison to other approaches available in the literature. The results of the experiment displayed accurate measurements of lift and drag. Research was performed with a free stream velocity of a wind tunnel within a range of 0.65<U∞<13.21 m/s for a flow over a two-dimensional rotating cylindrical object; with roughness variation at 5.0μm<Ra<77μm and circumferential velocity of the cylinder at range 0<∝<6 rad/s. It was found that the implementation of LWSM can measure forces in various rotating speed with less vibrating effect.

http://nopr.niscair.res.in/bitstream/123456789/42509/1/IJMS%2046%288%29%201673-1681.pdf

Experimental investigation of aerodynamic performances on a rotating cylinder with surface roughness using light weight smart motor (LWSM)

An iterative scheme of solving the Navier-Stokes equations for unsteady two-dimensional flow about a deforming and translating cylinder is given. In the &th iteration, the convected vorticity appears as the source term in an equation of transient diffusion of vorticity. A novel integral transform is used to reduce this transient vorticity equation to a ^-dimensional heat equation. The bounded solution of this equation is obtained with a general method of superposition for problems involving a moving boundary. An equation describing the migration of the separation point on a deforming cylinder in unsteady cross flows is derived from the analytically obtained velocity field. The radial expansion of the cylinder surface is shown to hasten the separation time and to increase the separation angle. The results imply that in the steady flow past a body that is moving forward and sinking at constant rates, the locus of points at which the azimuthal component of skin friction changes sign originates on the leeward ray at a point downstream of the front tip.

Migration of the separation point on a deforming cylinder. [boundary layer flow]

An iterative scheme of solving the Navier-Stokes equations for unsteady two-dimensional flow about a deforming and translating cylinder is given. In the kth iteration, the convected vorticity appears as the source term in an equation of transient diffusion of vorticity. A novel integral transform is used to reduce this transient vorticity equation to a k-dimensional heat equation. The bounded solution of this equation is obtained with a general method of superposition for problems involving a moving boundary. An equation describing the migration of the separation point on a deforming cylinder in unsteady cross flows is derived from the analytically obtained velocity field. The radial expansion of the cylinder surface is shown to hasten the separation time and to increase the separation angle. The results imply that in the steady flow past a body that is moving forward and sinking at constant rates, the locus of points at which the azimuthal component of skin friction changes sign originates on the leeward ray at a point downstream of the front tip.

Migration of the Separation Point on a Deforming Cylinder

The drag force acting on a circular cylinder fluctuating erratically in a viscous fluid is measured with a laser–cantilever force transducer. The experimental results compare very well with the theory.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112083002815

R. Shedden

Papers

Drag-force fluctuation on a cylinder