CFD Programming using Spectral Methods-Nek5000 Fortran Ubunto

***** OUTPUT FIELD SPECIFICATION *****

6 SPECIFICATIONS FOLLOW

T COORDINATES

T VELOCITY

T PRESSURE

T VORTICITY

F TEMPERATURE GRADIENT

T DRAG <---- DO we have to add this in rea file, to obtain Drag as a parameter?

T LIFT <--- DO we have to add this in rea file, to obtain LIFT as a parameter?

0 PASSIVE SCALARS

The complex yet efficient flight mechanism of Micro Air Vehicle flapping bird like regimes relies upon flexible wings. Effective vortex control, when flapping as well as low-drag gliding, may result from the wing’s flapping frequency. This hypothesis was tested by focusing research to capture lift, drag, and vortex shedding visualization for the flow over an oscillating 2D cylinder. The cylinder moved up and down for several cycles using the dynamic mesh properties of the Nek5000 CFD solver. Results are compared with Re = 500 and a fixed motion amplitude of ymax/D = 0.25. The simulation was ran until Time=100 seconds. An attempt to validate the calculations for the vortex shedding Frequency vs. Reynolds number is studied. A comparison of documented parameters such as those in "A study of two-dimensional flow past an oscillating cylinder" by Blackburn and Henderson will be compared to the simulations that are discussed in this report. The simulation is 2D, and irregular laminar to turbulent regions due to high Reynolds number (Re=500). Results for comparison such as vortex shedding, lift, and drag will be looked into. Identification of 2-3 regimes and computations for lift, drag, shedding frequency, cylinder pulsation will be studied. The computational model consisted of a single cylinder oscillating in 1 degree of freedom with a symmetric boundary condition Results are interpreted using an analysis of vorticity generation. Results showed that the embedded vortex size and shape generated over the cylinder depended on the frequency of oscillation.

Drag Vs. time for oscillating cylinder.

Blackburn and Henderson. Drag Vs. time for oscillating cylinder.

Numerical Vorticity Vs. time for oscillating cylinder.

Experimentally Flow Visualization for oscillating cylinder.

This shows that our St= 0.2 simulation is operating in mode A. This graph does

not explicitly show that for our Re=500 but has good indication that our regime is changing from

laminar to turbulent regime. From A. Jain. "Fluid Mechanical issues of the flow

around a bluff body". Indian Institute of technology kharagpur.

This shows that our Re=500 simulation is operating in the irregular flow regime.

From A. Jain. "Fluid Mechanical issues of the flow around a bluff body". Indian Institute of

technology kharagpur.

Strouhal number with Re plot. The simulation we are running correlates to Re=500. This plot

confirms that we are working in a turbulent regime for our cylinder validation case studies.

From J. Lienhard. "Synopsis of Lift, Drag, and Vortex Frequency, Data for Rigid

Circular Cylinders". Washington State University. Technical Extension Service.1966.

Regime of fluid flow across Circular Cylinder. From J. Lienhard. "Synopsis of Lift, Drag,

and Vortex Frequency, Data for Rigid Circular Cylinders". Washington State University.

Technical Extension Service.1966.

Strouhal no. = fvD/u

fo=frequency of cylinder cross-flow osscilations

fv=natural shedding frequency of the fixed cylinder

F=fo/fv

D= Diameter of the cylinder

A = amplitude ratio

Re= reynolds number

Regime 1. Validation Case Study 1 with F=0.89:

Flapping Cylinder Dynamic Mesh with Nek5000.

The parameters are Re=500, A=ymax/D=0.25, D=1,

Journal Parameter:

fo=0.89fv = 0.89*(0.2)

Nek5000 parameter input:

fo= 0.178

At Re=500 for circular oscillating cylinder eddies are shed continuously from each side of the body, forming rows of vortices in its wake. The vortices move downstream at a speed smaller than the upstream velocity U. A Von Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow over this bluff body. They are named after the engineer & fluid dynamicist, Theodore von Kármán. Vortices are shed in to the downstream flow from sides of the body. Karman investigated the phenomenon and concluded that a nonstaggered row of vortices is unstable, and a staggered row is stable only if the ratio of lateral distance between the vortices to their longitudinal distance is 0.28. From A. Jain. "Fluid Mechanical issues of the flow around a bluff body".

Indian Institute of technology kharagpur.

At about Re = 500 , multiple frequencies start showing up and the wake tends to become Chaotic. As the Reynolds number becomes higher, the boundary layer around the cylinder tends to become turbulent. The wake, shows fully turbulent characters. For larger Reynolds numbers, the boundary layer becomes turbulent. A turbulent boundary layer offers greater resistance to separation than a laminar boundary layer.