Speaker: C.H.K.Williamson 

Speaker's affiliation: Cornell University, Department of Mechanical and Aerospace Engineering

Date: Thursday, April 26, 2007

Time: 3:00pm - 4:00pm

Location: 5-314 

Abstract:  

In this presentation, we summarize phenomena concerning vortex-induced vibration (VIV) that have been discovered over the last few years (see for example, Williamson & Govardhan, Annual Review of Fluid Mechanics, 2004).  We pay special attention to the vortex dynamics and energy transfer that give rise to modes of vibration.  We present new vortex wake modes from several different flow-structure configurations (for example, involving 2 degrees of freedom, tethered bodies, pivoted bodies, or freely-falling bodies) often in the framework of the Williamson-Roshko (1988) map of vortex modes compiled from forced (controlled) vibration studies. New modes include the formation of vortex triplets, co-rotating vortices and vortex rings.  We have discovered a generic phenomenon in VIV whereby an elastically-mounted body can continue to resonate even as the normalised flow velocities becomes infinitely large, i.e. as the vibration frequency, f >> natural frequency, f_N, which is radically different from classical resonance, where f ~ f_N.  This is only possible if the mass of the structure falls just below a special critical value!  Correspondingly, we find that freely rising bodies (spheres and cylinders, for example) will only vibrate as they rise, if their relative density falls below a critical value, closely related to that found in our VIV studies. This contrasts with the general belief, in the case of spheres, that they vibrate for all rising conditions.   We shall throw light on the large unexplained scatter found in the classical Griffin plot (a plot of the peak vibration amplitudes versus mass-damping) over the last 30 years. There exists a distinct trend of increasing peak amplitude as Reynolds number increases.  If we go on to renormalise the axes of the plot to take account of Reynolds numbers, then we find a beautiful collapse of peak amplitude data in a "modified" Griffin plot.   Finally, we shall present some preliminary ultra high-resolution forced vibration experiments, which are able to predict catastrophic jumps and several other characteristics of free vibration response.

Biography:

Charles Williamson is a Full Professor at Cornell, and he has spent 16 long years there, having before spent 6 years at Caltech, 2 years in industry, and before that he received a PhD at Cambridge University.  Charles has received 11 Teaching Prizes (not grants), including one National one in 1994 for $10,000 from the Keck Foundation, including the 1999 Weiss Presidential Fellowship ($25,000), and he was awarded the 2006 Carnegie-CASE Professor of the Year award for New York State, meeting Senator Hillary Clinton in Washington DC in Nov. 2006.  He has published a number of papers, with a total of more than 2,500 citations, including an invitation to write two reviews of his fields of research in 1996 and 2004 for Annual Review of Fluid Mechanics. He has been Chairman (and founder) of a series of five international conferences (Bluff Body Wakes and Vortex-Induced Vibration BBVIV), the fifth one to be held in Costa do Sauipe, Brazil in Dec 2007.