Ph.D. Thesis Defense by Mark A. Trautman
Friday, November 19, 1999

(Dr. Ari Glezer, advisor)

"The Manipulation of Instabilities in a Natural Convection Boundary Layer Along a Heated, Inclined Plate"

Abstract

 In part I of the study flow instabilities leading to the formation of streamwise vortices in a natural convection boundary layer over a heated inclined plate submerged in a water tank are manipulated using spanwise arrays of surface-mounted heating elements. The flow over the plate is driven by a two-ply surface heater comprised of a uniform, constant-heat flux heater and a mosaic of 32 x 12 individually-controlled heating elements that are used as control actuators. Surface temperature distributions are measured using liquid crystal thermography and the fluid velocity in cross-stream planes is measured using particle image velocimetry (PIV). Time-invariant spanwise-periodic excitation over a range of spanwise wavelengths leads to the formation of arrays of counter-rotating streamwise vortex pairs and to substantial modification of the surface temperature and heat transfer.  The increase in surface heat transfer is accompanied by increased entrainment of ambient fluid and as a consequence higher streamwise flowrate. Subsequent spanwise-periodic merging of groups of vortices farther downstream results in a reduction in the heat transfer rate. Finally, the suppression of small amplitude spanwise disturbances by linear cancellation is demonstrated.

In part II of the study the evolution of a localized disturbance in a laminar natural convection boundary layer is investigated.  A disturbance comprised of localized regions of spanwise and vertical vorticity is introduced into the boundary layer by locally increasing the surface heat flux. It is found that the structure of the disturbance far from the source is similar to a wavepacket in a Blasius boundary layer.  Increasing the initial amplitude of the disturbance or increasing the plate inclination angle acts to accelerate the downstream evolution of the disturbance, but does not fundamentally change the various stages in the disturbance development. A mechanism for the production of vertical vorticity at increasingly small scales is identified, and its relevance in the production of turbulence is discussed.  An analysis of the disturbance velocity field suggests that the streamwise evolution of the disturbance can be divided into four distinct stages.  In the first stage the streamwise velocity disturbance is dominated by the transient part, and the cross-stream velocity is characterized by unstable 2-D wave modes.  Downstream the transient decays, and the second stage of growth is characterized by the amplification of the disturbance velocities through linear mechanisms.  In the third stage, spanwise vortex elements rapidly lift from the surface, and intense internal shear layers develop within the disturbance.  The enhanced shear regions promote the generation of small scale structures and in the final stage, the disturbance breaks down into turbulence.