3 research outputs found
Density Field Reconstruction of an Overexpanded Supersonic Jet using Tomographic Background-Oriented Schlieren
A Tomographic Background-Oriented Schlieren (TBOS) technique is developed to
aid in the visualization of compressible flows. An experimental setup was
devised around a sub-scale rocket nozzle, in which four cameras were set up in
a circular configuration with 30{\deg} angular spacing in azimuth. Measurements
were taken of the overexpanded supersonic jet plume at various nozzle pressure
ratios (NPR), corresponding to different flow regimes during the start-up and
shut-down of rocket nozzles. Measurements were also performed for different
camera parameters using different exposure times and f-stops in order to study
the effect of measurement accuracy. Density gradients and subsequently
two-dimensional line-of-sight integrated density fields for each of the camera
projections are recovered from the index of refraction field by solving a
Poisson equation. The results of this stage are then used to reconstruct
two-dimensional slices of the (time-averaged) density field using a tomographic
reconstruction algorithm employing the filtered back-projection and the
simultaneous algebraic reconstruction technique. By stacking these
two-dimensional slices, the (quasi-) three-dimensional density field is
obtained. The accuracy of the implemented method with a relatively low number
of sparse cameras is briefly assessed and basic flow features are extracted
such as the shock spacing in the overexpanded jet plume
An experimental realisation of steady spanwise forcing for turbulent drag reduction
We present an experimental realisation of spatial spanwise forcing in a
turbulent boundary layer flow, aimed at reducing the frictional drag. The
forcing is achieved by a series of spanwise running belts, running in
alternating spanwise direction, thereby generating a steady spatial square-wave
forcing. Stereoscopic particle image velocimetry is used to investigate the
impact of actuation on the flow in terms of turbulence statistics, performance
characteristics, and spanwise velocity profiles, for a waveform of . An extension of the classical spatial Stokes layer theory is proposed
based on the linear superposition of Fourier modes to describe the
non-sinusoidal boundary condition. The experimentally obtained spanwise
profiles show good agreement with the extended theoretical model. In line with
reported numerical studies, we confirm that a significant flow control effect
can be realised with this type of forcing. The results reveal a maximum drag
reduction of 26% and a maximum net power savings of 8%. In view of the limited
spatial extent of the actuation surface in the current setup, the drag
reduction is expected to increase further as a result of its streamwise
transient. The second-order turbulence statistics are attenuated up to a
wall-normal height of , with a maximum streamwise stress
reduction of 44% and a reduction of integral turbulence kinetic energy
production of 39%
Study of a Supercritical Roughness Element in a Hypersonic Laminar Boundary Layer
In this study, the mean flow organization ahead and behind a supercritical cylindrical roughness element immersed in an incoming laminar boundary layer at edge Mach number 6.48 is investigated by means of schlieren visualization, infrared thermography, and planar particle image velocimetry. The schlieren images provide a general overview of the shock-wave system developing around the roughness element. The surface heat transfer map obtained with infrared thermography provides an overall description of the near-wall flow organization in the streamwise and spanwise directions. The off-surface flow topology is inspected with particle image velocimetry in the symmetry plane of the recirculation region upstream of the roughness element. The flow approaching the roughness element separates, forming a main recirculation region adjacent to the stagnation line at the cylinder leading edge. The reattachment vortex is responsible for a heat flux local peak in front of the protuberance. Secondary, more complex local foci and stagnation points are observed upstream of the roughness element, which also correspond to the local maximum of turbulent kinetic energy and surface heat transfer