Showing papers by "Julio Soria published in 2019"

Nozzle external geometry as a boundary condition for the azimuthal mode selection in an impinging underexpanded jet

Abstract: The role of the external boundary conditions of the nozzle surface on the azimuthal mode selection of impinging supersonic jets is demonstrated for the first time. Jets emanating from thin- and infinite-lipped nozzles at a nozzle pressure ratio of is the nozzle exit diameter, are investigated using high resolution particle image velocimetry (PIV) and acoustic measurements. Proper orthogonal decomposition is applied to the PIV fields and a difference in dominant instability mode is found. To investigate possible explanations for the change in instability mode, additional nozzle external boundary conditions are investigated, including the addition of acoustic dampening foam. A difference in acoustic feedback path is suggested to be the cause for the change in dominant azimuthal modes between the flows. This is due to the thin-lip case containing a feedback path that is concluded to be closed exclusively by a reflection from the nozzle base surface, rather than directly to the nozzle lip. The ability of the flow to form a feedback path that maximises the impingement tone gain is discussed with consideration of the numerous acoustic feedback paths possible for the given nozzle external boundary conditions.

The structure and dynamics of backflow in turbulent channels

Abstract: A statistical description of flow regions with negative streamwise velocity is provided based on simulations of turbulent plane channels in the Reynolds number range -dependent dynamics further away from the wall.

High resolution volumetric dual-camera light-field PIV

Abstract: Light-field particle image velocimetry (LF-PIV) was recently introduced to measure three-dimensional, three-component velocity field with just a single light-field camera. One of the major challenges lies in the small viewing aperture affecting the depth resolution of such a single-camera based LF-PIV approach. In the present study, we show that this limitation may be mitigated by a dual light-field camera framework, one which includes a novel volumetric calibration model derived from Gaussian optics, a particle intensity reconstruction algorithm based on the multiplicative algebraic reconstruction technique and a post-processing technique for the reconstructed particle intensity field. The proposed approach was firstly validated with synthetic light-field particle images as well as experimental light-field images of five tiny glass beads imitating tracer particles. Secondly, parametric studies were conducted to analyze the influence of the viewing angle and seeding particle density on the reconstruction quality and spatial resolution. In particular, synthetic light-field particle images of a direct numerical simulation jet data set were utilized to compare the performance of single- and dual-camera LF-PIV techniques. Finally, experimental volumetric flow field results of a circular vortex-ring were also measured by single- and dual-camera LF-PIV techniques and compared. It is determined here that an additional light-field camera can mitigate the elongation effects of reconstructed particles and improve the measurement resolution in the depth direction.

Volumetric measurements of a self-similar adverse pressure gradient turbulent boundary layer using single-camera light-field particle image velocimetry

Abstract: As a novel volumetric particle image velocimetry technique, single-camera light-field PIV (LF-PIV) is able to acquire three-dimensional flow fields through a single camera. Compared with other multi-camera 3D PIV techniques, LF-PIV has distinct advantages, including concise hardware setup and low optical access requirements. Its capability has proven effective in many experimental investigations. In this study, the use of LF-PIV in measuring a self-similar adverse pressure-gradient turbulent boundary layer (APG-TBL) is demonstrated. Experiments are performed in a large water tunnel at the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC), Monash University. Sets of 250 light-field PIV image pairs are captured covering both the inner and outer regions of the boundary layer. Instantaneous 3D velocity fields are reconstructed using a GPU accelerated density ray tracing multiplicative reconstruction technique (DRT-MART) and three-dimensional cross-correlation methods. The LF-PIV results are compared with two-dimensional PIV (2D-PIV) measurements of the same flow. Comparable accuracy to 2D-PIV is achieved for first- and second-order velocity statistics above approximately $$ y/\delta_{1} = 1 $$ .

Outer scaling of self-similar adverse-pressure-gradient turbulent boundary layers

Abstract: The prediction of turbulent boundary layer (TBL) flow over a convex surface as in aircraft wings or gas turbine blades is a challenging problem. Finding a universal scaling law of turbulence statistics of TBLs over a wide range of adverse pressure gradients (APG) remains unresolved. Here, we introduce characteristic length and velocity scales for APG-TBLs and nondimensionalise the turbulence statistics of the recent canonical self-similar APG-TBLs by Kitsios {\it et al.} ({\it J. Fluid Mech.}, vol.829, 2018, pp. 392--419). The characteristic length scale, which is termed the `shear thickness', $\delta^\ast$, is defined as the location which corresponds to the end of an actively sheared region in a turbulent shear flow, where the nondimensional shear rate normalised by the kinetic energy and the dissipation rate is approximately constant. Next, we show a universal scaling using a mixed velocity, termed the `friction-pressure velocity', $u^\ast$, which is based on total shear stress. It is revealed that the velocity fluctuations and the Reynolds stresses in TBLs over a wide range of APGs agree well with those in TBLs with zero-pressure-gradient (ZPG). The present scaling is used to scale the kinetic energy balance in TBLs, and compare them to other shear flows. Furthermore, a scaling for small-scale properties, i.e. vorticities, using $\delta^\ast$ and $u^\ast$ is also obtained assuming the local equilibrium in the inertial range. The present scaling for wall-bounded shear flows, including TBLs over a wide range of pressure gradients, implies that the underlying instantaneous turbulence structures have common features under a proper scaling and is key to the development and application of turbulent models.