Showing papers by "Julio Soria published in 2005"

A comparison between snapshot POD analysis of PIV velocity and vorticity data

Abstract: Proper orthogonal decomposition (POD) was performed on both the fluctuating velocity and vorticity fields of a backward-facing step (BFS) flow at Reynolds numbers of 580 and 4,660 The data was obtained from particle image velocimetry (PIV) measurements The vorticity decomposition captured the fluctuating enstrophy more efficiently than the equivalent velocity field decomposition for a given number of modes Coherent structures in the flow are also more easily identifiable using vorticity-based POD A common structure of the low-order vorticity POD modes suggests that a large-scale similarity, independent of the Reynolds number, may be present for the BFS flow The POD modes obtained from a vorticity-based decomposition would help in determining a basis for constructing simplified vortex skeletons and low-order flow descriptions based on the vorticity of turbulent flows

Using stereo multigrid DPIV (SMDPIV) measurements to investigate the vortical skeleton behind a finite-span flapping wing

Abstract: The structure of the flow behind wings with finite span (3D) is significantly more complex than the flow behind infinite span (2D) wings. It has been shown that the presence of wingtip vortices behind finite span wings significantly modifies the geometry of the wake flow. It is felt that this modification alters the dynamics of interaction between leading and trailing edge vorticity in a manner that affects the ability of 2D flapping wings to produce thrust. A model of the mean flow skeleton has been proposed from qualitative flow visualization experiments. An unambiguous quantitative representation of the actual flow is required for comparison to the proposed model. To accomplish this the full 3D 3C velocity is required in the volume behind the 3D flapping wing. It is proposed to use stereoscopic multigrid digital particle image velocimetry (SMDPIV) measurements to investigate this unsteady oscillatory flow. This paper reports preliminary SMDPIV measurements along the plane of a symmetrical NACA-profile wing at a Strouhal number of 0.35. Phase averaged measurements are used to investigate the complex flow topology and the influence of the forcing flow on the evolution of the large scale structure of a jet-flow. This paper focuses on optimizing the SMDPIV experimental methodology applied to liquid flows. By refining the 2D 3C technique, the 3D topology of the flow can be investigated with a high degree of accuracy and repeatability. Preliminary results show that the flow is characterized by two pairs of coherent structures of positive and negative vorticity. The arrangement of these structures in the flow is controlled by the motion of the wing. Vorticity of opposite rotation is shed at the extreme heave and pitch positions of the aerofoil to set up a thrust indicative vortex street in support of the suggested topological model.

Non-Intrusive measurement of a density field using the Background Oriented Schlieren (BOS) method

Abstract: A method is described for measuring the density distribution in an inhomogeneous medium. This method is based upon the classical Schlieren technique, known as Background Oriented Schlieren (BOS) method. The method measures the density dependence of the refractivity through the detection of the deflecting angle of a number of light beams passing through the space to be measured. The results of some preliminary experiments are presented with detailed reference to experiment methodology and accuracy. The measurement technique is well suited to cases seeking non-intrusive robust and multi-point measurement.

Particle image velocimetry measurements of an underexpanded supersonic jet

Abstract: An investigation into the methodology of analyzing a supersonic jet via Particle Image Velocimetry (PIV) using solid seed particles is presented. An isothermal je t at Ma= 1.3 through a 2.5mm De Laval nozzle is examined. A Lore nz-Mie scattering analysis is used to determine the scalar s of the flow, while a Multigrid Cross Correlation Digital Particle Image Velocity (MCCDPIV) approach measures the velocity fie ld. A classical free turbulent jet structure is evident, wi h a double hump velocity profile characteristic of an underexp anded jet. The effectiveness of the flow seeding approach is a lso examined, with the specific momentum of the incomin g flow shown to be the critical factor in the operation of the seeder.

Velocity and scalar measurement of low swirl jet

Abstract: This paper presents preliminary velocity and scalar measurements of low swirl intensity jets in air. Swirling jets were generated using a micro-injector swirl generator. Velocity fields of the jets were obtained using Multi-grid Cross Correlation Digital Particle Image Velocimetry. Initial flow visualizations were performed using planar laser induced fluorescence with acetone as tracer species.

The topology of homogeneous isotropic turbulence with passive scalar transport

Abstract: Data obtained from direct numerical simulations of isotropic homogeneous turbulence and the diffusion of a scalar with an applied mean gradient is analysed using the topological techniques developed by Chong, Perry and Cantwell [ Physics of Fluids A , 2 :765-777, 1990]. Simulations were run at two different Taylor--Reynolds numbers and two different Schmidt numbers. Comparing the numerical results obtained, relationships between the scalar characteristics and the topological features of the flow are identified.

Visualization of Flow Development in Hybrid Rocket Motors with High Regression Rates

Abstract: The effect offuel surface regression on the structure of the flow in a hybrid rocket motor is examined at isothermal conditions using an expanding backward-facing step flow in a wind tunnel It is shown that the long timeoxidizer flow structure is almost independent of the simulated fuel regression rate During the regression phase and for cases of high ratio of the oxidizer freestream velocity to the simulated fuel regression rate, the oxidizer flow evolves in a quasi-stationary manner, where the instantaneous backward-facing step flow structure is akin to that of the static case

A technique for generating adverse pressure gradients using active and passive flow control methods

Abstract: This paper examines a technique for generating an adverse pressure gradient using a setup that consists of a cylinder and a flat plate airfoil. The results show that the extent and nature of flow separation along the flat plate surface depended on the velocity ratio and location of the spinning cylinder.

Digital in-line holography for micro-droplet diagnostics

Abstract: Digital in-line holography has been applied for the study of water droplets generated by a micro dispensing device. The micro dispenser allows the control of the droplet size by changing the pressure applied to a water reservoir. Two pressures have been used in this work: 51.7 and 103.4 kPa. The holograms acquired are mathematically reconstructed and the droplet images are analysed. The droplet sizes are found to be between 140-170 μm for a pressure of 51.7 kPa and around 100 μm for 103.4 kPa.

Investigation of flow past a cavity using PLIF and MCCDPIV

Abstract: A fundamental study of laminar flow over a two-dimensional rectangular cross section cavity is presented. To further understand the complex flow system an experimental investigation was performed in a small water tunnel. The cavity length (b) was varied, while the cavity depth (d) was kept constant at 10mm. The range of Reynolds numbers (Red) investigated was 90-730. Planar laser induced fluorescence (PLIF) was employed as a flow visualisation method. Pathline flow visualisation using laser sheet illumination of seed particles followed. These two flow visualisation techniques allowed the flow to be characterised into three regimes, after which particle image velocimetry (PIV) provided quantitative measurements of the complex velocity fields.

Non-intrusive PIV measurements of turbulence - application to a free round jet

Abstract: This paper presents a number of pertinent aspects related to nonintrusive PIV measurements of turbulence by using a free rou nd jet at a Reynolds number based on diameter ( D0) and average velocity (U0) at the orifice of 4000 developing in a water-based apparatus as an illustrative example. The near jet region up to x/d < 9 has been investigated using high spatial resolution 2C2D multi-grid cross-correlation digital PIV (MCCDPIV) and moderate spatial resolution 3C-2D stereo MCCDPIV (SMCCDPIV). This paper addresses issues of the optical arrangem ent for both 2C-2D MCCDPIV and 3C-2D SMCCDPIV, the optical calibration requirements and measurement accuracy for 3C2D SMCCDPIV, spatial resolution and sample requirements fo r turbulence statistics.

Electromagnetically Driven Turbulence - Experimental Study

Abstract: This project will employ laser-based diagnostics to study electromagnetically driven turbulence. The two-dimensional turbulent flow is created electromagnetically in a saline solution. The presence of the two-dimensional inverse energy cascade, as the initial turbulent region self-organises through the process of spectral condensation will be investigated. The spectral redistribution of the turbulent energy that accompanies the formation of large coherent structures and an overall increase in the energy of the system will be measured using high spatial resolution multigrid cross-correlation digital particle image velocimetry (MCCDPIV).