Showing papers by "Julio Soria published in 2007"

Digital holography for micro-droplet diagnostics

Abstract: Digital in-line holography and digital image plane holography (DIPH) are applied for the study of the droplets generated by a micro-dispensing device. This micro-dispenser provides droplets whose diameter can be changed by varying the pressure applied to the reservoir containing the substance to be atomized. The droplet diameters measured with both techniques show good agreement in the range between 10 and 20 psi. The diameter increases with the pressure and so does the dispersion. Droplet velocity has been measured with DIPH at 10 and 20 psi. It was found that the velocity increases linearly with the droplet size.

Morphology of the forced oscillatory flow past a finite-span wing at low Reynolds number

Abstract: A study of the morphology of the vortical skeleton behind a flapping NACA0030 wing with a finite aspect ratio of 3, is undertaken. The motivation for this work originates with the proposal that thrust can be efficiently produced by flapping aerofoils. The test condition corresponds to a Strouhal number of 0.35, Reynolds number, based on aerofoil chord, of 600 and an amplitude of flapping, equal to the chord length of the wing. This test condition corresponds to the optimal thrust-producing case in infinite-span flapping wings. This study investigates the effect of wing three-dimensionality on the structure of the wake-flow. This is accomplished here, by quantitatively describing the spatio-temporal variations in the velocity, vorticity and Reynolds stresses for the finite-span-wing case. Preliminary flow visualizations suggest that the presence of wingtip vortices for the three-dimensional-wing case, create a different vortical structure to the two-dimensional-wing case. In the case of a two-dimensional-wing, the flow is characterized by the interaction of leading- and trailing-edge vorticity, resulting in the formation of a clear reverse Karman vortex street at the selected test condition. In the case of a three-dimensional-wing, the flow exhibits a high degree of complexity and three-dimensionality, particularly in the midspan region. Using phase-averaged particle image velocimetry measurements of the forced oscillatory flow, a quantitative analysis in the plane of symmetry of the flapping aerofoil was undertaken. Using a triple decomposition of the measured velocities, the morphological characteristics of the spanwise vorticity is found to be phase correlated with the aerofoil kinematics. Reynolds stresses in the direction of oscillation are the dominant dissipative mechanism. The mean velocity profiles resemble a jet, indicative of thrust production. Pairs of strong counter-rotating vortices from the leading- and trailing-edge of the aerofoil are shed into the flow at each half-cycle. The large-scale structure of the flow is characterized by constructive merging of spanwise vorticity. The midspan region is populated by cross-sections of interconnected vortex rings.

Algebraic Reconstruction Techniques for Tomographic Particle Image Velocimetry

Abstract: Tomographic particle image velocimetry (Tomo-PIV) is a technique for three-component three-dimensional (3C-3D) velocity measurement based on the tomographic reconstruction of a volume intensity field from multiple two-dimensional projection. As such the performance and accuracy of this technique is highly dependant on the algorithm used for reconstruction. This paper presents an evaluation of four different tomographic reconstruction algorithms, namely multiplicative algebraic reconstruction techinique (MART); adaptive algebraic reconstruction technique (AART); improved iterative algorithm for sparse object reconstruction (IIASOR); and simultaneous iterative reconstruction technique (SIRT). Results indicate that the MART and AART algorithms provide considerably better particle field reconstructions for fewer iterations.

Thrust Measurements from a Finite-Span Flapping Wing

Abstract: The thrust per unit length behind a flapping NACA0030 airfoil with an aspect ratio of three is measured and presented. Aspects of the evolution of vorticity behind the thrust-producing wing are discussed based on quantitative experiments. Multiple planes of stereoscopic particle image velocimetry measurements are conducted at several locations along the span of the wing at a Strouhal number of 0.35. Of particular interest is the effect of wingtip vortices on the structure of the flow behind the oscillating wing. Wing kinematics is responsible for the flow structure in the 2-D airfoil case. Here, the spanwise distribution of vorticity is found to be dominated, in the large scale, by a single pair of intense counter-rotating vortices. Each member of the large-scale vortex pair is constituted by two smaller corotating vortices that constructively merge in the initial stages of flow separation. Toward the wingtips, three-dimensional effects are more significant. The spatiotemporal variations of transverse and spanwise vorticity in these regions suggest severe local flow deformation. Measurements reveal that flow morphology is highly complex and three-dimensional, unlike any previously observed 2-D wing-based vortex sheets. Furthermore, using 2-D particle image velocimetry data, a sinusoidal variation in thrust force, 90 deg out of phase with the airfoil motion, is measured in the midspan region of the airfoil. The largest measured thrust occurs at the maximum angles of attack, corresponding to the creation of strong leading-edge vortices.

Accelerated flow past a symmetric aerofoil: experiments and computations

Abstract: Accelerated flow past a NACA 0015 aerofoil is investigated experimentally and computationally for Reynolds number Re =7968 at an angle of attack α =30◦. Experiments are conducted in a specially designed piston-driven water tunnel capable of producing free-stream velocity with different ramp-type accelerations, and the DPIV technique is used to measure the resulting flow field past the aerofoil. Computations are also performed for other published data on flow past an NACA 0015 aerofoil in the range 5200�Re �35 000, at different angles of attack. One of the motivations is to see if the salient features of the flow captured experimentally can be reproduced numerically. These computations to solve the incompressible Navier–Stokes equation are performed using high-accuracy compact schemes. Load and moment coefficient variations with time are obtained by solving the Poisson equation for the total pressure in the flow field. Results have also been analysed using the proper orthogonal decomposition technique to understand better the evolving vorticity field and its dependence on Reynolds number and angle of attack. An energy-based stability analysis is performed to understand unsteady flow separation.

Study of underexpanded supersonic jets with optical techniques

Abstract: An experimental investigation of underexpanded axissymmetric supersonic jets is presented Particle Image Velocimetry is used to obtain quantitative measurements of the velocity field, while a high framerate shadowgraph technique is used to assess shock position and stability The PIV technique demonstrates the ability to consistently resolve the instantaneous velocity field, with major flow characteristics such as shock structures clearly evident The shadowgraph images show that at lower pressures the shock structures are highly unstable, demonstrating periodic oscillation in angle and position, while in the highly underexpanded condition the location of the Mach disk is stable A discussion of limitation due to optical resolution and particle fidelity is presented, concluding that the system is more limited by inadequate particle fidelity post-shock than sensor limitations

Multi-camera digital holographic PIV: Tomographic DHPIV

Abstract: The wide scale application of digital holographic particle image velocimetry (DHPIV) as a three-component three-dimensional (3C-3D) velocity field measurement tool is current restricted by the limited size and resolution of commercially available CCD arrays, resulting in a elongation of particle is the direction normal to the hologram plane. This elongation can be over an order of magnitude greater than the true particle diameter and posses significant problems for the cross-correlation analysis used in particle image velocimetry (PIV). In this paper we discuss a multi-camera method of tomographic digital holographic particle image velocimetry (Tomo-DHPIV) to reconstruct a 3D intensity field without a loss of resolution in the hologram normal direction. Application of this reconstruction technique is provided along with Monte Carlo simulations of the effects of various operating parameters.

Flow control in S-Shaped Air Intake using Zero-Net-Mass-Flow

Abstract: Flow control using zero-net-mass-flow jets in a twodimensional model of an S-Shaped air intake diffuser was investigated. Experiments were conducted in a channel flow facility at a Reynolds number of Re = 8×104 with particular image velocimetry measurements in the symmetry plane of the duct. In the natural configuration, separation of the boundary layer occurs in a region of the duct with a high degree of curvature. A stability analysis of the wall normal profile at the location of the applied control is presented and estimates the most effective frequency of the actuator. Time-averaged velocity fields show total reattachment of the boundary layer using active flow control.

Numerical simulations of an evaporating bio-oil droplet

Abstract: Bio-oil is a promising alternative fuel which contains a wide range of chemical components, including high molecular mass, organic compounds and a significant amount of water. This chemical structure can lead to a slow evaporation rate with the low boiling point compounds like water evaporating away from the surface while significant proportions still remain in the droplet core due to the relatively slow rate of liquid diffusion. As the droplet temperature increases, this water trapped in the core can reach a sufficiently high temperature to cause it to vapourise resulting in the droplet exploding. This study presents a numerical model of a stationary spherically symmetric evaporating bio-oil droplet in a hot ambient atmosphere. A diffusion limited model is used to investigate the effect of the large number components, including high boiling point chemicals, on the droplet evaporation rate. The numerical model simulates transient behaviour in the liquid and vapour phases by solving diffusion and heat transport equations in both phases using temperature dependent fluid properties.

Comparison of high spatial resolution stereo-PIV measurements in a turbulent boundary layer with available DNS dataset

Abstract: In the present contribution, the aptitude of Stereoscopic Particle Image Velocimetry (SPIV) and of Direct Numerical Simulations (DNS) to investigate coherent structures of near wall turbulence is evaluated. For this purpose, the general properties and constraints of the two techniques are first reviewed. Then, data obtained from stereo-PIV experiments in a boundary layer and DNS in a channel flow are considered. Some statistics of the velocity fields are computed, and the results obtained from the two approaches compared.

Cylinder wake - boundary layer interaction in the near field

Abstract: The interaction between the wake generated from separation off a cylinder and turbulent structures evident in a boundary layer are of significant importance in understanding the flows for cooling towers, submerged and semi-submerged vessels. This investigation was conducted on a wall mounted circular cylinder 25.4mm in diameter in the near wake region using MCCDPIV (multi-grid cross correlation digital particle image velocimetry) using a PCO4000 CCD array with full resolution of 4008 x 2672 pixels2 per image. The investigation was conducted for (ReD ≡ DU / ν) of 3600 and 5400. It was seen in the mean after taking the ensemble average of the instantaneous results that a stagnation line was formed between x / D = 1 and 1.5 downstream of the cylinder. The region of the cavity wake was highly turbulent having the largest velocity fluctuations in this region. The shape of the stagnation line was also seen to change, where for the ReD = 3600 the stagnation line position from the cylinder changed for a given height above the flat plate surface at y / D = 0.

Characterisation of a Low Reynolds Number Turbulent Boundary Layer using PIV

Abstract: An investigation by 2D-2C particle image velocimetry was carried out in the far downstream section (greater than 70 downstream boundary layer thicknesses, δ 99 measured in the region of interest) of a nominally zero pressure gradient 0.5msquare re-circulating water tunnel seeded with 11∝m Potter’s hollow glass spheres. The use of a PCO4000 camera (4008 by 2672 pixel 2 ) in combination with a Nikon Nikkor 200mm f4 IF MF lens and a PK-13 lens extension provides a measurement spatial resolution (in the streamwise-wall normal plane) of between 2 and 25 wall units for momentum Reynolds number between 200 and 2000 respectively. The paper will present characteristic flow quantities with results from the measurements non-dimensionalised by the skin friction velocities that are estimated by the Clauser method.

Spatio-temporal stability analysis of the separated flow past a NACA 0015 airfoil with ZNMF jet control

Abstract: The current paper presents the spatio-temporal stability analysis of an instance of laminar separation, with the intention of determining the most appropriate forcing frequency to initiate flow reattachment. The flow configuration is a NACA 0015 airfoil at an angle of attack (a) where laminar separation occurs immediately downstream of the leading edge. A zero-net-mass-flux (ZNMF) jet, normal to the surface and spanning the entire leading edge is used to achieve reattachment. The uncontrolled flow field was generated numerically in [5] using a three-dimensional (3-D) Large Eddy Simulation (LES) and compared to the complementary water tunnel experiments of [22]. Initial simulations of the uncontrolled case agree well with the PIV and force measurements. The stability analysis presented herein is undertaken on the mean velocity field of the LES. The frequency determined by the stability analysis to maximise spatial growth of the disturbance is finally compared to the forcing frequency that maximised lift enhancement in the experimental study.

Direct numerical simulation of passive scalars mixing in an axisymmetric swirl induced vortex breakdown flow

Abstract: Direct Numerical Simulation (DNS) of multiple passive scalars with varying molecular diffusivity in a spatially evolving axisymmetric vortex breakdown flow is performed at Reynolds number of 1500. Vortex breakdown is peculiar to swirling flows, and generally occurs when the ratio of azimuthal to axial velocity exceeds a certain level (Billant et al, 1998). Vortex breakdown is known to enhance mixing of scalars. However, little about the mixing characteristic and mechanism behind the transport of passive scalars in such flow is known. Recent studies of scalar transport in turbulent flows have shown that the evolution of scalar field in the flow depends on the turbulence transport as well as the molecular diffusivity of the scalar (Saylor & Sreenivasan, 1998). This process, called differential diffusion, is a potential complication of the mixing process. The present study investigates the role of scalar molecular diffusivity in passive scalar transport in an axisymmetric vortex breakdown flow in a swirling jet. Both instantaneous and mean scalar and velocity fields are analysed. The instantaneous radial profiles of velocity and passive scalar are also examined. The convective and diffusive budgets in the passive scalar transport equation are analysed, and diffusion-dominated regions are identified. DNS data reveals that differential diffusion has a significant effect on spatial evolution of a passive scalar field depending on its Schmidt number. A scalar with lower molecular diffusivity predominantly responds to transport by momentum of the flow. In contrast, molecular diffusion is the predominant transport mechanism for a scalar with higher molecular diffusivity. It is shown that turbulence preferentially transport scalars with lower molecular diffusivity relative to one with higher molecular diffusivity even in the presence of high turbulent stirring, e.g. vortex breakdown. Furthermore, the instantaneous scalar radial profiles are shown to deviate from the instantaneous velocity profile depending on the scalar Schmidt number. The result has direct implication on limitations of dye-flow visualisation and on transfer of heat in vortex breakdown flows.

A singularity-avoiding moving least squares scheme for two dimensional unstructured meshes

Abstract: Moving least squares interpolation schemes are in widespread use as a tool for numerical analysis on scattered data. In particular, they are often employed when solving partial differential equations on unstructured meshes, which are typically needed when the geometry defining the domain is complex. It is known that such schemes can be singular if the data points in the stencil happen to be in certain special geometric arrangements, however little research has addressed this issue specifically. In this paper, a moving least squares scheme is presented which is an appropriate tool for use when solving partial differential equations in two dimensions, and the precise conditions under which singularities occur are identified. The theory is then applied in the form of a stencil building algorithm which automatically detects singular stencils and corrects them in an efficient manner, while attempting to maintain stencil symmetry as closely as possible. Finally, the scheme is used in a convection-diffusion equation solver, and the results of a number of simulations are presented.

An Investigation of a Zero-Net-Mass-Flux Jet in Cross Flow using PIV

Abstract: Zero-Net-Mass-Flux Jets in cross flow can be used in a number of engineering applications, such as combustion in a gas turbine engine and the cooling of turbine blades. ZNMF-Jets have been seen to create two distinct flow regimes when impinging into a flow, either singular or multiple trajectories as characterised by the Strouhal Number. This paper shows two cases for a ZNMFJet in Cross-flow, when the flow is singular and when the flow shows multiple trajectories, using Digital Particle Image Velocimetry (DPIV). Time and Phase-averaged experiments were conducted in a vertical water tunnel, where the ZNMF-Jet was created using a stepper motor scotch-yoke piston arrangement of cylinder diameter 20mm to force fluid through a 10mm circular jet orifice. The Strouhal Number was taken at 0.11 and 0.56 with the Reynolds Number held constant at 1066 and the velocity ratio between the free stream and the r.m.s. fluctuating velocities of the ZNMF - Jet also held constant at 2. The depth of penetration of the ZNMF–Jet into the cross flow decreases with an increase in Strouhal Number, while the strength of the vortical structures increases with increasing Strouhal Number.