Jordi Estevadeordal

Parametric Characterization for Particle-Shadow Velocimetry (PSV)

The particle shadow velocimetry technique (PSV) is examined in terms of measurement depth and its application to multiple colors. Because this technique relies upon volumetric illumination, the depth of measurement (depth of correlation) is guided by the collection optics and the application of post processing filters. The partial coherence of the LED light sources used in PSV is shown to increase the measurement depth relative to other volume illumination techniques. It is demonstrated that the introduction of a spatial diffuser into the optics train reduces the partial coherence and improves the measurement depth. The application of color to the PSV technique requires that three issues be addressed including chromatic aberrations, color crosstalk, and the non-uniform color camera pixel distribution. Each of these issues is addressed and methods to minimize their effects presented.

An Investigation of Wake-Shock Interactions in a Transonic Compressor With DPIV and Time-Accurate CFD

The effects of varying axial gap on the unsteady flow field between the stator and rotor of a transonic compressor stage are important because they can result in significant changes in stage mass flow rate, pressure rise and efficiency. Some of these effects are analyzed with measurements using Digital Particle Image Velocimetry (DPIV) and with time-accurate simulations using the 3D unsteady Navier-Stokes CFD solver TURBO. Generally there is excellent agreement between the measurements and simulations, instilling confidence in both. Strong vortices of the wake can break up the rotor bow shock and contribute to loss. At close spacing vortices are shed from the trailing edge of the upstream stationary blade row in response to the unsteady, discontinuous pressure field generated by the downstream rotor bow shock. Shed vortices increase in size and strength and generate more loss as spacing decreases, a consequence of the effective increase in rotor bow shock strength at the stationary blade row trailing edge. A relationship for the change in shed vorticity as a function of rotor bow shock strength is presented that predicts the difference between close and far spacing TURBO simulations.Copyright

Kilo-Hertz Color Particle Shadow Velocimetry (PSV)

Particle Shadow Velocimetry is an inline velocimetry technique that utilizes low power LEDs as the excitation source. Because of the low power requirements to produce particle shadows as compared to particle light scattering, kilo-hertz repetition rates can be easily achieved. This paper presents high-speed velocity measurement results for a cuvette flow, a cylinder in cross flow, and a heart value. A high speed color camera in combination with an overdriven LED are used to make measurements at 1.1 kHz. The power from a one microsecond LED pulse is sufficient in the inline arrangement to produce particle shadows that can be utilized for velocimetry. The steps necessary to employ color for this technique are also discussed.

Streamwise Vorticity Effects in a Curved Diffuser with Slot Jet Flow Control

Abstract : A slot jet is used to increase the diffusion level of a curved, diffusing wind tunnel passage. The passage entrance coincides with the tunnel throat, which has respective Mach and Reynolds numbers of 0.7 and 187000 (based on throat height) and jet Reynolds numbers ranging from 10000 to 22000. Each of four presented configurations uses linear slot jet flow control at the same relative location. Three configurations include co- or counter-rotating vortex generator (VG) fins of varying heights upstream of the slot jet, while the other configuration has no VGs. The configurations are tested for varying flow control input and evaluated primarily on the basis of passage exit total pressure surveys. At baseline conditions (minimum slot jet flow), the shorter VG designs were found to provide some diffusion enhancement and loss reduction compared to the non-VG configuration, whereas the tall counter-rotating VGs exhibit an increase in loss. With the addition of slot jet flow, all configurations showed an initial decline followed by a net increase in performance of similar magnitude, with choked slot jet flows tending to lead to flow instabilities. Upon further increases in slot jet flow, all configurations eventually achieved a stable flow pattern. A noteworthy change in the flow pattern, with substantially improved uniformity, is documented near 6% flow control input and will be the subject of more detailed investigation.

PIV Study of Blade-Row Interactions in a Transonic Compressor

Details of the unsteady flow field between an upstream stator and a downstream rotor in a transonic compressor are obtained using Particle Image Velocimetry (PIV). Flowvisualization images and PIV data that facilitate analysis of vortex shedding, wake motion, wake deviation, rotor incidence, and wake-shock-interaction phenomena are presented. Such analysis not only aids the understanding of the effect of blade-row interactions on compressor performance but also allows verification of time-accurate CFD codes that are used to characterize transonic compressors. The present investigation introduces new methods for PIV implementation in complex turbomachinery environments. The PIV measurements are synchronized with various rotor-blade locations, and the instantaneous and averaged velocity fields of the flow are calculated. Stator wake and rotor-bow-shock flow interactions in the blade row are identified for various stator/rotor axial spacings and operating conditions. Using various post-processing methods, specific shed vortices and wake topological features are isolated and details of the shock-wake interaction captured. At far spacing, the vortices shed from the stator are phase-locked and shed as counterrotating pairs in the wake. Rotor-bow-shock strength varied, depending on the axial gap between the stator and rotor and the operating condition. Results show that as the rotorbow-shock is chopped by the stator TE, it turns more normal to the stator pressure surface and propagates upstream, validating a prior significant observation made with timeaccurate CFD.

DPIV MEASUREMENTS OF THE FLOW FIELD BETWEEN A TRANSONIC ROTOR AND AN UPSTREAM STATOR

The use of a planar non-intrusive measurement techniques such as Digital Particle Image Velocimetry (DPIV) have made it possible to investigate many aspects of unsteady flows previously considered difficult due to the effect of a measurement probe on the flow field, or too time consuming because of the pointwise nature of Laser Doppler Velocimetry or Laser Transit Anemometry. Furthermore, time-accurate CFD codes are being developed and are now commonly used to simulate compressors and investigate complex unsteady flow phenomenon. In this paper, DPIV measurements made in a transonic compressor stage are used to investigate interactions between an upstream stator and a downstream transonic rotor. In particular, the interaction between the rotor bow shock and the wake shed from the upstream stator are explored and offered as a test case for unsteady CFD comparison. Blade-row interactions are known to have a significant impact on the aeromechanical and aerodynamic performance of compressors. For example, Sanders and Fleeter [1] have shown shock-induced rearward forcing to elicit significant upstream surface-pressure amplitudes and a complicated forcing environment that contributes to High Cycle Fatigue (HCF). Numerous low speed and high speed experimental and numerical investigations [2], [3], [4], [5], [6], [7] have revealed how some blade row interactions improve stage pressure ratio and efficiency while others are detrimental to performance.

Slot Jet Flow Control for Diusion Enhancement: Endwall Observations and Improvements

wind tunnel passage. The entrance Mach number to the passage is 0.7 with jet Reynolds numbers in the 600012000 range and tunnel Reynolds number based on throat height of 188000. The throat aspect ratio (throat height to span ratio) is approximately 7:1 and the diusing section of the passage has an approximate radius of curvature of 5.08 cm for the convex surface. Three slot jet configurations are presented, each one introducing the jet in a dierent spanwise manner. The linear slot jet is first presented where the jet is introduced at a constant streamwise position across the entire span of the passage. It is observed that this jet has eective midspan control, but poor endwall performance as determined by total pressure exit traverse measurements. The endwall flowfield results in blockage similar and more explosive in nature than the uncontrolled flowfield. This is observed with Particle Image Velocimetry (PIV) and corroborated with exit total pressure data. The hypothesis is that this is a result breakdown of the turbulent shear layer due to the abrupt termination at the sidewalls. Other researchers have observed this phenomenon with laminar planar slot jets with jet Reynolds numbers of approximately 3000 where an unraveling of the vortex filament formed by the shear layer occurs due to rapid turbulent formation at the jet ends which quickly encompasses the planar slot jet. Interestingly, this phenomenon occurs with and without sidewalls. It is suggested that the abrupt termination of the shear layer is the cause of the breakdown. Therefore, two dierent slot jet configurations are presented whereby the slot jet is introduced in a parabolic fashion. One configuration introduces the jet earlier at the midspan than at the sidewalls, while the other introduces the jet at the sidewalls first and midspan later. These configurations are labeled rearward and forward swept respectively. Both configurations are compared with the linear slot jet configuration.

Fluidic Control Studies for Diffusion Enhancement in Axial Compression Systems

The design of an experimental research rig for evaluating candidate flow control concepts for diffusion enhancement in axial compression systems is presented. The research rig is modular in design and capable of continous flow in a Mach 0.7 environment with both diffusion and curvature. Candidate concepts can be evaluated inexpensively in a realistic axial compression system flow environment. Baseline results will be presented along with three preliminary flow control modules; all are variations on a theme of blowing only flow control using a slot jet behind a backward facing step. Two of the variations are preliminary investigations into the effect of the lip thickness of the backward facing step and its impact on the flow control effectiveness. The other introduces streamwise vorticity as a means of enhancing the interaction between the blowing jet and the core stream in an effort to reduce the secondary flow control flow fraction requirements. Detailed PIV, PSP and exit total pressure traverse measurements are presented and observations discussed.