Ebenezer Gnanamanickam

High spatial range velocity measurements in a high Reynolds number turbulent boundary layer

Here, we detail and analyse a multi-resolution particle image velocity measurement that resolves the wide range of scales prevalent in a zero pressure gradient turbulent boundary layer at high Reynolds numbers (up to Reτ ≈ 20 000). A unique configuration is utilised, where an array of eight high resolution cameras at two magnification levels are used simultaneously to obtain a large field of view, while still resolving the smaller scales prevalent in the flow. Additionally, a highly magnified field of view targeted at the near wall region is employed to capture the viscous sublayer and logarithmic region, with a spatial resolution of a few viscous length scales. Flow statistics from these measurements show good agreement with prior, well resolved hot-wire anemometry measurements. Analysis shows that the instantaneous wall shear stress can be reliably computed, which is historically known to be challenging in boundary layers. A statistical assessment of the wall shear stress shows good agreement with exist...

Direct measurement of large-strain deformation fields by particle tracking

A hybrid particle image velocimetry/particle tracking velocimetry (PIV/PTV) method is described for direct measurement of large-strain deformation fields using plane-strain machining as the model system. PIV/PTV is shown to accurately measure displacements and velocities with a spatial resolution of ~1/10th of a pixel, which is an order of magnitude improvement over a comparable PIV-based method. For the configuration studied here, this translates to about ~400 nm in terms of displacement and ~80 ?m s?1 in terms of velocity. Furthermore, the method is shown to be able to capture steep gradients in velocity, which are typical of deformation zones in machining. This enables accurate estimation of associated strain rates. Implications of the technique for measuring large-strain fields in deformation processes and indentation tests, and velocity gradients due to friction at sliding interfaces, are briefly discussed.

Manufacture of high aspect ratio micro-pillar wall shear stress sensor arrays

In the field of experimental fluid mechanics the measurement of unsteady, distributed wall shear stress has proved historically challenging. Recently, sensors based on an array of flexible micro-pillars have shown promise in carrying out such measurements. Similar sensors find use in other applications such as cellular mechanics. This work presents a manufacturing technique that can manufacture micro-pillar arrays of high aspect ratio. An electric discharge machine (EDM) is used to manufacture a micro-drilling tool. This micro-drilling tool is used to form holes in a wax sheet which acts as the mold for the micro-pillar array. Silicone rubber is cast in these molds to yield a micro-pillar array. Using this technique, micro-pillar arrays with a maximum aspect ratio of about 10 have been manufactured. Manufacturing issues encountered, steps to alleviate them and the potential of the process to manufacture similar micro-pillar arrays in a time-efficient manner are also discussed.

Measurement of turbulent wall shear-stress using micro-pillars

In experimental fluid mechanics, measuring spatially and temporally resolved wall shear-stress (WSS) has proved a challenging problem. The micro-pillar shear-stress sensor (MPS3) has been developed with the goal of filling this gap in measurement techniques. The MPS3 comprises an array of flexible micro-pillars flush mounted on the wall of a wall-bounded flow field. The deflection of these micro-pillars in the presence of a shear field is a direct measure of the WSS. This paper presents the MPS3 development work carried out by RWTH Aachen University and Purdue University. The sensor concept, static and dynamic characterization and data reduction issues are discussed. Also presented are demonstrative experiments where the MPS3 was used to measure the WSS in both water and air. The salient features of the measurement technique, sensor development issues, current capabilities and areas for improvement are highlighted.

Image based sensor for distributed wall shear stress measurement

There is a need for a sensor to measure global, spatially and temporally resolved wall shear stress on wall bounded ∞ows in various engineering flelds. A wall shear stress sensor using a micro pillar array made out of silicone rubber is presented. This sensor is based on the principle that, if such a pillar is inside the viscous sub layer the de∞ection of the pillar is proportional to the drag forced experienced by the pillar, which in turn is proportional to the wall shear stress. The displacements of individual pillars in the array are tracked to obtain the wall shear stress fleld in a turbulent boundary layer ∞ow. Design and manufacturing considerations are discussed along with typical sensor calibrations in a fully developed turbulent channel ∞ow. Based on the resolution needed the sensor can be tuned for various applications. To demonstrate the feasibility of these types of sensors, the turbulent statistics in a fully developed channel ∞ow is studied. The instantaneous wall shear stress distribution around a cylinder in cross ∞ow was also mapped. I. Introduction The measurement of skin friction or wall shear stress is important for several everyday engineering problems. The time averaged values of the wall shear stress are a measure of the global state of wall bounded ∞ows and is used to determine quantities such as skin friction drag on a body moving in a ∞uid. The time resolved measurement of wall shear stress gives an estimate of the turbulent activity in the ∞ow and describes the momentum transfer events between the body and the ∞uid. The instantaneous wall shear stress is a foot print of the individual unsteady ∞ow structures that transfer momentum to the wall. 14 Wall shear stress is signiflcant especially in improving the performance and e‐ciency of transportation vehicles by reducing drag. In the airline industry skin friction drag accounts for about 45 % of the drag on an aircraft at cruise conditions. 4 Measurement of skin friction, thus assumes signiflcance as a reduction in drag directly results in a reduction in fuel consumption. Likewise, skin friction is responsible for a great part of the power expended in pumping oil and natural gases through pipes across countries and even continents. These ∞ows fall under the broad classiflcation of ∞ows called high Reynolds number ∞ows. The flnancial implications of measuring wall shear stress in a spatially and temporally resolved manner in a high Reynolds number ∞ow is hence signiflcant. Skin friction is also an important measured quantity because it helps in characterizing the state of the turbulent boundary layer, which is important both to the fundamental understanding of these ∞ows and also to assist in the fleld of ∞ow control. Flow control deals with the controlling of these ∞ows by using spatially distributed values of the instantaneous wall shear stress in manner such as to efiect changes in the boundary

Deformation Field in Indentation of Granular Materials

A preliminary study has been made of the deformation field in indentation of a model granular material using particle tracking optical flow analyses. A continuum composed of spherical sand particles with average size of 0.4 mm is indented with a flat punch under plane‐strain conditions. The region around the indenter/indentation is imaged in situ using a Charge‐Coupled Device (CCD) imaging system. By applying this hybrid analysis technique to image sequences of the indentation parameters of the deformation field such as displacement, velocity, and velocity gradient are measured at high spatial resolution. Implications of the measurement technique for accurate determination of local strain and strain rate, and exploring phenomena such as friction shear bands, in granular solids are briefly discussed.

Variable-Frequency Fluidic Oscillator Driven by a Piezoelectric Bender

A new actuator for aerodynamic flow control applications is described and evaluated in this paper: the piezo-fluidic oscillator. This actuator is a fluidic device based on wall attachment of a fluid jet and modulated by piezoelectric devices. The piezo-fluidic oscillator successfully decouples the operating frequency from the flow characteristics of the device. The frequency is specified by an input electrical signal that is independent of pressure, making this actuator ideal for closed-loop flow control applications. The oscillator exhibits high bandwidth (up to 1.2 kHz), modulation rates up to 100%, and a velocity range reaching sonic conditions. Furthermore, the bistable actuator may be operated in a steady state, with momentum flux in one of two desired directions for flow vectoring purposes. The piezo-fluidic oscillator may be used in flow control applications in which synthetic jets or plasma actuators cannot provide enough momentum for control authority. This paper details the design and characterization of the piezo-fluidic oscillator. The dynamic response characteristics are evaluated with flow visualization and hot-film probe measurements on the output.

Material properties of piezoceramics at elevated temperatures

The material properties of different piezoelectric ceramics were studied at elevated temperatures using the resonance method. Specifically the behavior of the longitudinal and transverse charge coefficients, dielectric constant, compliance coefficient and coupling coefficient were investigated. The modes studied were the length expander modes with the field both parallel and perpendicular to the strain and the materials under investigation were PZT (Navy II), Lead Metaniobate and Bismuth Titanate. All the samples studied were from the same manufacturing batch. The measured values of the charge coefficient at room temperature were compared with those obtained through direct methods (d33 meter and laser interferometer) and were found to compare well. Each material is affected differently by temperature changes, though all show a general increase in the charge coefficient with an increase in temperature. The increase in values of the charge coefficient is seen to be mainly due to the increase in the dielectric constant with very little influence from the mechanical coupling and compliance, except close to the Curie temperature where the coupling coefficient goes to zero. Bismuth Titanate has the widest temperature range however, based on a temperature scale normalized by the Curie Temperature PZT Navy II and Lead Metaniobate show the more stable material properties.

Validation of an Image-Based Skin friction Sensor in a Fully Developed Channel Flow

A new technology for distributed measurements of pressure and skin friction has been developed. The active element for this sensor is a thin film made of an elastomer with known thickness and shear modulus. The film deforms under load but does not compress or yield. The measurement is accomplished by determining the surface normal and tangential deformations of the film and then converting these deformations into pressure and skin friction. This technology is currently being utilized in wind tunnels for investigation of a variety of flows including laminar separation bubbles on low speed airfoils, shock-boundary layer interactions, and plasma flow control. While reasonable skin friction measurements are being obtained, it is necessary to validate the sensor by conducting measurements in a well characterized shear field. A validation test has been conducted in a fully developed channel flow built at Purdue University. Several quantitative and qualitative experiments were conducted including measurements of skin friction on the wall of the channel and measurements of pressure and skin friction on the wall near a strut end-wall junction. Qualitative analysis of the data near the strut-endwall junction indicates that the film is responding to the direction and magnitude of the local skin friction. A quantitative comparison of the skin friction measurements from the film with those obtained using fluorescent oil agree to within 15 percent.

Roll-modes generated in turbulent boundary layers with passive surface modifications

A unique class of directional surfaces arranged in a converging-diverging (herringbone) pattern are studied experimentally in a zero pressure gradient turbulent boundary layers. Hot-wire measurements using both single and cross-wire show that these small surfaces are able to generate large-scale counter rotating roll-modes/vortices within the turbulent boundary layer, resulting in dramatic spanwise variation in the boundary layer thickness δ (50% variation for the strongest case). The results reveal that above the converging region, the local mean velocity decreases while the turbulence intensity increases, resulting in locally thicker boundary layer. Over the diverging region, the opposite situation occurs, where the mean velocity increases and the turbulence intensity decreases, resulting in a locally thinner boundary layer. The strong perturbation effect from these surfaces to the overall flow dynamics seems unusual, considering that their peak-to-trough height is approximately only 1% of the boundary layer thickness. This study, also investigates the behavior of the large-scale counter-rotating roll-modes when the surface reverts from the herringbone pattern back to the smooth wall, to see how far they persist over the smooth wall. Our preliminary results show that the roll-modes above the smooth wall still persist even at 40δ downstream. The results of this study show that the herringbone surface roughness pattern can act as a novel method of generating counter rotating roll-modes (vortices) for flow control purposes in various engineering applications.