Plane-marching PSE wavepacket models for perfectly expanded twin jets

The present paper reports PIV (Particle Image Velocimetry) measurements of turbulent velocity fluctuations statistics in development region of an axisymmetric free jet (Re = 28 000). To minimize measurement uncertainty, adaptive calibration, image processing and data post-processing algorithms were utilized. On the basis of theoretical analysis and direct measurements, the paper discusses effect of PIV spatial resolution on measured statistical characteristics of turbulent fluctuations. Underestimation of the second-order moments of velocity derivatives and of the turbulent kinetic energy dissipation rate due to a finite size of PIV interrogation area and finite thickness of laser sheet was analyzed from model spectra of turbulent velocity fluctuations. The results are in a good agreement with the measured experimental data. The paper also describes performance of possible ways to account for unresolved small-scale velocity fluctuations in PIV measurements of the dissipation rate. In particular, a turbulent viscosity model can be efficiently used to account for the unresolved pulsations in a free turbulent flow

Gappy proper orthogonal decomposition (GPOD) is assessed here as a data reconstruction technique for particle image velocimetry (PIV) measurements, specifically those applicable to gas turbine combustors (GTC). At practical operating conditions, PIV measurements are plagued with issues, such as low signal-to-noise ratios, that result in significant gaps in data. Four GPOD methods are studied here, including a new method that makes use of a median filter (MF) outlier detection technique to adaptively update reconstructions between iterations. The analysis of the performance of the GPOD methods is done by implementing them on a non-gappy PIV data set. Artificial gaps of varying amounts are added to this non-gappy data set in a manner similar to the gaps found in real experimental data. Additionally, two criteria to check for GPOD convergence are investigated. It was found that the MF method produced the lowest reconstruction error regardless of the amount of gappiness. Furthermore, the MF method was found to be relatively insensitive to the accuracy of the convergence criterion. The MF GPOD therefore is an effective method for filling in missing data in PIV measurements of gas turbine combustor flows.

This paper presents a comparison between the two- and three-dimensional particle image velocimetry (PIV) measurement of air flow over a scaled-down model of Bukit Gemok in Telok Kalong industrial area (TKIA). The 2D PIV was performed using a single camera setup, while the 3D PIV was performed stereoscopically using a two-camera setup. The surface terrain map was obtained from google earth, which was then converted to a CAD format and the scaled down model was made using a 3D printer. The PIV measurement was performed in a wind tunnel. The results on the axial and streamwise velocity profile shows that, there are difference in the measured velocities obtained from the 2D and 3D PIV. The difference is less pronounced for the streamwise velocity but a greater difference was observed for the mean axial velocity (∼30%). In addition, the 3D PIV method captures the wake flow and vortices much better than that of 2D PIV. The data obtained from this work may be useful for Computational Fluid Dynamics validation of flow over TKIA.

Numerical study of turbulent wall jet over multiple-inclined flat surface

Experimental analysis is presented of the flow structure and performance of axisymmetric synthetic jets. We report direct thrust and Particle Image Velocimetry (PIV) measurements at varying amplitude and frequency for Reynolds numbers in the range 200-2200. It is shown that the jet performance is determined by the flow length, L, defined as the length of fluid ejected during the output stroke of the synthetic jet. Two flow regimes are identified. At small flow lengths L/D ~3, the thrust equals the momentum flux ejected during the output stroke, and is proportional to L 2 . The entrainment rate of the synthetic jet was measured and found comparable to that of conventional axisymmetric turbulent jets. The mean and time-resolved flow structure in the near field of the synthetic jet varies with flow length. At small flow lengths, vortex rings form close to the orifice and loose some of the vorticity during the input stroke. At large flow lengths, vortex rings with some trailing vorticity form and move away from the orifice thus avoiding re-ingestion of vorticity during the input stroke. The amount of trailing vorticity does not affect the thrust produced by the synthetic jet.

Proper Orthogonal Decomposition (POD) analysis of high-resolution Particle Image Velocimetry (PIV) measurements is used to identify the most energetic coherent structures of the turbulent flow field resulting from the interaction of a turbulent boundary layer with a shallow cavity. The PIV measurements capture the flow structure and turbulence, upstream, over, and downstream of a shallow open cavity. Two sets of PIV measurements corresponding to a turbulent incoming boundary layer and a cavity length-to-depth ratio of four are used. The cavity depth based Reynolds numbers are 21,000 and 42,000. The first six POD modes are found to contain a substantial percentage of the overall turbulent energy, approximately 45.2% and 45.7% for Reynolds numbers of 21,000 and 42,000, respectively. The overall spatial and modal energy content distribution are almost identical for both Reynolds numbers. The spatial flow characteristics of POD modes 1–6 reveal the existence of vortical structures, developing in the mixing region, that grow in size as they approach the trailing edge of the cavity. POD mode number one, containing approximately 20% of the overall turbulent energy, represents non-impinging vortices interaction with the cavity trailing edge, while POD modes 2–6 capture various stages of the impinging vortices type of interaction.

Flow control on a circular cylinder using streamwise-oriented Dielectric Barrier Discharge (DBD) plasma actuators was examined. These actuators generated pairs of counter-rotating streamwise vortices that influence overall flow characteristics. Particle Image Velocimetry (PIV) and aerodynamic force measurements were performed to analyze the flow control effectiveness. PIV measurements show that a streamwise-oriented DBD plasma actuator produced good suppression of flow separation at ReD = 3300 and the actuation effect was still discernible at higher ReD = 6530. However, this remarkable result was limited to region directly downstream of the exposed electrode where a downwash was present. Subsequent force measurements reveal a rise in both lift and drag acting on the cylinder when DBD plasma actuation was implemented. A maximum increase in lift coefficient ∆CL and drag coefficient ∆CD of 0.04 and 0.16 (+16%) were recorded, respectively. Surface pressure distributions estimated from PIV measurements indicate a pressure drop on the downstream-facing surface of the cylinder. The increase in streamwise pressure differential across the cylinder signifies a rise in drag acting on the cylinder.

One of the solutions to overcome the lack of electricity problem in rural area is Crossflow pico-scale hydropower. In improving cross-flow turbine performance, understanding fluid phenomena is needed. Particle image velocimetry (PIV) measurement is one of the methods used to visualize the fluid flow phenomena inside the turbine. Expensive price of seeding particle leads to find the solution for this PIV method. The objective of this study is to use a cheap seeding particle from cheaper materials, mainly resin and rhodamine B. Besides that, in this study, PIV measurement is compared to 2D computational fluid dynamics (CFD) with multiphase volume of fluid model. From the results, several phenomenon can be observed at both of the methods (PIV and CFD) such as separation in the blade tip. Therefore, this study showed that an inexpensive material resin and rhodamine B as a seeding particle has a potential and ability to represent flow inside pico-hydro cross-flow turbine.

Particle image velocimetry (PIV) measurements were conducted in the Arc-Heated Combustion Tunnel (ACT-II) on a high-enthalpy Mach 4.5 nozzle flow. The primary motivation for the current work is to establish the feasibility of PIV as a high-quality flow diagnostic technique in hypersonic arc-heated flows and produce a high-resolution hypersonic nozzle flow characterization, including nozzle free jet shear layer velocity profiles. Due to the uncertain nature of the gas composition in the flowfield of interest, PIV measurements of a supersonic wedge flow were used to couple the flow velocities to the oblique shock relations. This enabled a PIV-derived total temperature calculation that does not require a priori assumptions of the exact gas composition. A PIV expansion module was developed for this study, and proper seed sizing and material were selected for the present flow. An analysis of the seed particle response from the PIV measurements provided an initial validation of the seed performance while revealing potential issues with consistent seed sizing. These measurements and derived conditions represent a means of acquiring high-resolution velocimetry and temperature datasets that consider uncertainties in the experimental generation of hypersonic high-enthalpy flows.

PIV measurements of turbulent flow in planar mixing layer

CFD simulations of two opposing plane wall jets in a generic empty airplane cabin: Comparison of RANS and LES

Particle Image Velocimetry (PIV) relies upon the introduction of particle tracers that scatter sufficient light and follow the flow accurately. The use of submillimetre helium-filled soap bubbles (HFSB) as flow tracers for PIV is investigated for the purpose of enabling velocity measurements in large-scale industrial wind tunnels. That soap bubbles reflect more light than scattered by small liquid droplets or solid particles, allowing larger volumes to be illuminated, is a long known fact and has caught the attention of aerodynamicists since the 1930s. The difficulty encountered during initial efforts on using soap bubbles for accurate measurements revolves around the lack of control during the generation of these tracers, and the failure in presenting evidence that they could accurately follow the flow. Proof of concept that HFSB could be used for accurate flow measurements in wind tunnels was presented in the year that preceded the beginning of this work. In this thesis, the generation and control of HFSB and their tracing fidelity are studied through a series of experiments and simulations, bringing large-scale PIV using HFSB to the technology maturity level required for industrial measurements. High-speed shadowgraphy at the bubble generator exit revealed the main regimes of bubble generation. A regular, periodic and controlled generation bubbles of monodisperse size distribution, namely, the bubbling regime, was obtained by properly tuning of the input flow rates. The relation of the later with the bubble size and production rate was also obtained from these visualizations. Measurements of the HFSB velocity in the stagnation region ahead of a cylinder, obtained with Particle Tracking Velocimetry (PTV), relative to the flow velocity (slip velocity) were used to retrieve the HFSB time response and the ratio of helium to soap flow rates that satisfy the neutral buoyancy condition, in which the soap bubble density equals that of the surrounding air flow. Simulations of the particle motion in a rectilinear oscillatory flow was used to quantify the importance of the unsteady forces acting on a particle and to derive empirical relations for estimating the HFSB slip velocity in flows where the unsteady forces are relevant. In this case, the particle slip velocity is shown to depend on three parameters: the particle Reynolds number, the ratio of particle-to-fluid density and the flow time-scale. These cannot be combined into a single non-dimensional Stokes number. The validity of the empirical relations were extended for the analysis of the slip velocity of a particle travelling around an object. Based on the later, a method for deriving the density of a nearly-neutrally-buoyant particle that comprises the effects of unsteady forces and allows mismatch of acceleration between the particle and the flow was described. The tools developed for slip velocity analysis using the simulations were applied to assess experimental data from large-scale PIV measurements performed at the Low-Speed Tunnel (LST) of the German-Dutch Wind Tunnels (DNW). The experiments were realized in the flow around an airfoil of 70 cm chord at free stream velocity up to 70 m/s, reaching a chord-based Reynolds number of 3.2 million. PIV measurements using HFSB at this speed and Reynolds number were unprecedented. The results have indicated variations of the bubble density (20-30%) occurring post-generation. The tracing fidelity of HFSB in wall-bounded turbulence is investigated by comparing measurements in a turbulent-boundary layer of the mean velocity and Reynolds stress profiles, with those obtained with micrometre oil droplets (reference) and submillimetre air-filled soap bubbles (AFSB). The results have shown that the statistics of the first and second moments of velocity are well captured by all three investigated tracers, even by the heavier-than-air AFSB, which were shown to be poor tracers in the stagnation of a cylinder. Mechanisms of preferential concentration in turbulence were attributed as the cause of the better traceability observed. The thesis is concluded with a successful industrial application in the Large Low-Speed Facility (LLF) of DNW (9.5×9.5 m2 test section) around a tiltrotor aircraft in three flight modes, hover, transition and cruise, and tunnel speeds up to 60 m/s. The bubbles were introduced into the flow using a 3×3 m2 seeding rake, containing 400 bubble generators. The PIV measurements were performed in stereoscopic configuration in a field-of-view of 1.1×1.1 m2.

The control authority of a nanosecond-pulse-driven dielectric-barrier discharge plasma actuator (ns-DBDPA) was evaluated via wind tunnel experiments with the simultaneous measurement of lift and drag forces, pressure on the airfoil surface, and particle image velocimetry (PIV) measurements. In these experiments, a Reynolds number of Re = 2.6 × 105 was applied with a freestream velocity of 40 m/s under atmospheric pressure. The force measurements revealed multiple peaks of lift force recovery and drag force modulation depending on the angle of attack, α, and non-dimensional frequency, F+. At the positive post-stall α close to stall α of approximately 16°, F+ values around 2.0 were effective for lift recovery and drag reduction. When the deep-stall angle α is larger than 20° (either positive or negative), relatively low F+ values around 0.25 were effective for lift recovery. When actuating at a deep-stall angle corresponding to F+ = 0.25, the surface pressure measurements showed that a near flat pressure distribution is formed on the suction side, and the PIV measurement showed that this near flat distribution is caused by the increase in backflow velocity near the surface of the airfoil. This backflow enhancement near the suction side surface leads to the reduction in pressure in separated flow, resulting in significant increases in the lift and drag coefficients. Thus, this simultaneous measurement of force, pressure, and PIV is capable of evaluating the multiple control modes underlying lift and drag control by ns-DBDPA.

A natural convection boundary layer undergoing natural transition to turbulence is investigated by particle image velocimetry (PIV) measurements for the first time. The boundary layer adjacent to an isothermally heated plate immersed in water is subjected to environmental noise, the transition of which is therefore regarded as a natural transition. Distinct stripe-like flow structures characterising the natural transition of the natural convection boundary layer are visualised for the first time by the PIV experiments. The spatial wavenumber and the frequency of the flow structures are further quantified by applying a two-dimensional Fourier transform to time series of the flow properties. A distinct transitional regime in the boundary layer is noted, in which the three-dimensionality of the flow grows exponentially in the streamwise direction. The characteristic frequency of the natural convection boundary layer is also obtained from the PIV measurements, which agrees well with a theoretical prediction reported previously.

This paper is devoted to experimentally investigating the influence of magnetic field intensity and gas temperature on the plasma jet deflection controlled by magneto hydrodynamics. The catalytic ionization seed CS2CO3 is injected into combustion gas by artificial forced ionization to obtain plasma fluid on a high-temperature magnetic fluid experimental platform. The plasma jet was deflected under the effect of an external magnetic field, forming a thrust-vector effect. Magnesium oxide was selected as a tracer particle, and a two-dimensional image of the jet flow field was collected using the particle image velocimetry (PIV) measurement method. Through image processing and velocity vector analysis of the flow field, the value of the jet deflection angle was obtained quantitatively to evaluate the thrust-vector effect. The variation of the jet deflection angle with the magnetic field intensity and gas temperature was studied under different experimental conditions. Experimental results show that the jet deflection angle increased gradually with a rise in gas temperature and then increased substantially when the gas temperature exceeded 2300 K. The jet deflection angle also increased with an increase in magnetic induction intensity. Experiments demonstrate it is feasible to use PIV test technology to study the thrust vector under magnetic control conditions.