Experimental Test Cases Research Articles

Experimental and numerical analyses on the effect of increasing inflow temperatures on the flow mixing behavior in a T-junction

Thermal degradation of piping induced by high cycle thermal fatigue (HCTF) is of significant importance as operating Nuclear Power Plants (NPP) become older and lifetime extension activities are initiated. In particular, HCTF incidents related to turbulent thermal mixing of fluids in a T-junction piping system are not well understood and could not be adequately monitored using common thermocouple instrumentation. To investigate this phenomenon, an experimental T-junction test facility was commissioned at the University of Stuttgart, known as the Fluid Structure Interaction (FSI) test facility. The paper presents the experimental investigation and the corresponding numerical validation using the large eddy simulation (LES) method to study T-junction flow mixing. Three experimental test cases are investigated with temperature differences (∆T) of 51.5K (Case 1), 76K (Case 2) and 97K (Case 3) between the mixing fluids. A constant mass flow rate ratio (main/branch) of 4:1 is maintained in all the investigated cases. Flow mixing is observed to be incomplete in all the cases, resulting in a thermally stratified flow with an oscillating stratification layer downstream of the T-junction. Mean temperature and root mean square (RMS) temperature fluctuations predicted by LES in the mixing region are found to be in good agreement with measurement data, with the exception of few positions. Amplitudes of temperature fluctuations are observed to be higher near the stratification layer, ranging from 6.3–9.9% of ∆T. Power spectral density (PSD) analyses of temperature fluctuations indicate no dominant frequency (spectral peak) under prevailing flow conditions, an important factor in thermal fatigue analysis, and the energy of these fluctuations are mainly contained in the frequency range of 0.1–2Hz for all the investigated cases. LES is performed using the CFD software ANSYS CFX 14.0.

Evaluation of the shape deviation of non rigid parts from optical measurements

This paper deals with an approach to identify geometrical deviations of flexible parts from optical measurements. Each step of the approach defines a specific issue which we try to respond to. The problem of measurement uncertainties is solved using an original filtering method, which permits to only consider a few number of points. These points are registered on a mesh of the CAD model of the constrained geometry. The shape resulting from deflection can be identified through the finite-element simulation of the part’s deformation due to its own weight and the measuring set-up. Finally, geometrical deviations are obtained by subtracting geometrical deflections to measured geometrical deviations. The method is illustrated in an experimental test case.

Shape anomaly detection for process monitoring of a sequencing batch reactor

Abstract Anomaly detection is critical to process modeling, monitoring, and control since successful execution of these engineering tasks depends on access to validated data. Classical methods for data validation are quantitative in nature and require either accurate process knowledge, large representative data sets, or both. In contrast, a small section of the fault diagnosis literature has focused on qualitative data and model representations. The major benefit of such methods is that imprecise but reliable results can be obtained under previously unseen process conditions. This work continues with a line of work focused on qualitative trend analysis which is the qualitative approach to data series analysis. An existing method based on shape-constrained spline function fitting is expanded to deal explicitly with discontinuities and is applied here for the first time for anomaly detection. An experimental test case and a comparison with the principal component analysis method bear out the benefits of the qualitative approach to process monitoring.

A posteriori uncertainty quantification of PIV-based pressure data

A methodology for a posteriori uncertainty quantification of pressure data retrieved from particle image velocimetry (PIV) is proposed. It relies upon the Bayesian framework, where the posterior distribution (probability distribution of the true velocity, given the PIV measurements) is obtained from the prior distribution (prior knowledge of properties of the velocity field, e.g., divergence-free) and the statistical model of PIV measurement uncertainty. Once the posterior covariance matrix of the velocity is known, it is propagated through the discretized Poisson equation for pressure. Numerical assessment of the proposed method on a steady Lamb–Oseen vortex shows excellent agreement with Monte Carlo simulations, while linear uncertainty propagation underestimates the uncertainty in the pressure by up to 30 %. The method is finally applied to an experimental test case of a turbulent boundary layer in air, obtained using time-resolved tomographic PIV. Simultaneously with the PIV measurements, microphone measurements were carried out at the wall. The pressure reconstructed from the tomographic PIV data is compared to the microphone measurements. Realizing that the uncertainty of the latter is significantly smaller than the PIV-based pressure, this allows us to obtain an estimate for the true error of the former. The comparison between true error and estimated uncertainty demonstrates the accuracy of the uncertainty estimates on the pressure. In addition, enforcing the divergence-free constraint is found to result in a significantly more accurate reconstructed pressure field. The estimated uncertainty confirms this result.

Combined Turbine and Cycle Optimization for Organic Rankine Cycle Power Systems—Part A: Turbine Model

Axial-flow turbines represent a well-established technology for a wide variety of power generation systems. Compactness, flexibility, reliability and high efficiency have been key factors for the extensive use of axial turbines in conventional power plants and, in the last decades, in organic Rankine cycle power systems. In this two-part paper, an overall cycle model and a model of an axial turbine were combined in order to provide a comprehensive preliminary design of the organic Rankine cycle unit, taking into account both cycle and turbine optimal designs. Part A presents the preliminary turbine design model, the details of the validation and a sensitivity analysis on the main parameters, in order to minimize the number of decision variables in the subsequent turbine design optimization. Part B analyzes the application of the combined turbine and cycle designs on a selected case study, which was performed in order to show the advantages of the adopted methodology. Part A presents a one-dimensional turbine model and the results of the validation using two experimental test cases from literature. The first case is a subsonic turbine operated with air and investigated at the University of Hannover. The second case is a small, supersonic turbine operated with an organic fluid and investigated by Verneau. In the first case, the results of the turbine model are also compared to those obtained using computational fluid dynamics simulations. The results of the validation suggest that the model can predict values of efficiency within ± 1.3%-points, which is in agreement with the reliability of classic turbine loss models such as the Craig and Cox correlations used in the present study. Values similar to computational fluid dynamics simulations at the midspan were obtained in the first case of validation. Discrepancy below 12 % was obtained in the estimation of the flow velocities and turbine geometry. The values are considered to be within a reasonable range for a preliminary design tool. The sensitivity analysis on the turbine model suggests that two of twelve decision variables of the model can be disregarded, thus further reducing the computational requirements of the optimization.

Experimental study on performance assessment of Savonius rotor type wave energy converter in an experimental wave flume

The power generation of regular water waves from a horizontal-axis single Savonius rotor is experimentally investigated in an experimental wave flume in the context of the performance assessment of the rotor in intermediate-to-shallow water depths for different parametric conditions. A number of prototypes with multiple blades of 3–5 with the same diameter and blade curvature angle are fabricated in-house and tested to measure both the long-term average torque and power generated at different submergence levels, wave heights and wave periods. The experiments are performed in the wave-current flume equipped with a piston-type wave maker with an active wave absorption capability. The energy performance of the rotor is comparatively assessed for each experimental test case based on quantitative comparison of obtained wave-to-mechanical energy conversion efficiency (ECE) to suggest a possible optimum blade number, positioning and wave physical conditions for the investigated range of wave flow characteristics. It is found that higher ECE values can be achieved at higher wave heights and blade numbers for the lower wave periods when the rotor is placed at the water surface and/or just below the surface. This study provides a proper guideline for performance analysis of such device(s) for further studies.

Validation of Conjugate Heat Transfer Model for Rocket Cooling with Supercritical Methane

A numerical solver able to describe a rocket engine cooling channel fed with supercritical methane is validated against experimental data coming from a test article conceived and tested by the Italian Aerospace Research Center. The multidimensional conjugate heat transfer model numerically solves the Reynolds-averaged Navier–Stokes equations for the coolant flow and the Fourier’s law of conduction for the heat transfer within the wall. In this study, an experimental test case is reproduced in detail in order to evaluate the influence of partially unknown parameters, such as surface roughness and wall thermal conductivity, and of operative parameter uncertainty, such as the coolant mass flow rate and input heat transfer rate. The comparison made with respect to the wall temperature and coolant pressure drop of the whole set of experimental data provides complementary information that allows better understanding of experiments and infers possible deviations from the expected behavior.

Hypervelocity ballistic reference models as experimental supersonic test cases

Hypervelocity ballistic reference wind tunnel models are of interest to researchers in the field of high-speed aerodynamics, both as the means to validate the measurement chains in wind tunnels, and as test cases for validation of software tools for aerodynamic analysis. Wind tunnel tests of these models and some numeric simulations were performed in the Military Technical Institute in Belgrade. In the analysis of the obtained data, certain issues appeared that urge caution in the use of these models as test cases. It is shown that supersonic starting loads and high dynamic pressures in a wind tunnel can prohibit the use of the slender ‘standard’ support sting, so that some reference data are from tests with various non-standardized stings of larger relative diameters. Besides, the available reference data are actually few and quite old, the only comprehensive, freely available dataset being that from the 1963/64 tests at the Arnold Engineering Development Centre, which seems to be a de-facto reference. However, results for the total axial-force coefficients from those tests differ significantly from results from several other sources, which group well. A need is expressed for a larger database of test results, and a ‘standard’ sting of larger relative diameter is proposed for future tests.

Adapted two-equation turbulence closures for actuator disk RANS simulations of wind & tidal turbine wakes

Reliable methods for modelling wake recovery within a farm of wind or tidal turbines are critical for obtaining accurate estimates of annual energy production, and for detailed farm layout optimization. These are important objectives for maximizing energy yield while minimizing costs. Computational fluid dynamics (CFD) simulation is rapidly being adopted as a tool for flow modelling in wind and tidal farms, gaining favour over more traditional and simpler empirically-determined wake models. The most practical methodology for CFD simulations of turbine farms uses an actuator disk (AD) representation for each rotor, which imposes the rotor forces by adding source terms to the governing equations rather than explicitly resolving the flow over the turbine blades. It is well understood that when using the AD approach, standard turbulence models tend to predict faster wake recovery than is observed in real flows. Thus, the standard CFD turbulence models must be adapted for use with the AD methodology. Additionally, because of the manner in which the AD approach distributes the rotor forces, it cannot resolve the system of discrete vortices trailed from the blade tips.This article presents two contributions to improving AD simulations of wind/tidal turbine wakes. The first is identifying that the well-established k-ω SST turbulence model is appropriate for AD simulations because it mitigates the problem of over-predicting the initial wake recovery rate. The second contribution is a method to include the typically un-modelled production of turbulent kinetic energy due to the breakdown of trailed vortices. This method was tuned to minimize the wake error for three experimental test cases with different rotors and different ambient turbulence intensities {3,10,15}%. The new model was validated and compared to existing turbulence methods for the wake of a second rotor in a tandem array configuration with different separation distances and ambient turbulence intensities. The different models were assessed using an error metric designed to estimate the error in predicting the power production of a turbine array. The reduction of this error by the new model varied from case to case, but was on the order of 3.5–10%, compared to the standard k-ε model.

Improved Korteweg & de Vries type equation with consistent shoaling characteristics

An improved Korteweg & de Vries type equation for uneven water depths with consistent linear shoaling characteristics is derived. The improvement of the equation is with respect to linear dispersion characteristics while consistency in linear shoaling characteristics is achieved via an exact agreement of the shoaling rate of the wave equation with that obtained from the principle of energy flux concept. Improvements in both linear dispersion and linear shoaling properties are demonstrated analytically and numerically by simulating a challenging experimental test case of nonlinear wave propagation over a submerged bar.

Three-dimensional coverage path planning via viewpoint resampling and tour optimization for aerial robots

This paper presents a new algorithm for three-dimensional coverage path planning for autonomous structural inspection operations using aerial robots. The proposed approach is capable of computing short inspection paths via an alternating two-step optimization algorithm according to which at every iteration it attempts to find a new and improved set of viewpoints that together provide full coverage with decreased path cost. The algorithm supports the integration of multiple sensors with different fields of view, the limitations of which are respected. Both fixed-wing as well as rotorcraft aerial robot configurations are supported and their motion constraints are respected at all optimization steps, while the algorithm operates on both mesh- and occupancy map-based representations of the environment. To thoroughly evaluate this new path planning strategy, a set of large-scale simulation scenarios was considered, followed by multiple real-life experimental test-cases using both vehicle configurations.

Solenoidal filtering of volumetric velocity measurements using Gaussian process regression

Volumetric velocity measurements of incompressible flows contain spurious divergence due to measurement noise, despite mass conservation dictating that the velocity field must be divergence-free (solenoidal). We investigate the use of Gaussian process regression to filter spurious divergence, returning analytically solenoidal velocity fields. We denote the filter solenoidal Gaussian process regression (SGPR) and formulate it within the Bayesian framework to allow a natural inclusion of measurement uncertainty. To enable efficient handling of large data sets on regular and near-regular grids, we propose a solution procedure that exploits the Toeplitz structure of the system matrix. We apply SGPR to two synthetic and two experimental test cases and compare it with two other recently proposed solenoidal filters. For the synthetic test cases, we find that SGPR consistently returns more accurate velocity, vorticity and pressure fields. From the experimental test cases, we draw two important conclusions. Firstly, it is found that including an accurate model for the local measurement uncertainty further improves the accuracy of the velocity field reconstructed with SGPR. Secondly, it is found that all solenoidal filters result in an improved reconstruction of the pressure field, as verified with microphone measurements. The results obtained with SGPR are insensitive to correlation length, demonstrating the robustness of the filter to its parameters.

Handaxe reduction and its influence on shape: An experimental test and archaeological case study

The interpretation of handaxe shape is one of the most prominent questions in Acheulean archaeological studies. Nowhere is this question as sharply defined as in Britain, where there are a number of distinct handaxe shape types. Recently reduction intensity has come to the fore as an explanation for the creation of different biface shapes, however many Acheulean researchers do not see compelling evidence for differential reduction at their sites. In this study we report an experiment in which knappers, naïve as to the goal of the experiment, reduced handaxes according to different protocols. Changes in shape and flake scar density were recorded as reduction progressed. These trajectories of shape change are compared to those seen at five British Acheulean sites: Boxgrove, High Lodge, Hitchin, Swanscombe, and Broom. Our results show that although there is evidence for differential reduction intensity at these sites, this did not have a strong influence on shape. Reduction was never exhaustive, suggesting that the life history of these tools was short. Temporally and spatially variable traditions are a better fit for the observed patterns of shape variation.

Measuring Plasticity in Confined Cu Thin Films at Different Geometries

We report quantitative measurement of plasticity in confined Cu thin films with a new micro-pillar testing protocol. Polycrystalline Cu and CrN thin films were sequentially sputter deposited onto Si (100) substrates, forming thin film assemblies in which polycrystalline Cu thin films of various thicknesses were confined between non-deforming Si and CrN. Cylindrical micro-pillars of CrN/Cu/Si were fabricated through scripted focused Ga+ ion beam milling, with the interfaces either normal to the axial direction or at a 45° inclination. The CrN/Cu/Si micro-pillars were compression loaded in the axial direction with a flat diamond punch on an instrumented nanoindenter. The axial compression loading caused extensive plasticity within the thin Cu interlayers with the interfaces in both the normal and inclined orientations, but with distinctly different responses. We show that significant plastic flow occurs within the confined Cu thin films in both normal compression and shear loading. The flow stress of the confined Cu films is dependent on the Cu layer thickness and the deformation geometry. The presently described micro-pillar testing protocol offers quantitative evaluation of the plastic response of thin metal films under different deformation geometries. The present results offer new experimental examples of scale-dependent plasticity in thin metal films, and new experimental test cases for non-local plasticity theories.

Moving Kriging shape function modeling of vector TARMA models for modal identification of linear time-varying structural systems

This work proposes a Moving Kriging (MK) shape function modeling method for modal identification of linear time-varying (LTV) structural systems based on vector time-dependent autoregressive moving average (VTARMA) models. It aims to avoid the functional subspaces selection of the conventional functional series VTARMA (FS-VTARMA) models. Instead of the common basis functions, it constructs the time-varying coefficients on the time nodes with the MK shape functions in a compact support domain. The merit of the MK shape function is to determine its shape parameters upon vector random vibration signals adaptively. Model identification is effectively dealt with through an optimization scheme that decomposes the identification problem into two subproblems: estimating model parameters via two-stage least squares (2SLS) method and estimating shape function parameters via a discrete-continuous-variable hybrid optimization. In addition, the model order selection is achieved by the optimization scheme. This method has been validated by a Monte Carlo study of simulation case and further by an experimental test case, and the performance and potential advantages are illustrated.

Tikhonov's regularization approach in mode shape curvature analysis applied to damage detection

In this paper an on-going research effort aimed at detecting and localizing damage in plate-like structures by using mode shape curvature based damage detection algorithm is described. The proposed damage index uses exclusively mode shape curvature data from the damaged structure. This method was originally developed for beam-like structures. In this paper, the method is generalized to plate-like structures which are characterized by two-dimensional mode shape curvature. To calculate mode shape curvatures from the measured mode shapes, three approaches are proposed: the first one is the well-known central difference approximation, the other two are classical approaches based on Tikhonov's regularization technique with smoothing functional. The applicability and effectiveness of the proposed damage detection algorithms are demonstrated experimentally on an aluminium plate containing mill-cut damage. The validity of the method is assessed by comparing the identification results of the experimental test case to the results obtained from the simulated test case. The modal frequencies and the corresponding mode shapes of the aluminium plate are obtained via finite element models for numerical simulations and by using a scanning laser vibrometer (SLV) for the experimental study.

Accurate Asymptotic Preserving Boundary Conditions for Kinetic Equations on Cartesian Grids

A simple second-order scheme on Cartesian grids for kinetic equations is presented, with emphasis on the accurate enforcement of wall boundary conditions on immersed bodies. This approach preserves at the discrete level the asymptotic limit towards Euler equations up to the wall, thus ensuring a smooth transition towards the hydrodynamic regime. We investigate exact, numerical and experimental test cases for the BGK model in order to assess the accuracy of the method.

A New Physically-Based Simulation Framework for Modelling Flow-Like Landslides

Flow-like landslides are one type of the most catastrophic natural hazards that often cause significant loss of life and property. Numerical modelling has become a powerful tool to facilitate better risk management of this type of natural hazards. In the past decades, a number of models have been developed to simulate flow-like landslides. Among them, the depth-integrated models based on continuum theory have gained wide-spreading popularity. However, these models are generally developed either on the locally curved coordinate system that prevents the direct use of geographic information system (GIS) data or by not taking into full account of the vertical acceleration on steep slopes that may lead to loss of accuracy. These limitations hinder the wider application of these models in simulating field-scale landslides with complex topography. This paper presents a new depth-integrated model for more robust simulation of flow-like landslides. The model is developed on a fixed global Cartesian coordinate system to enable the direct use of the widely available GIS data. To gain more favourable accuracy, the new model also takes into account the effect of the vertical acceleration. The improved performance of this new model is validated against an experimental test case.

Computational study of a complex three-dimensional shock boundary-layer interaction

Shock boundary–layer interactions occur in many high-speed aerodynamic flows and they can have a notable impact on design considerations due to the aerodynamic and heat transfer effects. Consequently there is a notable interest in understanding the ability of computational tools to calculate the complex flow fields that can arise in a range of engineering applications. Three-dimensional complex shock boundary layer interaction studies are expensive in both time and computational resources. Although recent studies have begun to focus on the use of more complex computational methods such as large eddy simulations, the aim of this research is to assess the ability of steady Reynolds averaged Navier Stokes turbulence models to simulate the interaction of a planar shock impinging on a cylindrical body under supersonic conditions and to determine if these models have a role to play in engineering design applications. The performance of both eddy viscosity and Reynolds stress models are evaluated relative to an established experimental test case. The impact of Reynolds number and impinging shock strength are also considered. Of the eddy viscosity models it was shown that the Spalart-Allmaras model is unsuitable for this complex interaction and that the k- and Reynolds stress methods both gave notably better agreement with the measured surface static pressures. Overall it was considered that the Reynolds stress method was the best model as it also provided better agreement with the measured surface flow topology. It was concluded that, although a steady Reynolds averaged Navier Stokes approach has known limitations for this type of complex interaction, within an engineering context it can also provide useful results when applied appropriately.

Simplification of feature-based 3D CAD assembly data of ship and offshore equipment using quantitative evaluation metrics

In the design process for ship outfitting and offshore plants, an equipment catalog database is compiled in order for shipyards to reutilize data effectively. However, the current procedure for building such a catalog causes wastage of time because the modelers in the shipyard must perform manual modeling of the 3D CAD data in order to decrease the size of 3D CAD data and adopt a different level of detail (LOD) depending on the purpose of its use. This problem arises because equipment suppliers are not willing to give all of their 3D CAD data to shipyards, out of fear of the loss of intellectual property. Moreover, the 3D CAD data of equipment suppliers have a high LOD, while a shipyard’s 3D CAD data have a relatively low LOD. Therefore, it is necessary to introduce an automated method to simplify the 3D CAD assembly data for equipment that is received from the equipment supplier. In addressing this problem, this study first proposes criteria for a simplification process and quantitative evaluation metrics for the simplification of 3D CAD assembly data, considering the characteristics of equipment data in the shipbuilding industry. Based on these findings, a simplification system was developed, and, four experimental test cases that were conducted on-site were used to verify the proposed system. The results showed that the data to be stored could be reduced to at least 25% of the original 3D CAD assembly data while ports, outer boundaries, and connectivity between CAD parts could be maintained.