Scale Model Experiments Research Articles

Analysing the acoustic performance of a nearly-enclosed noise barrier using scale model experiments and a 2.5-D BEM approach

This paper describes a scale modelling method to measure the acoustic performance of a nearly-enclosed barrier and the corresponding predictions using an existing 2.5-D Boundary Element Method (BEM) program. Preliminary investigation results show the deterioration in performance of a nearly-enclosed barrier due to resonance effects that led to high pressure levels radiating into the surroundings via the topped opening. Measurements on one-twentieth scale model of barriers, viaducts and vehicle structures were carried out outdoors under controlled conditions. The measured results show that the transmission loss of transparent panels on the top were not adequate to make the measured results as high as the predictions. A modified scale model, by coating all the surfaces with rubber, was remeasured. The measured results from the modified scale model and the numerical calculations were in good agreement each other, which indicate that the 2.5-D BEM code can provide a reliable description of the acoustic performance of a nearly-enclosed barrier. Then the program was used to investigate the barrier performance for different receiver positions in the surrounding environment. As expected, the attenuation of the nearly-enclosed barrier averaged around 15 dB in the near field and around 10 dB in the far field. The number effect of incoherent point sources on the performance is discussed as well for the study of railway traffic noise. The increased number of incoherent point sources can result in smoother and lower attenuations for the whole sound field.

Utilizing a spark gap generator as an impulsive noise source for scale model experiments investigating novel metamaterial arrays

Utilizing a spark gap generator as an impulsive noise source for scale model experiments investigating novel metamaterial arrays

Experimental Study on the Performance of an Innovative Tide-Induced Device for Artificial Downwelling

Hypoxia has been increasingly observed in estuaries and coastal marine ecosystems around the world. In this paper, a tide-powered artificial downwelling device is proposed to potentially alleviate hypoxia in bottom waters. The downwelling device mainly consists of a vertical square tube, a 90° bend sitting on the top of the tube, two symmetrical-guide plates which installed alongside the vertical tube, a static mixer, and an artificial reef. Scale model experiments are performed with respect to different density difference heads, horizontal current velocities, and tube geometries. The results show that the downwelling flow rate is dependent on horizontal current velocity, tube geometry parameters, and the density profile of ambient water. In addition, increasing the equivalent diameter and bend radius of the device can decrease the total loss coefficient in the tube, which in turns enhance the downwelling efficiency. The two symmetrical-guide plates also generate obvious downwelling of surface water which further improves the whole performance of the device. Further work will need to determine the influence of the other parts of the device, such as the static mixer and artificial reef, on the downwelling efficiency.

Scaling spray combustion processes in marine low-speed diesel engines

Scaled model experiments would be very effective in reducing cost, time and energy consumption in marine diesel engine development, however, relevant researches are rarely reported. The aim of this paper is to explore the potential of scaled model experiments for marine low-speed diesel engines, and provide references for researchers who attempt to conduct scaled model experiments in marine diesel engine development. At first, the computational fluid dynamics simulation model is established and calibrated against the experimental data from a marine low-speed diesel engine with 340 mm bore diameter. Then, three scaling laws for spray mixture formation and combustion processes as well as pollutant emissions are numerically studied with the baseline 340 mm-bore engine and an up-scaled 520 mm-bore engine at various injection timings. The results reveal that the spray mixture formation processes, in-cylinder temperature and pressure traces, and indicated thermal efficiency can be well scaled between the 340 and 520 mm-bore engines, while the three scaling laws show different degrees of similarity in soot and nitrogen oxides emissions. The increased heat transfer losses of the small engine are identified as a major obstacle to achieve the overall similarity, and the wall temperature setting methods to scale the heat transfer loss are theoretically derived and numerically verified. With the sophisticated control of the cylinder wall temperatures of the small engine, the discrepancy in soot and nitrogen oxides emissions between the large and small engines is reduced.

Scale model observations of coupled vertical modes in a translationally invariant wedge.

Scale-model tank experiments offer a controlled environment in which to make underwater acoustic propagation measurements that can provide high-quality data for comparison with numerical models. This paper presents results from a scale model experiment for a translationally invariant wedge with a 10° slope fabricated from closed-cell polyurethane foam to investigate three-dimensional (3-D) propagation effects. A computer controlled positioning system accurately located a receiving hydrophone in 3-D space to create a dense field of synthetic vertical line arrays, which are subsequently used to mode filter the measured pressure field. The resulting mode amplitudes show the modal propagation zone, the modal shadow zone, and the classical intra-mode interference pattern resulting from rays launched up and along the slope. The observed features of the measured data are compared to three different propagation models: an exact, closed-form solution for a point source in wedge with pressure release boundaries, a 3-D ray trace model, and an adiabatic model. Examination of the mode amplitudes produced by the models reveals how the effects of vertical mode coupling can be observed in the measured data.

Experimental and Numerical Study on the Strain Behavior of Buried Pipelines Subjected to an Impact Load

Long-distance oil and gas pipelines are inevitably impacted by rockfalls during geologic hazards such as mud-rock flow and landslides, which have a serious effect on the safe operation of pipelines. In view of this, an experimental and numerical study on the strain behavior of buried pipelines under the impact load of rockfall was developed. The impact load exerted on the soil, and the strains of buried pipeline caused by the impact load were theoretically derived. A scale model experiment was conducted using a self-designed soil-box to simulate the complex geological conditions of the buried pipeline. The simulation model of hammer–soil–pipeline was established to investigate the dynamic response of the buried pipeline. Based on the theoretical, experimental, and finite element analysis (FEA) results, the overall strain behavior of the buried pipeline was obtained and the effects of parameters on the strain developments of the pipelines were analyzed. Research results show that the theoretical calculation results of the impact load and the peak strain were in good agreement with the experimental and FEA results, which indicates that the mathematical formula and the finite element models are accurate for the prediction of pipeline response under the impact load. In addition, decreasing the diameter, as well as increasing the wall thickness of the pipeline and the buried depth above the pipeline, could improve the ability of the pipeline to resist the impact load. These results could provide a reference for seismic design of pipelines in engineering.

Fire experiment on temperature distribution in an underground vertical car park

The underground vertical car parks are usually highly closed with few entrances and exits, and moreover the compact parking of vehicles leads to dense combustible materials and great fire load. Therefore, it is of practical significance to investigate the temperature distribution of underground vertical car park. In this paper, temperature measurements were presented as obtained in an 1/5 scale-model experiment. Since the main objective of this paper was to investigate the impact of the heat release rate (HRR) and the smoke exhaust system on the temperature field in the underground car park fire. Different parameters were varied: the heat release rate (HRR) (the presence or not of the automatic sprinkler system); the smoke extraction system (on and off); the location of the measured point (above, opposite, or adjacent). The results showed that the heating effect of the car fire on the temperature of upper floors was greater than that of the opposite side floors; when the exhaust system turned on, the temperature of the floors above the fire would be increased, and the temperature of the floors opposite the fire changed inapparently.

Investigation on Re-independence of air flow and pollutant concentration field in the basement space of an underground sewage treatment plant

The basement space of underground sewage treatment plants is an enclosed flat large-space where workers conduct inspection and maintenance work. Mechanical ventilation system is necessarily installed to ensure indoor air quality. In order to provide theoretical support for scale-model experiments of basement space ventilation, in this paper, several scaled models of the basement space in an underground sewage treatment plant installed with ductless ventilation system are numerically established. The critical Reynolds number, Recrit, to achieve independent flow pattern and pollutant distribution characteristics is investigated mainly with Computational Fluid Dynamic (CFD) method and validated against experimental data. For a 1:40 scaled model, the dimensionless velocity and pollutant concentration field in the basement space are compared when Reynolds number increases from 85 to 12750. The critical Reynolds number is found to be around 6000. Two evaluation indexes, i.e. relative change rate of velocity (RCRV) and deviation rate (DR), are used to evaluate the critical Reynolds number (Recrit). A criterion of evaluation indexes less than 5% is used to quantitatively calculate Recrit. The influence of scale ratio of prototype on Recrit is investigated when it changes from 1:5 to 1:100. The results show that scale ratio has almost no impact on Recrit. It is useful for the design of scale-model experiments for such large space ventilation.

A CFD study on the performance of a passive ocean plastic collector under rough sea conditions

In this study, a commercial CFD code, STARCCM+, is used to analyze the performance of a passive ocean plastic collector under rough sea conditions. The CFD code was first validated by comparing with data from a scaled model experiment conducted in the towing water tank in National Cheng Kung University, and it was proven to return accurate catch rate. Then thirty-eight test cases were setup to investigate the effects of four different parameters, namely, ocean current speed, wave height, wave length, and plastic density, on the plastic collector's catch rate, which is the percentage of incoming plastic debris intercepted by the plastic collector. It was found that the parameters of wave length and plastic density posed very little effect on catch rate. In contrast, the effects of the other two parameters were significant. Two important thresholds were found, and they were ocean current speed of 2.5 ms−1 and wave height of 0.4 m. The catch rate remained at high level until these thresholds were reached. The information found in this study is invaluable for the design of a practical passive ocean plastic collector.

Study of convective heat transfer load induced by nozzle air supply in large spaces with thermal stratification based on Block-Gebhart model

The Block-Gebhart (B-G) model is proposed to predict the convective heat transfer load for the stratified air-conditioning system using nozzle air supply. The scale-model experiments were carried out in an enthalpy difference laboratory in Shanghai. The theoretical solutions of convective heat transfer load calculated by the B-G model were experimentally verified. The results showed the feasibility and accuracy of the B-G model in calculating the convective heat transfer load for the stratified air-conditioning system in large spaces. The key factors affecting the convective heat transfer load were analyzed. The nomogram of convective heat transfer load was made, which could be used to determine the relationship among the non-dimensional convective heat transfer load (ratio of convective heat transfer load to heat gain in non-air-conditioned area), the heat intensity ratio (ratio of heat intensity in non-air-conditioned area to heat intensity in air-conditioned area), and the exhaust heat ratio (ratio of exhaust heat to heat gain in non-air-conditioned area). Through this method, the convective heat transfer load can be effectively calculated in practical engineering applications, which provides the theoretical basis for the optimal design of airflow distribution in large spaces.

Performance evaluation of ductless ventilation system in basement space of an underground sewage treatment plant: A scaled model case study

In recent years, the number of underground sewage treatment plants has increased dramatically in densely populated cities. Mechanical ventilation system is necessary to ensure indoor air quality in the enclosed basement space of underground sewage treatment plants. Many disadvantages exist when using conventional overhead duct ventilation system. In this paper, a ductless ventilation system based on the concept of unidirectional piston flow is proposed for such flat large-space in underground sewage treatment plants. The influences of perforated plate porosity, installation of passage partitions, opening state of vehicle passage entrance doors and air supply mode on ventilation performance are investigated by using scale-model experiments. The results suggest that perforated plate porosity is recommended between 0.25 and 0.35 and partitions near vehicle passage entrance need to be installed. With a reasonable design, the unidirectional flow is formed in the whole level. No air leakage from the basement space to ambient environment happens when the pressure at vehicle passage entrance is 200 Pa higher than that at the outlets. However, serious airflow short circuit phenomenon occurs when the pressure difference reduces to −50 Pa in scale model. Present research provides a new option for the design of ventilation system in such kind of large and flat space.

Inverse tracking of an airborne pollutant source location in a residential apartment by joint simulation of CFD and a multizone model

Prompt identification of an indoor air pollutant source location is important for the safety of building residents in gaseous contaminant leakage incidents. Using the computational fluid dynamics (CFD) method in such inverse modeling is time consuming, especially for naturally ventilated residential buildings, which have multiple rooms and require consideration both indoor and outdoor environments. This paper compares the results of the pollutant source location identification and the simulation time based on two different inverse modeling methods: the CFD method and joint modeling of the multizone and CFD methods, to discuss the consumption of the computing time, as well as the accuracy of the location identification result. An instantaneous airborne pollutant source is assumed in a typical residential apartment that utilizes natural ventilation. A CFD model with the computational domain of the whole apartment and surrounding environment is built, for which the adjoint probability method is applied to simulate the source location probability from limited sensor readings. Meanwhile, a multizone model of the apartment is built to simulate and identify the room in which the source is located using the adjoint probability method. The CFD method is applied to the identified room afterwards to identify the exact location of the source within that room. The joint simulation of CFD and the multizone model is verified by a scaled model experiment of the apartment. It is found that the joint simulation method can significantly reduce the computing time and provides a good alternative for real-time inverse tracking of the indoor airborne pollutant.

Outdoor measurement of wall pressure on cubical scale model affected by atmospheric turbulent flow

Most studies on indoor ventilation have utilized wind-tunnel experiments (WTEs) or computational fluid dynamics based approaches under well-controlled flow conditions. However, the effects of urban boundary layer flow with variable wind directions and various turbulence scales on the ventilation driven by the pressure differences between the upwind and downwind sides of a building within a block array is still under discussion. Therefore, we conducted outdoor experiments at comprehensive outdoor scale model (COSMO) experiment sites in an urban climate to clarify the relationships between the building wall pressure differences and the approaching flow. The pressure coefficients for the outdoor site were comparable with those obtained during previous WTEs. Accordingly, temporal variations in the wind speed and pressure coefficient on the target block were investigated in detail using low-pass filtering operations. The relationships between the filtered wind speed and the pressure differences indicate that the slower temporal variations in the wall pressure showed good agreement with the filtered approaching flow. In addition, the correlation coefficient between the filtered wind speed and the wall pressure differences quantified the apparent coherence between the turbulent flow and the ventilation rate. Furthermore, the statistics of the ventilation rate were determined based on the conventional model to clarify the effects of the turbulent flow on the natural ventilation rate. The ratios between the mean and short-term ventilation rates imply that the short-term ventilation rate presented dramatic temporal fluctuations owing to the various scale turbulence generated by the atmospheric flow.

On the resistance and speed loss of full type ships in a seaway

ABSTRACTThis paper presents results of a systematic study on the resistance and speed loss of full type ships in waves. The percentage resistance-increase in waves against experimentally measured calm water resistance data was estimated by use of two alternative empirical formulas, namely one proposed by the authors and the other one as recommended by ITTC. The obtained results clearly show that the percentage resistance-increase and the associated (involuntary) speed loss are strongly dependent on ship’s absolute size, loading condition and the operating seaway conditions and they can be satisfactorily quantified. This should be taken into account in the ship design process, when determining the sea margin for the selection of the machinery and propulsion system, when analysing sea trials and/or when preparing scaled model experiments to assess ship’s performance in realistic seaway scenarios.

Electromagnetic Forces in Continuous Casting of Steel Slabs

This paper reviews the current state of the art in the application of electromagnetic forces to control fluid flow to improve quality in continuous casting of steel slabs. Many product defects are controlled by flow-related phenomena in the mold region, such as slag entrapment due to excessive surface velocity and level fluctuations, meniscus hook defects due to insufficient transport of flow and superheat to the meniscus region, and particle entrapment into the solidification front, which depends on transverse flow across the dendritic interface. Fluid flow also affects heat transfer, solidification, and solute transport, which greatly affect grain structure and internal quality of final steel products. Various electromagnetic systems can affect flow, including static magnetic fields and traveling fields which actively accelerate, slow down, or stir the flow in the mold or strand regions. Optimal electromagnetic effects to control flow depends greatly on the caster geometry and other operating conditions. Previous works on how to operate electromagnetic systems to reduce defects are discussed based on results from plant experiments, validated computational models, and lab scale model experiments.

Aspects of the measurement of the acoustic transfer function in a cavitation tunnel

Model scale experiments represent one of the most effective, and probably the most commonly used, approaches for the study of cavitating propeller noise. Despite this, the acquirement of consistent data of propeller noise from tests in model scale facilities is still a challenging task. Data collected with such an approach are usually affected by several scale effects and facility related issues which make such experiments still very complex. Among others, the effect of the facility itself as acoustical surrounding must be carefully addressed in order to properly estimate propeller source levels in ideal free field conditions. This is currently carried out, when possible, by measuring facility transfer functions and applying such functions as a correction to measured noise spectra. The development of robust procedures for the measurement of these transfer functions is thus very important. In the present work, this problem is addressed by discussing in details techniques, merits and possible problems related to this topic.

Model scale cavitation noise spectra prediction: Combining physical knowledge with data science

Ships underwater radiated noise is a subject of great and increasing interest in naval architecture because of its impacts on the environment and on the on-board comfort. Among the different noise sources, the propeller is usually the dominant one, especially when it cavitates. For this reason a lot of efforts have been spent in studying the cavitation noise spectra in order to be able to predict its main characteristics at design stage. Nowadays, in order to reach this goal, the state-of-the-art solution is to perform model scale experiments. Unfortunately, this requires time-consuming tests in a cavitation tunnel with a model of the propeller and results are affected by scale effects that must be correctly interpreted. For these reasons, in this paper authors propose a hybrid approach which may be adopted to predict the main characteristics of the cavitation noise spectra without requiring an actual experiment. Moreover, the same approach may be used also in order to overcome typical model scale problems. For this purpose a hybrid modelling approach able to exploit both the physical knowledge of the problem and the real data obtained from many cavitation tunnel experiments performed on different propellers in different working conditions have been developed. Results on real data in both interpolation and extrapolation tasks will support the validity and the effectiveness of the proposal.

Investigation of a steel connection to accommodate ductility demand of beams in fire

In fire conditions the provision of connection ductility is key to the prevention of brittle failure and progressive collapse of steel and steel-concrete composite framed structures. This paper describes the development and testing of a novel connection concept intended to provide appropriate ductility enhancement compared to that of conventional connection types. The connection consists of two connection pieces, each of which takes the form of a web cleat which includes a semi-cylindrical section. This section allows the beam-end to move towards or away from the column-face by deforming plastically. A simplified analytical model has been developed to simulate the mechanical behaviour of the proposed connection, and this model will eventually enable the incorporation of this new connection type into global frame analysis to be used in performance-based structural fire engineering design. The model has been tested against FEA simulations and against model-scale experiment results, indicating that it can predict the behaviour of the connection with satisfactory accuracy. Preliminary sub-frame analysis results indicate that the proposed connection behaves similarly to an idealized pinned connection under ambient-temperature conditions, but provide significantly larger ductility, and resistance to disproportionate collapse, compared to conventional connection types.

Toward Noise Certification during Design: Airframe Noise Simulations for Full-Scale, Complete Aircraft.

An overview of a recent, NASA-sponsored effort to substantially advance simulation-based airframe noise prediction is presented. An accurate characterization of this component of aircraft noise requires a high-fidelity representation of the finer geometrical details associated with the landing gear and wing high-lift devices, such as slats and flaps, which constitute major noise sources. To achieve this ambitious goal, a systematic approach was followed to extend our current state-of-the-art computational tools to a full-scale, complete aircraft in landing configuration within a realistic flight environment. The work involved several phases: high-fidelity, large-scale, unsteady flow simulations; model-scale experiments in ground-based facilities; and farfield noise prediction for a full-scale, complete aircraft. The comprehensive aeroacoustic database generated during the course of the effort provided a wealth of relevant information for full validation and benchmarking of the advanced computational tools used in the present work. The database will also foster the development of simulation methodologies with improved predictive capabilities.

Simulation and evaluation of leakage electric field of bounded-wave simulator

ABSTRACTThis paper presents a detailed analysis on the distribution of the electric field inside and outside the bounded-wave simulator excited by HEMP (early part), and the attenuation law of the leakage electric field is verified by a scale-model experiment. Furthermore, at the basis of the energy spectrum distribution of excitation source, the general public and occupational exposures values at different observation points are calculated, and then, the limits of safety ranges are determined according to the guidelines issued by the ICNCP in 1998. The results provide a guidance for people to effectively defend against electric field radiation derived from a bounded-wave simulator.