Technische Mechanik - European Journal of Engineering Mechanics

Steric Effect Induced Heat Transfer for Electroosmotic Flow of Carreau Fluid through a Wavy Microchannel

2023-02-16T07:01:15+00:00

We investigate the heat transfer and flow characteristics for an electroosmotic flow of Carreau fluid through a wavy microchannel, considering the finite size of ions i.e., steric effect. The flow of electrolytic liquid is considered steady, two-dimensional and incompressible. The modified Poisson-Boltzmann equation, Laplace equation, continuity equation, momentum equation, and energy equation are solved numerically using a finite element method-based solver. The computed flow and temperature fields are validated by comparison with published results. The flow and temperature fields and average Nusselt number are computed by varying the steric factor, Weissenberg number, dimensionless amplitude and Brinkman number in the following ranges: 0≤ υ ≤0.3, 0.01≤Wi ≤1, 0.1≤ α ≤ 0.5 and 10^-5≤ Br ≤10^-3, respectively. We found the locations of the local maxima and minima of Nusselt number at the convex and concave surfaces of the channel for a lower Brinkman number (=10^-5). In contrast, the corresponding locations are swapped at higher Brinkman number (=10^-3). The value of average Nusselt number increases with the increase in Weissenberg number and decreases with the steric factor for the smaller Brinkman number (=10^-5). Whereas, it decreases with Wi for non-zero values the of steric factor with higher Brinkman number (=10^-3). Moreover, the increase in amplitude enhances the average Nusselt number at higher Brinkman number (=10^-3).

Influence of Surface Wettability on Droplet Spreading Behaviour over a Solid Substrate

2023-02-15T16:24:38+00:00

The present study investigates the fluid-solid interaction phenomenon when a spherical droplet falls on the surface of a solid substrate. Numerical investigations were carried out in a 2D framework to analyse the influence of the wettability of the substrate and interfacial tension of the liquid droplet. The 2D solver establishes a good agreement with the reported experimental results. The droplet is considered to fall on the solid surface under the influence of a minimal velocity imposed on it. The results are presented in terms of droplet interface morphology and the spreading distance over the solid substrate. It is observed that the spreading tendency of a droplet is much more significant with a hydrophilic surface compared to a hydrophobic surface. It is also established that the droplet spreading increases with the decrease in Weber number. However, droplet spreading on a hydrophobic surface increases with the decrease in Weber number up to a certain limit, after which the droplet starts to contract, reducing the droplet spreading on the surface.

Fast jets from bubbles close to solid objects: examples from pillars in water to infinite planes in different liquids

2023-01-27T15:28:37+00:00

The dynamics of a single, laser-induced cavitation bubble on top of a solid cylinder and right at a plane solid boundary is studied
both experimentally and numerically. The most intriguing phenomenon that occurs for a millimeter sized bubble right at a flat
solid boundary in water is the formation of a fast jet that is directed towards the solid with a speed of the order of 1000 m/s.
Paradoxically, in this setting, fast jet formation causally is related to the viscosity of the liquid.Thus, results from numerical
simulations with varying liquid viscosity and bubble size are presented. Bubble dynamics and jet formation mechanisms are
discussed. It is shown, that fast jet formation persists for a wide range of liquid viscosities, including e.g. 50 cSt silicone oil. For
bubbles generated close to the flat top of a long, thin cylinder the parameter space of initial distance to the cylinder, bubble size and
cylinder radius is scanned numerically and partly compared to experiments. When the maximum radius of the bubble exceeds the
one of the cylinder the bubble collapses in the form of a mushroom or can resemble a trophy, depending on the values of the
geometry parameters. Complex patterns of jet formation with jet speeds ranging from the order of a few hundred m/s to several
thousand m/s are found.The results represent a contribution to understand the behavior of bubbles collapsing close to solid surfaces,
in particular, how thin, fast jets are generated.

Use of High-Order Curved Elements for Direct and Large Eddy Simulation of Flow over Rough Surfaces

2023-01-27T10:00:59+00:00

In the present study, the curved element capabilities of a high-order solver are scrutinized for use in scale-resolving simulations regarding roughness. The approach devised not only suggests a plausible way to adopt a body-fitted grid approach as an alternative to immersed boundary method (IBM), but also enables performing LES instead of DNS without under-resolving the roughness. The method is first tested using various polynomial degrees. Then, it is validated against reference DNS-IBM results from a rough channel flow setup having various Reynolds numbers corresponding to the entire roughness range. The results confirm the validity of the new approach. Finally, a highly loaded low-pressure turbine cascade is simulated under LES resolution with and without the roughness patch. Although a rougher surface is needed for producing a more pronounced impact on the flow, the viability of this method also for pressure-gradient boundary layers is proven.

Turbulent mixing simulation using the Hierarchical Parcel-Swapping (HiPS) model

2023-02-07T13:59:13+00:00

Turbulent mixing is an omnipresent phenomenon that permanently affects our everyday life. Mixing processes also
plays an important role in many industrial applications. The full resolution of all relevant flow scales often poses a major challenge to the numerical simulation and requires a modeling of the small-scale effects. In transported Probability Density Function (PDF) methods, the simplified modeling of the molecular mixing is a known weak point. At this place, the Hierarchical Parcel-Swapping (HiPS) model developed by A.R. Kerstein [J. Stat. Phys. 153, 142-161 (2013)] represents a computationally efficient and novel turbulent mixing model. HiPS simulates the effects of turbulence on time-evolving, diffusive scalar fields. The interpretation of the diffusive scalar fields or a state space as a binary tree structure is an alternative approach compared to existing mixing models. The characteristic feature of HiPS is that every level of the tree corresponds to a specific length and time scale, which is based on turbulence inertial range scaling. The state variables only reside at the base of the tree and are understood as fluid parcels. The effects of turbulent advection are represented by stochastic swaps of sub-trees at rates determined by turbulent time scales associated with the sub-trees. The mixing of adjacent fluid parcels is done at rates consistent with the prevailing diffusion time scales. In this work, a standalone HiPS model formulation for the simulation of passive scalar mixing is detailed first. The generated scalar power spectra with forced turbulence shows the known scaling law of Kolmogorov turbulence. Furthermore, results for the PDF of the passive scalar, mean square displacement and scalar dissipation rate are shown and reveal a reasonable agreement with experimental findings. The described possibility to account for variable Schmidt number effects is an important next development step for the HiPS formulation. This enables the incorporation of differential diffusion, which represents an immense advantage compared to the established mixing models. Using a binary structure allows HiPS to satisfy a large number of criteria for a good mixing model. Considering the reduced order and associated computational efficiency, HiPS is an attractive mixing model, which can contribute to an improved representation of the molecular mixing in transported PDF methods.

Impact of Flow Conditions and Geometrical Parameters on the Separation of Two Immiscible Liquids in Helical Pipes

2023-02-08T12:38:29+00:00

The separation of two immiscible liquids was studied in helical pipes with different flow and geometrical conditions. The main objective is to investigate the impact of the geometrical dimensions on the phase separation in helical pipes. The Volume Of Fluid (VOF) method was used to model the two-phase flow. The separation performance was quantified and compared using the average mixing coefficient of the two liquids. A perfect mixture of two liquids (water and amine) was always assumed at the inlet. Comparing different flow orientations, proper separation could only be obtained when the helical pipe is oriented horizontally. A laminar flow at the optimal range of Reynolds number for separation was considered (approximately Re = 225-563), where Re = 225 leads to a slightly better separation. The three key geometrical dimensions of helical pipes; the coil pitch, the pipe diameter, and the coil diameter were studied within the ranges of 16-60 mm, 5-15 mm, and 70-150 mm, respectively. The results show that changing the coil pitch has no significant effect on the separation behavior, while high enough pipe and coil diameters are needed to preserve an efficient separation of immiscible liquids and easier extraction of the lighter phase.

Langevin Dynamics Prediction of the Effect of Shear Rate on Polymer-Induced Flocculation

2023-02-15T11:17:20+00:00

A novel potential-based model for resolving polymer-particle interaction in flows is presented and used to study the
effect of shear rate on the adsorption dynamics of polymer chains onto a stationary spherical particle surface. The polymeric phase is modelled as a sequence of bead-spring components using Langevin dynamics with the finite extensible nonlinear elastic (FENE) potential to represent the molecular interactions within the polymer chain. The effects of steric interactions and the Kratky-Porod bending rigidity potential are also included in the calculations. Particles are modelled as rigid computational spheres which interact sterically with the polymer beads through a modified, truncated Lennard-Jones potential. Dependencies of conformation properties such as the mean radius of gyration and end-to-end distance on the diffusion coefficient, bending rigidity and the shear flow rate are discussed and their implications on the collision cross section for polymer-particle interactions are considered. Polymer-particle adsorption events are studied, and it is shown from Monte-Carlo studies that low shear encourages full adsorption at the point of collision, whereas increased shear hinders it, with moderate shear causing shorter tail-like structures upon adsorption. Increasing the bending rigidity potential strength leads to higher adsorption rates, with rigid polymers more likely to form tails. At both low and high FENE potential strengths, an increase in adsorption efficiency as well as the frequency of tail-like final conformities is observed. The findings of this study are of importance to the development of behavioural modification techniques where bulk system parameters are tuned to obtain a desired behaviour in important industrial processes such as flocculation and settling.

1D thermal modelling of a wheel bearing to investigate energy losses

2023-02-08T13:37:09+00:00

This paper presents the 1D thermal multi-mass simulation of a wheel bearing in order to estimate how the component
temperatures affect the energy loss during the Worldwide Harmonized Light Vehicles Test Cycle (WLTC). The simulation model
was developed in MATLAB/Simulink on the basis of the conductive and convective heat transfer equations. The ambient air and bearing components are represented by 7 simplified thermal mass points. The physical quantities necessary to create these points such as area, mass and material of the components were taken from the bearing’s 3D computer model. The heat transfer between the individual masses can be determined by setting different values of heat transfer coefficients. In this way it can be observed how much thermal insulation and proper material selection can improve the energy efficiency. The simulation model was created with speed, stability and robustness in mind in order to allow a level of accuracy that meets industrial and scientific expectations. To validate the model, the simulation results were compared to experimental data. A case study with different heat transfer parameters was concluded to quantify the effect of insulation and so, the energy saving potential.

Performance of hybrid turbulence models in OpenFOAM for numerical simulations of a confined backward-facing step flow at low Prandtl number

2023-01-30T11:16:40+00:00

To date, numerical simulation of complex turbulent flows with separation remains challenging. On the one hand,
turbulence models in Reynolds-averaged Navier-Stokes (RANS) equations struggle with correctly representing turbulent momentum transfer in such flows, whereas turbulence-resolving techniques such as large-eddy simulations (LES) carry high computational cost on the other hand. Alternatively, hybrid RANS–LES turbulence models promise to deliver scale-resolving accuracy at acceptable computational cost, yet their accuracy remains highly dependent on simulation setup and flow conditions. Here, we investigate hybrid turbulence models readily available in OpenFOAM, and benchmark their performance to Reynolds-averaged approaches and turbulence-resolving high-fidelity reference data for a confined backward-facing step flow at low Prandtl number and relatively low Reynolds number. Although temperature is generally well predicted by all considered setups, a comparison between RANS and LES shows that turbulence resolution can increase the accuracy for the considered flow case. Results show that scale-adaptive simulation techniques do not produce resolved turbulence and fail to outperform the baseline Reynolds-averaged simulations for the considered case. In contrast, detached-eddy variants do resolve turbulence in the separated shear layer, yet some configurations suffer from modeled-stress depletion due to late development of resolved turbulence. A grid coarsening study compares the degradation of accuracy for each approach, showcasing robustness of the standard RANS approach and the good performance of full LES even at surprisingly coarse resolutions. For each grid, the best-performing setup was either a RANS or an LES approach, but never a hybrid turbulence model setup. Finally, a Reynolds-number sensitivity is presented, indicating that resolved turbulence development is promoted at higher Reynolds numbers, thus leading to setups more amenable to hybrid turbulence models.

Stochastic Modeling of Electrohydrodynamically Enhanced Drag in One-Way and Fully Coupled Turbulent Poiseuille and Couette Flow

2023-01-29T00:04:33+00:00

Joint predictive modeling of hydrodynamics and electrokinetics is a standing numerical challenge but crucial for various applications in electrochemistry and power engineering. The present lack in modeling of electrohydrodynamic (EHD) turbulent flows lies in the treatment of small-scale processes and scale interactions. To overcome these limitations, a stochastic one-dimensional turbulence (ODT) model is utilized. The model aims to resolve all scales of the flow, but only on a notional line-of-sight, modeling turbulent advection by a stochastically sampled sequence of eddy events that punctuate deterministic molecular diffusive advancement. In this study, two canonical flow configurations are investigated that address different coupling strategies and flow physics. First, EHD effects in a variable-density vertical pipe flow of an ideal gas with an inner concentric electrode are investigated with a one-way coupled model formulation. Electric fields are generated by means of a corona discharge and the corresponding effect of a fixed ionic charge density field. Second, in order to reduce physical complexity, EHD effects the turbulent boundary layers in plane Couette flow of an isothermal univalent ionic liquid are investigated with a fully coupled model formulation. Both application cases demonstrate that ODT has predictive capabilities due to multiscale resolution of transport processes. Present results suggest that more expensive fully than one-way coupling of electrokinetics is crucial when charge relaxation times are significantly larger than the mean advection time scale.

LES of a non-premixed hydrogen flame stabilized by bluff-bodies of various shapes

2023-01-27T12:04:14+00:00

Dynamics of flames stabilized downstream of different shape bluff-bodies (cylindrical, square, star) with different wall topologies (flat, wavy) is investigated using large-eddy simulations (LES). A two-stage computational procedure involving the ANSYS software and an in-house academic high-order code is combined to model a flow in the vicinity of the bluff-bodies and a flame formed downstream. The fuel is nitrogen-diluted hydrogen and the oxidizer is hot air in which the fuel auto-ignites. After the ignition, the flame propagates towards the bluff-body surfaces and stabilizes in their vicinity. It is shown that the flames reflect the bluff-body shape due to large-scale strong vortices induced in the shear layer formed between the main recirculation zone and the oxidizer stream. The influence of the acute corners of the bluff-bodies on the flame dynamics is quantified by analysing instantaneous and time-averaged results. Compared to the classical conical bluff-body the largest differences in the temperature and velocity distributions are observed in the configuration with the square bluff-body. The main recirculation zone is shortened by approximately 15% and at its end temperature in the axis of the flame is almost 200~K larger. Simultaneously, their fluctuations are slightly larger than in the remaining cases. The influence of the wall topology (flat vs. wavy) in the configuration with the classical conical bluff-body turned out to be very small and it resulted in modifications of the flow and flame structures only in the direct vicinity of the bluff-body surface.

On the Stability of Direct Spring-Operated Pressure Relief Valves: Impact of Frozen Mixture Flow and Lift Restriction

2023-02-14T16:36:55+00:00

The current work explores the effect of (frozen) mixture flow and lift restriction on the stability of direct spring-operated pressure relief valves. First, we study the effect of frozen mixture (constant mass fraction) flow through a pressure relief valve with upstream piping. DIER's $\omega$ technique is employed to cope with the mixture parameters, notably sonic velocity and choked/non-choked flow through the nozzle. By means of one-dimensional simulation, we show that the change in sonic velocity plays a fundamental role in both the valve opening time and its stability. Due to the extremely low sonic velocities in certain range of water-air mass fraction, such valves will have a poor response time (slow opening) and chatter even for short inlet pipings. Next, we investigate the possibility of improving the dynamical performance of the safety valve by using a larger valve (i.e., a larger orifice) with a restricted lift option while keeping the vented mass flow rate through the valve constant. Our numerical investigations reveal that the additional restoring force emerging from the valve restriction at the upper stopper improves the stability behaviour in the case of gas applications but can hardly have any influence in the case of liquid service.

Addressing wind comfort in an urban area using an immersed boundary framework

2023-01-27T12:31:07+00:00

Considering wind, air and heat comfort in designing new urban areas is still a challenge for city planners. Urban heat islands, or the phenomena of locally increased temperatures in urban areas compared to their rural surroundings, are becoming increasingly problematic with global warming and the rise of urbanization. Therefore, new areas must be planned considering appropriate ventilation to mitigate these high-temperature regions and cooling strategies, such as green infrastructures, must be considered. Typically, most of the comfort criteria are evaluated and assessed in the final stages of urban planning when further strategic interventions are no longer possible. Here, a numerical framework is tested that urban planners can use as a future tool to analyze complex fluid dynamics and heat transfer in the early stages of urban planning. The framework solves the RANS equations using an immersed boundary approach to discretize the complex urban topography in a cartesian octree grid. The grid is automatically generated, eliminating the complex pre-processing of urban topographies and making the framework accessible to all users. The results are validated against experimental data from wind tunnel measurements of wind-driven ventilation in street canyons. After validation, we will apply the numerical framework to estimate the wind comfort in an idealized urban area. Finally, guidelines will be provided on the choice of minimum grid sizes required to capture the relevant flow structures inside a canyon accurately.

SURFACE LAYER’S SOUND SPEED PROFILES: CLIMATOLOGICAL ANALYSIS AND APPLICATION FOR THE CNOSSOS-EU NOISE MODEL

2023-01-27T14:04:43+00:00

Noise pollution and exposure are important environmental issues that need to be investigated and regulated. To do this,
we need to know about micrometeorology to figure out how noise travels from the source to the receiver. Accordingly, the sound propagation part of the common noise assessment methods (CNOSSOS-EU) developed by the European Commission for different sources of noise needs detailed meteorological databases. Using data from the SYNOP stations maintained by the Hungarian Meteorological Service (HMS) and the ERA5 meteorological reanalysis database, the standard noise propagation conditions are determined. The primary objective of this study is to ascertain the probability distribution of stability classes for a variety of source-receiver orientations, utilizing either 25 or 2 stability classes, and several different aggregation levels. Relative frequencies and year-to-year variability have been calculated for favourable noise propagation conditions where the sound speed profile grows with height (downward refraction condition) and unfavourable noise propagation conditions where the sound speed profile constant or decreases with height (so-called homogeneous conditions). Favourable noise propagation occurs in approximately one-third of cases during the daytime while in approximately two-third of cases during the evening and night-time where the noise exposure is increasing. Furthermore, using the SoundPLANnoise software, sound propagation model calculations were performed on a study area near Budapest, using different values of parameter pf describing the probability of occurrence of favourable conditions on sound propagation during different periods of the day. This area is crossed by Highway 4, which is a major road according to the 49/2002 EU Directive, as it has more than three million vehicles passing on the examined section every year. The results show considerable deviations in annual average A-weighted sound levels calculated using different versions of parameter pf. The largest difference between the A-weighted sound levels calculated with the highest and lowest generated annual pf values was 1.65 dB(A); 1.42 dB(A) and 0.75 dB(A) for day, evening and night periods, respectively.

A novel model for glaze ice accretion

2023-01-27T09:18:01+00:00

This paper introduces a novel model to predict ice accretion in glaze ice conditions due to supercooled water droplets. Glaze icing is controlled by a large number of interacting physical phenomena. The purpose of the suggested model was to offer a faster alternative to explicitly modelling these phenomena. The paper presents the suggested model and investigates the sensitivity of the predictions on the model parameters for three experimental cases in the literature. The results indicate a qualitatively correct behavior. Quantitatively, the model over-predicts the amount of accreted ice, the error being significantly larger in severe icing conditions. The errors are caused partly by the choice of faster numerical approaches and by the lack of possibility to account for detaching ice from the surface.

CFD Assessment of an Ultralight Aircraft Including In-Flight Test Data Comparison

2023-01-31T08:36:34+00:00

In-flight test campaign was conducted with an ultralight aircraft to gather static and total pressure data over the aircraft
surfaces and its surrounding using a purpose-built measurement system. The measured data served as the basis for the evaluation of the aircraft CFD simulation results. The CFD model considers the entire external geometry of the aircraft and utilises the in-house developed 3D corrected Virtual Blade Model to account for the propeller-induced flow field. The modelling approach and results are discussed and compared with the in-flight test data. A derivative of the baseline model was created with a detailed engine bay. The water and oil cooler devices were modelled with porous zones which properties were derived by explicit CFD simulation. The assessment of the flow field within the engine bay and the effect of the propeller-induced flow is discussed.

Preface

2023-02-08T16:34:11+00:00

This volume of Tech. Mech. contains selected papers presented at the 18th event of the international conference series on fluid flow technologies, referred to today as Conference on Modelling Fluid Flow (CMFF’22). This conference took place in Budapest (Hungary) between Aug. 30th and Sept. 2nd, 2022, with more than 100 participants from 14 countries. The next event is scheduled for September 2025. Please bookmark https://www.cmff.hu if you would like to be kept informed.