Bernhard Weigand

Universität Stuttgart · Institute of Aerospace Thermodynamics

As new manufacturing methods enable manufacturing of microstructured surfaces with varying structure geometries, questions remain on some effects on the wetting behavior and the resulting apparent contact angles. In this study, we report the manufacturing process using 3D Direct Laser Writing (3D-DLW) and hot embossing for Poly-methylmethacrylate (...

Many industrial applications require turbulent closure models that yield accurate predictions across a wide spectrum of flow regimes. In this study, we investigate how data-driven augmentations of popular eddy viscosity models affect their generalization properties. We perform a systematic generalization study with a particular closure model that w...

Most data-driven turbulence closures are based on the general structure of nonlinear eddy viscosity models. Although this structure can be embedded into the machine learning algorithm and the Reynolds stress tensor itself can be fit as a function of scalar- and tensor-valued inputs, there exists an alternative two-step approach. First, the spatial...

The dynamics of near-critical single droplets allow to investigate the transition from two-phase to single-phase mixing under well-defined conditions, devoid of the additional complications due to drop-drop interactions and combustion. Recently, an empirical regime map was proposed to predict the evolution of microscopic transcritical droplets. The...

While direct numerical simulation (DNS) of multiphase flows has been the focus of many research investigations in recent years, large eddy simulation (LES) of multiphase flows remains a challenge. There is no standardized set of governing equations for multiphase LES. Different approaches and formulations have been discussed in the literature, each...

Enhanced momentum and heat transport in a three-dimensional rotating disk boundary layer due to the consideration of a wavy disk surface is an already known fact in the literature. Such a transport enhancement is primarily associated with two fundamental attributes of the wavy disk: the increased surface area and the enhanced convection due to the...

Feature attribution methods (AMs) are a simple means to provide explanations for the predictions of black-box models like neural networks. Due to their conceptual differences, the numerous different methods, however, yield ambiguous explanations. While this allows for obtaining different insights into the model, it also complicates the decision whi...

Aerosol therapy is an established technique to administer drugs by inhalation, offering well established clinical and patient benefits. However, given the complex multifactorial nature of drug nebulization and delivery to target sites, multiple opportunities remain to optimize key elements of aerosol generation to achieve increased efficiency and e...

Event-based imaging is an emerging technology in thermodynamics and fluid dynamics. In this work, we are leveraging an event-based camera with a conventional schlieren set-up to investigate the vortex shedding dynamics behind a blunt central injector in a transonic nozzle flow. We offer different methods to evaluate the data stream of the event-bas...

This project aims to advance differentiable fluid dynamics for hypersonic coupled flow over porous media, demonstrating the potential of automatic differentiation (AD)-based optimization for end-to-end solutions. Leveraging AD efficiently handles high-dimensional optimization problems, offering a flexible alternative to traditional methods. We util...

The current paper presents a theoretical analysis of swirl flow stability, both inside a tube (vortex tube) and in a free annular swirl flow. The starting concept is the study of the evolution of velocity and temperature fluctuations. Methods of non-equilibrium thermodynamics are used to describe the magnitude of fluctuations and their properties....

We investigate numerically the influence of thixotropic effects on the impact of a drop onto a thin film, a fundamental process in many technical systems. Direct numerical simulations are performed with a Volume-of-Fluid (VOF) method based multiphase flow solver whose capabilities are expanded in order to enable simulations of a thixotropic liquid....

Droplet and particle grouping can be influenced by applying an acoustic field and have practical applications such as particle scavenging and aerosol filters of engine exhaust and air purifiers. The present work experimentally investigates the influence of a standing acoustic wave on a single stream of droplets. The experimental setup consists of a...

Considered is thermal protection of the concave surface within an internal passage, which models the leading edge cavity of vanes or blades of stationary or aero engine gas turbines. Included along the internal surface are different arrangements of V-shaped ribs to further augment surface cooling heat transfer rates. Impingement jets are employed f...

This work presents a systematic experimental study of droplet impact onto a wet substrate. Four different silicone oils are used, covering a range of Reynolds number between $$116< \text{Re} <1106$$ 116 < Re < 1106 at two different initial wall film heights. The objective is to characterize the temporal and radial evolution of the velocity field wi...

In-air microfluidics became a new method for technical production processes with ultra-high throughput formerly performed in micro channels. Direct Numerical Simulations (DNS) provide a valuable contribution for the fundamental understanding of multiphase flow and later application design. This chapter presents a feasibility study with first DNS re...

The fluid dynamics of liquid droplet impact on surfaces hold significant relevance to various industrial applications. However, high impact velocities introduce compressible effects, leading to material erosion. A gap in understanding and modeling these effects has motivated this study. We simulated droplet impacts on solid surfaces and proposed a...

A technique that involves the use of a cylindrical insert to increase the heat-transfer rate in an impinging plane jet is studied using Large-Eddy Simulations. Results are presented for a plane impinging jet without an insert and for a jet with an insert, both at Reynolds number (based on the bulk inlet velocity and nozzle width) of 20,000 and with...

We experimentally observe a new phenomenon, the formation of a toroidal region of lower film thickness in the center of the lamella formed during high Weber number water droplet impacts onto smooth heated walls. This region forms around the air bubble, which is entrapped during the initial impact phase at the impact center. Our study encompasses a...

The fluid dynamics of liquid droplet impact on surfaces hold significant relevance to various industrial applications. However, high impact velocities introduce compressible effects, leading to material erosion. A gap in understanding and modeling these effects has motivated this study. We simulated droplet impacts on surfaces and proposed a new an...

In many technical applications the impact of a single droplet onto a dry, structured surface plays an important role. Hereby, the impact dynamics are often affected also by the wettability of the respective surface. In order to gain a better understanding of the complex interplay between surface structure and wettability, the basic process of singl...

Internal swirl cooling and jet impingement cooling subjected to particulate deposition were evaluated and compared using a conjugate heat transfer method, with an emphasis on the thermal effects of the insulative deposits. To accomplish the goal, an unsteady conjugate mesh morphing simulation framework was developed and validated. The swirl and the...

In multiphase flows, the evolution of fluid-fluid interfaces is of interest in many applications. In addition to fluid dynamic forces governing the flow in the entire volume, surface tension determines droplet interfaces. Here, the analysis of interface kinematics can help in the investigation of interface deformation and the identification of pote...

Energy flux analysis of power-law fluid is a novel contribution to recent developments in computational fluid dynamics. The study of the unsteady two-dimensional flow with double diffusive effect inside a complex enclosure exhibits great potential in optimizing the heat transfer rate due to the wavy nature of the side walls. The enclosure is confin...

The paper presents a numerical study of the heat transfer, pressure loss and flow characteristics of swirl cooling in elliptical tubes, which are compared to the counterpart of swirl cooling in a circular tube with a diameter of D=50.0 mm under equal passage Reynolds numbers and equal jet Reynolds numbers. The swirl tubes with two kinds of fixed tu...

This paper is concerned with an innovative regularization strategy for data-driven turbulence models. By enforcing explicit constraints on the Lipschitz continuity of the neural networks (NNs) which represent variable coefficients of the closure, their sensitivity with respect to input perturbations can be significantly reduced. This can be efficie...

The tendency of initially separated droplets to convene, named droplet grouping, is relevant in many technical applications, such as 3D ink-jet printing, medical inhalators and spray coating. Recent comparison of numerical studies to experimental data has shown that droplet shape deformation and subsequent oscillation can have a significant influen...

The dynamics of near-critical single droplets allows to investigate the transition from two-phase to single-phase mixing under well-defined conditions, devoid of the additional complications due to drop-drop interactions and combustion. Recently, an empirical regime map was proposed to predict the evolution of microscopic transcritical droplets. Th...

Shear-driven liquid rivulet flow on a horizontal plate is experimentally investigated. In contrast to publications found in the literature, a two-side view on the rivulet is used to simultaneously capture the side and top view. This enables more accurate measurements of the rivulet dimensions and the subsequent calculation of derived quantities. Th...

With technical progress, combustion pressures have been increased over the years, frequently exceeding the critical pressure of the injected fluids. For conditions beyond the critical point of the injected fluids, the fundamental physics of mixing and evaporation processes is not yet fully understood. In particular, quantitative data for validation...

An annular liquid sheet sheared by a coaxial supersonic gas stream with a swirling effect is investigated using Large Eddy Simulation. Despite its wide applications in aerospace and medical devices, the instability and spatial characters have been barely investigated due to the high complexity under supersonic condition. Unlike the conventional use...

In this study, particle deposition effects within a swirl cooling system were modeled by an obstacle that partially blocked the flow through the system. The swirl cooling system consisted of a swirl generator with two tangential jet nozzles and a swirl tube. Three typical blockage cases with the obstacle at the throats of the jet nozzles, on the si...

Liquids with a non-Newtonian rheological behavior are frequently encountered in engineering applications, where multiple phases can be involved. A rather scarcely investigated non-Newtonian behavior is thixotropy, in which the fluid's viscosity depends on the shear rate as well as on the deformation history. In this study, a fundamental case of an...

The onset of supercritical, single phase diffusive mixing and the breakdown of the classical two-phase evaporation has increasingly become important in many combustion applications. In virtue of this fact, a non-equilibrium evaporation framework is presented to predict the breakdown of the two phase regime and the onset of supercritical diffusive m...

In this work we present a two-step approach for the experimental investigation of transcritical fluid jets by the means of elastic light scattering (ELS). The objective is to get a better insight into the phase formation processes of fluid expansions that penetrate the two-phase region close to the critical point. Structured illumination is employed...

The main objective of this work is to enhance the micromixing of different species transported through the electrokinetic mechanism applicable in lab-on-a-chip devices used in BioMEMS. In this process, it is essential to predict the efficiency and precision of the micromixture for the quick and correct mixing. In this paper, a numerical study is co...

Water ingestion in gas turbines is used in industrial applications to improve the thermal efficiency by cooling the air before and throughout compression. However, this also leads to interactions between the liquid droplets and the compressor parts, which cause a faster degradation of the structure. The current work addresses the numerical investig...

In order to further decrease fuel consumption and pollutant emission of modern, highly efficient gas turbine blades, the internal cooling air consumption needs to be reduced. At the same time, the solid temperature must not exceed a critical level to avoid thermal damage of the turbine blades. Different cooling methods such as impingement cooling,...

Confined swirling flows are a promising technique for cooling applications since they achieve high heat transfer rates. In such systems, however, an axial flow reversal can occur, which corresponds to the axisymmetric vortex breakdown phenomenon. This report presents a numerical study using Delayed Detached Eddy Simulations (DDES) in order to analy...

The existence of large-scale turbulent structures within a porous medium, generated through the interaction with a turbulent outer flow, is a highly debated topic, albeit most of the analysis is based on computational studies. This study contributes to the ongoing discussion by providing detailed point-measurements of the velocity in both regions b...

In monodisperse droplet streams, the inter-droplet distances can change in a manner that brings pairs or triplets of droplets closer to one another, a process known as grouping. In the advanced stages of this process, droplet pairs can coalesce to form larger droplets. The grouping mechanisms in these droplet streams are not yet fully understood. P...

In-air microfluidics became a new method for technical production processes with ultra-high throughput formerly performed in micro channels. Direct Numerical Simulations (DNS) provide a valuable contribution for the fundamental understanding of multiphase flow and later application design. This chapter presents a feasibility study with first DNS re...

The influence of surface roughness, especially regularly patterned micro-structures on the physical outcomes of droplet impacts, is far from fully understood. In order to get a deeper insight into the physics of the impact phenomena, a systematic experimental study of the morphology on regularly patterned micro-structured surfaces has been carried...

Particulate deposition effects on flow and heat transfer in an internal swirl tube subjected to fly ash ingestion were investigated by constructing an unsteady simulation framework, in which a particle-wall interaction model and a mesh morphing technique were implemented. Swirling flows in the swirl tube were induced by two tangential jet nozzles....

The present work is concerned with the electromigration interaction of non-Newtonian fluid in a rectangular micro-capillary under the influence of an external electric field to predict the spatiotemporal dynamics of the solute concentration due to an effective dispersion and migration velocity. The solute concentration is optimized by dispersion an...

Collisions between a stream of drops and a continuous jet of a different liquid are experimentally investigated. In contrast to previous studies, our work focuses on the effects of liquid miscibility and wettability on the collision outcomes. Thus, miscible and immiscible liquids providing total and partial wetting are used. We show that, as long a...

A data-driven modeling framework for non-linear eddy viscosity models is presented. In contrast to the majority of similar approaches, it splits the multivariate regression problem into a set of univariate problems and is, furthermore, independent of a specific machine learning algorithm. Instead of inferring the closure equation from high-fidelity...

This paper addresses a consistency problem in data-driven turbulence modeling, which arises as the hypotheses are inferred from high-fidelity data but evaluated within a low-fidelity RANS solver. After elaborating on its origin, which is the systematic difference of the turbulent scales predicted by a low- and a high-fidelity solver, the frozen-RAN...

A central task in aviation technology is the development of efficient cooling techniques for thermal highly loaded engine components. For an optimal design of the cooling mechanisms, the heat transfer characteristics have to be known and need to be describable. As a cooling concept for low-pressure turbine casings, complex systems of impinging jets...

The impact of a drop onto a liquid film is a highly relevant process which occurs in many technical systems and has been well studied for pure Newtonian liquids. Drop film interactions with non-Newtonian liquids are, however, comparatively seldom investigated, even though such complex fluids are used in many real applications. In this study, we inv...

Accurate numerical simulations of multiphase flow phenomena like jet breakup or drop film interactions require an accurate advection of the disperse phase. A proper validation of the used numerical advection scheme is therefore of key importance. In order to validate these schemes, it is standard practice to perform advection tests in which simple...

The exact knowledge of droplet clustering or grouping is important in many technical applications , such as fuel injection, ink-jet printing or in medical nebulizers. Despite their relevance, the fundamentals of grouping processes are yet to be fully understood. Recent studies comprise the experimental and numerical investigation of single monodisp...

Droplet interaction technologies find application in a large number of industrial processes. Those applications show a strong non-linear coupling between flow dynamics and transport processes as well as the presence of multiple characteristic length and time scales. The International Research Training Group "Droplet Interaction Technologies" (GRK 2...

Droplet impact on dry, structured surfaces is ubiquitous in nature and technology, e.g., in soil erosion, spray painting, anti-icing coatings or self-cleaning surfaces. These applications also depend on the wetting behaviour of the surface. To gain a better understanding of the complex interplay between surfaces structure and wettability, a single...

Understanding the interaction between droplets impacting onto surfaces are of major importance in a wide range of applications, such as spray cooling, agricultural sprays, spray coating and painting. Despite many relevant applications, studies on multiple droplets impact onto surfaces are still quite limited, and especially so on surfaces textured...

Phase change processes of supercooled droplets at different boundary conditions are presented. This study is a summary of the current developments within subproject B1 of the SFB-TRR 75 with the focus on evaporation, sublimation, and freezing of supercooled droplets. To this end, new numerical methods to describe the phase transition were developed...

A fundamental understanding of droplet dynamics is important for the prediction and optimization of technical systems involving drops and sprays. The Collaborative Research Center (CRC) SFB-TRR 75 was established in January 2010 to focus on the dynamics of basic drop processes, and in particular on processes involving extreme ambient conditions, fo...

This paper reports on particle image velocimetry (PIV) measurements in compressible accelerated wake flows generated by two different central injector types, which are mounted in a convergent-divergent nozzle. The injectors differ by the extent of their trailing edge located either in the subsonic (injector A) or supersonic flow region (injector B)...

Droplet grouping is important in technical applications and in nature where more than one droplet is seen. Despite its relevance for such problems, the fundamentals of the grouping processes are not yet fully understood. Initial conditions that expedite or impede the formation of droplet groups have been studied, but a thorough investigation of the...

The development of combustion processes to achieve higher efficiencies has led to a continuous increase in operating pressures that exceed the critical point of the injected fluids. Especially for supercritical atmospheric conditions with respect to the injected fluid, the fundamental physics of the occurring mixing and evaporation processes are no...

An investigation of the interactions between a turbulent duct flow above a 3D porous medium with a periodic topology is reported. Both spatially and temporally resolved point measurements of velocity in the free flow and at the interface of the porous medium using the Particle Image Velocimetry (PIV) technique are presented. A Schwarz Primitive tri...

This paper presents a comparative experimental and numerical study of heat transfer and pressure loss in a swirl cooling system with multi-convergent tubes. The results are compared with those of a baseline circular swirl tube. The two swirl tubes are both with multiple tangential jet slots, which can induce large-scale swirling flows and significa...

A numerical model was developed for predicting the heat and mass transfer during condensation within a charge air cooler under different operating conditions. Numerical modeling of the charge air cooler was performed in a commercial CFD software with the condensation model implemented as a User Code. The semi-theoretical condensation model is based...

The blade lean technique has been extensively employed in the design of modern gas turbine blades. Since swirl cooling is a promising alternative for internal cooling techniques at the blade leading edge due to its strongly-enhanced heat transfer, it is thereby vital to completely understand the fundamental flow and heat transfer behavior of swirli...

The condition for the formation of droplet groups in liquid sprays is poorly understood. This study looks at a simplified model system consisting of two iso-propanol droplets of equal diameter, Dd0, in tandem, separated initially by a center-to-center distance, a20, and moving in the direction of gravity with an initial velocity, Vd0>Vt, where Vt i...

An unsteady analytical solution is proposed to predict the spreading rate of the crown generated by an impacting droplet onto wetted walls. The modelling strategy is based on the direct integration of the boundary layer correction into the potential flow solution that leads to the well-established square-root time dependence. The original potential...

The development of modern gas turbines for aircraft propulsion and power generation demands ever-increasing efficiency, which can be achieved by rising the turbine inlet temperature. Therefore, turbine components and especially the leading edge of turbine blades are exposed to particular high thermal loads with temperatures that are well above the...

This work presents an improved estimation of the local measurement uncertainty of transient heat transfer experiments using Thermochromic Liquid Crystals (TLC). Additional signal analysis steps and a Monte Carlo (MC) approach are added to the evaluation routine. As a result, the local uncertainty in the indication time of the maximum green intensit...

Two-wire thermocouples are often used for temperature measurements. Under transient conditions, measurement errors can occur due to capacitive inertia and heat conduction along the stem of the thermocouples. The present study presents a correction of these thermocouple measurement errors caused by transient inertia and conductive effects using a si...

The motion of two dissimilar spherical non-evaporating liquid droplets moving vertically along their line of centers is analysed mathematically. The stimulus for the study is evidence from the literature that, under appropriate operating conditions, droplet grouping can occur. Carefully controlled experiments, aimed at providing a paradigm for unde...