"uuid","repository link","title","author","contributor","publication year","abstract","subject topic","language","publication type","publisher","isbn","issn","patent","patent status","bibliographic note","access restriction","embargo date","faculty","department","research group","programme","project","coordinates"
"uuid:e9db82ff-36f1-4068-9349-49a1ea6dd094","http://resolver.tudelft.nl/uuid:e9db82ff-36f1-4068-9349-49a1ea6dd094","Biomass segregation between biofilm and flocs improves the control of nitrite-oxidizing bacteria in mainstream partial nitritation and anammox processes","Laureni, M. (Swiss Federal Institute of Aquatic Science and Technology; ETH Zürich); Weissbrodt, D.G. (TU Delft BT/Environmental Biotechnology; TU Delft BT/Cell Systems Engineering; Aalborg University); Villez, Kris (Swiss Federal Institute of Aquatic Science and Technology); Robin, Orlane (Swiss Federal Institute of Aquatic Science and Technology); de Jonge, Nadieh (Aalborg University); Rosenthal, Alex (Northwestern University); Wells, George (Northwestern University); Nielsen, Jeppe Lund (Aalborg University); Morgenroth, Eberhard (Swiss Federal Institute of Aquatic Science and Technology; ETH Zürich); Joss, Adriano (Swiss Federal Institute of Aquatic Science and Technology)","","2019","The control of nitrite-oxidizing bacteria (NOB) challenges the implementation of partial nitritation and anammox (PN/A) processes under mainstream conditions. The aim of the present study was to understand how operating conditions impact microbial competition and the control of NOB in hybrid PN/A systems, where biofilm and flocs coexist. A hybrid PN/A moving-bed biofilm reactor (MBBR; also referred to as integrated fixed film activated sludge or IFAS) was operated at 15 °C on aerobically pre-treated municipal wastewater (23 mg _{NH4-N} L ^{−1} ). Ammonium-oxidizing bacteria (AOB) and NOB were enriched primarily in the flocs, and anammox bacteria (AMX) in the biofilm. After decreasing the dissolved oxygen concentration (DO) from 1.2 to 0.17 mg _{O2} L ^{−1} - with all other operating conditions unchanged - washout of NOB from the flocs was observed. The activity of the minor NOB fraction remaining in the biofilm was suppressed at low DO. As a result, low effluent NO _{3} ^{−} concentrations (0.5 mg _{N} L ^{−1} ) were consistently achieved at aerobic nitrogen removal rates (80 mg _{N} L ^{−1} d ^{−1} ) comparable to those of conventional treatment plants. A simple dynamic mathematical model, assuming perfect biomass segregation with AOB and NOB in the flocs and AMX in the biofilm, was able to qualitatively reproduce the selective washout of NOB from the flocs in response to the decrease in DO-setpoint. Similarly, numerical simulations indicated that flocs removal is an effective operational strategy to achieve the selective washout of NOB. The direct competition for NO _{2} ^{−} between NOB and AMX - the latter retained in the biofilm and acting as a “NO _{2} -sink” - was identified by the model as key mechanism leading to a difference in the actual growth rates of AOB and NOB (i.e., μ _{NOB} < μ _{AOB} in flocs) and allowing for the selective NOB washout over a broad range of simulated sludge retention times (SRT = 6.8–24.5 d). Experimental results and model predictions demonstrate the increased operational flexibility, in terms of variables that can be easily controlled by operators, offered by hybrid systems as compared to solely biofilm systems for the control of NOB in mainstream PN/A applications.","Biomass segregation; Hybrid system; IFAS; Mainstream anammox; Mathematical modelling; Nitrite sink; NOB washout; Partial nitritation/anammox","en","journal article","","","","","","","","","","","BT/Environmental Biotechnology","","",""
"uuid:43179649-5354-41f9-b2da-027d40795544","http://resolver.tudelft.nl/uuid:43179649-5354-41f9-b2da-027d40795544","Growth of Nitrosococcus-Related Ammonia Oxidizing Bacteria Coincides with Extremely Low pH Values in Wastewater with High Ammonia Content","Fumasoli, Alexandra (Swiss Federal Institute of Aquatic Science and Technology); Bürgmann, Helmut (Swiss Federal Institute of Aquatic Science and Technology); Weissbrodt, D.G. (TU Delft BT/Environmental Biotechnology; Swiss Federal Institute of Aquatic Science and Technology; ETH Zürich; Aalborg University); Wells, G.N. (Swiss Federal Institute of Aquatic Science and Technology; Northwestern University); Beck, Karin (Swiss Federal Institute of Aquatic Science and Technology); Mohn, Joachim (Swiss Federal Laboratories for Materials Science and Technology (Empa)); Morgenroth, Eberhard (Swiss Federal Institute of Aquatic Science and Technology; ETH Zürich); Udert, Kai M. (Swiss Federal Institute of Aquatic Science and Technology)","","2017","Ammonia oxidation decreases the pH in wastewaters where alkalinity is limited relative to total ammonia. The activity of ammonia oxidizing bacteria (AOB), however, typically decreases with pH and often ceases completely in slightly acidic wastewaters. Nevertheless, nitrification at low pH has been reported in reactors treating human urine, but it has been unclear which organisms are involved. In this study, we followed the population dynamics of ammonia oxidizing organisms and reactor performance in synthetic fully hydrolyzed urine as the pH decreased over time in response to a decrease in the loading rate. Populations of the β-proteobacterial Nitrosomonas europaea lineage were abundant at the initial pH close to 6, but the growth of a possibly novel Nitrosococcus-related AOB genus decreased the pH to the new level of 2.2, challenging the perception that nitrification is inhibited entirely at low pH values, or governed exclusively by β-proteobacterial AOB or archaea. With the pH shift, nitrite oxidizing bacteria were not further detected, but nitrous acid (HNO_{2}) was still removed through chemical decomposition to nitric oxide (NO) and nitrate. The growth of acid-tolerant γ-proteobacterial AOB should be prevented, by keeping the pH above 5.4, which is a typical pH limit for the N. europaea lineage. Otherwise, the microbial community responsible for high-rate nitrification can be lost, and strong emissions of hazardous volatile nitrogen compounds such as NO are likely.","","en","journal article","","","","","","","","","","","BT/Environmental Biotechnology","","",""
"uuid:aa951c86-2220-4cf1-a987-c90ff19693d2","http://resolver.tudelft.nl/uuid:aa951c86-2220-4cf1-a987-c90ff19693d2","The impact of future sea-level rise on the global tides","Pickering, M.D. (University of Southhampton); Horsburgh, K.J. (National Oceanography Center, Joseph Proudman Building, Liverpool); Blundell, J.R. (University of Southhampton); Hirschi, J.J.-M. (University of Southhampton); Nicholls, Robert J. (University of Southampton); Verlaan, M. (TU Delft Mathematical Physics; Deltares); Wells, N.C. (University of Southampton)","","2017","Tides are a key component in coastal extreme water levels. Possible changes in the tides caused by mean sea-level rise (SLR) are therefore of importance in the analysis of coastal flooding, as well as many other applications. We investigate the effect of future SLR on the tides globally using a fully global forward tidal model: OTISmpi. Statistical comparisons of the modelled and observed tidal solutions demonstrate the skill of the refined model setup with no reliance on data assimilation. We simulate the response of the four primary tidal constituents to various SLR scenarios. Particular attention is paid to future changes at the largest 136 coastal cities, where changes in water level would have the greatest impact. Spatially uniform SLR scenarios ranging from 0.5 to 10 m with fixed coastlines show that the tidal amplitudes in shelf seas globally respond strongly to SLR with spatially coherent areas of increase and decrease. Changes in the M2 and S2 constituents occur globally in most shelf seas, whereas changes in K1 and O1 are confined to Asian shelves. With higher SLR tidal changes are often not proportional to the SLR imposed and larger portions of mean high water (MHW) changes are above proportional. Changes in MHW exceed ±10% of the SLR at ~10% of coastal cities. SLR scenarios allowing for coastal recession tend increasingly to result in a reduction in tidal range. The fact that the fixed and recession shoreline scenarios result mainly in changes of opposing sign is explained by the effect of the perturbations on the natural period of oscillation of the basin. Our results suggest that coastal management strategies could influence the sign of the tidal amplitude change. The effect of a spatially varying SLR, in this case fingerprints of the initial elastic response to ice mass loss, modestly alters the tidal response with the largest differences at high latitudes.","Barotropic tides; Climatic changes; Flood forecasting; Sea level rise; Tidal change; Tidal power","en","journal article","","","","","","","","","","","Mathematical Physics","","",""
"uuid:ea3d6b17-93c7-4ca7-b412-b0a97e5257ee","http://resolver.tudelft.nl/uuid:ea3d6b17-93c7-4ca7-b412-b0a97e5257ee","Energy Stable and Momentum Conserving Hybrid Finite Element Method for the Incompressible Navier–Stokes Equations","Labeur, R.J.; Wells, G.N.","","2012","A hybrid method for the incompressible Navier–Stokes equations is presented. The ethod inherits the attractive stabilizing mechanism of upwinded discontinuous Galerkin methods hen momentum advection becomes significant, equal-order interpolations can be used for the velocity nd pressure fields, and mass can be conserved locally. Using continuous Lagrange multiplier paces to enforce flux continuity across cell facets, the number of global degrees of freedom is the same s for a continuous Galerkin method on the same mesh. Different from our earlier investigations on he approach for the Navier–Stokes equations, the pressure field in this work is discontinuous across ell boundaries. It is hown that this leads to very good local mass conservation and, for an appropriate hoice of finite element spaces, momentum conservation. Also, a new form of the momentum ransport terms for the method is constructed such that global energy stability is guaranteed, even n the absence of a pointwise solenoidal velocity field. Mass conservation, momentum conservation, nd global energy stability are proved for the time-continuous case and for a fully discrete scheme. he presented analysis results are supported by a range of numerical simulations.","finite element method; hybrid finite element methods; incompressible NavierStokes equations","en","journal article","Society for Industrial and Applied Mathematics","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:3800fde5-4b6e-459e-b47a-209f6906f50c","http://resolver.tudelft.nl/uuid:3800fde5-4b6e-459e-b47a-209f6906f50c","Automated Modelling of Evolving Discontinuities","Nikbakht, M.; Wells, G.N.","","2009","The automated approximation of solutions to differential equations which involve discontinuities across evolving surfaces is addressed. Finite element technology has developed to the point where it is now possible to model evolving discontinuities independently of the underlying mesh, which is particularly useful in simulating failure of solids. However, the approach remains tedious to program, particularly in the case of coupled problems where a variety of finite element bases are employed and where a mixture of continuous and discontinuous fields may be used. We tackle this point by exploring the scope for employing automated code generation techniques for modelling discontinuities. Function spaces and variational forms are defined in a language that resembles mathematical notation, and computer code for modelling discontinuities is automatically generated. Principles underlying the approach are elucidated and a number of two- and three-dimensional examples for different equations are presented.","partition of unit; extended finite element method; fracture; automation; form compiler","en","journal article","MDPI AG","","","","","","","","Civil Engineering and Geosciences","Design & Construction","","","",""
"uuid:07a23e06-d001-4273-aae3-5133b0bca26f","http://resolver.tudelft.nl/uuid:07a23e06-d001-4273-aae3-5133b0bca26f","Evidence against a charge density wave on Bi(111)","Kim, T.K.; Wells, J.; Kirkegaard, C.; Li, Z.; Hoffmann, S.V.; Gayone, J.E.; Fernandez-Torrente, I.; Häberle, P.; Pascual, J.I.; Moore, K.T.; Schwartz, A.J.; He, H.; Spence, J.C.H.; Downing, K.H.; Lazar, S.; Tichelaar, F.D.; Borisenko, S.V.; Knupfer, M.; Hofmann, Ph.","","2005","","","en","journal article","American Physical Society","","","","","","","","","","","","",""
"uuid:1409cb4d-167c-46da-aa82-1b4926fad230","http://resolver.tudelft.nl/uuid:1409cb4d-167c-46da-aa82-1b4926fad230","Energy transfers and spectral eddy viscosity in large-eddy simulations of homogeneous isotropic turbulence: Comparison of dynamic Smagorinsky and multiscale models over a range of discretizations","Hughes, T.J.R.; Wells, G.N.; Wray, A.A.","","2004","Energy transfers within large-eddy simulation (LES) and direct numerical simulation (DNS) grids are studied. The spectral eddy viscosity for conventional dynamic Smagorinsky and variational multiscale LES methods are compared with DNS results. Both models underestimate the DNS results for a very coarse LES, but the dynamic Smagorinsky model is significantly better. For moderately to well-refined LES, the dynamic Smagorinsky model overestimates the spectral eddy viscosity at low wave numbers. The multiscale model is in good agreement with DNS for these cases. The convergence of the multiscale model to the DNS with grid refinement is more rapid than for the dynamic Smagorinsky model.","flow simulation; turbulence; viscosity; convergence of numerical methods; differential equations","en","journal article","American Institute of Physics","","","","","","","","Civil Engineering and Geosciences","Design & Construction","","","",""
"uuid:9cdda996-0ab8-4a35-a346-ce912d6e6a8a","http://resolver.tudelft.nl/uuid:9cdda996-0ab8-4a35-a346-ce912d6e6a8a","Sensitivity of the scale partition for variational multiscale large-eddy simulation of channel flow","Holmen, J.; Hughes, T.J.R.; Oberai, A.A.; Wells, G.N.","","2004","The variational multiscale method has been shown to perform well for large-eddy simulation (LES) of turbulent flows. The method relies upon a partition of the resolved velocity field into large- and small-scale components. The subgrid model then acts only on the small scales of motion, unlike conventional LES models which act on all scales of motion. For homogeneous isotropic turbulence and turbulent channel flows, the multiscale model can outperform conventional LES formulations. An issue in the multiscale method for LES is choice of scale partition and sensitivity of the computed results to it. This is the topic of this investigation. The multiscale formulation for channel flows is briefly reviewed. Then, through the definition of an error measure relative to direct numerical simulation (DNS) results, the sensitivity of the method to the partition between large- and small-scale motions is examined. The error in channel flow simulations, relative to DNS results, is computed for various partitions between large- and small-scale spaces, and conclusions drawn from the results.","turbulence; channel flow; flow simulation; numerical analysis; variational techniques","en","journal article","American Institute of Physics","","","","","","","","Civil Engineering and Geosciences","Design & Construction","","","",""
"uuid:cae44a03-b6c8-4c18-b1cb-c7341c41b168","http://resolver.tudelft.nl/uuid:cae44a03-b6c8-4c18-b1cb-c7341c41b168","Combined experimental-computational study to discrete fracture of brittle materials","De Proft, K.; Wells, G.N.; Sluys, L.J.; De Wilde, H.P.","","2002","In this paper, a combined experimental-computational study of a double-edge notched specimen subjected to tensile loading is presented. In the experimental part, the loaddeformation response and the displacement field around the crack tip are recorded. An Electronic Speckle Pattern Interferometer (ESPI) is used to obtain the local displacement field. The experimental results are used to validate a numerical model for the description of fracture using finite elements. The numerical model uses displacement discontinuities to model cracks. The discontinuities are able to pass through finite elements. At the discontinuity, a plasticity-based and a combined damage-plasticity cohesive zone model are used, while the continuum remains elastic. Both local and global results from the numerical simulations are compared with experimental data.","","en","journal article","Delft University of Technology","","","","","","","","Civil Engineering and Geosciences","","","","",""
"uuid:42df5286-9ac1-4bef-8a38-63c25ae89710","http://resolver.tudelft.nl/uuid:42df5286-9ac1-4bef-8a38-63c25ae89710","Discontinuous modelling of strain localisation and failure","Wells, G.N.","De Borst, R. (promotor)","2001","","","en","doctoral thesis","","","","","","","","","Aerospace Engineering","","","","",""
"uuid:542fdde9-5b61-4b47-b104-4e41174acf31","http://resolver.tudelft.nl/uuid:542fdde9-5b61-4b47-b104-4e41174acf31","On the conceptual equivalence of embedded strong discontinuity and smeared crack formulations","Wells, G.N.; Sluys, L.J.","","2001","Embedded discontinuity formulations have been recently presented as a method of incorporating displacement discontinuities in standard finite elements. These elements allow inelastic deformations to be modelled using discrete constitutive models at an internal interface. However, the actual formulation of these models is identical in concept to more traditional fracture energy regularised smeared crack models. Rather than adjusting the hardening modulus depending on element size, the inelastic strain itself is made dependent on element size. It is shown here that the embedded discontinuity idea and smeared crack models are conceptually identical.","ebedded discontinuity; smeared cracks; concrete fracture","en","journal article","Delft University of Technology","","","","","","","","Civil Engineering and Geosciences","Mechanics, Materials and Structures","","","",""
"uuid:982997e8-4d8d-450f-b585-438919546060","http://resolver.tudelft.nl/uuid:982997e8-4d8d-450f-b585-438919546060","On the time varying horizontal water velocity of single, multiple, and random gravity wave trains","Wells, D.R.","Mostertman, L.J. (promotor); Verhagen, C.J.D.M. (promotor)","1964","In this dissertation some characteristics of the horizontal water velocity for single, multiple, and random gravity wave trains are studied. This work consists of two parts, an analogue study and hydraulic measurements. An important aspect in this work is to suggest the horizontal water velocity asymmetry as a criterion for sand movement by water waves. In the first chapter an analogue model is developed to measure the probability distribution of an ensemble of wave trains. The work in this chapter can apply to the water velocity at a point in space, E uler coordinates, as well as the water particle velocity, Lagrange coordinates, because the velocity is described as a function of the ratio of the first two terms of the Fourier series solution. Expressions are found for the second and third moments of the velocity of an endless wave train assuming linear superposition of such waves; interaction terms are neglected. The moments and distributions are discussed for application to sand movement by waves in shallow water. The second chapter deals with some direct measurements of the second and third moments of the horizontal water velocity on the bed of a shallow water wave tank. This was done for single, multiple, and random gravity wave trains with the purpose of observing the moments as a random wave field is built up out of the single wave trains. The analogue study of chapter 1 shows that forward movement of sand by waves should diminish as the number of superimposed wave trains is increased provided the total energy of the waves is constant. This is true only for this superposition of wave trains consisting of the first two terms of the Stokes theory. Furthermore, it has been found that the distribution of four or more superimposed wave trains is nearly Gaussian. A Gaussian random wave field can, under certain conditions, be approximated by the simultaneous generation of only four wave trains. The use of less than four wave trains can lead to some special effects as far as the statistical sorting of sand particles is concerned. For the hydraulic measurements of chapter 2, a positive asymmetry for the horizontal water velocity of single wave trains in shallow water was found. On the other hand it was mostly negative for the multiple and random wave trains studied here. The measurements indicate a negative asymmetry that increases in magnitude with increasing band width for random wave trains with a constant center frequency. It also appears that the asymmetry tends toward positive for an increasing wave length and constant band width. This predicts movement of sand in a seaward direction for short, broad band wave trains and landward movement for long, narrow band wave trains. This is in agreement with the known observation that long, quiet, relatively narrow band summer wave trains build beaches while short, broad band winter wave trains wash beaches away. It is thus possible that the beachward and seaward movement of sand in shallow water due to waves can be described by means of the asymmetry of the horizontal water velocity.","water waves; wave celerity; velocity; orbital movement","en","doctoral thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""