Visualizing largescale flow using synthetic aperture PIV Houwelingen, Josje van; Holten, Ad P.C. ; Clercx, Herman J.H. ; Kunnen, Rudie P.J. ; Molenaar, Jaap ; Water, Willem van de  \ 2020
Experiments in fluids 61 (2020)1.  ISSN 07234864 Abstract: We discuss the application of synthetic aperture particle image velocimetry for measuring the flow around human swimmers using small bubbles as tracer. We quantify the twodimensional projection of the velocity field in planes perpendicular to the viewing direction of an array of six cameras. With help of simulations, modelled after the experiment, we address questions about depth selectivity and occlusion in dense bubble fields. Using vortex rings in the swimming pool, we provide a proof of principle of the method. It is further illustrated by the vorticity field produced by a human swimmer. Graphic abstract: [Figure not available: see fulltext.]. 

Approach for designing contextspecific, locally owned interventions to reduce postharvest losses : Case study on tomato value chains in Nigeria Plaisier, Christine ; Sibomana, Milindi ; Waal, Johannes van der; Clercx, Luud ; Wagenberg, Coen P.A. van; Dijkxhoorn, Youri  \ 2019
Sustainability 11 (2019)1.  ISSN 20711050  16 p. Behavioural change  Contextspecific interventions  Nigeria  Participatory approach  Plastic crate  Postharvest losses  Raffia basket  Supply chain  Tomato  Value chain development
Development projects on interventions to reduce postharvest losses (PHL) are often implemented largely independently of the specific context and without sufficient adaptation to the needs of people who are supposed to use them. An approach is needed for the design and implementation of specific, locally owned interventions in development projects. Our approach is based on Participatory Development and includes Living Lab and World Cafés. We applied the approach in a case study on reducing PHL in tomato value chains in Nigeria. The approach consists of nine steps. After scoping the sector, selected value chain stakeholders (case: farmers, transporters, traders, retailers) were gathered in Living Lab workshops. In the workshop, participants analyzed the product, information, and monetary flows in their own value chain, identified causes for PHL, and selected potential interventions to reduce these (case: plastic crates instead of raffia baskets to transport tomatoes). Selected interventions were implemented, tested, and monitored in pilot projects with the workshop participants. This was followed by an evaluation workshop. At the end of the case study, 89% of participants bought crates to keep using them in their value chain. Our approach resulted in contextspecific, locally owned interventions to reduce PHL in the case study on tomato value chains in Nigeria. Its application in other countries, commodities, or interventions is needed to determine the effectiveness of the approach in a broader scope. 

Parameters for the collapse of turbulence in the stratified plane Couette flow Hooijdonk, Ivo G.S. van; Clercx, Herman J.H. ; Ansorge, Cedrick ; Moene, Arnold F. ; Wiel, Bas J.H. van de  \ 2018
Journal of the Atmospheric Sciences 75 (2018)9.  ISSN 00224928  p. 3211  3231. Boundary conditions  Boundary layer  Numerical analysis/modeling  Turbulence
We perform direct numerical simulation of the Couette flow as a model for the stable boundary layer. The flow evolution is investigated for combinations of the (bulk) Reynolds number and the imposed surface buoyancy flux. First, we establish what the similarities and differences are between applying a fixed buoyancy difference (Dirichlet) and a fixed buoyancy flux (Neumann) as boundary conditions. Moreover, two distinct parameters were recently proposed for the turbulenttolaminar transition: the Reynolds number based on the Obukhov length and the "shear capacity," a velocityscale ratio based on the buoyancy flux maximum. We study how these parameters relate to each other and to the atmospheric boundary layer. The results show that in a weakly stratified equilibrium state, the flow statistics are virtually the same between the different types of boundary conditions. However, at stronger stratification and, more generally, in nonequilibrium conditions, the flow statistics do depend on the type of boundary condition imposed. In the case of Neumann boundary conditions, a clear sensitivity to the initial stratification strength is observed because of the existence of multiple equilibriums, while for Dirichlet boundary conditions, only one statistically steady turbulent equilibrium exists for a particular set of boundary conditions. As in previous studies, we find that when the imposed surface flux is larger than the maximum buoyancy flux, no turbulent steady state occurs. Analytical investigation and simulation data indicate that this maximum buoyancy flux converges for increasing Reynolds numbers, which suggests a possible extrapolation to the atmospheric case. 

Nearsurface temperature inversion growth rate during the onset of the stable boundary layer Hooijdonk, Ivo G.S. van; Clercx, Herman J.H. ; Abraham, Carsten ; Holdsworth, Amber M. ; Monahan, Adam H. ; Vignon, Etienne ; Moene, Arnold F. ; Baas, Peter ; Wiel, Bas J.H. van de  \ 2017
Journal of the Atmospheric Sciences 74 (2017)10.  ISSN 00224928  p. 3433  3449. Boundary layer  Classification  In situ atmospheric observations  Radiative forcing  Single column models  Temperature
This study aims to find the typical growth rate of the temperature inversion during the onset of the stable boundary layer around sunset.The sunset transition is a very challenging period for numerical weather prediction, since neither accepted theories for the convective boundary layer nor those for the stable boundary layer appear to be applicable. To gainmore insight in this period, a systematic investigation of the temperature inversion growth rate is conducted. A statistical procedure is used to analyze almost 16 years of observations from the Cabauw observational tower, supported by observations from two additional sites (DomeCandKarlsruhe). The results show that, on average, the growth rate of the temperature inversion (normalized by the maximum inversion during the night) weakly declines with increasing wind speed. The observed growth rate is quantitatively consistent among the sites, and it appears insensitive to various other parameters. The results were also insensitive to the afternoon decay rate of the net radiation except when this decay rate was very weak. These observations are compared to numerical solutions of three models with increasing complexity: a bulk model, an idealized singlecolumn model (SCM), and an operationallevel SCM. It appears only the latter could reproduce qualitative features of the observations using a firstorder closure. Moreover, replacing this closure with a prognostic TKE scheme substantially improved the quantitative performance. This suggests that idealized models assuming instantaneous equilibrium fluxprofile relations may not aid in understanding this period, since history effects may qualitatively affect the dynamics. 

Early Warning Signals for Regime Transition in the Stable Boundary Layer : A Model Study Hooijdonk, I.G.S. van; Moene, A.F. ; Scheffer, M. ; Clercx, H.J.H. ; Wiel, B.J.H. van de  \ 2017
BoundaryLayer Meteorology 162 (2017)2.  ISSN 00068314  p. 283  306. Critical regime transition  Early warning signals  Maximum sustainable heat flux  Minimum wind speed  Nocturnal boundary layer
The evening transition is investigated in an idealized model for the nocturnal boundary layer. From earlier studies it is known that the nocturnal boundary layer may manifest itself in two distinct regimes, depending on the ambient synoptic conditions: strongwind or overcast conditions typically lead to weakly stable, turbulent nights; clearsky and weakwind conditions, on the other hand, lead to very stable, weakly turbulent conditions. Previously, the dynamical behaviour near the transition between these regimes was investigated in an idealized setting, relying on Monin–Obukhov (MO) similarity to describe turbulent transport. Here, we investigate a similar setup, using direct numerical simulation; in contrast to MObased models, this type of simulation does not need to rely on turbulence closure assumptions. We show that previous predictions are verified, but now independent of turbulence parametrizations. Also, it appears that a regime shift to the very stable state is signaled in advance by specific changes in the dynamics of the turbulent boundary layer. Here, we show how these changes may be used to infer a quantitative estimate of the transition point from the weakly stable boundary layer to the very stable boundary layer. In addition, it is shown that the idealized, nocturnal boundarylayer system shares important similarities with generic nonlinear dynamical systems that exhibit critical transitions. Therefore, the presence of other, generic early warning signals is tested as well. Indeed, indications are found that such signals are present in stably stratified turbulent flows. 

Early Warning Signals for Regime Transition in the Stable Boundary Layer: a Model Study Hooijdonk, I.G.S. ; Moene, A.F. ; Scheffer, M. ; Clercx, H.J.H. ; Wiel, B.J.H. van de  \ 2016


The maximum sustainable heat flux in stably stratified channel flows Donda, J. ; Hooijdonk, I. van; Moene, A.F. ; Heijst, G.J.F. van; Clercx, H. ; Wiel, B. van de  \ 2016
Quarterly Journal of the Royal Meteorological Society 142 (2016)695.  ISSN 00359009  p. 781  792. In analogy to the nocturnal atmospheric boundary layer a fluxdriven, cooled channel flow is studied using Direct Numerical Simulations. In agreement with earlier studies turbulence collapses when the surface cooling exceeds a critical value. In that case laminarization occurs. Here the socalled Maximum Sustainable Heat Flux hypothesis is tested. It explains why laminarization will occur at strong cooling rates. It states that in stratified flows, the downward heat flux is limited to a maximum, which, in turn, is determined by the momentum of the flow. If the heat extraction at the surface exceeds this maximum, nearsurface stability will rapidly increase, which further hampers efficient vertical heat transport. This positive feedback eventually causes turbulence to be fully suppressed by the intensive density stratification. The framework is used to predict the collapse of turbulence and a good agreement between theory and simulations is found. Therefore, it is concluded that Maximum Sustainable Heat Flux mechanism explains the collapse of turbulence in this kind of flows. In future work, there is a need for extension to more realistic configurations, allowing for Coriolis effects and more realistic surface boundary conditions.


The maximum sustainable heat flux in stably stratified channel flows Donda, J. ; Hooijdonk, I. van; Moene, A.F. ; Heijst, G.J.F. van; Clercx, H. ; Wiel, B. van de  \ 2015
In analogy to the nocturnal atmospheric boundary layer a fluxdriven, cooled channel flow is studied using Direct Numerical Simulations. Here, in particular, the mechanism behind the collapse of turbulence at large cooling rates is analysed. In agreement with earlier studies, the flow laminarizes at strong cooling rates. The mechanism for the cessation of turbulence is understood from a Maximum Sustainable Heat Flux (MSHF) theory, which is here tested against numerical simulations. In stratified flow the maximum heat flux that can be transported downward by turbulence at the onset of cooling is limited to a maximum, which, in turn, is determined by the initial momentum of the flow. If the heat extraction at the surface exceeds this maximum, nearsurface stability will rapidly increase, which further hampers efficient vertical heat transport. This positive feedback eventually causes turbulence to be fully suppressed by the intensive density stratification. It is shown that the collapse in the DNSsimulations is successfully predicted by the MSHFtheory. Apart from formal analysis, also a simplified methodology is presented, which is more useful in practice for prediction of regimetransitions in field observations.


Collapse of turbulence in stably stratified channel flow: a transient phenomenon Donda, J.M.M. ; Hooijdonk, I.G.S. ; Moene, A.F. ; Jonker, J.J. ; Heijst, G.J.F. van; Clercx, H.J.H. ; Wiel, B.J.H. van de  \ 2015
Quarterly Journal of the Royal Meteorological Society 141 (2015)691.  ISSN 00359009  p. 2137  2147. The collapse of turbulence in a pressuredriven, cooled channel flow is studied by using 3D direct numerical simulations (DNS) in combination with theoretical analysis using a local similarity model. Previous studies with DNS reported a definite collapse of turbulence in cases when the normalized surface cooling h/L (with h the channel depth and L the Obukhov length) exceeded a value of 0.5. A recent study by the present authors succeeded in explaining this collapse using the socalled maximum sustainable heat flux (MSHF) theory. This states that collapse may occur when the ambient momentum of the flow is too weak to transport enough heat downward to compensate for the surface cooling. The MSHF theory predicts that, in pressuredriven flows, acceleration of the fluid after collapse will eventually cause a regeneration of turbulence, in contrast with the aforementioned DNS results. It also predicts that the flow should be able to survive ‘supercritical’ cooling rates, in cases when sufficient momentum is applied to the initial state. Here, both predictions are confirmed using DNS simulations. It is also shown that in DNS a recovery of turbulence will occur naturally, provided that perturbations of finite amplitude are imposed on the laminarized state and provided that sufficient time for flow acceleration is allowed. As such, we conclude that the collapse of turbulence in this configuration is a temporary, transient phenomenon for which a universal cooling rate does not exist. Finally, in the present work a onetoone comparison between a parametrized, local similarity model and the turbulenceresolving model (DNS) is made. Although local similarity originates from observations that represent much larger Reynolds numbers than those covered by our DNS simulations, both methods appear to predict very similar mean velocity (and temperature) profiles. This suggests that indepth analysis with DNS can be an attractive complementary tool with which to study atmospheric physics, in addition to tools that are able to represent high Reynolds number flows like largeeddy simulations.


Predicting Nocturnal Boundary Layer Regimes: An Observational Study Hooijdonk, I.G.S. ; Donda, J.M.M. ; Bosveld, F.C. ; Moene, A.F. ; Clercx, H.J.H. ; Wiel, B.J.H. van de; Sun, J.  \ 2014


The minimum wind speed for sustainable turbulence in the nocturnal boundary layer Wiel, B.J.H. van de; Donda, J.M.M. ; Hooijdonk, I.G.S. ; Baas, P. ; Moene, A.F. ; Jonker, H.J.J. ; Heijst, G.J.F. van; Clercx, H.J.H. ; Sun, J.  \ 2014


Collapse of turbulence in stably stratified channel flow: a transient phenomenon Donda, J.M.M. ; Wiel, B.J.H. van de; Hooijdonk, I.G.S. ; Moene, A.F. ; Jonker, H.J.J. ; Heijst, G.J.F. van; Clercx, H.J.H.  \ 2014


Direct Numerical Simulation of laminarization in the atmospheric Donda, J. ; Wiel, B. van de; Moene, A.F. ; Heijst, G.J. van; Clercx, H.  \ 2013
In the present work the collapse of turbulence in the evening atmosphere is studied by means of direct numerical simulation (DNS). As physical properties of the land surface play a crucial role in either stimulating or moderating the collapse process, emphasis lies on the coupling of the flow model to a realistic surface model. Hereto we apply a heat budget equation at the surface, with contributions from conductive, radiative, and turbulent heat transport. The response of the atmosphere to weak mechanical forcing is studied for various surfaces like: short grass, snow covered surfaces, bare soil and water. Besides numerical simulation also theoretical analysis is used to understand the dynamic feedbacks between the lower stratified atmosphere and the underlying surface. Finally, results are compared with data from various atmospheric field experiments over the world.


Rural Finance, Poverty Alleviation and Sustainable Land Use: The Role of Credit for the Adoption of Agroforestry Systems in Occidental Honduras Ruben, R. ; Clercx, L.  \ 2003
Journal of Microfinance 5 (2003)2.  ISSN 15274314  p. 1  24. 