Unsteady flow

The unsteady flow of a viscoelastic fluid with the fractional Maxwell model, induced by a constantly accelerating plate between two side walls perpendicular to the plate, is investigated by means of the integral transforms. Exact solutions for the velocity field are presented under integral and series forms in terms of the derivatives of generalized Mittag–Leffler functions. The corresponding solutions for Maxwell fluids are obtained as limiting cases for β → 1. In the absence of the side walls, all solutions that have been determined reduce to those corresponding to the motion over an infinite plate.

Pipelines that are used to convey water, petroleum, gas or any other fluid across a river are generally burried under the bed. Interaction between a pipeline and an erodible bed assumes importance in engineering because of the high cost of laying such pipelines. The pipes may become partially exposed to the action of water currents where a flood causes a general scour in the bed of the river. Also as a direct consequence of rapid urbanisation, bridges in close proximity are likely to interfere with such pipelines enhancing their vulnerability to scour. It is agreed that difference in pressures on the upstream and downstream of the pipe initiates piping below the pipeline. Piping together with stagnation eddy combine to undermine the pipeline and mark the onset of scour. Maximum local scour below the pipe is observed when there is a small gap between the pipeline and the undisturbed sediment bed allowing jet like high velocity flow through the gap. As the gap increases, scour depth g...

Simulation results on dynamic yaw stability derivatives of a gull bird by means of computational fluid dynamics are presented. Two different kinds of motions are used for determining the dynamic yaw stability derivatives CNr and CNbeta . Concerning the first one, simple lateral translation and yaw rotary motions in yaw are considered. The second one consists of combined motions. To determine dynamic yaw stability derivatives of the bird, the simulation of an unsteady flow with a bird model showing a harmonic motion is performed. The unsteady flow solution for each time step is obtained by solving unsteady Euler equations based on a finite volume approach for a smaller reduced frequency. Then, an evaluation of unsteady forces and moments for one cycle is conducted using harmonic Fourier analysis. The results on the dynamic yaw stability derivatives for both simulations of the model motion show a good agreement.

An uncertainty analysis of the unsteady flow component (UNET) of the one-dimensional model HEC-RAS within the generalised likelihood uncertainty estimation (GLUE) is presented. For this, the model performance of runs with different sets of Manning roughness coefficients, chosen from a range between 0.001 and 0.9, are compared to inundation data and an outflow hydrograph. The influence of variation in the weighting coefficient of the numerical scheme is also investigated. For the latter, the empirical results show no advantage of using values below 1 and suggest the use of a fully implicit scheme (weighting parameter equals 1). The results of varying the reach scale roughnesses shows that many parameter sets can perform equally well (problem of equifinality) even with extreme values. However, this depends on the model region and boundary conditions. The necessity to distinguish between effective parameters and real physical parameters is emphasised. The study demonstrates that this analysis can be used to produce dynamic probability maps of flooding during an event and can be linked to a stopping criterion for GLUE.

Downpull on tunnel gates installed in the intake structure of a hydroelectric power plant was studied experimentally using a hydraulic model. The pressure distribution on the gate lip surface was measured, and the lip downpull was evaluated by surface-area integration of the measured pressure distribution. An easy to use lip downpull coefficient was defined as a function of the lip angle and gate opening. The lip downpull coefficient function is linked to a one-dimensional mathematical model of unsteady flow in the intake-penstock system. The model is based on the integral energy and continuity equations. Overflow through the gate spacings is also included in the model to compute the water level in the gate shaft and to evaluate the downpull component on the top face of the gate. Time-dependent calculation of the total downpull force acting on a closing gate is exemplified. The total downpull is also measured by the direct weighing method for fixed and closing gates. Predictions of the mathematical model compare favorably with the downpull obtained from the direct weighing method.

Modern hydraulic engineering has been and remains to a large extent based on experimental investigations. Mathematical modeling techniques have progressed rapidly due to the advent of high-speed digital computers, enabling the equations of motion coupled with semi-empirical relationships to be solved for complex flow such as pipe network analysis, pressure transients in pipelines, unsteady flow in rivers, etc. There are many cases, particularly where localized flow patterns cannot be mathematically solved, when physical models are steel needed. The hydraulic model investigation of the evacuation structures of dam "St. PETKA" on Treska River has been performed in Hydraulic laboratory at the Faculty of Civil Engineering in Skopje. The model was designed and constructed in scale 1:40. The evacuation structures that have been investigated are: diverse tunnel with intake structure, shaft spillway with spillway tunnel and output portal and bottom outlet with intake structure. Th...

We propose a mechanism by which massive ignimbrite and layered ignimbrite sequences — the latter liable to have been previously interpreted as multiple flow units-form by progressive aggradation during sustained passage of a single particulate flow. In the case of high-temperature eruptive products the mechanism simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variations in texture, including between non-welded, eutaxitic, rheomorphic (lineated) and lava-like. Agglutination can occur within the basal part of a hot density-stratified flow. During initial incursion of the flow, agglutinate chills and freezes against the ground. During sustained passage of the flow, agglutination continues so that the non-particulate (agglutinate) layer thickens (aggrades) and becomes mobile, susceptible to both gravity-induced motion and traction-shear imparted by the overriding particulate part of the flow. The particulate to non-particulate (P-NP) transition occurs in and just beneath a depositional boundary layer, where disruptive collisions of hot viscous droplets give way, via sticky grain interactions, to fluidal behavior following adhesion. Because they have different rheologies, the particulate and non-particulate flow components travel at different velocities and respond to topography in different ways. This may cause detachment and formation of two independent flows. The P-NP transition is controlled by factors that influence the rheological properties of individual erupted particles (strain rate, temperature, and composition including volatiles), by cooling and volatile exsolution during transport, and by the particle-size population and concentration characteristics of the depositional boundary layer. At any one location along the flow path one or more of these can change through time (unsteady flow). Thus the P-NP transition can develop momentarily or repeatedly during the passage of an unsteady flow, or it can occur continuously during the passage of a quasi-steady flow supplied by a sustained explosive eruption. Vertical facies successions developed in the deposit (high-grade ignimbrite) reflect temporal changes in flow steadiness and in material supplied at source. The P-NP transition is also influenced by factors that affect flow behaviour, such as topography. It may occur at any location laterally between a proximal site of deflation (e.g. a fountain-fed lava) and a flow's distal limit, but it most commonly occurs throughout a considerable length of the flow path. Up-sequence variations in welding-deformation fabric (between oblate uniaxial to triaxial and prolate) reflect evolving characteristics of the depositional boundary layer (e.g. fluctuations from direct suspension-sedimentation to deposition via traction carpets or traction plugs), as well as possible modifications resulting from subsequent, post-depositional hot loading and slumping. Similar processes can also account for lateral lithofacies gradations in conduits and vents filled with welded tuff. Our consideration of high-grade ignimbrites has implications for ignimbrite emplacement in general, and draws attention to the limitations of the widely accepted models of emplacement involving mainly high-concentration non-turbulent transport and en masse ‘freezing’ of high-yield-strength plug flows.

A strategy for adaptive control and energetic optimization of aerobic fermentors was implemented, with both air flow and agitation speed as manipulated variables. This strategy is separable in its components: control, optimization, estimation. We optimized parameter’s estimation (from the usual KLa correlation) using sinusoidal excitation of air flow and agitation speed. We have implemented parameter’s estimation trough recursive least squares algorithm with forgetting factor. We carried separate essays on control, optimization and estimation algorithms. We carried our essays using an original computational simulation environment, with noise and delay generating facilities for data sampling and filtering.

Our results show the convergence and robustness of the estimation algorithm used, improved with use of both forgetting factor and KLa dead-band facilities. Control algorithm used in our work compares favorably with PID using the integrated area criteria for deviation between oxygen molarity and critical molarity (set point). Optimization algorithm clearly reduces energetic consumption, respecting critical molarity. Integration of control, optimization and adaptive algorithms was implemented, but future work is needed for stability. Methods were defined and implemented for stability improvement. We have implemented data acquisition and computer manipulation of air flow and agitation speed for actual fermentors.

A new one-dimensional (1-D) numerical model for calculating flow and sediment transport in steep mountain streams is developed. 3 ST1D, which stands for Steep Stream Sediment Transport 1-D model, is applicable to unsteady flow conditions that occur over transcritical flow stream reaches such as flows over step-pool sequences. 3ST1D consists of two coupled components, the hydrodynamic and the sediment transport. The flow component is addressed here by solving the unsteady form of the Saint-Venant equations. The Total Variation Diminishing Dissipation (TVD)-MacCormack scheme, which is a shock-capturing scheme capable of rendering the solution oscillation free, is employed here to approximate the hydrodynamic solution over transcritical flow stream reaches. The sediment component of the model accounts for multifractional sediment transport and incorporates a series of various incipient motion criteria and frictional formulas applicable to mountain streams. In addition, sediment entrainability is estimated based on a state-of-the art formula that accounts for the bed porosity, turbulent bursting frequency, probability of occurrence of strong episodic turbulent events, and sediment availability in the unit bed area. The model at the end of each time step predicts the flow depth, velocity and shear stress distribution within a cell and calculates changes in bed evolution and grain size distribution. The overall performance of the model is evaluated by comparing its predictions with observations from two flume studies, two field investigations and against the predictions of the quasi-steady model of Lopez and Falcon developed for mountain streams. A sensitivity analysis is performed to assess the effects of cell size and Manning’s roughness coefficient in the predictive ability of the model.Un nouveau modèle numérique uni-dimensionnel (1-D) de calcul d’écoulement et de transport de sédiment dans les torrents rapides de montagne a été développé: 3ST1D, mis pour Steep Stream Sediment Transport 1-D model (modèle de transport 1-D de sédiment dans les torrents à forte pente), est applicable aux écoulements instationnaires qui se produisent dans les biefs critiques tels que les séquences de marches. 3ST1D se compose de deux composantes couplées, l’hydrodynamique et le transport de sédiment. Pour la composante écoulement on a recours à la forme instationnaire des équations de Saint-Venant Le schéma TVD de MacCormack (diminution de la variation totale de dissipation) qui est un schéma absorbant les chocs capable de rendre la solution libre d’oscillations, est utilisé pour approcher la solution hydrodynamique dans les biefs du torrent où l’écoulement est transcritique. La composante de sédiment du modèle tient compte de sédiments composites et incorpore une série de critères pour le début d’entraînement et de formules de frottement applicables aux torrents de montagne. En outre, l’entraînement de sédiment est basé sur l’état de l’art qui tient compte de la porosité de lit, de la fréquence des bouffées turbulentes, de la probabilité d’occurrence d’événements turbulents épisodiques forts, et de la mobilité des sédiments dans le secteur de lit. Le modèle à la fin de chaque pas de temps donne la profondeur, la distribution de vitesse et de contraintes de cisaillement dans une cellule et calcule l’évolution du lit et de la distribution des tailles de grains. La performance globale du modèle est évaluée en comparant ses prévisions aux observations de deux études en canal, de deux expériences en nature et des prévisions du modèle quasi-stationnaire de Lopez et de Falcon développé pour des torrents de montagne. Une analyse de sensibilité est exécutée pour évaluer les effets de la taille de maille et du coefficient de rugosité de Manning sur les capacités prédictives du modèle.

This paper presents the work carried out to predict the behavior of a 0.6 m Impulse turbine with fixed guide vanes with 0.6 hub-to-tip (H/T) ratio under real sea conditions. In order to predict the true performance of the actual Oscillating Water Column (OWC), the numerical technique has been fine tuned by incorporating the compressibility effect. Water surface elevation verses time history based on Pierson Moskowitz Spectra was used as the input data. Standard numerical techniques were employed to solve the non-linear behavior of the sea waves. The effect due to compressibility inside the air chamber and turbine performance under unsteady and irregular flow condition has been analyzed numerically. Considering the quasi-steady assumptions, unidirectional steady flow experimental data was used to simulate the turbine characteristics under irregular unsteady flow conditions. The results show that the performance of this type of turbine is quite stable and efficiency of air chamber and the mean conversion efficiency is reduced around 8% and 5% respectively, due to compressibility inside air chamber.

Helicopter tail shake phenomenon is still remained as a long dragged issue that adversely affected the overall performance, occupants ’ comfortness and handling qualities of helicopter. This problem is more severe in modern helicopter as it needs to fly faster and with high manoeuvrability, causing rigorous interactional aerodynamic (I/A) related vibration problems. The objective of this present research is to improve basic understanding of the viscous unsteady flow phenomenon observed behind the helicopter tail part. A good understanding of this matter is necessary as a typical aspect of tail shake that it has unsteady random character, indicating that the wake induced excitation is in also unsteady of nature. For this, a wind tunnel test had been conducted with a rigid 14 % model of generic Eurocopter 350Z helicopter in the Universiti Teknologi Malaysia – Low Speed Tunnel (UTM-LST) with a test section size of 2m x 1.5m x 5.8m and 288 km/hr maximum test wind speed. The model, suppl...

A new method for designing propeller blade sections is presented. A vortex lattice method is used to evaluate the performance and the time-dependent pressure distribution on the blade surface in a non-uniform flow, while efficient optimization algorithms are used to modify the blade sections. Two different designs were carried out in this study. The first was a design to realize a target pressure distribution in a rotating three-dimensional flow. A two-dimensional wing theory was used to obtain the target pressure distribution. The predicted increase in efficiency and the reduction in the cavity volume were confirmed by model experiments. The second was a design to maximize the propeller efficiency. By this method, the propeller efficiency was improved by 1.2% under the constrains of constant thrust and a prescribed margin for face cavitation.