Ardalan Javadi

Experimental and numerical investigation of the precessing helical vortex in a conical diffuser, with rotor-stator interaction

The flow unsteadiness generated in a swirling apparatus is investigated experimentally and numerically. The swirl apparatus has two parts: a swirl generator and a test section. The swirl generator includes two blade rows, one stationary and one rotating, is designed such that the emanating flow resembles that of a Francis hydro turbine operated at partial discharge. The test section consists of a conical diffuser similar to the draft tube cone of a Francis turbine. A new control method based on a magneto rheological brake is employed in the rotating section, runner, in order to produce several swirling flow regimes. The LDV measurements are performed along three survey axes in the test section. The measured mean velocity components and its fluctuating parts are used to validate the results of unsteady numerical simulations, conducted using the FOAM-extend-3.0 CFD code. A dynamic mesh is used together with the sliding General Grid Interfaces (GGI) to mimic the effect of the rotating runner. The delayed detached eddy simulation method, conjugated with the Spalart-Allmaras turbulence model (DDES-SA), is applied to achieve a deep insight about the ability of this advanced modeling technique and the physics of the flow. The RNG k-epsilon model is also used to represent state-of-the art of industrial turbulence modeling. Both models predict the mean velocity reasonably well while DDES-SA presents more realistic flow features at the highest and lowest rotational speeds. The highest level of turbulence occurs at the highest and lowest rotational speeds which DDES-SA is able to predict well in the conical diffuser. The special shape of the blade plays more prominent role at lower rotational speeds and creates coherent structures with opposite sign of vorticity. The vortex rope is captured by both turbulence models while DDES-SA presents more realistic one at higher rotational speeds.

Velocity and pressure fluctuations induced by the precessing helical vortex in a conical diffuser

The flow unsteadiness generated in the draft tube cone of hydraulic turbines affects the turbine operation. Therefore, several swirling flow configurations are investigated using a swirling apparatus in order to explore the unsteady phenomena. The swirl apparatus has two parts: the swirl generator and the test section. The swirl generator includes two blade rows being designed such that the exit velocity profile resembles that of a turbine with fixed pitch. The test section includes a divergent part similar to the draft tube cone of a Francis turbine. A new control method based on a magneto rheological brake is used in order to produce several swirling flow configurations. As a result, the investigations are performed for six operating regimes in order to quantify the flow from part load operation, corresponding to runaway speed, to overload operation, corresponding to minimum speed, at constant guide vane opening. The part load operation corresponds to 0.7 times the best efficiency discharge, while the overload operation corresponds to 1.54 times the best efficiency discharge. LDV measurements are performed along three survey axes in the test section. The first survey axis is located just downstream the runner in order to check the velocity field at the swirl generator exit, while the next two survey axes are located at the inlet and at the outlet of the draft tube cone. Two velocity components are simultaneously measured on each survey axis. The measured unsteady velocity components are used to validate the results of unsteady numerical simulations, conducted using the OpenFOAM CFD code. The computational domain covers the entire swirling apparatus, including strouts, guide vanes, runner, and the conical diffuser. A dynamic mesh is used together with sliding GGI interfaces to include the effect of the rotating runner. The Reynolds averaged Navier–Stokes equations coupled with the RNG k–e turbulence model are utilized to simulate the unsteady turbulent flow throughout the swirl generator.

LES and DES of Swirling Flow with Rotor-Stator Interaction

A highly swirling turbulent flow engendered by the rotor-stator interaction of a swirl generator is investigated using LES and DES. The delayed DES Spalart-Allmaras (DDES-SA), improved DDES-SA, shear stress transport DDES (DDES-SST) and a dynamic k-equation LES are studied. A mesh sensitivity study is performed on the hybrid methods, including the ability to capture the details of the flow field. It is shown that all the methods are capable of predicting the large-scale flow features, e.g. the vortex breakdown and the corresponding on-axis recirculation region. It is also shown that all the hybrid methods capture most of the small-scale coherent structures, even with a relatively coarse mesh resolution. The various shielding functions of the hybrid methods are analyzed, distinguishing the location of the transition between RANS and LES mode.

A comparative study of scale-adaptive and large-eddy simulations of highly swirling turbulent flow through an abrupt expansion

The strongly swirling turbulent flow through an abrupt expansion is investigated using highly resolved LES and SAS, to shed more light on the stagnation region and the helical vortex breakdown. The vortex breakdown in an abrupt expansion resembles the so-called vortex rope occurring in hydro power draft tubes. It is known that the large-scale helical vortex structures can be captured by regular RANS turbulence models. However, the spurious suppression of the small-scale structures should be avoided using less diffusive methods. The present work compares LES and SAS results with the experimental measurement of Dellenback et al. (1988). The computations are conducted using a general non-orthogonal finite-volume method with a fully collocated storage available in the OpenFOAM-2.1.x CFD code. The dynamics of the flow is studied at two Reynolds numbers, Re=6.0×104 and Re=105 , at the almost constant high swirl numbers of Sr=1.16 and Sr=1.23, respectively. The time-averaged velocity and pressure fields and the root mean square of the velocity fluctuations, are captured and investigated qualitatively. The flow with the lower Reynolds number gives a much weaker outburst although the frequency of the structures seems to be constant for the plateau swirl number.

Unsteady numerical simulation of the flow in the U9 Kaplan turbine model

The Reynolds-averaged Navier-Stokes equations with the RNG k-e turbulence model closure are utilized to simulate the unsteady turbulent flow throughout the whole flow passage of the U9 Kaplan turbine model. The U9 Kaplan turbine model comprises 20 stationary guide vanes and 6 rotating blades (700 RPM), working at full load (0.71 m3/s). The computations are conducted using a general finite volume method, using the OpenFOAM CFD code. A dynamic mesh is used together with a sliding GGI interface to include the effect of the rotating runner. The hub and tip clearances are included in the runner. An analysis is conducted of the unsteady behavior of the flow field, the pressure fluctuation in the draft tube, and the coherent structures of the flow. The tangential and axial velocity distributions at three sections in the draft tube are compared against LDV measurements. The numerical result is in reasonable agreement with the experimental data, and the important flow physics close to the hub in the draft tube is captured. The hub and tip vortices and an on-axis forced vortex are realistically captured. The numerical results show that the frequency of the forced vortex in 1/5 of the runner rotation.

Active flow control of the vortex rope and pressure pulsations in a swirl generator

ABSTRACT The vortex rope and pressure pulsations caused by a radial pressure gradient in the conical diffuser of a swirl generator is controlled using continuous slot jets with different momentum fluxes and angles injected from the runner crown. The swirl apparatus is designed to generate flows similar to those in the different operating conditions of a Francis turbine. The study is done with numerical modelling using the hybrid URANS-LES (Unsteady Reynolds-Averaged Navier–Stokes–Large Eddy Simulation) method with the rotor–stator interaction. The comprehensive studies of Javadi and Nilsson [Time-accurate numerical simulations of swirling flow with rotor–stator interaction. Flow, Turbulence and Combustion, Vol. 95, pp. 755–774], and Javadi, Bosioc, Nilsson, Muntean and Susan-Resiga [Experimental and numerical investigation of the precessing helical vortex in a conical diffuser, with rotor–stator interaction. ASME Journal of Fluids Engineering, doi:10.1115/1.4033416] are considered as the bench mark, and the capabilities of the technique is studied in the present work with the validated numerical results presented in those studies. The pressure pulsations caused by the pressure gradient generated by the swirl, present at off-design conditions, are cumbersome for hydropower structures. The investigation shows that the pressure pulsation, velocity fluctuations and the size of the vortex rope decrease when the jet is injected from the runner crown. The flow rate of the jet is less than 3% of the flow rate of the swirl generator. The momentum flux, angle of injection of the jet and the position of the slot are important factors for the effectiveness of the flow control technique.

Numerical Predictions of Slot Synthetic Jets in Quiescent Surroundings

A detailed numerical simulation is undertaken to investigate physical processes that are engendered in the injection of synthetic (zero-net-mass-flux) jets into quiescent surroundings. A complementary study of 3-D unsteady Reynolds-averaged Navier-Stokes (URANS) applied to a nominally 2-D problem is carried out and compared with experimental data that are obtained at corresponding conditions with the aim of achieving an improved understanding of fluid dynamics of synthetic jets flow fields in the quiescent surroundings. Making this investigation allows the computational framework to be verified, and so the basic properties of synthetic jets to be comprehended. Of particular interest is acquiring the turbulent structures from undigested experimental data. The hierarchy of established coherent structures presented here provides a credible explanation for the turbulent characteristics that are observed both in the experiments and the simulations. The computations are conducted by OpenFOAM C++ with two turbulence models, SST and RSM, are used to predict the synthetic jets flow fields. Although the models are capable of simulating time-averaged turbulent quantities, they underestimate phase-averaged turbulent quantities. As Reynolds number increases, the underestimates intensify.

Turbulence modelling of an unsteady periodic zero-net-mass flux jet

The purpose of this article is to present the results of a series of numerical simulations of zero-net-mass-flux (synthetic) jet actuators that are acquired by three turbulence models. Shear stress transport k–ω-based models, Reynolds stress ɛ-based models, and Reynolds stress ω-based models are applied and the results are compared along with the experimental data. Computations are performed with usual Reynolds-averaged Navier–Stokes equations solved in a time-dependent mode, for the simulation of a synthetic jet in quiescent environment. The collocated finite-volume approach is used by FOAM C++library. The present investigation focuses on the ability of the turbulence models to substantiate the phase-averaged Reynolds equations. The reason of the poor reflection of phase-averaged values by numerical modelling is the crux of the current contribution but time-averaged values are predicted to be more reasonable.