Basman Elhadidi

Experimental and computational study of perforated floor tile in data centers

Current CFD simulation studies of large data centers cannot model the detailed geometries of the perforated tiles due to grid size limitation. These studies often assume that the tile flow can be modeled as constant velocity based on a fully open tile. In this case, mass flux is enforced at the expense of under-predicting momentum flux; the error in momentum flux can be as high as a factor of four for a 25% open perforated tile. Since jet entrainment is a strong function of its initial momentum flux, this error can be significant with respect to predicting the mixing of the surrounding room air into the tile flow. Combined experimental and computational studies were carried out to quantify the importance of the detailed tile geometry, and it was found that proper prediction of the mixing process must account for the tile opening patterns. Suggestions of how to model the floor perforated tiles in data center CFD simulations are then presented.

Particle Levitation Due to a Uniformly Descending Flat Object

This article presents analytical and computational fluid-dynamics (CFD) solutions of the unsteady flow resulting from a horizontal circular disk moving downward at a constant velocity toward a horizontal floor seeded with spherical micro-particles, and the effect of this flow on particle detachment and levitation. The selected configuration is a simplification of numerous practical applications in which particle resuspension is important, for example a foot or an object impacting a dusty floor, or a squeeze film thrust bearing with particle contamination. The resulting radial and axial velocity field, coupled with a particle detachment model and the particle equations of motion were employed to compute particle trajectories in the gap. The CFD solutions were utilized to describe the high-speed radial wall jet and the vortices developing outside the disk and to explain their role in particle levitation and entrainment. It is shown that as the gap narrows the resulting radial velocity close to the disk perimeter is high enough to detach and levitate μ m-size particles, and that the vortices shed by the descending disk and its high-velocity radial wall jet create an upward convective motion that contributes to particle resuspension from the floor and entrainment in any far-field flow that might be present around the descending disk.

Correlations and wavelet based analysis of near-field and far-field pressure of a controlled high speed jet

Discussed within are the experimental results obtained via simultaneous near-field and far-field measurements of a controlled jet. A large database of both open-loop and closedloop control cases are carried out. The two most interesting investigations from the openloop (OL) and closed-loop (CL) studies are highlighted here and compared to the uncontrolled jet. For the open-loop forcing, the shear layer is seeded with an axisymmetric sinusoidal input with a frequency of 1700 Hz. For the closed-loop case, the shear layer is seeded with a sinusoidal forcing amplitude modulated with the mode-filtered near-field pressure sampled from the stream-wise location at x/D = 6. Data analysis looks at both averaged and instantaneous quantities. In an averaged sense the introduction of control significantly modifies the characteristics of the developing shear layer. Open-loop control is shown to modify the phase lag between both near-field stations with a penalty in an increase in the overall sound pressure level. The closed-loop control imparts more subtle changes to the flow field but yields a slight reduction in the acoustic pressure at the microphone closest to the jet axis. Wavelet based filtering exposes the evolution of the broadband intermittent events seen at both stations.

Preliminary investigation of the active flow control benefits on wind turbine blades

This paper investigates the bene t of ow control over a 2D airfoil specially designed for wind turbine applications. The experiments were carried out in Syracuse University’s Anechoic Wind Tunnel, both with and without large scale unsteadiness in the freestream. When there is no large scale unsteadiness introduced in the ow, under open loop ow control conditions with unsteady blowing, the leading edge separation was delayed and maximum lift coe cient was increased. For the cases where large scale unsteadiness was introduced into the ow, the experiments showed that both open loop and closed loop control cases were able to reduce lift uctuations by a measurable amount. However, only the closed loop control case which utilized surface pressure information from the airfoil near the leading edge was capable of consistently mitigating the uctuating load.

Aerodynamic Resuspension of Particles Due to a Falling Flat Disk

In this article, computational fluid dynamics (CFD) is used to compute the unsteady, compressible flow caused by a flat falling circular disk. CFD results are coupled with a particle trajectory model to determine particle trajectories for different particle sizes and examine which particles are trapped or ejected from the closing gap. CFD results show that the normalized radial velocity profiles are self similar for different Reynolds numbers based on the gap height, and that flow compressibility results in a density increase toward the center of the disk for decreasing gap height. To efficiently couple CFD results with the particle trajectory model, a regression model representing both the velocity field and density variation inside the gap is developed and coupled with a particle detachment model. Particle trajectories are shown to be sensitive to the flow compressibility and must be taken into account. The escaped and trapped particle distribution is computed for a thin mono-layer of Arizona Test Dust under the disk. The simulations show that higher impact velocities tend to cause more of the heavier and larger particles to be trapped under the disk because they are catapulted from the floor and are quickly driven down again by the descending disk.

Comparison of CFD with BEM Technique for Wind Turbine Simulation of Thin and Thick Rotor Blades

In this paper we compared computational fluid dynamics (CFD) simulations with the blade element momentum (BEM) model for two wind turbines. In the first turbine the blade t/c=15% is constant and in the second turbine t/c varies from 30% to 21% at the tip section. The CFD simulations were computed using an open source code (OpenFoam) and the BEM results were obtained from the NREL code HAWT_OPT. Results suggest that CFD and BEM compare well for the thicker turbine blades. The results compare well because both the lift and drag forces (mostly pressure drag in case of thick blades) is comparable to the experimental data used in the BEM model. The difference in the results between the CFD and BEM for the thin blades can be attributed to the errors in computing the aerodynamic drag, which is mostly skin friction drag.

Experimental Verification of Pitch Angles for Power Regulation of Variable Pitch Horizontal Axis Wind Turbine

In this paper a variable speed variable pitch horizontal axis wind turbine (HAWT) is designed using the blade element momentum (BEM) model and experimentally tested in an open section wind tunnel. The BEM offers a fast design approach since it is essentially a twodimensional strip theory model and is used for all preliminary designs and for real time wind turbine control. Two wind turbines were designed that have the same rated power (0.45 kW). In the first design the blade is composed of a constant thickness blade (t/c = 15%). In the second design the blade is composed of a variable thickness blade varying from t/c = 25% at the hub to t/c=21% at the tip. The model results show that thick blades are less sensitive to errors in the pitch angles as the turbine switches from variable speed to variable pitch control. The thick blades were also easier to manufacture using CNC machines. Three sets of experimental results were obtained for pitch angles of -5 o , 0 o and 10 o . The experimental measurements compare favorably for the 0 o and 10 o which correspond to cases in which the power coefficient is large (order of 10%). The experimental measurements for the -5 o does not compare well with the model. This is attributed to two issues. First the blades operate in the stall regime and hence the BEM model is not valid, and second the power coefficient is relatively small (order of 1%). Overall the BEM theory compares well with experiment, with a correlation coefficient, R 2 , of 0.9.

Efficient Domain Decomposition Technique for Solution of High Amplitude Acoustic Wave Scattering in Nonuniform Flows

Numerical results suggest that linear flow simulations are acceptable for very low amplitude acoustic pulses as expected from analytic theory and perturbation analysis. Tests were conducted to investigate the importance of mean flow gradients in a converging-diverging channel and in the vicinity of a stagnation point. The results show that accounting for nonlinear effects is not significant for low amplitude acoustic pulses in accelerating flows; however, it must be accounted for in the vicinity of stagnation points. The results show that accounting for nonlinear flow effects by using a domain decomposition technique can eliminate the need of computing the entire domain using as nonlinear solver and that both the computational and time requirements can be reduced by about 40%. The results show that the nonlinear domain around the scattering body does not need to be very thick, just a few cell widths in the normal direction to the body.

SOUND SCATTERING FROM ACOUSTICALLY TREATED BODIES USING A DOMAIN DECOMPOSITION TECHNIQUE

Airframe noise is a significant component of the aircraft signature. In this paper the impact of coating airfoils with an acoustic absorbing material is considered. The finite element method (FEM) is implemented to mo del the homogenous uniform air, the non -homogenous flow and the isotropic thin absorbing layer. Each domain has different governing equation which requires the use of a domain decomposition (DD) approach using appropriate coupling conditions. A ppropriate causality conditions are implemented at the far field to ensure no reflections of waves in the near field. The numerical technique was verified using the method of manufactured solution (MMS) and validated with some of the results published in literature. R esults show that the unsteady lift depends on the airfoil camber/chord ratio, reduced frequency of incident acoustic wave and direction , and absorbent impedance. The results show that a thin isotropic absorber coating an airfoil modifies both the acoustic pre ssure lobes in the far field, the unsteady lift and attenua tes the propagating sound power. The sound power attenuated for the NACA 2412 coated with a thin absorber with acoustic permittivity � = 5 -0.5i, subject to an incident plane wave with reduced frequency k=5, is 7.3 dB . Nomenclature