Jasim Ahmad

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Navier-Stokes Simulation of Local Winds Over the Earth’s Topography

Computational Fluid Dynamics 2006, 2009

A numerical approach is described that simplifies and automates the CFD solution process so that ... more A numerical approach is described that simplifies and automates the CFD solution process so that Earth scientists can utilize high-resolution Navier-Stokes flow solvers as a research tool to investigate wind events on the Earth's surface. The current approach utilizes the OVERFLOW-2 structured overset RANS code. A genetic algorithm is used to obtain an optimal multi-zone overset grid system that reduces the grid size and simulation time by maintaining high resolution over high-gradient land regions and lower resolution over low-gradient water regions. Flow simulations are presented that include flow separation and reattachment over mountainous terrain for coastal islands in Alaska (USA) and British Columbia (Canada).

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An Overset Grid

Computations of rotorcraft aeroacoustics with a Navier-Stokes/Kirchhoff method

Hovering Rotor and Wake Calculations with an Overset-Grid Navier-Stokes Solver

Rotorcraft Aeroacoustics Computations with Overset-Grid CFD Methods

Journal of the American Helicopter Society, 1999

Page 1. Rotorcraft Aeroacoustics Computations with Overset-Grid CFD Methods Roger C. Strawn Earl ... more

Computer Simulation Performed for Columbia Project Cooling System

This demo shows a high-fidelity simulation of the air flow in the main computer room housing the ... more This demo shows a high-fidelity simulation of the air flow in the main computer room housing the Columbia (10,024 intel titanium processors) system. The simulation asseses the performance of the cooling system and identified deficiencies, and recommended modifications to eliminate them. It used two in house software packages on NAS supercomputers: Chimera Grid tools to generate a geometric model of the computer room, OVERFLOW-2 code for fluid and thermal simulation. This state-of-the-art technology can be easily extended to provide a general capability for air flow analyses on any modern computer room. Columbia_CFD_black.tiff

The Side-by-Side Urban Air Taxi Concept

AIAA Aviation 2019 Forum

Calculation of viscous flows with separation using Newton's method and direct solver

26th Aerospace Sciences Meeting

An efficient and reliable numerical solution technique is presented for solving problems of stead... more An efficient and reliable numerical solution technique is presented for solving problems of steady viscous (laminar) incompressible flows in a plane. The fully-implicit technique combines Newton&amp;amp;#39;s method with a direct solver based on banded Gaussian elimination. The method is applied to solve an external and an internal flow problem using the Navier-Stokes equations in the primitive variable form, the stream-function

Numerical Simulations of an Unsteady Rocket Launch from the AH-64D Apache Longbow Helicopter

Computational modeling of hovering rotors and wakes

38th Aerospace Sciences Meeting and Exhibit, 2000

An overset grid Navier-Stokes/Kirchhoff-surface method for rotorcraft aeroacoustic predictions

34th Aerospace Sciences Meeting and Exhibit, 1996

This paper describes a new method for computing the flowfield and acoustic signature of arbitrary... more This paper describes a new method for computing the flowfield and acoustic signature of arbitrary rotors in forward flighr The overall scheme uses a finite-difference Navier-Stokes solver to compute the aerodynamic flowfield near the rotor blades. The equations are solved on a system of overset grids that allow for prescribed cyclic and flapping blade motions and capture the interactions between the rotor blades and wake. The far-field noise is computed with a Kirchhoff integration over a surface that completely encloses the rotor blades. FIowfield data are interpolated onto this Kirchhoff surface using the same overset-grid techniques that are used for the flowfieid solution. As a demonstration of the overall prediction scheme, computed results for far-field noise are compared with experimental data for both high-speed impulsive (HSI) and blade-vortex interaction (BVI) cases. The HS! case showed good agreement with experimental data while a preliminary attempt at the BVI case did not. The computations clearly show that temporal accuracy, spatial accuracy and grid resolution in the Navier-Stokes solver play key roles in the overall accuracy of the predicted noise. These findings will be addressed more closely in future BVi computations. Overall, the overset-grid CFD scheme provides a powerful new framework for the prediction of helicopter noise

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Progress Toward Generation of a Navier-Stokes Database for a Harrier in Ground Effect

2002 Biennial International Powered Lift Conference and Exhibit, 2002

A process is described that enables the generation of 35 time-dependent Navier-Stokes solutions f... more A process is described that enables the generation of 35 time-dependent Navier-Stokes solutions for a YAV-8B Harrier aircraft in ground effect in one week. The solution time is dramatically reduced through process automation and the use of large-scale parallel computers. A dual time-stepping algorithm is described which improves solution time accuracy and robustness. The static computations capture the suckdown and ground cushion effects. A method for computing the stability derivatives of lift variation with height and angle of attack is presented. Unsteady flow visualization and a discrete Fourier analysis are also used to identify and correlate key flow features with the time variation of lift.

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Automated CFD Database Generation for a 2nd Generation Glide Back Booster

21st AIAA Applied Aerodynamics Conference, 2003

A new software tool, AeroDB, is used to compute thousands of Euler and Navier-Stokes solutions fo... more A new software tool, AeroDB, is used to compute thousands of Euler and Navier-Stokes solutions for a 2 nd generation glide-back booster in one week. The solution process exploits a common job-submission grid environment using 13 computers located at 4 different geographical sites. Process automation and web-based access to the database greatly reduces the user workload, removing much of the tedium and tendency for user input errors. The database consists of forces, moments, and solution files obtained by varying the Mach number, angle of attack, and sideslip angle. The forces and moments compare well with experimental data. Stability derivatives are also computed using a monotone cubic spline procedure. Flow visualization and three-dimensional surface plots are used to interpret and characterize the nature of computed flow fields.

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Automated CFD Parameter Studies on Distributed Parallel Computers

16th AIAA Computational Fluid Dynamics Conference, 2003

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Numerical Simulation of Vortex Breakdown

Vortex breakdown is simulated by a three dimensional Lagrangian method using vortex filaments. Th... more Vortex breakdown is simulated by a three dimensional Lagrangian method using vortex filaments. The filaments are approximated by vortex elements and their velocity is computed by a Biot-Savart type law of interaction. The numerical calculations show the development of an axisymmetric bubble with a recirculation zone and resemble in many respects the results obtained in the physical experiments on vortex breakdown.

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Water Chemistry Models for Passur River of the Sunderban Mangrove Forest

Computer Visualization of Vortex Wake Systems

AIAA Journal, 1999

Navier-Stokes Simulation of Air-Conditioning Facility of a Large Modern Computer Room

A 3-D full Navier-Stokes simulation of a large scale computing facility at NASA Ames Research cen... more A 3-D full Navier-Stokes simulation of a large scale computing facility at NASA Ames Research center was carried out to assess the adequacy of the existing air handling and conditioning system. The flow simulation of this modern facility was modeled with a viscous, compressible flow solver code OVERFLOW -2 with low Mach number pre-conditioning. A script was created to automate geometry modeling, grid generation, and flow solver input preparation. A new set of airconditioning boundary conditions was developed and added to the flow solver. Detailed flow visualization was performed to show temperature distribution, air-flow streamlines and velocities in the computer room.

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Comparison of Computed and Measured Vortex Evolution for a UH-60A Rotor in Forward Flight

31st AIAA Applied Aerodynamics Conference, 2013

A Computational Fluid Dynamics (CFD) simulation using the Navier-Stokes equations was performed t... more A Computational Fluid Dynamics (CFD) simulation using the Navier-Stokes equations was performed to determine the evolutionary and dynamical characteristics of the vortex flowfield for a highly flexible aeroelastic UH-60A rotor in forward flight. The experimental wake data were acquired using Particle Image Velocimetry (PIV) during a test of the fullscale UH-60A rotor in the National Full-Scale Aerodynamics Complex 40-by 80-Foot Wind Tunnel. The PIV measurements were made in a stationary cross-flow plane at 90 deg rotor azimuth. The CFD simulation was performed using the OVERFLOW CFD solver loosely coupled with the rotorcraft comprehensive code CAMRAD II. Characteristics of vortices captured in the PIV plane from different blades are compared with CFD calculations. The blade airloads were calculated using two different turbulence models. A limited spatial, temporal, and CFD/comprehensive-code coupling sensitivity analysis was performed in order to verify the unsteady helicopter simulations with a moving rotor grid system.

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Visualization and Quantification of Rotor Tip Vortices in Helicopter Flows

53rd AIAA Aerospace Sciences Meeting, 2015

This paper presents an automated approach for extraction, visualization, and quantification of vo... more This paper presents an automated approach for extraction, visualization, and quantification of vortex core radii from the Navier-Stokes simulations of a UH-60A rotor in forward flight. We adopt a scaled Q criterion to determine vortex regions and then perform vortex core profiling in these regions to calculate vortex core radii. In particular, the vortex core radii are displayed graphically in a plane using a new color map scheme. This method represents an efficient way of visualizing and quantifying rotorcraft vortical flow fields.

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