Date of Award

7-2022

Degree Name

MS in Mechanical Engineering

Department/Program

Mechanical Engineering

College

College of Engineering

Advisor

Russell Westphal

Advisor Department

Mechanical Engineering

Advisor College

College of Engineering

Abstract

The primary objective for this work was to evaluate the reliability of computational fluid dynamics (CFD) tools in the prediction of upstream surface pressure disturbance and pressure drag of various instrument excrescence shapes for a small aircraft flight test device called the Boundary Layer Data System (BLDS). Insights on pressure disturbance will serve as a guide for the placement of BLDS probes/sensors, and pressure drag can be used to ensure sufficient adhesive is used to install BLDS instrumentation. The Mach number for all CFD cases was 0.12 and the Reynolds number based on excrescence height varied from 4 x 104 to 1 x 105. Excrescences studied have height to local boundary layer thickness ratios 0.75 < h/d < 1.9 and width to height ratios 3 ≤ w/h < 4.

Wind tunnel tests were first conducted in the Cal Poly Fluids Lab’s 2 x 2-foot wind tunnel to obtain measurements of the upstream pressure disturbance created by a blunt BLDS housing and a streamlined BLDS fairing. Upstream surface pressure data was measured for two-dimensional excrescences and for three-dimensional models of the blunt and streamlined housings. A rake measurement of the undisturbed boundary layer profile at the leading edge location of each excrescence was also obtained to compare to the computed boundary layer.

Prior to viscous modeling with CFD, potential flow theory was used to compute the inviscid upstream pressure disturbance for a generic excrescence on a smooth surface. A Rankine oval was generated using superposition, and a MATLAB program was written to evaluate ovals of varying chord and height. The potential flow results for the pressure distribution upstream of a Rankine oval were found to agree quite well with 2-D measurements and viscous CFD.

Ansys ICEM CFD and FLUENT were used for computational modeling. A viscous CFD model was first created in two-dimensions and validated by comparing the upstream pressure disturbance results to the two-dimensional experimental measurements. The validated FLUENT case set-up was extended to three-dimensions, and three-dimensional models were created for blunt and streamlined BLDS excrescences. ICEM CFD was used to generate meshes for 2-D and 3-D models and FLUENT was used to solve the Reynolds-Averaged Navier Stokes (RANS) equations in conjunction with the Spalart-Allmaras turbulence model. Mesh independence studies and evaluation of discretization error were conducted to ensure that the final mesh employed provided adequate spatial resolution. The computed flow features, and results for dimensionless pressure and drag, were compared to experimental measurements and classic aerodynamic principles to evaluate the CFD solutions.

It was concluded that CFD can accurately compute upstream pressure disturbances and pressure drag for excrescences mounted to a smooth surface. The viscous calculations showed that the effect of excrescence shape on upstream pressure field is only significant within 6 body heights of the leading edge. Beyond that, no significant difference in the pressure disturbance was observed between different excrescence configurations. Additionally, the spanwise pressure disturbance was found to become negligible at about 1-1.5 housing widths away from the upstream centerline of each excrescence regardless of its shape. Finally, all computed blunt housing models resulted in a pressure drag coefficient of about 0.5 which corroborates past experimental drag measurements. This thesis has set-up a working FLUENT CFD case that can be used for future computational studies related to the BLDS and provides detailed guidance for existing BLDS housing shapes beyond the rules of thumb currently used for informing housing designs.

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